[Hybrid Amp] Questions about the use of zener diodes

[Johnix]

Hi all,

I found this schematic yesterday :




Firstly i dont understand the goal of the relay, as described here :

http://www.homestead.com/whaan/files/page6463.html



And then, i was wondering what was the goal of the 2 12V Zener diodes between the BC556b and BC546b.

Thx

[anatech]

Hi Johnix,
Man, is that ever close to a Counterpoint SA-100!

The relay prevents bias current until the tube stabilises, a "thump" reducer.
The BC 546 / 556 form current sources. I did the same mod to the SA-100. The zeners form a voltage reference to the bias supplies to isolate the bias voltage from supply voltage hum and variations. They are also needed to set the zero voltage.
This thing is so close to the Counterpoint, slightly improved and worse, I'd have to call it a rip off. I think I will call it a copy. The only major difference is no feedback and therefore, one less stage, and the lack of a gate drive buffer.
If I want to be kind, I'll say the design borrows very heavily from the Counterpoint SA-100. I mean heavily.
-Chris

[sajti]

Originally posted by anatech
Hi Johnix,
Man, is that ever close to a Counterpoint SA-100!

The relay prevents bias current until the tube stabilises, a "thump" reducer.
The BC 546 / 556 form current sources. I did the same mod to the SA-100. The zeners form a voltage reference to the bias supplies to isolate the bias voltage from supply voltage hum and variations. They are also needed to set the zero voltage.
This thing is so close to the Counterpoint, slightly improved and worse, I'd have to call it a rip off. I think I will call it a copy. The only major difference is no feedback and therefore, one less stage, and the lack of a gate drive buffer.
If I want to be kind, I'll say the design borrows very heavily from the Counterpoint SA-100. I mean heavily.
-Chris

Almost SA-100. The valve stage is simplyfied, and avoid the feedback. The idea almost same...

sajti

[anatech]

Hi sajti,
This design needs some feedback. It sounds awful without it (I tried).
12 to 18 dB sounds the best.
-Chris

[sajti]

I made some hybrid design. It was E82CC drive IRFP240/9240 devices. I used about 14dB of nfb. And I run it in class A, with 1A bias...

sajti

[anatech]

Hi sajti,
How did you drive the gates? Gate charge is non-linear.
-Chris

[sajti]

Originally posted by anatech
Hi sajti,
How did you drive the gates? Gate charge is non-linear.
-Chris

I used the second E82CC with high current (6mA) to get enough speed. I didn't care with the nonlinearity. It sounds well, but I think that BJT output gives better sound...

sajti

[anatech]

Hi sajti,
The feedback plus the higher current made it okay. Bipolars are certainly easier to design for a tube front end. The output impedance would be lower too.
-Chris

[sajti]

With tube front end, I need at least triple darlington output stages to get high enough impedance. And - due the input current - not easy to connect the output stage to the tubes...

sajti

[anatech]

Hi sajti,
Capacitor coupling and a power diamond type buffer (three stage instead of two, extra diodes in first stage emitters). That would do a nice job.
-Chris