"Double powersupply" LM3875 gain clone - diyAudio
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Old 28th February 2005, 10:53 PM   #1
OA51 is offline OA51  Sweden
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Default "Double powersupply" LM3875 gain clone

Hi

I have put together a BGT kit were I tried to address the problems with weak bass...

It's basically a LM3875 BGT kit with an extra parallel power supply with larger caps...

The enclosure is almost the same as my previous 4 Hypex amps, byt with heat sinks instead of venting holes...

Click the image to open in full size.

Click the image to open in full size.

Here is the layout for the standard kit...
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Transformers are 2*24 V 165VA...


in parallel with the standard kit I made an extra power supply consisting of double rectifiers, 1 Ohm resistors, 15000µF BHC Aerovox caps, Rifa 4 µF metalized polyprop going to fuses (one for + and one for -, gives me the possibility to "disconnect" the extra power supply for different tests) and via diodes on to the BGT PCB...

The purpose of the diodes is to create a voltage drop compared to the standard PSU so that the big caps will only "kick in" when needed (transient music signal, a lot of bass etc.)
They also make it impossible for the standard BGT rectifier to charge the big caps... The resistors in the extra PSU will dampen any spikes in the rectifier, maybe making it a little slow, something I can afford since I have the fast BGT PSU intact...


Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

-it really seems to work

The bass is much better articulated compared to the standard setup and I have not noticed any sonic degradation of the mids and highs...

/Stefan
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Old 28th February 2005, 11:01 PM   #2
homer09 is offline homer09  Canada
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interesting.... i applaud your daring attempt

how does this compare to the snubber setup?
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Old 28th February 2005, 11:03 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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Those are quite possibly the nicest cases I have ever seen an amp in

I see you've added shielding to the interior, but there are some large gaps. Shielding is only effective for wavelengths much larger than the gaps, so I would suggest making them as small as possible.
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Old 28th February 2005, 11:19 PM   #4
OA51 is offline OA51  Sweden
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I have not tried the snubbers...
(wanted to do something different....)

/Stefan
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Old 1st March 2005, 12:24 AM   #5
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Originally posted by Mr Evil
Those are quite possibly the nicest cases I have ever seen an amp in

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Old 1st March 2005, 01:16 AM   #6
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Very, very clever idea! I may have to try it!

Do you bleed any current from the larger cap supply? This might keep the voltage from bouncing up when the diodes stop conducting.
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Old 1st March 2005, 06:48 AM   #7
OA51 is offline OA51  Sweden
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I don't bleed them at this point, seems like a good idea...
I'm also thinking of putting small 0.1µF caps over the diods...

Any size suggestions for bleeder resistors?

/Stefan
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Old 1st March 2005, 07:19 AM   #8
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Quote:
Originally posted by OA51
Any size suggestions for bleeder resistors?
I believe the idle current of the chips is around 35ma each, which seems like a good amount to bleed. With 24V rails that's roughly equivalent to 330 ohms across each rail. Use minimum 10W resistors to have a healthy derating.
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Old 1st March 2005, 07:25 AM   #9
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OA51, you couldn't help yourself in the long run.....

Good of you to show the rest of the world your projects instead of only to swedes.

Nice work indeed.
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Old 1st March 2005, 07:35 AM   #10
OA51 is offline OA51  Sweden
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Hi peranders...

Thanks...

See you in Gbg 2/4, will you bring any of your nice amps...

/Stefan
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