Tube DIY Asylum: My latest phono revision unveiled by Kurt Strain

Click below for the schematic of my latest choices of parts for my phono stage. Power supply is not included, I get it from my line stage. I can't think of a better way to make this phono stage from the point of maximum transparency, except maybe to change the output tube to a 417A or other low Rp tube with high gain just to drive the autoformer step attenuator better.

The tubes are some high frequency low noise high gain lowish Rp single triodes with exceptional detail and almost no sonic signature in this level of signal. The 6ER5 is a fine frame grid triode of that type of tube, and so is the 6GK5, a cousin to it. They have the same pinouts but slightly offsetting sonic signature. If I use two of the same kind I get a washed out sound as similar even order harmonics cancel out.

Most of the resistors are very transparent Caddocks, and not quite as sterile clean as Vishays. Tantalums would be nice I bet. The caps are as much teflon as I could put them in, with one 0.0033uF being too small to get in teflon, and so it's polystyrene. The equalization takes into account the little 3.18 usec time constant. The simulated error in RIAA taking this high time constant into account and neglecting the RC rolloff of the load of the line stage, can be seen at
http://www.sonic.net/~ktstrain/phonoeq.jpg

The power supply splits into the "C4S" or equivalent active constant current source loads with filtering to each CCS. This is necessary to stop small residual ringings that somehow get through to the power supply. Some opportunity to improve on the CCS is there.

If there was a way to avoid the big 249K series resistor between tubes this could sound better, but I have no practical means yet to do that. Something in series has to be there I believe unless you use a lot of transformers and inductors or whatever.

The EAR MC-3 is a very good MC step-up transformer and if you're going to go all out, why not? This adds about $1000 to the bill. The phono preamp after box and ground plane proto board and wiring and parts, etc. costs about $700 to make, plus power supply.

I think this phono preamp can beat most others on the market, but I haven't heard them all. This does beat the following ones I have heard:

Audible Illusions MOD3 and MOD3A with and without gold phono board.
Herron's first preamp offering (forgot that model)
Air Tight ATE-2
EAR 834P

The sound is characterized as very neutral in tone, great dynamics, great microdynamic shadings, and details, details, details, and very low noise. It doesn't impart much on the music at all. For character I use my line stage and driver stage of my amp to get into the finer tones.

Compared to the mentioned brands of phono stages, this one is not as dry as the Herron with better body and tone and seems about equal in detail and probably more so, not as dark as the Air Tight and more detailed, not as bright and sterile as the EAR, and definitely all around better than the Audible Illusions.

So there's some other data points to consider in a DIY phono stage.

Kurt

Phono schematic (Open in New Window)