meter scale...is this math right?

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regularjohn

Well-known member
Joined
Jan 23, 2009
Messages
321
Location
Raleigh, NC
Ok...I'm trying to figure out every option possible for a GSSL meter...and math was never my best subject.  So I might need some help here.
According to Jakob, with a 2K meter resistor, 50uA is equal to 1dB GR on a 20dB scale for a 1mA meter.  For 10dB scale on a 1mA meter it's 100uA per dB with a 1K resistor.

But, if I wanted to use a 2mA meter, with a 20dB scale, it's 100uA per dB for a 20dB scale with a 1K resistor, right?  But a 10dB scale on a 2mA meter would require 200uA per dB, so you'd need a 500 ohm resistor...is that correct?

And mathematically, you could use a 20mA meter.  It would be 1mA per dB of gain reduction for a 20dB scale, and you'd need a 100 ohm resistor.  Right?

Is my math ok, or there something I'm missing with these dB to GR to FS deflection ratios?
 
Here's what I do...

Math will take you so far, measurement will check the math.

I use a trim pot to set resistance in series with the meter. Then I MEASURE THE OUTPUT and advance the threshold control until the output drops by exactly 8dB.

Then I adjust the trim pot to make the meter read 8dB GR.

Then I check it at 4dB, 12dB etc.

A 5kΩ trim pot usually works fantastically.

In fact, in my latest 'mad scientist' test mule, I put trim pots EVERYWHERE...

I trim the unity gain of each channel to be EXACTLY unity.

I trim the distortion of each channel's VCA (naturally)

I trim the ratio to be EXACTLY correct.

Here's a shot of the inside of my current work-baby...

Mule-Inside.jpg


See the five trim pots on the main board.. also you can make out another trim pot alongside the crimp terminal on the end of the RED wire going to the API VU meter... right alongside the 3-button digital controller board. That sets the meter to read BANG on.

-IN this case, the VU meter is switchable between gain reduction and L+R output (summed). The meter has the trim pot for setting the meter correctly, then the VU GR meter drive board has a trim pot for setting the 0VU position of the meter in GR mode.

Seriously... Trim pots mean you can set EVERYTHING absolutely BANG-ON.

Keith
 
Damn, that's a great idea!  I'll definately try that.  
I've been doing math to get close, and then soldering and desoldering and resoldering resistors into place until I'm spot on...total waste of time!

Here's possibly a bonehead question: so to install the trimpots in spots where resistors normally go, are you just soldering the wiper and CCW (or CW) leg and leaving the third leg disconnected?
 
I usually short the third leg to the wiper, Since it prevents any intermitent wiper contact from allowing things to  go COMPLETELY wide-ass-all-the-way-open... it also helps with mechanical support since the holes are not in the EXACT place they should be.

Each GSSL I've made (client builds, test mules etc) has had trim pots everywhere. They've all been precisely calibrated for gain, ratio, distortion and meter reading.


Looking back, there's almost no other 'right' way to do it, now that I've started doing it that way!

Keith
 
Oh just a thought... 20mA meter etc... the values don't scale UP very linearly, because there's only so much 'work' a TL07x op-amp can do before it starts to grumble...

1mA or less (100µA for example) is fine. 1mA is usually 'faster' than 100µA, because it's a more efficient motor, but 100mA would be a silly load to impose... I don't think I'd go beyond 10mA.

Keith
 
Hi!
Maybe someone can help me to understand how to setup correctly my 1mA Vu meter

I use a trim pot to set resistance in series with the meter. Then I MEASURE THE OUTPUT and advance the threshold control until the output drops by exactly 8dB.

Then I adjust the trim pot to make the meter read 8dB GR.

I do not fully understand how i can apply this method. Below is what i did :

1/ I put a 5k trim pot in series with the 2K resistor
2/ I set the threshold control to its middle position
3/ I use TestGenerator from Cubase (500Hz Sine) and set the gain to -14.2dB as I can measure 0.614V on the output
4/ At this point, the Vu Meter is arround 3dB with R = 2K
5/ I move the threshold control as it drops 8dB => 3 + 8 = 11dB
6/ I adjust the trim pot as i can read 8dB (R =2K5)
7/ When I move back the threshold to its middle position, the Vu meter is arround 2dB and not 0dB

Thanks for your support!
 
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