Re: op amp-based effects loop circuit for guitar amplifier

On 17 Jul 2005 10:20:56 -0700, in sci.electronics.design
eerickson_1@xxxxxxxxx wrote:

>I need help with a circuit design for an effects loop in my tube guitar
>amp. Because I don't have space for another tube, I'd like to use an
>op amp based circuit. However, my knowledge re: op amp circuit design
>is limited - I'm a tube guy....
>
>After poking around I found this schematic:
>
>http://www.drtube.com/schematics/marshall/pcbp-00119-60-02.pdf
>
>Is this a reasonably decent design to clone?
>
>I think it would work well for me since it was designed for an
>amplifier very similar to mine and doesn't require an additional power
>transformer; just step down the 380VDC rail (B+) voltage with a power
>resistor and let the regulators do the rest.
>
>Are there any easy improvements that could be made to this design? Am
>I right to think it would be improved by using 7815 regulators over
>Zener diodes and, by using an improved op amp (Analog Devices OP249)?
>Any help is appreciated!
>
>Eric
>
>FWIW, an effects loop allows one to insert a low impedance signal
>processor into an optimal section of the amp's high impedance signal
>path.

Yep, looks reasonable at first sight.
But you are totally wrong in replacing the zeners. These are here to
prevent high voltage signals from zapping the opamps.

I'd just use 8 pin ic sockets on a homebrew, so you can easily change
the opamp. you may blow up a few prototyping. Stick with the TL072
until you have it working.

What the circuit doesn't show is the power supply. what are you going
to use?


martin
.

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