Everything going so well until...
I noticed a low level mains frequency hum on the right channel along with low level white noise. Left channel is silent as the night.
Haven't got a scope so any ideas? Difficult one this. My feeling is that it is the power supply but I have bypassed the diodes with a bridge and still the hum. Checked grounds and they are fine. Power transformers swapped and still same problem. Both power supplies are reading about 26V on each rail and no sign of AC.
Both channels are playing music quite happily and to high standard in fact I could live with it but it does suggest something ain't right. I am happy (not very happy really) to change the power supply rectifier circuit but has anyone experienced the white noise and hum that didn't come from the power supply? I haven't seen anyone mention this so far.
Incidentally offsets at Power Board and front end are fine as are all the other readings.
Moved transformers around chassis and this makes no difference.
Oh I forgot to mention; it is raining again!
I noticed a low level mains frequency hum on the right channel along with low level white noise. Left channel is silent as the night.
Haven't got a scope so any ideas? Difficult one this. My feeling is that it is the power supply but I have bypassed the diodes with a bridge and still the hum. Checked grounds and they are fine. Power transformers swapped and still same problem. Both power supplies are reading about 26V on each rail and no sign of AC.
Both channels are playing music quite happily and to high standard in fact I could live with it but it does suggest something ain't right. I am happy (not very happy really) to change the power supply rectifier circuit but has anyone experienced the white noise and hum that didn't come from the power supply? I haven't seen anyone mention this so far.
Incidentally offsets at Power Board and front end are fine as are all the other readings.
Moved transformers around chassis and this makes no difference.
Oh I forgot to mention; it is raining again!
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Pass DIY Addict
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I've been bending and wiggling all day and nothing. uF level is ok on the other channel and on other builds so I don't think the problem lies there.
I'm going to have to power up the right channel from the left supply and see if the problem is still there. If it is I have a spare BA-3 front end to see if that makes a difference. Lots of work tomorrow I think.
I'm going to have to power up the right channel from the left supply and see if the problem is still there. If it is I have a spare BA-3 front end to see if that makes a difference. Lots of work tomorrow I think.
Pass DIY Addict
Joined 2000
Paid Member
Well after replacing the jfets, the caps, testing every trace and connection without success over a couple of days the very last things I got round to were the output mosfets, SJ313 and SK2013. I knew I was finally there when the replacement made the interference worse. I thought they might be phony components so I tried a different batch of mosfets and the hum and white noise have gone.
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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.
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.
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.
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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Hi Bob, Have a read of Michael's gain structure article http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html
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!!
Tony.
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!!
Tony.
If ever you are thinking about selling your wife !!!
Give me first call.
Yeah, she's enthusiastic about learning about building stuff. And she doesn't hold my lack of enthusiasm for pop idol shows against me. I definitely appreciate her.
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.
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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