Advice: About to Embark on a Stage Center Reverb

[deathfaces]

I've got two pcbs with charge pumps i etched from GGG and am going to have a go at these in the next week or so.  I was wondering if anyone had any friendly advice to offer to get the best Stage Center possible.

I want something with a longer decay so i just emailed Ac@#$%ronics for a quote on a 9DB3C1D, which is spec'd the same as the recommended tank, but is a different spring configuration (Type 9) with long decay.

Note: Recommended tank is Accutronics 4DB2C1D (Type 4 with medium decay)

Thanks in advance!

[Mark Hammer]

Understand this: ANY reverb assumes you will be throwing lots of current at the driver transducer.  Indeed, that is why you'll see Fender units using power tubes to drive the pan, or solid state circuits using 386 power-chips to drive them.  What I infer from that is that using a charge pump to try and provide enough current from a 9v battery is not gonna do it.  If you plan on using a charge pump with a wall-wart, note that some charge pumps tend to crap out around 15ma or so.  So if you want to assure pleasing reverb tone, you will want to use a supply that can provide +/-20ma just to be on the same side.

This is not a hard and fast rule, and those who have had success with less should speak up, but fundamentally a reverb unit is trying to drive a speaker-like load and needs the same amount of current to do so.

[deathfaces]

Wall wart is a definite go, i dont use batteries for anything these days. I was planning on using a 9 volt, but it sounds that a 12 volt is at least in order.  But you're saying the crucial element is the power supply and not the stage center circuit itself? i assumed the stage center circuit would be driving the reverb so i'm a little confused.

[Strategy]

Search the forum for 'stage center reverb' and you will turn up a detailed set of Mark Hammer mods for the Stage Center. I didn't implement them all, just one so far.

I etched my own board, the layout that is on GGG but I cannot remember now if it's the same layout they use in the ones they sell. I used 2x 9V batteries, I use rechargeable batteries only and the wiring diagram shows that you use a power switch to deactivate the batteries so they don't drain each other. (note, it is not clearly marked in the wiring diagram, just 'mentioned'!) The pedal works well with that, but you have to make sure you don't leave the power on or it will drain quickly.

Of the mods implemented, I rolled off the bass going into the springs by replacing C4 with a 4700 pF capacitor. I am finding this has a high noise floor, and it's suggested to try different op amps. I have TL074 in there now, but I have an LM833 on the way to me this week to see if that makes a difference. I am also going to kludge in a high rolloff cap on the output resistor to see if that doesn't bring some of the hiss down.

I used a 3-spring, type 8 tank (9") and it comes close in cavernousness to the 17" while maintaining a super small footprint. I have a pedal sized one that I can take out. What I like about this circuit is that there's no hum or buzz, which I have even in some high end, synth/studio type spring reverbs (Vermona Retroverb for instance). But, the trade off is that hiss/noise floor.

I hope some of this helps, again there is a wealth of info in old threads on this one - worth taking an hour or so to go through them closely
- Strategy

[mth5044]

Here is a picture of the mods from Mark Hammer in red and two other ones in blue. I got it mostly to work, but the reverb signal has a terrible distortion on it. If it were to be a distorted reverb unit, it would be perfect. I am going to try to replace the IC, but besides that I can't seem to figure out the problem, so I'm going to have to say this picture isn't verified yet.

[deathfaces]

Thats pretty neat, but i was going to stick to the standard circuit to start. I've looked through the other posts, and just wanted a general vibe on the circuit to know if it was worth dropping the coin.  Success seems relative. I'm not looking for the perfect reverb (the music i make could be called "experimental") so i wasnt sure how to gauge other people's view of success.

i'm going to search for the mods because none of them are labeled here, but they give me pause since they arent verified.

[anchovie]

Ac@#$%ronics

I like how you mis-typed that in such a way that the auto-censor had to step in! 

[Strategy]

I play experimental music as well, and I built the circuit stock first and then did the mods. I would recommend going this way. What I think you will notice is that you really have to roll off the low end of the signal going into the springs. There was this nasty snapping/slamming sound from even a little bit of low end, like E string on a guitar or anything from synth or electronics putting out line level was worse. It was not musical and not even musical noise like in noise music. I would highly recommend changing the value of C4, the tonality of this reverb is in the mids and high mids.

That bass rolloff mod is easy, you just choose another value. I chose 4700pF, I saw another post where someone used 5600pF. Mark Hammer's mods, and Mth5044's mod diagram is pretty involved as well, I'd say start stock but don't be afraid to swap out parts and add mods bit by bit, this is a pretty easy circuit to experiment with, hard to wreck unless you do something wrong to the PCB really. As you decide to tweak the circuit, you can add them mod by mod instead of all at once.

- Strategy

[Mark Hammer]

Although longer springs have a tendency to wiggle and shiver for a longer period of time, that assumes that they will be pushed hard enough.  What has traditionally been called the "dwell" control on Fender reverbs is essentially a driver-level control.  The principle is that if you hit the reverb pan with a lower level signal, the springs stop vibrating sooner.  The corollary is that one can adjust the decay time for any reverb pan by means of adjustments to the gain stage just ahead of the springs.  Longer springs will still produce longer decay, but how much one approximates the longest possible decay for those springs will depend on drive/dwell.

Of course hitting the springs harder is not without its risks.  Too hard a "slap" tends to elicit misbehaviour from the springs and a kind of "gulp" sound at the start of every picked nte, as opposed to a nice ambient wash and "air".  There are several ways to avoid hitting the springs with too hard a transient.  One is to use a limiter as part of the driver circuitry (and a number of projects and commercial reverb units have done this).  Another is to filter out some of the low end when the biggest part of the transient amplitude lives.  Of course, since chopping out a big chunk of the bass will have the effect of reducing the overall ampitude of the initial pick envelope,that necessitates increasing the gain applied to whatever you have left.  It's a bit like asking everyone to whisper, but needing to amplify their whispers enough so as to be able to hear everyone.

The basic principle to adhere to is that springs are a mechanical device.  And much the way you would expect the strings of your guitar to produce different tones depending on how you pluck them, you can expect the springs of your pan to produce different tones depending on how you get them to start moving.  All you have to do, then, is figure out how you like them to move, and from that point you can figure out how to get them to move in the desired fashion.

[deathfaces]

So a shorter tank and springs would do me a greater good based on the limited nature of the SCR driver.  A type 8, like Strategy used, over the longer type 9. Loud and clear.

So if i still wanted a longer decay time, a 3 on the accutronics chart, what determines the length of decay time on a tank with shorter springs, and would i be shooting myself in the foot still and be better off with the recommended medium decay?

[Mark Hammer]

So a shorter tank and springs would do me a greater good based on the limited nature of the SCR driver.  A type 8, like Strategy used, over the longer type 9. Loud and clear.

Not necessarily, but my gut tells me it takes a better driver to push longer springs hard enough.  Remember, you are using your guitar signal to make a vibration work its way from one end of a piece of steel to the other.  Think of it like a long car ride with no gas stations along the way; either you have enough fuel to take you there...or you don't.

So if i still wanted a longer decay time, a 3 on the accutronics chart, what determines the length of decay time on a tank with shorter springs, and would i be shooting myself in the foot still and be better off with the recommended medium decay?

There are two aspects to decay.  One is determined by the actual length of time it takes for the sound wave to travel the full distance of the spring.  Since the sound wave will go in one direction, then perhaps back again (like the waves in the bathtub when you swish the water), the delay time will influence the decay time, in much the same way that the delay-time control on a delay pedal will make the echo linger longer.  How hard you push the springs will determine how long they quiver for.  In this respect, it is just like a guitar string: pick harder and the vibrating string takes longer to decay.

Longer springs tend to produce a different tonal quality, quite apart from the differences in their delay/decay time.  Where short springs tend to produce a more resonant reverb sound (because with a short distance to travel, the sound wave goes back and forth along the spring several times, akin to what you hear when using a short delay time and cranking the regeneration/feedback), longer springs produce a more diffuse reverb sound.  The "ideal" - if one seeks a lush non-resonant ambience - is more springs and longer ones.  That is not to say that the shorter set will produce crappy tone.  It is also one of the reasons that studio-quality professional spring reverb units will include some EQ (often semiparametric); to counteract the resonances of shorter springs...or create them.

Note, as well, that it is always possible to recirculate the reverb signal back to the input stage, in exactly the same way you would with an analog delay, such that the signal is re-fed to the pan to linger even longer.  Now, it is not advisable to feed back that much, but it is something you can experiment with to achieve different feels.

And just so we are clear, there is no spring unit that I know of which will produce those ultra-long cathedral reverbs like you hear on Lexicon units or some of the recent digital reverb pedals.  When one is relying on the transmission properties of a physical object, there are limitations as to what you can achieve in comparison to a digitally generated signal.  So, while you can get some bitchin' sounds out of a medium pan, be realistic in what you expect to be able to do, even with every conceivable mod.

[Strategy]

Mark, this is spring reverb wisdom! Us spring reverb fanatics really owe you one. (I am building the Gaussmarkov spring reverb right now too.) 

I use spring reverb a lot in my studio production for very up-front dub style effecting and the 3-short-springs, type 8 works well to get splashes of metallic brightness- it has plenty of decay but decay is less important in the overall picture than tonality. I like a certain amount of clang and bang for use on snare, hi hates, high funk guitar chord type sounds, electric organ vamping- For that brightness the short pan is great. I would like to experiment with a longer pan too at some point.

One way to go would be to (it would be spendy but would answer your questions) actually get one long pan and one short pan and see which you like better!

- Strategy

[Mark Hammer]

I can't link to it right now, but later this evening I'll post the links to an old POLYPHONY article written by Craig O'Donnell that I scanned and posted.  Craig's original comments (now some 28 years old) were my inspiration.  But thanks for the nod.

[Mark Hammer]

Here they are:
http://img.photobucket.com/albums/v474/mhammer/Reverb1.png
http://img.photobucket.com/albums/v474/mhammer/Reverb1.png

Pleasant reading.

[petemoore]

  I remember having less than smooth response when using boosted input>reverb. I tried a power mosfet follower IIRC to as buffer to output into the tank driver.
  Otherwise it could be adjusted to cool effect and nice sounding reverb.
  Considered a different chip-amp [LM3886, but haven't checked the output impedance possibilities] output to do the driver work, I think it might not even need a heat sink.

[Strategy]

I successfully modded my Stage Center Reverb last night - I added a 15pF ceramic capacitor to the 2M2 output resistor, just by wrapping the cap legs around the part of the resistor legs exposed on the top side of the board. it was a tight squeeze to solder with all the components and wiring being so close, but it helps the sound a lot with the aforementioned hissy noise floor. It does not completely eradicate it but seems to reduce it a lot. It has a really musical effect on the whole circuit, I recommend this mod.

- Strategy