Fairchild 661tl Gate... convert to Compressor?

[efinque]

The optocompressor/gate I built is very similar but it substitutes the transistorised (discrete) circuitry with a microcontroller.. basically when the signal peaks at a given threshold it turns on a light (LED, incandescent light bulb etc) to which the gain reduction circuitry LDR responds by lowering the series resistance.

So you'd only find a way to inverse the optocoupler LED/light operation so it turns off instead (which increases the resistance during peaks resulting in gain reduction) but it means the optocoupler light is on most of the time (ie. you need to service it more often)

As to how you do it with TTL (transistor-transistor logic) is another thing.. basically a logic NOT gate, you could also try a (solid-state) relay to open the circuit I think but it isn't much of an envelope follower then, like someone said you could shunt current from it to ground too I guess.

Modern LDRs from 150kOhm to 5kOhm were about the widest I found which are probably ok for line level signals (~1,2Vrms)

 

[Noth]

Hi,

I know it's a quite old thread but it seems that no detailed response were given.
I bought a 661 gate and plan to modify it as follow:

• switch it to a 663 compressor
• since there are two LDR on the 661, make it a stereo version with a side chain input
• add input and output buffers
• add a Make Up Gain pot
• replace the light bulb with a LED in order to reduce the compressor reaction time

I rewrote the 663 schematic on kicad (not really usefull since the found found on this post was already quite detailed):


I tried to rewrote the 661 schematic, based on the photo provided by dustbro. Some component values were unreadable but I assumed these were actually the same as the 663:


So basically we can see that:

• The 661 and 663 are passive effects, the affected signal does not go through the detection circuit
• The whole circuit is actually designed to light a bulb depending on the music level
• On the 661 gate, when the music is loud enough the LDR value lowers thus letting the current go from the input to the output directly. When the music level is low, the LDR value increases thus reducing the amount of signal from the input to the output. You may eventually call that a gate
• On the 663 compressor, the LDR wiring differs: now when the signal is high the LDR redirects the signal to the ground
• The 663 only has one LDR while the 661 has two of them
• the input wiring (pins 1 - 9) differs between the 661 and the 663
• the "R19" resistor location differs

Here is a (WIP) rewiring project mod, using one LDR as a compressor and keep the other one for the original gate feature. I'll try the original gate but actually I'm pretty sure I'll move to a stereo compressor version instead.



I'll post future updates here!

 

[Noth]

I've worked on the v2 version of my 661 mod.

I've changed the input pinout for something clearer
The Circuit is now powered using +9VDC line instead of the 6.3VAC (I may eventually bypass the diode bridge)
Added input buffers using a quad TL074 powered with the +9/-9 PSU.
Added output buffers with a gain potentiometer (+12dB)

Quick question, if someone can help: I used two distinct potentiometers for the gain adjustment instead of using just one and merging signals at the opamp outputs. Is that necessary?

This is still a work in progress, so not tested at all and I'll probably have to adjust some values.
Any advice is appreciated!

 

[eddie_ruff_]

Hey Noah, I did this 661 to 663 conversion fairly recently and it's a neat sound. I replaced the bulb and LDR with new equivalent parts and switched to a 9V supply as well. Works like a charm.

Adding some buffered inputs/outputs is a nice idea! Are 'Input R' and 'Output R' supposed to be connected together?

Below is a link to a quick Instagram story I did on the unit.

https://www.instagram.com/stories/highlights/18033183562469166/

 

[Noth]

Hi!
Thanks a lot for your reply and the Instagram story, nice to see that I'm on the proper path!

You're righ, Input R and Output R are not supposed to be connected, I'll fix the schematics.

I'll post further updates with the final schematics!

 

[Noth]

I'm currently recapping everything, since all caps were original (and dead).

If it can be useful to someone else, here is the actual board (661 noise gate) and the actual component values.

Note: the 4 gold diodes seems to be the rectifier, the diode next to Q4 is D3 (1N96) and.. I was not able to locate the Zener diode (D1).
Anyway, I plan to supply the whole board directly with 9VDC and skip the rectifier/Zener part.

 

[eddie_ruff_]

wow, I didn't know they had a circular circuit board like that. Is this from a larger unit or channel strip?

My unit also omitted D1. I put that value in, and it didn't do much regulation after I switched to pedal power and D1 just got hot, so I also omitted it.

I recommend getting new LDRs as mine were delaminating and had much higher resistance than they should have been to be useful in a 470R voltage divider.

 

[Noth]

Thanks for the tip about the LDR!
I guess that these have round PCBs in order to be reused on the "round" version of the 661.
I don't really know what is the use for these versions (airborne audio gates maybe ?)

 

[Noth]

Hey Noah, I did this 661 to 663 conversion fairly recently and it's a neat sound. I replaced the bulb and LDR with new equivalent parts and switched to a 9V supply as well. Works like a charm.

Adding some buffered inputs/outputs is a nice idea! Are 'Input R' and 'Output R' supposed to be connected together?

Below is a link to a quick Instagram story I did on the unit.

Fairchild 663 = @ruff_records_

Hi,
I converted my unit to DC and replaced all the capacitors. I naively thought that I would be able to replace my dead bulb with an LED, but it seems that it is not that simple!

Do you remember where you sourced a compatible replacement light bulb?

Thanks!

 

[eddie_ruff_]

The LED would need some type of current limiting otherwise it'll burn up. The original bulb won't have that issue. There could be other issues with an LED due to the diode drop, I think the bulb is actually emitting at much lower voltages.

I went with the following 2158D replacement bulb...although I don't know why the price is that high. Ilc Replacement for Light Bulb / Lamp 2158d replacement light bulb lamp 10 Pack 2158D LIGHT BULB / LAMP | Zoro

 

[MidnightArrakis]

Hey Noah, I did this 661 to 663 conversion fairly recently and it's a neat sound. I replaced the bulb and LDR with new equivalent parts and switched to a 9V supply as well. Works like a charm.

Adding some buffered inputs/outputs is a nice idea! Are 'Input R' and 'Output R' supposed to be connected together?

Below is a link to a quick Instagram story I did on the unit.

Fairchild 663 = @ruff_records_

Here's what I received when I went to your included InstaGram link:

Sorry, this page isn't available.
The link you followed may be broken, or the page may have been removed. Go back to Instagram.

/

 

[eddie_ruff_]

Here's what I received when I went to your included InstaGram link:

Sorry, this page isn't available.
The link you followed may be broken, or the page may have been removed. Go back to Instagram.

/

hmm, works for me, but i do have to be signed in. Do you have an account? Maybe I'll make a youtube video of it at some point.

 

[Noth]

The original light bulb (2158D) are nearly impossible to find, even at indecent prices.

I did test the following instead: https://www.aliexpress.com/item/1005004900146314.html (5mm / 6V version) and it actually works just fine!

You can have a pack of 50 replacement bulb for 1€ so.. you will have plenty of spares!


Next steps:

- test new LDRs (the older ones do not vary much thus have no audible effect)
- write a tutorial for upgrading a 661 to 663
- rack enclosure and power supply
- unbalanced I/O and make up gain knob

 

[Noth]

As promised, here is the full blog post about switching a Fairchild 661 to a Fairchild 663 (and going from 6,3VAC to 9VDC) !

Converting a Fairchild 661 Gate to a 663 Compressor – Radix Studio – Blog