8 Channel, 3U NEVE 1073 completed

[sgenevay]

Hi there...
Short question....
is it normal for you.... to have up to 0.8 dB difference between 2 board with a 1kHz signal at same gain settings...  ?
i mean, i don't think i have to worry about this cause it only means that gain resistors maybe don't have the same value.....  but... i was wondering...
Thx

Sylvain

 

[MicDaddy]

Hi there...
Short question....
is it normal for you.... to have up to 0.8 dB difference between 2 board with a 1kHz signal at same gain settings...  ?
i mean, i don't think i have to worry about this cause it only means that gain resistors maybe don't have the same value.....  but... i was wondering...
Thx

Sylvain

I would expect some variance.  Even with 1% metal films, hand selected, there's going to be some deviation from one unit to the next.  What prompted you to measure, could you hear a difference or did you measure just to see? 

I have only compared one to the next by ear, and they sounded to me to be all reasonably equal.

Comparing them to the X73i, there is definitely tonal/timbre differences, both are in the same ball park.  The level is much hotter on the EZ1290 than the X73i with switch at the correlating gain setting.  I.E. with both pres at 25dB the EZ is hotter.   

 

[sgenevay]

u can hardly hear the difference... but as i said, it's still 0.8 dB is the highest difference i have between the smallest and biggest level (channel 2 / channel 7)
u can see a little difference on recorded waveform and i measured the difference it in a protools plug.
Of course it's a 1 kHz sine and it's harder to feel the difference with a regular sound... but if it's a normal thing, i don't care and will let it to lives its life  )

Sylvain

 

[erikb1971]

Just fired up my dual 1290´s for the first time.. no smoke, no fire, no fuses blown! For me that means an improvement over the last time I switched on a DIY device. The external power supply is working fine, all the right voltages on the right pins.

I have done a small pre-listen, channel one is completely dead, channel two is working but distorted, but I can't really say something untill I have the correct cables (My mixer's xlr inputs are mic only, and I can't feed my osc. into the mic input, have to use the jack on front (go between with fet di). But, step by step, let's solder some cables!

I have bought a small Jyetech diy oscilloscope, so Iam ready for biassing. However, I have no clue where to measure the sound for biassing. is it at the output of the board or at the output of the complete pre amp?

Cheers

Erik

 

[MicDaddy]

erikb1971, yo if you are impatient and want to try to 'bias' them in the short term, just measure the Voltage on the case of the 2n3055 reference to 0V on the 1290PCB.

Dial this in via the trim pot to show 22.66ish and the pres will work (so long as everything else is working/wired proper), when you get your scope you can fine tune the trimmer.

 

[erikb1971]

erikb1971, yo if you are impatient and want to try to 'bias' them in the short term, just measure the Voltage on the case of the 2n3055 reference to 0V on the 1290PCB.

Dial this in via the trim pot to show 22.66ish and the pres will work (so long as everything else is working/wired proper), when you get your scope you can fine tune the trimmer.

Thank you for your reply! I can't get the voltage higher than 19.97 or so.. if I dial in more, the meter goes to 1...
I can't get the V higher then 21,6. If I dial in more, i hear a soft metalic click in the trim pot and the v does not go up anymore...

***edit*** without a 1k sinus wave into it, I can get it at 22.6v. If I plg in the siniswave, it drops to 20.6

 

[erikb1971]

Hi all

Just to be sure.. where do I check the waveform on attached picture? A, B or C?

 

[sgenevay]

personally i'm doing it on C point...    but.... personally...   

Sylvain

 

[erikb1971]

And with the same settings on your oscilloscope as you got the sinuswave at input?

 

[sgenevay]

I think you won't have same signal level at the O/P point and at the I/P point...  hopefully cause if they were the same.... i would have build 16 channel for nothing 
so your Oscillo settings may vary... just adapt the sensibility to the signal level...
Follow the martin's instructions in assembly pdf file to set the bias...
It's really useful....

Sylvain

 

[erikb1971]

oowww.... that was a stupid question! Do we have a thread somewhere for those?

This is all that was said in martins assembly guide about biassing right:

After you’ve hooked everything up you’ll need to set the bias of
the driver stage. Do this by setting the gain switch to full gain (75dB)
and then running a 1K sine wave into the mic input until clipping.
(It’s ok if you actually hear the mic pre at this point)
Look at the signal with an oscilloscope. More than likely the waveform
is only clipping on one side.
Adjust the trimpot until clipping is equal on both sides of the waveform.

 

[sgenevay]

this is normally all what u need... set the gain at max... plug your oscillo and send a 1kHz sine wave in the preamp... (adjusting the wave volume with the signal generator u use...  up to the signal to be clipping...  just a bit...)  and then turn the trimpot to have something looking like the draw on the pdf.  When i do this... i use the voltmeter to check V on TR3 too.... in the same time...

Sylvain

 

[erikb1971]

aaahhhhh now it starts to make sense... would this be a sort of reasonable waveform?

 

[sgenevay]

mmmmmmm it's clipping on bottom right ???
if yes u can maybe push the volume a bit more...  and then it's ok....  u can turn yout trim pot to have the same clipping on bottom and on top ...

Sylvain

 

[erikb1971]

Ha! I had the same waveform when I unplugged the oscillator... can't be good I think I have it figured out though, but have to wait for the cables to see how it sounds...

 

[sgenevay]

Ha! I had the same waveform when I unplugged the oscillator... can't be good

Didn't get it.....

Sylvain

 

[erikb1971]

when i pulled the plug of the oscilator out of the input, nothing changed on the oscillator...

 

[MicDaddy]

Are your preamps going into oscillation?  Mine were oscillating at the highest gain positions, and these oscillations were not letting me set bias with gain at max.

I had to dial in the trim pot at lower gain settings (if I remember correctly, there were some late nights.)

With no input signal, and gain at 20, what Voltage do you see on the shell of your 2N3055?

Stick a microphone to the input and monitor output at low levels (protect your ears), if you have audio, start to increase the gain.  See if you are passing audio or if your pres go into oscillation?

 

[erikb1971]

Are your preamps going into oscillation?

No

With no input signal, and gain at 20, what Voltage do you see on the shell of your 2N3055?

22,6V

Stick a microphone to the input and monitor output at low levels (protect your ears), if you have audio, start to increase the gain.  See if you are passing audio or if your pres go into oscillation?

I have audio, but it is distorted at low gain.... and it seems to distort more at higher gains... But it could be my soundcraft compact 4 mixing console, because I do not yet have the cables to plug the pre amp in as line input, so they are in the mic input right now... but defenitely at low gain I get distortion from the pre amp!

 

[sgenevay]

I have audio, but it is distorted at low gain....

I had this once when i mistook on a res...  did u double check the components value ?