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otl amps/speakers and zoebel networks

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Posted on May 8, 2023 at 18:44:01
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 A simple question really: in the absence of an output transformer on my amp, do I still need a zobel network?
Please assume that the last thing a speaker would "see" (if looking back into the amp) is a cap.
Therefore the question: would a zobel help? hinder? make no difference?
Thanks in advance.

 

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You may not have an output coupling cap- many OTLs don't. , posted on May 11, 2023 at 10:27:02
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 Which includes ours, which also do not have a ZOBEL network at their output.

Usually that network is there to stabilize an amplifier at high frequencies- in particular, ultrasonic. You're more likely to see such a network if the amplifier employs feedback. It really won't have much to do with whether a coupling cap is used by the amp or not. At any rate a ZOBEL network won't hinder things.

 

RE: You may not have an output coupling cap- many OTLs don't. , posted on May 11, 2023 at 14:17:30
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 Thank you for your quick reply. Am I to understand then, that you have an OTL amp already (without feedback) and that you do not use a zobel network?
Thanks again.

 

That's correct., posted on May 12, 2023 at 09:03:52
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 We've been making them for decades now.

 

RE: You may not have an output coupling cap- many OTLs don't. , posted on May 12, 2023 at 10:32:18
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 Thanks again for the clarification.
In some sense not having a zobel is a relief, especially if it would be redundant. But on the other hand, I still have a lingering feeling that without it (on the woofer of a full range speaker), the impedance of the woofer will rise with the higher frequencies to the woofer's sonic detriment.
Is this not a concern and would the zobel not correct for this?

 

RE: You may not have an output coupling cap- many OTLs don't. , posted on May 12, 2023 at 10:33:40
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 I do have an output coupling cap between the last tube and the speaker.

 

This really sounds like a crossover issue; here's something to consider, posted on May 12, 2023 at 10:47:27
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 If you don't crossover the woofer its impedance will be in parallel with that of the midrange 'full range' driver.

OTLs generally don't like lower impedances. If you have 8 Ohm drivers, the speaker will be nominally 4 Ohms thru the 'full range' driver region.

I know 'full range' driver guys don't like crossovers. But the simple truth is 'full range' drivers deserve the quotes because there really isn't any such thing (else you'd not be trying to use a woofer). If the woofer is active at higher frequencies, you'll get combing effects as the woofer is alternately in phase and out of phase with the other driver. The ear interprets this sort of thing as harshness.

So use a crossover. Getting the bass off of the 'full range' drive is a good idea, since that reduces Doppler Effect distortion.

The crossover will help linearize the impedance curve, which will make for an easier load for the amplifier. A ZOBEL network really has nothing to do with it!

 

RE: This really sounds like a crossover issue; here's something to consider, posted on May 12, 2023 at 13:36:11
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 Thanks. Your note helped clarify a couple of misconceptions I had about zobels and crossovers, etc.
The one remaining issue (perhaps it is a misconception too) that have is related to my original question which now, might have to be re-phrased. It was that an amplifier with a transformer can drive speakers (woofers with a zobel) easier because their impedance is more linear. So it was because of this that I asked about zobels and an otl amp.
But it seems that the hidden issue is as you say, about crossovers and not about transformers.
So it appears in fact that any system with a crossover would benefit from a zobel because of the effect that the zobel has on the impedance of the woofer as the frequency increases, transformers notwithstanding, and not the other way around.
If the above is correct, then thanks for the clarification.

 

You'd really have to build and measure it, posted on May 12, 2023 at 13:56:11
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 but I suspect that the ZOBEL network won't be needed in the context of a crossover. A lot will depend on the impedance curves of the drivers in question.

If the amplifier has no feedback, even with an output transformer you'll run into frequency variation due to load variation.

Some speakers are designed to expect a higher output impedance from the amplifier (IOW an amp with no feedback) as the assumption there is that the brain pays more attention to tonality induced by distortion than it does to tonality caused by frequency response errors. Of course, keeping in mind that all speakers have profound FR errors all by themselves.

 

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