Mcm 20.4

[pucho812]

Anyone ever look at these? Anyone have a schematic?
Got one on the bench that measures -68dBu noise +THD on the audio precission. Not too terrible compared to other equipment. But trying to reduce the noise as the owner finds it too noisy. So far the only suggestion from the manufacture yielded zero change.

 

[zamproject]

From manufacturer website...

• THD + N: Not more than 0.07 % from 50 Hz to 10 kHz at +20 dBu output (22 Hz to 22 kHz bandwidth) into 600 Ω.
• Noise, all faders up, all subgroups in, measured at the main output, better than -80 dBu.

No info about noise weighting measurement to achieve -80dBu, but your -68dBu don't look that good ...
This tool seem to accept and deliver quite high level, maybe they just mean -80dB (S/N)...and add a u at marketing dep ?

 

[pucho812]

From manufacturer website...

• THD + N: Not more than 0.07 % from 50 Hz to 10 kHz at +20 dBu output (22 Hz to 22 kHz bandwidth) into 600 Ω.
• Noise, all faders up, all subgroups in, measured at the main output, better than -80 dBu.

No info about noise weighting measurement to achieve -80dBu, but your -68dBu don't look that good ...
This tool seem to accept and deliver quite high level, maybe they just mean -80dB (S/N)...and add a u at marketing dep ?

Well maybe. Right now I am testing 2 channels. +4dBu in and +4dBu out.
The -68 is not so bad. Many a unit a vintage measures around that.
The Avalon stuff measures an average of -91dBu THD + N so everything else seems poor in comparison.

Sadly I reached out to heritage on it and they suggested adding resistors in key spots which offered negligible change and for the worse.

 

[Brian Roth]

From the very limited specs they posted, it appears it has unity gain from in to out (?). I got that from the pictures. Is that correct? If so, -68 dBu at the output of a 16 channel unity gain mixer really sucks.

Bri

 

[Brian Roth]

Lordie me!

https://www.altomusic.com/products/heritage-audio-mcm-20-4-mixer-hamcm20-4
https://www.guitarcenter.com/Heritage-Audio/MCM-204-20-channel-Summing-Mixer-1500000378017.gc
I will keep my PT Barnum comments to myself.

Bri

 

[pucho812]

From the very limited specs they posted, it appears it has unity gain from in to out (?). I got that from the pictures. Is that correct? If so, -68 dBu at the output of a 16 channel unity gain mixer really sucks.

Bri

I’m testing with just 2 channels. It goes beyond unity if I did more then 2 channels.

 

[Brian Roth]

I’m testing with just 2 channels. It goes beyond unity if I did more then 2 channels.

Not quite understanding the gain structure.....

With signal going into just one input pot set to WFO and hard panned to one output, with that output master pot WFO, what is the gain in to out?

Bri


PS, hope the flooding isn't impacting you.

 

[pucho812]

Not quite understanding the gain structure.....

With signal going into just one input pot set to WFO and hard panned to one output, with that output master pot WFO, what is the gain in to out?

Bri


PS, hope the flooding isn't impacting you.

With +4 input signal into a channel, channel panned hard left or right. Volume at full on channel and master, I get +4 at the output.

Not impacted by flooding but I have other things impacting my life right now. I’ll survive.

 

[Brian Roth]

Curious where the noise in coming from, especially since their -80 dBu spec says everything WFO. What;'s the noise floor in other combinations, like all inputs off, all masters off, etc.

Bri

 

[pucho812]

Curious where the noise in coming from, especially since their -80 dBu spec says everything WFO. What;'s the noise floor in other combinations, like all inputs off, all masters off, etc.

Bri

I am too. Noise doesn’t change when I lower the volume and measure with the AP. But I will test it again and confirm it tonight. External psu that is a switch-mode

 

[Brian Roth]

And what type of noise: hiss/hum/switcher supply crap/etc.

Bri

 

[Brian Roth]

Interested in any pix of the guts, if that is easy to do.

Bri

 

[pucho812]

Interested in any pix of the guts, if that is easy to do.

Bri

Will snap some tomorrow.
While they won’t provide a schematic.
Their new theory is faulty switches for mute and insert. Which I couldn’t see if they are not excercised when testing. If unmuted and not using the insert I don’t see how a switch would add to the noise when in a static position

 

[pucho812]

Update as I am waiting for a replacement VU meter shipping from overseas.
Measure from the insert send on the inputs and signal is good. -90dBu thd+N
Level is same on input and output.
So the AP and I are happy.

But stereo buss and insert send from the stereo bus measure the same -70dBu THD+N.
Adjustment of the master fader(a pot) shows change when level is off in THD+N, noise goes down to -75dBu

Hmmmm

 

[zamproject]

Any plot of the noise figure ?

Also seem the device offer -on- button per channel, guess it's a mute, are your result at master with all channel engaged ?

 

[pucho812]

Any plot of the noise figure ?

Also seem the device offer -on- button per channel, guess it's a mute, are your result at master with all channel engaged ?

I can show those. Yes on button is mute. So far testing has been with 2 channels out of 32. so I am using 1 X subgroup out of the 2.
The results are 2 channels up at full and the master at full so that I have +4 input test signal and +4 output, unity signal.

 

[Brian Roth]

Hmmmmm.....wish we had at least a block diagram.

But, in my "pea brain". it seems there is at most 16 channels feeding any bus.

Doing a simple calc with a virtual ground summing amp as a reference, I come up with a noise gain of 24 dB for the summing amp. I think even a crappy 741 opamp summer running at that gain would have a much lower noise floor than what you are reading.

Bri

 

[pucho812]

Hmmmmm.....wish we had at least a block diagram.

But, in my "pea brain". it seems there is at most 16 channels feeding any bus.

Doing a simple calc with a virtual ground summing amp as a reference, I come up with a noise gain of 24 dB for the summing amp. I think even a crappy 741 opamp summer running at that gain would have a much lower noise floor than what you are reading.

Bri

Block diagram

 

[pucho812]

The20.4 is essentially a voltage summing mixer with a 1272 as a makeup gain amp.

 

[pucho812]

Well after a month and a half of waiting for shipping from China, the meter I purchased from eBay finally arrived. This was because heritage refused to sell me a meter. their claim was that it would be more expensive.
Spare no expensive I say but anyway they wouldn’t do it.
I quickly wired the meter in, exactly the same as heritage wired in the old meter.
The meter had lights working.
Ok cool step 1 done I see the light.
No meter action when using test signals.
It has free movement so nothing is preventing it from moving.
The meter harness( 3 pin connector to bare wires for soldering) tests ok and if I swap meter connections( left and right). The working meter follows.
Currently meter for the right side of the stereo bus works. Left side( new meter) does not.

I can’t imagine the meter being faulty as I was able to get it to do action when I tested resistance between + and -.

Worst customer service I have experienced to date.