Bypassing Signal Coupling Caps

[Cressy Snr]

Now then,
After trying Greg's Dynamicaps for a while and having now taken them out and put back Phil's Green Russian Grenade caps, I have a conundrum.

In my amp, the Dynamicaps are far superior to the Russians, at the top end; clear and clean. It was this quality that impressed me so much on initial audition that I wanted some NOW! etc etc.

Buuuuut......the Russian caps are better than the Dynamicaps in terms of soundstage size; huge in fact, and are the equal of the Dynamicaps at the mid and bass level. The Russians are darker at the top end than the Dynamicaps of that there is no doubt, but not grainy or fuzzy, just less lit up, and the juicy, colourful overall presentation is very attractive; warming up the icy clarity of a pure pentode amplifier like this one to just the right colour temperature.

So I was wondering if anyone has ever tried bypassing signal coupling caps with low value, high quality, silver mica or polystyrene elements.

I have 0.22uF couplers in, so a 10,000pF polystyrene or silver mica would be just the job. In these low values, polystyrenes are relatively cheap.

I suppose what I need is the warming through characteristics of the Russians, combined with the top end clarity of the Dynamicaps.

I have a pair of big Russian, metal-clad, silver mica, 10,000pF caps, but obviously need another two of the buggers to be able to experiment.

Any thoughts would be appreciated.

[pre65]

I'm just off out for the afternoon, but I'll have a peek in the capacitor boxes tonight.

[pre65]

I'm just off out for the afternoon, but I'll have a peek in the capacitor boxes tonight.

I found this snipette.

Do not use bypass capacitors in the signal path. A single capacitor for DC blocking/AC coupling creates a simple path with one time constant. The signal quality will be compromised if a bypass or multiple bypass capacitors are added to a signal path capacitor. Bypass capacitors were used in the past to bypass low quality film capacitors or electrolytic capacitors. The bypass was the lesser of two evils. With the advent of better quality film capacitors the need for a bypass capacitor was eliminated. Bypass capacitors create multiple signal paths with multiple time constants. These time constants are very short but they can still be heard as a smear or overall loss of focus.

[Cressy Snr]

I'm just off out for the afternoon, but I'll have a peek in the capacitor boxes tonight.

I found this snipette.

Do not use bypass capacitors in the signal path. A single capacitor for DC blocking/AC coupling creates a simple path with one time constant. The signal quality will be compromised if a bypass or multiple bypass capacitors are added to a signal path capacitor. Bypass capacitors were used in the past to bypass low quality film capacitors or electrolytic capacitors. The bypass was the lesser of two evils. With the advent of better quality film capacitors the need for a bypass capacitor was eliminated. Bypass capacitors create multiple signal paths with multiple time constants. These time constants are very short but they can still be heard as a smear or overall loss of focus.

Yes, I was afraid of that Phil; which is why I ask if anyone has actually tried it.

It could be argued that the time constant smearing caused by speaker crossovers and driver positioning is of a far greater magnitude.

[Nick]

I'm just off out for the afternoon, but I'll have a peek in the capacitor boxes tonight.

I found this snipette.

Do not use bypass capacitors in the signal path. A single capacitor for DC blocking/AC coupling creates a simple path with one time constant. The signal quality will be compromised if a bypass or multiple bypass capacitors are added to a signal path capacitor. Bypass capacitors were used in the past to bypass low quality film capacitors or electrolytic capacitors. The bypass was the lesser of two evils. With the advent of better quality film capacitors the need for a bypass capacitor was eliminated. Bypass capacitors create multiple signal paths with multiple time constants. These time constants are very short but they can still be heard as a smear or overall loss of focus.

Its a tricky one that, on the one hand it sounds like tosh, but then again, so does caps sounding different. I would try it and see.

Your premise does assume the difference in sound is due to the signal passing through the cap, instead of the effect the cap has on the components on either side of it. If the latter, then adding another cap, may produce the worst of both worlds instead of best.

I would distrust any such snippet on the web though (including this one).

[Mike H]

Ditto! Does sound like tosh, or the time delay bit.

I have used polystyrene for exactly this, (my preference), try it and see.

[Nick]

Ditto! Does sound like tosh, or the time delay bit.

Not sure delay was mentioned. But the two caps will (assuming they are different value) have different time constants, and the proportion of the total current passing through them will vary with frequency.

[Cressy Snr]

Cheers chaps,
I'll order some polystyrene caps. Not too expensive, can't break anything and not a big financial hit if they don't deliver the goods.

[Cressy Snr]

Hmm..
Had a go with my pair of 10,000pF Russian, silvered mica caps across the greenies on one channel and it's good that I did.
The difference was clearly audible, even though only the one channel was affected, and it wasn't good.
A layer of God knows what, was added to the affected channel, resulting in a shrill quality to everything from the upper midrange onwards....fooking horrible.
The very top was lovely though....easily as good as the Dynamicaps, but this did not compensate for the failings introduced elsewhere and they were soon removed. Just shows how good though, those caps of Greg's are. In a triode amp I would think there is very little capable of beating them.

The silvered mica caps were probably too high in value, their influence reaching too far down, buggering up the lovely midrange quality the amp has.
I need a value that will extend the far treble without impinging on the beautiful midrange the green caps have.
Calculator out methinks before ordering anything

[simon]

Ah, I was just about to say Steve those big Russian silver micas sound like the ones I gave you - you're welcome to borrow a couple more to try, but I won't now .

Some swear by 1500V 0.22uF LCR PPs. And they're cheap as chips...

[pre65]

[quote="simon"

Some swear by 1500V 0.22uF LCR PPs. And they're cheap as chips...[/quote]

http://www.maplin.co.uk/p/lcr-metallise ... itor-rg24b

http://www.ebay.co.uk/itm/CAPACITOR-0-2 ... 564c9b932a

[pre65]

Was the old rule of thumb for bypass caps ten times less capacitance ?

So to bypass 0.22uf (220nf) = 22nf or 2200pf

[Paul Barker]

Morgan says 100:1 for modern components for old ones 100:1 then that 1 at 100:1. Past the three caps the inductance of the leeds brings negative returns.

So he gives example for old lytic of 220µF + 2.2µF + 2.2nF

In his diagram he shows this happening on the last cap in the B+ supply.

[Greg]

Was the old rule of thumb for bypass caps ten times less capacitance ?

So to bypass 0.22uf (220nf) = 22nf or 2200pf

Yep, that is my understanding and you can continue the ten times process for triple bypass if possible. However, and I admit I have no evidence to support this, my understanding from the past has always been bypassing signal coupling caps is a complete no no. The other 'advise' was if using Blackgate's, obviously in other applications, they do not benefit from bypassing.

None of this is IMHO. It's simply advise I have previously had and I have no explanation as to it's validity. Having said that, it has always worked for me

[simon]

There's a thread on DIYAudio somewhere that talks about bypassing caps. I don't remember the detail beyond it was a fair bit more complicated than I thought. In reality the "right" value was generally between 10:1 & 100:1, and 50:1 often being about right. Or something like that. Or maybe not.