[earplay]

Okay, I plan to replace the old MIT caps in my power amp. It was built with bypass caps on the MIT's.

I am not skilled with my hands so I do not want to attempt a reconstruction of the original circuit, but instead just replace the original main caps and bypass caps with a single modern cap.

How do I calculate the values for the intended single modern caps?

[S Clark]

If connected in parallel, simply add the values of the caps together.  Most bypass caps are used to change the quality rather than specifically to increase Faraday value.  What are the exact values of each cap?

[art]

The small bypass cap is intended to raise the frequency at which the capacitor(s) become inductive. (They all become inductive at some frequency.) Sounds good in theory, but in practice can lead to resonance problems, which actually makes things worse.

In the audio world, some feel that using a smaller cap of a different type of dielectric (or some other parameter) makes it sound better. Maybe, maybe not. Since all caps do sound different............well, you have to let your ears tell you.

OK........so why exactly do you want to change them? They are not electrolytic, and therefore won't have dried out with use. There must be some other reason that you want to test your skills.

Pat

[earplay]

Thanks for the replies.

The caps are 1987 vintage MIT RTX .22pf/600v. Upon closer inspection, they are not wired in parallel with bypass caps. They are wired in series to other components. The other components look like resistors to my amateur eyes.

I know it is a complex relationship between components in a circuit and the sound of the circuit. The amp sounds dry so I thought I could try rolling in some other caps. I've got Mundorf PIO in mind.

Any thoughts?

[art]

Probably low-value resistors, to avoid the problem that I mentioned above.

Some folks...............ok, mainly the DIY crowd.........play games with inserting low-value resistors in series with bypass caps. Largely tuned by listening, but most likely done without any detailed technical knowledge of what they are doing. It sounds better to them, and that is good enough. The fact that they may actually be doing something that has technical merits is probably a coincidence.

There are a ton of things that could make the amp sound "dry", and yes, the caps may be part of it. But that includes the big filter caps, as well. Which, by pure coincidence, might have dried out due to age and heat.

Picking main supply caps is something that we always drove us nuts. Ones that sounded clean often times were too overdamped in the bass. Caps that had "punch" were too often harsh and nasty. Keep that it the back of your mind.

Pat

[earplay]

Interesting. This is a Cary amp. The MIT caps were not stock, but the circuitry is unmodified, to my knowledge. I was the original owner and the caps were installed by a Cary factory tech.

I don't know which are the filter caps, but there are two big caps sticking up beside two of three transformers. It's a symmetrical layout and a big cap is on each side.

Regarding the MIT caps, I have read more than one person opine that MIT caps had a dry sound in their amps. So, I was approaching the problem from that angle.

I know the big caps are electrolytic and may be over age and need replacing. The amp has not had much play, so I am hoping the big electrolytics are still okay. They look fine. They are described as "computer grade" in a way that meant they are high quality caps. I don't know if they are in good condition, though. I think replacing them would be expensive, so I thought I should try replacing the smaller caps to begin.

I suppose I could take the amp to a shop to have the big caps' performance measured. Does that make sense?

[markC]

Fact is that the "big caps" will be cheaper than you think. Electrolytics are not nearly as expensive as boutique films. That being said, if there's no performance issue with the amp, the electrolytics are probably o.k.

[art]

Having the performance measured? Could be an invitation to be taken advantage of. The best test would be to measure the amount of ripple voltage present. But then, you would need to have an identical unit to know how much to expect. Odds are that if you don't have hum on the output, those caps are fine.

There are other ways to measure them, but I would not expect a repair shop to have the sort of lab gear someone like myself would use.

[earplay]

Thank you everyone for the information. I appreciate your suggestions and guidance.

Best wishes, earplay

[Danny Richie]

The reason that the smaller by-pass caps are used is for their lower dissipation factor and the results of using various types of by-pass caps can be dramatic. Don't let anyone downplay their effects.

[S Clark]

Danny,

"lower dissipation factor"-- What exactly is this?  Does this mean that it unloads stored charge faster?

[Danny Richie]

Yep, the smaller caps have a faster charge and discharge rate.