Engineer's Thumb Compressor

Discussion in 'General Electronics Chat' started by Geoffb, Mar 21, 2015.

  1. Geoffb

    Thread Starter New Member

    Nov 4, 2012
    Hi Folks,

    I've built an engineer's thumb using the JMK PCB rev 1.2 and I can't seem to get the circuit to start and compress until the input is greater than approx 600mV p-p. I'm looking for compression by monitoring the input waveform (1kHz) with one channel of a scope and the output with the second channel and I can't see the output compressing until the input is approx 600mV p-p.
    Here's a link to the circuit

    The following are DC voltage readings with no signal (9v power):

    pin 1 =4.69v
    pin 2 =4.51v
    pin 3 = 4.11
    pin 4 = 0v
    pin 5 = 4.11
    pin 6 = ?
    pin 7 = 4.56
    pin 8 = 3.86
    pin 9 = 4.4
    pin 10 = 4.02
    pin 12,13,14 = 4.5v

    pin 2 = 4.51v
    pin 3 = 4.51v
    pin 5 = 0.69
    pin 6 = 4.51

    I beleive the problem has something to do with the precision rectifier circuit as the waveform on pin 1 of the TL074 looks nothing like half wave rectification, in fact the waveform is still a full sinewave until the above 600mV input is reached and only then does the bottom of the negative half cycle start to show signs of a more negative spike. I've even tried simulating this circuit using 5Spice which seems to give the same results. All the precision rectifier circuits I've seem on the internet have the feedback to the same input as its signal input. The only changes I've made from the orginal circuit are using 1uF electro in place of C5 and C6 and a BC 327 in place of Q1.

    I would appreciate if someone with more knowledge of the type of circuit give me some advice, please?

  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    I'm impressed that you managed to build anything from this bizarrely drawn schematic.o_O
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    Hmm... The attached schematic doesn't appear to be a 'compressor' in the common (DSP technique) sense of the term, but, rather, some manner of audio (amplitude) limiter --- You wouldn't, perchance, be a 3-meter pirate? Tisk! tisk! tisk!;)

    With nostalgic regards

    PS FWIW, Not having 'waded' through the circuit, I nonetheless, point up the fact that Si Diodes enter forward bias at Ca 600mV to 700mV --- So... You may be 'on to something' as it were! :D
    Last edited: Mar 22, 2015
  4. ebeowulf17

    Active Member

    Aug 12, 2014
    Audio compressors have a threshold value for the input signal, below which they will pass the signal through unaltered (or at least not deliberately altered - there's always the possibility of some distortion.) Sounds like maybe your threshold is around 600mV?

    In the PDF they mention the possibility of changing R3 value or replacing it with a pot for threshold control. I'd be interested in hearing what happens if you change R3.