[sdiy] Driving high impedance reverb springs with IC amps + transformer?

[Dave Brown]

Think about driving it with current instead of voltage.  Put the input in
the feedback loop of an op-amp.  The reverb tank input needs to be floating
as neither end will be grounded.  My reverb design for my modular used a
tank with an 800R input impedance so it is similar.  There are a number of
good ap notes out there although I don't have any bookmarked.  You can see
my design on my site at
http://modularsynthesis.com/modules/DJB-002/djb002.htm

The schematics are there, but not much else.  It was my second module and I
wasn't putting that much detail there.  Even the schematics are hand drawn.

Dave

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of aankrom
Sent: Saturday, September 10, 2011 10:02 AM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] Driving high impedance reverb springs with IC amps +
transformer?

 I have a reverb tank with an input impedance of 600 Ohms. There's a  simple
circuit I found online to drive reverb tanks with an LM386 amp. 
 I've heard that driving a tank with too much DC bias can magnetize the
core of the input driver - which is probably bad. Also, since I plan to  use
+/- 9V, I probably won't have enough swing to really drive the tank. 
 Then I had an AHA! I could use one of those cheap 1KCT:8Ohm audio
transformers from Radio Shack. I could drive the 8 Ohm side with the amp  IC
(I like LM380's better...)and drive the tank with the secondary. If I  use
the tap it'd be 500 Ohms, which should be close enough. Also the  xformer
will isolate the tank from DC. The small size will cut low end,  but for
reverb, this is probably a good thing. I'm just wondering if I'm
overlooking anything, like maybe should I let the transformer output -->
reverb input float or should I ground it? I thought if I grounded the
center tap, I could switch the phase of the reverb audio by switching  which
primary lead I use. Will I really get enough swing using a +/-  supply?
Might I get too much? Should I load the amp IC with a resistor,  that is,
might this help protect the tank? If I burn an LM386, big deal. 
 I have a million of them.

 AA