Modifying a commercial amp (Peavey CS-800-B)

[RyanW]

I have an old Peavey amp (CS-800-B) and I would like to attempt to make a few tweaks. Are there any standard tweaks that typically apply to commercial grade equipment? I plan to use this amp on the bottom end of a bi-amped system with an active crossover at ~100 to 300Hz.

My thoughts:

1. Increase the bias: I don't see how to adjust the bias on this amp... there is no variable resistor. Is the bias set and controlled through some sort of self stabilizing thermal loop? The stabistor diodes (marked 13886) appear to be involved with regulating the bias. Any hints?

2. Check and adjust the DC offset: Perhaps this is not necessary. Again, I don't know how I would make this adjustment.

3. Replace and increase PSU Caps: This amp was built in 1977. Do Electrolytic caps last 30 years?

4. Replace the input Op-amp TL071 with a Burr Brown OPA134

5. Replace the 50k volume control pots: The contacts inside are badly corroded. Maybe a dual gang pot so I can control both channels together.

6. Eliminate some of the input wiring: There are all sorts of input wires for connecting X-overs, transformers, or the jumper plug (which is all I have). More wires and connections = more noise? I doubt I will ever use the balanced input. Can I therefore simplify things?

7. Protection circuitry: Could some of the protection circuitry be eliminated or modified? I believe there is a crowbar (Triac board) and clipping indicator?

8. Audio-grade replacement components: Replace critical caps and resistors with better audio grade components. Any recommendations? Perhaps any signal line electrolytics could be replaced with poly film caps? Again, do electrolytics degrade after 30 years?

A higher res. schematic is available for anyone intersted

[FastEddy]

Is it possible to get a better pdf file that the one above? = bad resolution, can't read the numbers, letters, words. Post a link if possible.

1) The bias is fixed. Changes would be w/ soldering iron (unless there is something I can't read on that pdf file = possible).
2) Got a PSU rail voltage measurement? Check the rail voltage first, before making any DC offset adjustment (right after you "improve" on the PSU caps).
3) Replace and increase PSU Caps: This amp was built in 1977. Electrolytic caps do not last 30 years? Might also consider adding some snubbing caps in parallel with those (new) electrolytics. Make 'em plastic MKT or polystyrene, >> 100 volt rating and about 0.1 % to 1 % of electro cap values (for 15,000 uF electro use 150 uF MKT, if there is room, smaller if not).
4) Ti for Burr Brown TI = good choice, much better distortion rating, slew rate > 20,
5) Replace the 50k volume control pots.
6) Eliminate some of the input wiring.
7) Protection circuitry: Could some of the protection circuitry be eliminated or modified? ... maybe later, for now you will probably want / need it for the final smoke tests.
8) Replace critical caps and resistors with better audio grade components. Any recommendations?
a) metal film or carbon film resistors in the signal path and any feed back circuits.
b) plastic snubbing caps around the op-amps, close coupled if possible. TI/Burr Brown engineers use 0.1 nF polystyrene directly across those opamp power pins in order to get those great numbers. (I spread these poly 0.1 nF caps around like bread crumbs across all PSU and PS Rail lines close to active elements, op-amps etc., etc.)

This could be a lot of fun. Ugly enough that no one will steal it, powerful enough to be useful, lots of room in that honking big case for improvements and remodeling ...

(You would be suprised how many Peavey amps are used on stage, most heavily modified. These ugly sticks get tricked out and used for ever by those road warriors in the know.)

[RyanW]

A higher resolution PDF is available. I will e-mail it to anyone who is intersted.

[RyanW]

1) Would one expect that adjusting the bias is worth the effort? How challenging would it be to tweak the "fixed" bias? I am not affraid of a soldering iron... but I would need support of the intelligent men (and women?) of DIYAudio.
2) I have measured the PSU rails in the past. From what I remember it agrees with the schematic at +-81V
3)Do you agree that electrolytic caps are probably past due after 30 years?
4)Do you expect that a hi-speed op-amp might get into oscillation problems?
5) I assume these should be log pots...
6) Agreed
7) Agreed
8) I may need help determining which are feedback loop components... I should be able to sort out signal path myself though.

Any recommendations other than what I have put forth?

[TomWaits]

Originally posted by RyanW
A higher resolution PDF is available. I will e-mail it to anyone who is intersted.

I am intersted. Send me an email...

Shawn.

[FastEddy]

"1) Would one expect that adjusting the bias is worth the effort? How challenging would it be to tweak the "fixed" bias? I am not affraid of a soldering iron... but I would need support of the intelligent men (and women?) of DIYAudio.

Don't change it until you know for sure how and why. (Don't experiemnt on this amp, find a trash amp to play with ... )

2) I have measured the PSU rails in the past. From what I remember it agrees with the schematic at +-81V

Good ... this should not change after installation of better caps, etc. ... Then check to within +/- 1% w/ a good meter ... absolute value of +/- 81 VDC X 0.01 = abs value within 0.8 volts on each rail ... if deviation is un signed equal value, don't change a thing. If deviation is more than about 1% (unsigned abs. 0.8 volts) high or low, one rail compared to the other, then there may be cause for the soldering iron. (Example: + 80.2 and - 81 = abs (0.8 volts) = needs fixin'. Ex: +80.5 and -81.1 = abs (0.6 volts) = no worry mate.)

3)Do you agree that electrolytic caps are probably past due after 30 years?

Yes. (Don't forget the snubbing caps.)

4)Do you expect that a hi-speed op-amp might get into oscillation problems?

Yes, but with caution and careful deliberation ... read the OPA134 app notes.

5) I assume these should be log pots...

or "audio taper" = common on stage equipment. Check the part numbers.

8) I may need help determining which are feedback loop components... I should be able to sort out signal path myself though.

If unsure, don't change the feedback = not very important except for final tweaks, maybe ... do fix the direct signal path with film resistors, etc. ... but this is not as important as the PSU caps and the pots.

[phase_accurate]

I have one of these as well. We used it for a small tri-amped four-way horn-loaded P.A. Since it wasn't the best performer in terms of bass reproduction we used it on the low-mid section.

I think there would be room for improvement regarding PSU capacity and the dimensioning of the coupling caps.

Regards

Charles

[FastEddy]

" ... I think there would be room for improvement regarding PSU capacity and the dimensioning of the coupling caps. ..."

Yes. (Don't forget the snubbing caps.) ...

[RyanW]

How much capacitance is recommended? Does a 50% increase over factory seem reasonable? What would you recommend for snubbing caps? Maybe a couple of electrolytic 1,000uF and poly 100nF on each rail?

What is meant by "dimensioning of the coupling caps"?

[darkfenriz]

This amp has output stage mixture of class AB+C quasi-complementary, you won't cure this by 'audio grade' resistors.
My advice is change electrolytics, it is not worth further upgrade, especially for bass range.