To use DC servo or not?

[jaycee]

Wow, forgot I started this one

I think, for now, I will make a 1-channel prototype of the circuit I intend to build, and test it with just a cap in the feedback path. If that works "good enough", perhaps with an offset adjust pot.. i will leave it at that. I can also rig up a small piece of Veroboard with a servo on it to try, i guess.

[peranders]

Originally posted by jtcc1015
peranders,

very much appreaciate your help. I will look into the LTSpice. I have actually thought of adding a speaker protection circuit before I connect the DC servo because I understand the fact that an Op-amp circuit can have higher failure rate than a simple RC filter. Therefore, I am working on both upgrade kits simultaneously.

An opamp is a reliable part. The failure rate is zero compared to the power amp itself. The only thing you have to think of is that the opamp is driven with lower voltage than the main amp. Therefore you must make some considerations both in protection but also in dimensioning the servo.

[jtcc1015]

anatech & forr,

interesting idea, I shall look into that.

perander,

Good suggestion, I shall add some protection to the input of the DC servo. Have tried out the LTSpice simulation with the Leach Amp feedback circuit inserted and confirmed that low-pass filter on the output of the DC servo is not necessary after all. The existing filter will do the job of filtering high frequency noise from the DC servo as well.

On the other note, from the what I've heard from other DIYers who has been using the DC servo in their power amp:

I quote "The DC servo do not only correct the DC offset condition on the output of my power amp, but also, depending on which Op-amp is being used, slightly changes the original sonic character the power amp itself.

To me, this is perfectly understandable as different Op-amp do perform differently in a given environment, and this is the "side effect" offered by the DC servo that I am also insterested to explore. Well, it may sounds like witch-craft to someone while unorthodox to others, but it does provide a very convenient way to teawk the sound character my power amp.

jaycee,

welcome back. Yes, you are the thread starter, I have no intention to steal it from you. Just that, I am in the verge of implementing a DC servo kit onto my Leach Superamp and got anxious to hear opinions and suggestions from your goodself and others.

I shall look forward to your further updates on your project.

BTW, I have just done soldering the boards. Attached is the photo of the one them:

[forr]

anatech ,
"forr is hit on something I have noticed over the years. If an amplifier is balanced and the diff pair doesn't have to do much work to maintain a DC balance, the amp sounds much better. I didn't know why that was until having the time to study this in the last few years. A DC servo circuit may interfere with the diff pair, throwing the collector current balance out."

jtcc1015
"anatech & forr,
interesting idea, I shall look into that."


I think the complementary differential input is fundamentally a wrong concept because, among other things, each input differential creates its own offset at its output and the two offsets of the upper and lower differentials can never be fully equal in value. Particularly in this case, a servo fed back to the Vas à la Perrot should do better than one fed back to the input.

~~~~~~~ Forr

§§§

[anatech]

Hi forr,
I agree, I have never liked the sound of amplifiers using complementary differential pairs. I have found they sound a whole lot better when all the diff pairs (NPN and PNP) are matched to the same DC beta. They will not operate perfectly equally as pairs, but that's better than the normal mis-matched mess I see.

-Chris

[amplifierguru]

Here's how you balance DC on a dual diff'l without any filtered supply chain or servo.

+0.6V to -0.6V low Z and modulated as the FB point. Weep!
100K VR and R.

Cheers,
Greg

[Leolabs]

Has anyone thought about duo-beta????Much simpler than DC servo.

[anatech]

Hi Greg,
The best way to balance that circuit is to match all the transistors together, and the current sources. That I think gives you the best sound quality possible from this configuration. Over 20 yrs experience with doing that.

The only problem is, I don't think it sounds very good compared to a single diff pair. I'll trade 10~30mV DC offset for a balanced differential pair any day.

Hi Leolabs,
Duo-beta (Luxman term) is a DC servo in any design I have seen, unless I am thinking of the wrong thing. It works by unbalancing a diff pair to correct for the natural DC offset.

-Chris

[amplifierguru]

Hi Anatech,

My dual diff'ls are always beta matched to < 5%. And they use the above DC offset adjust cct (patent pending). the beauty is that there is no need to filter supply lines - just the trimpot and a resistor - couldn't be simpler or work better. It can even be used for AC coupled so zero any DC to < 1mV.

This is what I call simple elegance folks.

Cheers,
Greg

PS wait a moment - doesn't a Leach have dual diff'ls ( and they sound no good - watch for the wrath of GOD, Anatech). Could it be we can improve it with this cct?

[mikeks]

Originally posted by jaycee
Is it worth the extra effort of using a DC servo on an amplifier ?

If the power amp. you've just designed needs a servo, or a pot. to trim DC offset, scrap the design and start again....