QE712 Stereo Balancing

The pultec stereo balancing thread was timely. I finally got my QE712's in a case with rotary switches for gain.

http://img.photobucket.com/albums/v357/plushmore/QE712/2b718f0d.jpg

The two sides are not matching up as well as I would like. The way I derived my resistor values was by soldering two 4k99 resistors together with the wiper lead connected in the middle to mimic the 10K adjustment pot. I did this for six of the seven bands. I then used a 10k 10 turn trimmer that I selected to be close to 10k (9k91). I measured and wrote down the values. I did this for a few of the frequencies and got similar values. I set the gain range trimmers on the board for 8dB of boost/cut. All went pretty well.

Now that I have all the switches done and can do more thorough tests the sides aren't matching up as well as I'd like. On some bands I can't get the "0" position to be at 0 with both the boost and cut set to 8dB. I have .5 db steps and they don't exactly match channel to channel.

I used 1% resistors for the 24pos switches. I haven't measured all the switches to see how close they are. Would this be the first place you would look. Do I have to get .1% resistors. I can't imangine selecting resistors so every switch was exactly the same. Should I look elsewhere?

> I used 1% resistors for the 24pos switches.

1% gives about 0.1dB accuracy. But over 24 steps, that could be several dB inaccuracy.

Cut 2 of the three wires to the switch. Use your best ohm-meter. Some of the "1%" resistors will be like 0.2% off, and some may be 0.9% off. If they are all off the same amount and in the same direction, it should be pretty close. If you find some -0.9% and some +0.9%, the gain will wander around a large piece of a dB.

You might end up testing, rejecting, selecting and replacing so many resistors that 0.1% might be the more-sane solution.

Looking further afield: be sure you have enough idle current in the push-pull output pairs Qs 4 5 9 10. Should be at least 2mA. 5mA might be better, 10mA if the transistors can take the heat.

Thanks PRR. Would you spell out how and where to measure the idle current through one of the transistors?