View Full Version : The piezo fun box
INTRODUCTION
I have different types of drivers from various manufacturers since I like to try a number of things. This simple project is a way for me of letting go some older drivers in good shape as replacements arrive.
Though Piezos are not "noble" and snobbed by some, they're still known drivers, of low-cost that can produce acceptable sound if they are properly accompagnied by a simple crossover network.
Motorola did sell tons of these not only for individuals, but also to some known speaker manufacturers, as well as less known ones, and still counting under other names (back on this later).
Among known manufacturers who used these: Community, Peavey, Fender, Sun (piezo source). They can't be all stupid in my view, though admittedly not high-end drivers, but can still do the job adequately. Motorola Engineers published in the JAES about their invention or discovery in 1975 if I recall correctly, and their specific design is patented. My understanding is they are deemed to be the high-end of piezos.
Motorola's most famous being the KSN 1005A 3 1/2" Super horn tweeter, which I'm tempted to call somewhat of a "poor man's version" (not a copy) of a 2402 for example. JBL Selenium Brazil did make lower cost "copies" of the latter, such as ST200, ST-304, ST-400, and ST-350 (50°X50°), as well as Sammi (China?) HF 200A, and of 2405 as HF 250.
The pair of boxes in the making are low-cost ones for the neighbor's ambience music outside on the deck/near the pool. No pretention of being great high-fi or sound reinforcement cabs. Attracting rave reviews isn't the intent, rather just sound ok in the particular context (e.g. city noise). Simply a fun project.
Btw, the two-way pair of speakers are given to him absolutely free. There's about $60+. of material per box, and my time. The main purpose of the project being to say goodbye to some working drivers that I didn't want to throw away, the same logic applies to other materials in stock such as plywood leftovers from other projects that can be used for smaller box sizes.
Last but not the least, these boxes should sound better than the table radio's bad sound he's projecting when making his pool and deck gatherings. So, in a way its an "investment"...
Of the six piezos I have, 4 of them are 1005 type (2X 100% original Motorola I keep; 1 "semi-original" Motorola given to me- element replaced with OEM?; 1 Asian copy available purchased to make a pair, the latter two going in the neighbor's boxes-see below), plus two KSN 1025A 2X6" mid/high horns, 75W, that are 100% original Motorola. They also made a high-power "Powerline" 400W version of this one. I'm first owner of all originals purchased new. (Back later on Motorola original VS others...).
Since Motorola originals cannot be purchased anymore (with a nuance), contrary to a large number of cheap "copies", I didn't want to give here (see below) "knock-offs" that anyone can easily purchase by themselves, plus for neighbor's use of the boxes it won't make much difference re ambient noise...
If someone wants the latter two 1025 type originals they are available for FREE, packaging included, shipping cost excluded. Since these are light and relatively small, shipment cost should be low, the carton they're in now/for shipping is about 0.25 cubic foot. Tested them today with music, still work ok, good condition. If interested to play with these just let me know by private message or in the thread. Actual ext. dimensions: about 3 X 7.5 X 4" deep. Then 4 of them will be gone, I keep only 2 X1005 originals.
If a person doesn't feel familiar enough "electrically" speaking to play with these, no problem as I can also provide free easy to read technical and simple crossover info on these from Motorola and others, including from a Peavey Loudspeaker Design Engineer. Though many people on the net have their own "recipe" on this, good or bad, I prefer to pass along the real thing from acceptable makers.
In follow-up article, as it comes, I'll describe with some detail the project parameters/tech info. Even if its a low-cost project, to me it's not just a matter of throwing a bunch of components together in a box. There's still some modeling, tweaking and testing work and choices/decisions to be made: e.g. though they don't REQUIRE a crossover, they perform/sound better and accept more power with a simple one... Regards,
Richard
1audiohack
08-25-2018, 11:41 PM
Sounds fun.
Piezo’s are crazy things aren’t they? For sure they sound better with a resistor across them and some form of crossover.
Barry.
Hi Barry,
Thanks for dropping in.
Sounds fun? You bet, something different, and outside is somewhat another challenge than building for the comfort of our homes... In this case MANY things to think about e.g. different acoustics, even the presence of many kids who's fingers want to touch everything, pool water flying around, easy carrying as not permanent install, etc. The box itself isn't too different but the context in which it is used IS different, so things need to be "adjusted".
Piezos ARE crazy things, and even then millions were sold, not rocket science though, in fact simple to high-pass, pad down, reduce top end a bit, protect amp from capacitance, just need to know how. Good manufacturers make it easy enough, as well as that Peavey Engineer. Regards,
Richard
I have a pair of motorola 1001 copies. I have put them in old Sanyo sx 170 which were given to me with blown tweeters. I replaced them with those piezos and they are good for use again. Piezos are not match to bass drivers in quality aspect, but speaker now sound decent... I am interested in your piezos because I heard that original motorolas plays bettert han copies. It is shame that those are not same model but I would like to give them a try.
Hi MoD,
Thanks for your post.
RE: "speaker now sound decent" That's exactly my point, decent, ok, correct, no pretention of being high-class. Though I seem to remember seeing somewhere a $3,500. commercial speaker using a piezo?
RE: "heard that original motorolas plays bettert han copies" stay tuned for that because I'll post shortly what I said in a previous post about Motorola VS others, from someone who should know...
Btw there's a small typo I need to correct in my first post, its not model 1025A/B that I give free but rather model 1025A. The B version has beveled edges that mine don't have, rest is same.
The ksn 1001 you have is a small rear mount model VS the larger 1025A model I give away is usually front mounted.
RE: "I am interested in your piezos ... I would like to give them a try." I don't have a problem with that in itself. However, Croatia being very far away from me, one must realize about shipping delays, gear condition on arrival, postal price, etc. when overseas shipment. I cannot guarantee such things, can only do my best re packaging, but overdoing that increases shipping price that YOU have to pay to me at cost to ship.
On the other hand, maybe its your lucky day currency exchange wise, if Croatia has adopted the Euro worth much more than the Canadian $, even more so than USD! Which would further reduce your expense...
Send me a private message I'll explain to you how to do it to minimize your out of pocket money for shipping cost, same principle as I did for the BGW amp and EQ given on this site recently.
RE: "It is shame that those are not same model" (as mine), more sad than shame I understand, but I'd like to hold-on to my pair of original Motorola KSN 1005A, as well as other tweeters I have, for possible future projects and these are somewhat difficult to replace with same... If I ever change my mind on that I'll let you know, but for the time being its the 1025A pair. Regards,
Richard
"Motorola no longer does the piezo's, CTS has taken over the piezo operation. They are still considered to be the premium piezo source, although there have been claims by some that CTS has not been maintaining the quality levels that Motorola did.
Le son, and many of the other sources, were distinctly second tier compared to the original Motorola parts.
Jon Risch" (High Efficiency Speaker Asylum, Forum, May 28, 2002). This should answer MoD's question.
Who is Jon Rish? I checked. Senior Project Engineer, Transducer Engineering Dept. at Peavey, with specialties in Speaker system design, Crossover filter design, Transducer analysis & development; there almost 30 years (Linked In). Education Electrical Engineering, etc. Was granted a few patents in audio.
Peavey used piezos from others in many of its speaker models and its unlikely they just threw these in their boxes without product knowledge and Engineering work.
The relation between Risch's years at Peavey and brand name piezos, is the latter were more widely seen/used in speaker systems years ago, making it quite feasible he was involved then with the drivers, plus its likely he designed himself systems using these. In my view he's a credible source regarding his piezo writings here and others he's written on how to use piezos: crossovers, padding, multiples, etc.
Finally, as mentioned by Risch, CTS took over the piezo business from Motorola, however when CTS ceased making these, then Piezo Source took over Motorola's piezo business in 2005: Eng., rights, designs, mfr line, molds, etc. THEY are NOW considered the premium piezo provider and sell to OEM plus directly online. There is on Piezo source's site a piezo technical document mostly from CTS (2nd half or so more accessible and practical).
More to follow. Regards,
Richard
EDITED for clarity
Hi MoD,
You haven't responded to my post about getting the transaction done for the free pair of original Motorola KSN 1025A piezo 2 X 6" mid/high horns in good condition.
In the meantime I've found out that Crotia's currency isn't the Euro (well known/traded here along with the USD) but rather the Crotian Kuna (seldom seen/traded here), which would make things much more complicated to cover the shipping cost. I assume its the explanation and have to consider you changed your mind about getting these.
Therefore, for the benefit of others, the two piezos ARE still available for free (packaging incl., shipping cost excl.), "while the offer lasts" as they say. If interested, send me a private message or inquire here. Regards,
Richard
EDIT to post # 7:
In order to give folks some idea about shipping cost, I recently sent by Accelerated postal service a BGW amp and EQ from eastern Canada to Nevada, USA which is pretty far from me. The smaller of these two cartons, the EQ one, had a volume of almost one cu. ft. and weight of 4.115 Kg (by Canada Post), shipping cost was 40 something Canadian dollars taxes included. The carton with the two piezos is 0.25 cu. ft. and weight is 1 Kg max. So I suppose shipping cost in North America for e.g. could be about $15 CAD (equivalent to $12 US/10 Euros). I can get a Canada Post shipping quote if one is interested by these.
Since I have three versions of the Motorola KSN 1005A tweeter on hand, one pair original, one semi-original unit and one unit Asian copy to make a pair with the semi, I was interested in knowing with a music listening test, comparing two at the time, how they sounded respectively and versus each other... No crossover, just as it comes "off the shelf".
The original's response has emphasis around 5 khz and some at 15-20 khz, both of which are relatively easy to cure with simple components (resistors and capacitor), more on this later.
The "copy" with no name is the disappointment. Whether compared to the original or semi, it is shy on very high-frequencies, lacking some output there even untreated. Not only less "hissy" than better ones, but also less high-end detail. Will address this issue with the components choice mentioned above.
On the other hand, the copy has more evident emphasis, than the other two, towards the low-end of its spectrum. For example, voices are definitely accentuated vs the other piezos. This one might need a little more correction around the usual 5 khz bump in response. A slight change in resistor value within the High-pass section should take care of that. Finally, the copy's slots directing sound to the horn's throat are of poor design/execution compared to Motorola's original nicely made ones (I opened both piezos to see). I'll try getting another cheap one here, hopefully from another bunch, just to see if its the same or better made.
The semi-original is the pleasant surprise (was keeping it as spare). Globally, its pretty close to the original's sound. Just a tiny bit less output and extreme highs. This one, given to me or taken out of a used speaker cab long ago, is a Motorola identified horn with a non-identified back cover (contrary to original ones) which makes me believe the element may have been changed with a non-OEM. Could be an element from one of the better copies. For example, GRS brand piezo tweeters sold by parts Express claim "Using a 22 mm ceramic piezo element, GRS piezo tweeters follow CTS' original recipe" (Not necessarily Motorola recipe).
Btw I don't recall ever seeing an official sound dispersion spec nor a coverage angle figure for this KSN 1005A horn tweeter. The angle finder tells me 60° should be the nominal number. (2402: 40°)
More to come.
Richard
I've changed course a bit, RE "I'll try getting another cheap one here, hopefully from another bunch, just to see if its the same or better made." from my last post. I ended-up purchasing not one but rather FOUR more low cost piezo 1005A copies for evaluation and testing. Might as well make this a little "educational" by the same token. Plus I've taken many pictures of inside a piezo tweeter, to compare original vs some copies. To be posted soon.
The first copy purchased recently to make a pair with a semi-original, mentioned in my previous post, had too much "mid" emphasis and was lacking some high end. This IS the black sheep among all copies now have, others ARE better.
Because of the poor experience with that first copy, that I opened to see inside and wasn't impressed by the six slots' craftsmanship, I brought along a small flashlight to look deep inside the horn when going to purchase more of these. Quite revealing.
Of the ten or so I looked at, four had serious machining/molding slot issues, like 2-3 or 4 slots partially blocked by halfway done cutting or leftovers. Discarded those. Others had minor similar issues and took home the four best ones seen. Morale: If you're going to buy piezo tweeter copies in a store bring some light with you to check the slots' shape at the bottom of the horn as seen from its mouth. This gives a clue on how well or bad its made and reduces the chances of buying a donkey... My black sheep, after slot corrections, remains the same so it may have a piezo ceramic element or "cone" issue.
The 6 slots are basically the sound's passage way leading to the horn's throat. How can the sound properly enter, go through this passage way and throat plus exit the horn's mouth correctly if the slots located at the beginning are poorly made to start with?
First thing after getting the four copies I opened each driver to check the slots more carefully and the front part of the piezo element (i.e. the "cone"), not the ceramic piezo thing behind it to avoid damaging. But I WILL go there soon with the black sheep tweeter and take pictures, plus hopefully measure the ceramic device VS the GRS mentioned 22 mm...
I corrected slightly with a small very sharp knife the slots incorrectly made, as these should be straight and rectangular shaped. I removed just enough excess plastic to get there, though a few tiny bits remain since too difficult or risky to remove. Too bad I thought about taking pictures only after correcting the first unit of four as it was the worst of the group, with few pretty weird looking slots initially...
Then a listening test of each new unit, no crossover, etc., with music, comparing each first with the black sheep to hear if better (yes, all four are much better sounding), after each new one again VS my semi-original Motorola which is pretty close sound wise to the fully original. The four corrected copies sound the same.
Honestly, the four slots corrected copies are close to the semi unit on sound quality, having more balanced sound than the black sheep- less mid more highs. The differences noted were the four copies have I'd say about 2db more output/sensitivity globally, so a little louder, and the very top end of the semi has just a little more detail/sweeter.
So, I'm somewhat surprised by the four copies being better than expected, tested after slots correction. Reason for getting four? Basically to give just a little validity to the sample. Had I got one only and it been differently poor than black sheep then what do you do/think? The other reason being trying to get two more closely matched units for the neighbor's boxes, instead of a pair made of the semi and black sheep, also meaning, if need be, doing identical electrical corrections in X-over. I didn't compare the four units to the fully originals I have as I thought initially that comparison would be unfair.
Bottom line, of the five 1005A copies on-hand, the black sheep will be scrapped on the "dissection table" for "educational" purposes (back later/picture), I'll take any two of the four remaining slots corrected copies for the project mentioned in post # 1, since they look and sound the same, and I'll put the remaining two in the carton with the originals 1025A units, all being given away for free here (excl. ship.), since I don't need them. The1005A do fit in same carton, I checked, and btw according to Piezo Source the 1005A weight is 75 grams and 1025A weight is 130 grams, total drivers weight = 410 grams, adding the pretty small carton weight I don't see how this would reach or exceed one Kg.
Remember, the initial project objective is to reduce driver stock, not increase it. I'll keep only the two 1005A Motorola originals, and the semi-original one as spare just in case... More on the way.
Richard
Was looking today at some well-known speaker forum where Q & A are given about hooking up piezo tweeters and doing crossover, pad, etc. The number of unbelieveable answers given is nothing short of alarming (e.g. use of an 8 ohms 10 W parallel resistor, over heating btw, to pad down such tweeter!!). Even though trying to help in all good faith MANY of the answers seen are plainly WRONG, doing no good or counter productive. These folks have certainly not looked at Motorola, CTS, Piezo Source and Jon Risch's stuff on these issues, but instead give their own "recipes". Hence my previous mention that I prefer to pass along info from credible sources...
Considering the above, as a preview or warning note, lets just say for now that in most cases you'll need only up to four easy to find components: two capacitors and two resistors, no requirement for inductors. One simple "safety" resistor (may also be used to modify response) in series re amp stability vs capacitive nature of piezos (one included in the 1025A), one series capacitor to pad down the tweeter if need be (not really a resistor as seen above), one resistor in parallel to make a load looking like a resistive one (may also be used to modify response), and one series capacitor that acts as a high-pass filter. Simple.
Will discuss the order of these in the signal path and their values further in a future post. Regards,
Richard
EDIT: The logic here being the lower cost driver is dealt with simple lower cost solutions the vast majority of times (e.g. crossover, pad, etc). Otherwise it would sort of defeat the purpose of that purchase. Clever Motorola and other Engineers...
I took 28 pics total, from which I'll post a dozen or so of the more revealing. Mostly grouped by subject or issue for comparison purposes (e.g. copy vs original or before vs after some correction).
First group: Pic 1 Original Motorola nicely made chamber, slots are pretty uniform size, straight and rectangular from top to bottom. Pic 2 "Nice" copy's chamber and slots before correction, small plastic bump above 6 o'clock slot; slots at 4, 8 and 10 o'clock curiously machined/molded at the bottom; slots at 8 & 12 o'clock aren't really straight. Pic 3 A copy's more uniform slots after some corrections and plastic bump removal by me with a sharp blade.
Other pics on the way
EDIT: order of pics came out wrong, damn it, first is #3, second #2 and third #1...So in this post one should look at the pictures from bottom to top.
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Second group: Pic 1 Nice Motorola original "cone" on front of piezo ceramic element. Btw it has more play/loose and "suspension" than one from the copies. Pic 2 A copy's cone, sorry pic not as clear as I would have liked.
Other pics on the way
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Third group: pic 1 Poor copy's view of the slots seen from horn's mouth with light from the back, light showing slots have a uniformity ussue (a revealing trick), that piezo copy is the one being scrapped for "educational" puposes in some later pictures. Pic 2 Sharp blade corrected slots on a relatively nice copy, again front view with light in the back, this is basically how it should look like, light seen being more uniform.
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Fourth group: Pic 1 Copy seen before slots correction, plus molding plastic bump present here under 12 o'clock slot. Pic 2 A copy seen after corrections of the above issues.
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Fifth group (last for the time being): Pic 1 The scrapped black sheep piezo's ceramic and cone elements pulled out. Pic 2 Ceramic element has 21 mm diameter, also confirmed with measuring tape VS CTS/GRS 22 mm mentioned previously. Close. Pic 3 View of copy's cone attached (glued?) to ceramic piezo element. Btw cone diameter measured at 40 mm (or 1.5+ inch).
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TomR3399
09-04-2018, 10:46 AM
Fifth group (last for the time being): Pic 1 The scrapped black sheep piezo's ceramic and cone elements pulled out. Pic 2 Ceramic element has 21 mm diameter, also confirmed with measuring tape VS CTS/GRS 22 mm mentioned previously. Close. Pic 3 View of copy's cone attached (glued?) to ceramic piezo element. Btw cone diameter measured at 40 mm (or 1.5+ inch).
822168221582214
This is my second post to the Forum ...so hopefully I get it right.
I am attaching a white paper on Piezo's that may be helpful understanding how they behave. The paper is not a cook book, but a overview
of ways to tame them. With some time, and measurement equipment, along with the white paper's general suggestions, I believe piezos can be made to sound good.
I was given the paper from a friend, who implemented the white paper guidelines with reported excellent results - not sure what driver model he used. My friend stated I need to do the hard work myself to truly understand how these they work. Unfortuntley I have not had the extra time to work on the XO's
Tom R
Hi Tom,
Thanks for Community's white paper. I had not seen this one. I glanced through it quickly, not really a beginner's paper though. Btw It refers numerous times to CTS' piezo application note (technical paper) I have among others.
From the piezo pics posted you probably have by now a pretty good picture of how they're made inside and out...
RE: "I believe piezos can be made to sound good." I tend to agree with that if done correctly.
RE: "friend stated I need to do the hard work myself to truly understand how these they work." I think going this route you're making it unduly difficult on yourself. I gave some hint already on this in post # 11, with much further details to follow, including what you need to know/how they work.
"Extra time to work on a crossover"? You certainly can make an extensive one if this is your wish, but a resistor and a capacitor is all you really need in practice here to make an acceptable one. Add another capacitor if pad is needed, and another resistor for "safety". Not a whole bunch of components...
There's other quality sources, like Community, which are easier to follow, understand and apply: Motorola (the inventor of these drivers), CTS who took over after Moto, now Piezo Source, plus one of the rare Engineers who wrote about these in public places with understandable language, Jon Risch of Peavey who used them, like Community. If I were you I'd stick with these and keep Community's paper for later. I'll mention later where some can be found. Regards,
Richard
P.S. If you want to practice yourself on some working copies of the KSN 1005A, and get at the same time original Motorolas KSN 1025A for FREE (excl. ship.) let me know. Offer won't last forever though, now a local fellow would gladly take them but since they were offered here first I respect my commitment, he's next...
A) I think folks who looked at the pictures posted quickly realized some copies are definitely not equivalent to original Motorola (or Piezo Source today). However, some copies may be "salvaged" or improved with a little work in order to reach an acceptable performance level.
B) While I use the word "cone" to describe the piezo's sound producing device, because it looks like one, the CTS Piezo Tweeter Application Note (technical document) more appropriately uses the following term:
"...a specially impregnated diaphragm which then works into a compression volume. Slots in the compression space direct the sound into the throat of the horn. The radial slots are transformed into a 3" circular mouth through the unique shape in the throat of the horn. The actual horn contour is a hybrid design between a pure exponential contour and a hyperbolic one. Again, this computer-generated geometry is optimized for the best acoustical output."
C) My testing of the various piezos on-hand has been carried out using an Onkyo receiver as an amp simply because this unit is very easily accessible, particularly for connections on the back, as it sits on a table, not in the pro gear equipment rack that I'd need to move everytime to access the rear since tests were not all done in one shot. Pure convenience.
On the other hand, the above-mentioned CTS technical paper indicates "Many amplifiers today boast outputs that extend to 100 khz. At those frequencies, ultrasonic resonances may occur between the amplifier and the tweeter, causing damage to one or the other or both." (even more so with piezo arrays). Hence the series wired protection resistor recommended, mentioned previously. The latter prevents this possibly "destructive" resonance issue.
Well, all my testing was voluntarily done without any such resistor, crossover, etc. even at some higher power levels. And, btw, I knew the receiver has response rated at 5hz-100khz +1/-3 db. Additionally, I even blocked totally the receiver's only ventilation at the top with a cardboard! Piezos are supposed to require only very little energy (very efficient) so little amp heat? Its an indirect way to try to assess energy really required...
If the amp heats up with ventilation blocked then the protection circuit should trigger. DON'T DUPLICATE THE ABOVE TEST, it was only for the experiment, at my own risk. Luckily for me ($) nothing ever happened, though it may be different with other equipment. Just wanted to test/see how sensitive modern gear could be to this capacitive load effect and energy/thermal issue. YES I always keep good gear ventilation plus do use a "safety" series resistor in a piezo box to prevent stability problems. A 50 cents or so peace of mind.
D) I think I found the reason why the "black sheep" piezo tweeter didn't sound correctly. While dismantling the device, removing the back cover with attached diaphragm gave some resistance, instead of "loose" fit. Closer inspection of the diaphragm showed its not perfectly round. In post # 16 on the third picture with device held with pliers it can be seen on the left side of it there's a small curve in diaphragm's edge caused by its diameter being a bit too large for the space in the compression chamber, the latter looking nicely round. Or it could be "cone"/ceramic was installed a bit off center in the back cover creating a too tight fit. Not having the necessary play/loose and suspension of a correct one it couldn't "move" properly. Anyway, that one is gone for "scientific" purposes... More to come.
Richard
Though the woofer section of the box isn't the project's main "attraction", there are still important and relevant issues to address here.
Sound reproduction outside, in a less controlled environment, has some different challenges, such as noise, weather, other dwellings around and unlike acoustics: e.g. less room issues such as sound reflexions. A somewhat limited sound coverage pattern may be preferable here, instead of "spilling all over the place". Plus, in free field sound level drops by 6 db per doubling of distance from speaker, per inverse square law.
Original intent was to use a different pair of woofers I have for this project, however upon inspection the foam surround on these needs replacement. Getting proper new foams from another country and do the refoam job would have delayed the project. So another pair on-hand, not first choice here, will have to do the job. And their surround is in good shape: rubberized foam!
That 6.5" woofer (sen. 90 db) is a little larger, therefore becoming directional a little sooner as frequency goes up, isn't necessarily a bad idea in this exterior use context with close-by neighbors in city and the use of a relatively directional piezo tweeter. Could be an indirect bonus helping keep peace with others.
However, the woofer has a rather higher Qts than I would have liked here. The plywood leftovers I have for the project allow making one pair of 15 L. boxes (net, after driver space, bracing, etc.) and Fb stands at 60 hz. The LF response with such Qts, as modeled with 2 Pi in Winspeakerz, is bumpy since the box should have some more volume. Tuning the box lower to get rid of the bump would get vent length too close to the back panel, plus the intent isn't to ask this size driver to reproduce very low frequencies... Then, the idea of creating additional virtual box volume with lots of damping material was put aside in view of the following.
Being mostly an exterior use speaker in a closer to 4 Pi (full space) environment than 2 Pi (half space), the effective LF response also modeled in approx. 4 Pi indicates it should drop by a number of db if there's absence of significant close boundaries where the speakers might be located (note 1). In this case, somewhat "saved" from bumpy sound by the outside acoustics. But if the 4 Pi loss is too high, then maybe shortent the vent a little (tune higher) to increase LF bumping?? Or preferably mitigate that LF drop with some alternative speaker placement compromise, like a table/rear wall distance + or - placement may be sufficient.
Actual on-site testing will give a clue about LF loss level, but for the time being the calculated transition frequency (note 2) at which the 4 Pi effect should START influencing LF response is 260 hz with loss increasing while going further down up to about Fb. Not all woofer response is affected by this 2 Pi/4 Pi placement issue, only the LF part of the spectrum.
Other calculated numbers of interest here: woofer upper frequency bound, where off-axis response BEGINS to fall off (note 3) is 1,176 hz; Directivity frequency (-6 db) (note 4) where DI raised no more than 6 db is 1,872 hz; Note that flat on-axis response can extend beyond these frequencies and it does. Maximum crossover frequency re driver size, as per "traditional formula" (note 5), is 2,948 hz. The initial INTENTION here is to X-over around 2 - 2.5 khz at 6 db/oct. low-pass, as I have inductors on-hand for this, plus to benefit from SOME of the driver's higher response, even if more directional, for reasons explained earlier.
Finally, considering woofer sensitivity, I may have to drop the piezo tweeter's sensitivity (about 94 db?) to minimize unbalanced sound levels between tweeter/woofer. More will follow.
Richard
1- Analogy to JBL's John Eargle 2 Pi vs 4 Pi designed small console top near field monitor speaker systems is interesting here regarding impact on LF response (Handbook of Sound System Design, P. 294-5).
2- Same, P. 22, 295
3- Same, P. 99
4- Same, P. 100
5- Speed of sound (inches per sec.) divided by effective cone diameter (inches) = Frequency
A) FIRST PART
Piezo devices are different than conventional drivers since they don't have nor need a magnet, voice coil, etc. making them light, simple and low cost. They're also different in that they don't present a mostly resistive load to the amplifier, but instead a more capacitive one which some amps may not like. Motorola even mentions in the spec sheet the tweeter in series with a 15 ohm resistor "... is a highly reactive load...". Be it my previous testing with a receiver showed no sign of complaint, stress nor damage.
Jon Risch of Peavey mentions "... what is nominally a pure capacitive load,..." (2002) Nominally, and this can be somewhat "modified" easily. He also refers to these as being "... normally voltage driven, not current as with the typical voice coil and magnet type speaker." (2009) "Power capacity" here is really in Volts applied (e.g. 15-35 Vrms) but that number is sometimes referenced to watts at 8 ohms equivalent in some spec sheets using this formula: voltage squared divided by resistance = power in watts. 8 ohms assumed there.
Keep in mind the tech info given regarding driver attenuation, HF response shaping and crossover is valid for original or so piezo tweeters, like those that come with sufficient specifications from the manufacturers (e.g. Motorola, CTS and Piezo Source). Low cost copies provide little or no info on the drivers, sometimes few very basic specs that look like duplicates of original makers'. Also cheap copies don't offer specific driver capacitance number useful for pad, no response curve useful for shaping and crossover, etc. So copies may require guessing some things, to "play it by ear" and/or do more trial/error-success.
Folks on LH routinely measure driver DC resistance on conventional drivers, could be with simple low cost multimeters, that seldom do capacitance assessment. One reason I purchased two more expensive digital multimeters that do measure capacitance, in addition to many other things (not all same on the two). Their residual capacitance, to be deducted from measurements, indicates one nano Farad, nano being one thousandth of micro Farad, the tiny impact would be on the third digit of uF numbers here. For resistance measurement their residual to deduct isn't significant either, between 0.0 and 0.1 ohm, compared to 0.6 ohm on a lower cost digital one I also have. New meters tolerances: capacitance 4% & 3%, resistance 1%.
First capacitances measured were on 1005A type piezos. The seven (except one) such piezos on-hand have measured capacitance between .118 uF and .135 uF, and I've kept a "capacitance matched pair" of piezo copies at .131 uF and .132 uF for the neighbor's free project boxes mentioned previously. With that close measurements for the pair this means, for example, the same value capacitor can be used in series for both piezos when pad is needed. Also found out my semi-original Motorola kept as spare looks sick at .063 uF, about half of where it should be.
Comparing the above capacitance measurements with those rated by manufacturers I see: Motorola says looks approx. 0.15 uF, Piezo Source appears as 0.13 uF and CTS appears like 0.12 uF. Jon Risch indicates most run in the 0.1 to 0.26 uF range (1999). So close enough here (except one), though a bit further from Motorola, but my two originals of that brand gave .130 and .135 uF.
Starting from the tweeter end of things and going towards the amp, the series resistor next to the piezo is first in line for that capacitance issue, i.e. making things more acceptable or safer for some amps. But that same resistor can also reduce high-end peaking around 15-20 khz (about +2db PS; +3db CTS with first crossover filter, fig 10), if its objectionable, simply by increasing that resistor's ohm value. Two birds with one stone!
(Remember that the 1025A type 2X6" mid/high 90° H horn usually doesn't require such resistor since a 30 ohms one (measured 30.8 ohms) is built into that horn's casing. I'll post shortly pictures of this. For more practical replacement reason some fellows remove this 30 ohms resistor from the 1025A casing as they prefer locating it , and replacing it if need be, outside of the driver. Though mine are still original I don't disagree with them for the same reason, the working space in the driver IS quite tight. You may cut the resistor wires as close as possible to the resistor body and solder instead a jumper (piece of wire) between the two remaining wires. Done forever, but don't forget using a 30 ohms series resistor outside of the driver...).
On DIY Audio Forum, Nelson Pass replied to a question re confusion on the use of a resistor considering the piezo tweeter can be used without a crossover (To increase power? Reduce sensitivity? Protect the piezo?). The question was about use of a 22 ohms resistor, etc. Though not mentioned, context shows its the series resistor involved here. With Jon Risch, Pass' reply is among the most informed ones I've seen yet on Forums:
"Piezos present a capacitive load, so often the resistance is a polite way of keeping the power amplifier from getting unduly excited.
It does not much affect the level in the audio band.
The resistor actually will roll off the high frequency response, and you might adjust this down as low as 2 ohms or so if you find that it improves the sound."
Personnaly, I prefer to go the other way around: instead of starting with a higher value resistor and then reduce that value according to taste, it seems more logical to me to start with a stock driver or so (minimal resistance) and then add more resistance as need be to improve high-end sound. Starting with a higher resistor value may not let you hear initially the tweeter's original sound as a benchmark. Could be its ok for you with 2 ohms only, then additional resistance may not be required here.
The 2 ohms resistance mentioned by Nelson Pass appears to come from Motorola's piezo tech sheet giving crossover examples: "The two ohm resistor has been added to insure amplifier stability and will not affect frequency response (unless more than eight tweeters are in parrallel)." "added" since its NOT part of the crossover itself but rather located before it when seen from the piezo end of things looking backwards. But the important thing to remember here is that a 2 ohm series resistor would not change top end frequency response. However, using much more resistance there will reduce these frequencies as shown by Motorola.
I will provide a list of references later of the piezo stuff I have, also posting here the material that I can, and indicate where members and others can easily read the copyrighted ones for free...
BTW the Motorola 1025A originals and copies of 1005A are no longer available to be given away free here since someone locally will take them.
More details on the above issues and others in the next post.
Richard
First pic, back cover can be separated into two parts, one side showing cone and resistor, the other side showing the grey piezo ceramic element on top of which there is an orange rubber ring. The pink stuff under the piezo element seems a lot like pink insulation. If I remember well this pink stuff is to damp some resonance. On this model the piezo element is larger at 32 mm dia. than on the 1005A type at 21-2 mm. But this driver does mid/high from about 1.8 khz and and up, the other driver from about 4 khz and up.
Second pic, rear cover removed showing cone or diaphragm, plus resistor with orange and silver lines at 3 o'clock, inserted into driver casing, not a whole lot of space to play there.
Third pic, unit's back cover, on cheap copies sometimes even the screws aren't as nice as these, often no name nor country of origin...
824088240982410
* Sorry for the delay to come up with this next part, quite busy these days...
B) SECOND PART
To go a little faster here with typing of the following info, Motorola = Mot, CTS is same, Piezo Source = PS and Jon Risch = JR.
1) Series resistance details
We know from the previous post here there's a 2-3 db or so HF response bump in the 15-20 khz region for 1005A type piezo tweeters (PS and CTS). Plus, that a 2 ohm series resistor won't affect top end response (Mot). So 2 ohms being the lower resistance number, what about the higher resistance value one may use?
Mot has a "circuit for high roll-off" chart on this and the effect such higher value resistors have on top end response (at 20 khz): 50 ohms/-2.5 db, 100 ohms/-5 db, 150 ohms/-7.5 db. I don't see why someone would want to use as high as 100-150 ohm resistor to correct a 2-3 db or so bump, but if taste requires it the info is here. For response correction of a db amount in between these numbers, a series resistor value in between the above resistances can be used.
As for others' resistor indications, CTS 50 ohms "without noticeably affecting the response", PS 30 ohms, JR "about 30-50 ohms to tame the very top end" (1999) and 50 ohms (2009). So JR mentions higher value series resistor tames the top end, like Mot says. CTS' note re "no" effect on response is curious vs Mot and JR.
JR (1999): "Adding resistance in series with a piezo will actually roll-off the highs a bit, adding more will roll-off the highs noticeably."
My choice, is in the range of 2-15 ohms initially (both of these numbers appear in Mot's spec sheet) for listening test purposes, to minimize impact on HF response, then I'll adjust value higher if need be.
With regards to resistor type and capacity (watts) to use, JR (2009) mentions "can be 5-10 W non-inductive type, metal oxyde, etc." CTS suggests 2 W, PS & Mot don't say. In my book 2 W seems pretty low and the price difference with higher capacity ones being a matter of cents, I'll use 10 Watts here.
I'll go with ceramic type resistors as I have many of those on-hand (10 and 25 watts), though I also have a bunch of 10 Watt metal oxyde type, but prefer to keep them for other projects I have. Btw a 25 Watt resistor will be required later for the crossover.
I tested the resistance of a good sample of both types of resistors (various values) with the 1% ohm accuracy multimeters I have, and even the less "noble", lower cost, ceramic ones showed good tolerance, except for the odd one. So no big surprises here and this is fine. Knowing their accuracy isn't really a critical issue for me since I have a habit of measuring each one I use on a project to assess its tolerance, and discard any donkey.
2) Attenuation (Pad) details
JR (1999) explains that two capacitors in series (piezo tweeter and capacitor) create a voltage divider (piezos are voltage driven) therefore reduce (Pad) tweeter output level. A series capacitor of the same capacitance value as that of the tweeter's (hence my interest in measuring tweeters earlier) will drop the overall tweeter output by 6 db.
Mot also has an "Attenuation without high roll-off" chart with effect on tweeter output level given based on series capacitor value used: 1.0 uF/-1 db; 0.5 uF/-2 db; 0.2 uF/-4 db; 0.1 uF/-7 db (remember their tweeter was rated at approx. 0.15 uF).
CTS also gives some straight level attenuation values using a non-polar series capacitor: 0.1 uF/-6 db; 0.04uF/-12 db (their tweeter was rated at 0.12 uF).
That series capacitor is located right after the "safety" resistor mentioned above. So up to now we have, looking towards the amp, the piezo tweeter, then the series resistor, followed by the series capacitor (if tweeter padding required).
I also tested capacitance on a good number of Solen capacitors on-hand (2%, 3% and 5% tolerances) and these were usually within 2-3%, even for the 5% ones. A few other lower quality/price ones (not Solen) were more like 10%.
3) Variable L-Pad
I have a pair of those L-Pads in a box somewhere but I don't plan on using variable 8 ohm L-Pads on the project's piezos. However, if I did I'd tend to follow JR and Weems' advice on this with the use of a 10 ohm parallel resistor to make a load for the L-Pad, instead of using an 8 ohm resistor shown by Mot. (more later).
For a 10 ohm resistor value with such an L-pad, Weems suggests crossover capacitor values: 2.2 uF/7200 hz; 4.7 uF/3400 hz. JR (2009) also suggests a 2.2 uF series capacitor.
BTW these tweeters have a bump of + 4 db centered at 5 khz that you may want to minimize. Therefore, the choice of the capacitor value for a 6 db/oct. high-pass roll-off, to reduce that bump, should then be kept in mind.
In Mot's and Weems' cases the L-Pad is seen wired before last, looking in the amp's direction: piezo, parallel resistor, variable L-Pad and finally the crossover capacitor. However, in JR's notes the L-Pad is the last item of the bunch: piezo, series resistor, Pad capacitor (if needed), parallel resistor, high-pass filter capacitor and L-Pad...
(David B. Weems, Designing, Building, and Testing Your Own Speaker System, 4th Ed., McGraw-Hill, 1997, P. 101).
4) Listening tests
For listening tests, instead of soldering the components (resistors/capacitors) in place right from the start, I use temporary connections, easy to do and undo, like using a small piece of 5/8-3/4"plywood and 3/8-1/2" screws but with a little larger flat head, to do a turn or two of the components' wires under the screw's head, then screw it down into the wood. The screws act as the junction between and holding the components in place for the time of the listening tests. Holds well, gives a good connection and is easy to undo to try another resistor and/or capacitor. It could even be done on the bottom panel of a cabinet.
More to come.
Richard
A) FIRST PART
I indicated earlier in the LF part of this project my intentions regarding the low-pass section of the X-over. So, I'll focus more here on the High-pass crossover section related to the piezo tweeter.
There are two ways to go about making a simple high-pass crossover for the piezo tweeter (one resistor and one capacitor).
The first one detailed involves determining the series capacitor value (uF), and then choose the parallel resistor value based on frequency response desired, e.g. LF roll-off point vs 5khz bump (Mot, CTS, Weems). CTS shows some response curves and Mot gives a small chart of db effect at 5 khz. More on these further.
The second method indicated involves determining the parallel resistor value (ohms), and then choose the series capacitor value according to LF roll-off you want (JR). Except for some JR words regarding effect on response in a few cases, there is no response curve shown and no db chart given here. This way of doing things may involve a little more trial/error/success listening tests.
However, there IS a small and simple crossover drawing available for this piezo second method, including a simple formula given to calculate the crossover capacitor value, the equation being partly solved since the resistor value is already determined (the series safety resistor and optional Pad capacitor are not shown on that drawing though). The author of it is clearly indicating the little drawing is copyrighted! I'll tell you where you can see this free.
So, one method is fixed capacitor value and you vary resistor value based on expected sound, the next method is the other way around (fixed resistor value and you play with capacitor value). The first one may be easier to implement considering the recipe details already available from Mot/CTS. For many cases though I tend to prefer JR's second method for power input reasons we'll see.
Since I have a variety of 25 Watt ceramic resistors available I'll meet JR's minimum wattage requirement for the load (parallel) resistor. Recall in a previous post the mention of a fellow in a forum, using a 10 W 8 ohm parallel resistor, complaining it was overheating... Well, unfortunately he had it wrong on resistor value (ohms) and capacity (Watts), specially for use with higher power.
The parallel resistor's value and power capacity are important because this is the one taking the heat, therefore a 25 Watt or more resistor, with higher resistance value, is recommended here by JR. My local electronics store doesn't have more than 25 W ceramic resistors though, but the project box isn't a high power one anyway.
More will follow.
Richard
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