AM8007 SH05 Envelopes


AM8007 Envelope Module

Overview The AM8007 module is a replica of the ADSR and AR envelopes used in the Roland SH-5 analog mono synthesizer from 1976. It use transistors to switch between the various phases of the envelope generator and a JFET transistor to act as an output buffer. The ADSR has a uni-junction transistor as its core, there are no Op Amps in the original design.

Euro Rack Module:   14HP wide
User Manual:          Download PDF

Detailed Description The AM8007 is an exact replica of the original circuit but with the addition of output Op Amps to buffer the signals and provide both positive and negative going envelopes.

The envelope section timing is all the same using 1 MA slide potentiometers, instead of the Release being twice as large as Attack and Decay. This is simply due the difficulty of getting 500K and 2M log potentiometers. The original Attack and Decay times are stated in the user manual as 0 to 7 seconds, with the Release at a maximum of 14 seconds.

Tests of the module showed the AR envelope using the original 3u3F capacitor was giving rather long maximum envelope times of 16 and 12 seconds, due to the smaller 1M log pots. So this has been reduced down to 2u2F in production modules. There is also a noticeable 8 ms delay to the envelope start caused by a 470 ohm loading resistor, this is reduced to 100 ohm to make the Attack a bit snappier.

AR Envelope

The ADSR has a 6 second maximum Attack time, and a long 12 second Release, this is close to the Roland original so no changes are made. The minimum Attack time in around 2 ms. The sustain level has been corrected for 12V operation, so that the maximum attack and sustain levels are the same.

ADSR Envelope

The envelopes are clearly exponential which blows away the myth that the SH-5 has linear envelopes, however the AR is a bit slow, and the reputation of not being “snappy” like the 100M envelope generator is correct. The same ADSR circuit appears in the Roland System 101 and 102 with a sub second Attack time of 0.8 ms, but the maximum time reduces accordingly down to just 3 seconds.

The gate signal of each envelope generator can be switched from the Doepfer Bus (Internal) to Panel Jack (External ), the push button is always active for initiating either generator. The presence of a gate signal is shown by the red panel LED. The sliders are Bourns PTL30 with red LED’s which are supplied by a separate +5V power supply and are always on.

Both envelope generators go through an Op Amp buffer to provide the negative going envelopes and some gain make up on the ADSR due to the lower 12V power rails. There is a single trimmer for setting the ADSR output at 0V at rest.

AM8007 Module

Euro Rack Module:   14HP wide
User Manual:              Download PDF

There are five 3.5 mm jack sockets on the base of the front panel:

  • External GATE input (Euro Rack standard of +5V)
  • AR envelope output, normal and inverted (AR-, AR+)
  • ADSR envelope output, normal and inverted (ADSR-, ADSR+)

Module Outcome & Availability The PCB’s were laid out in late 2017 and ordered as prototypes in March 2018. This worked correctly and a LM393 comparator chip was added to convert the lower Euro Rack +5V gate signals into the required +10V. The REV02 PCB was working on 02 December 2018 with a very pleasing result, the sustain level correction was made on 22 December. Transistor gain(hfe) is critical to the ADSR circuit working, with the low gain 2SC946-Q transistors required, along with one 2SC373. An easy to locate 2N6028 UJT replaces the N13-T1.

Production Status The AM8007 will be available in January 2019 as a completed module, PCB set and Hard to Find Kit.

 

Copyright AMSynths 2017