Bill and Will's Synth
Jürgen Haible "Subtle Chorus" -
with "DIM D" & "DIM T" Effects

July 2009 -

In November of 2008, Jürgen Haible began taking orders for his "Subtle Chorus" PCBs.  This set of two boards allows for several configurations - two of which enlist the filter ladders of two Tau Pipe Phaser PCBs.

Jürgen, again, offered us tireless generosity in answering our questions and for letting us post some of the images he created here on our page.  Our interest in doing so is to document his work and provide an idiot's guide to building this yet another of his marvelous gadgets.  We hope we succeed.

Although this circuit can be built with common TL072s and TL071s, a truly vintage version can be built using somewhat rare HA1457W opamps. Rare, but as of July, 2009 they can still be gotten at Small Bear Electronics for around 2 bucks a piece - you need  18 of them ($36) - that's what we did.

We also found the MN3101 BBD IC and the related MN3007 Timer IC difficult to find.  Again - Small Bear Electronics to the rescue - but these babies are pricey!  You need two of each - about $40.

Table of Contents

This page has become really long, so here's a table of contents that we hope will make it easier to traverse:

Background - presents Jurgen's initial description of the effect

Recapitulation of Construction/Feature Options - presents a simple list of the different possible implementations

Option Details - presents the details of our implementation - you'll need to consider the full set of options as presented by Jurgen in deciding how you'll build yours

Parts - presents a Bill of Materials and notes about it

Panel - presents how we came up with our panels' design - ultimately Scott Deyo at Bridechamber fabricated the MOTM format one

Construction Phase 1 - Resistors, Capacitors, IC Sockets, Semiconductors, Power Plugs, MTA headers

Construction Phase 2 - Trimmers, Panel connections

Set up / Testing

Use notes


Jürgen's site describes the effect:

"The "Subtle Chorus" is a variable project consisting of 3 different PCBs that can be combined for various functions.

  • You can build a rather simple, true-stereo, dual-BBD Chorus with voltage controlled Chorus Rate and voltage controlled Chorus Depth, using a single PCB, similar to what a famous Japanese company starting with "R" built into some of their analogue poysynths.
  • You can emulate the famous "Dim D" sound with it's unique filtering and presets, using two PCBs, the "Dim D". You even have the choice between vintage HA1457W opamps, or more common TL071 / TL072 opamps. I'm talking about a 2U rackmount enclosure with just some buttons to select presets.
  • You can build a similar effect that uses 20-pole all pass filters instead of BBDs, using three PCBs, the "Dim T".
  • And you can build an overkill effect device that allows to choose between the Dim D and Dim T effect, using 4 PCBs, the "Dim TD" version.

Learn more...

Fundamental Options:

Subtle Chorus Main Board - can be used to control a BBD Board and /or two Tau Boards. The board can be built with the "vintage" HA1475Ws (like we're going to do) or with TL072s & TL071s:

click on the image for higher resolution pic

You can see the schematics for the Main Board by clicking these links:

Schematic 1 - LFO and Power Supply
Schematic 2 - Input Section
Schematic 3 - Output Section HA1457W Version

Subtle Chorus BBD Board - also can be built with the "vintage" HA1475Ws (like we're going to do) or with TL072s & TL071s:

You can see the schematics for the BBD Board by clicking these links:

Schematic 1 - BBD
Schematic 2 - Additional LFO

Tau Pipe Phase Delay PCBs:

Option Details

1. Power Supply

The Main Board serves as the power source for the other boards - if +/-15V MOTM power is used, then the AC power supply components can be left off the PCB.  This diagram illustrates the parts that should be left off for the vintage version (in red):

2. Four Board Module Version - Connections

Now - you can build this thing with just the BBD Board.  This produces the "simple, true-stereo, dual-BBD Chorus with voltage controlled Chorus Rate and voltage controlled Chorus Depth." And You can build it with the BBD Board and the Main Board - to "emulate the famous 'Dim D' sound with it's unique filtering and presets, using two PCBs, the 'Dim D'." Etc, etc, etc.

But we're building the completely nutso version with the two Tau Pipe boards. Here's how Jürgen shows their connections:

3. Input Connections

XLR vs 1/4" Jack

You can build the module with XLR balanced inputs and outputs. We suppose you could, therefore provide balanced TSR jacks as well. But we're going to build it with regular old unbalanced jacks.

Jack ground-side switch

This option requires a switch in the ground (shank) side of the jacks. That switch can be accomplished with a 114BX type jack.

Stereo / Mono Switch

This connection diagram also shows the SPDT Stereo / Mono Switch:

here's how the Input 114BX jacks get wired up to accomplish the connections as Jürgen specifies

4. Controls

BBD Board LFO Panel Controls

Three Controls - completely optional; but we've got space on the panel

FREQ - 100K lin - shown as "Offset" on the BBD PCB schematic diagram.
RES - 100K lin - shown as "Resonance" on the BBD PCB schematic diagram.
DEPTH - 25K lin - shown as "Depth" on the BBD PCB schematic diagram.

BBD Board LFO CV Controls

These Trim the CV inputs - again, completely optional; but there's space

RES CV - 100K lin - in series with the "Resonance CV" input shown on the BBD PCB schematic diagram.
DEPTH CV - 25K lin - in series with the "Depth CV" input shown shown as "Depth CV" on the BBD PCB schematic diagram.


These Trim the CV inputs - again, completely optional; but there's space

INPUT Mono/Stereo - DPDT - switches between Mono vs Stereo inputs as shown on hookup diagram.
PHASE - SPDT - switches the chorus effect of the Right side in and out of phase with the left.
D/T - SPDT - switches between the BBD (Dim D) and the Tau Pipe (Dim T) effects.
Effect Bypass - SPDT - switches the effect off and on.


Will and I have developed a parts-list / bill-of-materials in the form of an XL spreadsheet. Jürgen has been very patient and helpful answering our many pesky questions.

In the BOM, the left-most column is the "part."  These parts we have a high (but not perfect) level of confidence that we've specified correctly - we caught a mistake or two in part numbers / prices as we were ordering.  please double-check us and let us know of mistakes you find.

Click here to see Jürgen's Bill of Materials.

Corrections to BOM:

None yet -


None yet -

Click here to download the spreadsheet (apx. 350K).

Mounting Bracket

In order to arrange the four PCBs behind the panel we/ll need to create a supplemental bracket to attach to a Stooge 3-pot Bracket.

To access all the adjustment trimmers, we'll arrange the boards in two layers like this - with the Tau boards below the BBM and Main Boards - but extending beyond - so the two trimmers on the Tau boards are accessible.  They'll take up about 9-3/4 in. depth and we'll create supports for the two top pcbs.

Here's how we'll do it:




We got some 26 gauge sheet steel at Home Depot and began:

we measured 2" from the front of a Bridechamber 3-pot long bracket -

and marked it for reference


we fit the Main and BBD PCBs on the sheet steel, marking the location of the holes

then we positioned the Tau boards, used them as reference for their hole locations

next, the 3-pot bracket

we cut the steel - it's pretty easy

we used the punch to get ready for drilling


For our FPD panel design, click here.


Again, here are the PCBs:

Main PCB front and back
(click on images for larger one)

BBD PCB front and back

Tau PCB front and back (two are required)

Construction Phase 1

All the stuff in Phase 1 gets soldered using "Organic" Solder.  At every break in the action, we wash the board off to get rid of the flux.



IC Sockets

Construction Phase 2

All the stuff in Phase 2 gets soldered using "No-Clean" Solder and the PCB doesn't get washed off from here on.

SMT caps / TL071 fix jumpers

ICs and Trimmers

Mounting Bracket

Test Mount

PCB Wiring

Bracket mounting

Bypass Switch

Output Jacks

Input Pot

Rotary Switch


Construction Done

Set up / Testing

Use Notes


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We are self-proclaimed idiots and any use of this site and any materials presented herein should be taken with a grain of Kosher salt. If the info is useful - more's the better.  Bill and Will

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