QRV-08 Building and test

[galouche]

yes, I've checked and tested every coponent ... very strange ... But the sound is very cool, the sinewave of the generator is ok at the output without apparent distortion, no pic, no DC offset just input bias on the right channel can't be adjust before C1 for exemple 600 mV behind C1 ( or over R1 or between r10/R11) and before C1 it's always 2.4 mV wich is a good value but not in the good way ... very curious I don't see why for the moment

[peranders]

Have connected the pot both positive and negative voltage? How about the voltage at "wiper" at P1?

[galouche]

I have connected the two pot at the negative voltage ( in fact the left channel don't need it at all so I'll desolder it ) the pot is well doing its job it brings -749 mV ( just after the pot ) to have a 2 mV input bias after C1 ( not easy to adjust with only one rotation, 25 rotation trimmer are more easy :) )

right opa output is -1.2 V

[peranders]

I'm not following you here. You said 600 mV and now it's only 2 mV? I think less than 5-10 mV is perfectly OK as voltage across R1. The rest takes the DC servos care of.

You should be able to see a shift in sign in voltage across R1 when you adjust the pot.

[galouche]

the 600 mV was an exemple it can be 1 V, -1V ... befor C1 it's always 2mV

[peranders]

You mean it's hard to adjust more accurate?

[galouche]

No, It exemples, I've just adjusted it :

0.2 mV after C1 so only - 0.30V at the opa output and always 2 mV before C1 on the wago. the pot bring - 686 mV

[peranders]

It looks good. Nearly 0 mV at the input (= you inject the needed input bias current) and the DC servo is working properly. How much output offset did you get?

[galouche]

The output offset with this adjuste is 8.4 mV.

when I turn the pot the output offset is changing so no prob with it.

with 1.6 V AC signal at input : before C1 2.4 mV DC                                  after C1: 0.2 mV DC   out offset : 8.4 mV
without AC signal at  input :                 3.5 V DC shuting down to 250 mV                 0.2                             8.4 mV

all those value are +- 5mV moving continuously

[peranders]

Do you use OPA134? You should have a bit lower offset, max 2 mV

[galouche]

when the input bias stay at 0.2 mV the output offset is 1 mA, 8.4 is the max output I've seen. I've find and repared 5 problems in my work on this pcb but this problem is very strange ... If there was a parasitic capacitance on the input like it seems when I test without input signal, turning the pot would change the input bias ... I don't understand at all  :?  :wink:

  Thanks for your help ... sorry to bother you ... I have several opa134  in my stock now but I don't think it comes from it ? it have a link with it because of the static input bias before C1 ( I see quick pick if I turn quickly the pot but it shuts downd very quickly to 2 mV input bias)

[peranders]

If T11 and T12 had exactly the current gain the total bas current would be zero but in real life they differ. This different results in a current which either goes in or out and this current is the one you add or substract with the pot.

[galouche]

If I understand well after T11/T12, the signel goes through T13/T14 then R10/r11 and finish in T19/T20. the T25/26 and T27/28 is the buffer.

I'm not completly familiar with opamp ( thanks for your jung pdf files I'm reading )could you explain me a little bit how it works in this schematic ?

[galouche]

looking a little bit on the schematic helped me to understand about how the opa work ( I think ) it mesures the dc offset between the 2 resistances and compensate / the 0V reference. thesignal is court circuited by the condensator so just the DC is adjust.
  If I'm right I can now study my problem knowing all I need ... Thanks for your patience ... :)

[galouche]

I've just listened the QRV-08 with the cd pro2 I've built ... very plaisant, very neutral, I hear for exemple for the first time a real bass sound (as I'm used to ear it in studio)... Thanks for this baby.

I'll put a 2 x 10k log at the input because behind the cd pro2, the sound is high ...

PS : I've not found the parasitic capacitance I have  but it doesn't affect the sound so I prefer to finish the pcm1794 DAC ...