JH String Filter Mods

1. Level Control and Patch Point For All 40 Channels

This modification turns the JH String Filter into a 40 channel filter bank. The changes are quite simple for each channel. But due to there being 40 channels in total, the amount of extra wiring is quite substantial. Before beginning this modification, make sure you have lots of coax cable on hand and prepare to spend a lot of time planning how to route all the wires so you don't end up with a rat's nest. I probably spent just as many hours planning how to construct this module as actually building it.

There are several choices for the 40 patch points. You can use standard TS phone jacks if you simply want an output for each of the 40 bandpass filters. If you use Switchcraft 114B jacks (or equivalent) you can create patch points for each of the 40 channels. This allows each channel to be an output-only, an input-only, or a SEND-RECEIVE patch point as found on many mixing boards. The latter requires INSERT cables that have a TRS plug on one end and two standard TS plugs for the SEND and RECEIVE signals (HOSA make these or you can make your own). However, if you wire the 114B jacks as described below you can still use standard TS phone plugs to get an output-only signal by inserting the plug halfway into the 114B jack so that the tip of the plug makes contact with the RING connection (not the TIP connection).

Circuit Description

Below is a sample of the original summing network (paraphrased from JH's schematics). There are four op-amps (U7A, U8A, U10A, U11A) that each create one of the four partials, only one of these is shown below (U11A). Each of these op-amps adds together the outputs from ten bandpass filters, only one of these is shown below (U140B).

Below is a sample of the modified summing network. Resistor Ry (470 ohm) protects the op-amp from being accidentally shorted to ground when inserting cables into the 114B jack. The bandpass filter output goes to the RING and the TIP SWITCH on the 114B jack. With nothing plugged into the jack, the bandpass filter output is routed to the top of the 50K pot. The wiper of the pot sends a proportion of the bandpass filter output to the partial summing amp U11A via RMx40.

Note that RMx40 and R58 were both changed from 10K to 27K. The reason for this is that, with a 50K linear pot, the original 10K resistors created a very lopsided loudness contour when the pot was rotated. I changed these to 27K because those values provided a fairly log-like response to my ears. You may choose to use a different value for RMx40 and R58, that's fine, just make sure they are both the same value. Also, if you use a different pot (e.g. 100K) you will likely want to use different values for RMx40 and R58. The value for Ry (470 ohm) is not critical, you can select anything from 100 ohm to 1K.

Keep in mind that the circuit above is only a SAMPLE of the modified summing network. There are four partial summing amps, so if you change R58 you also need to change R43, R45, and R56 to the same value. Plus there are 40 bandpass filters in total, so if you decide to change RMx40 you will need to change all 40 resistors that begin with "RMx" on the schematic and add 40 Ry protection resistors (one for each bandpass filter output).

Solder the RMx and Ry resistors to the circuit board standing up next to each other (see the construction photos for an example). Run coax cable to the jacks and pots to minimize crosstalk between filter channels. Connect one end of the coax shield to a nearby ground connection on each filter.

Construction Photos


Mod Detail	Board #2 Installed	Board #2 Installed

Board #1 Installed	Board #1 Installed	Finished

Panel Layout

Detailed panel cutout diagram (pdf) with notes for those who want to build their own panel.

Sample Sounds

Pink Noise Experiments

The following three selections all use pink noise as the input to the string filter.

Pink 1 - Starting from silence, each of the 40 channels are brought up from minimum to maximum, one at a time. With resonance set to maximum, the filter really sings (in stereo).
Pink 2 - Continuing from Pink1, the RES control is rotated from maximum to minimum and back, and the SPREAD control is used to collapse the stereo field to mono and then back to stereo.
Pink 3 - Continuing from Pink2, the PARTIAL switches are flipped to audition each of the 15 possible combinations that produce sound (in stereo).

Vocal Formant Experiments

The following three selections all use a sawtooth wave with a bit of frequency modulation as input to the string filter. On its own it sounds like this. Five of the level controls are set to produce a single vocal formant. The other 35 controls are set to zero. The individual outputs are used to create two additional vocal formants (using external mixers to set the levels). Five frequencies are used for each of these two formants as well. Some switching logic is used to sequence through the three formants.

Solo Voice - A Doomsday Machine is used to thicken up the single sawtooth wave while manually playing notes from a MIDI keyboard.
Choir Voice - Similar to the Solo Voice, but using three sawtooth oscillators to create chords, played manually from a MIDI keyboard. No Doomsday Machine was used on this one.
Bad Party - We've all been to one of these. This one was multitracked.