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Old 6th June 2009, 04:33 PM   #331
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Hi . Gareth. Your latest image looks like the Roenders's amp , at least in principle. He cascoded both LTP and the balanced VAS to come up with his final amp.

Quote:
Not sure yet if this common base has enough drive capability for the output.
Here you hit the "nail right on the head" concerning my design choices. I was "torn" between roender's amp and the symasym. With more than a couple OP pairs , I needed more drive. While I do admit that the common base sounded better (by a VERY small margin), I would of needed a triple EF and 4 extra components for the LTP/VAS (cascodes) to both be able to drive a large OP and run the common base precisely.

While I thought that using no typical Cdom was real cool (wow , the speed- perfect 50Khz squares with almost 100 v/us slew) , it was very hard to differentiate the sound from the enhanced symasym. (both sounded great)

Another point to make , the ideal balanced VAS does use a CM, not on the input pair , but right on the VAS. Some , like Roenders and MJL's , even use a Wilson mirror CM.
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Old 6th June 2009, 05:19 PM   #332
Bigun is offline Bigun  Canada
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Thanks OS. I think I am finally starting to understand some of these things, way more complicated than I thought only two weeks ago - I guess the more you know, the more you realize you don't know.

I am lucky enough to have a Patchwork amp on my desk, I agree, MJ's design is very nice.
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Old 6th June 2009, 07:30 PM   #333
Bigun is offline Bigun  Canada
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OK, it's sunny and I have to stop 'work' and go make a BBQ etc.

But I have started on the 2nd channel. No changes to the pcb. I will use these first two channels for the rear speakers (I will consider improvements for the front three).

And to prove that pcb making frustrations are behind me, (top image) toner transfer worked nicely first time, (middle image) etched and (bottom image) drilled (need to re-drill for large components):
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Old 7th June 2009, 11:57 PM   #334
Bigun is offline Bigun  Canada
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Default And finally

I now have a 2nd channel of TGM1. I checked dc offset (both channels too high of course), adjusted the output pair biass current, then connected it up. First listening test was with the DIY Fostex, a much cheaper 'fuse' than my PMC speakers

I find these Fostex 127 drivers really detailed and the amps have nothing bad to say about the music at all (except the hum that I haven't fixed yet which is louder with these more sensitive speakers). It sounded very good to my ears.



Still lots to do, but being able to sit and listen in stereo is a key milestone. And a good time to thank everyone again for their help and encouragement, you made a good amplifier!

I'm not sure if this is the last of the TGM1's - it wasn't intended to be one of those amps that others would build and the two channels I have will go into service for the rear HT channels. Next up I have a chance to try and build the TGM2 for the front channels and learn some more.

Still sounds good on the PMCs now, but my initial feeling is that the Fostex units make a surprisingly good match with this amplifier. We know this type of topology is a little warmer than some of the more accurate amps and these speakers sound to my ears to be very much at home with this type of amplifier. More listening time is needed.
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Old 8th June 2009, 12:00 AM   #335
AKSA is offline AKSA  Australia
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Congratulations, Gareth!

I do agree about the Fostex, too. These speakers are ideally suited to SETs, with their warm, low power. Could it be that your amp has some H2/H3 built in?

Cheers,

Hugh
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Old 8th June 2009, 01:02 AM   #336
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Quote:
Originally posted by AKSA



Besides, where are the double blind tests which clearly show 100% correlation between low measured, single tone THD and subjective listening pleasure?

Have a nice day!
I'm curious could you elaborate? Is that low as in some is bad and the absense is good or is that low as in none is bad and some low amount is pleasurable. I am really curious since you did not quote the source.

On vinyl it's moot there is so much low order tracking distortion for any real cartridge the electronics are irrelevant.
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Old 8th June 2009, 01:20 AM   #337
Bigun is offline Bigun  Canada
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Hugh,

You have found me out

It really hasn't been an objective/subjective issue with me, I came to this without the weight of all the historical discussions that have been going on in the audio world. All I needed to do was build it and listen to it in comparison with my Bryston (relatively free of harmonic colour).

Another hobby of mine is photography. Before it went ClassD (errr, I mean digital) people would evaluate different film stocks. Manufacturers publish detailed specifications for their film and add a subjective characterization by calling it 'vivid' or 'warm' etc. Photographers choose a film based on their subjective evaluation of the results. The strange thing is, they don't seem to have a conflict between objective and subjective.

I have decided that my next project will be stereo and I will design the amplifier to allow me to change its characteristics to suit the speaker and the type of music I am playing.
I'm sure that some people do this in the digital domain (like photographers) and then output via a colour-less amplifier - both solutions are valid.
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Old 8th June 2009, 03:57 AM   #338
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Hi Scott,

Quote:
I'm curious could you elaborate? Is that low as in some is bad and the absense is good or is that low as in none is bad and some low amount is pleasurable. I am really curious since you did not quote the source.
I asked for the double blinds that show a correlation. I have never seen any, have you by any chance?

All amps distort, and vinyl distorts horrifically as you mention, mostly H2/H3/H4.

Zero distortion would be good, agree. But it's almost unrealisable. And global negative feedback, while reducing amplitudes, seems to synthesize artefacts well into very high harmonics. Doubtless these beat, and produce audible outcomes.

I'm very concerned about very low levels, below -80dB, of H5, H7, H9. These to me are a primary concern, because they appear to subliminally cause problems.

Some low amount of H2/H3 appears to be pleasurable to many, viz the vinyl and tube set.

THD is an rms summation; gives no real indication of the profile of the distortion. This harks back to Hiraga's notions of the monotonically decreasing distortion profile, which is certainly what I aim for.

My source is normally the ubiquitous CD; I use a tube preamp to add a bit of low order, but the equation changes with a vinyl rig. You essentially need no tube preamp then.

Further, it's a fact that people's tastes vary. Some like utterly distortion free sound, others like a bit of 'warmth'. Consequently good design is something of a moving target.

Liked that recent circuit of yours, very clever,

Cheers,

Hugh
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Old 8th June 2009, 05:26 AM   #339
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Quote:
Originally posted by AKSA


I'm very concerned about very low levels, below -80dB, of H5, H7, H9. These to me are a primary concern, because they appear to subliminally cause problems.

Cheers,

Hugh

Hello Hugh

Maby those very low levels of H5, H7, H9 do mix-up the way the brain are doing the psychoacoustic reconstruction of the music we are listening. So we could lost some soundstage and feel some kind of discomfort wen there is those H5, H7, H9 distortions harmonics in a amp.

So assuming that the brain use high frequencies as psycoacoustic marker (or references) to find out what was the sound levels, the positions of instrument in space and the sound personality of them, if the distortion do have some high frequencies harmonics the psycoacoustic marker are poluted and the brain cannot reconise the fine details and the psycoacoustic references, so it try to reconstruct the music with some loss, difficulties and discomfort.

But if you mask those high frequencies harmonics distortions with some low level and low frequencies harmonics, maby the brain will reconstruct the music with less difficulties, because that the poluted high frequencies psycoacoustic marker are masked, and the brain can use it memories of sound to compensated and give a better or a less stressing result.

Bye

Gaetan
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Old 8th June 2009, 06:26 AM   #340
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Gaetan,

Certainly sounds plausible, but in the absence of measurement, I can't verify.

Insofar as this is detective work the thrill of the chase is good fun, isn't it?

Ciao,

Hugh
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