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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.
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.
Edits: 03/17/17Follow Ups:
'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.'
I really have no idea how you come to that conclusion regarding high order harmonics although I can take measurements using a wave analyser or FFT if need be . In terms of measurement , I do not posses a DVM with the sort of resolution to take such measurements so an HP mV meter was used
As for the rest in cut and paste format . There is no feedback in the filament supply . This consists of LCRC to a Coleman reg (buffered CCS) taking the cathode connection from the 4P1L filament centre tap
You stated that you heard a sonic improvement with a choke in place preceding the Coleman regulator. I took that at face value and my response was an attempt to explain why you heard such an improvement - that's all.
The reduced upper harmonics, is obviously a conjecture or an educated guess on my part with no concrete proof offered and the language I used reflects that. Having said that, I still think that the quite drastic reduction in the upper harmonic content put out by the choke supply, is responsible for the sonic improvement you hear, irrespective of the filament regulator. On my part I don't understand why you would think that it'd be that big of a leap to even entertain such a hypothesis. It's been common knowledge for decades now that upper harmonic distortion even at very low levels is quite detrimental.
As far as the regulator comment is concerned, I haven't seen Rod Coleman's circuit, but do you know of any regulator, be it either of the voltage or current reg variety, where the core working principle is not based on feedback? Olson's comment as quoted, is very much on the money IMO.
Maybe you misunderstood me, I don't know, but if you haven't taken a close look at the filament supply test that I linked to earlier, I would give it another shot.
The point I am making is that a Coleman reg presents a high impedence and hence a very large degree of isolation from the downstream PSU circuitry . It feeds the filament with a ring of two CCS one side and a gyrator/buffer the other . By using choke input , the measured noise at the filament pins drops by an insignificant amount . In terms of harmonics measured on the FFT , I can't see a difference in comparison to cap or resistor input but there is an unexplained improvement in sonics . Maybe I need better equipment to measure this but as far as I'm concerned , it's not worth it
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