Pearl DRX-1 Drumsynth

Pearl DRX-1 - front panel
Pearl DRX-1 – front panel

Pearl DRX-1 Overview

The Pearl DRX-1 has 5 Instruments which can be triggered by drumpads. Of course you can also feed some trigger-signals into it. All intruments have the same parameters and are therefore sounding identical, although the pitchranges are a little different. Beside Bassdrum there is the Snare and 3 Toms. Each has its own trigger-in and an individual out jack, 6.3mm. If you do not use the individual jacks, all sounds are summed together to a stereo-out. Very solid, professional and expensive hardware.

Pearl DRX-1 - back view
Pearl DRX-1 – back view

You have 8 kits to save your parametersets. the kit-selectionbuttons recall the kits immediately, you can switch them live in the song to change the sounds. You can hear that in the soundclip at the pagebottom.

To edit the sounds you select a pad (=sound), with two buttons (+/-) you select the parameter and with two other buttons you can increase/decrease the value. The range is from 1-19 for all parameter.

The Parameters

1. Pitch = the basic tune of the sound

2. Bend = the amount of pitchmodulation, the pitch is also velocity-dependant. heavy hit=more bend. Only positive (piuuuu).

3. Mix OSC/Noise = here you can blend from OSC-only to Noise-Only

4. Overtone = WOW, i love this parameter! It adds some ringmodulation sound to simulate the second drumhead of a tom.

5. Attack = Volume of Attack-Sound, a short enveloped Noise, not a Click. Makes the drumsound more defined and aggressive.

6. Filter = Lowpass for the Noise, also dampens the Attack-sound and even the Oscillator.

7. Decay = with high settings, the sound is very long, 808-like. With the shortest value, the decay is still quite long. It’s because this unit doesnt work with t-briges like the 808/cr78/er-0/dr55 a.s.o. machines but with oscillators that get modulated by enveloped VCAs like on the 909. It has 2×13900 (dual channel-OTAs) per channel.

8. Level = well ..

Pearl DRX-1 - inside
Pearl DRX-1 – inside

Here is the soundclip – first half BD,SD and 2 Toms, second half without the toms. The pattern comes from my modified fricke MFB-502 with trigger-outputs. Each two measures, i switched to the next kit so you will get a really good overview of what this thing is capable of.

Making the parameters Pot-tweakable

After selling my unit, i luckily re-grabbed a broken one on the bay quite cheap. I had to replace a PSU-voltageregulator for geting it to work again. I always wanted this thing to have pots instead of ++++/—- tapping-editing. After measuring a bit on the PCB i found the solderingpoints for tweaking the parameters with pots. The PCB is VERY clean and professional, Pearl did an excellent job on this drumsynth.

Pearl DRX-1 - modding cabeling
Pearl DRX-1 – modding cabeling

Each of the 8 parameters goes from 0-4.75 Volt on the 4 outputs of the 4046 OpAmps, there are 2 of these 4046 for each voice. They are amplifying the signals from the 4051 multiplexer. I tried to get one of these opamps desoldered – no chance. I found an easier way: I simply followed the OpAmps outputs and found 8 resistors which are very easily desoldered on ONE side to put cables on them. In the pic to the right, you find them in the first and third chip-row from above, i drawed red circled around them.

The housing

I decided to replace the metal top of the 19″-unit with a wood-U, well not really wood, it’s somewhat between wood and paper called MDF here in Germany. Its easy to drill and stable enough – and cheap, just 2 EUR. I glued it together, drilled the 40 holes, painted it with some paint i had left over – of course water-based and therefore environement-friendly. The pot-plate needed to be detachable from the unit for easier assembling, painting a.s.o. The unit itself got two 20-pin jacks on a holematrix-pcb.

Pearl DRX-1 - tweaking points for modding
Pearl DRX-1 – tweaking points for modding

The hotspots

click image to the right for a detailed view on the points where the cables need to be attached. I lifted of the resistors and soldered the cables directly to the open pin of the resistor. Just pitch is a bit different, to enlarge the pitchrange i replaced the lifted resistor (68k) completely with 10k before going to the pot. This enhances the range of the pot, if you keep 68k your pitch doenst go as high as i could. If you remove the 68k the range of your pot is to big and some turningway wont make the pitch any higher, 10k workes well.

The 8 cables of each channel then go to the middlepin of a standard resistor/slider. I grabbed about 200 from the bay quite cheap, they are 100k-Ohm, i took linear ones and it worked quite well.

All the resistors have mass on one pin and +5 Volt on the remaining pin. Thats it. If you like you can also tweak the parameters with external voltages. Although i would NOT exceed 8 Volt, thats the voltage this
unit is running on, Some modular systems work with even 15 volt, that might burn some chips.

Voltage Regulator for 5V mounted to case to improve heatsink
Voltage Regulator for 5V mounted to case to improve heatsink

The 7808

40 resistors in parallel cause an increase in currentflow. As a result the 7808-voltage regulator of the PSU gets quite hot. There is an unwritten rule which sais ‘as long as you can hold your finger on it, the regulator doesn’t get too hot’. Well, this was quite on the edge. I desoldered it and put it directly on the metal-housing, now it can get rid of its heat quite better.


Enlargement of pitch-range

The 68k-resistor at the pitch-tweakpoint can be replaced by 15k. This enlargend the range from lowpitch to highpitch.

Pearl DRX-1 - modding detail
Pearl DRX-1 – modding detail

Shortening of decay

The decay of the sounds is very long, even the shortest possible time is quite long, compared to the typical double-t-filter-sounds (cr8000 e.g.). But that can easily be changed: just put a 10k-15k-resistor in parallel to the 62k-resistor near the decay-tweakpoint and the decay gets very short. I added a switch to the each of the 3 toms to change between long and short decay. the decay pot works in both modes as before, just the timerange is changed by the switch.

Pearl DRX-1 - detail of tweaking point
Pearl DRX-1 – detail of tweaking point

Pushing pitchrange higher

The normal highest archivable pitch is even quite low. To simulate claves or congas i want to pitch to be much higher. Just put ~ 25k-resistor in parallel to the resistor above the 4016-ic at the bottom of the pcb (opposite to the printed labels ‘Snare’, ‘Bassdrum’ …), see pic to the right. Making this switchable lets choose you from normal pitchrange to a pitchrange much higher. As with the decay-pot, the pitchpot works perfect as before, you just have another pitchrange.

The resistor BELOW the 4016 is responsible for the overtone-oscillator. Change for your needs.

Pearl DRX-1 midi interface (right side) and 909 Hihats with Rim (left side)
Pearl DRX-1 midi interface (right side) and 909 Hihats with Rim (left side)

Last changes: Midi and some Hihat + Rim

After some time i got tired of hooking it up with an external midiinterface and made an internat midiinterface for it. It also offers flashing LEDs and manual triggerbuttons, easier to see which sound to edit. The interface has 8 channels, so 3 were left over. As i was on the fly i also made some hybrid hihats from the tr-909 schematics and a SM-Card. It has 6 pairs of open/closed hihats. Last was the tr-909 RIM-circuit which is quite easy to clone. Although everything works i am not fully satisfied with the sound of the hihats.

Finished Pearl DRX-1 with Mods, Midi and 909 Hihats with Rim
Finished Pearl DRX-1 with Mods, Midi and 909 Hihats with Rim

Here is a demovideo i uploaded to youTube: