Dual channel tube mic pre/ desk top version

[Holger]

A voice over artist here in Hamburg asked me if it would be possible to build a smaller desk top version tube pre amp. I thought it would be a good idea for someone without a studio to have a 230 mm wide desk top version with a separate PSU.

There was some space on the front so I added two input XLR and a selector switch for front/rear input.

 

[ruffrecords]

Very nice build as always Holger. I am not familiar with the rotary switches. They look a little like Grayhills.

I like the method of mounting the output trasnformers.

Cheers

Ian

 

[Holger]

Ian, the switches are NKK MR-K 112.

 

[Holger]

Interesting comparison Desktop versus 19":

Noise spectra, full gain, 150R terminated
Desktop, external PSU



19", internal PSU



some numbers

 

[Tubetec]

Any slots in the enclosure for air flow ?
Looks like its going to get awefully hot otherwise ,
A front panel that runs warm to the touch is fine , but you dont need it to cooks eggs on it either .
Housing the psu seperately obviously makes a big difference to noise levels ,
It takes heavy engineering to screen a psu as is done in the tele V series , very much a game of diminishing returns as the costs pile up .

 

[ruffrecords]

Ian, the switches are NKK MR-K 112.

OK, I was confused by the black things behind them.

Cheers

Ian

 

[ruffrecords]

Any slots in the enclosure for air flow ?
Looks like its going to get awefully hot otherwise ,

These tubes only consume a few watts because the output pair only runs at about 6mA (1.5W at 250V) and the input one at about 0.5mA (125mW) and the heaters only consume about 5.5W total. There should be less that 10watts of heat in there. You can fully close the module and the front panel gets barely warm - certainly not as warm as the front of the shaver transformer in our bathroom that charges up our toothbrushes. Remember these are preamp tubes not power tubes.

Cheers

Ian

 

[ruffrecords]

Interesting comparison Desktop versus 19":

Noise spectra, full gain, 150R terminated
Desktop, external PSU


some numbers

View attachment 108722

Proof once more that external power supplies are a 'good thing' and also proof the tube preamps don't have to be noisy.

Cheers

Ian

 

[Holger]

OK, I was confused by the black things behind them.

Cheers

Ian

Ian, the black thing is part of a little PCB system for the NKK switches. They eat up less space behind the front panel than a Lorlin switch.
Dimension is 17.15 x 19 mm.

 

[warpie]

Very nice build!

Do you get any interference (crosstalk?) between the two channels? I'm asking because the output transformers seem to be pretty close to each other.

 

[Holger]

Yes. (Or no?...)
When I put a -40 dBu 1kHz signal in channel one and amplify it by 40 dB and set channel two to 40 dB gain also and terminate input two with 150R, I can clearly see a 1k signal on channel two, measured with a Neutrik Minilyzer. The level is -83 dBu. Is it "any crosstalk"? Yes. Can I ignore it? From my point of view yes.
I'm happy with the built.

 

[warpie]

Yes. (Or no?...)
When I put a -40 dBu 1kHz signal in channel one and amplify it by 40 dB and set channel two to 40 dB gain also and terminate input two with 150R, I can clearly see a 1k signal on channel two, measured with a Neutrik Minilyzer. The level is -83 dBu. Is it "any crosstalk"? Yes. Can I ignore it? From my point of view yes.
I'm happy with the built.

Fair enough. The built looks great, no doubt. I was just curious

 

[Tubetec]

Might be interesting to look at the temperature of the unit over time , in general we want things to reach opperating temp in around 30 minutes and stay there , a closed box might take hours to reach stabillity , and as we know the sound changes during the warming up period .

hats off to Holger all the same , incredibly beautifull work ,