8 Channel, 3U NEVE 1073 completed

[ruffrecords]

You should not measure between the outputs and ground. The 1290 has a transformer output so it is floating with respect to ground. Measure the signal between pins 2 and 3 of the XLR.

Cheers

Ian

 

[Whoops]

The unit works fine and sound great until I set the gain to the last two positions (70 and 75 dB). Here the noise become unacceptable, I measured the output (+ to gnd) with the scope and found out that I got 23V on 70dB setting and almost 50V on 75dB setting.

like kidb said,
let us know specifically where are you measuring, is it the output of the PCB or the output from the transformer secondaries?
The 23V and 50V are AC or DC?
You don't need to use the scope just use a DMM for the voltage measurements

the transistor voltages are close enough to the tab on the assembly guide.

Show us your voltage chart anyway

You have a lot of worked involved in your build, quite a challenge, it's clear that you were really dedicated to that build. Specially the wiring for the gain switch, it's massive.
To be honest I would get pcb mount 2 switches and try at least in 1 channel with the proper switch soldered to the pcb instead of the wiring.
Did you talk with Colin from Audio Maintenance to see if he could get the switches?
If I remember correctly when I did this project I bought the switches from him

 

[M_TheDoc]

let us know specifically where are you measuring, is it the output of the PCB or the output from the transformer secondaries?
The 23V and 50V are AC or DC?
You don't need to use the scope just use a DMM for the voltage measurements

Measuring with the DMM between pin 2 and 3 of the output xlr (transformer's secondary) I'm reading 11VAC at 70dB and 20VAC at 75dB, without any input signal and with phantom power disabled. Not sure why I had a different reading on the scope, but it was anyway a AC signal measured between pin 1 and 2 of the output xlr.

Show us your voltage chart anyway

I checked transistors voltages on the 75dB setting and found something bad. Chart on the left is with gain at 55 and chart on the right is gain at 75. I think emitter voltages being higher than base voltages could be a problem.

I didn't ask Colin for the switches, I only found the panel mount option on Mouser and I thought them allowed a more flexible internal layout, but if I can't find the solution to this problem I will try swapping to the PCB mounted ones.

Anyway thank you guys for your suggestions, I'm quite lost on this troubleshooting and I really appreciate your help

 

[ruffrecords]

At very high gains, layout is very important. Unexpected stray capacitance can cause supersonic oscillation which is often audible as additional noise.

Cheers

Ian

 

[M_TheDoc]

At very high gains, layout is very important. Unexpected stray capacitance can cause supersonic oscillation which is often audible as additional noise.

Cheers

Ian

Thank you Ian, do you think that the gain switch cabling may be the cause of the stray capacitance? Or are you saying it as a general layout rule?

The annoying signal at 75db is 50kHz so I suppose it might be an unwanted oscillation, I still can't figure out why it's all wonderful working until a certain gain setting and then it's unusable