Hello all, I have a Mackie M1400i power amp that was having many issues that have mostly been resolved, but found some rail voltage where it shouldn’t have been and it was due to a faulty 270pF silver mica.
So I plan to replace both C18 and C20 in both channels. What I have on hand are 330pF silver mica and 220pF and 330pF in both polypropylene and ceramic, not sure if dielectric matters here. Ultimately I’d like to swap in four 330pF silver micas, but want to make sure that increasing 60pF won’t cause an issue. I’m trying to save the $25-$30 I’d have to spend for four new silver mica and shipping costs.
Should I be okay with the 330pF mica or would be better to go down to a 220pF poly or ceramic?
Thank you,
Dan
So I plan to replace both C18 and C20 in both channels. What I have on hand are 330pF silver mica and 220pF and 330pF in both polypropylene and ceramic, not sure if dielectric matters here. Ultimately I’d like to swap in four 330pF silver micas, but want to make sure that increasing 60pF won’t cause an issue. I’m trying to save the $25-$30 I’d have to spend for four new silver mica and shipping costs.
Should I be okay with the 330pF mica or would be better to go down to a 220pF poly or ceramic?
Thank you,
Dan
That's a good question. I think they are just snubbers to prevent oscillation. Try it and see. I really don't think it'll blow up.
That was my thought as well. Ran into the same thing with a CV amp where when driven to its limits would oscillate at the peaks of the waveform. Increased the capacitance of the snubbers and it went away.
But that was my main concern, hopefully someone can confirm I wouldn’t run into oscillation issues.
Dan
Bigger caps == lower frequency that's quenched... Try 330pF first 🙂 Best case, works fine, worst case, reduced HF bandwidth.
Is silver mica the dielectric to use here? Any issues with using ceramic? I’m guessing there is a reason they used silver mica.
I have a whole bunch of these ceramic 560pF 100v capacitors, two in series would yield me 280pF 200v. That’s pretty darn close to the 270pF. Or would it be best to use the silver mica?
Thank you,
Dan
I usually use mica for this kind of thing, but MKP (metallized polypropylene should be just as good here. It's AF, not RF. The general rule is use anything but a cheap ceramic disc cap (even if the original design had one -- upgrade it). Others can surely correct me if I'm mistaken.
They are to prevent oscillation, but not “snubbers”. They are basic lag compensation and the pF value should be respected. 280 pF is close enough, with 330 the open loop pole will be lower and you lose HF capability and compromise slew rate. 220 and you may risk oscillation. I tend to use micas in those positions because of the voltage swing, and the couple ohms of ESR isn’t the end of the world. Especially with the resistor in series.