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Old 25th July 2001, 04:28 PM   #11
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Pete,
We are in agreement about electrolytics--that's why I threw in the part about film caps, noting the superior high frequency performance. However, that's in the main bank. As far as caps directly across the diodes, I tend to use film there, too (polyester--nothing fancy or expensive, just garden variety critters), since I regard ceramic caps as suspect.
William,
You said you changed the diodes in your preamp...were there four diodes there to begin with, or two? If you went from a four diode rectifier to a two diode rectifier, your rail voltage changed (cut in half, actually), which would definitely give you sonic changes.
Yes, if a center tap is available, you can have full wave rectification with only two diodes. Both diodes should point the 'same way', i.e. with both cathodes (or anodes) towards the filter section, and the center tap serving as ground.
On a philosophical note--if it sounds better, it should sound better everywhere. I've always been wary of trying to balance colorations against one another to get a system to sound right. There are exceptions, viz. tube amps are rarely a good idea for subwoofers (regardless of strengths in midrange), although a good tube amp can hold its own down into the woofer range; poor ones give the classic bloated bass syndrome. If (and for me, the jury is still out) super-duper diodes are better in amps, they should also be better in preamps, CDs, etc. with the same strengths evident. I just haven't had time to play with it yet.

Grey
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Old 25th July 2001, 05:21 PM   #12
Geoff is offline Geoff  United Kingdom
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So far, we seem to have a lot of 'fence sitting', with very few people admitting to having tried the rectifier substitution. I will therefore give my experiences, for what they are worth.

Several years ago, I replaced the rectifiers in a pair of well-regarded, commercial Class-AB integrated amps that I was using in a bi-amped system. The replacement diodes were Schottky with a similar size and rating to the original standard diodes.

On completion of the modification (and no other changes were made at that time), the amplifiers appeared to be audibly better. The sound seemed 'smoother' and 'cleaner'. There are several possible reasons for this.

1. I had spent some hard earned money and wanted/expected an improvement.

2. In order to fit the diodes, all connections to the amps (mains, phono, speaker) had to be broken and remade and so any contact contamination/deterioration should have been removed (I had previously determined that regular cleaning of all connectors, including the mains plug, gave a perceived improvement)

3. The Schottky diodes actually gave an improvement.

About 12 months after this modification, I replaced the Schottkys with the original diodes, prior to selling the amps. On re-auditioning, in the same system, to check that the amps were working correctly, I could hear no deterioration in the sound quality due to reverting to the standard diode type. This tends to rule out options 2 and 3 above, and I am rather loath to accept that option 1 is the reason as I am sure I heard a difference!

This leaves me 'fence sitting', along with many other people.

A quick comment to Grey. If mains borne interference and noise can get through a power supply (simple or regulated) to affect the sound quality of an amp, then I'm sure diode switching noise can do so as well.

Geoff



[Edited by Geoff on 07-25-2001 at 08:22 PM]
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Old 25th July 2001, 05:36 PM   #13
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Dear Grey,

Thanks for your comments. I did have 4 rectifiers in my pre-amp before (that was before the old transformer failed). Since the replacement transformer has a centre tap, so I used two rectifiers instead as it would still have full-wave rectification. I did connect up correctly to produce 350V d.c. Yet, the result ended up to be unexpectedly poor. A friend of mine said that I should use 4 to make a bridge and he believed it would be better to have 4 working instead of two. As I have no electronic background, I just could not figure out the reason.

My expectation is that I may find some improvement (if any) but not sounding worse. As I do not have testing instruments, I can only believe my ears.

Regards,

William
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Old 26th July 2001, 12:42 AM   #14
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Gíday Grey,

While I would agree with your conclusion of ceramic capacitors when used at AF, I am hesitant to project my prejudice into RF applications. In this instance we are looking for capacitors capable of performing their intended function well into the MHz range, and I believe the characteristics of ceramics make them the right choice for this purpose. Personally, I would be hesitant to use anything but a very small value Polyester capacitor in this instance given their relatively low resonant frequency compared with ceramics.

Cheers,

Pete
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Old 26th July 2001, 02:31 AM   #15
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Geoff,
Your point about AC line noise vs. diode switching noise is well taken. Without trying to steer the thread in that direction, I threw together some quick-and-dirty AC conditioners a few years ago using a cap across the AC hot and neutral (bear in mind that I'm in the US with 120VAC, not 220V as you have), a surplus isolation toroid, followed by another cap and some other miscellaneous whatnots that I had on hand. What I got was a surprise--*much* quieter 'space between the notes.' Frankly, I hadn't expected it to do that much sonically, as my primary intention was to tame an unruly AC situation here at the house. Given the difference that such a simple (and cheap) circuit made, especially on the other side of the power transformer, I'd lean towards the idea that something closer to the circuit (both electrically and physically [possible air transmission of RF from the spikes?]) could be detrimental to the sound.
In the same vein...I would think that this would be (yet another) argument for inductors in the power supply (either L or PI), as the Z at such high frequencies is surely going to be a major impediment to any noise from diode spikes trying to travel downstream. What say ye?
William,
I hadn't realized that the transformer had been changed, too. Egads, I'm not ready to enter into a discussion on the 'sound' of power transformers, as I've done absolutely no fiddling in that direction at all. I don't recall ever hearing anyone say much along those lines, excepting a vague recollection that the old Peerless power transformers were reputed to sound better than their competitors. And--setting aside the issue of recovery time--what if the diodes themselves have a sound. It's true for transistors; why not diodes? If the power transformer failed, and presumably the diodes as well, did you lose the caps in the main bank? That might provide yet another variable. If you have the parts, the time, and the patience, by all means try a four diode rectifier if you can get the voltage to work out properly. If you do so, please report, as I'd be curious about the result.
Pete,
You're trying hard to complicate my life. (And succeeding.) I might need to sit down and try ceramic vs. polyester in situ to see if my mistrust of ceramics in that part of the circuit is justified.

Grey
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Old 26th July 2001, 02:44 AM   #16
jam is offline jam  United States
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Pete,

Any thoughts on silver-mica capacitors?


Grey,

Did someone not say 'Life without complications would be an error.'

Cheers,
Jam
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Old 26th July 2001, 03:48 AM   #17
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Default Dunno!

Grey,

Sorry for the curve ball

Regarding transformers I have read reports (I canít recall who from, possibly John Curl, probably not!) claming quite significant differences between different transformers. In this case not the type of transformer (although that brings to mind the different coupling characteristics at higher frequencies with regard noise injection Ö ergh, letís not go there!), however the power rating of the transformer. There appeared to be a direct correlation between size and audible performance, with the larger transformers providing a significant improvement. Absolutely nothing to do with the thread, but I do like tangents.

Jam,

I really donít have any experience with these capacitors in this application so Iíll have to hand this off to somebody more knowledgeable than myself. Personally I believe ceramics have pretty much ideal characteristics as snubbers so donít see any reason to use anything else.

I certainly donít want to hold myself out to be any sort of expert in this area, just base my judgement on experience and what I read. You may be interested in Walt Jungís article if you havenít already read it http://capacitors.com/pickcap/pickcap.htm however bear in mind that he is referring to audio frequencies. Another useful resource on this subject can be found at http://www.analog.com/publications/m...ersary/21.html indeed ADI do some very interesting articles on all sorts of toys.

Bit of a non-answer, sorry Ďbout that!

Cheers,

Pete
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Old 26th July 2001, 04:37 AM   #18
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(Just for the record, I'd like to note that Jam positively *loves* to torture me with oddball ideas that I ain't got time to follow up on. Now he's got Pete helping out. You guys gotta promise to come visit me when they cart me off to the insane asylum...just bring red wine [is there any other kind?], and/or single malt Scotch and something that makes decent music, okay?)
Jam,
I had thought about silver-mica caps also, but they're generally pretty small, and you'd have to parallel several to get up to the .1 uF range.

Grey
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Old 26th July 2001, 10:34 AM   #19
Geoff is offline Geoff  United Kingdom
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Grey

Your digression from rectifier noise to power supply/mains noise is not much of a tangent so I think we can continue discussing it here.

Your point regarding choke input or inductive Pi filters is very relevant with regard to mains borne and power supply noise. The amplifier PSRR and the voltage regulator (if used) ripple rejection is usually very good at the normal ripple frequencies of 100 or 120Hz. However, the situation changes considerably at higher frequencies. For example, the venerable LM337 has a ripple rejection of over 80dB at 100 to 400Hz but less than 20db at 1MHz (as low as 10db if the adjustment pin isn't bypassed). The inductor(s) should help to attenuate the higher frequency noise and this may well be why a number of people prefer choke input filters to capacitor input types.

Your findings on the suppression of mains noise are similar to my own, though I haven't gone to the extreme of a full mains conditioner. I have had good results from the simple (and cheap, £0.80 here in the UK) modification of adding an MOV transient suppressor across the mains input. This can be wired in the amp itself, in the mains plug or even in a separate plug inserted into an adjacent wall socket if there is one spare. Like your conditioner, this mod gives 'cleaner spaces'. In fact, a friend of mine was having considerable problems in that his system became virtually 'unlistenable to' at certain times during the day (it sounded 'rough' to say the least) and nothing he tried would rectify the problem. I fitted a £0.80 transient suppressor to his mains distribution block and he hasn't had a complaint since. The problem, I believe, was caused by mains borne noise from an adjacent semi-industrial area.

Geoff

[Edited by Geoff on 07-26-2001 at 11:35 AM]
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Old 26th July 2001, 03:57 PM   #20
ALW is offline ALW  United Kingdom
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Default Re : Mains Noise

I've had good results from transformers (toroidal) that incorporate an electrostatic shield - they can provide some significant attenuation of noise, but you don't often find them on off-the-self parts.

I also discovered recently that I get sonically better results from a full-wave bridge rectifier using a centre tapped transformer (2 diodes) than a more conventional (and efficient) bridge circuit.

Whether this relates to the lower diode drop, or the fact that the earth reference is moved back to the transformer on the centre tapped design I've yet to investigate.

I haven't yet had the opportunity to look for measured differences, but the audible ones are definitely there and they are not subtle.

Andy.

P.S. Both designs used the same standard bridge rectifier module.

P.P.S Both supplies were regulated


[Edited by ALW on 07-26-2001 at 11:07 AM]
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