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Old 29th December 2001, 11:17 AM   #1
diyAudio Member
Join Date: Dec 2001
Location: Berkeley
Unhappy Help me get rid of the HUMMMMMMM

I just got done building this amp (see attached) and I can't seem to get rid of this nasty hum that seems to dominate quiet passages and what should be silence between tracks. I've tried everything - star grounding, separate ground points for the power supply and the amp circuit - what am I missing? I know that this is a common problem, and I have built several amps now that have not had this problem.

The only change I made to the circuit is that I'm using a 50 - 80 mfd 450WV two-section electrolytic for C9. (It's what I had available in my junkbox).

Any help would be appreciated.
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Old 29th December 2001, 07:28 PM   #2
diyAudio Member
Join Date: Sep 2001
Location: Sth. Oz

OOps, pardon the pun...
I'm no expert by any means, but all things being equal, if it's in both channels I think it may be the ps cap which could be near the end of it's use by date...
Lash out and get a pair of 40mfd's and if that does not cure it then you could have a faulty solder joint somewhere.

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Old 31st December 2001, 04:46 AM   #3
TJ is offline TJ
diyAudio Member
Join Date: Sep 2001
Location: UK
I think the problem is the main voltage problem ! I have some suggestion for u ... after rectifier add a 220uf cap then following a choke (optional ..2 - 5 H 300ma will do). Add another 220uf after R8 .Change the R4 to 1W -2W type . Disconnect the heater to ground . Try connect like this ...
) /
) \ R1
) /
) \
6.3V ) ---------------> ground
) /
) \
) / R2
) \

R1 = R2 . 50 ohm or 100 ohm (3W) any value will do but R1 and R2 must be the same value ! or u can try voltage regulator in the main and heater . Connect the ground to the case . Hopefully this will help !
Happy Diy and happy NY !
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Old 31st December 2001, 09:55 AM   #4
diyAudio Member
Join Date: Dec 2001
Location: Berkeley
Smile Hum problem solved!

After much troubleshooting, headscratching, and second guessing I finally tracked the problem down to my preamp. I don't know what happened to it (an unmodified Dynaco Pas2) but after swapping out the preamp with another (a Scott 355 that had recently been serviced) the hum problem miraculously went away. NO hum whatsoever now and tha amp simply sings, especially with LPs and female vocalists. For an inexpensive little power amp, I have to say that I am impressed with this circuit. Currently my system consists of the 6BM8 single ended power amp, Scott 355 Preamp, and Klipsch Professional Model LaScalas with homemade silver wire speaker cables. Amp was constructed using 1% film resistors and coupling caps are matched orange drops from Torres Engineering. OPTs are $8.95 5K:8 ohm SE transformers from AES. I'll give the amp a few more hours to break in, but as it is now - simply fantastic. A very good amplifier for the first time DIYer.
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Old 8th January 2002, 05:26 AM   #5
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Join Date: Nov 2001
Location: Dallas,Tx
Could you post some more about this amp? I happen to like 6BM8s and this amp looks like it is easy to build. How much did parts cost you and how long to build? I ran across a Japanese club for building SE 6BM8 amps once and some of them were beautiful. I have some PP 6BM8s and know that the tubes sound good and are cheap to buy but have never had the chance to listen to SE.
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