"Double powersupply" LM3875 gain clone

OA51 · 28th February 2005, 11:53 PM

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...

Here is the layout for the standard kit...

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...

-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

homer09 · 1st March 2005, 12:01 AM

interesting.... i applaud your daring attempt

how does this compare to the snubber setup?

Mr Evil · 1st March 2005, 12:03 AM

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.

OA51 · 1st March 2005, 12:19 AM

I have not tried the snubbers...
(wanted to do something different....)

/Stefan

merlinx76 · 1st March 2005, 01:24 AM

Quote:

Originally posted by Mr Evil
Those are quite possibly the nicest cases I have ever seen an amp in

jeff mai · 1st March 2005, 02:16 AM

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.

OA51 · 1st March 2005, 07:48 AM

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

jeff mai · 1st March 2005, 08:19 AM

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.

peranders · 1st March 2005, 08:25 AM

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.

OA51 · 1st March 2005, 08:35 AM

Hi peranders...

Thanks...

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

/Stefan

28th February 2005, 11:53 PM	#1
OA51 diyAudio Member Join Date: Jun 2004 Location: Trollhattan (Saab town)	"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... Here is the layout for the standard kit... 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... -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 __________________ Electronic's - compressed smoke...

1st March 2005, 12:03 AM	#3
Mr Evil diyAudio Member Join Date: Aug 2004 Location: Behind you	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. __________________ http://mrevil.pwp.blueyonder.co.uk/

1st March 2005, 12:19 AM	#4
OA51 diyAudio Member Join Date: Jun 2004 Location: Trollhattan (Saab town)	I have not tried the snubbers... (wanted to do something different....) /Stefan __________________ Electronic's - compressed smoke...

1st March 2005, 07:48 AM	#7
OA51 diyAudio Member Join Date: Jun 2004 Location: Trollhattan (Saab town)	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 __________________ Electronic's - compressed smoke...

1st March 2005, 08:25 AM	#9
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me


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1st March 2005, 12:01 AM	#2
homer09 diyAudio Member Join Date: Jan 2004 Location: Montreal	interesting.... i applaud your daring attempt how does this compare to the snubber setup?

1st March 2005, 02:16 AM	#6
jeff mai diyAudio Member Join Date: Aug 2002 Location: Melbourne	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.

1st March 2005, 08:35 AM	#10
OA51 diyAudio Member Join Date: Jun 2004 Location: Trollhattan (Saab town)	Hi peranders... Thanks... See you in Gbg 2/4, will you bring any of your nice amps... /Stefan __________________ Electronic's - compressed smoke...

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