1mA vu meter on LA-2A ? How to do ?

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hugo

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Feb 26, 2016
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Hi,
I have those beautiful vu meters 1mA and i would like to use them on my opto 6 pair...
I tried one of them on my working La-2a point to point, and it does not work like it suppose to. Is there a way to make that vu meter fit one la2a ? Maybe by changing the 3.6k resistor ?.. When i switch on "GR", it is not possible to set the 0 at the roght position, i  can not go after the -20db display..
Thank you for your support.
Hugo
 
He also wants a DC meter to monitor GR current.

It is not clear at that JLM link if the buffer can do this.
 
The VU buffer could be inserted between output transformer and switch. It will be active only when the switch is in  VU position. Have a look at the federal limiter schematic as well.

http://www.preservationsound.com/wp-content/uploads/2014/02/AM-864_Schem.jpg

Bernd
 
There are multiple problems here.

The meters are surely DC? For "VU" we need AC meter. You add diodes. Good LOW-drop diodes are hard to get. Now you need to increase current to the meter by a factor of 10 or 20 to cover the jump from 100uA(?) to 1mA. That means decreasing total meter resistance from ~~7.5K (3.9K + meter resistance) to around 750 Ohms. This is a heavy load on a 600r output. And bad distortion because of the diodes.

Also the 1mA general-purpose meter will be much slower than a true VU meter. I would not try to read speech/music on such a sluggish needle.

But you can buffer and rectify to get a dancing level meter.

Now for DC--  the LA2 meter circuit is just plain wonky. I see how they got there but it is very difficult to follow the trail. What they got is a low-current bridge. It is what it is. It will NOT deliver more current. The output is neither a voltage or a current, but precisely scaled to the VU meter properties.

In both cases we seem to want to use op-amps to do math. But there is no convenient 10V-50V several-mA DC supply in here. Dropping 30V from the 275V rail is a lot of heat, also would sag the B+ voltage.

I've just spent time looking at many different tricks. EDIT . . . . . . .
 
ARGH!!

Right after talking myself out of solving your wrong-part problem, it came over me like an undercooked burrito.
 
The diodes inside the opamp loop make a sharp full-wave rectifier. It may not be dead-accurate in the upper audio octaves or down around the bottom stop, but more than good enough for speech/music VU-ing.

It does nothing about meter dynamics. If the meter over-shoots bad, you can play with 10uFd-200uFd across the meter, but "dynamics" is why you should use a for-purpose meter.

A full VU+3.9K is about 7500 Ohms. I implemented this real simple. The ratio of this to R117 R118 multiplies the current 10X or 20X to your 1mA area. Note the polarities at the opamp inputs (I flipped one to make the topology clear).

Power is a 100K 2 Watt resistor from the raw 350V DC point, to two 12V Zeners, to ground. No more filtering needed.

Opamps should be LM324 types. Need low supply current and to-gound inputs. LM358 is the dual of quad '324. Costs less than a postage stamp. The power supply is designed for the demand of LM358 plus meter, don't substitute without deep thought.

This will work if the opamp inputs are above zero V DC and below 20V DC.

So to make it work as VU, we must bias the inputs up into this range. 10uFd 50V N.P. caps and 100K resistors to +12V do this.

You should calibrate R117 in VU mode to read correctly on steady sine wave.

For the "+10 range" the 7.5K and the 6.8K make a 6dB (5.8dB?) pad as designed.

For "GR" we need to sense a voltage-difference across a 7500 resistor bridged across the Neon+LDR bridge. It works out that both nodes are at several Volts DC, within the opamps' input range.

Once the R117 is calibrated as VU, it should not be changed when calibrating the GR bridge. Zero Adjust should happen fine. LA2's R25 is a trim for the audio-chain's attenuator resistance (not 68K, but 68K||100K||100K=28K), plus/minus the difference between the audio LDR and the meter LDR. As they may be 50% different, R25 may have to be quite far off 28K to get -20dB GR to touch "-20" on the meter. (May be best to calibrate for -10dB GR, and let -20 fall where it will.)
 
Meter over-load--

A 1mA meter is "obviously" safe at 1mA, and is normally safe to 2X full scale or 2mA.

A well-powered LM358 can deliver 30+mA!

Here it can't. The 100K 2W passes at-most 3.2mA. The LM358 sucks 1.2mA nominal. This leaves 2mA available for meter current. Safe. A worst-case LM358 can suck 2mA, leaving 1.2mA, enough to pin the meter. There is no low limit on current that a '358 could suck, but basic function suggests it can't be less than 0.5mA, allowing 2.8mA meter current.

It would be good to avoid slamming the meter while testing. But I don't think it will be instant death.

If you meter the "+24V" while the meter is pounding you may see it droop toward 20V. The Zener is starved and dropping-out. This should do no harm. As the Zeners only get 24mW idle, 1/4W (0.25W) parts are ample, and "might" be less prone to dropout than fat 1W parts.
 
You can calibrate R117 by switching to "+4" and clipping a fresh "1.5V" battery across the junctions of the 100Ks and the 10uFds. Trim for "+2VU". (1.56V is 2dB above 1.23V or +4dBu.) There should be no signal in the LA2.
 

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