Thorens TEM 3200 clone

[tiefbassuebertr]

Originally Posted by audiodesign

I cannot apply the negative feedback because the second stage does not invert the phase.

The second stage is a potential shifters created with a screen driven pentode.

So probably Thorens use another configuration for this stage.

For a clone you will need at first the detailled (or st at least the simplified) schematic diagram of this Thorens power amp.
The serial NFB network from the circuit attached by post #6 consists of R14/6 for the left half and R15/7 for the right half
Voltage gain factor is additional dependend from R17/4/5 resp. R16/10/11.
If only the first stage does invert the phase, only a parallel global NFB or a local serial feedback by the first stage is possible (by the use of one voltage gain input stage and two follower/unity gain stages resp voltage shift stages).
If I had the simplified schematic from the TEM3200 I could say more details.

For your clone also ths thread could be of interest:
Biasing schemes for bipolar transistor circlotrons

P.S. Causing the wrong filing you don't get the right amount of replies. The right file for this thread is "Solid State" because only the front-end uses tubes.

[audiodesign]

I will test soon one channel of this amplifier.

This is the result of many simulations so I am interested to ear the sound.

The design allow:

1) extreme high slew-rate
2) low distortion with low feedback
3) high frequency range 0Hz to 1MHz at 0dB
4) no switching output stage like a pure class A
5) 4 trimmer to set zero offset and individual mosfet bias
6) all driver are single ended
7) the global feedback is managed by a tube
8) low voltage power supply are necessary +150, +150 and 6.3 for the filaments
9) open loop feedback easy to modify
10) no high power resistors

It is necessary a balanced input but this is not a problem because my new USB DAC 3 will have this type of output and all current production DAC chips are balanced.

[audiodesign]

I am still waiting an help for second stage of the TEM 3200 "clone".

[Zen Mod]

in schematic in post #12 you have one redundant stage in each half

[audiodesign]

What you mean ?

R5 and R10 set the dc offset for each output

R4 and R25 set the mosfet bias

[audiodesign]

I have got a copy of Linear Audio magazine - volume 0 (Linear Audio | your tech audio resource) to read the article of Frank Blöhbaum about New Low-Noise Circuit Approach for Pentodes.

[audiodesign]

This new schematic resolv the problem of feedback phase but using a differential stage the voltage gain is lower.

Open loop:

30Vpp with 2 x 2Vpp

10.6Vrms with 2 x 0.7Vrms

[audiodesign]

Another solution is cross the feedback

102Vpp with 2 x 2Vpp

36Vrms with 2 x 0.7Vrms

[audiodesign]

I don't like the cross feedback

[taniknight]

Hi Andrea,

I am following along and looking forward to reading about your findings. Thanks for posting.

Tani.