Author Topic: Serious imbalance with SSM Version  (Read 10915 times)

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Offline Cel

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Serious imbalance with SSM Version
« 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
« Last Edit: November 21, 2014, 06:15:16 PM by Cel »

Offline peranders

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Re: Serious imbalance with SSM Version
« Reply #1 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.
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Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #2 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

Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #3 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.


Offline peranders

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Re: Serious imbalance with SSM Version
« Reply #4 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.
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Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #5 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  :-(

Offline peranders

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Re: Serious imbalance with SSM Version
« Reply #6 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?
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Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #7 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?
« Last Edit: November 30, 2014, 05:08:33 PM by Cel »

Offline peranders

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Re: Serious imbalance with SSM Version
« Reply #8 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.
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Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #9 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.

Offline peranders

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Re: Serious imbalance with SSM Version
« Reply #10 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?
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Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #11 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.

Offline peranders

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Re: Serious imbalance with SSM Version
« Reply #12 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?
« Last Edit: December 01, 2014, 09:16:22 PM by peranders »
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Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #13 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.
« Last Edit: December 01, 2014, 09:51:10 PM by Cel »

Offline Cel

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Re: Serious imbalance with SSM Version
« Reply #14 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.