Opamp fader circuit level mismatch

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JAY X

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Hi,

I'm troubleshooting a small level mismatch between a pair of opamp based fader amplifiers of my summing mixer. The opamps are LM4562. rails voltages +/-17.5v.

The AC levels (RMS) comming in from the balanced return diff amps are nearly equal:

L return level: 0.183v  -12.53dbu
R return level: 0.189v  -12.25dbu.

The AC levels (RMS)measured after the stereo 10k linear pot are also nearly equal:

Left pot output level:  0.175v  -12.92dbu
Right pot output level: 0.177v  -12.82dbu

The AC levels (RMS) measured after the input coupling capacitor (2.2uf 63v) + 47k input resistor to gnd. These are the levels BEFORE reaching the non inverting opamp input, to clarify.

Left ch:  0.114v  -16.64dbu
Right Ch: 0.135v  -15.17dbu

So, the question is if the input coupling caps, if they are bad, ¿can they cause this level mismatch?

Any help appreciated!!

Jay x
 

JohnRoberts

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A useful troubleshooting technique is to reduce the number of variables. If two similar gain stages show different output levels connect the two inputs to only one source.

If they use faders maybe short the two wiper together insuring that the voltage there for both will be the same...

The difference should reveal what is causing it.

JR
 

abbey road d enfer

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JAY X said:
Left pot output level:  0.175v  -12.92dbu
Right pot output level: 0.177v  -12.82dbu

The AC levels (RMS) measured after the input coupling capacitor (2.2uf 63v) + 47k input resistor to gnd. These are the levels BEFORE reaching the non inverting opamp input, to clarify.

Left ch:  0.114v  -16.64dbu
Right Ch: 0.135v  -15.17dbu
That means that there is something that creates attenuation in both channels.
If I understand correctly, you don't have then same level on both sides of the capacitors? That's weird.
I think a bit of schemo would help us to help you.
 

JAY X

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Hi!

I run a signal from one output of the audio interface into the two returns with the same level, and the mismatch disappeared. I think it was due to the stereo sine signal not being perfectly matched on both channels.

Another question that maybe it is important or not... In mixing consoles all faders have a Unity gain position. ¿how is it determined? I read somewhere, that unity gain position is the position where the fader attenuates the same amount as the gain the amplifier gives... ??? In my case the output fader is a dual linear pot loaded with 47k input resistor to gnd.

Jay x
 

JohnRoberts

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JAY X said:
Hi!

I run a signal from one output of the audio interface into the two returns with the same level, and the mismatch disappeared. I think it was due to the stereo sine signal not being perfectly matched on both channels.

Another question that maybe it is important or not... In mixing consoles all faders have a Unity gain position. ¿how is it determined? I read somewhere, that unity gain position is the position where the fader attenuates the same amount as the gain the amplifier gives... ??? In my case the output fader is a dual linear pot loaded with 47k input resistor to gnd.

Jay x
The typical design target is for +10dB gain in post fader gain stages, to deliver unity, the fader would be attenuating -10dB to deliver the nominal 0dB combined.

JR
 

JAY X

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JohnRoberts said:
The typical design target is for +10dB gain in post fader gain stages, to deliver unity, the fader would be attenuating -10dB to deliver the nominal 0dB combined.

JR

OK, so if I have a 0db line receiver + 10db fader amp + 6 db balanced line driver = +16db of gain...So I should inject an input signal of -16dbu, rise the gain +10db, to get +6db at the balanced line driver output. And the potentiometer then will be at the Unity gain position. ¿correct?

Jay x
 

abbey road d enfer

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JAY X said:
OK, so if I have a 0db line receiver + 10db fader amp + 6 db balanced line driver = +16db of gain...
Correct.

So I should inject an input signal of -16dbu, rise the gain +10db, to get +6db at the balanced line driver output.
Correct too.

And the potentiometer then will be at the Unity gain position. ¿correct?
Unity gain means 0dB gain. this is in contradiction with your first sentence.
Unity gain is when you lower the fader by 16dB.
Are you confusing "unity" with " nominal"?
 

JAY X

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Hi!,

Attached, is the pdf schematic of the fader amplifier and line driver output and transformer output stages.

I decided to have both clean line driver output, and a more colored transformer output. The point is that I have a small level mismatch between both outputs, when i switch from one to another. The level mismatch is about 4db. The transformer output is 4db higher wrt the balanced line driver output. I reduced the feedback resistors to 5k to try to match the output level to the line driver output, but I still have  a 4db mismatch... ??? From the Lundahl datasheet, the mixed feedback driver is used in a 2:1 configuration. Maybe there is something i missed...

Jay x


 

JAY X

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Hi!

Yes, R7, R10, 10k. Ok.

Now I'm in a phase, considering making some changes after some suggestions.. For example, In most summing mixers I see, they don't have an output fader amplifier, but an attenuator, maybe because much gain is not needed. I could turn the fader amplifier into a buffer, or give it smaller gain, say 6db.

 

abbey road d enfer

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JAY X said:
For example, In most summing mixers I see, they don't have an output fader amplifier, but an attenuator, maybe because much gain is not needed. I could turn the fader amplifier into a buffer, or give it smaller gain, say 6db.
Many like to abuse their summing amps, by slightly overloading the output stage. that's why they use an attenuator at the output.
 

JAY X

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Hi Abbey!

Today I changed the 5k resistors in the R channel of the transformer buffer for the 10k resistors, and in the Left channel, i left the same 5k resistors, to compare. For the same input signal, the level with unity gain transformer buffer dropped by 6db, wrt the Left channel, and are now 2db below the output level of the line driver output.

Perhaps I miswired the transformer primary.... These lundahl trafos, after the datasheet, are driven with a mixed feedback driver with the first input stage non inverting, and the second stage inverting. In my case both stages are inverting types.... I don't know if this makes sense...

Jay x

 

JAY X

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Hi!

¡Thanks Abbey!

Attached is a drawing of my mistake and correction. I miswired the primary phases.

Now I'm figuring how to correct the wiring, on board...drilling holes, cutting traces... ::)
I Hope to have it corrected in the next days.

Jay x
 

Attachments

  • TRAFO WIRING.JPG
    TRAFO WIRING.JPG
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JAY X

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Hi!

Finally i could correct the primaries wiring, making the first buffer stage non-inverting. The secondaries are wired in paralell. So effectively i have the primaries in series and secondaries in paralell. This makes the transformer ratio: 2:1, halving the output level at the secondary. I could wire the secondaries in series, making the transformer ratio 2:2 or unity, But it is very difficult to desolder the transformer from the board.. :p.

Jay x




 
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