Help Needed To Make A Zobel Circuit.

[BugC]

Hi All!

Am new to this forum and would like to seek your help in making a Zobel circuit for my Altec 415A driver. I'm currently running this driver full range, with a JBL 2402 crossed at 3000hz using the JBL Power Pack 3102 and a Fostex FT17H crossed at around 18khz. The enclosure is a bass reflex having an internal volume of 6 cuft. The 2402, x-o and fostex are located outside the enclosure ... I plan to use the 415A's in a 2-way system as a LF driver upto 900hz or 1200hz along with an 811B horn mated with a 902-8A compression driver ... My problem is I don't know what the Le of the 415A is to make a zobel circuit. I tried the Altec forum but wasn't lucky enough to get any info on this. Am hoping someone in this forum have the info I need to get me going on my project. Thanks in advance for your help.

All the best!
BugC

[Ian Mackenzie]

If you have means run an impediance curve and measure the DCR of the coil. Then try some RC combinations till you get your target impediance.

Buy a woofer tester 11 from Parts Express if you are serious

Alternatively talk to Great Plains audio and see if anyone there knows.

Ian

[Hoerninger]

My problem is I don't know what the Le of the 415A is to make a zobel circuit.

Here is a hint:
http://audioheritage.org/vbulletin/s...1mH#post133724
____________
Peter

[Earl K]

Hi BugC

(A)
- What ( or whose ) Zobel formula are you going to use ?
- David Weems published a couple of formulas for determining a Zobel network. .

- For projects like this you should own a LCR meter . eBay has some available from Hong Kong based vendors . If you can't afford one ( @ $ 45.00 USD ) maybe you could find someone ( such as at a college ) to make a basic coil inductance measurement for you .



(i) The first formula is for use when you know the voice coils' inductance

C = Le / Re^
R = 1.25 * Re

- Re is the dcr of the woofer ( dc resistance )
- Le in the formula has to be transposed into Henry units ( so a coil given in mH values needs to be divided by 1000 to convert mH into Henries )
- C is "answered" in Farad units ( so multiply the answer by 1,000,000 to get a cap with a uF value )
- ( ^ is a standin for the "squared sign" )

(ii) The next formula is for use when you don't know the voice coils' inductance .

- Measure the woofers' DCR ( dc resistance ) . Call it Re .
- Using a signal generator, sweep up in frequency until you can measure the AC impedance that is twice the Re value. ( Obviously, one needs to know, how to make AC impedance measurements ). Do You ?
- Mark this point as "F" ( it's unit is stated in basic units of hertz / so, you'll record something like 2500 hz as the "F" ) .

- Then "C" ( again given in Farads ) is found by

C = 0.16 / ( Re x "F" )

(B) After poking around a bit ;

- Woofer has a ; 3" edgewound aluminum voicecoil, working within a deep gap ( so says the promo ) . The woofer handles 25 watts .
- So, I suspect the coils inductance is going to be in the 4.5 to 5.5 mH neighbourhood ( Altec has speced the Le for 515-8G coil as .58 mH / which is also a 3" aluminum voice-coil / though it handles triple the watts compared to the ancient 415a ) . ( The 515-8HP has a .68 mH coil and it handles 8 times the watts as compared to the 415a ).
- The DCR should be between 6 to 6.5 ohms for the 8 ohm , Altec 415a "BiFlex" .

- You can use the first formula to build yourself a range of guesstimated Zobels .
- I would start by building one with a 8.0 ohm resistor & a 13 uF cap .
- You could insert it ( across the woofer ) into your new lowpass network & then measure for the networks' "expected F3 point" . Make empirical adjustments from there .
- FWIW ; the N1209a crossover has a Zobel composed of a 7.5 ohms resistor with a 18uF cap. That network was designed to work with a bunch of their 70s' era woofers .

- Here are the basic 415a specs ( img linked to HifiLit ) . Total woofer weight is a bit heavier than the 803a ( the predecessor to the 416a ). Note the highish Fs .

[WTPRO]

Hi Ian

I know you and a few others have purchased WT2's from PE, but PE is not our distributor anymore. Either buy directly from us or get the WT2 (its the same box) through MCM where the WT2 is known as the 'Speaker Measurement Interface'.

If you have a look at our web page you will also see that we have moved into in-air microphone testing, Interactive Crossover Design (ICD) and High Power TS and box testing. In the case of the WT-Pro this is in one box, or we offer Speaker Tester for those that want to step up from the WT2.

Hope this helps,
Best regards,
Keith Larson (WTPro)

www.woofertester.com

[4313B]

Thanks for the update Keith

[Ian Mackenzie]

Keith,

You may have me confused with another member.

WT Pro sure looks groovy though. I'll put it on my wish list!

Thanks for the update.

Ian

[Robh3606]

Thanks Keith

I have been using the Beta software for WT2 and no issues for me. Your new set-up looks good.

Rob

[WTPRO]

Sorry for the small mixup there.

If you guys are interested, I can keep you in the loop with regards to things I am working on for the WT2, ST and WT-Pro. Just send me a personal email.

An an example of what is on my todo list, a simple Zobel calculator is pretty much a slam dunk. However Le is not really a simple 'L' but rather frequency dependent and a Zobel really only works best at one frequency. In my mind, I would want to see how a 'Zobel' works at all frequencies, becoming a simulator issue. This is a fork in the road and I can be an off road kind of guy. Asking for some directions might not hurt! On the other hand, I am an engineer. Its like when someone asks me if I am an optimist or pessimist... the if a glass is half full or half empty kind of thing. I am the third type that announces 'neither, the glass is twice as large as it should be!'

An example for the WT product line would be the Interactive Crossover Designer (ICD) tool. In the real world you end up tweaking Zobels and other XO components so the canned Zobel solution is really nothing more than a starting point. What you need to be able to do is modify the XO components in a virtual sense. Knowing I might want more than a traditional SPICE library, I wrote my own translator.

Anyhow, there are a number of these kinds of things in the WT products. The interface and what all may not be pretty, but we do try to think outside the box. And, I dont think anyone can argue that we have not improved our products over time.

Best regards,
Keith Larson (WT-Pro)
Smith & Larson Audio

PS: I can arrange a demo for anyone in the Boston area.

[WTPRO]

Might help if I posted my email address!
[email protected]

[Ian Mackenzie]

Thanks Keith for the heads up.

I am Down Under but it would be cool if some the local natives surfaced for some daylight and had a look at your new offerings.

Ian

[Paul C.]

Telling him to just "measure the inductuctance and use this formula" is not very helpful... few speaker builders have the equipment necessary to do that. If they did, we'd already have a chart here of VC Inductance for various Altec models that had been gathered by a number of speaker builders.

[4313B]

few speaker builders have the equipment necessary to do that.

Which is why they should probably take up another hobby.

Hmm, I think I'll pull the transmission out of my car today. Let's see, here's the vise grips, a slotted screw driver and a ball peen hammer. Git 'er done!

I tried the Altec forum but wasn't lucky enough to get any info on this.

Well, what the hell are they doing over there then? OT?

[Zilch]

Telling him to just "measure the inductuctance and use this formula" is not very helpful... few speaker builders have the equipment necessary to do that. If they did, we'd already have a chart here of VC Inductance for various Altec models that had been gathered by a number of speaker builders.

WT2 does it rather nicely.

[And a bunch of other stuff, too.... ]

[Earl K]

(B) After poking around a bit ;

- Woofer has a ; 3" edgewound aluminum voicecoil, working within a deep gap ( so says the promo ) . The woofer handles 25 watts .
- So, I suspect the coils inductance is going to be in the 4.5 to 5.5 mH neighbourhood ( Altec has speced the Le for 515-8G coil as .58 mH / which is also a 3" aluminum voice-coil / though it handles triple the watts compared to the ancient 415a ) . ( The 515-8HP has a .68 mH coil and it handles 8 times the watts as compared to the 415a ).
- The DCR should be between 6 to 6.5 ohms for the 8 ohm , Altec 415a "BiFlex" .

- Obviously my own proof-reading of the quoted post, just plain "sucked"

- The underlined red text shows my mistake .
- The text should have read ; .45 mH to .55 mH .

Telling him to just "measure the inductuctance and use this formula" is not very helpful... few speaker builders have the equipment necessary to do that. If they did, we'd already have a chart here of VC Inductance for various Altec models that had been gathered by a number of speaker builders.

- Okay, your scolding & "finger-wagging" is duly noted .
- I've offered my best guesstimate to this poster based on an assumption of possible limited access to technology ( if not limited financial means ) / due to his being from a 3rd-world country ( ie; "developing nation" ) ..

- Considering all this , what advice do you offer to "BugC" about his Zobel query?

<> Earl K