Moog Prodigy: do capacitors model/type affect the sound?

[GregoryK]

There are 6 electrolytic capacitors on the Moog Prodigy circuit board. And as I understand the schematics, 4 of them are in the audio circuit. And after 30+ years, I assume their performance is compromised. And since it *seems* to me that the sound of the synth has been slightly changed over the last time, I’m thinking to replace all electrolytic capacitors, using the same capacitance/voltage replacements, of course.

The question is, do the capacitors affect the actual sonic characteristics of the output signal? At least, in a case with Moog Prodigy?

My concern is that the modern capacitors (different series, different characteristics) will make the sound different. And I would like to keep the "original" coloration of the sound as much as possible.

I would greatly appreciate any input..

Thank you!

[latigid on]

In general it's the electrolytics in the power supply that you need to worry about. In the audio signal chain, the high-value caps are likely to be used for timing (more for CV though), or AC coupling (i.e. removing DC offset).

What do you want; "coloured sound" or something closer to the original? The "change" that you hear could be other sorts of component drift also, for instance trimmer pots.

[GregoryK]

Thank you very much for your reply!

I would like to keep the sound closer to the original as much as possible.

Here is the schematics of the Prodigy, where I highlighted (by green color) four electrolytic capacitors. Would you be so kind to explain me their functions in the synth, please? Currently, only C26 is the factory installed capacitor. The other three are Chinese caps of doubtful quality, though they are not leaking or something..
https://www.dropbox.com/s/ajfkeam9ahvb7 ... 0.jpg?dl=0

P.S. The Power Supply capacitors will be changed as well.

[MC]

Thank you very much for your reply!

I would like to keep the sound closer to the original as much as possible.

Here is the schematics of the Prodigy, where I highlighted (by green color) four electrolytic capacitors. Would you be so kind to explain me their functions in the synth, please? Currently, only C26 is the factory installed capacitor. The other three are Chinese caps of doubtful quality, though they are not leaking or something..
https://www.dropbox.com/s/ajfkeam9ahvb7 ... 0.jpg?dl=0

P.S. The Power Supply capacitors will be changed as well.

C26 and C36 are audio AC coupling caps. C35 and C45 are power supply filter caps.

If you are concerned about original tone color, you really should be looking at C27/C28/C29/C32/C33/C34. The correct dielectric is critical. I don't know what the originals would be, but if they are rectangular white blocks then they are polyester caps.

[noddyspuncture]

Thank you very much for your reply!

I would like to keep the sound closer to the original as much as possible.

Here is the schematics of the Prodigy, where I highlighted (by green color) four electrolytic capacitors. Would you be so kind to explain me their functions in the synth, please? Currently, only C26 is the factory installed capacitor. The other three are Chinese caps of doubtful quality, though they are not leaking or something..
https://www.dropbox.com/s/ajfkeam9ahvb7 ... 0.jpg?dl=0

P.S. The Power Supply capacitors will be changed as well.

C26 and C36 are audio AC coupling caps. C35 and C45 are power supply filter caps.

If you are concerned about original tone color, you really should be looking at C27/C28/C29/C32/C33/C34. The correct dielectric is critical. I don't know what the originals would be, but if they are rectangular white blocks then they are polyester caps.

I don't think that changing those non-electrolytic capacitors would be necessary.
But I do believe that changing the power supply caps (C38,39,42,43) would be a good idea.

I'd leave all the other electrolytics in the synth alone - unless of course they are leaking or test faulty/out of spec with an ESR meter.

I once completely recapped a Minimoog, replacing all the electrolytics and I really didn't like what it did to the sound.

Cheers,
Tom

[GregoryK]

C26 and C36 are audio AC coupling caps. C35 and C45 are power supply filter caps.

Thank you for your reply!

Did you mean that C42 and C43 are Power Supply filters filter caps? Because C35 is in the filter section and C45 is in the Envelopes section. I'm a bit puzzled. Would you be so kind to look at the schematics again?
https://www.dropbox.com/s/ajfkeam9ahvb7 ... 0.jpg?dl=0

[GregoryK]

I don't think that changing those non-electrolytic capacitors would be necessary.
But I do believe that changing the power supply caps (C38,39,42,43) would be a good idea.

I'd leave all the other electrolytics in the synth alone - unless of course they are leaking or test faulty/out of spec with an ESR meter.

I once completely recapped a Minimoog, replacing all the electrolytics and I really didn't like what it did to the sound.

Thank you for your message, Tom!

Are there any preferences in terms of the Tantalum replacements series/brand?

Your experience with Minimoog is indeed very interesting. The thing is I'm afraid the sound might be already "ruined" a bit by previous owner, because the electrolytic capacitors he installed look like cheap counterfeit. I couldn't even find any info for such brands like "Chang" or "Soli"

[thealien666]

My Minimoog D is 41 years old, and still has all its original, high quality Philips electrolytic capacitors inside. I checked all their ESR reading and they're all well below maximum tolerance (some even better than cheap new ones!). It has a few polyester capacitors (Mullard/Philips "tropical fish") on which the external shell has started to crack from age, but their reading is still good, so I didn't touch them either.
It sounds fabulous.

If it ain't broke, don't fix it.

I did, however, install the Opto-Key system to replace all the problematic keyboard contacts.

My two cents on it. Take it for what it's worth.

[Stevie Ray]

Oh yeah the 'tropical fish' - I still have a bag of them of all sorts of values somewhere

As a rule, leave the caps alone unless there is an obvious problem. They may look shonky, but if they are doing the job, leave them alone.

Generally the only caps that need replacing are the large value electrolytics in the power supply.

[GregoryK]

Thank you for you reply, Stevie!

Would it be better to use low-esr or regular capacitors in the Power Supply?

[noddyspuncture]

Thank you for you reply, Stevie!

Would it be better to use low-esr or regular capacitors in the Power Supply?

Low ESR are preferable... but also more costly.
Any top brand 105degree with a one-level-up voltage rating would do the job nicely...!

Cheerz,
Tom

[GregoryK]

Great, thank you very much!

[MC]

C26 and C36 are audio AC coupling caps. C35 and C45 are power supply filter caps.

Thank you for your reply!

Did you mean that C42 and C43 are Power Supply filters filter caps? Because C35 is in the filter section and C45 is in the Envelopes section. I'm a bit puzzled. Would you be so kind to look at the schematics again?
https://www.dropbox.com/s/ajfkeam9ahvb7 ... 0.jpg?dl=0

C42/C43 are not the only power supply filters. It is common for others throughout a circuit board.

C35 is the power supply filter for the DC bias divider in the ladder filter. It is also tied to the audio and regeneration inputs (base of both transistors in the bottom pair) through a 1K resistor, which forms a lowpass filter through C35 tied to ground. This is a protective circuit against frequencies above audio (IE RF). This doesn't have any significant impact on the tone.

C45 is not a timing or audio cap. It is a power supply filter for the positive rail going to the opamp.

[GregoryK]

Oh, I see. Thank you for detailed explanations!

[Stevie Ray]

Thank you for you reply, Stevie!

Would it be better to use low-esr or regular capacitors in the Power Supply?

Makes no difference in PSU applications. Just use high-quality caps and you can't go wrong.