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Old 4th April 2015, 06:38 AM   #1
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Default VU meter for Bias

how to setup VU meter for a lateral mosfet amplifier having a bias of as much as 0.5Amp per fet.


2pc Panel Vu Meter TR 35 0nu l l128 8V35x35 mmL EDLa mpNiss | eBay


The required voltage for the VU meter is 1.3V for 0db but here I dont see any series resistor with either source or drain. So to setup the bias we need either 1.3V across a resistor so where shall i put this resistor?

since its lateral mosfet output stage i havent used any source resistors.

how does nelson pass does it?
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Old 4th April 2015, 08:38 AM   #2
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A Volume Units meter like the one in your link, is a basic AC voltage meter, for placing across the output terminals of an audio device and suited specifically to line levels. For use as a simple power meter for amplifiers used with a fixed load impedance, you will need to use a voltage divider and probably should calibrate it with a low frequency signal against a reasonable standard like a quality DVM that is reliable at the frequency used. However, this approach only shows just how poor the vast majority of retro "VU" power meters of the 1970's -80s really were.

Bias current has nothing to do with VU or output power so using source resistors, which would sample DC bias+AC output current, would only provide a current sample. A cap will fillter DC but if you want to measure or integrate current with voltage as in a proper power meter, the design then becomes more complex. If you want any semblance of accuracy though, you should be researching audio power measurement, not just VU circuits for this.
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Old 4th April 2015, 10:21 AM   #3
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Why not use a 0 - 500mA moving coil meter?
I have no idea of your location so try;
Weston DC Round Panel Meter Ammeter Milli Amps Milliamperes 0 500 MA | eBay
or
Meter Moving Coil 500mA Panel Displays Instrumentation VM55466 | eBay
It is a simple task to use stencils and re calibrate your dial.
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Old 4th April 2015, 05:49 PM   #4
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Quote:
Originally Posted by Ian Finch View Post
A Volume Units meter like the one in your link, is a basic AC voltage meter, for placing across the output terminals of an audio device and suited specifically to line levels. For use as a simple power meter for amplifiers used with a fixed load impedance, you will need to use a voltage divider and probably should calibrate it with a low frequency signal against a reasonable standard like a quality DVM that is reliable at the frequency used. However, this approach only shows just how poor the vast majority of retro "VU" power meters of the 1970's -80s really were.

Bias current has nothing to do with VU or output power so using source resistors, which would sample DC bias+AC output current, would only provide a current sample. A cap will fillter DC but if you want to measure or integrate current with voltage as in a proper power meter, the design then becomes more complex. If you want any semblance of accuracy though, you should be researching audio power measurement, not just VU circuits for this.
why cant we just use a source resistor of value 0.33ohm and then get the voltage across it like consider for 0.5A so we get 0.165V across it and them amplify with the opamp to get the target voltage of 1.3V. wouldn`t that work?
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Old 4th April 2015, 08:42 PM   #5
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Originally Posted by rhythmsandy View Post
why cant we just use a source resistor of value 0.33ohm and then get the voltage across it like consider for 0.5A so we get 0.165V across it and them amplify with the opamp to get the target voltage of 1.3V. wouldn`t that work?
As said, sampling a power Mosfet source resistor or any series resistor's voltage drop gives a voltage which is proportional to the current flowing through it. If your meter is marked in VU though, it implies a voltage measurement which could be quite simply divided from the output, without the complications of bias current, adding source resistors or any need for amplification.

However, you haven't said what you are trying to indicate with your meter and what relevance bias had to the question. I guess you realize that you can't simply monitor DC bias whilst the amplifier is operating and I'm only assuming you are intending to use a VU meter in a power amplifier to indicate output voltage as somewhat analogous to perceived volume.

As Jon Snell suggests, if it's bias and/or output current you do want to measure, a simple AC milli-ammeter fitted with a linear current scale is more appropriate than a logarithmic VU scale. If fitted, one of the source resistors or preferably a precision, low value, high wattage resistor in one rail that supplies all output devices, would then be better for sampling total output current.

Although there are some confused, partial instructions for installing the meter shown at the Ebay site, most low cost analogue meters are the same basic DC micro-ammeter with various scales, cases and multiplier/shunt resistors either fitted internally or specified to be fitted. For AC scales, diodes may also need to be added but the instructions/diagrams for this are almost non-existent . It's hard to tell what you are getting there and I wouldn't bet on the results, wherever the product is made.
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Old 5th April 2015, 05:49 AM   #6
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A circuit and instructions for adapting a basic meter is here: Audio Wattmeter or Audio Power/Level Meter Circuit with Diagram. From your seller's advice, It seems all you get is a basic 450 uA DC meter and you have to build up the AC circuit. This should help, noting that R2 will be your actual speaker load and any power calibration with a fixed resistance will only be valid at that one, load impedance.

However, simply calling the measurement VU means you only expect a relative indication of power for a given load impedance, like most power meters you see on budget equipment. .
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Old Yesterday, 03:59 AM   #7
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Quote:
Originally Posted by Ian Finch View Post
As said, sampling a power Mosfet source resistor or any series resistor's voltage drop gives a voltage which is proportional to the current flowing through it. If your meter is marked in VU though, it implies a voltage measurement which could be quite simply divided from the output, without the complications of bias current, adding source resistors or any need for amplification.

However, you haven't said what you are trying to indicate with your meter and what relevance bias had to the question. I guess you realize that you can't simply monitor DC bias whilst the amplifier is operating and I'm only assuming you are intending to use a VU meter in a power amplifier to indicate output voltage as somewhat analogous to perceived volume.

As Jon Snell suggests, if it's bias and/or output current you do want to measure, a simple AC milli-ammeter fitted with a linear current scale is more appropriate than a logarithmic VU scale. If fitted, one of the source resistors or preferably a precision, low value, high wattage resistor in one rail that supplies all output devices, would then be better for sampling total output current.

Although there are some confused, partial instructions for installing the meter shown at the Ebay site, most low cost analogue meters are the same basic DC micro-ammeter with various scales, cases and multiplier/shunt resistors either fitted internally or specified to be fitted. For AC scales, diodes may also need to be added but the instructions/diagrams for this are almost non-existent . It's hard to tell what you are getting there and I wouldn't bet on the results, wherever the product is made.
im intended to use it for higher bias like .5A + per Mosfet so measuring the voltage across the source resistor wouldnt that be fine with a jfet input opamp to amplifiy the meausred voltage. I agree the fact about using a linear am meter but you hardly find a good looking ammeter but the problem here is that if we use ammeter in series at the power supply there wouldnt be a drop at that power supply rail and the other rail being unaltered?

I just need to get the needle being displayed just to make sure the amp is getting out of class A im not looking for linear operation here. Just the moment the needle moves then it just indicates that more current is being drawn as the amp just started to go out of class A.
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Old Yesterday, 03:34 PM   #8
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Originally Posted by rhythmsandy View Post
....I just need to get the needle being displayed just to make sure the amp is getting out of class A im not looking for linear operation here. Just the moment the needle moves then it just indicates that more current is being drawn as the amp just started to go out of class A.
So you are indeed trying to monitor the peak AC current in excess of the fixed DC bias, by measuring the ACV across a source resistor? Interesting

I don't think the AC output current somehow appears anywhere as small peaks above the DC bias current though. Output AC current flows through the load to power supply common, hence through the source resistors, but will just independently rise from 0 Amps to the maximum level, much as the output voltage does, with or without DC bias flowing rail to rail through the output devices.

A basic VU meter or any simple meter is not going to tell you anything about the adequacy of the bias current to ensure class A operation. For a demonstration, get an analogue audio millivoltmeter or a 'scope (Audio sensitivity of your handheld DMM usually drops away dismally above 400 Hz) and look for any step changes in the source resistor voltage drop as the output level sweeps from 0V up to and above the level at which 0.5A peak is being drawn by the speakers. A single, low frequency tone will certainly be easier to see results with but that really doesn't relate much to audio.

Another consideration is that the nominated 500mA bias current is only relevant to one fixed load impedance which means the point of leaving class A varies widely across the frequency spectrum with real speaker loads. For heavy bass it may be at modest levels but for treble, it might remain in class A up to quite high SPLs.
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Old Yesterday, 07:12 PM   #9
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Quote:
Originally Posted by Ian Finch View Post
So you are indeed trying to monitor the peak AC current in excess of the fixed DC bias, by measuring the ACV across a source resistor? Interesting

I don't think the AC output current somehow appears anywhere as small peaks above the DC bias current though. Output AC current flows through the load to power supply common, hence through the source resistors, but will just independently rise from 0 Amps to the maximum level, much as the output voltage does, with or without DC bias flowing rail to rail through the output devices.

A basic VU meter or any simple meter is not going to tell you anything about the adequacy of the bias current to ensure class A operation. For a demonstration, get an analogue audio millivoltmeter or a 'scope (Audio sensitivity of your handheld DMM usually drops away dismally above 400 Hz) and look for any step changes in the source resistor voltage drop as the output level sweeps from 0V up to and above the level at which 0.5A peak is being drawn by the speakers. A single, low frequency tone will certainly be easier to see results with but that really doesn't relate much to audio.

Another consideration is that the nominated 500mA bias current is only relevant to one fixed load impedance which means the point of leaving class A varies widely across the frequency spectrum with real speaker loads. For heavy bass it may be at modest levels but for treble, it might remain in class A up to quite high SPLs.
yeah you got my point so now what I say is want to make a 25W class A amplifier for which im using two fets per rail so with 0.5A x 2 fets =1A
1A+1A peak ^2 = 4 x 8 ohm load = 32W peak and approx 25W of class A so now im sure that till we cross 25W i,e till the current consumption exceeds 2 Amps the voltage across the source resistor will be the same after which the voltage changes across the source resistor. So may i use a fet opamp here to sense the voltage across the source resistor? im just finding this as one possibility i agree the fact that as you said after 400Hz it might fade away but why does this occur?
what is your suggested way to get the reading right in the case?
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