Neumann V475-2 level problem

[Audioman]

Hi All,
I'm building a passive summing mixer for a friend using a neumann V475-2 as summing amplifier,
I have a problem with the neumann card , the card loss 1 dbu on the output at the max output level but if i connect another card the problem disappear and i have a margin of 1 dbu more !!
I have look the two card and the difference is that the one that working correctly have changed two resistors the R8 and the R108 from the original 7K5 to 10K and all the electrolytics capacitors are recapped
Anyone have idea of what is the problem ??

 

[pucho812]

Sounds like you have it figured out.

You mention a resistor change and a recap. Unless the working one has problems, sounds to me like you need to change the resistors and recap.

 

[[silent:arts]]

There is a trim potentiometer on the card with a range of +/-3dB ...

 

[Audioman]

Hi Volker,
I know there is a 10k trimmer but if i turn the trimmer at the Max level the Loss is -1 dbu ....
this is the problem

There is a trim potentiometer on the card with a range of +/-3dB ...

 

[Harpo]

Hi Volker,
I know there is a 10k trimmer but if i turn the trimmer at the Max level the Loss is -1 dbu ....
this is the problem

...and the trimmers R9 and R109 come with +/-20% parts tolerances, giving a feedback resistor range of 7k5+10k*0.8 up to 7k5+10k*1.2 for the highest gain setting.
When tolerance is on the low side, you are short of enough ohm to get your head scratching/to bother you.
For an easy fix just increase the 7k5 feedback resistors R8 and R108 with 10k ... 12k resistors (instead of F.I.changing all summing resistors that connect to the V475-2 pins 10,11,20,21). YMMV.

 

[Audioman]

Hi Harpo,
I have changed the two resistors (from 7k5 to 10K) now i reach the +4 dbu at full trim level and i don't any gain margin.
If i change again resistors with two 12k can i reach the desired margin ??
About the recapping electrolytics, you advise me to change them ??

P.S. near the 10k trimmer there are another trimmer what these are ?? and how can i calibrate them

...and the trimmers R9 and R109 come with +/-20% parts tolerances, giving a feedback resistor range of 7k5+10k*0.8 up to 7k5+10k*1.2 for the highest gain setting.
When tolerance is on the low side, you are short of enough ohm to get your head scratching/to bother you.
For an easy fix just increase the 7k5 feedback resistors R8 and R108 with 10k ... 12k resistors (instead of F.I.changing all summing resistors that connect to the V475-2 pins 10,11,20,21). YMMV.

 

[Harpo]

12K resistors for R8 and R108 should give you the desired margin.
(for usual you would have fitted the rescaled input resistor values that compensate for your passive summer losses. The V475-2 is a stereo active summing amplifier.)
Recapping the (Roederstein) electrolytics is highly recommended. While at it, you might recap the tantalum caps as well.
The remaining trimmers C2 and C102 are varicaps in range 8pF...40pF in order to match values with C1 and C101. Without adequate measuring equipment and measuring wires for this pF range better don't touch ...

 

[Audioman]

Hi Harpo, you advise me to use the same brand of electrolytics (roederstein) or another brand is fine ??
What you mean when you Say adeguate measuring equipment ?? And how Can i check if the calibration is correct ??

12K resistors for R8 and R108 should give you the desired margin.
(for usual you would have fitted the rescaled input resistor values that compensate for your passive summer losses. The V475-2 is a stereo active summing amplifier.)
Recapping the (Roederstein) electrolytics is highly recommended. While at it, you might recap the tantalum caps as well.
The remaining trimmers C2 and C102 are varicaps in range 8pF...40pF in order to match values with C1 and C101. Without adequate measuring equipment and measuring wires for this pF range better don't touch ...

 

[Harpo]

Any brand with correct value, voltage rating, pin spacing and parts orientation should fit the bill.
Do you have a cap meter, capable of measuring this low pF values ? A meter of guitar cable probably has a higher capacitance than 8...40pF, hence the questioning of measuring wires ... (I wouldn't expect much if any rf contamination in this spot within a shielded enclosed box, so I'd probably leave them out, but as always YMMV).

 

[Audioman]

Hi Harpo,
I must respect the original voltages (eg 470uF 6V) or if i find a superior voltage (eg 470Uf 10V)  is fine ??
I have a Neutrik A2 in the lab of the radio station where i work

Any brand with correct value, voltage rating, pin spacing and parts orientation should fit the bill.
Do you have a cap meter, capable of measuring this low pF values ? A meter of guitar cable probably has a higher capacitance than 8...40pF, hence the questioning of measuring wires ... (I wouldn't expect much if any rf contamination in this spot within a shielded enclosed box, so I'd probably leave them out, but as always YMMV).

 

[Harpo]

I must respect the original voltages (eg 470uF 6V) or if i find a superior voltage (eg 470Uf 10V)  is fine ??

For this specific 470uF cap, connecting to a virtual ground node of a working opamp, any voltage rating (3.3V, 6.3V, 10V, 16V, 25V, ...) should do. For the remaining lytics pick same or higher voltage rating for replacement.

 

[Audioman]

About the Level problem i solved with the 12 k resistor change now is fine and i have a margin of 1 dbu on the trimmer

Thank you guys for help me !!