Passive Spring Reverb?

[zerotronics]

Fascinating discussion...

First a disclaimer: I am not here to extol the virtues of my products or to tell you how to build them.  That is something you will have to do for yourselves...

Now, on the subject of driver design and impedance matching: the name of the game is power transfer and as we all know (or at least, should know), the point of maximum power transfer occurs when the source and load impedances are equal.  In practice, a source impedance that is less than the load impedance works fine; it just means that you have more power than you need and that you can drive multiple loads, up to the point where the combined load impedance equals or exceeds the source...

So, to design any kind of driver, you need to know the load impedance and also, you need to have some idea of how much power (P = V x I) you need to deliver to the load.  Then you can work back to the required voltage and current by using V = sqrt(P x R) and I = sqrt(P / R).

For example, to deliver 100 mW of power to a 10 ohm load, you will need 1V @ 100 mA.  But to deliver the same amount of power to a 1925 ohm load, you will need about 14V @ 7 mA.  So, if you're designing a driver that's going to run off of a 9V battery or the 5V provided by USB, you need to choose the right load impedance that allows you to deliver the power that you need with the voltage and current that you have...

 

[abbey road d enfer]

Why would you need transformers, as the coils are already balanced and floating?

 

[PRR]

Some of the tanks are normally sold with RCA jacks and case tied to RCA shell.

Sure, you flip it over and un-solder the wires, you have two coils floating in space, to connect any way you want.

I'm not sure why the obsession with "passive". The transfer ratio of the Accutronics springs is like 1000:1, even allowing for impedance transformation, you NEED an amplifier. Usually one on each end. And I'm not sure these need to be "good" amplifiers... the sound off a spring is pretty funky, needs some twisting back into shape, and too much "fidelity" on the metallic artifacts may be annoying.

 

[plumsolly]

Hi Ralph - Thanks for chiming in! what I ended up with is a Champ-type ~3.5w amp driving a pair of 1925 ohm input/600 ohm output acutronics spring tanks wired in parallel. On the output of each tank I have a 1:2 transformer wired backwards. (at least I think they are 1:2. They are Tamuras I stole out of some ampex tape machine electronics. They give me ~5 db gain/loss.) I use mic preamps for the makeup gain. This setup seems to work great except that the output transformers pick up crazy hum from my console power supply. I am going to try some shielded transformers and see if that helps. I will also give no transformers a try - my tanks have the rca jack's shell tied to the chassis so i will have to unsolder them, but you can order them with jacks that are not like this.
-Ben

 

[zerotronics]

Several years ago ('05?) I read an interview with engineer/producer Mark Linett in TapeOp magazine.  He's a noted spring reverb aficionado and he said that springs, if they are set up properly, can sound like chambers...

The conventional wisdom is that you should drive the springs as hard as you can.  This makes sense if the reverb is mounted at the bottom of an amp cabinet, sitting under a pair of 10" speakers.  You want to create a high signal to noise ratio and minimize the necessary return gain...

But it is that high drive level that creates those springy metallic artifacts that make spring reverbs sound like springs...

When you have control over both sides of the equation (drive level and return gain) you will find, if you drive the springs gently with lots of return gain, that you can get sounds that compare favorably with those of a plate, or even a chamber...

But to get that kind of sound, you need a good amplifier.  Something with a balanced low-impedance input, low noise and variable gain.  Hey, how about a mic pre?

 

[plumsolly]

Hi Ralph - Thanks for stopping back by. Since my last post I have tried a couple of different things. First I got rid of my transformers - If I isolate the cold on the input and output of the transducers it seems to work great without them and I am not picking up nearly as much hum. Then I hooked up the two tanks like this to cancel out some of the hum the transducers were picking up

This arrangement did minimize the hum - but the reverb didn't sound right (probably because I tied the outputs of the reverb tanks directly together with no summing resistor?) I also was getting less noise by just driving the tanks with the direct outs from my console (D-A into console line driver, direct out into tanks in parallel. So now I am back to no transformers, separate outputs from each tank and driving them with my console's direct out's. I have to keep the box away from my rack because those transducers pick up hum from various power supplies.
Best, Ben

 

[zerotronics]

With two reverb tanks you have four connection possibilities on each side: series or parallel and in-phase or out-of-phase, so a total of 16 possible combinations that you should try.  It's not that hard, just a couple of DPDT switches on each side will do it.  But it does require isolated connectors and a separate ground path.  You should not need any summing resistors.

And although your ears should be the final judge, it's good to have some kind of objective measurement, like a real-time frequency analyzer.