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Pentode Phono Preamp
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I've been working on a passive RIAA pentode phono preamp. I really like the way it sounds, but as you might expect, using a pentode in the front end is not the quietest thing you can do. I find it acceptable, but others may or may not... the noise floor is about what I get for surface noise on a brand new test record. On most of the vinyl I have (granted, not all pristine) I can't hear the noise contribution of the preamp.
The first stage is a pentode-connected low-noise pentode. One of the nice things about using a pentode here is that since the plate resistance of the pentode is so high, it has little effect on the source impedance driving the RIAA network: that is dominated by the plate load resistor. Response is very accurate as shown in these measurement graphs - error is within a dB 20-20k:
Following the RIAA stage is a triode-connected EF184/5EJ7 with a Ixys CCS as a plate load. this is a very cheap and under-rated tube, especially connected as a triode! I am currently using the Ixys part "mu-connected", which gives a very low output impedance.
I've been comparing GE and RCA 5879 input tubes, as well as some miniature tubes. They are nearly equally noisy, but different. I really need to try some EF86's for comparison as well.
One unique thing I did here was to run the thing off of a 12V DC "wall-wart" power supply, and use a DC-DC converter to generate the B+. The beauty of this is that there is no 60Hz anywhere near the unit - the B+ switcher runs at 100kHz, well above anything audible. So the only noise you hear is from the tubes themselves.
Here's a schematic (or download a PDF file of it):
I'm also posting the Eagle CAD PCB, SCH, and Gerber files (330k ZIP archive), if you'd like to build your own or take the design and modify it.
And a photo of a PCB with just the power supply stuffed for testing.
The big round shiny circles are spots for Sowter moving-coil step-up transformers. The power transformer can be seen near the right-hand edge. The heatsinks that are not installed are for the IXCP10M45S CCS parts.
The power supply transformer is actually an off-the-shelf unit designed for off-line flyback use (120/240VAC -> 12VDC). Here it is being used "backwards" to get 300VDC from 12VDC. It's not as optimal as a custom design would be, but it's cheap and available (from Coilcraft, Y8848-A). Here's the data sheet. The switcher controller is a "simple switcher" part from National Semiconductor, the LM2586.
The power supply is not quite perfect. I think I would be better off with a purpose-designed transformer here... things are running a little close to the edge, and the AC stability is a little variable. Of three I built, one seemed to have either LF instability and/or poor ripple rejection of the incoming DC. Perhaps just a bit more work and thought would straighten it out, but I'm not actively working on this at the moment, as I'm very happy with what I have.
Also, here's a .XLS file bill of materials. Pretty much everything came from the "usual sources".
Although it isn't the quietest phono stage around, I think it sounds very nice indeed. The noise that is there is a mostly "tube noise", as opposed to 60Hz hum or buzz or other sounds that are annoying. Quite a lot of folks listened to it at ETF, and I had nobody say anything about the noise, nor could I hear it in the listening room there.