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Old 1st September 2004, 04:07 PM   #1
Cappy is offline Cappy  United States
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Default Is There Any Downside to Lowering Gain?

I built a set of LM3875 monoblocks using Brian's PC boards and Peter's chassis.

I am using the monoblocks as bass amps for my Oris Horn bi-amped setup. I've got pots on the gainclones to pad down the low-passed signal coming from the main 45 tube amps.

My question -- instead of padding down the signal using the resistors in the pots, I wonder if it makes sense to lower the gain of the amps themselves? Optimally I would like to lower the gain somewhat and then use the pot to make final adjustments. This way there will be less resistance in the signal and I would expect more transparency.

What do you'all think of this plan, lowering the gain from the standard 33db to 20 to 24 or so? Is there any downside?

I see Peter mentioned in a post back in April 30-33 might be optimal, but I am weighing that against the (slight) loss in signal quality from more resistance in the signal.
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Old 1st September 2004, 06:16 PM   #2
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I did not do any comparison with regards to gain in NI amp. I just used what is in datasheet (and what seemingly 47Labs are using). It sounds well, so I didn't see a reason to research it further. I also need that gain as the amp is mostly used without line stage.

Now, I did some comparisons with inverting amp, by changing feedback resistor value, and it was definitely affecting the sonics. While 10k input resistance didn't change, I was substituting 220k, 250k and 300k for feedback. To me, it seemed like 250k in feedback sounded the best.

I wouldn't expect anybody giving you a definite answer. Best approach would be to change the gain and compare it to a your current setup: what sounds better to you?

Here a quote I noticed previously regarding the importance of proper gain setting:

Quote:
Originally posted by Joe Rasmussen
... But it also means you cannot set the feedback exactly where you want it and you can’t easily reduce it. I note that the original 47 Labs Gaincard has fixed gain slightly above 30dB, so the feedback values they must have used were not the ones shown in National Semiconductors PDF data file. I believe they did this because it sounds better. But in Thorsten’s circuit the gain drops a whopping 13dB which means that typically 13dB + more feedback relative to the Gaincard.
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Old 1st September 2004, 06:23 PM   #3
Cappy is offline Cappy  United States
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Peter, thanks....

I'll try lowering the gain and report back.

By the way, the amps sound superb.
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Old 1st September 2004, 08:32 PM   #4
GregGC is offline GregGC  Canada
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I don't think you'd get any sonic dif. especially if you use it for a bass amp. 20dB gain should be fine. Stability would be the only issue, so gain of 10 is fine.

/Greg
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Old 2nd September 2004, 06:04 AM   #5
Mad_K is offline Mad_K  Norway
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For bass amps it would be preferable to have lower gain (10x). Then you'll have more feedback and more control of the woofers. That's objectively; subjectively you might like it the other way around
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Old 2nd September 2004, 07:08 AM   #6
Franz G is offline Franz G  Switzerland
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Imho there are two downsides of lowering the gain:

- you apply more feedback and therefore you loose some transparency.

- as the open loop frequency vs gain is not linear, the more feedback the more linear up to RF, what is a disadvantage for audio purposes and eventually some LPF is needed.

Very low gain is interesting for subwoofers, like mentioned above.

Franz
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