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Old 18th November 2003, 01:46 PM   #1
Bricolo is offline Bricolo  France
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Default Mi big gainglone

After many months, it's now 95% finished


I'm using a TDA7294 with Peter's IGC schematic. Basic metal film resistors, a wima coupling cap, FC supply caps.
The toriod is a 2*18V 225VA, made by Velleman. The diodes are MBR10100 (schottkys).

Signal wire is shielded monacor OFC. Supply wire is a basic 0.75mm˛ one. Output is Qed Quoudos (not sure about the spelling)


The chassis required a lot of work. A lot! It's made from 6mm aluminium plates. Self made knobs and feets.


More pictures will come during the next few days.

I have 4 solder points to do, and it's finished and fully working.
After this, I'll have to totally finish the chassis (adding damping material under the feets and a led)

I'm also thinking about anodising it, because you can see every fingerprint on it. But I would have to dismount everything...
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Old 18th November 2003, 01:48 PM   #2
Bricolo is offline Bricolo  France
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The amp hasn't been tested on a real loudspeaker now, so I can't tell you about the sound. I'm testing with a 0.01$ pc speaker
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Old 18th November 2003, 01:52 PM   #3
GregGC is offline GregGC  Canada
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Wow...!!!
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Old 18th November 2003, 01:53 PM   #4
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Hi Bricolo,

Very nice work!

But...
Have you powered it and tried it as it is?
It seams to me that you'll have a huge ground loop.
One single transformer and the two chips too far from each other.
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Old 18th November 2003, 02:02 PM   #5
Bricolo is offline Bricolo  France
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Carlos, where do you see a ground loop?

I'll post a diagram of the grounding when I'll find the time.
There are 3 local grounds in the amp:

-2 power grounds, one on each chip
it's very close to the chip, at the point where the 2 caps meet. They are connected to: the caps, the speaker+, (the muting and stdby circuits ground) and to the diode bridge's ground (this point is the star ground)

-a signal ground, where all the RCAs ground go to. The pot is connected to it. The non inverting inputs are connected to it. And all the signal cable shields. This point is connected to the diode bridge's ground


I can see no loop.


As for the power supply, I twisted the +/- wires together
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Old 18th November 2003, 02:13 PM   #6
joensd is offline joensd  Germany
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Very nice chassis!
Where did you get your metal parts from?

Cheers
Jens
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Old 18th November 2003, 02:15 PM   #7
Bricolo is offline Bricolo  France
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Quote:
Originally posted by joensd
Very nice chassis!
Where did you get your metal parts from?

Cheers
Jens

I got it from a local company who does aluminium windows
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Old 18th November 2003, 02:24 PM   #8
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Bricolo,

In my experience, it's much better to keep the two chips close one from the other, joint the two power grounds (the caps) and make it the star ground.
It's a question of centimeters, the more you move them apart, the more noise you have.
Close together as I say, the amp is dead quiet.
It's better to join the grounds locally than run a wire from each chip to the PSU.
That is with one transformer.
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Old 18th November 2003, 02:41 PM   #9
Bricolo is offline Bricolo  France
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I switched the scope on.
The amp looks quiet. More than when I tested it without chassis.

Before, I had small 100Hz peaks on the output, now I don't see them anymore.
I have some 50Hz, but the case isn't grounded (when I touch the case, the 50Hz is doubled), and I my portable cassette player has always be an antenna...
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Old 18th November 2003, 07:34 PM   #10
Bricolo is offline Bricolo  France
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Ouch!

I turned the thing on, and got signal on the right channel, but not in the left one.
They were both connected to 2 10R 10W resistors in //

After 10 secs, I saw (and smelled) some smoke. I immediately switched it off.

After some verifications, the left channel's +in wasn't connected to ground anymore, it was floating. The left channel was oscillating like mad, it burned my 2 10W resistors! (they still work, but they'r not green anymore but 50% green 50% brown)

When I had fixed everything, and controlled twice, I switched it on and the left channel didn't work, no signal. Oh, man! The left channel is probably fired

But I was lucky, it seems that the protection don't simply switch off when you unpuwer the chip. After 2 minutes it worked again

I think it's enough for this evening, I'll continue tomorrow
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