My new Preamp!

T

tube-a-lou

Addicted Member
Hi,
Picked up one these off ebay and I received it yesterday, it's a
Dun Mei Audio AV38 Line Stage Pre amp. So far it's very nice sounding,
it takes two 6922 tubes (Sovtek) which I'll find some more tube for 6DJ8 Bugle
boys seem to be of good supply and good cost. I did a little cap change because
of a slight 60hz hum, I took out the two 330uf 450volt power supply caps
caps and replaced them with two Ecpos caps which made it a little lower
but only hear it when I'm close to the speakers. Someone said to increase
the value of the 330uf to something higher but I'm not sure, I did add a wire
from the inputs to the ground which made it worse so I took it out. I did replace
the volume pot today, they had one of those detented type that when I turned
it up one step it got loud so I put in a Bourns Black Beauty 100K pot and it so
buttery smooth, anyway here's a pick and if anybody have any way to fix it let
me know.

Thanks

77B91F14-9257-4865-8CB4-744075B06CFB_1_201_a.jpeg
 
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No when I play music I don't hear the hum the music covers it, I was playing it
for about a hour or so today and it sounds really nice. I just ordered some
Electro-Harmonic 6922 tubes see how they work out, I like your website.
 
well then you gotta decide if that is acceptable to you or not. Standing next to a speaker and hearing some hum, well that is not really a practical 'test'. What is important, is hum audible under normal listening conditions. If you hear hum between songs at normal levels and listening position, then that is not acceptable, or, it should not be acceptable.
I see that the R-Core transformer is not shielded at all from the rest of the circuitry. To me, that is a no-no!!! If a transformer is inside the chassis, like yours, it must be shielded, either by it being enclosed inside a metal cover, or, a metal shield. made from aluminum or steel, should be placed between the transformer and the rest of the chassis. This metal shield should be grounded, usually, bit being screwed to the chassis, will accomplish that.
Attached is a photo of a good example of shielding a transformer, which sits inside the chassis, from the rest of the circuit.

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tube-a-lou said:
You don't sell shields do you?
Click to expand...

hehehe...no I don't. But hardware stores like ACE and the like, they do sell various sizes of aluminum sheets, that is were I get mine from, then I cut them to size, I also buy the 'L' shape brackets to mount the shield to the chassis. A bit drilling etc., but I think it is a worth while experiment to try out. Of course, you'll need to drill into the preamp chassis as well, so keep that in mind
 
I have *NEVER fixed a linestage's hum by shielding. Stupid grounding mistakes, and or ground looping yes. And it turns out my 'never' was a bit premature, but this for a linestage with output TX's that pick up the field from the PS irons...
cheers,
Douglas
 
PakProtector said:
I have *NEVER fixed a linestage's hum by shielding. Stupid grounding mistakes, and or ground looping yes. And it turns out my 'never' was a bit premature, but this for a linestage with output TX's that pick up the field from the PS irons...
cheers,
Douglas
Click to expand...

Oh, I disagree, Douglas. I built an SP14 preamp from scratch, without the offered chassis and blessed trannies, and shielding/grounding is definitely an issue.
 
zackthedog said:
Oh, I disagree, Douglas. I built an SP14 preamp from scratch, without the offered chassis and blessed trannies, and shielding/grounding is definitely an issue.
Click to expand...

yeah I have to agree. Sometimes one may get away with not shielding a transformer inside a chassis, but in close to 50 years in this industry, NOT shielding a transformer inside a chassis, especially with high gain tube circuitry, will cause hum issues, sometimes that hum may be subtle, but it is there non the less, and shielding the transformer will get rid of most of that hum.
Of course hum can be caused by many other issues as well, untidy wiring, DC wires too close to AC, AC wires not twisted, too many grounding points, aka ground loop, dry solder joints and so on and so on.....and this is not even counting bad/faulty components, although in new builds, this is not very common.....
 
Erhard-Audio said:
yeah I have to agree. Sometimes one may get away with not shielding a transformer inside a chassis, but in close to 50 years in this industry, NOT shielding a transformer inside a chassis, especially with high gain tube circuitry, will cause hum issues, sometimes that hum may be subtle, but it is there non the less, and shielding the transformer will get rid of most of that hum.
Of course hum can be caused by many other issues as well, untidy wiring, DC wires too close to AC, AC wires not twisted, too many grounding points, aka ground loop, dry solder joints and so on and so on.....and this is not even counting bad/faulty components, although in new builds, this is not very common.....
Click to expand...

The wiring is all twisted I'm going to check the rest of the wiring tonight, I have two projects going on this preamp and
restoring a Fisher X202C which I have to replace the transformer on it.
 
I would think a steel box would be more effective than one made of aluminum for shielding magnetic leakage.

I think it should be grounded too.

Am I right?
 
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