matching preamp input/output impedances for spring reverb?

[rjd2]

hi folks, wondering if any of you have any ideas on something. i got a dirt cheap spring reverb at a garage sale that had the amp section blown. so i just wired the actual spring leads to input and output of the chassis, and i drive both ends with mic pres. it works, and it actually sounds alot nicer than my next closest spring reverb(peavey valverb), however, its noisy. this is probably because i need to really drive the back end preamp.

im wondering if i could more "properly" match the impedances so i would get more gain off this.

the preamps im using are im GUESSING 10k ohm input(?), 600 ohm output. the actual spring reads around 175 ohms. so it looks like this:

preamp >spring>preamp
10k/600 > 175 > 10k

what would a more optimal impedance match look like? thanks much.

[nclayton]

Was the reverb amplifier tube or solid state?

How are you measuring the resistance for the reverb pan? It actually has two separate motors, one on the input, one on the output, and each motor's coil will have its own resistance. Is the 175 a measurement from input to ground, output to ground, or something else?

Ned

[rjd2]

it was a shitty little solid state homemade "amp", breadboarded up-3"x5". very very homebrew. i was told when i bought it for dirt cheap the amp wasnt working in it, and after seeing it, i didnt even bother trying to fix it.

so i wired up the leads of actual spring to the in/out jacks, and just plug the ins and outs of the mic pres to it.

that impedance reading would be from signal to ground-same on each side, of course. thanks for the help!

[The Scum]

Measuring the DC resistance with an ohmmeter doesn't tell you might about the audio impedance of the tank. The driver is probably inductive, and will read different in an AC test.

If they're accutronics tanks, you can look up the part number at the accutronics website, and they'll tell you the impedances. Most tanks have a pretty low input impedance, and take some cleverness to drive.

They've also got some fairly simple schematics for drivers and stuff.

[rjd2]

thanks scum. when i opened up the casing, its just a spring-the same size that's in a twin-in a vinyl casing thing. there's no other mechanics to it than a spring, and the metal chassis that holds the spring. i physically checked it, when wiring,that there was nothing between the jack and the actual spring mechanism. could there still be a driver there? and is measuring signal to ground, or tip to sleeve, not the correct way to measure the actual springs resistance?

[nclayton]

Usually on these things the motors are very small and located right next to the spring, since they're what the spring's ends actually attach to. It's definitely possible to look at them and assume they're just mounts for the spring. They'll just be a few small steel laminations, a little magnet, and a very small coil of wire. One motor works like a speaker and jiggles the spring, one motor works like a microphone and picks up the jiggles. So when you measure the resistance from tip to sleeve you're not actually measuring the resistance of the spring, you're measuring the resistance of the motor coil. That's OK -- the resistance of the spring is totally irrelevant. The resistance of the coil is semi-useful. But there are two coils, one for input and one for output. Not all reverbs use identical motors for input and output, but it sounds like yours might, since the resistances are the same.

If the resistance is 175 ohms, the actual impedance is a good bit higher than that, probably at least double, but maybe it's supposed to be 600 ohms.

I was hoping you'd say the output coil had a lot lower resistance than the input coil, because that might explain your noise problem. If both coils are 175 ohms, then I think your setup should probably work OK. I can't think of what the problem might be other than you must need to drive the reverb's input harder somehow. What kind of preamps are you using?

Ned

[rjd2]

thanks ned, that's helpful. entirely possible a driver was hidden in the vinyl casing.

well i measured both sides, and they are not the same, but negligible? 185 vs. 192 ohms.

the pre's are an rca ba-31 on the front end, and i run it into my trident fleximix console pre. i'll put it this way-i need to run the gain on the trident channel at about +40db or so to get it usefully loud, and with no signal running, you can definitely tell whether that reverb channel is on or off. again, its useable, it would just be great to kill the noise that it generates.

[newfuturevintage]

some thoughts...take em with a grain of salt, for I know very little about what i'm going to say, and could very easily be wrong:

1) can you hit the front end of the tank harder? Maybe with a headphone amp or small amp instead of a line output?

2) I think the output of the tank should be like a guitar pickup or dynamic mic, and would put out a balanced signal, but half of the signal gets grounded for the feed into the single-ended reverb return in a guitar amp.
I wonder if you could gain signal level and s/n ratio by floating the output of the tank: put the output wires that hit its RCA jack to an insulated output, say an XLR's pins 2 and 3. Ground the chassis to pin 1. It seems like it's already a lowish impedance output, and might not need a transformer.

3) can you EQ some of the noise out on the return channel? I would think the noise is mainly high frequency, but the spring wouldn't be putting out much HF information?

[Skipwave]

Nice discussion all. I have an old Altec 1206 mixer with a spring tank in it, and i was going to hacking into the existing driver circuit. The ZeroTronics passive Coolsprings got me thinking about going the external preamps route.

Cool info and graphics on the ZeroTronics site:

http://zerotronics.com/coolsprings/index.html

They use built in transformers to impedance match with the outside world.