Friday, 1 August 2014

TTSH Synth - Two Thousand Six Hundred - 4020 ADSR (DIY)

The envelope generator build of the TTSH synthesizer. 
This is part 8 of my TTSH (Two thousand six hundred) build which is an ARP 2600 clone synth.
You can see my full build thread HERE

There are two awesome TTSH threads in Muffs.
2600 clone - Two Thousand Six Hundred (TTSH)
Two Thousand Six Hundred (TTSH) Project General Build Thread

And of course the official build thread is here

The ARP 2600 has two envelope generators: an AR & ADSR.

 The EG section of the TTSH faceplate.

Vintage 2600s use a 4020 submodule in the ADSR section.
The TTSH synth has this built into the main PCB.

These are pics of a vintage 4020 submodule from an old 2600

The large silver circular component is a LM301

The TTSH uses the modern version.

Trying to match up the TTSH BOM with the old schematics of the 4020.... they are pretty close.
(I've read there were some (intentional?) errors in the 4020 schematic... might be just a rumor though).

The TTSH BOM of Integrated circuits & semiconductors is:
       1N4148 x 8 (CR1-CR8)
       2N3904 x 1 ???
       2N3906 x 3 (Q1,Q5)
       2N4392 x 1  (Q3)
       2N5172 x 3 (Q2,Q6,Q7,Q8)
       2N5460 x 1 (Q4)
       LM301 x 1 (A1)

  • 120 x 1 (R20)
  • 1k x 1 (R12)
  • 1k5 x 1 (R24)
  • 4k7 x 1 (R15)
  • 9k1 x 1  ??
  • 10k x 2 (R7 & ?)
  • 12k x 1 (R23)
  • 15k x 1 (R5)
  • 22k x 1 (R2)
  • 33k x 1 (R8)
  • 39k x 1
  • 40k2 x 1 (R11)
  • 68k x 2 (R17 - 19)
  • 68k1 x 1 (R13)
  • 100k x 2 (R14 & ?)
  • 120k x 1 (R10)
  • 180k x 2 (R4,R22)
  • 270k x 1 (R1)
  • 470k x 1 (R18)
  • 1M x 2 (R6,R9)
  • 1M2 x 1 (R3)
In the ADSR section, at the input of the LM301 (A1), the resistor going to GND [R7] is 10K in the original schemo. The TTSH uses a 33k instead.

A work of art.

Maybe the envelopes on the 2600 could be improved ???
Without modifications, their maximum Attack, Decay and Release times are short (esp the attack ---> 1.5 seconds on mine). The Decay & Release were about 10 secs. This could be a good place to start future modifications.

Phil Cirocco of Discrete Synthesizer offers EG upgrades to 2600s.
He describes the ADSR in a stock 2600 as "quick".
His upgrade looks cool:

"ADSR: The ADSR is quick in a stock 2600. The 3 position switch we install gives you a x1(factory speed), a x2 and a x3 speed, providing much longer times previously unavailable on a 2600.
AR: Conversely, the AR generator in a 2600 is quite sluggish in certain situations. This mod now includes an upgrade of the AR circuit. In addition to the normal X1 mode, the high quality 3 position switch provides a new high speed mode for sharper attacks, as well as a X2 mode for longer attack and release times."
Anyway, below are pics of the TTSH (envelope section) PCBs before population with most of the components:

 The rear PCB.

The front PCB.

For now, I'll build the EG section to TTSH specs.

Resistors & diodes first as usual.

Caps next. The TTSH schematic uses a 30pF cap here in The AR section.
However the BOM specifies the 33pF cap from Mouser.
Hopefully it won't make too much difference.

The  2N5460 is a P-channel JFET.designed primarily for low level audio and general purpose applications with high impedance signal sources. In the TTSH it's used in the AR envelope section. 
It was also used in the early ARP2500 1004 VCO module.

 AR section. Two 1M sliders & the SPST Off(On) Pushbutton Switch.

The 2N4392
N-Channel JFET used in the ADSR section.It's identical to the JFET used on my original 4020 module.

The Decay slider in the ADSR is 100K linear. The rest are 1M Audio.

All components are in. Ready to test.

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