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
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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fender4 said:
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.
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
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
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.
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.
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
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
analog_sa said:
if resistors from different batches and isolation pads can have audioable impact on sound, it would only be natural to see the same for pcb layout and better wires.
Next thing you know, people are going to claim that the use of Honda lawnmowers two towns over will make their amps sound nice, airy and open open the soundstage,
All i use is battery P/S. One time , just for grins and giggles, i made a 3 component amp.10k res-300k res and the chip(no caps anywhere).pin 7 went straight to ground.It worked very well but i felt the need for more control and the offset was higher than i really wanted (.028 v) And not having a cap on the input makes me a little nervous.Of course it had to have a pre.
ron
ron
All i use is p2p wiring.Never will ave a problem if you are careful.I believe analog is correct.
I use a 500k multi turn trimmer between pin 7 and ground to corrct for offset.If the amp has been run in and normal operation temp. i just dial in the offset.I have found out that it will drift slightly but only about .001v.
ron
I use a 500k multi turn trimmer between pin 7 and ground to corrct for offset.If the amp has been run in and normal operation temp. i just dial in the offset.I have found out that it will drift slightly but only about .001v.
ron
I learn something new every day! I just assumed that the change in offset was due to the new layout, but after some reading, analog_sa is correct in that this probably didn't have any effect on the offset.
I know I'm not an electronics expert. I just like simple projects like this to "learn through experience".
Ron, I think I might try the trimmer pot idea. I would feel much better about trying new (other's) speakers with a minimized DC offset. (Sorry analog_sa, no Lowthers...yet ).
-f4
I know I'm not an electronics expert. I just like simple projects like this to "learn through experience".
Ron, I think I might try the trimmer pot idea. I would feel much better about trying new (other's) speakers with a minimized DC offset. (Sorry analog_sa, no Lowthers...yet
).-f4
I don't think i'd ever go back to Lowthers; informative and fast as they are, the price is incredible headache after even a short listen. Even with PX25 SETs. I wouldn't mind using a pair just to audition amps, passive components, cables etc; for music: no thanks. Others certainly disagree.
cheers
peter
Heard lowthers before, an acousta cab wih 300b set amps.Yes the sound was dang good , fast and all but no bass to speak of.
At present i am using fostex Fe103e horns (my own design) and combined with the GC the sound is fanstastic. although i am forced to admit that SPL is limited in the small fostex.
ron
At present i am using fostex Fe103e horns (my own design) and combined with the GC the sound is fanstastic. although i am forced to admit that SPL is limited in the small fostex.
ron
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