High impedance spring tank driver

[anotherjim]

Found time to trial a solution using standard 9v DC pedal power.

I have a 17" 2 spring tank from an old home organ. In that, it got driven from a medium power transistor class A off a 38v supply. Both the input and output transducers measure around 160R so I suppose the drive at least is classed at high impedance. Although from a Farfisa who were known to use piezo effect transducers, it appears to be an absolutely standard magnetic Hammond style spring line.

To drive it, I tried the JRC2073 bridge mode amp scheme right off the datasheet...

...only I used a 50k trimpot on the input instead of those resistors and I left it DC coupled for the test. I didn't fit the Zobel across the output and DC coupled it to the tank input.
*Important* Don't connect any of the amp output pins to the tank chassis. Chassis should only ground at one place anyway, which is going to be the cable screen from the output transducer. If the tank has RCA phono jacks, ensure the phono ground isn't chassis ground. The input driver cable doesn't need to be screened anyway.

To test, I connected the input to my little Korg Monotron synth and the output of the spring directly to a small guitar amp (standard 1 meg input).

It works just fine with plenty of drive available for the tank and the amp didn't need anymore gain than a guitar would.
In fact, it sounded really nice and as you might expect with that synth, very 50's sci-fi, even when the JRC2073 was clipping on the 'scope.
Come to think of it, I don't think I've ever heard a spring line 100% wet before. It has a fuller bottom end than I had expected. When I get around to building it as a standalone unit, I'll have to include a full band 100% wet option.
The JRC2073 makes a nice low part count bridge amp for the job

Some Q's...
Should it need any Zobel stability and/or flyback protection on the amp output? I can see any ill effects of it being absent.
Can full range audio damage the spring unit?

*Note - typo above, the chip is NJM2073, JRC is the make. TDA2822 from Tru-Semi appears to be equivalent.

[mac]

I am using a LM386 to drive a speaker coil + small neo magnet. Plenty of power, no preamp needed. Lots of lows and mids. Output cap as the low pass filter.
I dropped the RC output network.

160 ohm DC will lower the output power. Is the input coil wire angel hair or thicker?

mac

PS: I will drive the Accu tank of my amp with a LM386

[PRR]

> transducers measure around 160R

It will be 1k-2K at 1KHz. You don't really need a "power" amp; you can drive it with an opamp eating +/-9V to +/-15V (or single 18V-30V).

[anotherjim]

160 ohm DC will lower the output power. Is the input coil wire angel hair or thicker?

mac

It's probably the same wire gauge as the output coil. The coil bobbins look identical although the output DCR reads a little higher at 167R.
Driver coils as high as 200R DC can be had. See the impedance chart on this helpful page...
https://www.amplifiedparts.com/tech-articles/spring-reverb-tanks-explained-and-compared

> transducers measure around 160R

It will be 1k-2K at 1KHz. You don't really need a "power" amp; you can drive it with an opamp eating +/-9V to +/-15V (or single 18V-30V).

For sure, but...
In that old organ, the class A driver amp collector is idle biased to about 16v which probably defines the peak swing on the transducer. A single ended drive from 9v supply would be lucky to reach 4.5v swing. The BTL connection doubles that and it's enough to drive this spring tank.
It's a standard 9v power low-part solution and a little simpler than making an op-amp equivalent BTL amp.