THE ENGINEER'S THUMB... At last, a better compressor!

[rankot]

Most of the modern vactrols have much faster rise time and it's not easy to find some with 500ms rise anymore. 

GL55 series LDR have fall/rise time in the range of 20-30ms. So that wouldn't be the problem for NG.

[jhsa]

You might find that an LDR is not quite fast enough for a gate operation, they typically go low resistance in about 10-20ms, and high resistance in about 0.5s, this is almost ideal for an audio compressor, but might cut off the beginning of notes when used in a fast application like a noise gate. I do like LDRs for compression, but I would generally stick to a JFET, THAT VCA, OTA or something similar for very fast opening of a gate.

Thanks for your input Jonny. very much appreciated. The LDR seems quick enough to me. Perhaps my hear is not good enough but it seems to be OK. I have just finished building it and installed it on my PedalBoard. Please follow the link below to the project's topic. All schematics are there.

Be careful with regards to the body diode if you are using MOSFETs as a variable resistance. I generally prefer JFETs for this purpose. Let me know how you get on!

I used two Mosfets connected Source to Source, so the body diodes will cancel each other. No audio will pass unless the Mosfets turn ON
jFETs have quite high resistance (a few hundred ohms) when ON if I want to use them to mute the Audio to GND. This is why I do prefer the Mosfets. I use this configuration on my Pedal's relay bypass to mute the audio while the relay is switching. It works quite well.

Please check the link below. The latest schematics are on the last few posts as I cannot edit the first post unfortunately.

https://www.diystompboxes.com/smfforum/index.php?topic=128883.msg1244523#msg1244523

[PRR]

.....go low resistance in about 10-20ms, and high resistance in about 0.5s, ....might cut off the beginning of notes when used in a fast application like a noise gate. ...

LDRs can float. Put the LDR in the series element of an L-pad. Get signal, light beats on the crystal, resistance falls fast, signal passes. Signal fades, crystal's trapped charge slowly drains out, signal slowly goes away.

And as said, modern crap LDRs are fast both ways, so you can tailor the electronic time constant to taste.

[jhsa]

After building my Noise Gate with LDR and playing with it for a while, yesterday while messing with my PedalBoard, I did notice some very light slow attack when playing very high pitched notes only. I am talking about the high G or A notes on the first string. Perhaps I will try to decrease the resistor I have in series with the attack pot, and see if it helps. If not, then it is the LDR. Currently I have a 2k2 resistor in series with the Pot.

[DUCKFACE]

IV build the noisegate part from the Redux but only thing that gets out of the effect is a sound like a fart. Iv changed the LM 13700 with 13600 and the fart is more pleasent .. but still a fart. Anyone has the same problem and whats the sollution. The schematic is as follow :

edit:
power supply is -9 and +17.5v

[changes]

Hi guys

I've built the ET and love it!

I would like to put a VU meter with a needle to show the amount of compression in db

Any thoughts or circuit schematic on how to do this???

Thanks in advance

[merlinb]

I would like to put a VU meter with a needle to show the amount of compression in db
Any thoughts or circuit schematic on how to do this???

Your best option is to take the voltage this point (blue arrow), buffer it with another opamp and use that to drive your meter somehow.* The voltage at that point is proportional to gain reduction, and should vary from 4.5V (no reduction) down to about 2.5V (max gain reduction).


EDIT: *My first guess at a circuit for a moving coil meter, notice the meter polarity!


BTW my latest ET has a simple LED compression indicator, so you could try that instead.
http://valvewizard.co.uk/engineersthumb4.html

[changes]

Wow thank you so much Merlin

I will try that and get back to you

A quick question that came to mind

Is there any chance the ratio could be controlled from the sidechain instead of the opamp's feedback loop?

[merlinb]

Is there any chance the ratio could be controlled from the sidechain instead of the opamp's feedback loop?

I can't see any obvious way to do that, no

[changes]

Is there any chance the ratio could be controlled from the sidechain instead of the opamp's feedback loop?

I can't see any obvious way to do that, no

Thanks again Merlin!

I thought that it would be a good way to have the ratio control before the vu meter / gain reduction meter so that it would show the actual reduction.

If I get it right the way it is setup now if the ratio control is all the way down the vu meter will still show that there is gain reduction going on

Do you think that if the vu meter would go where the compression indicator LED is on v4 of ET would be more accurate???

[merlinb]

Do you think that if the vu meter would go where the compression indicator LED is on v4 of ET would be more accurate???

I think it would be the same either way.
An actual reduction meter is very rare on any compressor, because it's not easy to implement -I've never even seen one. You would typically need to subtract the output signal from the input signal, and use the difference to drive the meter, but that only works when you have a guaranteed unity-gain signal path (when no compression is happening) which the ET doesn't do. A simple gain reduction meter, like I showed, is more tyipical.

[PRR]

> subtract the output signal from the input signal, and use the difference

A log-ratio computation seems best.

However we don't find a true ratio display on even the best classic limiters (like Barry Blesser's swiss army knife, Blesser, B. and Kent, A. (1968) Analysis of a feedback-controlled limiter using a logarithmic measuring scale. IEEE Transaction of Audio and Electroacoustics 16:481-485).

And this is for guitar!! Do we have a pluck-force meter? Calibrated for the six different strings? Because you know you pluck different depending on mood, health, enthusiasm, vitamins..... A precision limiter meter sounds awful OCD to me.

While a GR Depth indication is useful for device commissioning, for night after night monitoring IMHO a simple green/yel/red LED tells what you need to know and easier. Did that moderate chord blip the red? Did I mean for it to be ducked? Are the loud parts failing to blip at all? Maybe I can turn-up a bit?

[changes]

> subtract the output signal from the input signal, and use the difference

A log-ratio computation seems best.

However we don't find a true ratio display on even the best classic limiters (like Barry Blesser's swiss army knife, Blesser, B. and Kent, A. (1968) Analysis of a feedback-controlled limiter using a logarithmic measuring scale. IEEE Transaction of Audio and Electroacoustics 16:481-485).

And this is for guitar!! Do we have a pluck-force meter? Calibrated for the six different strings? Because you know you pluck different depending on mood, health, enthusiasm, vitamins..... A precision limiter meter sounds awful OCD to me.

While a GR Depth indication is useful for device commissioning, for night after night monitoring IMHO a simple green/yel/red LED tells what you need to know and easier. Did that moderate chord blip the red? Did I mean for it to be ducked? Are the loud parts failing to blip at all? Maybe I can turn-up a bit?

Do you think that if the vu meter would go where the compression indicator LED is on v4 of ET would be more accurate???

I think it would be the same either way.
An actual reduction meter is very rare on any compressor, because it's not easy to implement -I've never even seen one. You would typically need to subtract the output signal from the input signal, and use the difference to drive the meter, but that only works when you have a guaranteed unity-gain signal path (when no compression is happening) which the ET doesn't do. A simple gain reduction meter, like I showed, is more tyipical.

Thanks again guys

I'll see if I can come up with something that would treat my OCD 

Cheers

[tuckster]

Hi Guys,
In V4 the attack pot got changed to a 10k but the performance description below "(using the recommended 100k pot)" is still mentioning the old value.
Do I still get the same attack time with the new design and a 10k pot?

[merlinb]

Hi Guys,
In V4 the attack pot got changed to a 10k but the performance description below "(using the recommended 100k pot)" is still mentioning the old value.
Do I still get the same attack time with the new design and a 10k pot?

10k is plenty to give you access to very long attack times, longer than you're likely to use in practice. Stick with 10k.

[Djonz]

I would like to put a VU meter with a needle to show the amount of compression in db
Any thoughts or circuit schematic on how to do this???

Your best option is to take the voltage this point (blue arrow), buffer it with another opamp and use that to drive your meter somehow.* The voltage at that point is proportional to gain reduction, and should vary from 4.5V (no reduction) down to about 2.5V (max gain reduction).


EDIT: *My first guess at a circuit for a moving coil meter, notice the meter polarity!


BTW my latest ET has a simple LED compression indicator, so you could try that instead.
http://valvewizard.co.uk/engineersthumb4.html

How come you place the VU Meter cisrcuit at the output of OpAmp C, while the metering LED in ET v4 is placed at the output of OpAmp D?

[Blackhawkamplifiers]

Thanks Jonny, perhaps sometime I will come back to this circuit. Meanwhile I ended up using a Led/LDR combo to mute the audio. I am waiting for the PCBs to arrive
Actually, tomorrow I will play in a place that is terribly noisy, so I have just built a much simplified version of the circuit I used.
It has the same peak detector /comparator circuit of your design, but I replaced the attack and release pots with a couple resistors, and instead of the LDR I have used two BS170 mosfets connected source to source to mute the signal. one of the drains connect to signal, the other connect to GND. It has an input buffer only. It seems to work, but the real test will be tomorrow
Thanks again

So I'm determined to come back to this LM13700 OTA version rather than going with a vactrol. Can you explain exactly what was going on with why you didn't like the OTA? Was it just the little bit of signal sneaking through?

[jmwreck]

Just finished the Engineer's Thumb from pedalpcb.

[snk]

Wow, it looks wonderful!
Is the text silkscreened?

[Hypernurbs]

Hi, How would I connect a sinus LFO 0V to 5V to get a sort of tremelo effect? I tried disconnecting the 1K and 1uF and connecting the LFO to the + of opamp C with a voltage divider pot but that doesn't seem to work. I tried other ways to connect to pin 1 of VCA F but with varying results, not really satisfactory.