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Old 15th September 2013, 10:50 AM   #1331
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OK a dumb question on changing the default gain of the BA3 gain stage.

I'd like to drop it from 30 X down to about 20 X I initially got confused until I read zenmods correction that it is R13 divided by R10.

My dumb question comes about because I'm uncertain whether I should decrease R13 or increase R10 (or a combo of both). My gut feeling is I should increase R10. So If I did that, and I understand things correctly I should increase it to 33 ohms. If it is better to decrease R13 then I should drop it to 220 Ohms?

I'm just finalising a BOM for my build (which probably won't happen for quite some time) and wanted to sort this out

Tony.
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Old 15th September 2013, 10:54 AM   #1332
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decrease R13

for starters

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Old 15th September 2013, 10:58 AM   #1333
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OK Thanks Zen Mod That's easier as I don't need to do R11 as well I was a bit worried about the dissipation in R13. I assume leaving it as a 3W is ok.

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Old 15th September 2013, 11:23 AM   #1334
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Tony, I'm curious about the reason you want to reduce the BA-3 gain. Are you trying to match a system component - pre or speakers or other? Or is it a sound quality factor? I haven't reached that point yet, but I'm aware increasing the bias by reducing the values of resistors on the output boards reduces distortion. Is adjusting the default gain a similar technique?

I got my amp back up yesterday and needed to add a JC-2 pre clone to get volume levels to my comfort zone. It is loud enough as is for small groups and vocals - but just under what's needed for large orchestral and dynamic big band pieces.
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Old 15th September 2013, 11:42 AM   #1335
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Hi Bob, Have a read of Michael's gain structure article What is Gain Structure?

My current main amp has a gain of 20, and I will be using it for my stereo subs and the BA3 for the MTM's. Having the gain matched means not having to do too much adjustment (ie attenuation) in the crossover.

I may be being naive here, but even a gain of 20 is probably a bit much.. I could probably drop to 15 (based on the plan to run 30V rails in my BA3). Rational is that these days I pretty much exclusively use digital sources which will provide 2V input so 30X gain is way more than I likely need. Of course some recordings will be at a lower level so I may regret that when listening to quieter recordings!!

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Old 15th September 2013, 11:59 AM   #1336
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Understood, Thanks. I'll read the reference later today.
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Old 15th September 2013, 04:59 PM   #1337
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The build went smoothly until my wife pointed out that I had forgotten to order the trim pots from Mouser. So we made a quick trip to RS and bought a pair of audio 5k pots................... My wife had never soldered or drilled metal before, so she was delighted to learn how. And of course I had my own nefarious plan to get her hooked on this DIY thing . . ...............................
If ever you are thinking about selling your wife !!!
Give me first call.
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Old 15th September 2013, 05:01 PM   #1338
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Crap. I see I accidentally posted my "review," post 1331, in the wrong thread. Can a mod move it to the ACA thread?
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Old 15th September 2013, 05:06 PM   #1339
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Old 15th September 2013, 09:22 PM   #1340
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Just had a minor OOPS!

I blew a fuse - but it was after playing for several hours yesterday and about three hours this morning. The toroids are 500 VA/18 with CL-60s on all four primaries. The PS caps (four per side) are 27,000 uF/35 on a standard Pass layout. Have two 5A slo-blo fuses in this AC entry module. All this is feeding 24 output transistors.

So my question is whether to just increase the fuse value or should I be looking for a problem elsewhere? The amp is fine with a new fuse, but the long delay before burn-out has me concerned.
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