Re: Using Op-Amp to drive Amp Gauge with Audio Signal
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Tue, 26 Feb 2008 10:54:26 -0800
Tom wrote:
I currently have a small audio amplifier in a custom case I built. InDo a web search for "VU Meter". That's what you'd expect a meter connected to the input would be reading, and you may even be able to find one that's designed for a 1mA movement.
order for it to look cooler, I decided to find an old analog gauge to
add to the case. Well, my local surplus store had a REALLY cool
looking gauge that I HAD to have. The gauge is a 0 - 1mA analog
gauge, 130ohm internal resistance. It is a bit small in range. What
I would like for the gauge to do of course is move in step with the
audio input to the amp. I did not want to hook it to anything on the
amplification portion of the amp, as it might reduce/change some audio
quality. Someone suggested a great idea that I split out the input
signal (Note: only 1 channel would be needed) and "use an op-amp to
drive a dummy load with the gauge inline". The source of the audio is
a computer audio out jack.
This sounded like a great idea, so I started researching op-amps and
their use in circuits. I have learned quite a bit, but I am by no
means an electrical engineer, so I am turning to you for help. Here
are my current stumbling blocks.
1. What circuit to use? Voltage gain, Voltage to Current...??? (Note
I built a simple Voltage to current amp last night, The gauge went to
about .8mA and stayed there. There was no fluctuation that I could
see based on the audio input)
2. I gather that I am using the op-amp mainly to act as a high
impedance buffer only, I do not think I need any gain that I can think
of. Does this sound correct?
3. I would prefer using a single power supply (24V off of the amp
power supply). If I do this I would need to add a DC offset to the
signal. I would like to avoid this, as I do not want to get any DC
sent back to the main amp. I guess I could use a capacitor if need
be, but I would like to avoid it, so was going to use resistors to
split the 24V into +12V, and -12V to feed the op-amp so that it can
handle +/- swings. Perhaps add the DC offset after the op-amp?
4. I currently am testing this with a lm741 op-amp. Is the 741
bandwidth good enough for this (It only goes to 1kHz)? Should I be
using a different op-amp?
Any help or even a good "Right path" would reeally help me out. I am
having quite a bit of fun learning this, but a nudge in the right
direction could help.
Thanks,
Tom
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
.
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