J99's bias adjustment not happening

[thebearingedge]

Hello.

I just finished building 2 A12 kits and 2 J99 kits. The A12's went together pretty well and although some of the soldering felt kind of tricky they have fired up right away without any issues.

I felt like I did a MUCH cleaner job on the J99's and I worked VERY slowly and checked the values carefully. Then I realized the revisions and referred to the instructions on the sticky. No problem. I set the voltage across the opamp sockets to +/-20V (well within the tolerance of Millenia's MM99, measured the voltages on U3 and got within <0.5V of the recommended 12V. Powered down, installed opamps, powered up and began the bias adjustments.

I read 575mV on U1 with SW2 at full CCW and over 900mV with SW2 at full CW. I try to match the second measurement to the first, then turn the switch all the way back to CCW. Measure this new voltage at CCW and then switch full CW again and try to set the voltage to match the new CCW value. The variable resistor won't let me adjust anymore. It maxes out and I can't get less than 20mV difference. I don't know how close they should be.

U2 on both J99's has much higher voltages 1.3V at full CCW and over 3.0V at full CW. I can't get these values anywhere near matched.
Any ideas?

[tpryan]

The MM99s are JFET input op-amps and have orders of magnitude lower bias current than bipolar input op-amps like the AD797 and JH990C. Try starting with the wipers of R45 and R50 adjusted as close as possible to 0v and limit your adjusting to just a few turns in either direction. Still, you may find that the bias compensation is not effective with these particular op-amps.

[thebearingedge]

Hi, Tim. Thanks for the reply.

I pretty much gave up on adjusting the MM99's. I tried using the AD797 op-amps packaged with the J99 and ran into the same issue of a VERY large spread between the U8 pin 6 voltage at Full CCW and Full CW. However, I moved jumpers J6 and J8 away from the ADJ position and checked the voltages again. As per the old instructions, these measurements dropped. U8 dropped to about 8mV and U9 to nearly 0mV.

Upon listening I discovered that there is a bit of a noise floor with no mic plugged in and when I get to positions 10 - 12 on the gain switch there is a slight buzzing. Are either of these symptoms due to my U8 pin 6 reading higher than 5mV?

Am I on the right track at least? If the voltage at U9 gets very low (below 5mV) can I just leave R50 alone now and only worry about R45? Must I do R45 first?

Also I wanted to confirm that a slight pop in either normal or abnormal when adjusting the Gain Switch SW2. I don't get this with my A12's.


Thanks again, I really appreciate the help.

[dandeurloo]

I'm having very similar issues? Where and how are you metering your kit. I think I am getting better readings by grounding the negitave lead of my DMM to the chassis (stick it in a screw hole) and then my positive lead on pin 6. That seems to be working a little better? Maybe that will help you a little.

[thebearingedge]

Hey,

Actually I've done like you and grounded my negative lead but I was able to get clear readings just grounding to the U1 pin 1 instead of the chassis. Tonight I'm gonna try some more adjustments and see what I've got.

[dandeurloo]

cool, let me know what you find. I have been able to get U1 on both of my boards pretty close. U2 seems a lot harder for some reason. I think I have narrowed it down to the switches? It seems like each stage of the gain from the switch is relatively close but around the last three stages of full gain the jumps are really large. Also, they are not very regular? Each time I go back and check them the voltages are different from the previous time. I have check to see that I get constant power and I am. So, I feel something in the last few steps of gain are causing the crazy voltages. I have triple checked everything else and it seems like this makes the most sense. I have not hooked them up to anything yet but I am out of options so I think I might have to see how they sound with out being exactly where I would like them to be.

I also, noticed that in the sticky, Tim said to get U2 as close as possible. I wonder if that just means U2 is harder to adjust and it may never get as close as U1.

Let me know you get any different or similiar results.

[thebearingedge]

Hey dandeurloo

Just got done working on one J99. With the AD797's, not my Millenia op-amps, I was able to get U8 (first op-amp) to 0mV at CCW and 0mV at CW. YAY!!!

As you've pointed out, U9 is a TOTALLY different story! I can't get smaller than a 300mV gap! And to make things even more interesting, correct bias adjustment throws out my Op-amp power. Now I've got +18V/-19.3V. This can't be adjusted with the proper pot. With U8 calibrated, it's stuck! What the HELL... On the plus side, since the preamp is now "calibrated," the Voltages across pins 4 and 6 of the op-amps drops much more quickly.

So, what else could I do? GIVE A LISTEN of course

The noise floor of this preamp still seems a bit high without any mic plugged in but its comparable to my A12 which I'm pretty happy with so I really can't complain in this area. Plus I start clipping my converters with a low-ouput mic by the 8th gain position so the noise doesn't really come into play.

So a few questions remain....

*Is the asymmetric voltage across my op-amps going to hurt them?

*How much whitenoise should I be hearing when the pre is at full gain with no mic plugged in?

*Is it normal to hear a small click or pop when turning the gain switch?

Thanks!

[tpryan]

Is the asymmetric voltage across my op-amps going to hurt them?

No, but you should be able to adjust the voltages easily. Are you sure you're turning the right pot?

How much whitenoise should I be hearing when the pre is at full gain with no mic plugged in?

This is a meaningless test. You have to terminate the inputs, usually with a 150 ohm resistor when dealing with mic preamps. Even so, you will hear more noise at a given switch setting compared to your A12 because the J99 is delivering more gain.

Is it normal to hear a small click or pop when turning the gain switch?

Yes.

[thebearingedge]

Thanks Tim

This is just what I wanted to hear! I'll add any developments.