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Old 6th April 2013, 09:30 AM   #81
Zen Mod is offline Zen Mod  Serbia
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just match same polarity ones
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Old 6th April 2013, 10:58 AM   #82
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thanks & Хвала
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Old 6th April 2013, 02:25 PM   #83
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Quote:
Originally Posted by 6L6 View Post
In what I have experienced, yes.

The first time I got it stable was with the 1000pF compensation cap ( marked C3 and C4 in the V3 schematic) and 680ohm gate resistors (R13-16 in the V2 schematic)
have you tried this amp with high idds j-fets? say 8-10mA.
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Old 6th April 2013, 02:29 PM   #84
6L6 is offline 6L6  United States
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If I recall correctly, (I'm not near it right now...) the Idss of the driver pair is in the 8-9 range.
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Old 6th April 2013, 05:13 PM   #85
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For those of use without all of the insight most of you have, could you explain what C2 is doing in the circuit? I've read how Nelson states it may be needed to control ringing, and I'm looking at the schematic of V3 where C2 is shown as C3 and C4. Is it simply passing AC at a given bandwidth to the drains of the MOSFET where the given unwanted signal adds to negative feedback through the feedback resistors to the JFETs? Please excuse this simplistic explanation if it is way off base. Just trying to learn here. Thanks.
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Old 9th April 2013, 08:26 PM   #86
6L6 is offline 6L6  United States
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A bit more progress -- Well, actually a bunch of progress, as I have a fully-functioning stereo amplifier...

Click the image to open in full size.

Click the image to open in full size.

The good news - it is stable, it is working, the 5U chassis is keeping the heatsinks around 120F (49C) with .35v bias across the source resistors. There is room for more bias current.

The bad news is I currently have a ground loop problem, most likely due to the PSU wiring and general lack of wire organization. I knew I had a layout problem before I started, and wanted to utilize the DIY-friendly base as much as possible, but it's not optimal for this in it's current iteration. I have to get the bridge closer to the input caps and the FE board further from the transformer. I would like to have the wiring bundle a bit more organized, but that will happen regardless, I just plain cut some wires too long.

Concerning layout I have 2 ideas --
A) Place the FE board on the back panel. There's plenty of room in-between the RCA and Speaker jacks. That would get the driver board away from the transformer, but the wire bundle is still long.

B) Make a mount from long standoffs and some non-conductive material (Waferboard/perfboard comes to mind) and attach to the heatsink in-between and higher than the output boards. The wires would all be short. That can't be bad.

With either of those two options it will give me the space to tighten up the PSU, which is where I think the bulk of the problems are coming from. I knew the layout wasn't ideal, but vainly hoped that it would all work out. It needs a bit of work, but I'm sure it will be fine in the end.

The sound, for those of you who are curious, is completely fantastic. No surprise, really.
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Old 9th April 2013, 09:07 PM   #87
WalterW is offline WalterW  Netherlands
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Idea 2 is the right idea, I guess.
Shorter leads from the FE-boards to the gates of the MOSFETs is always better.
As a RF-engineer I don't like the long leads from the FE-board to the output-boards, even twisted with other (output) wires... brrrr.
I guess that is what is causing oscillation.
Remember it's a very wideband amp up to 1 MHz, it can even amplify AM signals
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Last edited by WalterW; 9th April 2013 at 09:12 PM.
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Old 9th April 2013, 09:48 PM   #88
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Looking good 6L6. Glad to hear of success. On the ground loop issue. Are you are sure it's ground loop or overtaxed toroid? V2 is a beast and 600VA should e a minimum. I used 400vA and had slight hum, but I think it was toroid really being pushed.
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Old 9th April 2013, 09:52 PM   #89
6L6 is offline 6L6  United States
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It's a loop. There is noticeable hum when the inputs are attached. It's quieter with them open or shorted.

The Toroid is 600VA and has a bit of mechanical hum, but I bet dual rectifier bridges will help with that.
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Old 9th April 2013, 10:00 PM   #90
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Got ya. Does sound like GL. Not going to be easy to solve.
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