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Projects => Phono RIAA amplifiers => Topic started by: Cel on November 20, 2014, 01:30:37 PM

Title: Serious imbalance with SSM Version
Post by: Cel on November 20, 2014, 01:30:37 PM
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
Title: Re: Serious imbalance with SSM Version
Post by: peranders on November 23, 2014, 07:56:41 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: Cel on November 25, 2014, 03:11:52 PM
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  (%)
DZ16,96V6,75V6,75V-3-3
DZ26,96V6,76V6,75V-3-3
DZ37,08V6,76V6,72V-5-5
DZ47,09V6,82V6,71V-4-5
R1-9,5mV0V0V-100-100
R6491mV442mV462mV-10-6
R7438mV518mV508mV1816
R85,85V5,71V5,68V-2-3
R95,88V5,63V5,64V-4-4
R105,86V5,73V5,70V-2-3
R115,90V5,64V5,66V-4-4
R124,80V4,71V4,65V-2-3
R134,83V4,63V4,62V-4-4
R14200mV266mV272mV3336
R15299mV274mV281mV-8-6
R165,64V5,39V5,41V-4-4
R175,61V5,47V5,44V-2-3
R185,64V5,40V5,41V-4-4
R195,61V5,47V5,44V-2-3
R205,64V5,40V5,42V-4-4
R215,61V5,48V5,45V-2-3
R225,64V5,41V5,42V-4-4
R235,61V5,48V5,46V-2-3
R24508mV529mV529mV44
R25518mV526mV526mV22
R261,65V1,712V1,714V44
R279,15V823mV829mV88958960
R283,65V13,51V13,53V270271
R299,12V812mV825mV88048946
R303,62V13,51V13,50V273273
R311,68V1,701V1,702V11
R321,56V1,646V1,645V65
R331,58V1,638V1,641V44
R34926mV1,008V1,003V98
R35926mV1,001V1,009V89
R38834mV1,071V1,151V2838
R39855mV1,308V1,324V5355
R42325mV362mV371mV1114
R43319mV389mV369mV2216
R44321mV381mV372mV1916
R45319mV365mV374mV1417
R46316mV368mV381mV1621
R47323mV366mV368mV1314
C36,46V6,21V6,23V-4-4
C46,40V6,29V6,27V-2-2
C195,28V4,63V4,49V-12-15
C205,30V4,79V4,62V-10-13
H11,82V1,84V1,84V11
Title: Re: Serious imbalance with SSM Version
Post by: Cel on November 29, 2014, 07:30:13 PM
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.

Title: Re: Serious imbalance with SSM Version
Post by: peranders on November 30, 2014, 10:15:45 AM
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.
Title: Re: Serious imbalance with SSM Version
Post by: Cel on November 30, 2014, 11:03:05 AM
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  :-(
Title: Re: Serious imbalance with SSM Version
Post by: peranders on November 30, 2014, 02:25:36 PM
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?
Title: Re: Serious imbalance with SSM Version
Post by: Cel on November 30, 2014, 05:03:23 PM
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?
Title: Re: Serious imbalance with SSM Version
Post by: peranders on November 30, 2014, 09:40:49 PM
Connect 1 kohm across the input and measure the voltage between R96 and R97. There you have 270k + 270k.
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 01, 2014, 06:57:48 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: peranders on December 01, 2014, 07:10:23 PM
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?
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 01, 2014, 08:09:57 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: peranders on December 01, 2014, 09:14:44 PM
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?
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 01, 2014, 09:47:21 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 01, 2014, 09:53:35 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 01, 2014, 10:08:31 PM
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).
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 01, 2014, 10:45:33 PM
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)
Title: Re: Serious imbalance with SSM Version
Post by: peranders on December 02, 2014, 06:18:38 AM
The oscillations could be temperature variations. Are you sure that the oscillations is sinusoidal?
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 02, 2014, 12:32:13 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: peranders on December 02, 2014, 05:56:26 PM
 How much is the output drifting? A millivolt or a fraction of it?
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 02, 2014, 10:51:19 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 03, 2014, 08:59:08 PM
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?
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 07, 2014, 06:48:57 PM
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.
Title: Re: Serious imbalance with SSM Version
Post by: peranders on December 07, 2014, 09:32:36 PM
When the servos are working properly you should have the output as the opamp has, a few millivolts.
Title: Re: Serious imbalance with SSM Version
Post by: Cel on December 08, 2014, 12:02:41 AM
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  :-)




Title: Re: Serious imbalance with SSM Version
Post by: peranders on December 31, 2014, 09:07:17 AM
I'm glad that it works for you and you can use my stuff to what ever you'll like.