Bypassing a 100uF output electroytic - which value?

[johnm]

Hi folks.

I've just inherited a Quad 34/FM4/606 system purchased around 1988/89. I am recapping each item and they're all pretty straightforward.

Before anyone says anything, yes I am aware the Quad 34 hasn't got the best reputation for sound quality, but for sentimental reasons (they were owned by my Grandfather who got me into hi-fi in the first place) I want to keep them running without any major mods (I have other systems for that!).

The output caps on the Quad 34 are 100uF cheap electros (used to be 100uF tants on earlier models). I know that these are often placed in error on the circuit board (wrong polarity markings on circuit board which never appeared to be corrected, which I find odd), and can leak causing trouble - luckily my 34 is totally clean inside. These are C77 and C78. Just to be on the safe side I am replacing the caps with non-polar Nichicon ES which I already have. I have read it's best to bypass these (to clean up the treble) with a small value polypropylene or polystyrene cap.

My question is which is best? 0.1uF or 0.01uF for bypassing the 100uF signal capacitors? I have done a search but most posts appear to relate to PSU use, not signal.

Many thanks,

- John

[sandyK]

John
Why not try all 3 options, and see which sounds best to your ears ?
You may even prefer the no bypass option.
SandyK

[rdf]

Personally I wouldn't go less than 1/100 the main cap value, or 1 uF. It literally depends on cap and trace length and where the bypass is applied but going anything less (in ideal Spice world) risks the bypass resonating against the main cap's inductance.

[johnm]

Hi Sandy - I own an older version of the Quad 34 too (which has been fully modded), and from experience I know the tracks on these lift VERY easily.

I really don't want to risk that on this inherited Quad 34. I'd rather take guidance from someone else who's bypassed signal path electros.

[johnm]

Thanks for that RDF - so everyone using 0.1 or 0.01uF bypasses are actually making things worse then by the sounds of it?

[richie00boy]

If they are not leaking and given the risk of lifting traces, I would seriously consider just leaving them alone. In the position where they are they will pass very little current and never get even warm, so their lifetime will be many thousands of hours.

Also I agree with rdf's 1/100 rule.

[rdf]

Well, they're probably doing something they didn't intend and is 'technically' worse. Whether it sounds worse or not in that specific circuit application is a different question. I never found tiny bypasses transformative and, as sandyK appears to hint, eventually they get pulled out in favour or either no bypass or one approaching the 1/10th the main cap's value.

All that said, this may be less applicable to electrolytic caps than film-on-film. Someone did a series of measurements on bypassed electrolytic caps revealing little or no resonant behaviour. My own tests on a bypassed 100uF motor run film/oil cap showed definite resonant behaviour at high frequencies, which is why I tend to shy from them now. Could be the right answer is 'solder and listen'.

[Eva]

Bipolar electrolytics have typically double the ESR, double the leakage current and higher dissipation factor (probably double too). Definitely not recommended over a standard polar when there is a known DC bias and you are free to choose.

Paralleling capacitors may lead to resonances easily. The tale about erratic behaviour of electrolytics in the upper audio frequencies is a pure myth. In the beginning I was putting 100n or 1u films everywhere, until I learned to measure and calculate things and discovered that in some circuits they were not doing anything useful and in others they were doing plain harm. Paralleling is only useful when low impedance in the Mhz range is required and ESR and ESL have to be considered to avoid resonance (electrolytics with not too low ESR get well with either film or ceramic).

[johnm]

I'm more confused than ever now - I remember reading an article (by Cyril Bateman in Electronics World) saying Non polar types are to be preferred in the signal path for lower distortion?

Damn this hobby can have you running around in circles

Here's the specs for the Nichicon ES non-polar which I was going to use:

www.nichicon.co.jp/english/products/pdf/e-es.pdf

Does this indeed look worse than a polar cap? I'm afraid I'm out of my depth with capacitor spec sheets.

[JC951t]

Just my 2 cents. Never like the sound of bypassing irregardless
of bypass caps used. Film or BG NX's. It just does not sound right