Preamp Circuit

[cm961]

I'm trying to build a preamp for my new amplifier and I have the schematic I wish to use. However, the schematic calls for a non-polar 10uF 250V capacitor. Does anyone know what kind of capacitor this is and where to get it? I think it could be NP electrolyte but nobody sells them as high as 250V. I think the obvious answer would be a ceramic disc, but nobody has one that high voltage either. It's getting a bit bothersome trying to figure this out.

On another note, does anyone have any links to sites with more preamplifier designs. I would like to do something a little more elaborate for this new amp.

 

[GRollins]

What they're telling you is not to use a standard electrolytic capacitor. The usual electrolytics have a +/- marking on them to tell you which way to orient them in the circuit. Don't hook them up reversed.
There is, however, such a thing as a non-polarized electrolytic.
You didn't specify where in the circuit the cap is located, but I'd imagine that it's somewhere in the signal path. When possible, avoid electrolytics in the signal path. They don't sound good.
Ceramic caps are readily available to 1000V and more, but you won't find a 10uF ceramic; they're more in the pF range. They sound lousy, too.
Your best bet is a film cap. By definition, film caps are non-polar. Solen makes pretty good metallized polypropylene caps and are available in the 250V range (and on up to 630V, should you need such a thing). Film caps sound better, also.
Would it be safe to assume that this is a tube circuit?

Grey

 

[jam]

Non-Polar Capacitors

Cm961,

A non-polarised 10uf capacitor can be made by hooking two 20uf polarized capacitors back to back. ie tie the positive leads togther and use the negative leads as signal in and out.
Remember equal value series capacitors halve in value. I would use Black Gates for best sonics, this combination can be bypassed with a high quality film capacitor if required.

Jam

 

[dorkus]

You didn't specify where in the circuit the cap is located, but I'd imagine that it's somewhere in the signal path. When possible, avoid electrolytics in the signal path. They don't sound good.
Grey

i take issue with audiophiles blindly objecting to electrolytic caps in the signal path. i used to be the same way, if i saw an electrolytic in the circuit i reflexively replaced it with a decent-quality film cap. but then i started seeing a lot of very good-sounding gear using electrolytics in the signal path, usually for output or feedback-shunt decoupling. i also noted that such gear tended to have a warmer, more relaxed, more natural sound than a lot of similar high-end gear that used film caps. so i played around a bit, and i found that when properly used, a quality electrolytic can sound pretty damn good. the key is to have some DC bias on the cap if it's polarized, or to use a NP cap. but i've played around comparing films like Solen and SCR polypropylenes to good electros (Black Gate, Nichicon Muse) and in the right applications have come away preferring the electros - metallized films can sound etched and tinny in comparison. and for circuits where there simply isn't enough room for a film, the volumetric efficiency of a electro can't be beat. the Nichicon Muse NP, for instance, has a very nice, full, smooth sound, though it may give up a little treble detail to the film caps. so don't fret if you can't fit a film cap in, a good electrolytic will work just fine and cost less to boot.

 

[Samuel Jayaraj]

Electros in signal path

There is truth in what GRollins and Dorkus say about electrolytic caps in signal paths. Earlier in my experience, electros did sound bad and I found much improvement using films caps. But of late the quality of the electros seem to have improved and connecting two electros back to back to form a non-polar and shunting with a film cap gives very good results, where caps cannot be avoided in the signal path. No capacitor at all is better if the circuit permits that.

I have found ELNA caps to be much superior to some of the cheap brands from the far-east. I am presently trying out a single ended class-A mosfet amplifier (Andrea Ciufolli I think is the name of the designer) which uses an electrolytic cap at the output (4700uF). I am going to try ELNA, Panasonic and Nichicon and hear the difference. Will post on the results.

Any comments?

[Edited by Samuel Jayaraj on 06-14-2001 at 03:41 AM]

 

[jam]

Electrolytic Capacitors

Electrolytic capacitor technology has advanced greatly in the last few years. The choice of capacitors should depend on the circuit and impedances involved.
For example, solid state equipment with their low impedances
require larger coupling capacitors than tube circuits and electrolytics have a clear advantage here. The capacitor in the shunt leg of a feedback circuit can be as high as 1000uf,clearly impractical value for a film capacitor.On the other hand I would never dream of using an electolytic in a high frequency compensation circuit where a silver mica or polystyrene would be superior.
Large value film caps have their problems and are not the cure all that their proponents claim, because of their size they are prone to noise pick-up and microphonics.
I myself use Black Gates as coupling capacitors and find them in many ways superior to film capacitors, they require a longer time to break in and as Dorkus said they work best with a slight DC bias on them.
Hi-Fi World (a UK publication) did a survey on coupling capacitors a few years ago, and the Black Gate was the winner beating out even some very expensive oil filled capacitors.
Several manufactures use electrolytics as coupling capacitors because then can sound more dynamic in the lower end of the frequency spectrum even compared to similar value film capacitors.

Jam

[Edited by jam on 06-14-2001 at 06:55 AM]

 

[GRollins]

I've never found a circumstance where I *preferred* the sound of an electrolytic over a good quality film cap. The best I've ever found were cases where it was such a minor improvement that it was a coin toss as to whether to drop the extra money.
However, I readily concede their size and price advantages.
The Aleph circuit has three electrolytics in the front end--all 220uF. I used Panasonic FC series electrolytics. To say that the sound of the circuit is excellent rather misses the point. How much *better* could it sound if film caps were substituted? (At least for the one cap on the -IN. Arguably, the other two on the current source are somewhat less critical.) However, it's clear space-wise that film caps would dwarf all other componentry--at least doubling the size of my front end board. Even as fanatical as I get sometimes, that's a little further than I want to go, especially when I'm pressed for time wanting to move on to other projects.
In cases where I must use electrolytics, I try to bypass them with film caps. (I haven't done so with the Alephs yet, as I wanted to hear the circuit 'stock' first. [Yes, I'm saying stock with tongue in cheek, knowing that I modified the circuit a bit, pulling in bits of Volksamp where it suited me.]) It doesn't take a mathematical genius to look up the ESR figure, apply it to the rated capacitance, and crank the handle on your long-suffering calculator: Electrolytic caps roll off at absurdly low frequencies. A film cap is more nearly flat in frequency response. Even the numbers crowd can sink their teeth into that.
I forgot earlier--there was a question in cm961's original post about other preamp designs. Well...preamp designs are as common as grass, a quick stroll through the links section here will net probably ten or twenty schematics in as many minutes, and there are scads more out there. Were you looking for tube or solid state? With or without phono stage? Passive or active? Give us a bit of information as to what you're looking for, and we can be more helpful.

Grey

 

[dorkus]

I've never found a circumstance where I *preferred* the sound of an electrolytic over a good quality film cap. The best I've ever found were cases where it was such a minor improvement that it was a coin toss as to whether to drop the extra money.

agreed about the latter part, i'm not saying film caps sound bad, they in fact sound very good. but 'lytics are not all that bad and have been unjustly hammered by audiophiles. in many cases (given a little DC bias) they sound just as good, albeit different. i think films definitely have better HF definition due to the lower ESR at increasing frequency and lower dissipation factor, and have a more "hi-fi" sound that many audiophiles prefers. me, i'm willing to give up a little HF detail in exchange for a smoother and more organic midrange and upper bass, as well as (oftentimes) better perceived bass extension. just a personal preference...

In cases where I must use electrolytics, I try to bypass them with film caps.

this is usually not a bad idea, however there are some people (a guy in the Audio Note SE tube camp comes to mind) who feel that bypassing 'lytics w/films is just playing around with colorations and can adversely affect the coherence of the sound. in some applications, e.g. a simple signal decoupling function, it's better to just pick one good cap, be it film or 'lytic, and not try to band-aid it with bypasses. usually you need bypasses where really really wide bandwidth is required, but for audio signal the bandwidth is relatively small so one cap should suffice, if it's a good one. sometimes i think bypassing one cap with a radically different type of cap (e.g. 'lytic and film) can confuse the sound a little, esp. when a large band of the audio signal is actually parelleling through the bypass. of course, power supply bypassing is a different matter...

It doesn't take a mathematical genius to look up the ESR figure, apply it to the rated capacitance, and crank the handle on your long-suffering calculator: Electrolytic caps roll off at absurdly low frequencies.

true, ESR rises much faster w/increasing frequency with 'lytics, but again, do we need all that bandwidth? if a coupling cap can maitain relatively low ESR up to 20kHz (and the good ones like Nichicon Music, Black Gate seem to), then we should be ok. again, supply bypasses will need wider bandwidth and here a film cap comes in handy...

anyway, it would be cool if one of us did a signal coupling capacitor comparison, say Black Gate vs. Nichicon Muse vs. Solen film vs. Hovland film or something... to be completely fair to the 'lytics, it should be with some DC bias, at least a couple volts i would think. i'm pretty sure the Hovland will come out on top pretty handily but it would be interesting to see hwo the 'lytics compare to the Solen...


marc

 

[GRollins]

Marc,
I regard bandwidth less than, say, 75kHz as highly suspect. Not that I claim to be able to hear that high as a discrete tone, but there have been numerous studies showing that it's audible on things with a sudden rise time (drums, cymbals, what have you--no guarantees on flutes). Straight Fourier analysis of a complex waveform shows harmonics going *way* out. Turns out it's audible (hell, even the numbers folks can get behind this--go look up the formula for a square wave).
Keep in mind that 75kHz is less than two octaves beyond 20kHz. Most circuits are good down to 1 or 2Hz on the bottom...a good 3 to 4 octaves below 20Hz.
A somewhat more subtle problem is that where there's a rolloff, there's a phase shift. And the phase shift is present (and measurable, for those who care to do so) way below the -3dB point. This is why you see equipment with specs for phase shift at 20kHz; sometimes also at 20Hz. Phase shifts are often overlooked because you've got to have a scope to see them. But if you bring in a waveform (let's assume lagging, in the case of high frequencies) an eighth of a cycle behind the rest of its companions, you're going to get transient phase-related frequency cancellations. I.e. your frequency response is no longer flat, and the waveform you put in ain't what comes out, hence a transient distortion mechanism that's completely ignored by steady state tests. Square waves are a good start for testing such things; impulse testing is better still.
The long and the short of it...I'll take as wide a bandwidth as I can get. (With a nod towards stability, let's go for the 100-200kHz range.)
Dialectric absorbtion and dissipation factor, too. Charge an electrolytic to rated voltage. Let it sit for a minute. Discharge it quickly, using a low value resistor. Then hook a meter to it and watch the voltage as it climbs. Voltage? *What* voltage? There's no voltage there...you just discharged it, right? It was at 0V. Only, it isn't. It's at 1.5V and still climbing. Now 2V. Those electrons were hiding in the dialectric. Now they come creeping back out. Now 2.5V. This is not good. We have a passive component acting as an active component, injecting current flow into the signal that shouldn't be there.
Try it. If nothing else, put a 9V battery on a 15V cap. (Although it's easier to see at higher voltages.)
Then try the same thing with a good film cap.
Have I heard good sounding equipment with electrolytics in it? Yes. And progress has been made on electrolytics, true. But like I said, the best case scenario for electrolytics I've ever heard were a few times where it was only a slight improvement, hardly worth the (admittedly very high) price for good film caps. Lower price film caps may or may not be such an improvement, though I've got a small box of old Sprague Orange Drops that I toss into things when I'm short of the good stuff. Even humble polyester has been an improvement in most cases. (I eventually order in something decent and toss the Orange Drops back in the box until the next time.)

Grey

 

[dorkus]

hi grey,

i agree with you completely on many of your points. phase shift is an often overlooked aspect of signal fidelity; some meter-readers will insist that human phase accuity is negligible above 2kHz or so, but they are basing their assumptions on studies performed by bell labs YEARS ago. haven't more recent tests proven that humans do have some amplitude and phase accuity above 20kHz, though it may be at sort of a subliminal level? or something like that...

so yes, 'lytics tend to be pretty crappy at higher frequencies and have some non-linear characteristics. still though, they can sound surprisingly good in some circuits and in some ways preferrable to more common metallized films. the best film-and-foils will always win, but they are bulky and expensive esp. w/anything above 1uF or so. again, this is all personal preference, and i have no argument with your preference of even cheap films over 'lytics. i just don't think audiophiles should hold their nose if they're force to use a 'lytic due to practical considerations; if you use a good one it can sound perfectly decent. and i've heard plenty of cheap mylars that sound pretty crappy in a particular circuit compared to a quality 'lytic - weak bass, tinny treble, and lacking body. have you tried a Black Gate or Nichicon Muse NP for coupling? i think you will be surprised at the performance you can get with a little DC bias.


marc