Passive Spring Reverb and impedence

As someone with more good preamps (JLM and 7th Cirlce stuff, among others) than I'm ever actually using at one time, I've started building passive pieces and using my extra pres as sources of makeup gain.

I've been fooling around with some of the cheaper Accutronics reverb tanks and transformers to make a passive reverb, and have gotten some decent results for low fi effects. I want to build something that would be a more hi-fi piece that maybe could rival the quality a a low end plate.

I'm looking for suggestions as what input and output impedence the tanks I should have for this unit (http://www.accutronicsreverb.com/prodspec.htm). I would be using type 9 tanks. I guess I'm wondering which tanks would work best with typical input and output transformers and the type of preamps I have (mostly transformer in, about half transformer out).

Thanks,

Craig

if you are buying a new tank to DIY a passive reverb unit try to get something close to a 150 ohm output, since that is what a mic amp wants to see.

ive done this kind of thing alot. my technique is to use a guitar amp as outboard spring reverb. you send via a reamp box to the amp input. pull the output cable from the reverb tank and connect it right into a mic preamp. all you need is an RCA-male to XLR male cable. the low-z tanks work the best for this, usually from solid state amps. but it doesn't matter. I just plug it in and listen. if it is really low in level it might be too high-Z of an output. in that case I guess you could try a DI box. or just crank the input more.

edit: It does not have to be lo-fi! here is a song I recorded that the only reverb in the track is a musicman 112RD connected this way. It is on her vocal. the song is "goodbye"
http://www.myspace.com/elephantparade

Depending what you are driving it with, you will probably want a low input impedance, under 100 ohms if by a transistor driver stage.

Depending what you are driving it with, you will probably want a low input impedance, under 100 ohms if by a transistor driver stage.

Click to expand...

I'd say I will be driving it with either DOA based pres, a couple vintage tube devices, and occosionally the outputs of my audio interface.

lots of information at the site even schematics

http://www.accutronicsreverb.com/rvbapps.htm

Speaking of reverb, does anybody here know how the Hollies got that sound on the intro to Long Cool Women?
I would sure be interested.

That bass just rocks, too.

I found this, how do you hook up the gate?

"Noise gating (which is just what it sounds like. A noise gate placed after a reverb device gets you that Hollies' "Long Cool Woman" vocal sound, or the ubiquitous Phil Collins "In The Air Tonight" drum sound). "

link

I guess the input impedance is what I'm most concerned about. My options are 150, 200, 250, 600, or 1475 ohms. I'm guessing if its taking the output out of vintage style, transformer balanced pres it should be one of the last two.

Mike, the reverb on your tune is definately not lo-fi!

[quote author="CJ"]I found this, how do you hook up the gate?

"Noise gating (which is just what it sounds like. A noise gate placed after a reverb device gets you that Hollies' "Long Cool Woman" vocal sound, or the ubiquitous Phil Collins "In The Air Tonight" drum sound). "

link[/quote]

I believe you just put the gate after the reverb and adjust it to cut off as much of the reverb tail as you find aesthetically pleasing. On snares, it seems to give the sound a bit more bite as the gate chops off the decay abruptly, which seems to add to the definition by temporarily adding some "space" in the mix.

Cheers,
--
Don

Note you could drive the reverb tank 'as usual' (as done in most gtr-amps) or 'current driven' (enclosed in the feedback-loop of an opamp, as for instance on page 2 of this 1987 'studio reverb': http://www.d-r.nl/dnrsite/Service Parts/Reverb-Spring-Service.PDF )
The latter wil give more BW.

Regards,

Peter

Based on the very general information you posted, I'd say that 600-ohm input and 500-ohm output are your best bet (referring to type 1 and 4 tanks at http://www.accutronicsreverb.com/ioic.htm).

Most decent pro audio gear can deliver reasonable power--100mW or more--into 600 ohms without distorting excessively, blowing up, etc. Yes, I said power--we're driving a transducer, after all. In your case, using a transducer with a lower impedance is risky; most preamps and the like are not designed to work with loads below 600 ohms. As for output impedance, 150-200 ohms would be a better match to the typical mic preamp input, but 500 ohms (the lowest impedance offered) should give good results.

Consider using a limiter in-line before the drive amplifier if you want to avoid that "sproingy-ness" that usually characterizes the sound of a cheap spring reverb. Also, high-pass filtering of some sort at the input side is usually helpful.

Thanks for the advice, Dave!

Could I use an output transformer to step down to 150 ohms on the output? If I did, would a 600:150 work with the 500 ohm output of the tank?

Craig

I wouldn't bother. The signal from the pickup xducer is miniscule as it is--you don't really want to introduce 6dB loss if you don't have to. Besides, this is not full-bandwidth audio and ruler-flat frequency response is not necessary. In fact, rolloff at the extreme ends of the band is a good thing in this application.

Would input transformers be uneccessary as well? I was thinking transformer in and out because of a unit I saw doing the same thing. Are they putting the transformers in there just to say it's tranformer balanced and increase the cost?

There is a reverb transformer on the driver side of most git amps, Ido not know what the specs are.
I have one at home if you need a turns ratio.

That's to match the high impedance of the driver tube plate to the low-Z of the reverb transducer.

The sort of devices Craig wants to use already have a low source impedance and a step-down transformer is unnecessary--as long as the xducer nominal impedance is 600 ohms or greater.

Craig, don't bother with in/out xfmrs. When you order your tank, ask Accutronics to make it with both connectors--input and output--not grounded to the case. This is connector code D: "Input Insulated / Output Insulated."

When you build your reverb, wire up your balanced I/O to the RCA connectors (XLR pin 2 to center conductor, pin 3 to outer conductor) and connect the cable shield (pin 1) to the tank case. This will in fact fit the usual working definition of a "balanced*" I/O, RCA connectors notwithstanding, since neither end of the transducer is grounded. Don't forget to ground the tank case, otherwise it's noise city. (Ever used a mic cable with an open shield? You get the idea).

(* "Balanced" is often used as shorthand to mean any transmission line where the "cold" end is not grounded, although the more technically accurate definition could be stated as "symmetrical to ground." Our application would be called "balanced floating" since no part of the transducer coil is connected to ground; the waveforms in the two lines are symmetrical but not ground-referenced).

Dave,

I'm going to order some tanks and wire them up as you suggest. If it doesn't sound like I want, I can always add transformers later. Plus making it without the transfomers makes my wife happier! Thanks for your help.

Craig

[quote author="craigmorris74"]As someone with more good preamps (JLM and 7th Cirlce stuff, among others) than I'm ever actually using at one time, I've started building passive pieces and using my extra pres as sources of makeup gain.

I've been fooling around with some of the cheaper Accutronics reverb tanks and transformers to make a passive reverb, and have gotten some decent results for low fi effects. I want to build something that would be a more hi-fi piece that maybe could rival the quality a a low end plate.

I'm looking for suggestions as what input and output impedence the tanks I should have for this unit (http://www.accutronicsreverb.com/prodspec.htm). I would be using type 9 tanks. I guess I'm wondering which tanks would work best with typical input and output transformers and the type of preamps I have (mostly transformer in, about half transformer out).

Thanks,

Craig[/quote]

Typical spring drivers like to driven by currents rather than voltages. This can be accomplished with conventional opamps (with adequate drive capability) as simply as connecting the spring driver between opamp output and minus input with a reisitance from minus input to ground (preferably cap coupled to keep DC out of spring). If you overdrive a spring even transiently it will make a nasty audible artifact that persists bouncing around in the springs until it decays.

Even the best spring will not IMO approach a plate for smoothness. But more springs and mixed spring lengths will help make them more diffuse. I seem to recall at least one DIY plate project published in a recording mag years back.

JR

Ideally, I would like to have a plate, but there's no way I have room with my current setup. It's a shame because buiding one looks like fun. I'm aware of the problems of using spring verbs on sources with large transients. I will problably limit peaks heavily before they pass into this unit.

Fender made that stand a lone reverb, you might want to check that out.
I have some Kendrick mods for that, tach and dwell, you know them Longhorn types...hot rods and giutars, thats all they know.

There are a couple of xfmr's , or transducers as you call them, on the send and recieve sides of the springs, so you may get some free isolation there.
Current driven-thats why they tie the two plates together on the driver tube.
Plus, it's an unused half if they don't.

Fender made that stand a lone reverb, you might want to check that out.
I have some Kendrick mods for that

Click to expand...

Funny, I haven't even looked at any schematics for the Fender units. I suppose I could learn something there! I may hit you up for the mod notes based on what I see.