Engineer's Thumb Compressor Problems

[Geoffb]

Hi Folks newbe hear.

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    http://jmkpcbs.com/wp-content/uploads/2014/08/Engineers-Thumb.pdf

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

TL074
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

CA3080
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 bottem 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?

Regards
Geoff

[merlinb]

Your voltages look reasonable, so if you think it is the rectifier then my guess is that you've put in D3/D4 backwards...

[Geoffb]

Hi Merlinb,
Many thanks for the prompt response.

I've checked the polarity of the diodes and they are both correct. If I monitor the voltage on C5 (1uf cap), I can see the voltage going neg. by a few mV when the signal increases. What I can't understand is why is the waveform on the op amp output still a full sine wave until the input signal reaches 600mV p-p. The precision rectifier circuit doesn't seem to be doing much at all. The diodes and the op amp have also been changes. I've even built this precision rectifier circuit on a breadboard in order to try and undertand the circuit better but the output waveform is still the same.
I have a picture of the output but don't know how to attached.

Regards
Geoff

[Transmogrifox]

Pin 1 on the TL074 will approximately follow the input.  The rectified output will be seen at the anode of D3.

As you increase the level on the signal generator from 0 on up you should see the DC voltage at Pin 10 decrease.  That will confirm the rectifier is working correctly.

Having D5 reversed could also cause an appearance that there is very little compression until the signal level gets much higher.  Double check D5 polarity.

[merlinb]

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.

The BC327 has a different pinout from the 2N3906. It needs to be flipped 180 degrees if you want to put it in the same holes. Did you do this?

[tubegeek]

To post a pic:

• upload the pic to some location on the internet - either a free image hosting service like imgur or photobucket, or if you have a personal website somewhere that'll be fine too.

• copy the URL of the pic to your clipboard once it is uploaded.

• in the button selection above the posting window there is a button for the "IMG" tags - it has a picture frame icon. Two places to the right of the YouTube button. Put the pair of "IMG" tags where you want the pic to appear in your post.

• paste the URL between the "IMG" and "/IMG" tags

• preview your post before hitting "Post" to verify all is well.

[Geoffb]

Hi MerlinB,
Many thanks for your comments, however, double checked to polarity of D3,4 & 5 -all ok. Also understand the pin-out difference between Q1 and its correct.
The signal levels are:
Input 100mV p-p
TL074 levels
pin 1 = 152mV
pin 2 = 100mV
pin 3 = 100mV

When measuring the DC voltage on pin 8 of the TL704, I can see the voltage go more negative by 200mV but only when the input level is 500mV.
Al can you tell me how to post an image on this forum, please?

Does this provide any more clues?
Regards
Geoff

[Geoffb]

Hi Merlinb,

Here's two pics of the waveform I see on pin 1 of the TL 074.

The waveform goes positive 0.2v and neg 0.6v with 100mV input.





Regards
Geoff

[tubegeek]

On the right side of Photobucket you have four URL choices: use the one called "direct" to paste between the "IMG" tags.

[Geoffb]

Hi,

Thanks for the info but I cannot see any url choices within Photobucket. Please help as I've never used Photobucket before.

Regards
Geoff

[tubegeek]

When I double-click on any photo, I get a display of that photo only (not an "album" style view but a single-photo view.) In that window, off to the right there is a "SHARE LINKS" heading where I see 4 different versions of the photo URL: "Email & IM," "Direct," "HTML," and "IMG."

Click on one of your images and see if you can get to that display type. Then click on the "Direct" link and it should get copied to your clipboard.

Give that a try?

[Geoffb]

Hi Tubegeek,

Many thanks. I'll attach hear to see if it works


Regards
Geoff

[tubegeek]

[merlinb]

The precision rectifier is very sensitive to being loaded by scope probes / multimeters, but your oscillograms look plausible. Post photos of the circuit board. Since you're using a ready made PCB there is nothing that can go wrong with the layout, so the culrpit is probably wrong part/wrong place/wrong offboard wiring.

[Geoffb]

As requested, I've posted a picture of the completed circuit board. In relation to checking the component values, I've double checked the resistor values by sight and measurement. Taking into account any small variation with in-circuit measurement, they seem Ok.



With regard to the earlier comments relating to possible circuit loading with a scope or multimeter, I decided to look at the problem from another angle. Assuming the precision rectifier and control current circuit is working, I should see the voltage across R8 (current into the CA3080) increase with signal input and as this point should be low impedance any reading should be OK.
Here are my results:

Input signal (1Khz, p-p)
0V =       124uA
50mV =  173uA
100mv = 183uA
500mV = 226uA

Now I must confess I'm not sure I'm reading the datasheet correctly, but it does seem that for the CA3080 to reach full gain (max feedback /compression) it needs 500uA into the control pin 5.
and this would point to the problem I'm seeing, that of no compression until a high input signal is present.

As I'm now running out of ideas, I'v ordered some non electo caps for C5&6 to see what difference this makes.

However any further comments would be very much appreciate.

Regards
Geoff

[samhay]

if you short the protection diode (1N4148) next to the BJT then U2a becomes a buffer that you can use to monitor the envelope voltage without loading the rectifier. If you are going to try this, I suggest you remove the OTA from its circuit first to keep it safe.

[Geoffb]

Hi Samhay,
Many thanks for your comments howerver I tried this but still don't get the range of neg. voltage swing I need to activate the CA3080.
I'm not seeing any reduction in the voltage at base of Q1 until the input is greater than 500mV. Which bring me back the the precision rectifier circuit not doing its job.

One addition posible problem I'm noticed is the lack of control when using the "ratio" pot and measuring across this 1meg pot when in circuit, I measure approx. 21K. There is a constant 21k path via R12,R12,R10 and R6. The circuit is therefore putting a constant 21k feedback between pins 6 and 7 of the op amp and a 1meg ratio pot is doing nothing. Can anyone explain, please?

I wonder if  MerlinB could try and repeat my measurment to see if they match?

Getting totally frustrated now.
Geoff.

[merlinb]

There is a constant 21k path via R12,R12,R10 and R6. The circuit is therefore putting a constant 21k feedback between pins 6 and 7 of the op amp and a 1meg ratio pot is doing nothing. Can anyone explain, please?

The junction of R10 - R12 is bypassed to ground via the Vref supply, so it is not a feedback path as far as AC signals are concerned. Only the pot allows AC feedback.

I wonder if  MerlinB could try and repeat my measurment to see if they match?

I'll try and do it tomorrow.

[Geoffb]

Hi Merlinb,

Many thanks for your comments re the feedback loop. I think I've spent too long looking at this circuit and become blind to the obvious.

I thank you for your continued help with this.

Beast regards
Geoff

[merlinb]

OK, I measured the voltage at the base of the transistor (pin 8 of the opamp in your circuit), so what you see is a buffered version of the control voltage.

1kHz, all controls at maximum
Vin:
1Vpp -----------3.46V at base of transistor
500mVpp------3.73V
200mVpp------3.91V
100mVpp------3.98V
40mVpp-------4.03V (already compressing)
0V--------------4.88V (opamp is trying to drive the transistor hard off)

(It's times like these that my pedal-sized sinewave generator comes in real handy... )