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Old 10th June 2012, 11:17 AM   #11
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Dear all, I have put together the circuit using R1 = R2 = 22K, and have made sure that the volume pots which came with the kit have been turned up full. This allows the signal to pass through when unpowered, however when powered the opamp seems to get rid of the signal, muting the output. I am currently using a battery to power the circuit, however I previously used a wall power supply (12V , 300mA) to power the circuit; when I did this the opamp started smoking!

Do you think the opamp is dead? Will the wall power supply try and 'push' 300mA through the circuit, frying the opamp, or would it just provide a voltage and allow whatever current might be drawn from it by the resistance across it?
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Old 10th June 2012, 01:14 PM   #12
godfrey is offline godfrey  South Africa
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Originally Posted by mfurseman View Post
I previously used a wall power supply (12V , 300mA) to power the circuit; when I did this the opamp started smoking!

Do you think the opamp is dead?
Yes, definitely. My guess is you either used an AC adapter, or you connected it with the wrong polarity.
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Old 16th June 2012, 06:05 PM   #13
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I have got a brand new opamp, connected it to a 9V battery, carefully checking the polarity; this results in the output signal being muted. When I use batteries with lower voltages the signal is not 'muted' as much, and the signal becomes very distroted.

I have checked for dry joints, and the values of resistance in the circuit with a basic mutlimeter. I don't have access to a function generator, oscillascopeor more advanced multimeter to check the capcitors.

Any idea why this is happening!?
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Old 16th June 2012, 08:49 PM   #14
M Gregg is offline M Gregg  United Kingdom
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Possibly ,

R2 and R6 need to be 1K..if the gain is to high it will distort.

What are you feeding it with..ie what is the input connected to?

You must use a smooth DC supply ie like your battery or it will hum badly.
Polarity is very important..wrong polarity will burn out the IC..

If the input level is two high then it will distort no matter what you do!

You also say you are driving powered speakers with this?
Are you sure that you need a preamp with gain?
Are you sure that just via a volume pot (no preamp) to the input of the speakers will not work?
Normaly you would have the volume control on the input to the preamp board and the output out to the power amp.
This means that the IC is driving the cable to the main amp and not being wasted in the volume pot.

A wall wart power plug is not smooth enough for a supply of a preamp...the output of a plug in power supply is rubbish<<realy bad..if you can find a laptop power supply with correct polarity and Voltage it will work OK. (get it working with a battery first)..Always put a fuse in the supply to any project you build.<<that includes connection to low voltage laptop or battery supplies. At least it will protect something like a laptop PSU if you short it out. To protect polarity you can fit a Diode in line with the + supply like this.. battery + -I>I- to project. Hope it all goes OK..


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M. Gregg
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Old 16th June 2012, 09:10 PM   #15
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Dear Gregg,

Both R1 and R2 are set to 22k, therefore the gain should be unity, or 0 dB, so I don't think this is the reason for the distortion. The potentiometer to control volume is attached before the inputs shown on the schematic for the kit.

The only reason I wanted to use active electronics is so that the output impednace could be lowered to drive the signal along quite long cables (~5m) to the powered speakers.

The input is a Behringer UCA202, which has 400 ohm output at 2 dBV max.
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Old 16th June 2012, 09:17 PM   #16
M Gregg is offline M Gregg  United Kingdom
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OK how have you wired the input volume pot,

Is it wiper to the preamp or wiper to the input?
What value is it?

Are you sure that when the IC burnt out it did not drain the battery<<<?
IE battery output is now low? or is it a new battery?

If you connect the pot on the input direct to the output with circuit board disconnected ie to powered speakers is it distorted or is it clean with volume control working (no pre amp in the line)

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M. Gregg
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Old 16th June 2012, 09:22 PM   #17
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Is it wiper to the preamp or wiper to the input?
What value is it?
Wiper to preamp, the input is connected across the potentiometer to ground; the pot is 10K log.
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Old 16th June 2012, 09:30 PM   #18
M Gregg is offline M Gregg  United Kingdom
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So as above; wiper of pot on the input connected to output no circuit connected does the volume work with a clean signal to the powered speakers?

Please hit your refresh page button every now and then to show further posts after you have posted...

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Old 16th June 2012, 09:37 PM   #19
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So as above; wiper of pot on the input connected to output no circuit connected does the volume work with a clean signal to the powered speakers?
Yup, the volume pot works as expected, with a clean undistorted signal, when the opamp is unpowered, however the signal is a lot quieter than when the input is directly connected to the powered speakers.

Last edited by mfurseman; 16th June 2012 at 09:40 PM.
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Old 16th June 2012, 09:42 PM   #20
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by mfurseman View Post
Yup, the volume pot works as expected, with a clean undistorted signal, when the opamp is unpowered, however the signal is a lot quieter than when the input is directly connected to the powered speakers.
OK,

So with the volume off and board powered turn it just up so you can hear the sound, is it distorted?

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