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In Reply to: RE: Bottom View posted by bcherry on April 11, 2010 at 21:57:09
Very nice end result Brian. Very nice layout with flexibility to spare. I think your customers will be in for a very nice surprise when they put one of these in their systems. Congrats on getting the noise way down as you did. It is probably one of the most difficult things to tackle with a DHT preamp but necessary to get all of the wonderful detail that they are capable of giving the listener.
The overall look is your usual understated but professional looking piece of gear. I have not seen your listing for the unit but in my experience using silver wire in such a revealing piece of gear is almost a must especially since the runs are pretty short. A good option at the very least. If I did not have one (DHT pre)already I would certainly give this one a try.
I just have one question about the circuit layout. Is the input capacitor there just to prevent possible DC contamination from upstream components? If that is the case then perhaps a note to builders that it may be bypassed if they are sure upstream gear does not have DC. Just one more step in clarity if it is possible.
Again, congrats on a very fine looking unit.
Tony
Follow Ups:
Thanks Tony, for the encouraging words. We're enjoying the direct flow of music from this one.
regards
Brian
DIY - Done Right!
Hi,
As I had a little itty bitty hand in this, one or two comments.
First, all the signal and ground wiring is silver (soft annaeled) in PTFE Sleeving, 0.2mm for the signal, 1mm for the Ground. The RCA connectors are also silver plated!
The input capacitors are mandatory. In this circuit the DHT operates in fixed bias mode, so cathode straight to ground (okay, via a 12 Ohm bias sense resistor).
In this case the input capacitor (0.047uF PTFE & Tinfoil - only sensible upgrade would be AN Copper or Silver) is essential, without it the volume control would short out the bias and/or send the bias voltage to the source.
The benefit of using fixed bias is that we can dispense with the cathode Resistor and capacitor usually needed. And trust me, a 0.047uF PTFE & Tinfoil Capacitor in front of the grid sounds much less intrusive than a 100uF electrolytic capacitor (or a Nicad Battery) on the cathode... :-)
As it stands the signal goes:
Input ->
Volume Control ->
0.047uF PTFE & Tinfoil ->
DHT Stage ->
10uF Obbigato Copper in parallel with 2.2uF Polypropylene & Tinfoil ->
Output
It is hard to better this without bring in worse problems through the back-door.
Ciao T
Thank you Thorsten. I did not mean for you to give away the store so to speak by posting the schematic (in a sense). Glad to have it cleared up though. As usual, your answers are clear and concise.
The reason I asked though is that I built a 26 preamp using Gary Pimm's schematic which does not use an input cap and is also fixed bias. I agree that fixed bias is the way to go especially in a preamp as the need for amplification is usually very small if needed at all.
Regards,
Tony
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