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Old 1st February 2007, 07:06 PM   #1
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Default critique my amp layout please

Hi all,

FINALLY got my LM3875 together (using Peter's boards and parts). I am very happy with the results, although I find it a little bright and perhaps lacking some bass umph. I am unsure which of the following could be the cause of this:

- choice of parts (electronics)
- grounding layout
- smallish supply voltage of +-21V
- maybe something is not hooked up just right
- maybe it's just my speakers (70's Rega, dome tweeters, 7" woofer, ported)

Could you please look over the picture attached below and let me know if you see anything strange. I also have a circuit sketch of my layout which I can scan if needed, but I think the left channel in the picture is pretty clear.

All comments or suggestions are welcome!

Thanks
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File Type: jpg amp_1_sm.jpg (87.0 KB, 534 views)
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Old 1st February 2007, 07:14 PM   #2
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Default circuit sketch

actually, maybe this will be useful
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File Type: jpg circuit sketch.jpg (68.2 KB, 439 views)
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Old 1st February 2007, 07:52 PM   #3
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It looks very tidy.

It might be worth having a search on here for disconnecting networks to link your chassis ground point to earth. Nordic has posted plenty of useful info on these.
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Old 1st February 2007, 08:16 PM   #4
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Quote:
Originally posted by ianpengelly
It looks very tidy.

It might be worth having a search on here for disconnecting networks to link your chassis ground point to earth. Nordic has posted plenty of useful info on these.
this sounds interesting, could you please provide a link?
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Old 1st February 2007, 08:25 PM   #5
CarlosT is offline CarlosT  United States
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Looks good...all the capacitors are in the right way? Do you have a lotta capacitance at the power supply or are you just using Peter's rectifier board?

Interesting...two Alps pots...one for each channel.

Very tidy...I mighta punched out some holes underneath the heatsink and on the cover and I mighta twisted the wires coming in from the RCA jacks but that's just nitpicky esoterics...

Is it quiet with the little resistors going to chassis ground? Why have a chassis ground at all?
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Old 1st February 2007, 09:00 PM   #6
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I found this quote from AndrewT:

"Use a disconnecting network to connect the safety earth to the audio ground. The disconnecting network can consist of some or all of the following wired in parallel:-

Power diode wired in inverse parallel.
Power resistor.
High frequency capacitor.
Power Thermistor"

Hope this helps!
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Old 1st February 2007, 11:40 PM   #7
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Quote:
Originally posted by CarlosT
Looks good...all the capacitors are in the right way? Do you have a lotta capacitance at the power supply or are you just using Peter's rectifier board?

Interesting...two Alps pots...one for each channel.

Very tidy...I mighta punched out some holes underneath the heatsink and on the cover and I mighta twisted the wires coming in from the RCA jacks but that's just nitpicky esoterics...

Is it quiet with the little resistors going to chassis ground? Why have a chassis ground at all?
I'm using Peter's rectifier boards 1 per channel (1 xformer per channel also) with rather small caps on the boards (10uF Panasonic FC Capacitors). Can i swap these small caps out for something bigger for more bass? Will I notice much difference in swapping out the metal film resistors for better ones? (what is better?)

The amp is very quiet, no noticeable hum, although the DC offsets are different between channels

Regarding DC offset, should it be measured with the pots open or closed? My measurements are (using 10.3R resistor across output with no signal):

Left Channel: 14.7mV pot closed; 56mV max while opening; 35.6mV pot open

Right Channel: 26.8mV pot closed; 67mV max while opening; 44.8mV pot open

I burned up a resistor trying to measure DC offset with signal going through and the pot open, so I assume I'm not supposed to do that.

Do I NEED to connect CHG to chassis ground at all? Would it be adviseable to take those off? I only did it because Peter said to use 10R resistors before CHG on the boards ... Hmm ...

Quote:
Originally posted by ianpengelly
I found this quote from AndrewT:

"Use a disconnecting network to connect the safety earth to the audio ground. The disconnecting network can consist of some or all of the following wired in parallel:-

Power diode wired in inverse parallel.
Power resistor.
High frequency capacitor.
Power Thermistor"

Hope this helps!
I don't understand that at all. I'll have to do some research on my own about that. Thanks anyhow.
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Old 2nd February 2007, 12:26 AM   #8
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I just re-measured DC offset to find the right channel has 0mV regardless of pot position, while left channel remained as before. Strange ... so I measured the CHG resistors to find 10.3R on left channel, and 1R on the right channel. What? I guess that resistor was the one that sizzled.

Does this mean that I should replace the CHG resistors with wire and thus have 0mV DC offset for both channels?
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Old 2nd February 2007, 04:08 AM   #9
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The only thing I would recomend is moving the input wires further away from power wires.

Josh
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Old 2nd February 2007, 04:24 AM   #10
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"I'm using Peter's rectifier boards 1 per channel (1 xformer per channel also) with rather small caps on the boards (10uF Panasonic FC Capacitors)"

You are using only 10uf caps on the power supply boards?? This seems very small, I think a minimum of 1000uf would be more like it. Do you have pictures of the power supply??
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