heaveno P-A,
I'm building a QSXM2 SSM Version for MM pickup, power supply is JSR04 +/-15V. I have two problems right now:
1.I have actually finished the board itself, and as I have gone through the voltages I have measured some really disturbing values around the servos IC1/IC3. I think I have a huge imbalance on both channel:
R27 0.755V
R29 0.762V
R28 13.44V
R30 13.44V
DC-ADJUST_L to ground measures 14.27V
Preamp out is on 9mV
The exact same happens on the right side as well. What could cause this?
2. Another problem are the voltages of R42-R47/E42-47, they all measure over 500mV. The temperature of transistors T31-T36/V31-V36 goes up to 85-90 celsius. I understand in this case one should shunt the LED with a resistor. I measure 2V on my LED, in the VAS stage runs 11mA(?) , so my LED represents around 182Ohm resistance. Normal voltage would be 1.6V so I need to bring down this resistance to around 146Ohm. That would mean a shunt resistor of 1Kohm (with the LED it would make 154Ohm). Is that correct?
Otherwise everything seems to work, other voltages seem to be fine.
Thanks in advance!
Cheers,
Marcell
The pre amp seems to work and you have the same error in the correction amps. It's a build error I think. Do you use BC transistors for the input stage of the corrections amp?
I have measured in almost all points. Please measure yourself and see where you start to have a big difference.
Short the input when you do the measurements.
heaveno P-A,
thank you for your reply. I use BC550C and BC560C everywhere except for the couple of SSM-s. I have measured all the points, most of them are around 5%-10% of your values, except for R27-R30 and maybe R38,R39 and R14. I have remeasured some of the resistors around of them, but I could not find anything suspicious.
Should I try to disconnect R93 (RIAA amp) and measure everything without the IC1 and IC3 in place? Or this would damage something?
I post all my values, both L and R side (on R side I measured the equivalent parts of course), input shorted:
Measured part | Reference | MyAmpChnLeft | MyAmpChnRight | DeltaL (%) | DeltaR (%) |
DZ1 | 6,96V | 6,75V | 6,75V | -3 | -3 |
DZ2 | 6,96V | 6,76V | 6,75V | -3 | -3 |
DZ3 | 7,08V | 6,76V | 6,72V | -5 | -5 |
DZ4 | 7,09V | 6,82V | 6,71V | -4 | -5 |
R1 | -9,5mV | 0V | 0V | -100 | -100 |
R6 | 491mV | 442mV | 462mV | -10 | -6 |
R7 | 438mV | 518mV | 508mV | 18 | 16 |
R8 | 5,85V | 5,71V | 5,68V | -2 | -3 |
R9 | 5,88V | 5,63V | 5,64V | -4 | -4 |
R10 | 5,86V | 5,73V | 5,70V | -2 | -3 |
R11 | 5,90V | 5,64V | 5,66V | -4 | -4 |
R12 | 4,80V | 4,71V | 4,65V | -2 | -3 |
R13 | 4,83V | 4,63V | 4,62V | -4 | -4 |
R14 | 200mV | 266mV | 272mV | 33 | 36 |
R15 | 299mV | 274mV | 281mV | -8 | -6 |
R16 | 5,64V | 5,39V | 5,41V | -4 | -4 |
R17 | 5,61V | 5,47V | 5,44V | -2 | -3 |
R18 | 5,64V | 5,40V | 5,41V | -4 | -4 |
R19 | 5,61V | 5,47V | 5,44V | -2 | -3 |
R20 | 5,64V | 5,40V | 5,42V | -4 | -4 |
R21 | 5,61V | 5,48V | 5,45V | -2 | -3 |
R22 | 5,64V | 5,41V | 5,42V | -4 | -4 |
R23 | 5,61V | 5,48V | 5,46V | -2 | -3 |
R24 | 508mV | 529mV | 529mV | 4 | 4 |
R25 | 518mV | 526mV | 526mV | 2 | 2 |
R26 | 1,65V | 1,712V | 1,714V | 4 | 4 |
R27 | 9,15V | 823mV | 829mV | 8895 | 8960 |
R28 | 3,65V | 13,51V | 13,53V | 270 | 271 |
R29 | 9,12V | 812mV | 825mV | 8804 | 8946 |
R30 | 3,62V | 13,51V | 13,50V | 273 | 273 |
R31 | 1,68V | 1,701V | 1,702V | 1 | 1 |
R32 | 1,56V | 1,646V | 1,645V | 6 | 5 |
R33 | 1,58V | 1,638V | 1,641V | 4 | 4 |
R34 | 926mV | 1,008V | 1,003V | 9 | 8 |
R35 | 926mV | 1,001V | 1,009V | 8 | 9 |
R38 | 834mV | 1,071V | 1,151V | 28 | 38 |
R39 | 855mV | 1,308V | 1,324V | 53 | 55 |
R42 | 325mV | 362mV | 371mV | 11 | 14 |
R43 | 319mV | 389mV | 369mV | 22 | 16 |
R44 | 321mV | 381mV | 372mV | 19 | 16 |
R45 | 319mV | 365mV | 374mV | 14 | 17 |
R46 | 316mV | 368mV | 381mV | 16 | 21 |
R47 | 323mV | 366mV | 368mV | 13 | 14 |
C3 | 6,46V | 6,21V | 6,23V | -4 | -4 |
C4 | 6,40V | 6,29V | 6,27V | -2 | -2 |
C19 | 5,28V | 4,63V | 4,49V | -12 | -15 |
C20 | 5,30V | 4,79V | 4,62V | -10 | -13 |
H1 | 1,82V | 1,84V | 1,84V | 1 | 1 |
heaveno again,
in the meantime I have measured all the resistors in the preamp section and compared the values with my excel BOM and with the schematic as well, but all checks out well. I have checked all the transistors also, they are all soldered where they belong. As far as I can tell, everything seems to be fine.
Could it be, that the difference between the SSM2220 and the SSM2210 alone cause this much of imbalance? Could I just reduce the imbalance with changing some resistors so the servos have some more headroom, and leave it at that? Probably not a cold solder joint or damaged transistor or something like that, because both channels have the same problem.
Or should I try to measure without the servos, as I've written in my previous post? I'm a little bit worried, because I'm running out of time as well (this would be a Christmas present).
I have tested it with small signals (not with a scope yet), and it works without any audible distortion.
Try to reduce R27 and R28, connect 100 k in parallel over those resistors and see what happens. I think the reason is that SSM2220 and SSM2210 has so different current gain.
I have tried the 100KOhms parallel.The Voltage of R28 has gone down to 9V R27 gone down to 553mV, however DC-ADJUST_L to ground hasn't changed :-(
You are still outside the range of the DC-servo. Can you try to measure the base current to see how much it could be?
Sorry but I'm not sure, where can I measure the base current? You mean the current from the JSR04? I can measure around 170-180mA on the +15V output of the JSR04. Or you mean the current of the base of one of the transistors?
Connect 1 kohm across the input and measure the voltage between R96 and R97. There you have 270k + 270k.
I have removed the short from the input and put 1Kohm between input and ground. I can measure between ground and R97/R96 (so practically the voltage on C63) -1,132V on the left channel and -1,143V on the right.
1.14V/(270k+270k) = 2.1 uA rather normal since the SSM2220 has a rather low Hfe.
Do you have one SSM2220 and one SSM2210 per channel?
If you short the input, do you have zero volts across R2? R2 = 1kohm?
I have tried it with one SSM2220/2210 per channel (with 3.9Kohm-s) as well, but had the same problem with the servos. Right now I have put in the full 4 pairs (with 15KOhm-s) per channel. Rather unnecessary I now, I think I will go with 2x pairs per channel and 7.5Kohm at the end.
Yes R2 is 1Kohm right now and I measure 4mV on it with input shorted.
Let roll back a bit. You have 9 mV offset voltage on the preamp output? The only bad thing is that you could have a few millivolt less offset if the servo not was saturated.
Notice that the SSM's need 1 mA or more in collector current in order to give low noise. The BC's need 200 uA. Without changing too much one pair SSM's will be enough.
You could experiment with using lower values on the 270k resistors. Which servo opamps do you use?
Do you use red LED's?
Yes,I have 9mV on preamp out.
So one SSM2220 and one SSSM2210 with 3,9Kohm-s should be the way to go?
I use the OP07 for all six opamp.
Yes I use red LED-s, although I had to solder 220Ohms parallel, because on the LED there was over 2V, and I had over 500mV on the resistors R42-R47.
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).
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)
The oscillations could be temperature variations. Are you sure that the oscillations is sinusoidal?
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.
How much is the output drifting? A millivolt or a fraction of it?
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.
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?
Quote from: peranders on December 02, 2014, 05:56:26 PM
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.
When the servos are working properly you should have the output as the opamp has, a few millivolts.
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 :-)
I'm glad that it works for you and you can use my stuff to what ever you'll like.