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Old 10th July 2003, 08:00 PM   #1
fender4 is offline fender4  United States
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Default Is my Gainclone legit?

I built a pair of LM3875 "Gainclones" over 1 year ago based on the Thorsten schematic, and I have enjoyed it since day one. Just out of curiousity, I built a new channel up today...a stripped-down version. The schematic is below. Other than the cheapo 12-0-12 Radio Shack tranny and bridge, it only has 4 parts: 2X1000uf supply caps, a 220k resistor between pins 3 and 8, and a 10k resistor from 8 to input. No volume pot right now. All grounds meet at a "star" ground.

I didn't expect anything much, and a certainly didn't expect any stability. To my amazement, the "Gainclone Lite" has been running with CD music source for about 4 hours now with clean sound, NO noise, and 0.01mv DC offset.

I don't have access to a scope, but if this is any indication of stability--it's been running all this time without a heatsink! That's not intentional...I was just testing it initially, but it never really got that hot and the protection never kicked in. BTW, the chip is the LM3875TF, isolated package. I haven't really assessed the quality of the sound because it is currently hooked up to my test speakers (junk).

Is this just a fluke? I'm really enjoying the novelty of the minimalist approach, but this is almost TOO easy . Any input?

-f4
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Old 10th July 2003, 08:03 PM   #2
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Default Re: Is my Gainclone legit?

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Originally posted by fender4
Any input?

-f4

f4, my version is even simpler than yours: it doesn't have the two electrolytic caps (but I do have two in the power supply).

so mine has one chip and two resistors. (on a heatsink, ). and it does run. Couldn't tell if it runs well or not. putting in two decoupling caps makes no difference but I do plan on having them there.
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Old 10th July 2003, 08:09 PM   #3
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Hi!

Mine look (principically, since I use BB chips) exactly the same... don't need volume pot and decoupling cap...

And since the rule so far seems to be "the less parts you need to achieve stability and no-noise, the better", I thing your GC is very legit ...


Bye,

Arndt
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Old 10th July 2003, 08:22 PM   #4
fender4 is offline fender4  United States
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Millwood,
I guess we have the same thing because the two caps I listed ARE the power supply caps. I just included them in the schematic to illustrate the need to place them close to the chip. I have found that keeping the leads as short as possible between the caps and the chip keep RF noise to a minimum.

I'm glad to see others have had success with the same approach...that makes the decision to keep it the way it is much easier! I'm excited about getting this one up-to-speed with a better transformer and case (and heatsink) and comparing it to the more "complicated" GC that I already have.

-f4
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Old 11th July 2003, 09:07 AM   #5
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Hi fender4,

Let's see...
You are using the chip without a cooler and it doesn't get hot...
But I wouldn't assume that the reason is that it's not oscillating.
The reason is you're using a low voltage PSU and you're listening at low levels.
Just don't push it hard, or it will get hot.
But never near as hot as with a +/- 30~35V PSU.
With that PSU you should have around 10 watts RMS.
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Old 11th July 2003, 01:55 PM   #6
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how can he push it hard? it had no volume control.
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Old 11th July 2003, 02:00 PM   #7
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I always thought that volume control is a device that prevents from pushing it hard.
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Old 11th July 2003, 02:08 PM   #8
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f4

My clone is very similar; as all my sources are cap or transformer coupled i also have no need for input cap. Still, your claim for 0.01mV offset seems very optimistic, or should i say quite impossible?

cheers
peter
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Old 11th July 2003, 08:26 PM   #9
fender4 is offline fender4  United States
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carlosfm,
True, running relatively cool is a poor indicator of stability...there are many factors to consider with heat dissipation. I was pushing it "hard" in that I have no volume control, but if anything was being taxed it was the cheapo Radio Shack 12-0-12v tranny. It was warmer than the chip . Things are much more civil now--the chip has a heatsink, and I'm using a passive pre for volume control. I just need to find a nice transformer.

Peter (analog_sa),
Your suspicion is very valid...I meant to write 10mv, which is not that great. Since then, however, I have replaced the alligator clip leads with better wire and a tight layout. DC offset is now 7mv. Thanks for bringing that to my attention.

Thanks for the comments, everyone!

-f4
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Old 11th July 2003, 08:40 PM   #10
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f4

You may be keen to learn that tight layout and better wire have absolutely no effect on offset. Warming up (or cooling down) the chip and changing the PS voltage, otoh, does. Either way 10mV is perfectly fine by me if you're not using Lowthers

cheers
peter
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