Adding a DC-Servo to an Existing Amp

[EchoWars]

I've been fooling with matchng transistors (which don't stay matched) and adding nulling circuits to eliminate offset, and the servo approach seems to kick-butt!

I'd rather not like to get into a discussion on the pro's and con's of servo's, rather I'd like some input on how one might add a servo such as the one below to an existing amplifier which utilizes a differential pair input.

Yes I searched....all over the web, and here in the forums going back nearly two years. There was some good discussion a while back (1 1/2+ years ago), but the pics don't load and all the links are long dead.

OK...here is how one amp company does it... It works very very well. Here they use a NPN pair...

[EchoWars]

...here is the input stage of an amp I built a while back (PNP input). On this amp I have +/- 15V supplies handy, so would power the servo from there. All I have done here is to redraw the input and add the servo. What else should be changed?

The 56K bias resistor ought to be made larger I assume, and could probably toss the grounding cap on the feedback transistor, but this seems too simple...what did I miss...

Thanks ya'all...

[SY]

In similar amps, I've used non-inverting integrators brought back to the "normal" feedback, with good success. That way, you can get rid of the input cap and be truly direct coupled. Yes, you can get rid of the big elytic in the feedback path, one of the primary benefits of the servo.

I agree that servos are the way to go.

[OliverD]

Should work.

Often you see the servo output attached to the inverting input of the diff pair (with the inputs of the opamp swapped of course). Like SY said, it's an integrator parallel to the normal (negative) feedback path. In your drawing, it's an inverting integrator connected as a positive feedback. Does the same actually and you can't go much simpler anyway.

However, in your circuit the integrator seems to be faster than it should be - this could cause quite audible phase shifts in the bass.

[EchoWars]

Too fast? The 4.7M amd the .1µf cap make it about 2Hz. Anyway, the example I posted in the first pic is about all the firsthand experiance I have had with servo's.

Maybe if it is too fast I could substitute a .18µf cap for the .1µf (I have a bunch of .18µf caps, but no .1µf).

Lastly, can someone explain to the clueless (me..) why the servo allows you to dump the cap on the feedback path? The reasons do not just jump out at me...

[OliverD]

Originally posted by EchoWars
Too fast? The 4.7M amd the .1µf cap make it about 2Hz.

Too fast for a DC servo IMHO. You can really hear them. It acts like a simple low pass with fg=2Hz which means it will feed the amp with much higher frequencies than 2Hz, only at a lower level. Disconnect its output from the rest of the circuit and watch it with an oscilloscope while sweeping the amp and you will see what I mean.

Originally posted by EchoWars
Lastly, can someone explain to the clueless (me..) why the servo allows you to dump the cap on the feedback path? The reasons do not just jump out at me...

Sure, SY was talking about the input cap, not the feedback cap. You won't need it as your DC servo will correct a small DC offset even at the input of the amp (which otherwise would get amplified and could damage your speakers which is why you have the cap)

Another hint: I recommend two anti-parallel or anti-series zeners of say 10V each at the input of the servo opamp to protect it in case your output stage fails.

[SY]

AMT- I meant both.

EW- As AMT said, the big cap in the feedback loop reduces output offset by knocking the amp gain down to unity at DC.

[EchoWars]

Originally posted by SY
Yes, you can get rid of the big elytic in the feedback path, one of the primary benefits of the servo.

This was the statement I was referring to, the elimination of the (in this case) the 47µf cap on the base of the feedback tranny. The elimination of the input cap makes sense to me, the cap on the feedback transistor does not.

So...what's a good time constant? 1sec? Even slower?

[SY]

Well, that cap is superfluous- the servo acts to drop the amp gain to near-zero at DC, so is even better than the cap in that respect. Replace that nasty cap with a piece of your favorite wire. If you decide to use a noninverting integrator, you can get rid of the input cap, too (assuming you absolutely KNOW that nothing feeding the amp can POSSIBLY have output offset or could fail to one of its DC rails). For an inverting integrator, you need that input cap to prevent the servo from trying to feed DC back to the device driving the input.

Two Hz is well low enough. I mean, how low do you expect your speakers to go? If you're worried about contamination of the audio signal from the output of the servo, an RC low pass network with a pole at about 50 Hz can be put between the servo output and the amp node that it's feeding. Since its more than a decade above the servo time constant, it's unlikely to give the amp a stability problem.

[OliverD]

Neither saw the big el'lytic nor your comment about it. It's 5'o clock in the morning here

You will need the big caps in the servo then. My approach is to take the biggest film caps that I can fit (2.2..10µF, low voltage) and resistors a bit lower than yours (noise).

You can tweak your time constant by listening, too. Select one of those recordings they use to show off their subwoofers and increase your time constant until the bass sounds as clean and tight as without the servo.

Some discussion on this subject:
DC Servos - Why Are They Badly Regarded ?