Hello everyone,
thank you so much for all your input and information.
I was only able to the the Crosstalk listening over the weekend, I did 2 videos showing the crosstalk at different output transformer positions.
Here are the Videos, be prepared for a minute of distortion:
https://www.youtube.com/watch?v=6yXgkk52bUo
https://www.youtube.com/watch?v=R_dqkKAFTKE
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INFO
This unit is not my build, I was just asked to do some maintenance.
It consists in 5 original Neve 1272 amplifiers that were racked as Mic Pres, it has a 6th channel that is a DIY 1272.
It was done 12 years ago or more, so before the widespread internet information on racking Neve and DIY Neve related stuff.
It is well built.
Test/Recording
I fed music from my iPod left channel into channel 2 (channel 1 is out at the moment) and listened the output of channel 3
iPod Classic Headphone output, Euro model with volume limit. The Output Volume was set to Max
This is the info I could find regarding the output signal of the iPod Heaphone amp:
"iPod Classic's headphone out into a high impedance load is Up ≈ 0.47 Vp (peak)
This means Urms ≈ 0.332 Vrms and thus Lu ≈ -7.3 dBu"
"20Hz to 20kHz freq. responce, 30mW per channel power output, and 24 ohms"
Channel 2 (input channel)
Gain was set at 6, it's a 12 position switch, I dont have a scheme of the rack up so I don't know what that translates to gain. It should be 35db's or 25db's. (I can measure the switch resistors to know for sure but it would not make a difference for my goals)
I could listen to Crosstalk even if Gain was set at 1, I increased the Gain to help monitoring levels and recording levels.
Channel 3 (Output crosstalk channel)
Gain was set at minimum, changing gain didn't make a difference.
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Let me just be clear:
My Goals are not measuring crosstalk, the only thing I'm looking for is to know which could be the best positions for the 6 output transformers in the free space I have available in this box.
I'm not trying to get rid of crosstalk, and I know the amount of amplification involved in my recording is huge, so this doesn't tell us anything about a crosstalk figure, it just helps me in comparing the crosstalk for different positions and decide if there's a certain positioning inside the limited space I have in this box that could have the crosstalk minimized even if the crosstalk level at the worst case scenario was marginal or negligible in a studio session.
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Conclusions:
- for all the possible positions I tried, the present positioning with all the transformers next to each other is the worst possible one
- shielding greatly reduces the crosstalk when the transformers are next to each other , in Neve desks thereswas 2 sheets of steel between the Transformers (modules cases), I used toroidal trafo shield from don audio for the test, it works fine
- Rotating the transformers 90 degrees decreases the the crosstalk to "roughly" the same level as having them next to each other with shielding
- Best position I found is having one transformer elevated and positioned horizontally. It's easier to see in the video or the drawings.