A Heretical Unity gain line stage

[SY]

Questions are encouraged- it helps me learn.

The 1M resistor from the volume control wiper to ground is intended to keep things stable in the event the control’s wiper goes bad, a common occurrence.

[Stocker]

That was posted in the text. Perhaps some clarification would be helpful.

Sometimes, potentiometers (even high-dollar units) will fail. They can have big spots in their rotation that will be open or less usually short circuits, or they can have a spot that is all one resistance value along a large portion of the rotation.

That resistor will help keep the circuits from freaking out should the pot die a particularly nasty death.

*********

Speaking of nasty,

There but for the grace of God goes SY, (unless he is named Pepper, too )... so He at least loves you that much...

[Bill*B]

Thanks! Makes sense. Stability and reliability are essential to long term enjoyment.

[rdf]

Originally posted by SY
...there's precious little (ok, NO) evidence that extending bandwidth beyond 20-25kHz is audible.

In the context of this circuit exposition (very educational BTW, thx!) irrelevant since the trannies are good past 90 but this is possibly not completely correct. There's a paper floating around which was presented to the American Physiological Society (Inaudble High-Frequency Sounds Affect Brain Activity: Hypersonic Effect) detailing brain wave activity of subjects played band-limited and full range Gamelan music of Bali, which has strong harmonic components well past 50 kHz. The subjects were all pre-tested and none could discern discrete tones above 20 kHz yet the study found a significant statistical correlations between certain brain wave patterns and exposure to harmonics beyond 22 kHz. OK, not exactly like winning the State Perceptual Lottery but it still rules out 100% certainty.

Back to the circuit, for the sand-phobic is a big plate choke with DCR in the 170 ohm range a suitable sub for the CCS? Higher costs but also less potential noise, higher immunity to power supply issues, lower complexity, etc., but at a significant impact on distortion? You've mentioned input transformers in other posts and the specs for some of the Cinemags are extremely impressive. Do you find the isolation has (touchy ground here) audible benefits under most general operating conditions? I was also wondering about the benfits of a 1:2 tx at the front end for some free gain at the expense of ultimate HF bandwidth (not that I've found and suitable transformers except for old Hammonds.)

Originally posted by SY
I think Jesus may be reconsidering whether or not he even likes me.

Not to worry, audiophiles don't normally get past the Gate anyway. Thanks again for the posts.

[SY]

Good questions.

First, yes, I've seen references to that paper (though I haven't read it). AFAIK, they still haven't shown audibility; this is a very indirect, and as yet unreplicated, measurement. But I would agree, 100% certainty is not appropriate. I think 99.9% is.

OK, the choke. If it's enough henries to give a good solid load at 10 Hz and you return it to the -12V supply, it will probably work fine, though you'll have to add some series resistance to get DC into balance.

But let's get real: with the multimegohm source impedance of the CCS, the cathode's load is totally dominated by the cable and power amp input impedance. And if it's not, make it so by changing the power amp's input resistor to a lower value; it's OK, you've got a nice cathode follower to drive it.

Isolation made a big difference- the horrible grounding of my satellite TV system made the old one buzz a bit. That's gone now. I was probably not clear enough, and I'll edit the post later to fix this, that the input and output grounds are totally isolated from one another. That's got nothing but good sonic consequences: the disappearing buzz bears testament to that.

[Bill*B]

Maybe I'm jumping ahead to later changes - but did you ever consider adding a JT-10K61-1M output transformer as well?

[SY]

No. For 10K/1nF worst-case load, the output transformer brings nothing to the table that I can see.

After the intermission (I'll post power supply info today), the next two steps are getting rid of that output cap.

[Danko]

Hello!

Sy, you may remember me, I was looking for some preamplifier for my SS amplifier. When I saw your design in the first post, i told me "This is IT! It's almost MY requirenent!" :-)

Thank you, I will build this preamp. But I'm going to leave the isolating transformer, becouse I have one sound-source, my PC, and my PC and amplifier is in the same wall socket.

[Danko]

oh, I forgot one thing:
could you recommend me some PCB-design guidances for this preamplifier?

[SY]

Yes, you can build one without the input transformer, but you may find it of great value even with your simple setup. I'd probably RC couple if I were to go the non-transformer route.

There's at least one board design in the works at the moment. My next debate with the guy laying it out will no doubt be about which regulator to use.