F6 Amplifier

[Antoinel]

Originally Posted by lhquam

Does anyone know how much DC current through the transformer primary can be tolerated without added distortion? The question is motivated by the issue of the input JFETs not necessarily being well matched in IDSS. A potentiometer could be added to trim the input stage offset voltage at the cost of increasing the impedance driving the transformer.

lhquam: Why are you and other DIYers hung up on the exact topology of Conceptual F6? This schematic is a concept. Y'all have the option to practice the diverse teaching of its old parent classic design; shown in many posts like #38, 1082, and 1085 etc. Does the front-end need to be a complementary common drain JFET design? Why can't it be a bjt or jfet operating in a common source configuration instead? I read in an earlier post that buzzforb proposed driving the transformer differentially with an Op Amp; this is thinking outside the box of the Concept. This concept design has tied hands. One can't do this and can't do that. What [else] in heaven can you do? I advocate regaining your freedom, by distancing "your design" from that of Conceptual F6. Do you really want your design to be known in audio circles as "A copy of F6? You and others have choices now. Y'all can always [and easily] copy the [unique] actual working schematic of Mr. Pass when its publishes. By contrast, Mr. Pass will not copy your design. Ponder this. What is probability that your DIY copy of F6 will sound exactly the same in the reference loudspeaker of Mr. Pass like an authentic one he assembled?. You do care; but how will you ever know? A question like it was asked somewhere else regarding F5.

[lhquam]

Originally Posted by bobodioulasso

Some square waves from the output of your proto, Ihquam?

No, direct measurements of another, standalone JT-123-FLPCH as described in the post.

[Zen Mod]

Originally Posted by Antoinel

.... This concept design has tied hands. One can't do this and can't do that........

I think you got wrong impression what some thinks and what majority think

besides that , do not forget that one need pretty low driving impedance , to make repeater happy

[buzzforb]

Opa opa opa.

[Zen Mod]

moi like it more on other side of xformer

as quick'n'dirt output for xformer based I/V

[buzzforb]

I guess that means the trafo has no trouble driving low Z of Opa. Me got some ideas now. Talking to BudP about trafos. I am getting excited and haven't even heard anything. I assume your repeaters get better extension than the Jensens. Kinda discouraged to see the 15K roll off, not that my ears work that well.

[Rush]

Originally Posted by buzzforb

I guess that means the trafo has no trouble driving low Z of Opa. Me got some ideas now. Talking to BudP about trafos. I am getting excited and haven't even heard anything. I assume your repeaters get better extension than the Jensens. Kinda discouraged to see the 15K roll off, not that my ears work that well.

I think we (I) should build it and listen without feedback and with feedback.
Sometimes you find the finest midrange you ever heard (Western Electric) without feedback and you extend the range with a little feedback and stop when you are happy with the trade off. Or maybe you are happy with the extension as is.
The feedback is a play thing for this amp. The problem was the bias, I think this is handled now.
I have a preamp (w/crossover) project that I must finish before I start yet another amp.

Rush

[lhquam]

Sure, the input stage could be something other than a complementary follower, but I think there are good reasons for that choice. The input stage needs to have a fairly low output impedance, because of the load imposed by the transformer, which is about 100R at 20Hz and 200R at 40Hz. This suggests a complementary follower stage rather than common drain. Sure, an opamp could be used and would provide the benefit of more voltage gain. But that would lead us off of the Pass Path.

Originally Posted by Antoinel

lhquam: Why are you and other DIYers hung up on the exact topology of Conceptual F6? This schematic is a concept. Y'all have the option to practice the diverse teaching of its old parent classic design; shown in many posts like #38, 1082, and 1085 etc. Does the front-end need to be a complementary common drain JFET design? Why can't it be a bjt or jfet operating in a common source configuration instead? I read in an earlier post that buzzforb proposed driving the transformer differentially with an Op Amp; this is thinking outside the box of the Concept. This concept design has tied hands. One can't do this and can't do that. What [else] in heaven can you do? I advocate regaining your freedom, by distancing "your design" from that of Conceptual F6. Do you really want your design to be known in audio circles as "A copy of F6? You and others have choices now. Y'all can always [and easily] copy the [unique] actual working schematic of Mr. Pass when its publishes. By contrast, Mr. Pass will not copy your design. Ponder this. What is probability that your DIY copy of F6 will sound exactly the same in the reference loudspeaker of Mr. Pass like an authentic one he assembled?. You do care; but how will you ever know? A question like it was asked somewhere else regarding F5.

[lhquam]

Correction: Direct measurement of the transformer get about 870ohms at 40Hz, not 200ohms. The 200ohm figure came from a Spice simulation with an unverified inductance parameter.

Originally Posted by lhquam

... The input stage needs to have a fairly low output impedance, because of the load imposed by the transformer, which is about 100R at 20Hz and 200R at 40Hz. ...

[jackinnj]

Originally Posted by lhquam

Does anyone know how much DC current through the transformer primary can be tolerated without added distortion? The question is motivated by the issue of the input JFETs not necessarily being well matched in IDSS. A potentiometer could be added to trim the input stage offset voltage at the cost of increasing the impedance driving the transformer.

Audio Precision used the bigger version +28dBu @20Hz JT-123-BLCF in testing power supplies for PSRR and they got a few hundred mA. The smaller version you're using probably won't do as well. Here's a link to the application note with charts of THD vs DC Load. AP High Performance Audio Analyzer & Audio Test Instruments : Service & Support

I've used the transformer method for PSRR as well -- but in the great regulator bake-off opted for Walt Jung's "Current Injector".