Spring Reverb driver question

[craftyjam]

Hello!
I just wanted to get some advice about a circuit ive been thinking about:

The main jist is that I have a reverb tank sitting around with an input impedence of 600 ohms, and I don't have a 15+/- power supply for op amps in order to drive it.
My idea was if I could use an LM386 into an 8ohm/1k ohm audio transformer (feeding the output of the lm386 into the 8ohm side, and using the coil split around 500 ohms for the output to the reverb tank.) My thinking is that this would allow the 386 to drive the higher impedence tank. I would then have a recovery circuit of some kind with a dry/wet mix.

I'm somewhat new to this, so I was wondering if this idea had any merrit.

Thank you!

[Rob Strand]

It should work.   A straight LM386 driver is going to be a "voltage drive" which will probably sound muddy.

You can use a series resistor on the output,  like R5 in figure 2.   

http://www.radanpro.com/Radan2400/Music/Spring%20Reverb%20Unit%20For%20Guitar%20-%20ESP%20Projects%20Page.htm

A value of 10 ohm is a good start.   22 ohm is probably better but will end-up with a trade-off between signal swing and tweaking the top-end tone of the reverb.

The LM386 circuit as shown should have enough gain.  (No need to add the the gain cap see LM386 datasheet).

A 100uF output is also a good start, up to 220uF for more bass, reduce to 47uF for less.

 

[craftyjam]

Thank you for the response 

So you are saying that I should have a series resistor between the output of the 386 and the audio transformer?

Wouldn't that mess with the impedance matching between the 386 and the transformer or am I missing something?

[razabri]

As I was into working out a spring reverb driver myself at another thread, and Rob also helped a lot, I've found this circuit that doesn't need positive and negative voltages and it seems that it actually works well.


https://imgur.com/gallery/LyoOGgQ?fbclid=IwAR1VP6rkGI9FJCxVv13SZ8VwiDhGt2isZ_r747Igh3yp0Uacqzl5Q3xHcqI

If you decide to use it you can follow this image and contact the guy who has recently made it if you have any issues:
https://www.facebook.com/photo.php?fbid=3090902844287277&set=gm.2662475417195992&type=3&eid=ARDpbt1vchYkP2vfQex5exzQgNv7S8WGq-1rAEhTeediAPTqLyLT0T1rnpC4f8PkpASDdz_tchOGxh22&ifg=1

You may need to build a +18v charge pump too... Should be simple enough though.

[11-90-an]

Easy charge pump...

https://www.diystompboxes.com/smfforum/index.php?topic=124739.0

[Rob Strand]

 

So you are saying that I should have a series resistor between the output of the 386 and the audio transformer?

Yes.  It makes the reverb tank sound right.   You can read-up on voltage drive and current drive of reverb tanks here,

https://sound-au.com/project34.htm

The series resistor is a way of approximating current drive.   

Wouldn't that mess with the impedance matching between the 386 and the transformer or am I missing something?

No.   If you had an 8 ohm reverb tank then you would drive it using say a 10 ohm series resistor in order to approximate current mode.   By putting the 8 ohm to 500 ohm transformer before the 600 ohm reverb tank it just makes the reverb look roughly like an 8 ohm reverb tank.

A transformer is really just turns ratio thing which transforms the impedance.   The 8 ohm to 500 ohm transformer looks like 8 ohm at the input when 500 ohms is placed on the output.  However it will like 16 ohms when 1000 ohms is placed on the output and 1.2 ohm when 75 ohms is placed on the output.     The "8 ohms" spec is fairly nominal, normally worked out based on losses but not really anything specific about 8 ohms.

BTW a 600 ohm reverb tank doesn't look like a 600 ohm resistor at all.  It looks like an inductor with 600 ohms at 1kHz.   At 100Hz it looks like about 75 ohms (part resistive and part inductive).  So at low frequencies the impedance at the "8 ohm" side of the transformer is low quite low, near 1.2 ohms.

The presence of the series resistor actually helps the amplifier see the right impedance at low frequencies.   It also factors out a lot of details like the DC resistance of the transformer windings.

[craftyjam]

Wow! Thank you so much for all this info. I'm gonna try a few things out and let you know if I get something working.