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Old 25th February 2003, 03:21 AM   #1
cowanrg is offline cowanrg  United States
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Default gainklone problems, maybe...

hey everyone. i jumped on the bandwagon and threw together a gainclone. i used the minimized schematic, or rather the peter daniel schematic. unfortunately, im pretty much an idiot with electronics, and im having trouble with the schematic. i know its dead simple, but there is just a couple things i cant figure out.

i figured out the whole schematic, and put it together really close p2p, and it turned out well. however, i dont get the input part of the schematic. peter has been trying to help me out, but im just such a novice, i hate wasting his time. maybe someone else in here can help me out.

from pin 8, i have a 220K resistor connected to pin 3. then i have 10K, 1W resistor connected. but then what? i know there is supposed to be a 2.2uf (or 4.7uf) non polar cap there, but then where does that go? that whole area of the schematic just doesnt make sense... can someone just simple spell it all out for me? like, part A goes into part B? i looked on numerous websites trying to figure out how the diagrams work, and how to put them together, but no luck.

i hastily tried a few things, and got the amplifier to produce sound, but it was more like noise. you can tell what is playing, but its a bit tinny, but more importantly, buzzy and noisy. (however, this could be due to the fact i was using 2 12V 8AH lead acids for a temporary power supply...) I have yet to find a transformer to use for cheap. however, when it was playing, the sound beneath the "crap" was very nice. i was using a cheap old aiwa speaker to test, and i have NEVER heard such tight and pronounced bass from it, ever. i didnt think it was capable.

thanks to everyone in advance. i would really like to figure this thing out, becuase i think it could be an awesome project! im really trying to learn amplifier/electronics design, but im still such a beginner.
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Old 25th February 2003, 03:31 AM   #2
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This is the schematic you should use. 220K resistor is between pin 3 and 8. From your RCA jack the signal goes to the pot (if not using pot connect the input with 50k resistor to ground), then you connect the cap, the other pin of the cap connects to 10K resistor and the other lead of resistor connects to pin 8. So, pin 8 is connected with 2 resistors: 10k and 220k.
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Old 25th February 2003, 04:13 AM   #3
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Quote:
Originally posted by Peter Daniel
...(if not using pot connect the input with 50k resistor to ground)...
Ummm, why?

I assume you mean to replace the pot with a fixed resistor, correct?

se
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Old 25th February 2003, 04:23 AM   #4
cowanrg is offline cowanrg  United States
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ok, i was doing it wrong...

ill try fixing it and see. will using that lead acid power supply make the sound really bad, or do you just think its the resistor configuration i had?

btw, would this xformer be a good one? (just for something cheap... obviously a nice toriodal would be better) but would this at least give it enough power?

http://www.allelectronics.com/cgi-bi...245&type=store
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Old 25th February 2003, 04:35 AM   #5
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Quote:
Originally posted by cowanrg
will using that lead acid power supply make the sound really bad, or do you just think its the resistor configuration i had?
No reason the batteries would make it sound bad. Unless they're dead of course.

Be aware that with +/- 12 volt supplies, you're not going to get much but about 5 or 6 watts out of it. And unless you're loudspeakers are fairly efficient, don't expect to get much volume.

Be sure you're not driving the amp into clipping by trying to get enough volume out of your speakers.

se
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Old 25th February 2003, 04:38 AM   #6
cowanrg is offline cowanrg  United States
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yeah, im using 2 12 volts for a total of 24VDC, so there is enough voltage, but as far as power, who knows...

maybe it is clipping... i had the pre-amp running at a level that i would use for my normal speakers. but even at very low volumes, the sound was still crap. oh well, soon enough i will figure out whats wrong...
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Old 25th February 2003, 04:43 AM   #7
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Quote:
Originally posted by Steve Eddy


I assume you mean to replace the pot with a fixed resistor, correct?

Yes. That what I would do, but I didn't try it yet.
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Old 25th February 2003, 04:44 AM   #8
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Quote:
Originally posted by cowanrg
yeah, im using 2 12 volts for a total of 24VDC, so there is enough voltage, but as far as power, who knows...
Um, that circuit needs to be powered with a bipolar supply. In other words, +V and -V. If you're using two 12 volt batteries in series and running a straight 24 volts to the power supply pins on the chip, it's not going to work properly with that circuit.

You'll need to use the batteries like this:

<center>
<img src="http://www.q-audio.com/images/bipolar.jpg">
</center>

Quote:
maybe it is clipping... i had the pre-amp running at a level that i would use for my normal speakers. but even at very low volumes, the sound was still crap. oh well, soon enough i will figure out whats wrong...
It sounds like you may have it hooked up to a single 24 volt supply.

The LM-3875 can operate from a single supply, but not using Peter's circuit.

se
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Old 25th February 2003, 04:46 AM   #9
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Quote:
Originally posted by Peter Daniel
Yes. That what I would do, but I didn't try it yet.
Ok. But the question remains, why? What's the purpose of having that 50k ohm resistor to ground at the input?

se
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Old 25th February 2003, 04:51 AM   #10
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I tried it with my Aleph X and it definitely influences sonics. With the resistor in place, the amp seems to be cleaner (or smoother) sounding. I don't really know the reason behind it.
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