Studio Reference Amplifier

[thimios]

checking as in 'measured', and not 'just' by looking at color codes ?

or else, measure a similar spare component/resistor, and compare color codes with the ones mounted

just checking

R12,R13 =47R/+-1% MEASURED an color code checking.
kirchhoff say I(R11)=I(R12)+I(R13)
1.65mA=0.51mA+1.14mA
V(R11)=0.00165*8200=13.53V Measurment on R11=13.8v

 

[overall feedback]

R12,R13 =47R/+-1% MEASURED an color code checking.
kirchhoff say I(R11)=I(R12)+I(R13)
1.65mA=0.51mA+1.14mA
V(R11)=0.00165*8200=13.53V Measurment on R11=13.8v

Just asking, is there 2x470uF capacitor on it's place?

 

[thimios]

Just asking, is there 2x470uF capacitor on it's place?

In series with R15
Thanks for replay.

 

[thimios]

sr200 studio refer.

I would ask anyone has built this amp to measure VR12,VR13.Just two measurments without putting servo out.

 

[JOHN BALI]

Hi Thimios, I've build it too but not finish yet waiting for transformer to use... what voltage do you use it without servo? I will check my SR200 too so you can compare to it...

 

[AndrewT]

resistor checked, input LTP Vbe checked.
There is still serious imbalance in the LTP.

Now check the Vbe of the mirrors and the VAS.

 

[thimios]

sr200 studio refer.

Hi Thimios, I've build it too but not finish yet waiting for transformer to use... what voltage do you use it without servo? I will check my SR200 too so you can compare to it...

I use +/-45V with and without servo.
Keep in mind that amplifier is functional .
Thanks for replay.
good luck.

 

[thimios]

resistor checked, input LTP Vbe checked.
There is still serious imbalance in the LTP.

Now check the Vbe of the mirrors and the VAS.

Tkanks Mr, Andrew .
I can check this as soon as i can.

 

[thimios]

sr200 studio refer.

Hi Mr Andrew
Here are measurments.

 

[AndrewT]

None of those are broken. But too much variation.
Now do Q5, 6, 10 & it's partner.

 

[thimios]

sr200 studio refer.

None of those are broken. But too much variation.
Now do Q5, 6, 10 & it's partner.

Ok here again.

 

[overall feedback]

Ok here again.

0mV on base Q2 vs gnd looks impossible, please measure it again and check for errors. Anyway, if it works ok why bother?

 

[thimios]

sr200 studio refer.

0mV on base Q2 vs gnd looks impossible, please measure it again and check for errors. Anyway, if it works ok why bother?

I try to remove servo .
Apex say .....without servo and with unmatched LTP transistors the expected
offset is about +/-10mV.
In mine case offset without servo is 100mV with matched transistors
Thanks for replay.

 

[overall feedback]

I try to remove servo .
Apex say .....without servo and with unmatched LTP transistors the expected
offset is about +/-10mV.
In mine case offset without servo is 100mV with matched transistors
Thanks for replay.

Suppose output offset is +100mV. Left pin R14 is lower potential and current flows where? Capacitor blocks it from R15, and reverse base-emitter Q2 blocks it there. Even more weird, 54mV R13 suggest counter current. If measurements are not accurate, maybe problem is not accurate too?

 

[overall feedback]

Suppose output offset is +100mV. Left pin R14 is lower potential and current flows where? Capacitor blocks it from R15, and reverse base-emitter Q2 blocks it there. Even more weird, 54mV R13 suggest counter current. If measurements are not accurate, maybe problem is not accurate too?

And if suppose -100mV output offset, R13 is prime suspect. Look for some short circuits on layout, R13 to emitter Q2 connection.

 

[JOHN BALI]

I try to remove servo .
Apex say .....without servo and with unmatched LTP transistors the expected
offset is about +/-10mV.
In mine case offset without servo is 100mV with matched transistors
Thanks for replay.

Hi Thimios,
this what I got:
with servo & rail +/-43V
R11 =14,96V (15,59V on the zener)
R12 = 23,3mV
R13 = 23,6mV
R16 = 1,531V
R17 = 1,548V
R25 = 880mV
R26 = 856mV
R27 = 860mV
R28 = 882mV
DCO = 0.1mV (with TL071)
without servo anything not changed to much only DCO = 1,6mV
(I only lift R5 & add a bipolar cap on R15 see attachment)
I try to match all transistor to it partner, they are closely matched
that why I get very small DCO when servo is disconnected

I have my own problem, when I connect input to ground with cable I have smoke on the zobel resistor without speaker connected.
When the speaker connected it all okay but when I disconnect that happen...
Now I must find what wrong with this btw maybe because the cable is to long

 

[thimios]

sr200 studio refer.

Thanks for all.
Now it's clear that it's my problem.
I have a another qwestion .Is any 18pf capacitor or similar connected from collector Q9 to bace Q2 in your amplifier?

 

[JOHN BALI]

No, I use standard from Apex's schematic no changes... I will try that to solve my oscillation problem
I changed R24 to 2W & C 9 to 250V, high voltage there

 

[thimios]

No, I use standard from Apex's schematic no changes... I will try that to solve my oscillation problem
I changed R24 to 2W & C 9 to 250V, high voltage there

If you have see all posts Apex suggest to me this modification because there is an overshoot .
I must try to remove this again

 

[thimios]

0mV on base Q2 vs gnd looks impossible, please measure it again and check for errors. Anyway, if it works ok why bother?

I apologize for this.
V from base to ground Q2=-10mV
V across R14(feedback res.)=100mV.
Sorry for this.