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Old 1st November 2002, 03:16 PM   #21
PedroPO is offline PedroPO  Portugal
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hummm... I dunno..

It seams simpler to me to just rectify the ac.

If you want I can post the Audionote rectifier.

Are you using feedback in your amp?
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Old 1st November 2002, 03:25 PM   #22
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Plenty DIY'ers seem to agree that DC sounds worse...But I have seen quite a few..(like Morgan Jones) explanations why that is...

Something to do with the differential noise on the filament that DC does not take away...

From a layman's point of view then..placing a common mode filter after the DC would give you DC.. that also sounds good.

Like I posted before...I do not have this from firsthand experience..as I have not used DHT'S YET! :-).

Plus...are you sure it is really the amp itself..I thought so to after building my latest amp..but then I plugged the CD (Variable output) straight into my amp bypassing the preamp...and was stunned by how little hum there was...)

So it could also be a ground loop in your system..

Hope any of this help..

Good luck..
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Old 2nd November 2002, 11:54 AM   #23
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Hi guys,

Why not take a look at this thread:

http://www.diyvideo.com/forums/showt...?threadid=6082

Cheers,

Frank.
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Old 2nd November 2002, 01:34 PM   #24
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Bas,

There's no groundloop. The amp is sitting on my workbench just connected to a speaker with the input shorted, so it can't be that. I've tried separating the stages but no joy.
desparate......
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Old 2nd November 2002, 01:44 PM   #25
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Hello Little Tomato,

How is the 6SN7GTB heated?
If AC pull it out for 30 secs and see if the hum dissapears.
Don't do this for too long since your B+ is going to rise and may take a cap or two out.
Also put the speaker at least 70 cm away from the amp,it's magnetism may upset the iron in your amp.
Cheers,
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Old 2nd November 2002, 01:59 PM   #26
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Frank,

I tried it and the hum was still there but less. What does this signify ?
BTW thanks for letting me have the alternate link to the forum.

<Hello Little Tomato,> doesn't sound the same in english, does it ?
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Old 2nd November 2002, 02:13 PM   #27
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Tomatito,

Basically it means you suffer from inductive coupling between those two heaters.
Are they run from the same xformer?
What about the heating of the rectifier?
That should always be run from a separate winding,better still a separate xformer alltogether.
As a last resort: try to run them away from each other taking good care you don't approach any signal carrying wires and push them as close to the chassis. (fix 'em there with a stick-on clip,e.g.)

If that doesn't help try to reorientate your choke with respect to the xformers.
This is hard to crack from a distance but we'll give it a try.
What the efficiency of your speakers anyway?
Do you feel the woofers' membrane slightly trembling to the touch?

Cheers,
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Old 2nd November 2002, 02:32 PM   #28
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Frank,

Yes, they are run from the same transformer. Someone suggested I ground the transformer itself from one of the bolts. Is this a valid suggestion ?
Do you mean separating the heater wires from the 6SN7 ?
The test speaker is a very old Dynaudio with (guessing) no more than 85 db. sensitivity. I don 't feel the membrane trembling.

Thanks for your time and effort

Martin
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Old 2nd November 2002, 02:36 PM   #29
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BTW I tried bringing down the B+ a bit by placing a resistor between the choke and the first filter cap because it was about 40V over the given value. This brought the hum up with about 1mV to 5,5mV though so I took it out again.

Martin
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Old 2nd November 2002, 03:15 PM   #30
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Tomatito,

Quote:
Do you mean separating the heater wires from the 6SN7 ?
I mean arranging the twisted wires for both heaters (6SN7 +300B) in such a way that their far apart (at least a couple of cm) from each other.

Quote:
Someone suggested I ground the transformer itself from one of the bolts. Is this a valid suggestion ?
No,you've got a ground already there with the midpoint (loper in Dutch) of your rheostat (like on Steve Benchs' site),don't you?


Quote:
BTW I tried bringing down the B+ a bit by placing a resistor between the choke and the first filter cap because it was about 40V over the given value. This brought the hum up with about 1mV to 5,5mV
Sure,that's no good.
You had CRL.
Here's what you can do:disconnect the first cap after the rectifier
and put the R after the cap that was after the choke,then add another cap after that.
What was that first cap?A polyprop?
If so,you better use an elco there.

Good luck,
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