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Old 22nd April 2009, 01:37 AM   #1
240z4u is offline 240z4u  United States
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Default Troubleshooting hum in 12b4 preamp.. finally almost done!!

I bumped my other thread but wanted to show pictures of this preamp that you guys helped me with in the other thread. Thanks Eli and Douglas in particular.

I am having trouble with hum in the output, and it is not changing with the volume control. I have everything grounded to a common spot in the preamp.

Thoughts on where to start?

Click the image to open in full size.

Click the image to open in full size.

So far, this thing sounds absolutely great. If I can get rid of the hum I will build a second one to keep (this one is for my dad) and sell off some of my other gear.

Thanks!
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Old 22nd April 2009, 12:12 PM   #2
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Hi !
Where is the power supply /PS/?
You must put in PS a choke 5-10 H, after rectifire between first and
second elektrol. caps. And put parallel to the choke non elektrol. cap
1 uF/160 v.
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Old 22nd April 2009, 12:33 PM   #3
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It is in a separate enclosure.

http://www.diyaudio.com/forums/attac...amp=1191802078

That is a schematic of the power supply I used. I did use a battery bias'ed CCS that I got from another member that replaced the top part of that schematic (the part with the tube connections).

Evan
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Old 22nd April 2009, 01:20 PM   #4
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OK !
Put parallel every chokes caps 1 uF/160V.
/I prefer You connect between ampl. and AC main, LF & HF filter
from PC PS 300-500 wts/.
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Old 22nd April 2009, 09:23 PM   #5
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Quote:
Originally posted by azazello
Hi !
Where is the power supply /PS/?
You must put in PS a choke 5-10 H, after rectifire between first and
second elektrol. caps. And put parallel to the choke non elektrol. cap
1 uF/160 v.
Cap in parallel with the PS chokes? Why?
cheers,
Douglas
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Old 22nd April 2009, 09:35 PM   #6
240z4u is offline 240z4u  United States
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Hey buddy!

Bandersnatch is another character who helped me get this thing functioning.

Evan
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Old 23rd April 2009, 01:56 AM   #7
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Evan,

By chance, did you forget to bias the 12B4 heater CTs off B+? If said bias is present, switching to DC heaters may be in order.

Don't forget the "routine" checks of lead dress and cold solder joints.
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Old 23rd April 2009, 03:31 AM   #8
240z4u is offline 240z4u  United States
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Thanks Eli and everyone else. Ill go back and check the basics.. it never even occurred to me until today that it may be a simple ground loop in my system. It will be a few days before I can get back to it though.

Thanks guys. I am enjoying the sound. I had a buddy over that was absolutely shocked at the change and asked if I was willing to build him one. I said sure, it will be ready in 2011 LOL.

Ill get back to this soon.

Evan
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Old 23rd April 2009, 11:23 AM   #9
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Quote:
Originally posted by Bandersnatch


Cap in parallel with the PS chokes? Why?
cheers,
Douglas
Please, think!
The cap has 2 plates, if on the first there is +U. on the second - -U....
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Old 23rd April 2009, 11:33 AM   #10
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Quote:
Originally posted by azazello

Please, think!
The cap has 2 plates, if on the first there is +U. on the second - -U....
Obviously...so why shunt all the chokes with a cap? Looking for a resonance point to block a particular frequency of interest? Probably best to do that on only one choke, and not the first.
cheers,
Douglas
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