Calibrating de bar graphs on TAC Matchless

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Calibrating de bar graphs on TAC Matchless

Postby Neve » Thu Apr 07, 2011 7:01 am

Hi,

Can anyone tell me how to properly calibrate the meters on a TAC Matchless?
I do not have an oscilloscope, nor do I know how to use one.
However I have a nice little software called Electroacoustics Toolbox ( http://www.faberacoustical.com/products ... s_toolbox/ )

The console I/O are connected to 3 x RME-FF800 (which are setup at +4db = 0)
It's been fully recapped and fitted with AML preamp upgrades.
I love this console. But I'd like to make it quieter. Could be a ground loop?

Thank's for guiding me through the calibration process.

Cheers
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Re: Calibrating de bar graphs on TAC Matchless

Postby Matt Syson » Sat Apr 09, 2011 6:15 am

Hi
Ground loops only 'create'** hum or possibly make it better at picking up radio.
I presume by noise you mean 'hiss' which is down to the op amps and transistors used (and the circuitry around them).
There are quieter op amps around which may make some difference if applied correctly which may involve changing other components too.
Calibration, a multimeter would do, or nominate 1 channel of a DAW as being 'calibrated'.
Set an oacillator and adjust relevent controls to give 1.228 Volts (0VU which is +4dBu)) at ther outputs feeding the 'recorder'. Now adjust the meter so it reads 0VU. Job done.
Note: check that your multimeter is accurate at the frequency you use as many are NOT CALIBRATED at greater than 500Hz.
All should be 'close enough' if you use 400Hz.
A 'real' multimeter may be preferred to 'virtual' equipment unless you KNOW that the signal presented to it is what is displayed. A 'soundcard' CAN be used but if there are user adjustments available anywhere then it may not really be correct.

** A loop does not create a hum but it acts to receive radiated magnetic signals and add them to your audio signals.
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Re: Calibrating de bar graphs on TAC Matchless

Postby Neve » Mon Apr 11, 2011 5:21 pm

Thank you Matt !!!
Helpful as usual.

I'll try to follow your instructions.

Regarding the "hum", it's more noise as a matter of fact.
Not sure what the cause of it is. The PSU maybe?
It's a rather large PSU. An AMEK 2000 series.

What about those OPAMPS you were talking about.
Which are the best?

I'll keep you posted.
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Re: Calibrating de bar graphs on TAC Matchless

Postby Jim Williams » Tue Apr 12, 2011 9:49 am

Good choice to lower noise are Burrbrown OPA1642's. Those draw the same current as the 072 opamps now there. You may need to add some local .1 uf caps from pin's 4 and 8 to ground, but otherwise they should work fine. They are SOIC so you need adaptors to fit them in.
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Re: Calibrating de bar graphs on TAC Matchless

Postby Neve » Thu Apr 14, 2011 1:59 am

Thanks Jim. Is there a direct replacement for the OPAMPS?
Can I just add the .1 caps to the existing 072 to reduce noise?
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Re: Calibrating de bar graphs on TAC Matchless

Postby Jim Williams » Thu Apr 14, 2011 10:05 am

A direct replacement without some extra work is dodgy. Amek was pretty good at adding feedback caps on each stage, but there are only a pair of .01 uf's for power decoupling for the entire strip. I would add those .1 uf caps as I mentioned. You do want to be sure the leads are short and that the ground is a good solid ground, not a thin ground trace that travels a ways before hitting the main ground trace.

That will help the 072's as well and it usually helps lower crosstalk on the master mix. Those BB opamps are fet input so they will not add source noise and they are only 5 nv noise, compare to 22 nv noise for an 072 opamp. Their offsets are also low so you may be able to remove some electrolytic coupling caps as well. A BB OPA1612 is ideal for the stereo sum opamp as it's very low noise, 1 nv, high slew rate and it's rail to rail, it has better headroom and will allow you to slam the stereo mix without clipping. It's also precision so you should be able to yank out the coupling caps around it. I usually replace the stereo sum input cap with a ferrite bead for extra rf filtration.

Then there is that EQ section...
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Re: Calibrating de bar graphs on TAC Matchless

Postby nebichou » Sat Oct 22, 2011 8:24 am

Hi,

About the bargraphs calibrating, I tried to do that way :
- activating internal Oscillo of the Matchless with the swith "state" as the signal is sent to all channels
- use a little screwdriver to adjust the bargraph trim on each channel

I noticed that the calibrating was de-calibrating quite fast... what could be the reason ?
For example, I adjust all channels and when done, I can notice that channel 9 is at -4 whereas it should be at 0...or channel 17 is at +2 instead of 0...

THanks !

Ben
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Re: Calibrating de bar graphs on TAC Matchless

Postby Matt Syson » Sat Oct 22, 2011 11:10 am

Hi
The oscillator can drift so put it on and have a cup of coffee while it gets to a sensible temperature.
If individual channels (busses) are 'drifting there are faults around, either dirty connections or maybe other things.
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Re: Calibrating de bar graphs on TAC Matchless

Postby nebichou » Sat Oct 22, 2011 4:30 pm

As it appears to be only on few channels, I think the oscillo is not in cause..
I'll check the trims, I think they are tired... like half the pots on the desk...

Thanks Matt !

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Re: Calibrating de bar graphs on TAC Matchless

Postby Matt Syson » Sat Oct 22, 2011 5:11 pm

Hi
The level drift is normally up to about 1dB maximum and would be from desk powered off cold, to the normal running temp say half an hour or thereabouts. Once it has got to it's usual temperature it is then pretty constant. I did a 'mod' for this on the Classic and BC2 desks and I think Mozart but it was not corrected on these. The BC3 used a different design as did other later desks.
All busses should be trimable to a level then remain constant. They would all drift together if it is the oscillator that is causing the problem.
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