C3g Aikido Phono

[Nick]

Just look at the phono circuit and think about the voltages you have and then think about the currents required to generate those voltages.

[simon]

Will do. It'll be a journey of discovery! Maybe.

[simon]

Looking at the Tubecad blog on the humming amp thread I noticed the SS phono preamps towards the bottom of the page suggested instead of step up transformers.

https://www.tubecad.com/2019/11/blog0482.htm

They look quite interesting, I'd need 10x gain rather 4x, but any thoughts on how good the circuit is and how well it would work with the C3g phono?

This is the DC servo version
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Simple MC Pre-Preamp with DC Servo.png (54.07 KiB) Viewed 1542 times

[Nick]

Looks like fun, but going from *4 to *10 is not that trivial.

[simon]

Ah right, not just a case of changing the ratio of the 100R and 330R resistors then?

[Nick]

Maybe.

[simon]

Okay, I've measured the voltages again and worked through the currents and I think I understand what you mean now Nick, and why the shunt reg should be connected, the return at least, close to the link to the output RCA phono.
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20240211_180624.jpg (79.74 KiB) Viewed 1461 times

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The HV PS (shunt reg with Force and Sense + and - wires) is grounded via the audio circuit, which in turn is grounded to mains earth via the 100R resistor attached between the output phono and the phono stage earth, which is wired to IEC mains earth via some solid copper from some twin and earth.

So by connecting the shunt reg closest to the 100R resistor earth currents are minimised around the C3g, which is where earth currents would have maximum affect as the signal is at its smallest here.

Is that something like?

[Nick]

Yes, and also, because the input is now floating (assuming the cartridge is wired as is normal so the output is not connected to earth), any voltage generated in the 0v line down to the ground point at the output RCA's is not seen by the input valve as its common mode WRT the input.

[simon]

Mmm, good point. I need to check but I think I had to ground the step ups as I couldn't get rid of the hum years ago. Hopefully now I understand more I can get them balanced if they aren't.

[Nick]

You should be able to ground the stepup metalwork, while leaving the windings floating. It may be ok to ground the primary if needed, but try and leave the secondary grounded via the phono stage.

[simon]

It's that long ago I really can't remember what I did. I need to investigate.

[izzy wizzy]

You should be able to ground the stepup metalwork, while leaving the windings floating. It may be ok to ground the primary if needed, but try and leave the secondary grounded via the phono stage.

2nd'd. It's how mine is and has worked through the many iterations. I took the stepup can to dirty earth or mains earth but it might work to 0V earth as well where the shunt reg 0V is. I used a clip lead to move it around to find the best place.

[simon]

A quick check - the primaries are grounded to the same AC mains earth as the cans and screens. The secondaries are floating.

[simon]

I thought it might be fun to try to model the pre-preamp in spice to see if I could understand how it works. But I don't think I can have modelled it correctly. If I understand the output correctly then there's no gain but much attenuation.
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TubeCAD Simple Pre-Preamp.png (105.05 KiB) Viewed 1353 times

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The plots below are taken at the coloured nodes above
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TubeCAD Simple Pre-Preamp Sim.png (190.65 KiB) Viewed 1353 times

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I found models for the 2N4401 and 2N4403 but they didn't make an obvious difference to the generic npn and pnp models I initially used.

Anything obvious I've got wrong?

[Nick]

Q2 is upside down.