Rack Spring Reverb Unit

Keefy

I love spring reverb and I like to record with it. I have a 1980s Frontline spring reverb desktop unit, but it's not great, so I've started to build another one that I hope will be better.

The circuit will be based on Craig Anderton's Center Stage - the modified version found here. I have collected together all the parts I need, including a suitable (correct impedances) reverb tank scavenged from a scrapped combo, and this faulty Behringer rack unit, which will provide the case and (with any luck) the power supply:



Let the build commence!

Adam_MD

Sweet I love projects like this.

paulnb57

I wont understand anything you do, but Ill still watch progress!

Keefy

I have stripped out everything I don't need. The front PCB is the one with pots and LED ladders - I just saved the push-on knobs. I sawed through the rear PCB to leave the +/-15V power lines - all I saved from the hardware was the nuts and washers!

The spring tank is an Accutronics 8DB2C1D unit, which has the right input and output impedances to work with the TL071-based driver and recovery circuit. Here I am just checking it will fit in the case.



I will be building the circuit onto strip board. This is my first attempt at a layout. I need to go away and sleep before coming back to it. The original circuit generates +/-9V from a single 9V battery, but the higher voltages available should give me more headroom.



I'm not convinced that the Dwell control is anything more than another mix control, so I won't commit to putting it on the front panel until I'm sure.

ICBM

Keefy said:

I'm not convinced that the Dwell control is anything more than another mix control, so I won't commit to putting it on the front panel until I'm sure.

Dwell is (or should be!) a send level control, which increases the reverb decay time by driving the springs harder, basically - not at all the same as a mix control which is a return level or balance with the dry signal.

Keefy

ICBM said:

Keefy said:

I'm not convinced that the Dwell control is anything more than another mix control, so I won't commit to putting it on the front panel until I'm sure.

Dwell is (or should be!) a send level control, which increases the reverb decay time by driving the springs harder, basically - not at all the same as a mix control which is a return level or balance with the dry signal.

That's exactly what I thought! There is also a gain control on the 'send' side of the circuit which is intended to be a trim pot, but I am going to bring it out to a front panel 'Drive' control.

@ICBM could you take a look at the schematic here and see what you think?

ICBM

Keefy said:

That's exactly what I thought! There is also a gain control on the 'send' side of the circuit which is intended to be a trim pot, but I am going to bring it out to a front panel 'Drive' control.

@ICBM could you take a look at the schematic here and see what you think?

Yes, they're mislabeled. 'Mix' is in fact the *dry* level, Dwell is the reverb level, and the actual dwell control is R7.

You will need the reverb level control on the front panel as well as the drive/dwell - the one you may not need is the dry level, but it would probably be useful to have it.

Keefy

ICBM said:

Keefy said:

That's exactly what I thought! There is also a gain control on the 'send' side of the circuit which is intended to be a trim pot, but I am going to bring it out to a front panel 'Drive' control.

@ICBM could you take a look at the schematic here and see what you think?

Yes, they're mislabeled. 'Mix' is in fact the *dry* level, Dwell is the reverb level, and the actual dwell control is R7.

You will need the reverb level control on the front panel as well as the drive/dwell - the one you may not need is the dry level, but it would probably be useful to have it.

Thanks so much for that @ICBM. I will use all 3 controls. For recording it will be 100% wet, but I will want a mixed output if I am using it between my amp sim setup (Revival Drive + Torpedo CAB) and studio monitors.

Keefy

I checked my layout diagram and found a couple of mistakes. I cut a piece of IC stripboard a bit bigger than I needed, masked off the unused areas. and drilled a couple of mounting holes. Then I set about populating the board in the following order:

• IC sockets
• Wire links
• Resistors
• Capacitors

It's been a while since I built on stripboard - I quickly changed my soldering iron bit over from chisel to fine point! This is what it looks like now with the ICs installed:

Keefy

The build to date. All earth connections go to a single point on the case. The reverb tank is insulated from the case as it gets its earth from one of the signal leads.There was no room for sockets on the back panel so input and output will be on the front.

It doesn't work yet. I have confirmed there is dry-through signal, a good strong driver signal, and the recovery circuit works. Everything points to the tank, so I have ordered a compatible one with 800ohm input impedance and long reverb time. Everything else has been built from scrap parts, so I'm not too bothered about the £20 outlay for a new tank!

joeyowen

Great work so far! 

Keefy

Doesn't work with new tank either. Bugger. Well at least I can eliminate that from my trouble-shooting.

Keefy

This project has been taunting me, cluttering up either the floor or my desk, saying 'Ha ha, you can't make me work, you don't even know what's wrong with me!' In the meantime I've done a quick-and-dirty test on both tanks by feeding my cable tester's test signal to the input, and the output to my computer interface. The new tank definitely worked, wasn't so sure about the old one.

Having spent the entire weekend working on guitar and bass material for 3 different bands, today I am waiting in for Virgin to come and fit a new cable box. I can't make a racket because I need to hear the doorbell.

For the umpteenth time I went through my layout, checking it against the schematic, but this time I found my idiot error - no connection to the non-inverting input of the reverb recovery op-amp, hence no reverb. The schematic calls for this pin to be grounded, so I fitted a new link, and - damn thing hums like a mother, more so if I touch the tank, but still no reverb sound. This style of tank has its output connected to the metal tank itself, but it clearly wasn't grounded. Continuity testing established that although the screen of the shielded cable was in the right place on the circuit board, it wasn't making a connection to earth. It must have become retracted when I turned the board over to solder it. I remade the connection properly and - yes, we have reverb!

It's a nice long reverb with that much-discussed 'drip'. I've only tested it by tapping the jack plug on the input, but it does sound quite dark. I'll test it with a guitar after the Virgin operative has been and gone. Looking at the circuit diagram, capacitor C2 will be dumping treble to ground via R2. I may be able to create a simple tone control by interposing a suitable pot (say 100k?).

Out of curiosity, I swapped in the old tank, and sure enough it's shonky - it just creates a short dull clank. If I can diagnose the fault I may be able to fix it, but for the time being it will go in the loft with all the other broken stuff that, ahem, might come in useful one day.

To recap, that was 3 faults altogether:

• Reverb tank Donald Ducked (I wrongly assumed it would work) 
• Board layout error (my bad)
• Ineffective solder joint (my bad again)

There could be a pattern there...

Now to work on the front and rear panel overlays!

Keefy

It turned out that C2 just dumped some hiss - with it out of circuit, that's the only difference I could tell, so I re-attached it.

I cracked on and completed the build. I covered the front and rear panels with strips of plastic sheet material that I glued into place, and gave the top a couple of coats of cheap spray paint. I used or enlarged existing front panel holes for the sockets and pots. Some recycled amp knobs and printed labels finish the job. The observant will note the lack of a pilot light and model name. I might add an LED at some point, but printing a big label that says 'REVERB' seems a bit pointless to me.

With Strat > Reverb unit > Princeton Reverb (with the reverb off, natch!) I proceeded to have a play. I had chosen the long decay tank, and boy is this a long reverb! With the Dwell control up the sound bounces around for ages. At lower dwell settings it is more restrained and less boomy, but still quite a lengthy decay. The character is very different to the PR's inbuilt reverb, but then it's a completely different tank and circuit.

I might buy an equivalent medium decay tank and try that too. In the meantime I'm calling this a win.

Next project is a Fender 6G15 standalone reverb. I've already started collecting the parts...

PeteC

Very nice project.   Outboard Spring reverb is very underrated in today’s plug in obsessed home  studios.   Sometimes a nice funky hardware effect can really make the track.    

Keefy

PeteC said:

Very nice project.   Outboard Spring reverb is very underrated in today’s plug in obsessed home  studios.   Sometimes a nice funky hardware effect can really make the track.    

Quite so. I like to get a great sound and then record it - I do very little ‘in the box’.

oaf

Nice work, good to see it completed  

Keefy

oaf said:

Nice work, good to see it completed  

Cheers, I had a longer play with it last night, patched between my Revival Drive and Torpedo CAB, and through my studio monitors. There's quite a range of sounds available, and it will sit nicely in the background to provide some 'air'.

PeteC

There is/was a good blog on the Preservation Sound website about using traditional spring reverb.  From memory I think he used headphone amps as drivers for accutronics springs , or similar.  I do the same and it works really well. A change from the Waves plugins etc.    I like using stuff this way - I love Sylvia Massey's Recording Unhinged book for ideas like this. 

Keefy

PeteC said:

There is/was a good blog on the Preservation Sound website about using traditional spring reverb.  From memory I think he used headphone amps as drivers for accutronics springs , or similar.  I do the same and it works really well. A change from the Waves plugins etc.    I like using stuff this way - I love Sylvia Massey's Recording Unhinged book for ideas like this. 

The headphone amp thing is interesting. It would depend on the input impedance of the tank input. For anyone interested in spring tanks and what the 7-digit Accutronics codes mean, this article is a good place to start.

Keefy

Got the medium decay tank today and did a comparison with the long decay - the medium decay has a much more usable range for guitar purposes and is a bit brighter, so it's staying in the unit.

Toying with the idea of adding sockets and a switch to select between tanks, but one of them would have to be external. I have already done the same thing with my Frontline unit - it works ok with 4AB__ and similar units.