Allen & Heath System 8 Mk1/2 recap and mods

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Lasselakken

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I plan to do some recapping and maybe a few mods on an Allen & Heath System 8 mixer. My mixer is a Mk1, but here I have been using the Mk2 schematics since they are mostly the same. I have been reading quite a lot on these kinds of jobs, and have come up with a plan for the input modules. I’d love to get some input from you guys, though.

Electrolytic caps
I have marked the electrolytic capacitors on the attached schematic, and here is what I plan to do:
  • C1+C2 – 10uF line input – no change, just some fresh 10uF
  • C3+C4 – 47uF Mic input phantom blocking caps – change to 220uF
  • C7 – 220uF just after mic gain – change to 1000uF
  • C8+C9 – 10uF just after transistors – no change, just some fresh 10uF – or could these be changed to polyester caps 2,2uF, 3,3uF or 4,7uF?
  • C11 – 10uF coupling cap – no change, just a fresh 10uF
  • C13+C14 – 100uF on power rails – change to 1000uF
  • C23 – 10uF coupling cap – no change, just a fresh 10uF
  • C27 – 10uF coupling cap – no change, just a fresh 10uF
  • C29 – 47uF at insert return – no change, just a fresh 47uF

I plan on using mostly Panasonic FC electrolytic caps.

On top of that I plan to bypass all electrolytics in the signal path with 0,1uF Wima MKS2 on solder side of pcb.

Should I be changing the 10uF coupling caps to higher values too?

Could C8 and C9 be changed to a smaller value polyester cap? (Wima does make 10uF polyester caps, but they are much more expensive). Could other electrolytic caps be substituted by polyesters?

Transistors
Would it be a good idea to change out the transistors (Q1-4) and if so, what would be good replacements?

ICs
I also plan to do local power rail decoupling at the ICs by adding a 0,1uF Z5U cap between +/- (pins 4 and 8) and ground for each IC.
Later on, I will try different op amps like OPA2134, LM833 or LME49720. Any suggestions or warnings regarding ICs would be very welcome. Can I expect any improvement from just changing the old TL072 to new ones?

PSU
I’m aware that the original PSU is too weak to handle more power hungry ICs and I plan to upgrade it (probably just building or buying a new one).



Actually my first move will be to recap the master channels, but I think that once I have a good plan for the input modules, I can apply same principles to the master section.

Please chime in with any suggestions.
 

Attachments

  • System 8 Mk2 input schematic.jpg
    System 8 Mk2 input schematic.jpg
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Whoops

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On top of that I plan to bypass all electrolytics in the signal path with 0,1uF Wima MKS2 on solder side of pcb.

What are the benefits of the added bypass caps?

Transistors
Would it be a good idea to change out the transistors (Q1-4) and if so, what would be good replacements?

Maybe they are fine,
what transistors does it uses for Q1-4 ?

ICs
I also plan to do local power rail decoupling at the ICs by adding a 0,1uF Z5U cap between +/- (pins 4 and 8) and ground for each IC.
Later on, I will try different op amps like OPA2134, LM833 or LME49720. Any suggestions or warnings regarding ICs would be very welcome. Can I expect any improvement from just changing the old TL072 to new ones?

From my experience in similar threads, I have to be honest and tell you that people around here are a bit skeptical about possible improvements related to changing the IC's, but maybe someone might chime in.
 

Lasselakken

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Thanks Whoops.
The benefit from the polyester bypass caps is better high frequency response, if I have understood that right. And the higher values for the electrolytics should get me better low frequency response.

The four transistors are all ZTX214 according to the schematics.

I am aware that there is a lot of “snake oil” talk about the op amps, so that’s not my main focus. But since I need to go in there to change the electrolytics (some of them have failed, so I guess the others might need a change too), I might as well give it some tweaks.

My main concern right now is the sizes of the new electrolytics. By comparing with advice from other threads in here, I’m pretty sure about the 47uF coupling caps and the power rail caps, but I’m still not sure about the 10uF caps in this circuit. Would I gain something from changing them to ie. 47uF?

I can of course just try it out, and I might experiment anyways, but I’d love to hear opinions from anyone who understands these circuits better than me.
 

moamps

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I would make the following changes:

- all 10uF/16V change to 22uF/25V except C11 - 100uF/25V
- C3, C4 - 47uF/63V
- C7 - 2200uF/10V bipolar if fits
- C13, 14 - 220uF/25V, not higher
- C29 - 47uF/25V (can be also 22uF/25V)

- all caps Nichicon UFG type

IC1, IC3 OPA2134, IC2 TL072, install sockets

- replace LED1 with another that has the highest sensitivity you can find, change R42 to 22k or higher

P.S:
You can use the same value for capacitors C11, 13, 14, 29 of 220uF/25V, that way you will reduce the total cost because you will get a better discount for larger purchase.
 

Lasselakken

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Thank you very much for the advice, Moamps.

Since you recommend Nichicon UFG-caps - does the quality of those caps eliminate the need for bypassing with film caps?

What is the benefit of changing Led1 and R42?
 

IBarfoot

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Totally agree on the Nichicon UFG they can be a little hard to find but in my experience Mouser usually have a good range and stock of them, also they are now being fitted to some of the new A&H mic input channels certainly the "Prime " range.
In my own limited experience different op-amps can change the character of the sound, but by how much is hard to quantify, if it is "Snake oil" then I may have to own up to being fooled into paying out for premium op-amps in various HPA's that I have built. Never quite made it to the likes of Burson disctrete op-amps though.
 

jensenmann

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The original transistors are fine, leave them in. The ZTX 214 had better specs than the BC214 which was considered to be one of the best low-noise transistors back then.

Leave the phantom blocking ccaps at 47uF. Increasing them could create other problems. 47uF is plenty.

@Whoops: if you consider the electrolytic distortion mechanism as relevant then it´s basically an AC voltage source. Soldering a film cap in parallel will short this voltage source, hence getting rid of the distortion.
 

Whoops

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- all caps Nichicon UFG type

As we are not into "snake oil" over here,
any Panasonic series Lytic capacitor rated at 105degrees is pretty good and recommended not better or worse than any Nichicon cap.
They're all good, Nichicon or Panasonic

Since you recommend Nichicon UFG-caps - does the quality of those caps eliminate the need for bypassing with film caps?

From what you and Jensenmann wrote, then the answer is No.
"The benefit from the polyester bypass caps is better high frequency response"
"Soldering a film cap in parallel will short this voltage source, hence getting rid of the distortion."

If the improvement in High Freq response and lower distortion is achieved by a bypassing an Electrolytic cap with a film caps then no Electrolytic cap from any brand will achieve that alone
 

musgrave

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I'm probably an idiot, I go up to 470, never under 100 in coupling. I don't bother with the parallel hf cap. I put a pair of 4700/25's where the power comes in and I use opa2134/314 for most buffers and opa228's for mic pre's, summing and following VCA's. I beef up the ground with #16 wire connecting ground planes and leave the power disrto as is. I add 33R/100u/25v to each opamp - cutting traces and inserting if necessary. and drop the 50-100 ohm resistors at the amp outs to 22r. the opamps's up the HF, sonics and noise floor and the big ol caps and ground beefing tighten and extend the lf. people always love the sound... but what do I know... lol great music has been made on crappy gear...
 

moamps

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As we are not into "snake oil" over here,

I do not advocate any "snake oil" theory, but I keep my eyes and ears wide open, I like to experiment, so far I have made modifications and services on a lot of professional and consumer devices, I also design my own devices, and have a really solid background of the theory of electronics and especially audio science, from hearing theory to speaker theory.
My suggestions and advice therefore arose only from my knowledge and experience. Maybe they can help someone.

any Panasonic series Lytic capacitor rated at 105degrees is pretty good and recommended not better or worse than any Nichicon cap.
They're all good, Nichicon or Panasonic

To know all the quality and weight of this claim of yours, can you tell me why capacitors have to be rated for 105 degC?
 

Whoops

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To know all the quality and weight of this claim of yours, can you tell me why capacitors have to be rated for 105 degC?

They don't have to, as they don't have to be Nichicon UFG.
I just advised the 105 rated, because all Panasonic series rated at 105 degrees are pretty good and that can help the Op not have to worry about which series to choose.
 

moamps

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They don't have to, as they don't have to be Nichicon UFG.

So that's how it is.

I didn’t say they had to be Nichicon UFG, but I suggested it based on my experience. I could also suggest the UFW series or MUSE here (although that series is not the best for me). As for the Panasonic capacitors, the FC series is good by default, but still slightly worse than the Nichicon UFG, IMHO.
 

Whoops

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As for the Panasonic capacitors, the FC series is good by default, but still slightly worse than the Nichicon UFG, IMHO.

"Worse" in what sense?
Can you define what you mean by "slightly worse" and provide technical proof of that?
 

Lasselakken

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Thanks everyone - your considerations are much appreciated.

- C7 - 2200uF/10V bipolar if fits
Why is bipolar important here?


I add 33R/100u/25v to each opamp - cutting traces and inserting if necessary.
Could you explain exactly where you add that 33R/100uF and why?
and drop the 50-100 ohm resistors at the amp outs to 22r.
In the schematic posted above, most op-amps are followed by a 22r resistor, so I guess that’s not necessary here?
 

gyraf

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..bipolar for C7, because actual DC polarity at this point is unpredictable.

In our old Amek, we'd measure DC polarity and mount the capacitor accordingly (the polarity follows a given set of transistors, so only need to be re-measured if a transistor is replaced)..
 

moamps

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"Worse" in what sense?
Can you define what you mean by "slightly worse" and provide technical proof of that?

I said “based on my experience”. My experience, thank God, is different from yours so it doesn’t need to annoy you so much, what’s worse for me may be the same for you, and that’s perfectly fine.
 

Newmarket

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ICs
I also plan to do local power rail decoupling at the ICs by adding a 0,1uF Z5U cap between +/- (pins 4 and 8) and ground for each IC.

Please chime in with any suggestions.

Why Z5U caps ? Poor voltage and temp coefficients. Only benefit is volumetric but I can't see that as an issue on this board.
I recommend minimum spec of X7R dielectric.
 

Lasselakken

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..bipolar for C7, because actual DC polarity at this point is unpredictable.
Thanks for clearing that up
Why Z5U caps ? Poor voltage and temp coefficients. Only benefit is volumetric but I can't see that as an issue on this board.
I recommend minimum spec of X7R dielectric.
I mentioned Z5U because I have a good source of these in axial form, that seemed practical. I’ll get X7R instead.

Is local power rail decoupling by the ICs generally advisable, or is it only necessary if I change to different op-amps?

Do you think I could improve anything by putting in new TL072’s, or would they perform exactly the same as the original ones from ca.1984?
 
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