What voltage rating do you think these caps are in the Sequential Circuits Pro-1 VCF?

[MaxDM]

Judging from the size, what would be your guess? 50 Volts?

 

[Ike Zimbel]

The ceramic caps in the foreground? 50 or higher is probably right, but they are extremely unlikely to have failed.

 

[mjrippe]

Most analog synths ran on +/-15VDC or less. Even a 35V cap would be pretty foolproof. As Ike said, ceramic disc caps rarely fail (I have seen it exactly once). If the electrolytics are 16V replace them with 25V.

 

[MaxDM]

Can anyone identify what the manufacturer's code is?

I googled but could not find

 

[mjrippe]

Which cap? There are several in the photo - ceramic, electrolytic, and possibly film. Not that you need the same brand anyhow...

 

[MaxDM]

 

[JohnRoberts]

What voltage do they need to be..? Looks like low voltage digital circuitry.

JR

 

[MaxDM]

What voltage do they need to be..? Looks like low voltage digital circuitry.

JR

Well, any voltage would do really, it's a Sequential Circuits Pro-1 VCF, and those are the caps used in conjunction with the CEM3320.

The Prophet-5, which is almost the same circuit, used more expensive poly capacitors.

It looks like the diameter of the caps is around 7-8 mm, which I am guessing, for a 1980 component would be a 100V rating? (150 pf)

The reason I ask is because I do have some older disk caps, but they are noticeably larger, older types, and although they would work, I am guessing that the lower voltage creates a tad different sound, especially when the resonance feedback is high.

I have a Beringher Pro-1 copy and the circuit looks identical, but the caps are minute SMT caps, and the filter has a different quality, which I am guessing is in part the capacitors.

I am sourcing the closest caps possible before I attempt to replace the SMT's with disk caps.

Obviously, the less time I heat up the circuit board, the less likely that I damage the traces, so I would like to get something very similar, just because...

The SMZ code doesn't match with anything I have found on the net. I have no idea if these are typical class 1 dielectric or something else..

Today's ceramic caps of that size handle much higher voltages, and I believe they will sound a tad different.

I am looking at Farnell, digikey etc. and I find mostly the dipped high voltage caps.

 

[MaxDM]

BTW here is the copy and the original

 

[soapfoot]

I’d caution that there are likely other factors besides the capacitors’ voltage rating that could be responsible for any change in performance you’ve noticed.

 

[JohnRoberts]

Well, any voltage would do really, it's a Sequential Circuits Pro-1 VCF, and those are the caps used in conjunction with the CEM3320.

The Prophet-5, which is almost the same circuit, used more expensive poly capacitors.

It looks like the diameter of the caps is around 7-8 mm, which I am guessing, for a 1980 component would be a 100V rating? (150 pf)

The reason I ask is because I do have some older disk caps, but they are noticeably larger, older types, and although they would work, I am guessing that the lower voltage creates a tad different sound, especially when the resonance feedback is high.

I have a Beringher Pro-1 copy and the circuit looks identical, but the caps are minute SMT caps, and the filter has a different quality, which I am guessing is in part the capacitors.

I am sourcing the closest caps possible before I attempt to replace the SMT's with disk caps.

Obviously, the less time I heat up the circuit board, the less likely that I damage the traces, so I would like to get something very similar, just because...

The SMZ code doesn't match with anything I have found on the net. I have no idea if these are typical class 1 dielectric or something else..

Today's ceramic caps of that size handle much higher voltages, and I believe they will sound a tad different.

I am looking at Farnell, digikey etc. and I find mostly the dipped high voltage caps.

OK I had to search that... forget about voltage... I agree with using film caps.

For SMT use NPO/COG caps that are linear.

JR

 

[MaxDM]

I’d caution that there are likely other factors besides the capacitors’ voltage rating that could be responsible for any change in performance you’ve noticed.

of course, but I want to keep things as non-invasive as possible.

Usually in this kind of circuit, capacitor type has an influence on the resonant sound of the filter, and low-voltage disk caps of old are among the most non-linear cap types, including high-voltage disk ceramics of the same era which tend to sound better.

OK I had to search that... forget about voltage... I agree with using film caps.

For SMT use NPO/COG caps that are linear.

JR

I am not sure that the sound of this particular filter is known for it's linear qualities, unlike it's 'better' twin found in the Prophet 5

 

[JohnRoberts]

of course, but I want to keep things as non-invasive as possible.

Usually in this kind of circuit, capacitor type has an influence on the resonant sound of the filter, and low-voltage disk caps of old are among the most non-linear cap types, including high-voltage disk ceramics of the same era which tend to sound better.



I am not sure that the sound of this particular filter is known for it's linear qualities, unlike it's 'better' twin found in the Prophet 5

If you want to get ceramic disc capacitor distortion look at data sheets for "voltage coefficient". I don't typically make distortion on purpose so I can't help you.

JR

 

[mjrippe]

I highly doubt that 150pf caps are in the audio path. This is a synthesizer, not precision test equipment and you are most likely wasting your time and money on pointless details.

 

[MaxDM]

I highly doubt that 150pf caps are in the audio path. This is a synthesizer, not precision test equipment and you are most likely wasting your time and money on pointless details.

the 150 pf caps that you claim are pointless details are part of a four stage/pole OTA-based Voltage Controlled Low-pass filter with a resonant feedback loop.

It is the heart of the sound of the synth, because those caps are what shape the sound by ‘subtracting’ harmonics from the waveforms.

a subtractive synth is only as good as it’s filter, or filters…. Unless you like to hear raw sawtooth or pulse or triangle waves etc.

 

[soapfoot]

The “SMZ” markings don’t look familiar to me, sorry to say.

The most logical thing, in my view would be to connect one of the caps you already have on hand and evaluate.

If we’ve got no concrete basis for assuming the voltage rating of the cap (or even the dielectric material) will change performance, proceed as though it won’t, and be prepared to change course if it does.

I’m not saying “parts is parts.” Sometimes capacitor type can impact performance, but even if that’s the case here I’d be far more focused on getting the class of ceramic right than the voltage rating.

A 50v Z5U ceramic probably won’t be very different from a 100v Z5U, but could be quite different from a 50v C0G (especially if temperature is elevated or changing).

But—and I know this as well as anyone—it is very possible to overthink these things and let our imaginations/superstitions/anxieties/fears run away with us.

Install your best guess and pretend nothing ever happened. If you try to listen for a difference, you’ll hear one whether it exists or not.

 

[Script]

I wouldn't be surprised if those caps were just off-the-shelf bulk at the time. But maybe not.

Maybe also ask over at this forum
https://forums.syntaur.com/And I think there's yet another forum focussing on analog synths, but I don't recall.

Or: just audition several caps and pick the one that sounds best. Maybe it'll sound even better than before?

(Had an analog synth from the 1970s professionally repaired some 25 years ago and when it came back it indeed sounded slightly different -- it sounded like new, I guessed.)

 

[MaxDM]

The “SMZ” markings don’t look familiar to me, sorry to say.

The most logical thing, in my view would be to connect one of the caps you already have on hand and evaluate.

If we’ve got no concrete basis for assuming the voltage rating of the cap (or even the dielectric material) will change performance, proceed as though it won’t, and be prepared to change course if it does.

I’m not saying “parts is parts.” Sometimes capacitor type can impact performance, but even if that’s the case here I’d be far more focused on getting the class of ceramic right than the voltage rating.

A 50v Z5U ceramic probably won’t be very different from a 100v Z5U, but could be quite different from a 50v C0G (especially if temperature is elevated or changing).

But—and I know this as well as anyone—it is very possible to overthink these things and let our imaginations/superstitions/anxieties/fears run away with us.

Install your best guess and pretend nothing ever happened. If you try to listen for a difference, you’ll hear one whether it exists or not.

Lots of instances when people have changed caps in VCF filters and resonant quality was affected.

You can't connect 'one of the caps' because it is a 4-pole filter, so that all the caps are equally involved and work together as a unit.

How much of a change is another thing, of course, and it will be interesting to find out, because I plan on doing it to other SMT synths as well if it is a significant difference.

I think it sounds patronizing to say that 'I can't hear a difference' if I'm paying attention.. if the difference is subtle enough that I have a hard time noticing it, that is an indication that I should not invest any more time into doing the same with other synths, and probably will put the synth back to original state.

Will need my watchmaker's glasses, cause those caps are very small...

 

[MaxDM]

I wouldn't be surprised if those caps were just off-the-shelf bulk at the time. But maybe not.

Maybe also ask over at this forum
https://forums.syntaur.com/And I think there's yet another forum focussing on analog synths, but I don't recall.

Or: just audition several caps and pick the one that sounds best. Maybe it'll sound even better than before?

(Had an analog synth from the 1970s professionally repaired some 25 years ago and when it came back it indeed sounded slightly different -- it sounded like new, I guessed.)

Yes, I am pretty sure that they were going for low cost parts, especially considering the overall build quality of the original synth.

 

[Newmarket]

the 150 pf caps that you claim are pointless details are part of a four stage/pole OTA-based Voltage Controlled Low-pass filter with a resonant feedback loop.

It is the heart of the sound of the synth, because those caps are what shape the sound by ‘subtracting’ harmonics from the waveforms.

a subtractive synth is only as good as it’s filter, or filters…. Unless you like to hear raw sawtooth or pulse or triangle waves etc.

But I think the point is that such values may not be relevant to the audio filter function itself. Rather they may be related to amplifier stage stability.
Do you have the relevant schematic ?