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.
No, direct measurements of another, standalone JT-123-FLPCH as described in the post.
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
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
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.
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.
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".
The F6 Amplifier thread is Research and Development by DYIers, and therapy too. Their look at the bigger picture of the old and new approaches is always healthy. After all, Mr. Pass and DYIers are resurrecting a classic design which has been extinct for 50 years. Ideally, one needs to understand this dinosaur to best infuse it with new tech devices and know-how. I believe that Mr. Pass has in his pocket a thorough understanding of the old and the new. The DIYer can also learn it and have it it too.
Here's an example of limiting freedom in this R&D experience by locking an exact transformer. Questions like this came up. Can I pass through its secondaries 10 uA DC; how about an offset 2 mA flowing through its primary due to a mismatch in Idss of the front end JFETs. I'll never be as good as Mr. Pass in matching JFETs; or buy them at extra expense. Thus a choice of several suitable transformers will accomodate the scope of mods and whims of DIYers. This proposed flexibility is freedom to experiment. Besides, design concerns with such a transformer were sorted out in the old designs. DC flowed freely through its primary and secondaries. Distortions? Audiophiles loved this old classic.
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Thanks for the pointer. The JT-123-BMCF is an impressive transformer. but at a cost about 3X that of the JT-123-FLPCH (I know you were not suggesting using the BMCF instead of the FLPCH for the F6) . It looks like DC current should not be a problem at the levels of input JFET IDDS mismatch.
IMHO, the underlined also includes the advice of Mr. Pass to explore and experiment; but never "my way or the highway". Simply stay within the scope and spirit of the concept.
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Not if the input stage has other advantages and the output stage is run as a follower. Just an idea. Much Kudos to you for building it. Maybe i will get my screwed up version running this weekend. Probably not.
lhquam. what is the output impedance of your build [and/or sim] with and w/o feedback? Are they significantly different to impact speaker damping and thus influence the underlined subjective experience?
For experimentation, replace the bottom leg of the feedback network with a 10R to 20R potentiometer (2-5 watts). That provides a simple adjustment the gain and amount of feedback. The pot at zero gives no feedback and full open-loop gain. At 10R you get a a gain of about 5.
I will see if the board layout can fit a pot.
buzzforb: Please visit the Headphone Systems forum and find the thread "A headphone Transconductance amp for a change". I show in posts 22 and 23 three schematics which maybe along your suggestion to drive a transformer in differential manner by an Op Amp or better yet a small bridge power amp.
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OPA1632 is SUSY chip - diff in , diff out
it demands pretty lowish impedance feedback net, and it's having low output impedance
I was using it connected to sec of Beyerdynamic 1:10 mic xformer ; xformer ditto on outputs of two (diff connected ) PCM1704 , then feedback net defining reflected impedance back to chips , to have linear freq through audio band
driving xformer with small bridge power amp , to drive output of small power amp ........
........ may I laugh ..... just a little ?
it demands pretty lowish impedance feedback net, and it's having low output impedance
I was using it connected to sec of Beyerdynamic 1:10 mic xformer ; xformer ditto on outputs of two (diff connected ) PCM1704 , then feedback net defining reflected impedance back to chips , to have linear freq through audio band
driving xformer with small bridge power amp , to drive output of small power amp ........
........ may I laugh ..... just a little ?