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MV rect, filter tuning and sound (long)

Posted by John Swenson (A) on June 17, 2005 at 23:40:43

The last week has been a major revelation to me. I added MV recifiers to my amps, it significantly improved some aspects but the added RF chokes and caps ("snubbers" to get rid of RF hash), added HF roughness and glare.

I tried a whole bunh of things but they didn't seem to make sense, the change in the sound was not correlated to the reduction in RF hash. Then I started playing around with PSUD with a small stepped current rsponse and tried modeling the different snubbers I'd been trying. What I found was the filters I'd been using were under damped, they "rang" when a transient hit, the snubbers were exacerbating the ringing (it was already there but not as bad). The situations with the most awful HF problems were the ones with the greatest ringing of the filter. Aha, I'm on to something!!

So I started playing around with PSUD trying different choke inductances, DCRs and ap values. What I ound out was that my cap value was quite a bit too small. If I went from 100uf to 500uf I wound up with a 'critically" damped filter, more caused the voltage to slowly change, less caused ringing, right at the critical value the step caused the voltage to quickly change level with out ringing or overshoot, this looked like the place to be. With this cap value adding the snubbers made no differentce.

So I tried it out, I tried a number of different cap values and listened, wow I was quite surprised at the radical change in sound from a fairly small change in cap value. At 600uf the sound was dull and lifeless sounding, at 300 it was bright and glary, at 400 the glare was gone and the sound was very alive sounding, at 500 everything just sounded right, natural, not unbalanced in any way. Its kind of a toss up to which I prefer, the 400 "sparkles" a little more, its just a little more "wow" sounding, but after a couple hours of listening it starts to get fatiguing. The 500 I can listen to all night, with 600 I get up and leave after 10 minutes because its too boring.

I tried the same thing with a couple other amps and got similar results. Changing the PS caps changes the sound, I cn "tune" it with a fairly small change in cap value. Strangely enough with SS rectifiers the effect is not as strong as it is with MV rectifiers. I'm not sure why this is so. The peroperly tuned MV amp sounds quite a bit better than the best tuned SS amp, but with the SS amp you can make big changes without changing the sound that much.

Another interesting aspect of this is how choke DCR and cap value interact. A number of people here tout low DCR, but I found that when you use low DCR you need quite high cap to keep the filter at the critical damping point. If you have high DCR you can use much lower cap values. So it seems to be an interesting trade off. With low DCR you need large caps, which usually mean electrolytics, which conventional wisdom say don't sound as good as film caps. But if you use the lower value film caps, you need to go with high DCR chokes, an interesting balancing act.

All this has been with a simple LC filtere, as you start cascading stages it starts getting very complicated! With an LCLC I could eventually come up with a good set of values, but it was much more time consuming. The amp I put the MV in originally had a LCL filter, but it sounded better with just LC, no wonder, the second stage was ringing like mad! It woul be interesting to try a proprly setup two stage, but I don't have any room left, the amp is completely full right now.

So there you have it, I'm now a believer in power supply tuning.

John S.

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