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Old 5th September 2005, 08:59 PM   #1
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Default Free Lunch/Monkey/DRD inspired 2A3

So I made a prototype of the attached circuit. Must say it sounds pretty nice, but I get a hum (or is that a buzz?). It's a quiet hum. But if I listen real close (and backwards) it seems to sound like this "you don't know how to hook up a/c heaters." Weird, huh?

Iíve never heard a DHT before this and I donít quite understand humbuckers, so any comments are very welcome.

I know a bit of the parts are over the top, but what can I be if not a wasteful American?

The 417A doesnít fully drive the 2A3. Maybe a bit more out of the phono stage is in order. (Haven't listened to it with the CD yet.)

Iím going to try to figure out the hum, and then make a proper monoblock for stereo listening!

Dan
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Old 5th September 2005, 10:22 PM   #2
kmtang is offline kmtang  Canada
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Default Schematic please

Hi,

It is rather difficult to answer your question without schematic and photo of your amp.

Do you employ the hum null potentiometer at 2A3's filament if it employs AC filament supply.


Johnny
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Old 6th September 2005, 12:47 PM   #3
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here's Johnny....
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Old 6th September 2005, 01:33 PM   #4
kmtang is offline kmtang  Canada
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Default Add a hum pot to 2A3 filament

Hi Dan,

Surely it will hum. It is so strange that the schematic doesn't include a hum pot for killing hum at the 2A3's filament. A 50-ohm 2W potentiometer could be placed in parallel with the two 100R resistor. You may refer to the attached schematic.

By the way, the Hammond 266M2 may produce excessive voltage greater than 2.5Vrms for the 2A3 filament. Check the voltage!!! You may use a dropping resistor to reduce the voltage to 2.5V if necessary. My very simple calculation of the resistor would be:-

R = (Vout - 2.5V)/2.5

You would find a resistor that will give the voltage very close to 2.5Vrms across the filament.

Johnnny
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Old 9th September 2005, 03:26 AM   #5
kmtang is offline kmtang  Canada
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Hi Dan,

Wonder if you have fixed the hum problem?

Johnny
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Old 9th September 2005, 09:22 AM   #6
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I've received the humbuckers and will put them in this weekend. I'll report back hopefully a great result.

Dan
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Old 9th September 2005, 12:05 PM   #7
kmtang is offline kmtang  Canada
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You can probably get the hum noise down to 1-2mV at the speaker output after the adjustment is done. Of course, it is important to have clean B+ supply.

Johnny
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Old 10th September 2005, 01:16 PM   #8
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I wired up a 50R humbucker from RS (173-164). With the dial in the centre, the same buzz/hum. When I turned the knob right, I started to hear a hum. Turn it back and to the left, I started to hear a hum.

With the knob in the centre - no hum, just a buzz.

So now I guess I know what hum is. So maybe it is not the AC.

If I put a 100k from input to earth, buzz is gone, but so is the music. Well most of it. If I put a 1M, no effect. (Just guessing now)

But with my power supply, I just don't see it being the B+

Maybe swap the input valve........

Dan
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Old 10th September 2005, 02:40 PM   #9
jlsem is offline jlsem  United States
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Quote:
Maybe swap the input valve........
Try a simple redesign using EC86, E86C, etc. Biased at -2V it will drive the 2A3 better than the 417A. May sound better as well...

John
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Old 10th September 2005, 03:24 PM   #10
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referenced the 417A's heater to earth....and silence!!!!!!!! Time to start building up a real monoblock.

Dan
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