0Vu = 1.625v

[moamps]

+6 dBm = 0VU (= -9dBfs) is broadcast standard in Germany 

Are you sure about that (then the  0dBfs is only 4.5VRMS )?

 

[[silent:arts]]

Are you sure about that (then the  0dBfs is only 4.5VRMS )?

Yes.

 

[guze]

Lower the value of R22 (22K), IC3c.
And R10 (22K),  IC3d.

16K.ohm should be about right.

If i change these resistors it should match 0Vu=+4dbu, right?

 

[Bo Deadly]

If i change these resistors it should match 0Vu=+4dbu, right?

Yes. Based on the information that you have provided it sounds like you're at a little more than +6dBu. So changing those resistors should bring it back to ~+4dBu.

 

[Gold]

So it doesn't really matter to which level an old and lazy VU-meter is calibrated because it is not an measurement instrument it is more an "estimation-iron" as we say to it in german.

The VU meter was designed as a loudness meter oriented to the human voice. It was designed by eye by a group of engineers. When it looked right they wrote the spec.

 

[abbey road d enfer]

Yes. Based on the information that you have provided it sounds like you're at a little more than +6dBu. So changing those resistors should bring it back to ~+4dBu.

It would change the overall system gain, but if the meters are connected at the output, with the wrong resistor, they will still be out of whack. Thus it is paramount to know how and where from the meters are fed.

 

[Bo Deadly]

It would change the overall system gain, but if the meters are connected at the output, with the wrong resistor, they will still be out of whack. Thus it is paramount to know how and where from the meters are fed.

Just to be clear, we're talking about two different things. The original question was how to change the output from 1.625Vrms which is +6.44dBu to +4dBu. That is a difference of 0.755x which can be had by adjusting said resistors from 22K to ~15.8K. This has nothing to do with what the meters say. The schem doesn't show where the meters are tapping into but you'll know if they drop by 2.4dB or not in which case they will need to be adjusted or not.

 

[ruffrecords]

Just to be clear, we're talking about two different things. The original question was how to change the output from 1.625Vrms which is +6.44dBu to +4dBu. That is a difference of 0.755x which can be had by adjusting said resistors from 22K to ~15.8K. This has nothing to do with what the meters say. The schem doesn't show where the meters are tapping into but you'll know if they drop by 2.4dB or not in which case they will need to be adjusted or not.

To be fair, the OP's original question was a little vague. He just measured his output at 1.625V without mentioning the conditions. It is not unreasonable to assume the conditions were with the meters reading 0VU. How else would he know it was too hot?

Cheers

Ian

 

[guze]

What i want is for the vu at 0 to match +4dbu so i don't need to pad before i hit the ADC and also so i can better use the meters near 0

Wich recomendation for the opamp at that spot?
Opa4132 (fet) or ne5532 (bipolar) or a 5532 and 2134?

 

[abbey road d enfer]

What i want is for the vu at 0 to match +4dbu so i don't need to pad before i hit the ADC and also so i can better use the meters near 0

Since I don't know where the meters are fed from, I can't answer thai question.

 

[abbey road d enfer]

Just to be clear, we're talking about two different things. The original question was how to change the output from 1.625Vrms which is +6.44dBu to +4dBu. That is a difference of 0.755x which can be had by adjusting said resistors from 22K to ~15.8K.

This coulm be achieved many ways: as I suggested earlier, just reducing the input would do it.

This has nothing to do with what the meters say.

Of course it has to do with the meters, as the OP has just confirmed again.

 

[Bo Deadly]

This coulm be achieved many ways: as I suggested earlier, just reducing the input would do it.
Of course it has to do with the meters, as the OP has just confirmed again.

Ok. So yes, the meter could be adjusted instead. And actually that seems to me like it might be the better way to do it from an SNR perspective.

 

[Bo Deadly]

Wich recomendation for the opamp at that spot?
Opa4132 (fet) or ne5532 (bipolar) or a 5532 and 2134?

Unfortunately based on your schem, I'm not sure you can change that amp at all. If there are no bypass caps at the supply pins of each amp, that rules out anything vaguely high speed. So no OPA4132. And most bipolar are going to have a bit of an output offset which could make faders and such scratchy.

 

[abbey road d enfer]

Wich recomendation for the opamp at that spot?
Opa4132 (fet) or ne5532 (bipolar) or a 5532 and 2134?

OPA2134 would be a definite iùmprovement; it is 10dB quieter than TL08x and has about 3 times the drive capability. Being a JFET type, it has negligible bias and noise current, which makes it compatible with the absence of coupling caps.
As squarewave mentioned, you should add decoupling caps (100nF ceramic) close to the chips.

 

[Bo Deadly]

There could be amps that would work without coupling caps. I have never used TLV2172 (or TLV4172) but my impression is that those are sort of the next gen of part that's like the TL0XX. Noise is great and they have major drive. There's actually a bunch of chips like TLV2172 that are related that use more or less power (TLV2172 only uses 1.5mA quiescent) with a little different noise and THD accordingly. They're the "sound plus" chips. There is language in the datasheet about using bypass caps but there's one bit that reads:

  "Place 0.1-μF bypass capacitors close to the power-supply pins to reduce errors coupling in from noisy or high impedance power supplies"

So it doesn't sound like a hard requirement or that they would be unstable which is what you would really need to worry about.

They are SOIC though so you would have to put them on surf-boards. But then you could tack on an axial ceramic to the bottom across the power pins. That could be good enough to keep it all stable and you wouldn't be hacking on the PCBs. Take careful noise and THD measurements, drop two in place of the output amps and measure again. If it makes a difference, consider other positions.

 

[Whoops]

You should get the complete schematics so its possible to understand the meter circuit and where is the signal being tapped for the meter.

Ted Fletcher has a forum an he still helps people out with old Alice consoles and documents:

https://tfpro.com/forums/forum/ask-ted/

 

[guze]

Hi,
I've been working a lot and no time to open the mixer.
I already spoke with Ted Fletcher when i bought the mixer years ago, he doesn't have any schematics for it.

Only yesterday i could open it.

The monitor out is connected to a similar circuit as the master in the monitor card.

The Sifam vu meter had a 2.7k resistor before it. with no resistor it actually came down 2 db, but then i read that the vu needs a 3.6k resistor. With a 3.6k the level is the same at 1.62v at 0vu.

In the picture the wires in the yellow circle are the ones that go the vu meter

Should i change the 22k resistors for 16k in both the master and monitor? or is there another option?

Thanks and sorry for the delay

 

[Bo Deadly]

I don't know anything about VU meters. I might question whether or not it really "needs" the 3.6K resistor. If taking out that one little resistor gets you to 0dBu, I might just roll with that.