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 Post subject: Gainclone critical path
PostPosted: 20 May 2010, 15:06 
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Joined: 18 May 2010, 09:33
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Hi all,

My question is what are the most critical paths in the Gainclone Amp Assembly? I'm pretty sure form reading this forum that the feed back resistor path is the MOST critical and should be as short as possible, so I plan on soldering that directly between the pins of the LM3875's.

The caps I have are screw terminal 10000 Uf 100V about 3 + 3/4 tall, so there will be some length of wire run from the caps to the amp, I was thinking about using some small poly caps right at the amp pins the BIG caps connect at. Does this sound like a "good thing?"

Then from the bridge to the caps, again some length of wire will be needed.

So to recap , what wire runs are most critical?

Thanks!

Alan


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PostPosted: 21 May 2010, 16:19 
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Location: Duesseldorf, Germany
Hi Alan ;-).

IMHO - the length of the wires is not really very important. It's important how you connect them and it's important
to have a ground rail that's able to handle the current-backflow to the rectifiers.. The ground rails should be really
massive or at one point (star grounding) to prevent loops or hum...

Poly caps are always fine. The LM has a very good ripple rejection and 2x10000uF is big enough.
The poly caps can help to reduce ripple currents, because they support the e-caps.
You should solder the poly caps close to the e-caps. The wire lenght from the power supply to the chips
is not very important.

The best way for the wiring is to twist all AC wires and don't mount any signal wires in parallel with the power
supply wires or the mains wiring... Build the wiring "straight" without any loops. Wire loops are the best "antennas"
or transmitting and resonant circuits...

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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 21 May 2010, 16:58 
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Joined: 18 May 2010, 09:33
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Thanks!,

That was very informative, I have not started the lay out yet but I am getting close and the cable issue was holding me back in some areas. What are your thoughts on the necessity of adding 10uf Polypropylene caps to the power caps. Its hard to source them here in Maryland, but someone suggested to use them?

Alan


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PostPosted: 21 May 2010, 18:55 
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The input / DC blocking cap is the most important IMO. Then the feedback resistor (keep this as physically short as possible - solder right to the chip pins) and then the resistors in the signal path.
Cheers

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 21 May 2010, 19:36 
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Wow, I wasn't even planning on using a input / DC blocking cap, most of the drawings I have seen do not use one, is this for speaker protection?

Alan


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PostPosted: 23 May 2010, 18:34 
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Hi Alan,

where did you see 10uF poly caps for snubbing the smoothing / tank caps??
Usually they are 10nF or 100nF (max. 220nF). They should be available nearly everywhere...

The input DC caps prevent the LM input section from incoming DC (from the source / player) and they also
protect the source against DC. (DC > Source - if it happens...).
The LM chip amplifies the incoming DC and you'll have that DC on your speakers. Not very funny!
You could ommit the input cap if you're very(!) sure that your source is free of DC.

I'm using input caps (nearly) everywhere and it never was a drawback ...

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Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 23 May 2010, 19:03 
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Great, thanks,

What is the recommended type and value of cap?

Here is the reference of the 10Uf Polypropylene caps.

Attachment:
5-23-2010 7-57-06 PM.jpg


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PostPosted: 23 May 2010, 20:12 
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Ah, now i got - Mark's Synergy... ;-).

You can use 10nF as snubber caps for the bridge rectifier. Good thing so far...
Wimas, orange drops, vishays, ROE, Evox-Rifa, Epcos or other metalized poly caps (MKP /MKS / MKT) are good here.
Snubbing the filter / tank caps (or line caps) with 100nF is more than OK (same types like above).
It helps out with the things Mark already wrote. The 10uF caps are not neccessary and hard to get (IMO).
The LM3875 has a very good supply and ripple rejection. The same like on the LM3886's i'm using.
I'd rather suggest to add a 4K7 / 2W bleeder resistor and a "boucherot network" (10R + 100nF MKP) to the supply.
In the same manner like the TDA2050 power supply (link below - .sig). That sorts out the most problems.

And if you don't use a Line filter, add a 220nF / 400VAC "X2" cap between L and N - plus a 10nF in parallel to the power switch.

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Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 24 May 2010, 10:21 
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fotomaticon wrote:
Wow, I wasn't even planning on using a input / DC blocking cap, most of the drawings I have seen do not use one, is this for speaker protection?

That cap is optional and the best sounding cap is no cap at all. Just note that if you don't use a DC blocking cap you must be sure your source is free of DC otherwise the circuit will amplify the DC and cook your speakers. Satellite receivers can be bad for this. Once your gainclone is built, the first thing you should do is check DC offset before you hook it up.
Cheers

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 24 May 2010, 11:01 
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Ok, If I do go with a DC blocking cap what is the best type of cap and what value should be used.

Alan


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