Serious imbalance with SSM Version

[Cel]

I have changed R96/R97 to 150KOhm, but it makes no difference IC1 pin 7 to ground measures still around 14,40V. (Input shorted, parallel resistors from R27, R28 have been removed for now, still 4 pair of SSM-s in with 15Kohms as R16-R23).

[Cel]

I have done some more experimenting on the left channel:

0.Starting with 150Kohms on R96/R97 still in, input shorted: servo 14,40V
1. changed R17/R16 to 3.9KOhm and left only the first SSM2220 and SSM2210 in: Servo started from 14,50V and gone down to around 14,40V again.
2. shorted R2: Servo started from 14,30 settled fast around 14,33V
3. soldered back the 100K-s parallel to R27 and R28: Servo gone up a bit to 14,38-14,40V again

I have 5,65V and 5,57V on R17/R16, so I got ~1.45mA on the SSM-s now.

Update1: I have left it turned on that way, the output of the servo gone down slowly (ca. 5-10mV/sec). After ca. 10 min I measure 11,90V, altough now it slowly oscillates between 12,30V and ca. 11,80V (again with ca. 5-10mV/sec, however it slows down at the upper and lower limits like a sinus).

In the meantime right channel (no modifications, 4 pair off SSMs with 15Kohm) is stable on 14,42V

update2: Still oscillates slowly on the left. (15min later)

[peranders]

The oscillations could be temperature variations. Are you sure that the oscillations is sinusoidal?

[Cel]

It is hard to say without the right tools, but it seems to be a sinus with a cycle time around 1-2 minutes (I would have to measure it, if it is important). It definitelly slows down around the limits (11,70-11,80 and around 12,30-12,40). The limits vary slightly with time. About half an hour later, I remember measuring only 12,01V as upper limit.

Unfortunatelly I can`t say which change caused this change in the servo output,  because I have waited way to little while I measured the values, but it shouldn`t be too hard to find out. I will undo the steps one after the other, when I get home, and see when it disappears.

[peranders]

 How much is the output drifting? A millivolt or a fraction of it?

[Cel]

Ok, so I measured it again, this time I have left the multimeter on it for the whole time and just watched what happens. It is definitely no sinus, but a temperature drift.

1.After turning it on, the servo voltage is around 14,40V, after 5-10 minutes it goes down to around 12,20V.
2.If I touch for example the transistor T33, the voltage on the servo output goes down a bit to 11,70. If I release it, it goes back and settles aroun 12,20V.
3.If I blow on the transistors T31-T36 the servo  voltage goes up.

If I move my hands over the circuit while holding the measuring headers, the small airflow generated by my movements triggers voltage drifts, that's what I did not recognized the first time. When I stay completely still the voltage always settles somewhere around 12V.

Unfortunately the voltage was too high again on R42-R46 (~480mV), so now I have put an even smaller resistor parallel with the LED, which brought back the voltages of R42-46, but caused the servo output to settle around 14,40V again. So I am back where I started :)

Update: I left it turned on for an hour, servo output  settled now around 12V, R42-R46 gone up a bit, but not so much (420mv). Tomorrow I will try to get the voltage of R42-R46  down to 330mV with an even lower resistor and see what happens.

[Cel]

I have tried it out today. So if I bring down the voltage of R42-R46 to around 330mV the servo output just stays on ~14,40V, and doesn't go down at all. I have left it turned on longer, but no change. I'm back where I started :(

Is there anything else I could try?

[Cel]

How much is the output drifting? A millivolt or a fraction of it?

I have measured the preoutput with the modifications No. 2 from my comment "December 01, 2014, 10:45:33 PM":

directly after turning on:
Servo output: 14,35V
preout offset -84mV

after warm up (R42-R46 400mV):
Servo output: 11,43V
preout offset: -40mV

Main output has an offset of 6V now.

On the Right channel with no modifications: 0V

I think I will undo the modifications, and just pay attention to the output. If I don't have any DC with the cartridge connected, I think I will leave it as it is.

[peranders]

When the servos are working properly you should have the output as the opamp has, a few millivolts.

[Cel]

Which opamp do you mean? IC5 and IC6?

The output of the servo on the second stage (IC2) is only at 4,5V with this 45-50mV offset on the pre amp. IC4 on the right channel (no mods, and only offset 9mV on the preout) is almost 0V (0.5mV) I think  the second stage swallows the little offset that the preamp cannot cope with. So I think I will just undo the mods and hope that with a turntable it won't be much more than that, and then the riaa part can easily correct the little offset, like it does now. How much DC could typically come from the turntable? Unfortunately I do not have a record player right now to test it with. :(

Actually I did some listening tests with a PCM1704 dac chip with a small resistor as current/voltage converter, because PCM1704 is very sensitive to load, and mostly unsuited for passive IV, because to keep the THD at bay, one would need to use a tiny resistor value, and in this case the signal is way too small. I figured in a way, this problem is quite compare with the tiny signals from a record player, so long story short I have just put 50 Ohm on the PCM1704 as IV resistor and hooked it up to the QSXM2 to test it. After some measurements I've seen that everything seems to work fine, so I connected  the preout of the QSX2 to my EHHA headphone amp and tried it with some music with my HD650. I have to say, the sound was astonishing! I have never heard anything even close to this on my setup. I was completely shocked!

As you might have noticed, I have ordered another QSXM2, and I'm planing to use it with my PCM1704 DAC. If you don't mind my sacrilege, using a phono amp to digital stuff, I would probably open a new topic on this, because I would need some suggestions on how to use the QSX2 this way. ( For example shorting or changing the filter elements of the RIAA amp, and maybe use a poti somehow between the preamp and the riaa section for volume control, if it could be done without destabilizing the amp)
I have bought  a truckload of BC transistors, and I have a couple of SSM-s as well, so I would try it out either way, just to play around a bit with this servo problematic, just in case. Probably starting with it next year, but for now I concentrate on the first QSX2.

So anyway, fantastic amp! I can't wait to try it with a turntable and speakers. Now I will try to build a case for this gem, but of course in the mean time , I am open for any suggestions, if you think I should try something. Christmas is near, I have to get cracking  :-)

[peranders]

I'm glad that it works for you and you can use my stuff to what ever you'll like.