I need some help biasing my Neves please

[corgan4321]

I have 4 of Madriannse's Neve style EZ1290 boards. They all sound and work great. I biased them originally so that the collector of the output transistor measured 22.6V

Today I set up my tone generator and oscilloscope to do a proper biasing and got results I didn't know how to deal with.

You can see in the first photo the negative side of the signal is just starting to see some more pronounced 2nd and 3rd harmonic distortion while the top side is just flatlined.

You can see the effect clearly once I turned up the amplitude of the signal generator slightly in photo 2.

If you guys could help me it'd be much appreciated! Thank you

 

[corgan4321]

Photo2

 

[critterkllr]

I'm curious to see if this is an issue. Both of mine do the same thing, but with the positive end of the waveform. I assumed the signal generator was too hot and only raised the volume on it enough to start clipping the signal. Perhaps your signal generator is too high?

 

[corgan4321]

Thanks for the reply.
That first photo was taken with the EZ1290 gain all the way up as it should be, with the signal generator's amplitude up just enough to see that much clipping.
I don't think that should be too hot of a signal out of the generator. It was only about 60mv p-p

 

[critterkllr]

Taking a closer look at the first pic, mine is the exact same, but flipped. So you're not the only one.

 

[corgan4321]

critterkllr, you inspired me to go back and try with a smaller amplitude signal. I think it worked. I turned it down till i got just a small bit of clipping on both positive and negative and then adjusted the trimpot. The resulting collector voltage on the output transistor ended up being 22.6V or 22.7 volts. I think this is the way to do it.

I'm owing that non-linear distortion at extreme gain to the fact that Neve circuits are not perfectly linear and probably exhibit that fact at super high gains. Regardless, I'm happy

The fact that your results are 180 degrees out of phase from mine is troublesome though. Perhaps one of us wired incorrectly resulting in phase reversal? I'll check that out and get back to you.

 

[critterkllr]

critterkllr, you inspired me to go back and try with a smaller amplitude signal. I think it worked. I turned it down till i got just a small bit of clipping on both positive and negative and then adjusted the trimpot. The resulting collector voltage on the output transistor ended up being 22.6V or 22.7 volts. I think this is the way to do it.

I'm owing that non-linear distortion at extreme gain to the fact that Neve circuits are not perfectly linear and probably exhibit that fact at super high gains. Regardless, I'm happy

The fact that your results are 180 degrees out of phase from mine is troublesome though. Perhaps one of us wired incorrectly resulting in phase reversal? I'll check that out and get back to you.

I ended up doing the same thing and came to the same conclusion as well.

I'm not sure if it's a phase issue or if simple component value tolerances tend to make mine different. I thought it was strange because the screenshots in the build guide show exactly how yours are. It could have easily been a mixup error on my part from the signal generator leads or the oscope probe.

 

[alexc]

You need to adjust the bias pot such that the clipping is symmetrical at top and bottom.

Not important is the input level, except to the extent that visual comparison is made easier.

It's also good idea to monitor the collector current while doing it  - I've used from around 65mA to 90mA.

Beyond that is not so good 

My experience has been with Sowter and Carnhill gapped output traffos and Motorola 2N3055

Cheers

 

[corgan4321]

Hi alexc,

thanks for the input. I guess what critterkllr and I are having isues with is that the positive clipping and the negative clipping aren't symmetrical no matter what. I was able to get similar amounts of clipping but the type of clipping is different (different amounts of harmonics).

 

[critterkllr]

Here's a link to a thread with a similar issue. PRR (who has helped me a bunch of times and definitely knows his stuff) mentioned that the impedance may play a role in the symmetrical clipping issue: http://recording.org/diy-pro-audio-forum/40572-neve-1272-bias-issues.html

My signal generator is 600 ohm output impedance and the EZ1290 was set to stock. I just installed an impedance switch on the EZ1290s and can see if that makes any type of difference.

 

[alexc]

Sorry - I see what you mean, the asymetry remains despite the bias.

I would expect the rising degree of asymetry to happen at the higher gains as various distortions begin to take hold.
But if it's not adjustable to a reasonable symetry at most settings, then that's not right.

The last set I did with these as line amps was around 70mA thru the 2n3055 into new carnhill and it was basically distortion free till the highest gains of a then the distortion gently rising.

From memory, it was putting out around +15dBu into a 10K load before I saw much of any distortion at all.

I would check at what stage the distortion appears to be originating - could be before the class A line stage.

It's also interesting to look with an RTA application at the spectrum of the output as you change the gains for various input levels. Gives a feel for the gain structure/distortion results you can get.