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Old 22nd January 2013, 08:28 AM   #1211
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I have no pet, so no cat at all

unfortunately I have no photo of my boards, but if I look at them (sink on the right hand side) all of the white stripes (locat minus of the caps) look straight up.

Other suggestions?
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Old 22nd January 2013, 10:02 AM   #1212
quan is offline quan  Australia
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Originally Posted by hesener View Post
Hi,

the test points all have a square layout and are labelled, except the test points right next to the emitter resistors of the current sources. The test point you are referring to might be round, in which case I think it is a "via" (connection between top and bottom layer of the PCB). Yes, that may be the output connection of the servo (== pin 6 of the opamp), this line is pretty long and winding....

The servo giving such a large voltage means it has to compensate a certain offset of the input stage. Remember the servo's impact is pretty weak, and deliberately so, so that voltage can reach several volts both ways. It should stay within +/-12 V though, in order to not leave the linear output range of that opamp. 5V is therefore OK. Best would be 0V, but that only happens with perfectly matched input stage, current mirrors, and output buffer with no offset.

While the mirrors's design is very clever so their impact on offset is negligible, the other two sources of offset may be the cause. On the output buffer, if the JFET Q99 has a high IDSS, the voltage on its gate will have to be negative so it operates at the current dictated by Q91+R15. So in order to have zero output offset, the voltage at its gate will have to be negative, so the servo must steer it that way. In my case, the voltage there is -1.5V (as documented in the annotated schematic in the assembly guide). Now, you could run the output buffer at higher current, by reducing R15, and thereby reducing this offset, but thats not needed for performance or sound.

On the input stage, if the current gains are not matched well enough between the NPNs and the PNPs, you may see that type of offset as well. In fact, it should be different for different values of input load resistance (you would probably have to look at values of 10K or higher to see something). It may also be that there is one transistor that exhibits base-collector leakage current, this is VERY rare but sometimes individual transistors excape the test program at manufacturing. You may want to try input resistors of 10K and 20K just to see if the offset changes. If it doesnt than just live with it, you should be fine. If it does, you may want to check if you can come up with another set of input stage transistors.

I would also suggest you remove that 10 Ohm resistor in the ground wire. The ground connection from the star ground to the ground connection on the PCB should be as low-ohmic as possible. Rather put the 10 Ohm in the ground connection of the power supply. In fact, the assembly guide has a detailed schematic from Frans on how to do the grounding on the power supply side which is very good. You want to have the TT ground, the phono preamp case ground and the PCB grounds as closely linked as possible. Please do connect the output RCAs to the same star ground. The power supply wiring on your picture is good.

hope that helps.....
Thank you Alfred for your comprehensive explanation and i understand only some of it due to my limited knowledge. To clarify a few things: so if i change the input loading to 20K it should make no difference to the voltage at pin6 of IC then just leave it alone? and if it rises then may have to change the input transistors due to leakage current of BC junction?
Quan
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Old 22nd January 2013, 10:51 AM   #1213
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by Libby123 View Post
I have no pet, so no cat at all

unfortunately I have no photo of my boards, but if I look at them (sink on the right hand side) all of the white stripes (locat minus of the caps) look straight up.

Other suggestions?
Did you make any measurements on the boards just after power on ?
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Old 22nd January 2013, 11:33 AM   #1214
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No, just V+,V- and the power in (+24,7V and -24,3V)
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Old 22nd January 2013, 11:54 AM   #1215
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No, just V+,V- and the power in (+24,7V and -24,3V)
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Old 22nd January 2013, 12:08 PM   #1216
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by Libby123 View Post
No, just V+,V- and the power in (+24,7V and -24,3V)
See PradiseBuilders #155 Vdc input voltage minimum simulated. it would be nice to have about a volt more, but it will work (The linked message states that 24V is marginal). (Do I have to post this twice )

Last edited by FdW; 22nd January 2013 at 12:11 PM.
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Old 22nd January 2013, 12:34 PM   #1217
hesener is offline hesener  Germany
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Quote:
Originally Posted by quan View Post
Thank you Alfred for your comprehensive explanation and i understand only some of it due to my limited knowledge. To clarify a few things: so if i change the input loading to 20K it should make no difference to the voltage at pin6 of IC then just leave it alone? and if it rises then may have to change the input transistors due to leakage current of BC junction?
Quan
yes exactly
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Old 22nd January 2013, 12:37 PM   #1218
hesener is offline hesener  Germany
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Originally Posted by JBdV View Post
Hello Alfred,
I second Benedetto's request ....
"Hello Alfred,
is it possible to you to put the link to your
fine, constantly actualized Builders Guide in your underline ?"

pleeeeeeaaaazzzzzz ;-)

It will make things much easier, you would even get less support questions...

JBdV
will figure it out
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Old 22nd January 2013, 12:46 PM   #1219
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by Libby123 View Post
No, just V+,V- and the power in (+24,7V and -24,3V)
Well, normaly if EL caps boil, they are with wrong polarity.

We need more measurements to be able to help.... You can remove the "cooked" caps and power the boards so measurements can be made.
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Old 22nd January 2013, 01:03 PM   #1220
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Thanks Frans, but what does that mean? 24V is minimum. but I have 24,3V so i am over, right?
Will this happen (leakage caps), when the Voltage drops under 23V?
I meassured yesterday evening (before bet) and this morning I saw the wet Caps and pulled of the plug. So I could not meassure.
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