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Old 10th September 2003, 01:25 PM   #1
00940 is offline 00940  Belgium
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Default Really simple preamp, could someone check it ?

Is there anything wrong with this ? I need a small switch box and volume control till I can complete a more ambitious tube-based buffer.
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Old 10th September 2003, 01:27 PM   #2
00940 is offline 00940  Belgium
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and the complementary PS :
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Old 10th September 2003, 01:33 PM   #3
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Please use a 100 k or 220 k resistor from the opamp outputs to ground and use a series resistor of 100 Ohm from the opamp outputs to the RCA's/muting circuit. Maybe you can consider to use the muting relay as a shunt to ground to avoid it influencing your precious signal. This, of course, is only possible when you use the 100 Ohm series resistor before the new muting circuit otherwise your opamps won't like it at power up.

An output cap of about 10 uF ( depending on what input impedance your power amp has ) for cancelling DC wouldn't hurt either. It can be omitted if the following power amp has input caps. If you want to use the cap it is best to place it directly after the opamp output and then continue with the above suggested resistors.

A "real" symmetric power supply would be my first choice, or use the 2 batteries in series for +/- 9V at least. +/- 4.5 V does not leave much headroom does it ? Don't forget to put smaller value ( 4.7 to 100 uF good electrolytics or 0.1 uF MKP or the like ) caps very near the chips power connections for decoupling.

A 10 k volume potentiometer will make your buffer a difficult load for some cdplayers. A 100 k or 50 k would be a better choice IMO.
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Old 10th September 2003, 01:38 PM   #4
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Default Re: Really simple preamp, could someone check it ?

Quote:
Originally posted by 00940
Is there anything wrong with this ? I need a small switch box and volume control till I can complete a more ambitious tube-based buffer.
As you said it's not really a preamp but a buffer.

For the supply, if you are using just two batteries, you may build a simpler +/- supply using them with nothing else. As it is you could later replace them with two rechargeable 12v batteries.


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Old 10th September 2003, 01:42 PM   #5
OliverD is offline OliverD  Germany
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I suggest adding a relay shorting the outputs to ground (after the 100 Ohm resistors) until some seconds after power-up.

Also, I'd use input resistors to ground on all four inputs (47k or something).
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Old 10th September 2003, 01:44 PM   #6
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Quote:
Also, I'd use input resistors to ground on all four inputs (47k or something).
Good advice.
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Old 10th September 2003, 01:44 PM   #7
moamps is offline moamps  Croatia
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And disconnect - input and output from ground.
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Old 10th September 2003, 01:47 PM   #8
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Quote:
Originally posted by moamps
And disconnect - input and output from ground.
I did not see that at first glance. Leave the - input connected to the output of the opamp however or your loudspeakers will protest

It is good practice to use an input RC filter to avoid RF creeping in your buffer/poweramp. This filter should be placed in FRONT of the volume potentiometer otherwise you'll have a variable filter as seen in many cheap amps. A 1.2 k resistor and a 1 nF cap will do in most cases.
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Old 10th September 2003, 05:28 PM   #9
00940 is offline 00940  Belgium
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Here is the modified version, is this allright now ?

Thanks a lot to all of you
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Old 10th September 2003, 06:06 PM   #10
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Quote:
Originally posted by 00940
Here is the modified version, is this allright now ?

Not really. The joint point between the batteries should go to ground too.


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