CQ CQ.....Compressor

[mozz]

CQ Compressor. Well, it is a ham radio microphone compressor i guess. Date codes in the area of 1967. 3 transistors, 2N3415 npn-si, gain of about 180. Think i am going to replace the 2 electrolytics and fire it up with 1/4" jack on the input. Should i go to the trouble of drawing up the schematic? Any simple 3 transistor compressors for guitar or bass already out there? Even if it needs some tweaking for guitar it might be fun. I do not know what the last component is, i see 2 wires but it may have leads underneath. The input and output caps and inductors were probably for keeping RF out and may have been added later.

[ElectricDruid]

Wow! So nice and clean inside! Everything I open that's even half that old is full of dust, fluff, and dead spiders. 

[Mark Hammer]

Many of the older circuit collections, like the Rudolf Graf books, will often have very simple AGC and compressor circuits for handheld CB and ham radio mics.  I've never tried any, but their simplicity is intriguing.  Here's one from 1981.

[mozz]

Wow! So nice and clean inside! Everything I open that's even half that old is full of dust, fluff, and dead spiders. 

Old tube radios = mouse turds. Yeah this was pretty clean inside. And now the story of me buying a empty bass cabinet, when i got it home, opened the hatch and a %^&*roach went running. Doused the vehicle with bug spray and luckily i never found any in the house or garage.

Looking at the resistor values, i see a lot of triples. So i am taking it the stages are the same and the black thing (maybe ldr, varistor or cds) is adjusting the gain. Have to search some ham radio sites, most likely a common circuit just someone decided to market it and make a few.

[amz-fx]

Many of the older circuit collections, like the Rudolf Graf books, will often have very simple AGC and compressor circuits for handheld CB and ham radio mics.  I've never tried any, but their simplicity is intriguing.  Here's one from 1981.

Like many of the older circuit collections, this one has a fatal error. R2 and R3 are just in series between the voltage rails. I suspect that the wire that is shown jumping over the base connection should actually be tied to the base, so that R2/R3 provide the bias for the transistor.

You could use 560k for both resistors and get the same response. Also, it will always be below unity gain and should be driven by a low impedance source.

Best regards, Jack

[Kipper4]

The Really Cheap Compressor is a viable op amp design, that's a good starting point from the point of view of a guitar.

There's a thread with schematic here.

https://www.diystompboxes.com/smfforum/index.php?topic=110831.0

[mozz]

Was wondering if there were any 2,3,4 transistor versions of a pedal compressor, if not I will draw this out so it can be breadboarded and tweaked.

[PRR]

The black lump is an early Vactrol. As you say, the coils are typical when working audio near a radio transmitter. More than likely the audio is lightly gained and buffered to drive the Vactrol's LED (or incandescent? but I think they were larger), the LDR at an early stage to short the audio.

--Ah, LED because I bet the four diodes are an audio rectifier.

[cspar]

According to this ad the vactrol is incandescent.



I found this ad too.



I'm curious about the schematic.

Is that a 2n340 in the heatshrink driving the bulb?

That'd make it a 4 transistor compressor.

Do the coils on the jacks have something to do with phantom power?

[duck_arse]

Is that a 2n340 in the heatshrink driving the bulb?

That'd make it a 4 transistor compressor.

that's quite a job of spotting - but you left off the trailing "2". 3402.

[mozz]

Thanks for the ads.  I figured the 2n3402 was power supply but probably driving the vactrol.  I doubt LED because of 1967 era, so probably a bulb.  If I get time this weekend I'll fire it up with new supply caps.