Reverb Drive And Recovery Amplifier

The P113 headphone reverb amplifier now has this page (and a 'new' project number).  It is simple to build, relatively inexpensive, and provides a level of performance that exceeds most commercial offerings.  The board measures only 76 x 42mm, so it can easily be installed into any suitable box or case.

The project as presented here has a couple of minor changes from the original, but is otherwise the same.  The feedback capacitor is a standard (polarised) electrolytic, and the current feedback is configured specifically for reverb tank drive.  This provides essentially constant current drive, which is optimal for reverb tanks.  The performance with a normal spring reverb tank is surprisingly good, and I tested it in my workshop with music (off-air from an FM broadcast).  The reverb is very clean, and potentially comes close to a pro-audio plate reverb (this depends on the reverb tank of course).

The circuit can use an existing ±15V supply, or may be powered from a P05 or P05-Mini supply.  The input, optional tone controls mixer and output circuits are shown in Project 94-RVB, using the P94 ('universal' preamp/ mixer PCB).  The wiring is straightforward and provides everything other than a compressor/ limiter.

Photo of Completed Reverb PCB

The above photo shows a fully assembled PCB, and as shown (and tested) no heatsink is needed.  I used 10 Ohm resistors for R7 and R8 (L+R) as a matter of expedience, and this configuration has been tested thoroughly.  Feel free to do the same if you wish.  The amp is built exactly as described below, configured for an 8Ω drive coil.  The changes for reverb driver and recovery amp are greatly simplified compared to the earlier P113 boards.  I used PCB headers for inputs and DC, and included pins for output and CFB (current feedback).

A single board can be used as a complete reverb sub-system, with the Left channel used as a reverb driver, and the Right channel used for the recovery amplifier.  The circuit provides more than enough current to drive all low impedance reverb tanks.  As designed, the board is suitable only for dual supplies (+/-15V), and single supply operation is not recommended.  The circuitry used was the basis for the Project 203 Guitar/ Studio Spring Reverb, but in the form shown here it has a little less recovery gain and no limiter.  The gain can be increased with another stage, and limiting can be applied if desired.

The output impedance of the reverb driver circuit is nominally around 330Ω (8Ω drive coil), but is higher for other coil impedances (see Table 1).  You will normally use the circuit directly coupled to your reverb tank.  High impedance tank drive coils need a small transformer to boost the output level.  Results will be different with different tanks, and you may need to experiment a little.  With an input voltage of around 1V RMS, expect an average output level of about 100mV RMS, but be aware that this is highly variable in practice.

Figure 1 - Complete Reverb Amp (8Ω Drive Coil)

This is the schematic of the reverb amp.  The right channel is wired completely differently, as most of the parts are omitted and the second half of U1 is used as the recovery amplifier.  The gain of the right channel is about 40 (32dB) which is sufficient to get a usable reverb level to inject into the external circuitry (guitar amplifier, mixer, etc.).  Also shown are the bypass capacitors - namely 2 x 33µF electrolytics and 3 x 100nF multilayer ceramics.  R4L is selected based on the tank impedance as shown in Table 1.  A value of 22k is used for all tank impedances except 8Ω types.

The value of R3L depends on the impedance of the reverb tank.  In theory, R4L should also be changed, but in reality it makes little difference except for 8Ω tanks.  The recommended values for these two resistors are shown in the following table.  The maximum recommended tank drive coil impedance is 250Ω, and coils with a higher impedance will be unable to get enough drive level to prevent distortion.  A small transformer (as described in Care and Feeding of Spring Reverb Tanks) can be used with high impedance coils.  The circuit should be configured for an 8Ω tank if a transformer is used.

Note that a heatsink may be needed for the driver amplifier, although it's unlikely.  If the transistors run hot (so you can't hold them indefinitely) then they are too hot and a heatsink is necessary.  This is unlikely, but quiescent current depends on the forward voltage of the two diodes, and it can vary.  Using 10Ω emitter resistors limits the worst-case quiescent current to no more than 10mA at 25°C.  The 'mA/ V' column shows the approximate drive coil RMS current with an input of 1V RMS.  It's approximate because some of the current flows through R4L.

Connect the reverb drive coil 'hot' terminal to the output, and the 'cold' terminal to the CFB input.  The suggested values for R3L and R4L are shown above, with alternatives in Table 2 in the article Care and Feeding of Spring Reverb Tanks.  The resistor designations are different in the article, but you'll see which ones are affected easily as the drawings are similar.  The values suggested here are 'rationalised', and experiments have shown that the values are quite acceptable.

For example, one tank I have uses a 166 ohm coil, so I used a 100 ohm resistor for R3L, and I used 33Ω with my 8Ω tank.  Be aware that the circuit will have massive gain with the tank disconnected - with a 100 ohm resistor, open loop (no load) gain is 220 times.  This is reduced when the tank is connected, and varies with frequency.  By connecting the tank's input coil between the output and current feedback terminals, the amplifier operates in constant current mode.  For a given input voltage, the amp delivers a relatively constant current, with output voltage determined almost completely by the tank coil's impedance and applied frequency.

Note that the tank's drive coil must be isolated from the chassis to be able to connect it this way.  There are many reverb tanks configured for just this purpose.  If you already have one that is not isolated, it must be modified or it will not work.  I suggest that you use 10Ω resistors (R7L and R8L) rather than anything lower, as this minimises dissipation in the transistors.  Although the rated input current for an 8Ω tank is 28mA, in reality it can handle much more.  In my tests I drove it at up to ±60mA without any sign of overload.  With a 33Ω resistor for R3L, the reverb coil current is 30mA/ volt.

You can reduce C2L and C2R to 10µF, and C1L, C1R can also be reduced to minimise extreme low frequencies (you will need to experiment with your reverb tank).  Conversely, the value of C2L is shown as either 100µF or 33µF.  The higher value is warranted for studio use with an 8Ω tank, as it provides response down to about 50Hz (C1L and C1R would need to be increased too, as they provide a cutoff frequency of 72Hz).  For all higher impedance tank drive coils, the 33µF cap is more than enough.  Low frequency response should be tailored using C1L, which ensures that the voltage across the polarised electro remains low at any frequency.

The circuit shown above has been tested using the 8Ω reverb tank I have, and it sounds really good, but you may wish to make some modifications for the tank you use, and to get the sound you want.  R3L and R4L must be selected to suit the tank's drive coil impedance, and you also need to select C1L to get the required bass response.  Feel free to use 100µF for C2L with any coil impedance.

Note that this circuit is not suitable for driving high impedance tanks (nominally 1,475Ω).  It is best suited to 8 ohm or 150 ohm tanks, but will usually be fine with tanks with drive coils up to 250 ohms.

After assembly, the board must be tested to ensure that there are no wiring errors.  If you have a suitable bench power supply this can be used, but most builders will not have access to such equipment.  If you do not have a bench power supply, use the following method.

Connect the amp to a ±15V power supply using 100 ohm resistors in series with each supply lead.  Make sure you connect the ground!

Apply power, and measure the supply voltage at the supply pins of the PCB.  It should be not less than ±10V, and should be symmetrical.  Check the voltage at each output pin of the opamp (pins 1 and 7).  The voltage should be less than 10mV.

Note that the current drawn by the NE5532 is quite high, so a lower voltage than indicated may be seen.  Do not panic!  Unless it is zero volts, the chances are that all is well as long as both supplies read the same.

If the above conditions are not met, there is something wrong with the wiring.  Check that all electrolytic caps, transistors, diodes and the IC are installed the right way around, and look for solder bridges between tracks.

When the circuit is working to your satisfaction, remove the safety resistor(s) and connect the supply permanently.

Although it might seem like this article is a re-hash of Project 34, it's included to highlight the ease of converting the latest P113 board to reverb driver and recovery.  The earlier P113 boards were more difficult to re-configure, which I suspect turned potential constructors away.  The latest revision of P113 (Rev-A) almost looks like it was designed for the job, but as it turns out that was more by accident than design (I could claim differently, but that would be a fib. )

Either way, it is a very neat arrangement which is completely painless to put together.  That it also works extremely well is to be expected, since it's based on well established principles and tried and proven circuitry.  Full construction details (including a complete BoM) have been added to the secure section, available to PCB purchasers.

Main Index

Projects Index