OTL preamplifier

[filippo22]

Hi guys
I found some input transformer with separate secondary windings, good also for a phase splitter. Starting from my first idea, a friend made the following circuit for an OTL preamplifier.

I'm waiting for building the first prototipe and any previous comment is sincerly appreciate

thanks a lot
Filippo
www.audiofanatic.it

 

[analog_sa]

Filippo

This is one weird circuit. Or i do not understand it Why do you call it OTL? After the splitting transformer both phases seem to pass through their respective 100uF caps, right? This can't be too good...


peter

 

[filippo22]

I called OTL only because the stage is like a tipical OTL amplifier, the HT is made with a dual PS, and the signal is taken at a virtual zero volt point between the two tubes. Anyway I put a condenser, because a little offset may occour, due to the tolerance of tubes.

The 100mF condenser is only a cap for avoid some residual hum because the fixed neg bias is taken directly from the heater supply and regulated with the pots.

Yes, is a weird scheme, I attach the first simplest idea when the basic circuit is better showed

... and sorry for my very maccheronic english
)))))))))))

ciao
Filippo
www.audiofanatic.it

 

[EC8010]

Confused.

It's a balanced input White cathode follower. Why? You've gone to a lot of trouble to design an expenisive active stage having a gain of one.

 

[Sch3mat1c]

Actually it is not a white CF. It has absolutely no NFB, aside from that in the triodes themselves. They are fed in such a way as to negate the characteristics of a CF. For this reason, the circuit seen in the 3rd post in this thread is incorrect; the top tube is a cathode follower, while the lower is not.

It's just an elementary form of a SEPP circuit, thought the arragement of the schematic is a little confusing.

Tim

 

[EC8010]

Sorry, but it is. If you look at the patent, you will find that there are two distinct circuits given. One has a feedback loop down to the lower valve, and the other is this one.

 

[Sch3mat1c]

The other is which? The post #3 circuit?

Tim

 

[EC8010]

Yes, UK patent 564, 250 includes the circuit in post #3 (although not with a transformer phase-splitter).