Tube DIY Asylum

RE: 4P1L preamp DIY

207.38.166.145

This Post Has Been Edited by the Author

In Reply to: RE: 4P1L preamp DIY posted by GeorgePope on March 17, 2017 at 07:50:25

I can see you have Coleman regs in there . A good way of squeezing out the maximum is to use a choke input filament supply with a separate filament transformer for each channel . Hardly anything in it measurement wise (a few tens of uV ripple/noise) but to me , it does sound better

The reason for the better sonic results with the choke, probably stems from the reduction in the upper harmonics, from say the 20th and beyond. This is something that a basic DVM measurement will not reveal, but the scope will. Look at the following two reports on DHT DC filament supplies.

http://www.clarisonus.com/Research%20Reports/RR002-DCFilterTest/RR002-DCFilSupplyTest.html

http://web.archive.org/web/20130201030256/http://www.clarisonus.com/blog/?p=169

Here's a quote from Lynn Olson's conclusion at the end of the second linked article above:

Now, maybe you have a lot of faith in an expensive active voltage regulator, but I wonder if it would really deliver a 55 dB improvement at 3kHz? Remember, regulators use feedback to do their magic, and the feedback is most effective at DC, and decreases at 6 dB/octave as frequency goes up. By contrast, the choke filter gets better as frequency increases, up to the point where stray capacitance makes the choke resonant.

Considering that elsewhere in a power amp we'd be struggling for a 6 dB improvement in noise (especially buzz and hash levels), hand-selecting tubes, careful wiring layout, etc. etc., how can anyone dismiss a 55 dB improvement?

If you were really hard-core, there would be four chokes: a pair on the high and low-side between the bridge and first cap, and a second pair on the high and low-side between the first and second cap. The first pair, since it emits magnetic noise, would be physically isolated from the audio circuit (on the far side of the chassis, close to the dedicated filament transformer), and the second pair would be close to the DHT, since it is part of a LC filter circuit. Another improvement would be to insert an RF common-mode choke of the type seen in the input of computer power supplies. This would reduce the common-mode RF noise that can sneak through the large power-frequency chokes.

Follow Ups: