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Old 15th July 2003, 12:57 PM   #1
Mel is offline Mel  United Kingdom
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Thumbs up new gaincloner- question about bypassing

Hi there

my first post, and my first gainclone just built. For the moment, it's lashed-up on a scrap of wood while I tweak, but sounds good from the outset, if a little light in the bass (by the way, I normally run 300B valve monoblocks). The design broadly follows the Decibel Dungeon layout. It uses a separate preamp- Bruce Rozenblit's grounded-grid design actually, which I feel works well with this amp.

For now, I have some quick questions if I may...

(1) At the moment, I haven't added any bypassing caps across the 1000uF caps- should I fit them?

(2) Some designs show a 1uf cap across pins 1 and 4. Does this perform the same function as bypassing the 1000uF caps? (i.e. presumably it is not worthwhile adding 1uF cap, if 1000uF caps are bypassed).

(3) It seems to work OK without the 0.22R resistor on the output. Any point in adding it?

(3) Would increasing the value of the 1000uF caps add bass?

thanks in advance

Mel
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Old 15th July 2003, 07:20 PM   #2
Elkaid is offline Elkaid  Canada
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Quote:
(3) It seems to work OK without the 0.22R resistor on the output. Any point in adding it?
Not everyone are using them. Personally, I don't use them and I don't suffer from instability or other issues.

Quote:
(4) Would increasing the value of the 1000uF caps add bass?
Depending on you supply rail, some guys here said that the bass reproduction with LM3875 is better when using +/- 18 V instead of +/- 25V.

Many people here tend to prefer the sound using only 1000uF caps. In my case, I found that my gainclone was lacking a bit of bass to my taste so I added 4 X 10 000uF I had on hand which improved a lot the bass response.

I hope this helps !
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Old 15th July 2003, 09:35 PM   #3
Nuuk is offline Nuuk  United Kingdom
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Hi Mel,

Yes, it's really a question of trying these options to see which sounds best in your system.

If you don't have stability problems, you can forget the cap across pins 1 and 4, and the 0.22R on the output.

Try bypassing the 1000uFs - I think that it is better that way, others don't.

Bypassing the 1000uFs is for sound quality; the cap across pins 1 and 4 is for stability.

The 0.22R can help stabilty or improve the sound so you could try that if you want but I believe that Thorsten (who drew up the inverted GC design) strongly recommends using a high quality resistor in that position.
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Old 16th July 2003, 10:54 PM   #4
Mel is offline Mel  United Kingdom
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thanks for your comments- very helpful. As you say, the best thing is for me to try the options and see. Regarding the cap across 1 and 4, I thought that it effectively provided a 'short circuit' to certain frequencies, so that they bypass the chip i.e. performing a similar function to the bypass caps.

By the way, the amp has been in place for four days now, and I don't feel in any hurry to put the 300B monoblocks back! I'm not suggesting the chip amp is better, just different- both amps have their strengths and weaknesses. I have to say that I'm genuinly suprised at how good this amp sounds, considering!

thanks

Mel
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Old 17th July 2003, 07:52 PM   #5
Nuuk is offline Nuuk  United Kingdom
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Quote:
I have to say that I'm genuinly suprised at how good this amp sounds, considering!
If I had a pound for each time that I have heard similar comments about the GC, I could buy a Krell (not that I would want one as I am using granite for my GC platforms)
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Old 17th July 2003, 08:26 PM   #6
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Default Re: new gaincloner- question about bypassing

Quote:
Originally posted by Mel
(1) At the moment, I haven't added any bypassing caps across the 1000uF caps- should I fit them?

Mel

as a minimalist, I have tried a gainclone with no by-passing caps (none whatsoever), and one with just one 0.1uf on each rail. Mine did not suffer from oscillation so it didn't make a difference.

However, if you are trying some tough load, I would suggest at least putting in a small by-passing cap. My experience is that a small fast cap does a much better job providing stability than a big slow (electrolytic for example) cap. a ceramic disc or polypropylene would do.
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Old 17th July 2003, 08:30 PM   #7
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Quote:
Originally posted by Nuuk
If I had a pound for each time that I have heard similar comments about the GC, I could buy a Krell (not that I would want one as I am using granite for my GC platforms)
If I had a buck each time I heard people talking about their Accords outdoing Ferraresi, I would have enough money to buy several Ferraries over, .

Not that I don't trust people, but we are biased towards our own "babies". Mine is good but no match for even my lowly Kenwood at high volume.
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Old 18th July 2003, 09:56 AM   #8
Mel is offline Mel  United Kingdom
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Quote:
I have tried a gainclone with no by-passing caps, and one with just one 0.1uf on each rail. Mine did not suffer from oscillation so it didn't make a difference.
This seem to be at odds with Nuuk's comment that bypassing the 1000uFs is for sound quality, rather than stabilty.. I would try adding bypasses and seeing for myself, but the way my lashed-up amp has grown makes access for soldering a bit tricky. Time to rebuild methinks .

One other question if I may: The Decibel Dungeon site shows 120uF smoothing caps on the power supply. What effect do they have on the performance of the amp?

thanks
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Old 18th July 2003, 11:49 AM   #9
Mel is offline Mel  United Kingdom
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By the way,

here's a piccy of my lashed-up version (I've tried attaching a jpeg- hope it works). It's standing next top one of the 300B monoblocks it replaces- it started off neat enough, but after a few tweaks it became the spaghetti you see here ('a complete horlicks' to borrow a recent phrase from a UK politician!) .

cheers, Mel
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Old 19th July 2003, 06:54 PM   #10
Nuuk is offline Nuuk  United Kingdom
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Quote:
One other question if I may: The Decibel Dungeon site shows 120uF smoothing caps on the power supply. What effect do they have on the performance of the amp?
I put them there as my first tweak and noticed a small improvement in the bas. That was acouple of months ago and I did initend to take them out to see if I noticed any difference but have not yet got around to it.

I think that they may be a good idea for those of us using umbilicals between PSU case and amps but not so important for the 'all-in-one-box' brigade.
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