Cary Audio Preamp Phono Section (SLP-70)

[fred76]

Oh, sorry -- forgot to mention that I had added C1 (just a decoupling cap) and reduced the value of C6 to flatten the simulated bass response. You can remove C1 and increase C6 to 0.1uF and not much would change.,

Increasing Cout to 0.47uF shouldn't change much either.

If B+ is down around 230V, then that will reduce performance a bit more.

Does hat Rload of 50k represent a volume control pot?

The EQ looks fine, of course. I'm sure there are better ways to go about it, but this is a pre-existing circuit, so I understand. Tweaking.

Hello.. 25k log pot with 1M resistor to ground.. Yes, it’s a ~30 year old commercial unit from Cary Audio..

 

[rongon]

Is that 25k log pot in parallel with the 1M resistor to ground? (It probably is, but just checking to make sure).

If yes, that means the phono stage is running into an approximately 24.4k ohm load. 😦

 

[fred76]

Yep, it is.. I ordered some ubiquitou blue Alps 250k and 500k pots to try out (what works best) and replace the old Noble 25k stereo log pot (still works fine).. The value Cary used is a head scratcher indeed..

 

[jackinnj]

a single grocery store battery (1.56 VDC) can bias all four triodes fine. Bolt those cathodes to chassis and live free, brothers!

Topic for another discussion -- but even simpler, a MOSFET gate driver/optocoupler can be used to bias the grid -- if you recall, Scott Wurcer biased the gate of a JFET with a Vishay VO1263. I tried to develop the topic here: https://www.diyaudio.com/community/threads/opto-to-bias-triode.302335/ Other gate drivers can be substituted for the VO1263 which has long lead-time

 

[JoeAlders]

A very interesting thread!
Is someone willing to show his measured RIAA response (or the measured RIAA accuracy) of this circuit build? Of course LTspice sims of the circuit are
already shown here, but I am very curious looking at the measured response.