[JohnRoberts]

Hi!

It works...randomly...depending on where you touch with the probes, the leds may light off. It seems a layout problem. But even when it worked, with 4k7/470R it light up at -14dbu.

Jay x

The scope probe will introduce a high impedance DC path (1 Mohm) to ground, also some very small capacitance.

JR 

[JAY X]

Hi,

The input coupling caps were backwards. ...Now It works!

[JAY X]

Hi!

I hope you are all well!.

Now that the vumeter and peak lead work well, i have some doubts about adjusting the VU buffer gain. These round vumeters read the level post-fader. So, the VU buffer should be unity gain?. G=1.? Otherwise it would be difficult to match input levels...¿Right?

Note: My VU buffer schematics are at post Reply#37.

As a second question, i noticed that on those JLM SD39 round vumeters, the needle, without signal, does not rest at the same mark place on both vumeters. One rests at the -20dbu mark, and the other slightly lower.
¿Are the front VU meter screws for adjusting this ?

Attached is a SD39 vumeter picture.(not JLM).

Jay x

[zamproject]

Hello

Now that the vumeter and peak lead work well, i have some doubts about adjusting the VU buffer gain. These round vumeters read the level post-fader. So, the VU buffer should be unity gain?. G=1.? Otherwise it would be difficult to match input levels...¿Right?

master post fader right ? Then the Vu buffer gain should be whatever needed to read 0Vu when master output is @+4dBu  (if you want to align your desk at +4Vu)

¿Are the front VU meter screws for adjusting this ?

Usually yes, this is mechanical adjustement

Best
Zam

[JAY X]

Hi!

DAW meters are peak meters ¿Right?. and the level they show is dBfs ¿correct?.

So, to align my summing mixer at +4dbu (1.23v rms) to the daw, i should read -14dbfs at DAW inputs. (EBU standard).

The gain setting process would be (correct me if wrong), something like this:

I play a stereo file at -14dbfs in the daw channel--> Summing inputs---> fader amp--> set the gain of the fader amp to match the -14dbfs in the DAW channel.

If this is correct i will try it tomorrow ASAP!.

Jay x.

[zamproject]

Hi!

DAW meters are peak meters ¿Right?. and the level they show is dBfs ¿correct?.

So, to align my summing mixer at +4dbu (1.23v rms) to the daw, i should read -14dbfs at DAW inputs. (EBU standard).

The gain setting process would be (correct me if wrong), something like this:

I play a stereo file at -14dbfs in the daw channel--> Summing inputs---> fader amp--> set the gain of the fader amp to match the -14dbfs in the DAW channel.

If this is correct i will try it tomorrow ASAP!.

Jay x.

For I/O unity you don't need to know the level (assuming your AD and DA have same 0dBfs relative to dBu)
Send a tone (DAW out), set your fader at unity (usually -10dB pos) and adjust the buffer to read back the same value at DAW in.

For Vu alignment you need to know the 0dBfs<=>dBu value
Typically 0vu<=>+4dBu but if your ADDA is not capable of +22dBu=0dBfs you may be little hot.
You can also consider room over 0Vu (and don't care 0vu<=>+4dBu value) and align your 0Vu to -18dBfs.

-14dBfs for peak ref (which is also your line level then) is very conservative, you can go to -10dBfs

Best
Zam

[JAY X]

Hi Zam!

I will try both!!: -14 and -10dbfs. My interface is a simple behringer UMC1820.
Max input level: +11dbu. Max output level: +18dbu.

Now i see that as different user may work at different levels, if I start selling my summing mixer, I would have to calibrate the meters to their needs/interface levels...¿right? There is no "one fits all" level..

Jay x

[zamproject]

Max input level: +11dbu.

Sure of that ? that's very low...

Best
Zam

[JAY X]

Sure of that ? that's very low...

Best
Zam

Yes, i checked the user manual...is correct. Mic input Max level is the same. Only instrument input level is +18dbu...
Jay x.

[JAY X]

Hi!

Ok, I calibrated the summing mixer vumeters to +8dbu / 1.95vrms / -10dBfs.

My question is if the DAW channels must be calibrated at the same level?? to align both mixer and daw.

Jay x

[zamproject]

Ok, I calibrated the summing mixer vumeters to +8dbu / 1.95vrms / -10dBfs.

Hello

0Vu<=>-10dBfs is very hot !!!
It will be unusable in practice

I told you, a 0Vu<=>-18dBfs is a good start, but request a +22dBu capable converter if you want the "+4Vu" alignment.

Best
Zam

[JAY X]

Hi!

I found at http://www.sengpielaudio.com/MeterComparisonTable.htm

that the vumeters can be calibrated at different levels: 0db/775mv/-4VU/-18dbfs. or 4db/1.228v/0VU/-14dbfs. So i will try at -18dbfs.

The only thing that bothers me is that as i have a single stereo pot to control the output level, as there is a slight gain mismatch between channels due to component tolerances, when one buffer reads 775mv the other is slightly higher...

To overcome this, when i finish to adjust one channel to the right voltage, i turn down the fader to setup the other channel to the same voltage... I don't know if this is the right procedure, but it should work. I will try this afternoon.

Jay x

[zamproject]

that the vumeters can be calibrated at different levels: 0db/775mv/-4VU/-18dbfs. or 4db/1.228v/0VU/-14dbfs. So i will try at -18dbfs.

Both are exactly the same...

Best
Zam

[JAY X]

Hi Zam!

Both are exactly the same...

Yes, now I see... 

What do you think about the adjusting procedure. When you have a single stereo pot to adjust the level on both channels, one channel may have the right level, while the other is higher:

Left channel: 775mv Right channel 825mv.

In the DAW, the source channels have the correct levels on both channels. The mixer output levels produce this slight mismatch.

What i did is set the left vumeter at 0vu, and then lower the fader on the right channel to match  the 775mv, and set the vumeter 0vu. ¿Is this adjusting procedure correct?

Thanks a lot!
Jay x

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