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
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.PeteMcK said:
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
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
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
It puts less distortion into global feedback
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.
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.
Bias Regulator AC Bypass Cap Quality.....
"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.
"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.
PeteMcK said:
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.
AKSA said:
Indeed, the HF properties are the one which are important (> 100 kHz). In this respect 100nF/63V Roederstein/BC/Wima/ etc are very good!
Indeed, the HF properties are the one which are important (> 100 kHz).
Ummm, that's why I said ceramic. 🙄
Eric.
Ummm, that's why I said ceramic. 🙄
Eric.
Re: Indeed, the HF properties are the one which are important (> 100 kHz).
Yes, but still, SMALL polyester caps aren't so bad is this respect.
mrfeedback said:
Yes, but still, SMALL polyester caps aren't so bad is this respect.
"Yes, but still, SMALL polyester caps aren't so bad is this respect."
Yeah, they can be, partly due to internal connections.
Ceramics can be essentially distortionless and lossless.
Eric.
Yeah, they can be, partly due to internal connections.
Ceramics can be essentially distortionless and lossless.
Eric.
The cap across the bias network of your typical push-
pull follower can be a crummy cap since it doesn't
actually carry signal.
pull follower can be a crummy cap since it doesn't
actually carry signal.
As Monty Python once said "That takes all the mystery out of life". Or in other words - where's the fun in replacing all the caps with BlackGates (or other exotica) if half the time it doesn't matter! There can be great personal satisfaction in remaining ignorant.
There can be great personal satisfaction in remaining ignorant.
But much, much, much more fun in experimenting and listening. I never felt the need to have to ask someone else to do my thinking or hearing for me. Good thing too.....
Fred
But much, much, much more fun in experimenting and listening. I never felt the need to have to ask someone else to do my thinking or hearing for me. Good thing too.....
Fred
As usual - succint and to the point
I am very glad to have that info.
Now if I will be able to remember it at the appropriate time, I will be even happier
Ken
As always, I wish to thank Nelson Pass for his generous support of this forum
Nelson Pass said:
I am very glad to have that info.
Now if I will be able to remember it at the appropriate time, I will be even happier
Ken
As always, I wish to thank Nelson Pass for his generous support of this forum
Vbe Multiplier impedance
"The cap across the bias network of your typical push-
pull follower can be a crummy cap since it doesn't
actually carry signal."
Nelson, are you saying that the impedance across the Vbe multiplier seen by the signal is so low that a cap will make no difference?
Perhaps thats's why Rod Elliot's 3A amp doesn't use one??
Cheers,
Pete McK
P.S. thanks for the discussion guys...
"The cap across the bias network of your typical push-
pull follower can be a crummy cap since it doesn't
actually carry signal."
Nelson, are you saying that the impedance across the Vbe multiplier seen by the signal is so low that a cap will make no difference?
Perhaps thats's why Rod Elliot's 3A amp doesn't use one??
Cheers,
Pete McK
P.S. thanks for the discussion guys...
CAPS
Hi
I use polyester or pp caps. I see them as a speed up help, to remove the charge from the Base of the transistor you want to turn off. Is the wrong to see it this way?
\Jens
Hi
I use polyester or pp caps. I see them as a speed up help, to remove the charge from the Base of the transistor you want to turn off. Is the wrong to see it this way?
\Jens
PeteMcK has asked the crucial question. 
Peranders, some calculations are needed across the frequency band of interest in order to establish how true the truth is.
Peranders, some calculations are needed across the frequency band of interest in order to establish how true the truth is.
Spice
Hi,
Just simulated the P3A, with and without the discussed cap.
It turns out, that there is a small increase in THD at 1Khz with a 1 µF (ideal) cap across the Cbe transistor. Dont know if this applies to real life also.....
\Jens
Hi,
Just simulated the P3A, with and without the discussed cap.
It turns out, that there is a small increase in THD at 1Khz with a 1 µF (ideal) cap across the Cbe transistor. Dont know if this applies to real life also.....
\Jens
- Status
- Not open for further replies.