Universal Preamp / Mixer For Reverb Amplifier (Project 211)

This is a special adaptation of the P94 board, specifically for use with the Project 211 version of the P113 headphone amp for use with spring reverb tanks.  The only thing missing is the compressor/ limiter, which is (for most applications) not often used.  Because the P94 board is so flexible, it's an easy matter to adapt it for this role, with no PCB alterations and a few component changes.

While the following drawings show the Left channel for reverb and the Right channel for the 'dry' (original) signal, this is up to the constructor.  It can be reversed (Left channel for dry), and this makes no difference.  I also omitted the tone controls from the dry channel, as it's unlikely that the reverb sub-system will be used for EQ.

The circuit is very simple, and the PCB is nice and small (approx 50 x 75 mm).  The idea is that one PCB would be wired with all components (Figure 1 and Figure 4), with the P113 board wired as shown in Figure 3.  You can select the inputs you need, and add the appropriate input circuits, such as phono preamps, mic preamps, etc.  Indeed, the range of uses is determined more by imagination than any 'limitations' in the circuitry itself.

Note that none of the pots are mounted on the PCB.  Everyone (including me) hates running wires, but using PCB mount pots would seriously reduce the flexibility of the board.  All pots will be single-gang, as the reverb system is mono.  All pots are all linear, and 10k is suggested for all of them.  If this means that the input impedance is too low, use a 100k pot for VR201, and a 100k resistor for R202.

The first stage (U1) is a buffer, but provides a gain of 2 (6dB) as shown.  The gain is easily changed by changing the value of R104 (and R204 in the 'B' Channel) - a higher value gives less gain, and vice versa.  I don't recommend that the gain be increased beyond about 4 times (12dB), or DC offset may become a problem with some opamps.  A value of 3k3 (3.3k) for R104/204 will give a stage gain of 4.03 (12.1dB) which should be more than enough.  The idea is that for serious use, the input pot (VR201) and the output pot (VR104) will be centred, giving unity gain.  Normally, only small adjustments will be required in practice.

Figure 1 - Input and Tone Controls

The second stage is a standard Baxandall feedback tone control, and will give an almost dead flat frequency response with the controls in the centre position.  The response has been tailored for use with reverb, but you can change it easily by varying the capacitor values.  The system is mono, so single pots are used in each location.  The tone control response curves are shown in Figure 2.  The small markings on the pots (e.g. B1A, B2A and B3A, shown in blue) are references to the PCB connections.  If you don't need tone control, then U2A will be wired as a buffer, with a link from U1A pin 1 to U2A pin 2.  All parts for the tone control section can be omitted (R106, 107, 108, 109, 110 and C103, 104).

Figure 2 - Tone Control Response

Figure 2 shows the frequency response with the controls at 25% intervals.  The centre frequency is deliberately set lower than 'normal', because it's a reverb system that has limited response at the frequency extremes.  Bass response may be changed by using a different value for C103 (higher value, lower frequency), and likewise C104 controls the high frequency point (lower value, higher frequency).  I expect that most users will find the values to their liking as shown, but it can be changed quite easily.  Note that the tone control section of the Right ('B') channel is not used, so pins 6 and 7 of U2 should be joined, with a link between R208 and R210.

Figure 3 - Reverb Unit (P211)

The circuit for the reverb drive and recovery amps has been included for reference.  Connection points are labelled so you can see where each point goes to/ comes from.  The drive level will normally be set for 50% (assuming a linear pot), as that gives unity gain for a 'line level' system.  If the reverb driver gets too much level, the peak current may saturate the drive magnetic circuit, leading to distortion.  The drive level should normally be no more than 2V RMS, which equates to 60mA drive current (roughly double the 28mA recommended drive for an 8Ω coil).

Figure 4 - Mixer Amplifier

The mixer is the common 'virtual earth' mixing amplifier, and there is nothing special about it.  As shown, the mixer has unity gain, but again this can easily be changed.  Making R115 22k sets the gain at -2.2 (i.e. inverted).  Because the main ('dry') signal is inverted by the mixer stage, the second mixer should be configured as an inverter to ensure correct polarity of the dry signal.  The polarity of the reverb signal is not important, as it's random by nature.  The Dry 'on/ off' switch is optional.  If the reverb is used in conjunction with a stage or recording mixer, the reverb signal would normally be on a separate input, and no Dry signal is required (and it's undesirable when reverb is used with a mixer).

Output impedance is 100Ω with the inverter stage is included, and about 2.5kΩ if it's left out.  If you need to have a balanced output, use the P87B balanced line driver.  Likewise, P87A can be used if you need a balanced input.  The output level pot is optional, and it may not be necessary for your application.

All potentiometers are preferably linear taper, although log pots can be used if preferred (note that tone control pots must be linear).  The resistor values are selected to give a (roughly) linear response, so that with the signal input pot centred, the overall gain is unity.  I suggest 10k for all pots, and resistor values are based on this.  Different values can be used, but you will need to re-calculate tone control resistors and capacitors.

Photo of Completed PCB (No Wiring Shown)

The photo of the PCB shows a fully populated P94 board.  This isn't what you'll end up with when it's wired as shown here, but you can see the connection points clearly.  Unused mixing resistors will be omitted.  Even a fully populated P94 (using the values shown in the original P94 project) will still work fine, but there's no good reason to install parts that will never be used.

If the ESP boards are used, construction is very easy.  They are small, but laid out very logically so it is easy to construct.  No pots are mounted on the PCB - not because I like running wires (and I don't expect you do either) but because this gives you far greater flexibility for your version of the project.  If I designed the board with the pots, then you would have to use the same type as I designed for, and the same spacings and layout.  This is very restricting - especially if you can't get (or don't want to use) the same type of pot.

The power supply should be ±15V using the P05 or P05-Mini power supply.  Any dual supply may be used, so if you have one already, it may be used as long as the voltage is at least ±12V (preferably ±15V).  Higher or lower voltages are not recommended, although some opamps will work reliably with lower voltages.  Feel free to experiment as required.

I have shown the circuit with TL072 opamps, but you may use anything you like (they must be an industry standard dual through-hole opamp though).  Suitable devices include NE5532, RC4558, NJM2068, LM4562, or OPA2134.  Feel free to use your favourite opamp if you have one, but I specifically recommend that you do not use LM833 types, as they have a tendency to be unstable in many circuits.

The standard pinout for a dual opamp is shown on the left.  If the opamps are installed backwards, they will almost certainly fail, so be careful. The suggested TL072 opamps will be quite satisfactory for most work, but if you prefer to use ultra low noise or wide bandwidth devices, that choice is yours.  The 4558 dual opamp is a staple for a great many guitar amps and pedals, and should work well.

Remember that the supply earth (ground) must be connected!  When powering up for the first time, use 100 ohm or 560 ohm 'safety' resistors in series with each supply to limit the current - this will prevent (most) damage if you have made a mistake in the wiring.  Remember that if you purchase the PCB, full construction details are provided, and there's some additional information available as well.

Part 1 - Project 94

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