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My experiments with high voltage regulators

Posted by John Swenson (A) on May 7, 2007 at 14:41:42

In Reply to: Low Hy? How about no Hy? posted by Ray Moth on May 5, 2007 at 16:16:00:

I've recently been doing a bunch of experiments along these lines. The upshot is that it IS possible to get very good sounding results using high voltage regulators, but not just any old regulator will do.

I tried a bunch of different designs I could find on the web and non of them were doing all that great until I got to the last group. I tried series regulators and shunt regulators, tube, BJT and MOSFET. My previous best result was a shunt regulator driving a series regulator (I got VERY good line rejection from the shunt, but the output impedance was not as low as I liked, the series had very low output impedance, but not so great line rejection). This combo sounded pretty good but was very complicated and needed large heatsinks, it wound up wasting a lot of power.

What changed this was a bunch of threads on regulator design over at diyhifi.org, this got me really analyzing the designs and finding the good and pad parts. I wound up with a modification of the infamous Gary Pimm CCS to turn it into a very nice series high voltage regulator. The sonic results of this are way better than anything else I've ever tried. The line rejection is quite high and very low output impedance from very low frequencies to well above the audio range.

So far I have tried this on B+ and screen supplies for several lower power applications, (12 watts max) primarily because I don't have a big enough heatsink yet to try it on some higher power amps. Unfortunately I'm a little afraid to try it on the big 813 amp, the 700V B+ is a little too close for comfort to the max rating of 1000V MOSFETs. On preamps its absolutely stunning.

Now on the aspect of choke input, cap input etc it gets rather interesting. It IS possible to use a cap input filter with a small cap, say 2-4uf that still keeps the diodes and transformers etc cunducting for a good portion of cycle. I did some experiments with large C, small C and critical inductance choke input and mapping out the junk they spewed into the environment. As has been mentioned the large C sprays a lot of stuff around inside the chasis and into the power cord etc. The choke input gets rid of almost all of that. The small C was almost as good as the choke input. At one point I was trying .47uf, I could not tell any difference with this.

The problem with the small C is a very large ripple after the C. So the transformer voltage has to be high enough that the bottom of the ripple is sufficiently high above the droput voltage of the regulator that you always stay above regulation no matter what the load is. This also means that you are converting that entire portion of ripple above the output voltage to heat, so you need a good size heatsink. So yes you can get away without a big choke, but then you need a big heatsink. Its all about tradeoffs.

Of course you can use a choke input filter WITH the regulator, there is nothing to prevent that. Just go with a simple LC first stage followed by the regulator. The single stage LC still has pretty high ripple but the regulator takes care of that.

I haven't finished all the experiments yet, but things are leaning towards the conclusion that results from using this regulator are sonically better than what I got with the low DCR, low H C etc design. As far as I could tell what that supply gave me was a quite low output impedance over the whole audio range with very fast recovery for large transients. The regulator design does the same thing, very low output impedance over the whole audio range, and no transient issues at all as long as the supply driving it has the power to handle the largest transients.

So it looks like a choke input design that I would normally not like because of fairly high output impedance in some parts of the spectrum can be used very effectively with a GOOD regulator. Of course you still need a supply that can handle the largest current spikes without dropping below the regulator dropout voltage. This still needs pretty good size chokes and transformers etc. Or you could go with a low C input and live with the somewhat larger heatink.

I'm currently in the process of trying both of these out on my preamp. It will be awhile though before I get both built and tried out. I really want to try this out on my big amp but I have not yet come up with a good way to handle the voltage to my satisfaction.

Oh yeah the schematic! Right now its just a bunch of scrawls on a pad. I'm going to try and get this on the computer soon and post it here. My computer has been down for a week so I haven't gotten it done yet. I wasn't quite ready to post about all this, but this thread was just too tempting!

John S.

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