Solen, Black Gate, etc for Power Supply

[greg]

In reading some information about Carr amps, there was a statement made to the affect of "once your guitar signal passes through the preamp stage, all you are hearing is the power supply - so, you should build an excellent power supply". Carr believes that the Solen Fast caps provide that better than electrolytics.

This sounds reasonable to me, but I want some input from you tweakers before shelling out the cash for my own tests.

My question, of course, is simply this: what are your experiences with these (or other) audiophile grade caps in the power supply?

A related question would be whether or not different types of cathode bypass cap types would have any affect on the audio signal. I can't see how it would make any difference, but what do I know.

Thanks

Greg

[Steve Conner]

Electrolytics were good enough for Hendrix, Clapton, and even Eric Johnson I honestly don't think the ESR or whatever matters. If you think of the main B+ feed going to your power amp, it's going through the windings of the output transformer that have a DC resistance of maybe 200 ohms. So what does it matter if the filter cap on that node is a crappy old electrolytic with an ESR of 2 ohms, or a $50 PP film cap with 0.01 ohms? It's only going to make 1% difference. I'd spend the money on a bunch of different speakers to try instead. (JM2C)

[Enzo]

"All you are hearing is the power supply" is a lot of glib sloganeering in my view. Sure you want a well designed power supply made with good parts. But "all you are hearing" is a bit much.

[greg]

Fast Cap to tighten low end response??

Here is what is making me interested in this: The amp in question is a 6 watt single ended El84 thingy that is much like a Matchless Spitfire using a 6CA4 rectifier and all 22uf power supply caps).

The amp sounds really cool cranked (Brian May style gain and mid-heavy eq) and is gainy (and loud enough) to use live for lead playing with a strat - but it has just a bit too much sag on the low end for chording on the low strings. I haven't tried it with a solid state rectifier yet, but I never seem to like the tone of SS (or FREDs).

My thought was that maybe these "fast" filter caps would deliver current a little quicker and give a bit more solidity to the low end. I am not looking for an "upgraded audiophile tone" necessarily - just a bit more solidity on the low strings.

Any ideas?

Thanks
Greg

[Ray Ivers]

Greg,

FWIW, my buddy replaced the e'lytics in his amp with black-body Solen axials, he reported an improvement in clarity and bass tightness IIRC.

As you know, the signal path and power supply form a series loop, with the PS filter caps tasked with passing the audio signal around the power supply components. I'm sure you can figure out for yourself whether this is an important task or not and also whether it should be completely entrusted to super-leaky paste-filled diodes... er, I mean electrolytic filter caps. Yep, they're good enough for rock 'n roll; the question is, are they good enough for you?

I've replaced e'lytic coupling caps in SS guitar amps and FX devices with plastic caps, and heard very real improvements in clarity - and my ears are by no means golden. I also routinely bypass e'lytic PS filter caps with smaller-value plastic caps - not to hear a wider unveiled rhythmic soundstage, just to eliminate the PS filter section as a possible sonic/RF issue.

All just my $.02, YMMV, etc. You really won't know if they'll make an improvement in your particular amp until you try them, and they're often expensive and bulky as well.

Ray

P.S. I came across a cool circuit a while back in Glass Audio (now AudioXPress) magazine; it was a single-ended amp, using a conventional PS w/electrolytic cap, etc. - but there was an additional high-quality plastic cap run from B+ directly to the power tube's cathode; IMO it made a powerful, understandable visual case for the whole idea behind bypassing e'lytics with plastic caps.

[greg]

bypassing electrolytics with plastic caps

What type of values would be used for something like this?

As for your comment "As you know, the signal path and power supply form a series loop, with the PS filter caps tasked with passing the audio signal around the power supply components"

I have never thought about it that way - I simply assumed it was there to filter ripple and distribute voltage. I suppose you are right - I won't know until I try - and the 400v Solen caps don't seem to be too expensive (about $9 each, if I am looking at the right part).

I'll post my results if I decide to take the plunge. Thanks for the input everyone.

Greg

[Ray Ivers]

Greg,

I can't remember where I saw it, but the only "rule of thumb" I've seen regarding this is to bypass the main e'lytic filter cap with a plastic cap 1/10 the e'lytic's value - so if you used a 50uF main 'elytic filter cap, you would bypass it with a 5uF plastic cap (this value is not critical at all).

On a side note; I recently played a well-known boutique amp, and got the unmistakable impression I was "listening to the power supply", strange as that seems - I actually stopped playing and just sat there listening to substantial 60Hz/120Hz/harmonics combined with what sounded like large electric motors turning on and off, light dimmers, power-line spikes, etc. - like a distant atonal symphony just begging for a Buckethead solo.

Ray

[Amp Kat]

Before you start buying Solens you better check out there size. The polyprops are extremely large especially the 630 volters. I have a 100uf Solen that is bigger than my Toroidal power tranny. FWIW I'm with Steve and Enzo and Ray that even though the power supply is in series with the output Electrolytics have proven themselves liable in guitar amps period ! I had a London Power Studio that had an array of Solens and it sounded ok as far as the caps but IMHO they aren't as full of life as the E-caps. I think they would be an excellent candidate for a Hi-Fi amp that depends on a clean filtered supply to get a good signal to noise ratio but a guitar amp that promotes distortion may not and IMO does not simply need it.

[Arthur B.]

P.S. I came across a cool circuit a while back in Glass Audio (now AudioXPress) magazine; it was a single-ended amp, using a conventional PS w/electrolytic cap, etc. - but there was an additional high-quality plastic cap run from B+ directly to the power tube's cathode; IMO it made a powerful, understandable visual case for the whole idea behind bypassing e'lytics with plastic caps.

That sounds like a form of cathode feedback. The capacitor couples the ripple to the cathode, which is then subtracted from the input. But the grounded cathode invertsthe input, so it adds to the ripple.

[Ray Ivers]

Arthur,

Just going from memory... the way the article explained it, they were basically trying to get around the shortcomings of electrolytic caps at both the PS-filter and cathode-bypass locations in one fell swoop, without having to resort to giant, expensive plastic caps everywhere (and a series cap connection). IIRC they used a pretty low value of cap - 1uF, I think? - which was sufficient to bypass down to single-digit frequencies since the impedances on both ends of the single cap were so different than the per-cap impedances of the dual-cap setup would have been.

Ray

[Smitty]

I'm with Ray

Ray has been recommending bypassing e'lytics with plastic for some time. I tried it a few years ago and really liked it. I think it's a big part of the sound of my amp.

First time I plugged into a Carr I could tell he was using plastic power supply caps because of the tone. I think the plastic caps lower the impedence and smooth the response in the upper part of the spectrum while the elytics give a little better punch in the low end.

You may want to try this instead. Tack in some diodes on the pins of your rectifier socket. Remove the rectifier. Tack in another 22uF cap in parallel on the input of your filter section which will bump it up to 44uF. It will give you a little more low end punch. If you like it then just work it into your chassis.

[galibier_numero_un]

Arthur,

Just going from memory... the way the article explained it, they were basically trying to get around the shortcomings of electrolytic caps at both the PS-filter and cathode-bypass locations in one fell swoop, without having to resort to giant, expensive plastic caps everywhere (and a series cap connection).
Ray

Hi Ray, Arthur ...

I can't trace the article, but the description sounds like the old Western Electric "Ultrapath" configuration.

Some folks on the audio reproduction side of things (as opposed to guitar amps) have hooked onto this circuit. Some go so far as to claim that it makes the circuit immune to differences in types of rectifiers.

P-S noise immunity - at least in audio amps has not been my experience with this cap. I'm not claiming that it does any harm, but it ain't no silver bullet either. It's pretty easy to try in the context of your circuit, and perhaps my results are not universal.

Our friend John Broski at TubeCAD journal goes through an analysis of this circuit in the April, 1999 issue (http://www.tubecad.com/april99/page4.html) as well as the math of sizing this last cap which includes in its calculation the size of the cathode bypass resistor). I recall him writing more on this, but can't find the link at this late hour.

Cheers,
Thom

[Steve Conner]

When you say your 6 watt amp has too much sag for heavy chords on the low strings, I'd be inclined to think that's just the nature of the amp. It probably hasn't got enough power to reproduce loud low notes, and just distorts them into a farting mess. Well, at least, I have a 3 watt SE EL84 amp in my collection and that's what mine does when cranked up. I keep it around because I actually quite enjoy the big sloppy mud pie of sound that it serves up (especially with the tremolo on)

I'd try modifying the early stages to cut some bass out so that the power amp isn't driven so hard at low frequencies. I doubt plastic caps will help, as they won't actually increase the output power of the amp. Current delivery of the PS is a red herring with Class-A power amps anyway, IMO, since they draw pretty much the same current whether they're idle or wailing to 11. (I believe some actually draw LESS current when dimed)

[greg]

My 6 watt amp + Solen Cap report

Yea, I am sure that this is a limitation that is inherent in the circuit. It is just so close...I'll try some substitutions in the coupling caps. Being that I am playing it through a G12H, I am sure I can lose a little low end in the circuit and still end up with a balanced sound. I can't say that I have a lot of experience with small amps, but I have never heard anything in this power class that is anywhere near this loud and beefy and detailed (it is considerably more loud and punchy than my buddies 15 watt, Class A Ampeg Jet reissue). It truly does smoke and weighs next to nothing.

By the way - back to the Solen Cap debate - I did buy them (4 @ 24uf). They didn't fit in this little head, so I used them in a new amp. This one is more or less a Dr Z Z28 (which I didn't have a schematic for) so I used preamp values close to the Route 66 and made the power section that of a Top Hat Club Deluxe (cathode biased, unlike the Z28). The amp turned out GREAT and, although I didn't try it with electrolytics at all, there is definitely something different going on here which I want to attribute to the caps. A different sense of immediacy that I haven't experienced in anything I have played\built before. It is undoubtedly smoother and more capable of withstanding hard hits to big full chords when cranked up (it doesn't get ugly me when I add in those 3rds and 6ths, etc). Certainly more friendly and predictable for this kind of stuff than anything else I have made (Ghia, Marshall Lead 20, Spitfire, Lightning). Of course, I could also have stumbled upon a lucky mix of values which just allows the amp to deliver what I am asking of it without caving in on itself, but at the very least, I don't feel that the extra $20 in caps was wasted on the Solens.

Greg

[greg]

Question for Ray Ivers: Bypassing E'lytics with Plastic

What value of plastic cap would be appropriate for something like this? I found the Solens to be way too large to work in my 6 watt amp, so I am interested in trying this approach. Of course, if this does have a noticable affect for me, then I open another can of worms - I'll be tempted to find out if Oil caps, Sprague 715, Mallory sound better, etc....

Sounds fun!

Thanks,
Greg