Synsonics Pro Dual Kit Drum schematic

    I'm always looking for drum synths to document, and I recently found a "Synsonics Pro Dual Kit" from Japan. Not to be confused with any other drum by a toy manufacturer, or guitar for that matter.

    It consists of two identical drum voices, so I've drawn one of them. Sadly, there's nothing too novel here. We have a decay-only envelope, a triangle VCO, and an OTA-based VCA.

Synsonics Pro Dual schematic

    The VCO design pops up a lot when you look for simple, DIY designs. Despite this, I realized that I had never taken the time to learn how it works. As always, it has an integrator and a schmitt trigger; How different could it be?

Basic Oscillator

    Let's start by looking at a non-voltage controlled version of the circuit. Rt sets the frequency of the oscillator by limiting the current flowing between the schmitt's output and the integrator's input.

Voltage Control

    We could maybe put a voltage controlled current sink in place of Rt to achieve voltage control. The trouble is that we don't only need to sink current; Half the time we need to source current instead. 

    Here's one solution to that, courtesy of the LM13700 datasheet. The righthand OTA is again just a schmitt trigger. The lefthand OTA is used as a voltage controlled current sink/source. You can specify the direction (sink/source) via its non-inverting input. The addition of the capacitor turns it into an integrator of sorts.

    Here's a closer look at this funny integrator. It has two controls: one for rate (amount of current), and one for the direction (inversion).

OTA integrator

    Really, the "rate" control is a normal integrator input, except it can't go negative. The "direction" input is what specifies that the input is negative (or positive).

The Synsonics Version

    We can see that the Synsonics version doesn't use an OTA, but it still has an integrator that can be inverted. How do they pull it off? The integrator works very much like a summing amp, and they're making it do arithmetic.

    The transistor is acting as a switch that can connect R17 to ground (0V). R15 and R16 form a voltage divider that puts half of our "Rate" voltage at the non-inverting input of the op-amp. We can substitute those in the simulator to simplify things. 

simplified circuit

The Math

    The voltage at the non-inverting input (non-inv) is subtracted from each of the inputs. Each difference drops across its respective resistor, and the resulting currents are summed.

We can represent a single input's current like this: (input - non-inv)/resistor

    In our circuit, the non-inv input will always be half the rate voltage, so we can put that in the formula. Now we can calculate the current for the regular rate input: (rate - rate/2)/100kΩ = (rate/2)/100kΩ

If we plug in 10V, we get this: (10V/2)/100kΩ = 5V/100kΩ = 50uA

Rate current

    Here's the second input when it's connected to ground: (0V - rate/2)/50kΩ = (-rate/2)/50kΩ

We plug in our 10V again to get: (-10V/2)/50kΩ = -5V/50kΩ = -100uA

0V current

    So, the second input's current is twice the magnitude of the first, but negative. Remember these currents get summed together, giving us this: current - 2*current = -current

Inverted current

That's how the integrator is inverted, by subtracting twice the normal current. The rest of the oscillator is standard, so we'll leave off with a Falstad simulator link.

Falstad simulation

Ibanez EM5 and DL5 schematic comparison

     After redrawing the Ibanez DL5 schematic, I moved on to the EM5 from the same line. The EM5 has become sought after, and commands hundreds of dollars. The DL5 goes for tens.

    There is an official schematic available for the EM5, but I thought it would be interesting to rearrange it to match my DL5 layout. This highlights differences and similarities. So, here they both are for a-b-ing.

Redrawn EM5 schematic

DL5 schematic

Delay Line

    The biggest visible difference is the delay chip, M50195 vs M65831. The DL5's M50195 uses external RAM and an external comparator, but it functions the same way. Both chips are built around the same adaptive delta modulation converters and 1-bit delay line. The external RAM is larger, at 64kb vs 48kb.

    The input filters got moved around a little, but they're identical 5.8kHz lowpasses. The integrator caps in the modulator/demodulator are also the same. The M65831 datasheet specifies that it needs 30Ω series resistors for some reason.

Clock

    The clock is exactly the same, and validates what we found when investigating that part of the DL5.

    For some reason, the extra gates from the inverter weren't used for the bypass logic this time. Instead, they added transistors set up as inverters. These pedals are a bit unreliable when it comes to switching on/off, so maybe this was an improvement to the circuit.

Filters

    The input stage, preemphasis, and deemphasis stages are all basically the same. The frequencies that are boosted and attenuated haven't changed.

    We already compared in the delay line's input filters, and found that they're the same. We also have output filters though, and they're different. The DL5 cuts at 5.8kHz, matching its input filter. The EM5 cuts lower at 3.9kHz. Immediately after that, there's an additional lowpass formed by R16 and C19. Together these give the EM5 a darker sound, and probably help filter the additional noise that comes from using less memory for the same length of delay.

    The EM5's output filter also has soft-clipping diodes, presumably to prevent runaway echos that get louder and louder. There's more gain in the feedback loop (lower R17 value) to compensate for this lower output. Strangely, R20 has been more than doubled, limiting the maximum volume of the repeats.

In Closing

    The two pedals don't have any major differences that explain the 10x price gap, at least not to my satisfaction. I suspect people happen to like the darker sound, and maybe the clipping of the EM5. It should be trivial to make the DL5 sound the same though. Since it's built on the same tech, I believe a humble PT2399 delay could also sound the same.