Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

Thanks Ralph!

Posted by Triode_Kingdom (A) on August 10, 2017 at 10:29:07

In Reply to: RE: This is similar to the old Berning TF-10 posted by Ralph on August 8, 2017 at 13:42:54:

Bypassing the output of the IC is effective though, if you are worried about 'solid state contamination' though :)

Sorry, I dismissed this too soon. I've come full-circle now after researching the possibility of using resistive optocouplers for better isolation. A simple two-section filter (RC-RC) at the output of the last opamp will increase isolation to 65dB at 20 Hz and nearly 100dB at 100 Hz. That will absolutely render the servo inaudible.

You are also better off with really slow timing constants in the servo. You really are only compensating for drift in the tube once the whole thing warms up. So there is no need for the servo to have any speed to it.

The servo loop is planned to consist of two active stages. The first will be a 1 Hz shelving filter with its non-inverting input attached to the cathode. No feedback resistance or capacitance will be attached to that pin. This will ensure the amplifier presents a totally benign, high-Z resistive load to the cathode. The output of the first stage will drive both the servo amplifier - which further filters and inverts - and a uPC1237 speaker protection IC.

I recommend an additional circuit that lights an LED or the like, in case the tube is unresponsive to the servo. Very handy for detecting a bad tube and you don't want DC at the output in any event.

The uPC1237 provides a large number of features in a small package, including startup delay, positive and negative DC offset detection, latching option, power-off disconnect and relay driver. I'll add a gain stage ahead of it so smaller than normal offset voltages will be detected. A LED wired to the bottom of the relay coil can act as a fault alert.

Funny, I didn't realize how many of your ideas I was chasing down until I went back and read your post. Thanks for the good advice!

This post is made possible by the generous support of people like you and our sponsors:

Herbie's Audio Lab

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---