OT HIFI amp mod

[Don Tillman]

   From: "Roel Das" <Roel.Das at student.groept.be>
   Date: Mon, 5 Jun 2000 16:33:04 +0200

   One of my friends is playing guitar through an old hifi
   amplifier. He uses an aux input, I think. The build in radio sounds
   alot louder (ofcourse).  Someone told him that it would sound
   better if he 'bypassed' the amp (that's what he told me, however,
   it probably sounds best if he just puts on the radio... :) The
   amplifier is stereo, he only uses one channel at the moment...  Now
   my question; what did the bypassing guy mean, and how do I do it?
   Any other mods, considerations on this?

There's no "bypassing" involved here.  I think the guy just had a
challange to use the word "bypass" in a sentence.

A Stratocaster will put out about 0.5 volt peak-peak when playing a
loud chord.  A Gibson with humbucking pickups will put out a bit more.
A line level input to a receiver is typically -10dBu, or about 
0.7 volt peak-peak, so voltagewise it's in the ballpark.  If you want
to do something more subtle than a loud chord you'll need more gain.

But that's voltage, the big issue is impedance.  Guitars like to see
an impedance of at least 1Mohm, preferably higher, while a hifi
receiver's input is likely to be around 20kohm.  So the signal is
getting badly swamped.

You need a guitar preamp.

   From: Toby Paddock <tpaddock at seanet.com>
   Date: Mon, 5 Jun 2000 20:34:33 -0700

   What would happen if you used something like the 
   passive 'connect line-level signal to phono input' 
   circuit by Tomi Engdahl and tweaked the values for less
   attenuation?
   http://www.hut.fi/Misc/Electronics/circuits/phono.html

It gets worse.  Not only does this circuit have a lower input
impedance, the input impedance changes with frequency.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com