PME40X Tone and Volume Stereo Control Module

A brand new analog module for the Korg PME40X pedalboard 40 years later the commercialization?

Yes Sir :)

In this instructables I will report my adventure in the development of the latest addition to the PME40X collection of modules: a stereo tone and volume control "box" based on the (now obsolete, but anyway available) TDA1524A integrated circuit.

How did it went? Well...

The prototype circuit give credits mainly to the Philips TDA1524A datasheet from 1987, but also takes inspiration from a very nice project from Tyler.

In particular, I adopted the idea of feeding the output of one TDA1524A channel to the input of the other channel to double up the tone control (boosted mono) and the introduction of an attenuated feedback loop.

As you probably already know, the PME40X pedalboard has two kind of modules: stereo and mono. Three out of four slots are mono, the latest (leftmost) is stereo. I didn't want to loose the possibility to use both options, so I designed the circuit so that one can select between the two operation modes by a simple switch movement: classic stereo or boosted mono.

The TDA1524A analog integrated circuit can be powered directly by the PME40X +15V DC without further regulators. This is very important considering that an additional on-board power regulator would occupy vital space (and dissipate power). Yes, we are close to the 16.5V upper voltage limit, but withing specs nonetheless.

The IC provides a fixed voltage (3.8V circa) and this is used by all the potentiometers to provide an adjustable DC voltage to the appropriate control pins. Current sensing is used to provide a linear response when this pin is grounded through a resistor, and a loudness contour when disconnected. I placed a ON-ON switch to activate/deactivate loudness.

The PME40X still has some noteworthy modules in its arsenal: the Octave-V and Dist Wah, just to cite the most famous. This is why I liked the idea of a not trivial feature like a feedback loop that would push the IC out of it's confort zone. My will to keep the stereo functionality made things more complex, but I solved (sort of... se later) by using only one of the two signal outs as loop signal.

In other words: when the mode switch is set to "Boost", right and left channel are in series (in this order) and the last channel output (left cannel out) is sent to the first channel input (right channel) of the integrated circuit. In stereo mode the two are in parallel and the left channel out is sent to both right channel in and left channel in for resonance.

A simple TL072 based mixer helps mixing the input from the pedalboard and the feedback singal to the two parallel channels inputs. Being powered by the single 15V rail, I had to virtual ground the non inverting input through two resistors (see attached schematic for a single rail, inverting, opamp mixer in it's simplest form). When using such a configuration it's good practice to place decoupling capacitors at the mixer inputs. This is because the input is biased by half the opamp supply voltage. In this module decoupling capacitors are all over the place, so to save some space I left them out.

About module dimensions: I already had developed a simple send/return module for the PME40X, so PCB dimensions, lock-hole position and connector pinout where already available and "tested" in my pedalboard.

When I finished assembling the module I had a lot of expectations. The integrated circuit itself is not to scream about (its was intended as a cheap tone control for consumer commodities like TV's and radios), but all the features I padded it with and the joy for having a new PME-40X module to share increased my own expectations.

Unfortunately you can't win them all.

First issue: the "loudness" control makes nothing (or it's so minimal I couldn't hear the difference). Not something attributable to my design, but the integrated circuit itself.

Second: resonance circuit doesn't work as expected. This is very likely an issue on my side and I will dig this point out (Eh...).

Third: feeding the first channel to the second boosts the tone controls as expected, but has a limited range before unpleasant distortions pop up.

All in all the IC used as per datasheet makes a nice, ordinary, tone control module.

By removing the feedback loop and the useless loudness control (but keeping the "boost" loop) I ended up with a revised, sharable, board design (see picture in the next step).

So, stereo and "boosted" are the only modes of operation still present. When the mode switch is set to "Boost", right and left channel are in series (in this order). Left channel input is disabled. In stereo mode, the two channels are in parallel. This mode of operations is the most conventional (and usable, in my opinion).

The revised PCB also has four holes for spacers. These could be used to rise the position of the module and make potentiometers closer to the other modules surface. Nothing special, but I will adopt these in all my future PME40X modules (if any).

Gerber files for the revised (untested!) PCB are available >>HERE<< (Github). You can see the board layout in the next step.

Here is the BOM for the revised board (pictured).

Components values are silkscreened on the board, so a reference sheet is not necessary ;)

Integrated Circuits

1x TDA1524A

Capacitors      

5x 100nF

2x 15nF

4x 56nF

1x 100uF (electrolitic)

2x 2.2uF (electrolitic)

2x 4.7uF (electrolitic)

Resistors

2x 10Kohm

2x 220 ohm

4x 47Kohm 9mm single turn potentiometer

Others

1x DPDT mini slide switch ON-ON (MS22D16)

8x pinheaders, angled

Korg released 15 modules for the PME 40X during the time the pedalboard was commercialized (16 if you count the blank “Filler" cartridge). Even if I could not include all the features I had planned, a new one adds to the list!

The printed circuit board pictured in this Instructable was sponsored by JLCPCB, a high-tech manufacturer specializing in the production of high-reliable and cost-effective PCBs.

Their customer service is very good and PCBs a great value for the money!

Their contribution to the realization of this module was essential sooo... many thanks! :)

By registering at JLCPCB site via THIS LINK (affiliated link) you will receive a series of coupons for your orders.

Registering costs nothing, so it could be a nice opportunity to give their service a try ;)