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Old 9th October 2004, 12:54 AM   #11
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This design is almost exactly the circuit with germanium transistors that I have in my almost 40 year old Telefunken Bajazzo portable radio. Just used it today. Sounds great! PS Dick Sequerra (hi end guru) was a consultant on that project. This portable radio runs rings around ANY portable component that I have ever heard, including Class D portable circuits.
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Old 9th October 2004, 01:14 AM   #12
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Default This Telefunken is wonderfull

I had one once....wonderfull sound!

Carlos
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Old 9th October 2004, 01:28 AM   #13
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I have owned 5 or more! Bought my first in Germany, 1965. Second in Berlin 1967, etc Found this one in a pawn shop in San Francisco, CA USA in 1972. Gave it to friend, and he gave it back to me when it developed problems. Fixed it, and I won't give it back.
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Old 9th October 2004, 01:48 AM   #14
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Quote:
This design is almost exactly the circuit with germanium transistors
The circuits that I see from the sixties ,with germanium transistors and output transformers, usually use the transistors in common emitter stage (With voltage gain) as a push pull tube stage.
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Old 9th October 2004, 01:49 AM   #15
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Hi Susan,

Novel circuit - outstanding! Have you considered running a phase splitter tube for input voltage amplification? This would enable lower impedance drive to the gates, keeping input impedance high....... And would it be possible to drive the speaker from across the mosfet sources, eschewing the secondary of the transformer, using it only as a centre tapped choke?

Interesting comment from Eva on bias imbalance, and nice to hear JC likes the configuration!

I'm sure you considered these aspects, but I'd be interested in your response.

Cheers,

Hugh
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Old 9th October 2004, 05:13 AM   #16
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I have seen this somewhere before using tubes but I can't remember where anymore. It's still a sweet design.
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Old 9th October 2004, 05:42 AM   #17
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Quote:
Have you tried playing piano notes at high volumes?
I have confusing problem with this design I attached. It is the similiar to Susan's design after the NJM386 (which is simply a preamp)
Bias can be adjusted independently. The output trafo is Jensen, There is also a small driver's trafo, after the NJM386. The supply is +50V single pole, going thru cap multiplier, driving OT in push-pull.

This design also have difficulty in matching with speaker. You must tweak your speaker system (passive xover, put LCR everywhere to make the response flat) to make them linear before you use this kind of power amp.
Put it with ordinary factory built speaker (like standard Dynaudio Reference)... the sound is strange.

Very long time for break-in. More than 300hours for the sound to settle. (Infact I dont know, who's breakin-in. The speakers or the power amp?)

Eva tell me that big current will make the core saturate. Even in push-pull configuration, any slight mismatch will saturate the core. So the design won't like class A. EVA seems don't suggest transformer anywhere in audio reproduction. (while she's good with transformers). She don't like the non-linearities for audio reproduction

Transformers theoritically are not linear, huge distortions for audio reproduction. But somehow, I like sound of transformers

Quote:
This portable radio runs rings around ANY portable component that I have ever heard, including Class D portable circuits.
Mr.Curl,
Is it possible that "Classical" designs are not worse than "Modern" Hifi designs?
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Old 9th October 2004, 07:02 AM   #18
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Tube dude, you are correct, I misread the circuit, BUT I do have both the driver and output transformer, AND same type transistors on the output. For some reason, it sounds pretty darn good. Maybe lack of too much negative feedback.
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Old 9th October 2004, 10:20 AM   #19
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Looking at the pictures I see a massive output transformer, I think it may whitstand 100W at 50Hz before saturation, but it's obviously overrated since I think this amplifier does not provide such a big output swing

The circuit also uses MOSFETs as followers, providing the Vg-s variations as the only error voltage introduced in the output signal [also transformer distortion], as opposed to lumanauw's common source configuration with higher distortion and poor speaker damping [almost current drive]. Also, class A biasing avoids crossing the low current zone of the MOSFETs where they show extremely poor linearity [MOSFET Id is proportional to Vgs^2]

So this circuit may work somewhat better than lumanauw's one at the expense of higher dissipation and the requirement of a custom step-up drive transformer instead of a simple 2:1:1, 1:1:1 etc...
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Old 9th October 2004, 10:46 AM   #20
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Default Re: Zero Feedback Impedance Amplifiers

Quote:
Originally posted by Susan-Parker


This amplifier is based on Impedance Amplification and has similar distortion characteristics to valve triode designs.

P.S. Yes, it does work.
Susan,

have you got any evidence for this statement (spectral measurement).

How does this amp sound when playing fff of the philharmonic orchestra, how is the resolution and clarity.

Best regards,
Pavel
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