Tube Phono Amps

[Primalsea]

One of the problems I was finding was that there are many scems on the web but when you plug them into spice they all seemed to show inaccurate riaa correction, usually low in the bass and had different phase responses which is important also. You need to correct for the phase of the riaa just as much as the amplitude.

The the TL Phono is good I'll definately consider it.

If you dont mind I'll PM you with my e-mail for the 6C45 model

Cheers

[Andrew]

Hi Paul,

Here's TL's phono and the FR graph from Spice. If you prefer 3 stages the JRB is very flat also, has more gain I think too. (V3 is for the sim only to model the grid leak bias voltage.)

cheers,

-- Andrew

[Primalsea]

Thanks for that.

What do you recommend for V3. Passive, Active Tube or Active SS.

Also I have noticed that single stage EQ seems to have a different phase responce to 2 stage eq.

Which one is actually the reverse of RIAA at time of cutting, or was it not that standardised anyway so you never really know exactly what was applied by the factory

[Andrew]

Hi Paul,

Sorry, crossed wires, V3 is there to support the sim only, you don't need a V3 when you build. Spice models for valves don't support grid-leak bias so you have to 'hack' it to make it look like it works. I have included Thorsten's original circuit for reference, it might make more sense to see the circuit minus the Spice extras.

If you do a google on grid leak bias you'll get a better worded explanation of why grid leak bias works than I can give you. Suffice to say in this instance it reduces Zout since there's no contribution from an unbypassed cathode resistor to the stages Zout value. The last stage is also driven quite hard to increase mu and again reduce Zout. This stage should have no problem driving most, if not all, pre-amps.

cheers,

-- Andrew

[Primalsea]

Sorry, that was my fault, I read V3 but it somehow became V1 in my head by the time I looked at the circuit. Then I though you meant the 20mV sine wave Which I guess is for estimating PSU rejection of the circuit.

[Andrew]

Hi Paul,

Yes, I always add a bit of PSU ripple to see what it does. A friend of mine says that the PSU should be labelled source, which is a good way to look at it.

I'm happy to send the .asc to you if you want.

Other circuits with good eq are the Bottlehead Seduction and the JRB one that Dave Dove recently built and the old WAD Phono II. I agree there some dross out there that pretends to do RIAA properly and really doesn't.

The Thorsten jobbie responds very well to tweaking as well. It's kinda what I use in a somewhat tweaked format. I like its single stage eq, which lends itself to all sorts of experimentation. Personally, I don't like cathode followers, which many three stage phono's use, they always sound edgy to me.

With regards to your question on cutting heads, I'm not sure if they are one or two stage but I do know that many used an LCR circuit.

PS if you model circuits in Spice using the Duncan Amp ECC88 then there used to be a problem with it, it has too much source impedance, it might be fixed now, I don't know as I use Koren's 6DJ8 model instead; his stuff is very good.

cheers,

-- Andrew