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1970s Design Indulgence
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Graham Slee View Drop Down
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Points: 16097 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 2023 at 12:41pm
This is another thing that's been troubling me (taken from the NJL3281/1302 application note):

"The noise floor of the amplifier will also improve through the elimination of the oscillation at the zero volt crossing caused by the slightly under-biased condition created by the original circuitry"

Now, I know this doesn't use the NJL devices, it uses the MJL devices, but so does the circuit in fig.1.

My own thoughts on this is nothing to do with under-biasing as the circuit has more bias than just what the diodes allow. It is optimally biased as regards distortion.

It's just that by changing one MUR120 for a 1N4148 diode, meaning less of a voltage drop across the trimmer, in turn resulting in a lower trimmer resistance (roughly 5 ohms), I could swear it made a difference to the sound.

Could this be anything to do with "the oscillation at the zero volt crossing"?

I have never been able to detect any oscillation, but I assume as the transistor manufacturer is saying there is, it must be.

Rather odd though, that I've never been able to measure any oscillation. Having said that, there is always a hump of 50kHz noise in the FFT display, whether measuring the amplifier, or just with the probes dangling and no amplifier in the room.

I am now wondering if the scope probes have a problem? Or is it simply that the APx525 analyser is connected to the laptop it needs to be able to read and view?

What I can say is there has been no such thing as luck with this design.
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Fatmangolf View Drop Down
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Points: 8623 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 2023 at 7:55pm
Originally posted by morris_minor morris_minor wrote:

Many thanks to Ifor for bringing along the amp! As the attached photo shows it's now installed in my living room system hooked up to a pair of KEF LS50s and driven by an Audiolab AV processor/pre-amp with sources of vinyl (Reflex M and Elevator, Yamaha PX-3/Zyx R100) and digital from a Roon driven Squeezebox Classic using the Audiolab DAC. I normally use a sub with the LS50s so will be trying the amp with and without this. Normally the amp I use is a venerable Audiolab 8000A from the era Graham's amp harks back to - so an appropriate comparison in many ways.



Nice.


Edited by Fatmangolf - 08 Feb 2023 at 7:56pm
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Graham Slee View Drop Down
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Points: 16097 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 2023 at 9:56pm
This seems to have solved it:

no stereo ground loop
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Graham Slee View Drop Down
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Points: 16097 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 09 Feb 2023 at 9:42am
Remember this?

artus ground
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Graham Slee View Drop Down
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Points: 16097 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 09 Feb 2023 at 10:11am
(from above)

That's from the radio laboratory of Decca, in 1965.

And the Quad 303 persisted with the "Dinsdale resistor" from 1967 to 1985...

In studio equipment the chassis ground went wherever it was found the noise spec became lowest.

And, where I once worked, the chassis ground went to the pot., and if there wasn't a pot., like in a power amp, it was anybodies guess! And chassis to power supply ground was always 100R||100n.

And take a look at Self's and Cordell's books.

Also Ed Cherry's designs.

BUT, a little info gathered here and there, often from chance encounters, this being due to me looking at a particular JVC circuit, came "why would they do it that way?"

So I drew it two posts ago, and it seemed to make sense. So I chanced it on my 7eventy, and it worked!

Signal to noise became identical on both channels, and distortion dropped away a little more. S/N was better too.

This being due to no opposite channel "pull" of ground current, so no stereo loop.

Shall we start again?
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ICL1P View Drop Down
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Points: 2324 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 09 Feb 2023 at 10:19am
I don’t understand, but this sounds positive.
Ifor
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Reflex M & ACCESSION M, CuSat50, Majestic DAC, a Proprius pair.
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Points: 457 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 09 Feb 2023 at 11:46am
Yes  .....this '' bite size " post is chunky in itself but digestible 
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