Spring Reverb Documentation
Contents
1. Spring Reverb Documentation PCB rev 2 1 Summer 2013 by Tom Whitwell Block Diagram a Tilt EQ Audio in C Vin 4 Attenuverter Blend Level e Works with a variety of spring reverb tanks or with the Accutronics BTDR 2 reverb brick designed by Brian Neunaber e Vactrol crossfade between dry and wet e Tilt EQ on the reverb send to tune the voice of the reverb from warm and deep to bright and light e X Fade input so the module can also be used as a standalone vactrol crossfader e Attenuverter on the crossfade CV input centre off anticlockwise inverted signal clockwise normal signal e Options for reverb tank connection and placement front or rear panel phono sockets and PCB connections e Optional unbuffered PCB connections for 100 wet and post EQ dry outputs A flexible easy to build voltage controlled DIY mono spring reverb module It s a simple build but there are quite a few options The more of this document you read before you start the less likely you are to make a mistake KAO Spring Tank O Crossfade in Crossfader gt Blend out e Ghp wide NB If using the brick mounted horizontally the brick extends 5mm over the PCB to the left of the module around 25mm back from the front panel so mount the reverb next to a shallow module e Around 60mm deep including a phono plug for the reverb tank coming from the back e Just 35mm deep if you mount the reverb conn2. back of your case probably the best way to mount your tank is outside Hanging a tank on the wall behind your modular is a good way to cut down unwanted vibration and avoid transformer hum or the tank can be left on top of the modular where then can Summary Which values should use in the unmarked spaces R7 depends on the vactrol Silonex 32SR3 4700 VTL5C3 47k Tested spring Specs are Input 1500 to 800Q Output 2 2500 to 2 5750 Insulated input Grounded output Beyond this range you may need to do some research or experimentation be shaken or strummed or next to a speaker to encourage feedback Out of the front of the case There is an alternative version of the front panel with space for two chassis mounted phono sockets So you can patch the reverb tank into the front It makes for a snug front panel but is the shallowest and most skiff friendly solution 4a Which Tank Short version f you re buying a new tank I d recommend one of these fairly common models Accutronics 9EB2C16B A big 17 long reverb tank with six springs Accutronics 8EB2C1B A smaller 10 version with three springs This is the tank from a Fender Blues Junior amp so is very common Other makes with the same number i e MOD SEB2C1B will work in the same way The Ruby 3EB2C1B also looks nice Long version The circuit has to be tailored to the Specific tank you use It s designed to be flexible but don t have h
3. like a filter or waveshaper Small changes in the input sound can be amplified and enhanced by the reverb e Choice of spring tank is not absolutely critical Big long six spring tanks will sound bigger than small tanks but I ve found that at least with my limited selection it is less important than the other factors in this list got pretty good reverb from the tiny plastic boxed new reverb that Belton are developing You can use EQ to compensate for the choice of tank to some extent e Hum is caused and can often be cured by the exact position of the tank A reverb tank is not unlike a big highly amplified single coil guitar pickup It will pick up 50 60hz mains hum particularly from transformers but probably also from dimmer switches or fluorescent tubes That hum can often be stopped completely by moving the tank away from the source or often more practically by just changing the position of the tank Try shifting it through 90 degrees on whichever axis is practical and often the hum will drop If you want to mount your tank inside a case experiment with the exact position to find the quietest spot before fixing anything e Noise hiss white noise not hum is caused by the high gain op amp in the recovery section ve tried to reduce it as far as possible by using low impedance resistors also tried two lower gain op amps in series which seriously increased the noise It may be that using other dual
4. op amps could reduce this noise even further Or just think of it as tape hiss adding mojo e Spring reverb is chaotic and hard to predict Spring reverb is a strange gnarly physical effect When tried to calibrate and measure this circuit I d find that a sine wave at for example 400hz might barely excite the springs at all while a sine at 405hz would bust them into fuzzy overdrive Change a component and that response changes completely How to use the controls Tilt Is a hifi style EQ control based on the Ambler circuit from 1970 It controls the signal sent to the reverb but has no effect on the dry signal At 12 o clock it is flat Turned clockwise it boosts frequencies above 1 000hz and cuts frequencies below by up to 8dB Turned anti clockwise it cuts treble and boosts bass Blend Controls the mix between clean and reverb outputs between dry and wet Control This attenuates or inverts the CV input for the blend control At 12 o clock the CV is attenuated although if you want it to go completely it s much easier to pull out the plug Clockwise voltages are added to the blend control high input voltage more reverb Anti clockwise they re inverted high input voltage less reverb X Fade Input Audio input here will replace the wet reverb signal in the crossfader so the module can be used as a standalone vactrol crossfader Reverb cannot be heard in this mode Building
5. bit so that it s clean You want it as loud as possible before distortion e Sweep the oscillator frequency up and down to ensure it doesn t distort If you like try a few other oscillators in your system or an oscillating filter Try whatever makes loud noises to check it won t overdrive the spring e Sweep the blend control back to 100 dry Hopefully the dry and wet signal levels are reasonably well balanced If not maybe push the spring a little harder e After a while using the reverb you may want to go back to tweak the gain to taste LEDs I ve tested the board with both superbright and regular 3mm LEDs both seem to work fine Be careful about changing R11 to make the LED brighter or dimmer as it will also change the Vactrol LED brightness potentially changing the sound Populating the board is hopefully straightforward build in the normal way resistors first then capacitors transistors sockets hardware Front board the board with sockets and pots Back board the board with the power header Take care to study the silkscreen and photographs to ensure components are on the correct side Both boards are double sided with components on both sides Particularly watch out for Type 1 PCB mounted phono Jacks for the spring reverb tank on the back back Type 2 An Accutronics reverb brick on the C12 the capacitor behind the the sockets on the front board must be soldered before the s
6. ections on the front panel e Modular PCB design the back PCB is a self contained system to drive a spring tank or brick making it easy to design simpler or more complex reverb systems e Power consumption Around 25 ma spring or 5ma brick Using the module and how to get the best possible sound I m very happy with how this circuit sounds particularly in this demo http snd sc 1606GvG But it s not always easy to coax such smooth tones from a spring reverb Don t assume there is a fault with your module if you don t immediately get this kind of sound made that mistake a few times on breadboard Here s what I ve learned about getting a good rich warm clean sound e What you get out depends on what you put in By far the biggest factor in the tone of the reverb is obviously the tone being fed into it Software reverbs can put the same smooth mushy sound over whatever your run into them This isn t like that at all Remember that it s a physical system moving about driven by magnets and electricity and springs lve found that smooth clean FM ed sinewaves work very well with the springs On a scope the output looks similar to the input but more complicated On the other hand square waves try to bash the springs about There s no way that a magnet can move a spring in anything like a square wave so the sound coming out will sound little like a square wave On a scope it s more
7. et it right first time go back and try again Once the module is fully built with all the ICs in their sockets connect the module to power in your case or a separate PSU if you have one Connect the spring reverb being careful to get the ins and outs the right way around Don t screw the module into the case you ll need access to the trimmer on the back e Connect an oscillator ideally the loudest sine or triangle oscillator in your modular to the input e Connect the spring module output to your output mixer etc Turn down your speakers It might get loud it might also be very quiet e Set the oscillator to a middle low frequency not sub bass or very high pitched Use an LFO or a sequencer to constantly vary the pitch It doesn t matter what it s doing but you can t really hear reverb on a static pitch e Set the CV and Tilt controls to 12 o clock e Pan the blend control from 7 o clock to 5 o clock You should be able to hear the clean dry signal at 7 and some kind of reverbed signal at 5 The reverb level might be very low or very loud and distorted e With the blend control at 5 o clock fully wet start moving the screw on the trim pot clockwise e Listen to the reverb output which should get gradually either louder or quieter If it doesn t change start moving anticlockwise instead e Once you ve got the hang of it adjust the reverb output until it just starts to distort then move back a
8. f parts but the parallel PCB construction means some components are very specific Sockets The board is designed for 3 5mm vertical jacks from Erthenvar or Thonk model PJ 301b Other jacks may work you ll probably need to wire the grounds together Potentiometers Many 9mm vertical mount pots will fit on the board Ideally for maximum module strength and stability use pots that mount to the panel like the Alpha RD9O1F 40 15R range available from Smallbear 7mm mounting holes or the Alps RKOQL range 9mm mounting holes The Blend pot is a voltage divider any Linear pot above 10k should work fine The CV Control attenuverter circuit is setup for a 50k pot but I ve used 100k in this slot without a big impact The EQ circuit is set up for 10k with values deliberately low to reduce noise The circuit will still work with other values but the maximum gain cut may be more extreme causing clipping at some settings Op Amps specified a NE5532 for the EQ section but another TLO72 will perform fine Unfortunately had little luck with the 5532 in the critical spring recovery circuit where the lower noise would be helpful Vactrols The Silonex NSL 32SR3 has a white spot next to the negative LED lead The resistor leads are the long thin ones It s a pretty tight fit but the Silonex and VTL5C3 will both fit on the board First run calibration The calibration procedure is pretty straightforward If you don t g
9. ly the wrong way round It still worked but certainly sounded better the right way around 1 Locking Devices B Mounting orientation The circuit doesn t care about either of these 4b Which Brick There are two types of BIDR 2 depending on the orientation Either should work but I ve only tested mine with a BTDR 2H which lies parallel to the PCB The BTDR 2H will lie flat against the back of the module poking 5mm out of the left side The BTDR 2V will poke out of the back of the module extending the depth to around 60mm and poking out a couple of millimeters on the right hand side Either solder the brick directly to the PCB which is sturdy and low profile or mount it in a bit of female header which adds a couple of centimeters of depth You probably wouldn t solder a chip worth 15 to a board without a socket would you Other Options There are lots of options on the board to help you build the reverb you want All the sockets are pulled out to tags on the front board On the back board there are outputs for the Post EQ pre Reverb signal and the Post reverb pre Blend 100 wet signal The rear board is a stand alone reverb circuit and the front board should act as a modular level EQ Blend circuit receiving power from the rear board Two Accutronics Blue reverbs mounted in a Hammond 1590XX enclosure Building the circuit Bill of Materials Most of the components are straightforward off the shel
10. ockets e The little 5 pin jumper that runs between the front and back boards Fit the male and female halves together put it in place screw the boards together then solder It doesn t matter which side is male or female e The back board is particularly double sided the phono jacks power header trimmer spring brick jumper and Brick are all mounted on the rear of the back board All other components including the rest of the power circuit are on the front of the back board between the two boards e lve accidentally soldered the power header to the wrong side of the board before It s a real pain to remove and you have to sacrifice the entire header to get it out It s best to attach the LED right at the end When the front board is finished put the LED into the board the longer lead goes into the hole but don t solder it Loosely fit the front panel and finger tighten a couple of nuts Place the LED Isic ing dular Type 3 Phono jacks for the PCB mounted on the front panel into the hole in the front panel then solder it into place before assembling the rest of the module Troubleshooting amp mods If the reverb level is too low after calibration consider changing R6 from 22k to 47k R6 sets the gain for the recovery circuit The downside is that this may cause increased hum and risk of feedback It is possible to build both spring and brick circuits then attach a simple SPDT switch to the jum
11. patch ideas to tom whitwell gmail com Open Source Hardware This is an open hardware project You are free to build this hardware sell it or make any changes to it so long as you share your changes and release them under the same license You should also credit me Tom Whitwell All of the original design files Eagle CAD Illustrator etc are shared These files are licensed under a Creative Commons Attribution Share Alike license which allows for both personal and commercial derivative works as long as they credit me and release your own interpretation under the same license CLV This work is licensed under the Creative Commons Attribution ShareAlike 3 0 Unported License To view a copy of this license visit http creativecommons org licenses by Ssa 3 0 or send a letter to Creative SA Commons 444 Castro Street Suite 900 Mountain View California 94041 USA
12. per to switch between the two reverb types There are solder points on the back board for Post EQ and 100 outputs which could be pulled out to front panel sockets These aren t buffered so inserting a plug may change the reverbed output It would be easy enough to hack two buffers from the TLO72 in the Brick section Patch ideas Feedback Ingredients Spring Reverb Filter with two input channels or filter mixer 1 Mult the Spring output to your speakers and to one input channel on the filter 2 Take the input source if required to the other filter input 3 Connect the filter output to the spring input 4 Turn the volumes up carefully until the whole circuit starts to feed back 5 Bandpass filters are particularly fun here sweeping the filter tunes the feedback Gated reverse reverb Ingredients Spring Reverb Make Noise Maths oscillator VCA 1 Set up the right side of a Maths as an envelope controlling the VCA to create a string of short audio pulses blips 2 Connect EOC to the left side TRIG Ensure left side is not in cycle mode 3 Connect the EOR output to the Spring CV input Set the blend to 7 o clock and Control to 5 o clock Listen to the spring reverb output 4 The Spring LED should now flash in time with the audio pulses Left hand Rise control sets the length of the reverb 5 Switch the Spring CV input from EOR to the envelope out to get pseudo reverse reverb Send your
13. the module and choices you ll need to make before you start Before you start This PCB was built to be flexible Before building you ll have to take four decisions 1 Tank or Brick The rear PCB has space for two separate drive amp recovery circuits one for a reverb tank and one for the Accutronics BTDR 2 reverb brick The real tank sounds better but is physically bigger and liable to hum The brick is small and self contained with a distinctive reverb based on PT2399 delay chips Only build the circuit you will be using If building for a spring ignore all components in the Brick area If building for a brick ignore all components in the Spring area It is also possible to build both circuits and attach a switch in place of the jumper to make the module switchable between spring and brick 2 Which vactrol I ve tested the circuit with VIL5C3 and Silonex 32SR3 vactrols Both work well and the value of R7 needs to change Silonex 32SR3 4700 VTL5C3 47k If you re using a DIY vactrol or another brand R7 should be chosen experimentally 3 Where will you put the tank Inside the case The rear PCB has space for phono sockets so that you can easily mount a reverb tank either inside your case There s a risk that your PSU will create hum that is picked up by the tank so experiment with the position and orientation of the tank Out the back of the case If you have or can make a Suitable hole in the
14. undreds of tanks lying around to test it with Here s how Accutronics Belton codes work using the top tank as an example 9EB2C1B 9 The type of tank the size number of springs etc The circuit doesn t care but the sound will be different E The input impedance A 8 10 Q B 150 190 QO C 200 240 QO D 250 310 Q E 600 800 Q F 1 475 1 925 0 The lower numbers are for Type 4 tanks higher for Type 8 amp 9 tanks I ve tested the circuit with various impedances between 150 and 800 Q Very low impedances may pull more current than is available from the TLO 2 driver Very high impedances might require swapping the 10k trimmer for a 100k trimmer B Output Impedance A 500 600 2 B 2 250 2 575 Q C 10 000 12 000 Q lve only tested this circuit with B type tanks To adapt to other impedances you d need to change the values of R4 and R6 2 Decay Time The circuit doesn t care C Connections A Input Grounded Output Grounded B Input Grounded Output Insulated C Input Insulated Output Grounded D Input Insulated Output Insulated NB This board assumes Red Output Check your tank carefully The mini tanks that come with a Doepfer A199 are sometimes coded differently This circuit is designed for C type tanks You might need to change the circuit significantly to use other types That said I ve run my Blues Junior with the reverb connected entire
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