resistor parallel over compression drivers

[Earl K]

Are you utterly convinced the shunt makes this much difference..was the level exactly the same?

- I've had similar results/experiences with conjugate resistors across the terminals of biamped JBL drivers. Like Frank, I too include a buildout / "series" resistor ( forming an Lpad ) to maintain the circuits' load impedance . To my ears , and dependant on the value of the conjugate used, the driver/resistor combo can start to sound like a form of applied "soft knee" compression with the threshold set to "always on" . I've accepted the conjugates' necessity in my passive circuitry when using JBL drivers. OTOH, I don't enjoy the resistors' mitigating effect ( on dynamics ) when applied in the same fashion , to Altec compression drivers.

- I feel the two differing experiences result from my observation that JBLs' "Sound Reinforcement" compression drivers typically sound underdamped ( to me ) while Altec drivers sound critically damped ( both used on smallish horns ) . So I feel, the conjugate can be used to somewhat tame the JBL driver ( but then , so can aquaplas dusted diaphragms +/or frequency specific; series type, LCR resonance traps ) .

- Just a personal opinion mind you

[aust-ted]

Hi Ted,

How large are your combined capacitors for each of the 2450 and 2405?

Polypropylene capacitors mostly are of a sufficient quality. I like paper-oil caps myself, but they can be very expensive and big [I've got some that are the size of a 2405 and provide only 4uF each].

If you need large values to keep them outside of the passband, polyprops can be a good alternative.

But DC protection can also be done through transformers I think, like the outputs of single ended [tube] amps that are also used to downconvert the tube's voltages. Then you wouldn't need caps.

Greetings, Frank

Hi Frank

I have 60 uF in series with the 2450s and 9 uF in series with the 2404. The active xover freqs are 800Hz and 5KHz. The caps I am using are polyprop fluoro caps which I acquired very cheaply.

What you write about DC protection using transformers is correct. However I am also trying to protect the drivers against my own stupidity (eg incorrect connection where the bass channel ends up being connected to the tweeter or mid say after some change in gear).

Regards
Ted

[aust-ted]

[QUOTE=Ian Mackenzie]Very interesting discussion brewing here chaps.

Ted,

Yes, have you tried charge coupling those caps at this stage....nine volt battery with 2-3 megs of series resistance?..worth a try.

Ian

I have read the discussions on charge coupling in the forum with great interest and intend to try it out some time with passive xovers.

However in this case I just want to make sure you are aware I am using active xovers (currently M553, but am in the process of swapping over to a dEQX) and only use the caps outside the passband to protect the drivers.

If you think the caps could be affecting the sound, I could try a bypass switch which shorts out the caps after I am sure the system is working ok. That way I could minimise an accident.

Regards
Ted

[Ian Mackenzie]

- I've had similar results/experiences with conjugate resistors across the terminals of biamped JBL drivers. Like Frank, I too include a buildout / "series" resistor ( forming an Lpad ) to maintain the circuits' load impedance . To my ears , and dependant on the value of the conjugate used, the driver/resistor combo can start to sound like a form of applied "soft knee" compression with the threshold set to "always on" . I've accepted the conjugates' necessity in my passive circuitry when using JBL drivers. OTOH, I don't enjoy their mitigating effect ( on dynamics ) when listening to Altec compression drivers.

- I feel the two differing experiences result from my observation that JBLs' "Sound Reinforcement" compression drivers typically sound underdamped ( to me ) while Altec drivers sound critically damped ( both used on smallish horns ) . So I feel, the conjugate can be used to somewhat tame the JBL driver ( but then , so can aquaplas dusted diaphragms +/or frequency specific; series type, LCR resonance traps ) .

- Just a personal opinion mind you

But a valued opinion my friend.

I mean JBL has imho done much experiements of an empirical nature...none of which is printed but remains in their giant heads (to coin a phrase) ..the fine tuning and voicing...the networks ..hand selecting matched pairs of drivers..therefore mere engineering for thought is not enough...one must make use of the ear as they do ....then measure the result and compute scientifically..after the fact.

Macka

[Ian Mackenzie]

[QUOTE=aust-ted]

Very interesting discussion brewing here chaps.

Ted,

Yes, have you tried charge coupling those caps at this stage....nine volt battery with 2-3 megs of series resistance?..worth a try.

Ian

I have read the discussions on charge coupling in the forum with great interest and intend to try it out some time with passive xovers.

However in this case I just want to make sure you are aware I am using active xovers (currently M553, but am in the process of swapping over to a dEQX) and only use the caps outside the passband to protect the drivers.

If you think the caps could be affecting the sound, I could try a bypass switch which shorts out the caps after I am sure the system is working ok. That way I could minimise an accident.

Regards
Ted

A very good idea.

Please keep us posted.....

[Ian Mackenzie]

Shall we call this the JBL tweakers asylum?

Ian

[Ian Mackenzie]

Hi Frank

I have 60 uF in series with the 2450s and 9 uF in series with the 2404. The active xover freqs are 800Hz and 5KHz. The caps I am using are polyprop fluoro caps which I acquired very cheaply.

What you write about DC protection using transformers is correct. However I am also trying to protect the drivers against my own stupidity (eg incorrect connection where the bass channel ends up being connected to the tweeter or mid say after some change in gear).

Regards
Ted

Ted,

I would like to wager that a Solen fast cap bypassed with 0.01 film foil and charge coupled would create the prefered JBL sound for your 60 uf .

Ian

[frank23]

I was going to post my passive crossover layout. Hereunder it is. I use only paper-oil capacitors as they sound the most real to me.

I am not sure about the level of the red curve [high] as compared tot the green curve [low], I think it should be a few dB higher and therefore in reality the crossover point moves a little to the left.

The blue curve is an alternative high crossover using only one capacitor as opposed to the two caps in the red curve [the red is the one I actually use].

By the way, I used an 6 ohm resistor for modelling the 2420 driver, which I know is not 100% accurate, but because there is such a low resistor in parallel, it doesn't make a big difference in the models results.

And the Davis 20klv8A almost has no self-inductance due to copper rings, so a single inductor suffices and no compensation is needed.

Earl, I am thinking about your remarks on the "compression knee", I think I'll try a layout with a somewhat lower parallel resistor once.

greetings, Frank

[Earl K]

Earl, I am thinking about your remarks on the "compression knee", I think I'll try a layout with a somewhat lower parallel resistor once.

- For the sake of clarity ; I didn't actually suggest that you tinker with your present scheme of "current dump" or try to lessen the apparent "dynamic-compression" effect .

- But if you are looking to hear a bit "less" dynamic loss / then raise ( not lower ) the value of the conjugate from your present 1.2 ohms to something like twice the rated AC impedance of the driver . ( JBL would typically choose in the 10, 20 to 30 ohms range ). The "effect" should become quite minimal , if present at all, when using these other, higher impedance choices .

Note : adding a 30 ohm conjugate in place of your present 1.2 ohm will raise the overall ( impedance ) load presented by the "driver/resistor combo" to everything in the circuit before them. The load will go up towards 8 ohms, ( created by paralleling 30ohms with @ 11ohms ). A new load impedance means you'll need to rework all your RC ( & L ) elements "source side" towards the amp / ( just to recreate the HiPass you presently enjoy ) .

[frank23]

I've just built a simple 1st order crossover for the 2440/2382A combi instead of the active crossover setup.

I have used the same idea that I used on my 2420/2344A combo. It tops out at about 20kHz, so you compensate for the rolloff above 4kHz. Effective acoustic crossover frequency will probably be somewhere around 1000-1500Hz.

This is much more like it! I don't know if it is due to the fact that the active crossover [M552] isn't any good, or it doesn't compensate well for the 2382A rolloff above 4kHz, but the active setup didn't work. Even my wife didn't like the sound.

I am still bi-amping because I haven't come to the finetuning of the sound levels, so I am using the same two amps that I was using in the active setup, same cables and same quality capacitors.

Maybe I can only live with 1st order crossovers...

Now I have to try what Earl meant by altering the value of the resistor network.

greetings, Frank

[ralphs99]

Hi Frank,

I have also included a low valued resistor connected across my compression drivers to minimise impedance variations and as half of an L-pad attenuator. But I think there's even more to the story....

I have done quite a bit testing and feel, like you, that it makes a big contribution to the sound, especially in an active setup. It started me thinking why this might be so. I also tried some experiments with charged coupling capacitors and wondered why I couldn't hear much difference.I also tried experiments with different capacitor dialectrics and again couldn't hear the differences that others had been reporting.

Then the idea struck me that the observations might be related in some way....

Compression drivers used in home environments typically have milliwatts of power circulating through them. This is because they are used for high frequencies where less power is required and because of their intrinsically high effiiciency. (we all know that much) The level of the drive signal never gets much away from the zero crossing as a result. Driving a compression driver with an amp directly, forces the amp to operate in the range where it least linear- near the zero crossing point (at least for push-pull topologies, class A amps being the exception). With a passive network in between, the situation is improved somewhat.

By using a an attenuator with a very low value resistor across the diaphram and a high attenuation factor causes much more power to flow from the amplifier as it also has to drive the attenuator, and also causes higher currents to flow as back EMF. This then means that the coupling capacitors work harder as well. The non-linearities of the coupling capacitors and the non-linearities of the amplifiers are thus proportionately reduced.

This, I think, explains my observations.

Sure I waste power in the attenuator by driving the amp and the network harder, but at typical listening levels, it's only a few watts, and that's a small price to pay for the improved linearity.

Comments invited!

Cheers, Ralph.

[Tom Loizeaux]

My understandinmg is that the "rule of thumb" is that Zobel networks (resistors between + & -) on tweeters should be at least double the impedance of the driver.

But then "my understanding" is always subject to challange.

Tom

[Ian Mackenzie]

Ralph,


That certainly fuels the discussions.


I agree class b and a/b amps , particularly large amps are least linear with gross crossover distortion in the millwatt range compared to say 1 - 10 watts.


I use a Passlabs Aleph and its light and day in every respect. The first watt is the most important watt....now who's quote is that?


Its the bass and midrange frequencies that create 3rd, 5th,7th and 9th harmonics from large amounts of negative feedback that cause the cheese grater effect in the horns. No wonder people snarled at the slant plate horns and slots in the 70's and 80's. Then pump it all through dirty capacitors and crap terminations and crap x crap x crap = very seriously bad sound.

But back to your point about loading capacitors, Finch and Marsh state that D/A and other capacitors distortions are reduced with reduced load impedances. Unfortunately this also means larger value capacitors which tend to have a higher magnitude of such problems.


From my own observations the run of the mill active crossovers which use mass market chips in the active circuits and low quality capacitors are the real villains in our quest to make our horns sound nice.


If when using an active crossover if you thing the sound is sterile , scrubbed or processed sounding compared to a passive crossover its the active crossover that is partly to blame.


Most designs use several chip opamps with 100% negative feedback buffers (TL072s, Ne5532, LM833s) nasty electo coupling caps and nasty 3 pin regulators. You would never dream of using a crappy preamp would you, well that's about the status quo of active crossovers used in pro audio.


I have built a prototype class A discrete crossover ..the results have renewed
enthusiasm for active crossovers. (Most v.good preamps have a single ended discrete class A stage.....why do manufacturers skimp on active crossovers..because they assume they will be used in a PA system)

Anyway, all the above does leave latitude for a very veiled electronic sound that is miles from the original signal.


While the above idea of using a shunt resister is unorthodox and may get sniggers from the engineering textbook types who time and time again choose plagiarise chapter and verse all over these forums and have absolutely no capacity think outside the Sq 3, I have for one have no reason to believe this approach does not offer real benefits.


In fact some of the most respected designers in the audio business apply radical ideas in their products because it works....and they are the trade secrets...(aka Nelson Pass...John Curl)

Ian

[Earl K]

But then "my understanding" is always subject to challenge.

Okay, Thanks for an oppotunity to blab .

My understanding is that the "rule of thumb" is that Zobel networks (resistors between + & -) on tweeters should be at least double the impedance of the driver.

Ummmm, ( that is just a JBL convention or "rule of thumb" ) & about Zobels', well,,,,

- a Zobel ( an "Inductance Compensating Network" ) is composed of a capacitor and a resistor wired in series with each other . These two elements are then wired parallel across the ( + and - ) load lines. The location of a Zobel is typically right next to the transducer ( electrically speaking ). If a conjugate load resistor is also present, the Zobel then takes up a new location one position closer to the source ( amplfier ) / leaving the conjugate resistor to occupy the space, closest to the transducer .
- Zobels are more commonly used with woofers since woofers can have quite high values of inductance with their massive voice coils .

- A simple conjugate or load resistor ( as Giskard said many times ) can be used to tame ( lower ) some of the wilder ( highish ) impedance peaks found in the ( AC ) impedance curve of a transducer/box/horn system . With many amplifiers this can translate into sonic benefits ( think simplified "tracking" of the impedance curve ) . Also, a flatter AC impedance curve greatly simplies the design process when creating passive networks .

- JBL circuit design ( of a certain vintage ) does make pretty consistent use of conjugates that are twice the average AC impedance of the transducer when applied to horn drivers.
- One can also find quite consistent evidence of load resistors on woofers that are five to ten times the nominal AC impedance . ( These are found in the newer designs ).
- These approaches are "JBLs" particular design conventions, for use with their specific drivers.
- That doesn't mean that there's some electrical rule dictating those same values be used with other manufacturers drivers ( or even JBLs ).
- As Frank found out, using other values is doable ( with JBL ) / but with some sonic consequences.
- Exploring those "consequences" can be fulltime work/fun for the tweaker .

- I play around with Altec drivers more than JBL drivers and I'll assert that some of JBLs' design conventions shouldn't be applied to Altec drivers .
- For instance; my ears have told me that I don't like conjugate resistors wired across the terminals of Altec drivers.
- OTOH, I feel they are sonically beneficial with many JBL drivers.

- Unfortunately this forum, lacks an Altec "seer" or "oracle" who is on par with the retired Giskard. Without some comparable insights we may never know the reasoning behind some of the interesting passive design approaches found in Altecs' legacy crossovers ( they are not all boring & straightforward 2 pole butterworth designs ).

- For instance ; here's the schematic for Altecs' model 19 network. I feel it is a good example of Altecs' best design thinking for the day. It is roughly comparable in electrical function and vintage, to JBLs' N3131a network .
- These two network examples underscore two very different approaches taken to deliver listenable sound from vintage 2-way systems ( both using compression drivers ) .
- In the N3131b, one will find the inclusion of a typical Zobel on the 2231h in the 3131a network . Also present, is a 20 ohm conjugate that conforms to the mentioned 2 to 1 impedance ratio ( JBL "rule of thumb" ) . A 2420/le85 will be an @ 10-11 ohm driver ( according to Zilches measurements ).
- The N1201a has neither a Zobel, nor a compression driver "conjugate" .

- Let the ears be the final arbriter over the inclusion of these so-called passive parts and/or the implementation of these different approaches to network design .



ps ; I'll try to edit this later for brevities sake .

[Ian Mackenzie]

Earl,

I am not a horn or compression drive guru by any means but I wonder what mechanical and magneting damping is at work with a typical driver / horn?

Also in some cases absolute damping is desirable in some types of drivers ie the 2405 is apparently meant to work far better if directly coupled to amp amp (ref Giskard). Mind you that is a ring radiator, not a compression driver per say.

Ian