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Old 19th June 2003, 10:55 AM   #1
PMM is offline PMM
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Question what do you think of this schematic?

here goes with the attachment, very sorry about image quality but I have had to shrink it a lot to get it to post. I may try to cut it in half if it is too difficult to read.
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File Type: jpg aem6000_4.jpg (82.8 KB, 4312 views)
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Old 19th June 2003, 11:18 AM   #2
raindog is offline raindog  Romania
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that looks like a VERY complicated amp especialy the input stage. is it an industrial build one?if can you obtain a pcb and you have the experience go ahead but otherwise i wouldn't advise you to undertake such a project
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Old 19th June 2003, 11:32 AM   #3
PMA is offline PMA  Europe
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Might be a good one. Differential and complementary symmetrical topology, JFET input stage and cascode circuit (may result in a good sound) - I would give it a try in case that I have had more time. But just in case that you are an experienced guy and have measuring instruments (scope, generator etc.).
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Old 19th June 2003, 11:44 AM   #4
sajti is offline sajti  Hungary
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Hmmmmm how can I tell You?

If You have some sponsor to order all the devices.....
I think the input stages are overkill for this output stage. Maybe JFETs are good as input stage, but this is the typical example for "less is more".
With the complementary differetial stage (Q11-14) the input stages are not necessary to use.
My point of view that You can get better quality if You redesign this amplifier and remove Q1-Q8.

Sajti
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Old 19th June 2003, 09:46 PM   #5
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A three gain-stage design. The final gain stage is notorious for quiescent current instability.

Outside of a configuration with gain in the output stage, i cannot conceive of any circumstances in which three gain stages are necessary, or required.
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Old 19th June 2003, 10:00 PM   #6
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yes you can get PCB's and yes i have built it. I did not mean this to be a guessing game, it is in fact an old "Australian Electronics Monthly" (now defunct magazine) design by David Tilbrook circa 1986.
I had to crop the edges of the page to get the image to post otherwise I would have left those details on.
The designer ackowledges that it is complicated but he was after best possible performance. My impression is that after years of diy audio as a hobby it is still the best sounding amp I have built and has been in regular use now for best part of 9 years.
My original question still remains though for the following reasons. Most circuits i have built have been based on Australian magazines as pcb's have been offered and also availability of parts for those circuits. The Australian mag circuits I have found to be a little unambitious in terms of sonic performance and are optimised for lowest risk in terms of performance when made by constructors of varying skill. I dont mean that to be a criticism either. We do get Elektor down here at some places but I always found the semi's difficult to source.
I guess i am after new challanges and that is a circuit that is going to be little more challanging but has to give better performance, or, do I build a second one of these and go active?
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Old 19th June 2003, 10:15 PM   #7
mlloyd1 is offline mlloyd1  United States
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this looks like one of those topologies with lots of gain stages to acheive a high open loop gain and compensate for the lower gain of the output stage.

was the intent of "best possible performance" really to get the lowest THD number possible?

That's what I thought

Be careful if you build it; sometimes those numerous gain stages conspire to make a wonderful oscillator.

mlloyd1
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Old 19th June 2003, 10:23 PM   #8
PMA is offline PMA  Europe
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PMM,
so I did guess well . I have built a similar one in the eighties ...
Pavel
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Old 19th June 2003, 10:29 PM   #9
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Hi mlloyd1,
yes one of the designers aims was to get lowest THD, remember it was 86 and that was everything back then.
It is still however the best sounding loudspeaker amp i have built( to date), mosfet inputs & outputs, multiple stages, older semis and all.
I dont have a scope but it does not appear to oscillate though i admit my only test is what i percieve to be excessive heat or not & whether or not it seems to be operating okay in general.
As far as acoustic performance what of the designs that seem to appear on this forum with regularity would, in your opinion, better this design?
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Old 19th June 2003, 11:06 PM   #10
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Question Three gain stages

The second gain stage appears to be to convert the signal from jfet diff pair to a push pull signal to pull the gates of the output mosfets too within a few volts of the rail. The second stage uses a lot of degeneration and appears to have a gain of about 7 for each of the complementary diff pairs. This stage has a very low gain and does the conversion of the signal and DC levels to swing the output mosfets close to the power supply rails. A pretty simple and linear gain stage for two diff pairs and two current sources. The final voltage gain stage also has emitter degeneration and a large enough resistor from the diff pairs to the rails to allow decent quiescent current stability. The final gain stage has 15 pF of base to collect compensation and the 10 ohm degeneration resistors and looks competently designed to me. I see no real basis for the assumption of high frequency stability or bias stability concerns at first look. Zip the schematic and repost post the file so I can be sure of the component values. This would be required to make any of the judgments expressed in the previous post with any degree of credible analysis. It is really not that complicated a design as one would assume at first glance and looks like a reasonable design to me. Gain stages with degeneration may be better than two high gain stages a higher and less linear gain. Three is much more to amplifier design than the number of gain stages. The design of each of the gain stages and sources is of much greater importance than something as simplistic as the number of gain stages.

Gene
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