Cap across bias stabiliser

[PeteMcK]

Hi Guys,

I'm wondering if there's any sonic advantage to placing a cap across the bias stabilising circuit in the output stage of an amp,
e.g. in Rod Elliots 3A amp, between the bases of the drivers Q5,Q6.
or would this interfere with the stabilising action of Q9?
I beleive the AKSA amp does something like this?

cheers,
Pete McK

[halojoy]

Originally posted by PeteMcK
Hi Guys,

I'm wondering if there's any sonic advantage to placing a cap across the bias stabilising circuit in the output stage of an amp,
e.g. in Rod Elliots 3A amp, between the bases of the drivers Q5,Q6.
or would this interfere with the stabilising action of Q9?
I beleive the AKSA amp does something like this?

cheers,
Pete McK

A good question.
I have seen very different values used in different amps.
From no cap to 100 uF.
If it is sonically is questionalble.
I would guess, that the value of the cap
is determined by a check with oscilloscope in the final trim.

It will not interfer with the current sensor,
that is a very slow process, compared to the upper audio frequency

[AKSA]

Hi Halojoy,

Almost all PP amps use a capacitor across the bias generator, bridging the two driver bases. This is common practice, and has been for forty years since SS PP amps first appeared.

It is done for two reasons.

First, any signal which appears at the bottom of the bias generator is transferred unchanged to the top, so that both driver bases see exactly the same AC signal. Obviously if the positive devices saw a different signal to the negative devices, then asymmetrical distortion would be introduced on otherwise perfectly symmetrical waveforms. This might be corrected by the negative feedback loop, and probably would be, but it's always best to avoid sources of distortion if you can.

Second, when the amp is first powerd up, this capacitor has to charge up. This happens over a few milliseconds, and ensures that the bias current on the amp slowly comes up to speed. This means any asymmetries in the turn on spike will remain infrasonic, and won't be heard.

Yes, the AKSA is no exception to this, and uses a 10uF electrolytic in this role. A properly biased Vbe multiplier (fancy name for bias generator!) will be reasonably AC transparent, but a cap guarantees it, and this is good for overall quality. However, the quality of this cap is much less important than say the quality of the input cap.

Cheers,

Hugh

www.aksaonline.com

[halojoy]

Every distortion/unlinearity/assymetry has to be cancelled out, sooner or later.
Better sooner/locally, than later/globally.

Thanks very much AKSA. I am now a wiser man.
And I would think PeteMcK thanks you too.

[mrfeedback]

"However, the quality of this cap is much less important than say the quality of the input cap."

I have bridged this cap (bias regulator bypas - usually electro or polyester) with a 0.1 or 0.22 ceramic and found cleaner and nicer upper mids and highs in a good number of amps.

Eric.

[peranders]

Originally posted by PeteMcK
Hi Guys,

I'm wondering if there's any sonic advantage to placing a cap across the bias stabilising circuit in the output stage of an amp,
e.g. in Rod Elliots 3A amp, between the bases of the drivers Q5,Q6.
or would this interfere with the stabilising action of Q9?
I beleive the AKSA amp does something like this?

100 nF/63 V polyester (works really good) or ceramics of some sort is a good start, plus maybe 10µF-100 µF electrolythic cap. The circuit simulates a voltage source so you can't get too low impedance.

[peranders]

Originally posted by AKSA
However, the quality of this cap is much less important than say the quality of the input cap.

Indeed, the HF properties are the one which are important (> 100 kHz). In this respect 100nF/63V Roederstein/BC/Wima/ etc are very good!

[mrfeedback]

Ummm, that's why I said ceramic.

Eric.

[peranders]

Originally posted by mrfeedback
Ummm, that's why I said ceramic.

Yes, but still, SMALL polyester caps aren't so bad is this respect.

[mrfeedback]

Yeah, they can be, partly due to internal connections.
Ceramics can be essentially distortionless and lossless.

Eric.