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EH 7550 Deluxe Memory Man
| PCB | EH 7550 |
|---|---|
| Panel Text | DELUXE MEMORY MAN SOLID DATE ECHO/ANALOG DELAY LINE |
| Panel Colors | Red and Blue |
| Knobs | 4 |
| Switch | Squelch |
| BBD IC | SAD1024 |
| Compander | Yes |
Note it is called “Memory Man Deluxe” on the schematics as opposed to “Deluxe Memory Man” for EH 7850.
As far as I know, this is the correct schematic for EH 1307 Memory Man boards.
Demystifying the Squelch
The squelch switch connects pin 16 of the compander to ground through 470 kΩ. This side of the compander is the expander side which is the delay output. There is a visible cut in the delay output when engaged. I have read somewhere on the internet that this was an attempt to control runaway feedback, but I’ve never seen an official manual or EH explanation.
2025 update: a reader alerted me to a posting of the original manual. In the overview diagram, the control is labeled “NOISE GATE (SQUELCH)”. The instructions say “The effect of the NOISE GATE is subtle. When ON it minimizes noise when no signal is present/ When OFF it permits a maximum amount of undistorted low level echoes but does not affect noise.”
The earliest design features an OTA based noise gate implemented with a CA3094, so I suspect the compander revision kept “noise gate” as a feature specification.
Pin 16 allows access to the control current for the expander’s gain cell. Grounding this pin will mute the expander’s output while feeding it high voltage through a resistor should increase output. More info on this can be found in the NE570/571 application notes.
The EH 7811 circuit appeared to take this “squelch” idea and turn it into a “mute” function. For further study: NE570/571.
Calibration Guide
Information on calibrating the EH-7550 using the trim pots.
In this series: Electro-Harmonix Memory Man
- Electro-Harmonix Memory Man
- Electro-Harmonix BBD Echo Issue A (Memory Man version 1)
- EH 7500 Memory Man
- EH 1309A Memory Man
- EH 1309B Memory Man
- EH 7550 Deluxe Memory Man (You are here)
- EH 7550 Memory Man Calibration
- EH 1307 Deluxe Memory Man with Squelch
- EH 1307B Memory Man
- EH-7810 Memory Man
- EH 7811 Stereo Memory Man
- EH 7811B Stereo Memory Man with Chorus
- EH 7811C Stereo Memory Man
- EH 7850 Deluxe Memory Man with Chorus/Vibrato
- EH 1343B Deluxe Memory Man
- EH 7850 Calibration
- EH 7850C
- EC 2000 Deluxe Memory Man Reissue
- EC 2002 Deluxe Memory Man Reissue