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Old 5th February 2010, 07:38 PM   #1
iaia976 is offline iaia976  Italy
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Default Guitar preamp envelope follower

Hi,

I would like your advice about the following question:

I need to connect my electric guitar to an external input (preamp and envelope follower) of a modular synth (Dopefer). The Doepfer module A-119 have these specs:

The pre-amp has two inputs: an unbalanced input for line level signals, with a gain factor of from 0 to 20, and a balanced input with a gain factor of from 0 to 500, for insertion of low level signals, for instance from a microphone or electric guitar.

I found out, looking at the pcb board, that the impedance of the inputs is 47k for the line input and 4k7 for the mic input, useless for guitar application. What the best solution in order to maintain the best signal integrity?

- Connect the guitar at the balanced input via (passive) DI box.

- Build a simple opamp preamp to drive the guitar directly at line level.

- Build a simple buffer to lower the output impedance of the guitar signal.

- Don't waste time with it and buy a mutron III!

Thank you for any suggestion!!!

Franz


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Old 6th February 2010, 09:13 PM   #2
mjf is offline mjf  Austria
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What the best solution in order to maintain the best signal integrity? [/SIZE][/LEFT]

- Build a simple opamp preamp to drive the guitar directly at line level.[/SIZE][/LEFT]

Build a simple buffer to lower the output impedance of the guitar signal

hello.
........best signal integrity?........not easily to answer.
perhaps it depends on your electronic skills.
i think so:
if the output of your pickup is big enough to drive the unbal. input you can try out a buffer...........
and if not you will have to use an opamp with gain.
the input impedance of these preamps should be around 1 megohm ........so a fet opamp is recommended (tl071,lf351,lf356,.......).and the power supply should have a clean output voltage (....with voltregs ).
greetings
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Old 7th February 2010, 08:39 AM   #3
iaia976 is offline iaia976  Italy
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Thank you mjf,

My electronic skills are not so good, but here's a sketch of a guitar TL071 preamp I drew rearranging some project found here and there...

What do you think about?


Click the image to open in full size.
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Old 7th February 2010, 09:50 AM   #4
mjf is offline mjf  Austria
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hello.
yes,that should work.........only the psu decoupling caps i am missing (100nf between the two 12v supply rails, or so.........).
modifications:
the gain of this preamp is big (around 15),if you want to lower it increase r4 (up to 10k or so.....).
c2=120pf is a little bit big,i think 22pf..........47pf is enough (if you do not like it leave it out).
greetings

Last edited by mjf; 7th February 2010 at 10:10 AM.
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Old 8th February 2010, 02:46 PM   #5
iaia976 is offline iaia976  Italy
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Thank you very much for the help mjf,

I'll try to carry out this plan!

Bye
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