Compensation capacitor Cdom

[steve_mak]

Hi,

I need to place a Cdom in the Vas stage of a new amplifier I'm building. I am wanting to use a boosted design so I'm using 2 transistors. I believe the local feedback needs to be around the whole lot but I don't know where to connect it.

If you can help, please show me how.

Diagram attached.

Cheers,

[richie00boy]

You answered your own question. Round the whole lot, i.e. move it from the collector of the first transistor to the emitter of the second transistor where the resistor is.

I have ask if you stumbled on this should you be trying to design an amp? Especially as this one looks as though it might be a little complicated and difficult to stabilise. Maybe try starting with something simpler.

[steve_mak]

Thanks Richie,

I have seen it written that boosting the Vas transistor has benefits but I've seen mainly standard Darlington designs. The trouble with the Darlington is that it is biased at 1.2V. This would mean large resistors in the current mirror that precedes this part of the circuit. If I go with this Sziklai combination it allows me a 0.6V bias.

I'm hoping it will work out!

Cheers,

[AndrewT]

Hi,
there are alternatives.
Cdom can return HF feedback from the emitter of the compound pair to the input base of the pair. But there is another out of phase point that the HF feed back can be returned to, the inverting input of the long tail pair.
There are others, but these two are the most commonly used.
The Cdom is the easy and very effective solution that virtually guarantees stable operation of a three stage "LIN" topology. But the downside is reputedly poor sound quality. Try the non-inverting input for improved sound quality. And use a very low capacitance transistor for VAS to reduce the current that gets sucked out of the LTP.

[Conrad Hoffman]

I hesitate to reinforce what may be an old wives tale, but when I stabilized an old Tiger amp with a Cdom on the vas stage, the sound quality appeared to deteriorate noticeably. I went back to a rolloff cap across the global feedback resistor, and everything went back to normal. IMO, though the response and distortion numbers were similar, an SA of the distortion probably wouldn't be. Since the amp has rather high distortion, it makes sense. For a design with fundamentally lower distortion, it might not matter. As for the actual question, you really need to order up Doug Self's book on audio power amps. It will answer your questions and bring up issues you'd take a decade to come across on your own.

[KSTR]

An increased Cdom shifts the whole Aol to the left, reducing the gain available for global feedback at all frequencies above the pole. Whereas a global roll-off cap only reduces Acl above the corresponding pole (to get a rate-of-closure of ~20dB/decade at the Aol 1/beta intersect where additional Aol poles raise their heads), way above audio frequencies. Hence with the Cdom compensation one quite often gets higher distortion in the audio band for a given closed-loop gain.

Mr.Self's website, in particular his excellent distortion article, might suffice for Steve's question.
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

Regards, Klaus

[steve_mak]

Cheers guys,

I do have the latest version of Mr Self's book but I couldn't see any examples of my configuration. My reading is now steering me to the discussion of matching the slew-rate performance which I haven't paid great attention to. I wonder how important it is to match the positive and negative slew rates ...

I shall try to get further from the Self site concerning distortion too thanks KSTR.

Andrew, the transistor downwind from the differential pair is a 2SA970 or 2SC2240. Both of these were selected for their reputedly linear response and their low capacitance. All the low power transistors are either of these.

I assume the 'non-inverting' input is the feedback side of the diff pair? Or is it depending on the configuration? I wonder where I could find some good examples of the Cdom configuration that taps into this 'inverting-input'?

Thanks for your thoughts guys.

Rgds,

Steve

[AndrewT]

Hi,
all these don't use the Cdom method.
JLH 80W mosfet,
Crimson series,
Sugden p128
Is it by coincidence they are all British?
Someone here once suggested that JLH may have been consulting to various manufacturers.
The JLH and Sugden are posted here.
Self does not discuss the alternatives to Cdom in detail.
He became blinkered with 0.0006% distortion target.

He completely messed up his treatment/solution/failure to properly analyse his asymmetric slew rate problem.
Read someone else for balance.

'non-inverting' input is the feedback side of the diff pair?

the two bases either side of the LTP are anti-phase (differential) inputs.

[AKSA]

Wow, Andrew,

you worked that out impressively.......

Hugh

[AndrewT]

Originally posted by AKSA
Wow, Andrew,

you worked that out impressively.......

Hugh

Hi Aksa,
to which bit(s) do you refer?
Does your comment mean you agree or disagree?