Retroflect Build Guide
Bill of Materials
Resistors
R1 = 1m
R2 = 20k
R3, R6 = 47k
R4, R5 = 33k
R7 = 100k
R8, R13 = 1k
R9 = 4.7k
R10 = 470k
R11 = 100r
R12 = 47r
R14 = 10r
CLR (on footswitch PCB) = current limiting resistor, sets brightness of LED. 4.7k is a common choice. Ignore c1 on the footswitch PCB!
Caps
C1 = 68n
C2 = 470n
C3 = 1n
C4 = 470p
C5 = 10n
C6, C7 = 47n
C8, C9, C15 = 100n
C10 = 100u
C11, C14 = 47u
C12 = 2.2u
C13 = 1u
Misc
D1, D2, D4 = 1n34a (the diodes icluded with the kit)
D3 = 1n4001
REG = 78L05
NOISE switch = ON-OFF-ON
Pots
WOW = B5k
SAT, VOL = A100k
MIX = B25k
SENS" is a 50k trimpot (or 30k for more accuracy!) that sets the max range for the "WOW" pot, which in turn controls the modulation depth. Every JFET is different, and the trimpot allows you to tune the circuit for the JFET you have.
The PCB is designed for this kind of trimpot - (Link)
To set the trimpot, turn the WOW pot to max, then play your instrument as loud as you ever will while adjusting the trimpot so that the modulation never goes too far, you'll see what I mean by "too far"! Roughly 9 o'clock to 12 o'clock is the usable trimpot range.
Wiring Diagram
I recommend using this type of DC socket - https://www.taydaelectronics.com/dc-power-jack-2-1mm-enclosed-frame-with-switch-external.html - to save room in the enclosure.
Schematic
This image of the schematic is a lot clearer! - https://imgur.com/a/YAo9KLv
Drilling
This drilling template was kindly provided by Jose Pedro Matos: PDF link It's been tested and should work fine printed on A4. Lumberg jacks will fit with this template, otherwise measure your own holes for the input and output jacks on the sides, moving them a few millimetres towards the footswitch.
The layout is symmetrical across the orange line, but slightly assymetrical across the green line:
A couple of notes (not mandatory reading!)...
1. The diodes included in the kit aren't 1n34a's but they're the diodes I used to design the circuit and the ones I use in every Retroflect I make. They were sold to me as 1n34a's by a reasonably reputable supplier so not sure what's happening there. I guess they're BAT__ something. The forward voltage is very similar to 1n34a's.
2. As of September 2022 the reverse voltage protection diode (D3) on the PCB is now in series with the power line, so between the top "+" pad and VCC for the circuit. It's still a 1n4001 but it faces the other direction.
Circuit explanation
The JFET is a voltage controlled resistor that lets +5v flow to the VCO. It's controlled by the guitar input via an envelope follower (1n34a, 470k & 1uf). The trimpot & 4.7k are a kind of limiting circuit to set the max modulation.
The clipping stage is supposed to simulate magnetic tape saturation by clipping the very highest frequencies first, hence the 1n cap. The saturation pot turns the gain up, at the same time allowing more low frequencies to get clipped through the 100k.
The noise switch syphons white noise from the chip's voltage bias. I'm still not exactly sure how it works, but the PT2399 is a notoriously noisy IC. I believe this is digital noise that has coupled to the VREF and the capacitor is "decoupling" it straight to the signal path, where it is then amplified. Lowering the value of either the 100r or the 47r will increase the volume of the noise (one resistor controls the volume for each side of the noise switch).
The audio signal is high-pass filtered before the delay chip, and low-pass filtered afterwards, in an effort to copy the "emphasis... de-emphasis" technique of old analogue equipment, where the high frequencies are boosted to achieve a better signal to noise ratio.
A few pics of people's builds!
