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I had a reply to this, but it disappeared. On the subject of diodes, non-linearity and swamping current. I keep assuming that the DC cathode current already swamps AC cathode current in a first stage phono application.

For the 6C45, assuming a LOMC with an output of 0.9mV, a SUT with voltage ratio of 1 to 30, then 0.9mV x 30 x 45mA/V = 1.2mA, surely 30mA DC swamps that???

For a 12AX7, same cartridge/SUT, 0.027V x 0.0016A/V = 0.0000432A. Doesn't 1mA pretty much swamp that. It's at least a 33dB ratio, AC to DC. Not sure if any effect would be apparent here. Surely it's better than a parallel RC cathode device.

Am I wrong?

Stuart

As I see it the reason for building with the lowest distortion triodes available is to get the lowest distortion without the feedback band-aid. An RC network instead of the diode would provide less distortion, even with an electrolytic, provided that it was very large in value. The reason for the large value is to suppress the non-ideal issues with electrolytics. In Guy's application, 10000 uF cap rated for switching power supply applications would be suitable. I know that this seems like an absurdly large value to most, but given the high transconductance and high current, the cathode impedance will be quite low, and the capacitor needs to have a very low impedance to accommodate. If the diode is used, that's fine, but the approach I mentioned earlier is needed to suppress its non linearity. Then either approach should allow the triode to perform at its best. Now I am being a perfectionist here, but I know that when most DIYers go to the trouble of building something they want to do the best they can, cost allowing. In my circuits I have used both approaches, and any preferences is based on ease of implementation.

David

@David B.,
I understand what you're saying, and can only explain my forgetting diode characteristics by saying I'm getting older. I also appreciate your points made in your last posting about RC networks in the cathode circuit compared to a diode (w and w/o bias). I need to clear off my 'bench' and apply some power to see what I see. Thanks!

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- Guy

Maybe build both ways and measure distortion products? I assume Dave B. has suitable equipment to measure. Could even listen to both, then decide.......

Roscoe,
Thanks for the point about not paralleling VR tubes. What you say makes sense, and shows that you've done this more than I have. I'll also do some experimenting and see what happens when I apply different filters.

David B,
Thanks for the cautions and insights on VR types and associated noise. Minimal noise is very important to a pre-amp! I haven't checked the last two links yet, but I will (don't want to be caught at work!)

Thank goodness this is not a final product, so I'll have opportunities to work on the weak points (assuming there are enough parts left after an Event!) It will be interesting to show my results (the good and the bad) to everyone. Maybe I can prevent someone from releasing the Magic Smoke like folks here have helped me!

_________________
- Guy

I forgot to mention that one should be most concerned about the dynamic range required of the input stage when passive EQ is used. For many of my designs I use passive, and I find that the input stage overloads first. This is because the phono cartridge (or tape head) has its highest output at high frequency. The stages that follow the EQ have an easier time because the highs are cut. So the linearity of the input stage is critical, and needs to be considered over its full dynamic range.

David

Just for grins, if anyone wants to try a solid state phono preamp, I built a stand alone unit that I use for burning LPs to digital. It uses OP27 op amps for the RIAA section and OPA134 op amps for additional gain if needed and buffering. It also has a gain control pot. The first stage DC power supply is in a separate housing that can be separated from the preamp by up to 6 feet. There is one set of RCA jacks for the input and 3 sets of paralleled RCA jacks for output. I use one output to go to an equalizer and then to the computer sound card, another to go to a headphone amp for monitoring, and a third for anything else like scope monitoring. Here is a photo of the circuit board and links to the spec sheets on the OP27 and OPA134 in case you are interested.

http://www.analog.com/static/imported-f ... s/OP27.pdf

http://www.ti.com/lit/ds/symlink/opa134.pdf

Tom

Back to topic!!....Guy, the second stage I built also used a 6AM4 as the gain element. I really like the sound of that tube, it will serve you well. What I don't like is the sound of coupling caps. Of the caps I've heard, the Deulund cast caps are in a whole other world when it comes to sound and, unfortunately, price. I've tried to minimize the use of any coupling cap, so that one I pick a final design, the Duelunds can be fitted as a crowning touch.

Having few caps as a goal led me to choose a LTP as a second stage. The "left" tube in the pair has the DC coupled signal from the plate of the first tube (EC8010 is my first tube, similar high gm tube to the 6C45P), the second grid is AC grounded and at the 1st plate's DC potential. In the tail is a 10M45S CCS returned to ground.

I also used high current LED bias, but a conventional plate resistor (Mills) in the first stage. So the two pairs of "signal path" caps are the grid grounding caps on the "right" grid of the LTP and the output of the LTP (taken at the "right" plate; advantage is elimination of Miller Effect rolloff in this high gain stage), which is a V-Cap (0.01uF) input to the 6SN7 follower, which is implemented the same Broskie way as is yours.

I think your preamp will sound amazing, don't keep us waiting too long to hear it!!

Your preamp actually has a lot in common with my phono stage and Charlie's. Those two are the best I've heard. I have one more in the pipeline, the latest, DC coupled version of Izzy Wizzy. That preamp has only an output transformer and RIAA caps. Whichever sounds best will serve me for a long time to come. If it is the EC8010/6AM4 preamp, then Deulunds it will be.

Since your stage has two sets of coupling caps, you have lots of money to spend!!

Please consider this experience in making your phono stage choices. don't make the mistakes I've made of fitting it to a box right away. Learn from Charlie and build it on an experimental platform. when it is to your satisfaction, then encase it!

Picture of the 6AM4 LTP stage is included. The whole audio section is included, which also shows the servo/6SN7 stage that you are building. Maybe this will give you ides from which to choose how you arrange yours. As you can see, I did it modularly, and never did get around to building the second stage on Teflon, like the other stages. Trust me, Teflon sounds better and it's a PITA to work with. But I went through several second stages before coming to this one (using the same 6AM4 as you, GREAT CHOICE!), and I can tell you, avoid Nuvistors!

Stuart

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Sorry, not much action recently.

Holy Smoke! (Not magic smoke.) I just looked at Stuart Yaniger's article called, "His Master's Noise Phono Preamp" <http://syclotron.com/?p=143>: his input stages have the same architecture as mine! Uses a dual-DN2540 CCS to drive a pentode (D3a) that is LED biased. He has all the inverse-RIAA in one circuit, followed by a 6DJ8 second stage and 6DJ8 driver.

Well, at least my driver-stage circuit is different, and I'm not using an input transformer for a MM cartridge. Interesting how the circuits mirror each other - I may be on a good path!

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- Guy