Are the electrolytic caps necessary in my gainclone? - diyAudio
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Old 27th July 2003, 01:18 PM   #1
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Default Are the electrolytic caps necessary in my gainclone?

I am using a regulated DC lab power supply for my LM-3875 gainclone.

Is it possible to just omit the electrolytic caps and make a more 'minimal' gainclone?

By the way, did anyone compare the quality of the DC coming out from a regulated power supply and the conventional design, on the oscilloscope or other way?

Thank you in advance for your kind help.


Best wishes,
Russell SIT
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Old 27th July 2003, 01:58 PM   #2
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Default Re: Are the electrolytic caps necessary in my gainclone?

Quote:
Originally posted by Russell Sit
I am using a regulated DC lab power supply for my LM-3875 gainclone.

Is it possible to just omit the electrolytic caps and make a more 'minimal' gainclone?

By the way, did anyone compare the quality of the DC coming out from a regulated power supply and the conventional design, on the oscilloscope or other way?

Thank you in advance for your kind help.


Best wishes,
Russell SIT


If you reduce the bypass capacitance too much you run a very real risk of oscillation. If you don't have an oscilloscope to measure the effects I wouldn't try it. Personally, I wouldn't try it anyway. I just re-read the section on supply bypassing on the LM-3875 data sheet, and maybe it's because I haven't had my coffee yet, but it wasn't particularly clear. They seem to recommend anywhere from 10//.1uF to 470uF LOCAL supply bypassing depending on how you read it. (This isn't the same as the bulk capacitance used at the bridge rectifier.)

I've looked at the output of regulated supplies, and the cleanliness varies widely amongst the brands. Some cheap ones are really good, some cheap ones are really bad, some expensive ones are mediocre and some are amazing. It all depends on the model.

Scott
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Old 28th July 2003, 03:32 AM   #3
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Thank you for your reply, Scott.

The power supply is a Goodwill Instek GPC-1850D. I am running my gainclone at +/- 18v dc, which is probably the lowest voltage used among the other gainclones here.

Though manufacturer's published spec may not represent the actual performance, following is the from the spec sheet:

Constant Voltage Operation:
Line Regulation: <0.01% +3mV
Load Regulation: <0.01% +3mV (rating current £3A)
<0.02% +5mV (rating current £10A)
Ripple and Noise: <1mVrms 5Hz ~ 1MHz
Recovery Time: <100µs (50% load change, minimum load 0.5A)

I do not have a scope and so have no idea whether it is up to spec or not.

I raised this question because of the high power supply rejection ratio of the LM3875. If my power supply is good enough, probably I can eliminate the electrolytic caps, and the gainclone is then basically a chip with 2 resistors plus the dc power supply. Alternatively, if capacitance is still needed but of a much smaller value, then I will probably give those big polystryene capacitors a try.

Thank you very much for reading this thread.



Best wishes,
Russell SIT
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Old 28th July 2003, 04:08 AM   #4
ronc is offline ronc  United States
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I just use +12v,-12v battery P/S.The sound is exceptionally clear,fast and has very tight bass. You can even use no caps on the P/S legs but i run 6.8uf because it appears to make the highs brighter at higher volume.
ron
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Old 28th July 2003, 07:26 AM   #5
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Quote:
I am using a regulated DC lab power supply for my LM-3875 gainclone.
The 3875 powered by regulated supply is certainly not a gainclone. Nor is it minimal. It may be better, it may be worse, but a gainclone it aint. Very few regulated supplies will do justice to a good GC. While there will no doubt be improvements in the bass and image stability, many other areas will suffer. And no, the oscilloscope won't tell you much.

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peter
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Old 31st July 2003, 12:24 AM   #6
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Quote:
The 3875 powered by regulated supply is certainly not a gainclone. Nor is it minimal. It may be better, it may be worse, but a gainclone it aint. Very few regulated supplies will do justice to a good GC. While there will no doubt be improvements in the bass and image stability, many other areas will suffer. And no, the oscilloscope won't tell you much
Care to elaborate on what makes a "real" gainclone? What "many" other areas will suffer from a regulated supply? How could a regulated supply make any difference if coupling capacitors are still used?
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Old 31st July 2003, 12:43 AM   #7
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Hi,

Quote:
Very few regulated supplies will do justice to a good GC. While there will no doubt be improvements in the bass and image stability, many other areas will suffer.
Sorry, but I tend to disagree to a point.

It sounds like a sweeping generalistion even though I'm sure you didn't mean it that way.

As far as electrolytic caps in the PSU go, yes, I'd use them to the best of their benefit...their imperfections are such a great asset for filtering out unwanted interferences.

Horses for courses fellas..
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Old 31st July 2003, 01:56 AM   #8
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hmmm...since russel hasint been back to acknowledge any replies I guess I wont give him any feedback LOL
But if anyone cares I would atleast use some 470 uf caps and bypassed with some 0.1 uf caps very close to the chip to smooth out any transients that are picked up between the power supply and amplifier. I am a big fan on tantulum caps near the supply pins as I have had great results with them.


DIRT®
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Old 31st July 2003, 03:16 AM   #9
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I'm sure Russ will soon be back to check in on his question. Things here seem to stay dormant and then explode with activity when one is not looking. . .

As I understand the question, you are using a packaged regulated DC supply to power the op-amp (as by conventional means this isn't a GC). From my reading here, a GC would require minimal filtering, due to the chip's high rejection ratio. Proponents could say this makes the amp feel 'faster' whereas those that do a more robust filtering job (larger caps) seem to feel that the bass improves.

I would suspect that if it is a lab supply, it probably has a lot of built in filtering, so I would suspect that additional capacitance would be redundant.

I think there are a lot of ways to make an amp based on the
LM 3XXX or similar op-amps. I think the term GC invokes many emotions from various people, so you might receive many different responses.

I think, technically, to make what is considered a GC, you need small filter caps right at the chip, and that's it for filtering. To make an implementation of a chipamp which you can readily enjoy, a DC regulated supply could be a viable source, while it might not be considered optimum by many.

Sandy.
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Old 31st July 2003, 03:52 AM   #10
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Quote:
Originally posted by Russell Sit


I do not have a scope and so have no idea whether it is up to spec or not.

I raised this question because of the high power supply rejection ratio of the LM3875. If my power supply is good enough, probably I can eliminate the electrolytic caps, and the gainclone is then basically a chip with 2 resistors plus the dc power supply. Alternatively, if capacitance is still needed but of a much smaller value, then I will probably give those big polystryene capacitors a try.

Thank you very much for reading this thread.



Best wishes,
Russell SIT

While the part does have very high PSRR, it is also a wide bandwidth, high gain, high power, high current device. All these things indicate instability when noise is about. Noise can creep in through a bad solder joint, wiring (even when it's done well), speaker cables, etc.

In electronics if a chip vendor recommends a part somewhere I tend to take that seriously, as 99.9999% of their customers are looking to save a 1/15 of a penny by pulling this part off here, and that part off there. If you have too many external components compared to your competitor, you will lose market share.

Since you don't have an o-scope, I would be more than hesitant to pull of those caps.

Scott
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