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Old 19th October 2005, 10:21 AM   #1
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Default Wintermute's Gainclone

Well I haven't built it yet (but I am making progress) hopefully it will eventuate a little quicker than my MTM's are......

I decided to start a thread specifically about this amp so when I have questions I'll dump them in, whether asking questions this way will work or not I guess I'll have to see

OK so I have just whipped up a dodgy chassis for it (first try at doing my own chassis, and this is only a test amp, so don't be too harsh.... the way it is going it'll probably end up in the ugly amp thread anyway ).... It's going to be relatively compact and I'm wondering about the orientation of the torroidal (its a 300VA one).

I've taken a pic of where I think I'll mount it ( the box is sitting on it's front in the pic)

The blue rectangles represent where the LM3886's (I'm doing it p2p) will be (the other option is to use a shortened version of the heat spreader that came with the sink and they will then be in roughly the same place but hanging down....

Now I read that torroids radiate pretty much in a narrow band straight through the center so I'm thinking that this orientation should be ok.... any cautions?

The main thing I don't like about this orientation is that it will not be symetrical ie the toroid is closer to one chip than the other, my original thought was to mount the torroid vertically on the front wall smack bang in the center.

I'm not worried about the asthetics as this amp will be used as a test bench amp

Tony.
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Old 19th October 2005, 10:58 AM   #2
inguz is offline inguz  United States
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If you're building P2P, you might want to leave yourself a *lot* of space around the chips. Repairs & adjustments are much trickier when you can't just lift up a PCB. So I'd consider putting the trafo on the base or behind the front panel.
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Old 19th October 2005, 11:12 AM   #3
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Thanks inguz

behind the front panel will certainly leave a lot more room around the chips, and also has the advantage that then both sides can be removed.... so that is probably a better bet. I'm a little paranoid about transformer induced noise as I have had problems with my other amp with this.

The base (as a mount point) is a bit tricky in that it makes it harder to mount my PS board because the base actually has the smallest dimentions (because I decided to put the heat sink on top) and the ps board wont fit beside the toroid!

Tony.
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Old 20th October 2005, 06:15 AM   #4
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OK another question, (bet no one saw that coming

when implementing the signal ground star point in p2p what method do people use?

I'm thinking iether just solder all the leads together at one point (messy and possibly dangerous, due to possibility of shorts), or use some sort of anchor point and connect to that. How have you done it

the PS star point is taken care of already

Tony.
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Old 20th October 2005, 08:21 AM   #5
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Quote:
Originally posted by wintermute
OK another question, (bet no one saw that coming

when implementing the signal ground star point in p2p what method do people use?
I solder all the leads together on the appropriate phono-out ground tag.
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Old 20th October 2005, 11:38 AM   #6
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Thanks Ropie, that's something I hadn't considered at all. I better start considering the layout of my rear panel too

Tony.
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Old 20th October 2005, 01:16 PM   #7
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ok still on the signal ground......how important is it that all leads terminate at the exact same point?? I've got 7 gnd connections to do, I'm thinking of making pin 7 on the chip the star earth point, though this could result in not all leads soldered to the same exact point..... bad idea??? I'm thinking this may minimise the length of the runs to the signal ground star...

Every time I look at the chip I'm thinking "what have I got myself into" the pins are a bit closer together than what I'm used to for p2p work!! Tansistors with three pins can have their leads spread apart easily

edit make that 6 I'll run the cz gnd separately back to the ps gnd actually thinking about it, should I run the supply bypass cap gnds back to the PS gnd separately too?

I'm using brians NI schematic..... I was originally thinking everything except the spkr returns terminated in a single star which then runs back to the PS ground..... now I'm thinking R2, NFB cap, input gnd, pin 7, local star with link back to PS. each bypass cap pair, wire back to ps. Cz wire back to ps. mute cap wire back to ps. I've just re-read the datasheet, and my the answer to my first question is probably a big fat "don't it's a bad idea"



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Old 22nd October 2005, 12:23 AM   #8
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Default Progres report

well I decided that I better start actually soldering something

This is what I have so far...... taking my time, this is fiddly work!!

Tony.
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Old 22nd October 2005, 06:39 PM   #9
Vikash is offline Vikash  United Kingdom
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Keep going Tony, I'm a following Here's my LM3886 P2P work for some more data, and : Mark's intricate LM4780 work is inspiring.
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Old 23rd October 2005, 01:38 AM   #10
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Thnaks Vikash I hadn't seen either yours or Marks , I've been trying to see as many as possible to give me inspiration I'm trying to get it as compact (around the chip) as I can without going crazy

Decided to have three earth returns..... one is the PS bypass caps and the mute cap. One is the NFB, chip ground, and signal grounds, and the other is the output compensation, the speakers will also have their own runs.

I've made some more progress, but when I checked my camera I don't have the latest pic (at GF's place now), I've attached the latest pic I took which has the rest of the FB components on the chip, I now have the input resistors and signal ground return wire too. only think left to do (for the first channel) is to put the output comp resistor and cap on, and solder the bypass caps and mute circuit to the chip

please excuse the poor quality and angle of this pic

Tony.
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