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Old 17th August 2007, 06:46 PM   #1
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Default transformer hum / buzz getting into B+?

I am working from home today, which means that the house is relatively quiet. So, I am playing some music through my Aikido preamp and for some reason, the slight barely discernible hum that I get seems a little louder. Now this could be due to the house being quieter.

When I get close to the Aikido, with the main amp off, I notice that there is a slight buzzing from one of the power transformers. I use a separate 6.3VAC transformer that is regulated to DC for the heaters. During the pre-heat, I can hear no buzz. When B+ gets switched-on, the slight transformer begins buzzing.

This makes me think that the 125-0-125 Hammond that supplies the B+ is the source of the buzz, although maybe it is the effect of referencing the 6.3VAC transformer's center tap to 1/4 B+ that is causing the buzz. The two transformers are fairly close together and orientated in different directions.

The other thing. The transformers are rated for 110VAC. I typically have 118 - 124 VAC. Today with the AC cycled off, I am reading 122VAC. Either way, both transformers get very warm, almost hot to the touch.

1. Could the transformer hum translate to hum from speakers? I think the answer is a maybe.

2. Could the 1/4B+ reference to the center tap of the filament transformer be causing it to hum?

3. Could my input voltage be too high?

4. Could both transformers be interacting to cause buzzing?

As I say, the buzzing on the transformers is not all that loud. In fact it may not be noticeable without reference to its absence during filament heating. The hum from the speakers is very quiet, certainly not noticeable when music is playing.

Thanks,
Charlie
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Old 17th August 2007, 06:52 PM   #2
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Sorry, the transformers are rated at 115VAC, not 110VAC.

Charlie
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Old 17th August 2007, 08:09 PM   #3
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Join Date: Jun 2005
Location: Tempe, AZ
NOW we are getting somewhere Charlie. Hammond transformers and chokes are known to buzz a bit. When I turn on my pre (same transformer as you), before I turn on my power transformer B+, I get a little buzz. It is only audible up close. Totally quiet from my listening position. Since my power supply is outboard the vibration doesn't couple to my input tubes. Since you are not getting much hum or buzz thru your speakers you also seem in good shape there. Isolating it with rubber washers probably will do no good. If it really bothers you (which I'm guessing it does) maybe some type of after market transformer cover would help. Seems like I've seen some nice ones out of Hong Kong? First make sure if the buzz is from the power transformer or the first choke.
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Old 18th August 2007, 02:49 AM   #4
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Hello Bruce,

You know, I was only really aware of the buzzing because, it was very quiet here today, we had a little rain last night - the storm fizzled before it got out to your place in Tempe. Anyway, it meant that it got down to 85F last night. So this morning, the AC was nicely cycling off.

No AC, no baby, the dogs asleep, meant that I could hear the buzz quite audibly. Of course, it these things always sound louder when you're listening for them.

It is definately a transformer, not a choke. But could the buzz be getting into the tubes, as they seem very non-microphonic? I can live with it.

One thing that I must experiment with when I re-house my Aikido (which will not be this year) is to try the PSU that Hagermann uses for the Cornet2 phono-pre. It seems like a simple single cap filtered PSU. The 550V secondaries go into the heaters of the tube rectifier, the output goes into another secondary of the transformer, and the center-tap of that winding becomes B+.

Now my guess is that given the 36dB gain of the Cornet2, any problems with the PSU would be very, very obvious. Yet it is dead quiet.

I did check my B+ on the Aikido, it is 285VDC, so I may tweak it a little. I have a spare 0.1uF Wilma rated to 400V. I can parallel it with the 0.68 tuning cap. I guess that the voltage will climb some.

Regards,
Charlie
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