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-   -   LM3875 Kit Finished - Could use some more bass (http://www.diyaudio.com/forums/chip-amps/119513-lm3875-kit-finished-bass.html)

johan851 16th March 2008 06:08 AM

LM3875 Kit Finished - Could use some more bass
 
Hey all. I just finished building my first chipamp, the LM3875 kit from audiosector. It sounds pretty good, but it's a little bright and harsh in places where it shouldn't be. Vocals, occasionally, and cymbal crashes. I'm hoping that that will decrease with some burn in.

The bass seems lacking compared to the commercial amp I was using, and the bass I get in some cases seems distorted and uncontrolled. I was thinking that some additional capacitance might help with that, since the kit comes with only 1500uF per rail per channel. Would 3300uF capacitors be an appropriate replacement? And if so, what should I get? I don't see anything from Panasonic FC that goes that high. Thanks!

I should also point out that the chips don't really get warm at all. I have them on big heatsinks with good thermal paste. The transformer is an Avel-Lindberg 250VA 18V+18V toroidal.

ttan98 16th March 2008 01:05 PM

change the 22k feedback resistor to carbon type not metal film, this reduce brightness at the top end.

mount the DC brigde rectifier with bank of capacitors close to the chipamp, this will improve bass response.

analog_sa 16th March 2008 01:49 PM

And get some really chip-friendly speakers :) Lowthers, big old Tannoys will all sound great.

AndrewT 16th March 2008 01:58 PM

Hi,
point us to the schematic you have used/built.

I believe that PSU quality is critical to amp performance.
CarlosFM's implementation comes closest to this goal and uses +-27.2mF per channel not +-1.5mF per channel.

valleyman 16th March 2008 05:41 PM

The audiosector kits have a very minimal PSU. I would consider buying a (couple of) PSU kit from chipamp.com. That PSU is a snubberized design with large smoothing caps and I think most people agree it's a pretty decent PSU offering good performance without being too complicated or expensive.
I'd wait for some confirmation from someone with more experienced but I would be surprised if you didn't see improvement with that PSU

johan851 16th March 2008 08:48 PM

Quote:

Originally posted by AndrewT
Hi,
point us to the schematic you have used/built.

I believe that PSU quality is critical to amp performance.
CarlosFM's implementation comes closest to this goal and uses +-27.2mF per channel not +-1.5mF per channel.

The schematic is the stock 'classic' implementation from Audiosector. Here's the .pdf - schematic is on page 3. http://www.audiosector.com/nigc_kit-users_guide.pdf

Yeah, I was looking at the Chipamp.com PSU, because 1500uF seems like a pretty small amount of capacitance for a power amp. The audiosector kit puts a 1500uF capacitor on each rail of each channel, so 3000uF per channel. The PS board just has the rectifier diodes and some 4.7uF caps. It is nice that the 1500uF caps are close to the amp boards, though. If I switch out this PS for the chipamp one, can I just leave the 1500uF caps on the amp board?

I'll try using the carbon feedback resistor instead - that may help too.

seventenths 16th March 2008 09:06 PM

Yes you can...
I have been using 1000uF per rail at the amp and 16,000-20,000uF on the PSU board to good effect. As a matter of fact, it is usually suggested to have 1000uF or so right at the chip when using a larger bank of caps at the PSU


7/10

edit to add: Johan, I just noticed that you are a Seattle local. It would be interesting to compare implementations sometime...


johan851 16th March 2008 09:07 PM

Quote:

Originally posted by seventenths
Yes you can...
I have been using 1000uF per rail at the amp and 16,000-20,000uF on the PSU board to good effect. As a matter of fact, it is usually suggested to have 1000uF or so right at the chip when using a larger bank of caps at the PSU

Ok, great. That's what I figured, but I wanted to be sure. Lots of smaller caps in parallel provides a lower ESR anyway, I think, so having those smaller caps close by should help.

Peter Daniel 16th March 2008 09:08 PM

2 Attachment(s)
You can use more capacitance with my PS boards, if you want. You can also implement snubbers and Zobels, if that's your cup of tea:

http://www.diyaudio.com/forums/showt...429#post584429

johan851 16th March 2008 09:28 PM

Quote:

Originally posted by Peter Daniel
You can use more capacitance with my PS boards, if you want. You can also implement snubbers and Zobels, if that's your cup of tea:

http://www.diyaudio.com/forums/showt...429#post584429

Ok. I'm looking at the PCB here:
http://www.audiosector.com/images/lm3875_se_pcb.gif
And it's kind of hard to see what should go where. R3 is a resistor for the LED, right? C3 and C4 should be 0.1uF polypropylene, and what about Cz on the amp boards? What about R1/R2 on the PS board? And what terminals do the big 35mm caps connect to?


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