Power supply bypass cap values.

[kgoss]

Hi everyone,

I'm thinking about bypassing the last power supply caps on my S.E.X. amp since that is the only recommended upgrade I have left other than the Magnequest Iron.  I have the version before the current 2.1 kit and I read in another forum thread that Paul recommends bypassing the 22uF/450V (terminals 16,17 and 36,37) as well as the 1000uF/35V (terminals 12,15 and 32,35).  My question is what uF and voltage value should I use to bypass the two stock values with?

Thanks in advance for your response.
Ken

 

[Grainger49]

First the important thing, stay with the same voltage rating Bottlehead used or greater.  Nowhere in the SEX circuit are they using a "stock" cap that has a higher voltage rating than is needed. 

In general a bypass can be 1/100 of the capacitor value.  That is a general rule.  So for the 22uF try a 0.22uF, etc.  Try it with jumpers first and do some comparisons.

 

[kgoss]

Thanks for the advise Grainger.
I will have to reconsider bypassing the 1000uF caps then.  Fitting a 10uF cap in at the front of the amp with the C4S boards there will take some creative planning.
Its probably best to do that after the iron upgrade anyway.  I have been holding off ordering the Magnequest Iron to see if the S.E.X. 2.1 OTs will be offered as an upgrade part.

Ken

 

[Grainger49]

Nah, try a 1.0uF or 0.1uF film.  The 100:1 ratio is just a guideline.  Or go with a 10uF and just extend the leads and put the cap wherever you have room.

 

[adamct]

Just to confirm, bypassing the 1,000uf caps is considered advisable, correct? Meaning there is some theoretical benefit to doing so, and the 1,000uf caps are not caps that are irrelevant for audible purposes?

Best regards,
Adam

 

[Grainger49]

The amplified signal is the power supply modulated.  That said, quoting PJ, "I have never made an improvement to a power supply that wasn't audible."

So, yes, bypass the 1000uF caps.  I assume that they are in the power supply, not a signal cap, right?

 

[adamct]

Grainger,

That's my understanding from the first post in this thread, although I haven't been able to track down the thread that the OP is referring to where Paul first recommended this.

Best regards,
Adam

 

[Paul Birkeland]

I'm thinking about bypassing the last power supply caps on my S.E.X. amp since that is the only recommended upgrade I have left other than the Magnequest Iron. 

The SEX 2.1 iron will be available shortly for older SEX amps.

-PB

 

[adamct]

So......I'm an idiot. I wanted to bypass the 22uf caps with film caps. And following the 1% rule of thumb, my plan was to bypass them with 0.22uf caps. But somehow I got mixed up with some 2.2uf caps I ordered for my Quickie, and wound up ordering 2.2uf instead of .22uf. What's an order of magnitude between friends, right?  :

So here is my question: is it still possible/advisable/recommended/not-stupid to bypass the 22uf electrolytics with 2.2uf film caps, or does that somehow create a potential problem, and I need to send these caps back and get 0.22uf caps like I originally intended?

 

[Paul Birkeland]

If you have the C4S upgrade, you could try just the 2.2uF caps and no electrolytic.

-PB

 

[adamct]

Is there any reason to believe the sound would be compromised vs. having the 22uf electrolytic in?

 

[Paul Birkeland]

If we consider the RC time constant of the power supply:

We have 680 Ohms into 100uF, we have a time constant of 0.068S.

If we look at 8.2K into 22uF, we have a time constant of 0.1804S. 

A higher time constant will really shave down the noise, but at the expense of potentially dropping voltage or paying a lot for caps.  (I shoot for 0.1S under most circumstances)

8.2K into 2.2uF gives you 0.018S, which isn't stellar, but there are more dimensions to the problem than just the time constant, which is reason enough to give it a shot.  Sonically, I'd expect it to be quite good, but you may not be happy with the noise performance (which would be less of an issue with the C4S in the circuit).

-PB

 

[earwaxxer]

Hey Adam - there is no real hard fast rule with bypass cap techniques. The main thing is the synergy of the two, or three etc. When combined in parallel they just add together in value. The small value cap adds little in terms of value. You have to try them to see. I have a 2.2uf Mundorf S/O 'bypassing' a 10uf P/O in my speaker crossover. The sound was significantly improved. A 2.2uf there did add a significant value to the crossover point, but it was more than that. The very top end was better. Go figure.

 

[Paul Birkeland]

I have a 2.2uf Mundorf S/O 'bypassing' a 10uf P/O in my speaker crossover.

That is pretty significant in a crossover...

If we assume that the 10uF cap is for a first order high pass crossover on an 8 Ohm driver, you will go from a 1.99K crossover point to a 1.6K crossover point with the additional 2.2uF.

If you bypass that 10uF cap with a 0.1uF cap, you will only go down to 1.97K; this is well within the tolerance of the 10uF cap itself.

 

[adamct]

Hmmmm......I may try it, but I think I'll also order some .22uf caps as well. Any value in using the 2.2uf caps to bypass the 220uf electrolytics, or is that unlike to have any audible result?

 

[Paul Joppa]

There are no reliable "rules" about bypassing power supply caps. Sometimes they help, sometimes the hinder, and sometimes you can't tell the difference. The "1% rule" is really more like "something between 0.1% and 10%" and even then exceptions are not at all uncommon.

The only reliably useful answer is "try it and see!"

I could go into technical explanations of what can work and what can cause problems, but frankly I think there are so many effects we DON'T understand that it would be deceptive to talk of the few we DO. Just try it.  :^)

 

[adamct]

Cool. I'll try them in both positions and see how it goes.

 

[earwaxxer]

I have a 2.2uf Mundorf S/O 'bypassing' a 10uf P/O in my speaker crossover.

That is pretty significant in a crossover...

True.. I probably shouldnt call it a 'bypass' because I was combining the values on purpose to reach the 12-15uf or so that I was looking for. I perceived that there was a benefit beyond just lowering the crossover point for the ribbon tweeter. So, when I combined them I was hoping I could kill two birds with one stone (bypass benefit, and significantly lower the crossover frequency). That seems to be the case, lucky for me!

 

[adamct]

If we consider the RC time constant of the power supply:

We have 680 Ohms into 100uF, we have a time constant of 0.068S.

If we look at 8.2K into 22uF, we have a time constant of 0.1804S. 

A higher time constant will really shave down the noise, but at the expense of potentially dropping voltage or paying a lot for caps.  (I shoot for 0.1S under most circumstances)

-PB

Hmmmm. This is very helpful in understanding how you choose the values you use. Voltage affects volume, is that right? And why does increasing the time constant reduce noise? Why wouldn't it affect noise and the music signal equally? Other than cost of caps and reduced voltage, does increasing capacitance have negative effects on the sound? (I'm no longer trying to figure out what caps to install, just trying to understand how this stuff works.)

Best regards,
Adam

 

[corndog71]

I have a 2.2uf Mundorf S/O 'bypassing' a 10uf P/O in my speaker crossover.

That is pretty significant in a crossover...

True.. I probably shouldnt call it a 'bypass' because I was combining the values on purpose to reach the 12-15uf or so that I was looking for. I perceived that there was a benefit beyond just lowering the crossover point for the ribbon tweeter. So, when I combined them I was hoping I could kill two birds with one stone (bypass benefit, and significantly lower the crossover frequency). That seems to be the case, lucky for me!

Hey Eric,
When combining caps in a speaker crossover you should try to balance the caps equally.  For instance, if you're shooting for 12uF then you should use a pair of 6uf caps and if you still want a bypass then use a .1uf.  This is the technique used by a speaker designer I follow. 

Combining a 10uf with a 2uf will cause smearing of detail.