PLEASE take a look at this circuit and offer a suitable alternative

[DJAE]

Hi,

New to the forum so please be gentle!

I am working an a very basic analogue processor (essentially a compressor, limiter eq and noise gate!). I have the compressor and limiter sorted.


I need help with a simple downward expander and/or noise gate. I thought about a non selective noise gate by simply using an opamp comparator to pull the signal down to gnd when the signal falls below a certain threshold (using an RC timing network on the input). However i would MUCH rather have a proper downward expander in there too.

The attached image is supposed to be a noise gate by by my reckoning it looks more like a voltage controlled attenuator (the larger the siganl the more it attenuates), i want the opposite: a circuit which begins to attenuate higest frequencies as volume descreases past a given point.

Thanks in advance.
AE
View attachment 64107

 

[DJAE]

BTW i don't own the copyright to that image!

It is also intended simply as a refernce and will not be the circuit i will build.

 

[unclejed613]

first of all, ditch the 741. a 741 is horrible for audio. use a TL071 instead. even though the op amp is not in the signal path, it's terrible at high frequencies, and it may not open the gate if your input is much above 2khz

 

[DJAE]

first of all, ditch the 741. a 741 is horrible for audio. use a TL071 instead. even though the op amp is not in the signal path, it's terrible at high frequencies, and it may not open the gate if your input is much above 2khz

oh good God yeah I would.. I'm using 74's throughout (quad 71).
But please explain how this gates because it looks like signal is pulled low with increasing volume to me, or have I missed something?

Thanks

 

[JimB]

But please explain how this gates because it looks like signal is pulled low with increasing volume to me, or have I missed something?

You have missed something.

When there is no signal through IC1, there is no rectified DC from the D1/D2 pair and so TR1 is not conducting.
This allows current through R8, R9 and R10 to turn on T2 and T3.
T2 and T3 being turned on presents a low resistance at the right hand side of R11 and R12 and the signal is attenuated.

When there is signal through IC1, it is rectified by D1/D2 and creates a current to turn on T1.
When T1 is turned on, the voltage at the junction of R8, R9 and R10 is pulled down to a low value so that there is insufficient available to turn on T2 and T3.
T2 and T3 being turned off presents a high resistance at the right hand side of R11 and R12 and the signal is not attenuated.

JimB

 

[DJAE]

I see, thanks.

So the transistor pair at the output CONDUCT when the first transistor isn't? And the pot sets the point where this will happen by varying the amount of signal available at the input.

is the electrolytic and resistor acting as a release or are they to simply to smooth the rectified voltage and limit base current to t1, or both?

Thanks

 

[MrAl]

Hi,

This circuit is really bogus for a number of reasons. It would be best to drop it entirely and start over.
Besides the other problems, it's not a good idea to use a transistor collector to cut an audio signal anyway, so the whole thing is not great to start with. The audio, if it is referenced to 0v (ground), only gets cut on the positive peaks which would produce really bad distortion. If it is referenced to some negative voltage then the non linearity of the transistors would still produce unacceptable distortion.

The modern approach would be to use DSP, but dated slightly before that the acceptable approach would be to use an LDR excited by some light source like an LED. As the LED is driven by the rectified and filtered audio, the LDR resistance decreases and thus cuts the audio. Since the LDR is a resistance, it shows the same conductance for both positive and negative signals and so cuts both polarities of the audio equally. The non linearity of the LDR is not with respect to the polarity of the signals, but with respect to the light level vs resistance which will not add any distortion as long as the time constant of the DC filter is longer than say 10 times the slowest audio signal.

 

[unclejed613]

Denon, Pioneer, and Yamaha to name a few, all use a transistor in a similar manner as a mute switch. we're talking signal levels of less than 1.5V, and they are quite effective for muting.

 

[DJAE]

Hi,

This circuit is really bogus for a number of reasons. It would be best to drop it entirely and start over.
Besides the other problems, it's not a good idea to use a transistor collector to cut an audio signal anyway, so the whole thing is not great to start with. The audio, if it is referenced to 0v (ground), only gets cut on the positive peaks which would produce really bad distortion. If it is referenced to some negative voltage then the non linearity of the transistors would still produce unacceptable distortion.

The modern approach would be to use DSP, but dated slightly before that the acceptable approach would be to use an LDR excited by some light source like an LED. As the LED is driven by the rectified and filtered audio, the LDR resistance decreases and thus cuts the audio. Since the LDR is a resistance, it shows the same conductance for both positive and negative signals and so cuts both polarities of the audio equally. The non linearity of the LDR is not with respect to the polarity of the signals, but with respect to the light level vs resistance which will not add any distortion as long as the time constant of the DC filter is longer than say 10 times the slowest audio signal.

Hi,

Thanks for reply: I did say at the satrt of the post that i wanted this to be ANALOGUE, and it is intended to solidify my learning of component function. If i am at the satge where i am learning transistor biasing, opamp circuits and time constants i don't really think i am ready for encoding and decoding for DSPs do you??

As for the LDR: Surely this would be much better served using an opto? The results would surely be alot more repaetable and matching between channels much easier? I know this creates a lovely warm analogue sound, but i am not looking for a vca/attenuator, i have designed a FET system, what i would like is a gate (on or off), or a downward expander which attenuates higher frequencies first to limit noise.

I know that bipolar transistor muting can be a little 'clicky', but this is meant as a learning process.

Thanks for you suggestions.

Can anyone amswer my question about the 10u/1M? Are they simply smoothing/current limiting, or do they serve a timing function?

Has anoyone got an alterantive circuit?

 

[unclejed613]

the cap and resistor slow the switch down so you get a seamless mute. otherwise you would get a click or a thump when the muting transistors activate.

 

[MrAl]

Hi,

Thanks for reply: I did say at the satrt of the post that i wanted this to be ANALOGUE, and it is intended to solidify my learning of component function. If i am at the satge where i am learning transistor biasing, opamp circuits and time constants i don't really think i am ready for encoding and decoding for DSPs do you??

As for the LDR: Surely this would be much better served using an opto? The results would surely be alot more repaetable and matching between channels much easier? I know this creates a lovely warm analogue sound, but i am not looking for a vca/attenuator, i have designed a FET system, what i would like is a gate (on or off), or a downward expander which attenuates higher frequencies first to limit noise.

I know that bipolar transistor muting can be a little 'clicky', but this is meant as a learning process.

Thanks for you suggestions.

Can anyone amswer my question about the 10u/1M? Are they simply smoothing/current limiting, or do they serve a timing function?

Has anoyone got an alterantive circuit?

Hi again,

If you are just going to do muting that should be easier, but from your first post it sounded like you want to do compression.
The LDR is linear with regard to the polarity while an opto coupler with an output transistor is not, unless you know of an opto coupler with an LDR output.

 

[DJAE]

unless you know of an opto coupler with an LDR output.

They are indeed out there Sir, though admittedly not common. There is a company makes them specifically for audio (guitar pedals I guess), and they quote audio characteristics in their data sheets!
Look here:
http://www.silonex.com/audiohm/levelcontrol.html


Just to clarify I am relatively new to electronics, I am learning in stages... I am working with audio right now having started with rf and realised what a massive undertaking that was! Thats where the interest in compression comes from though.

I didn't just want to build someone elses design so I did a lot of research and eventually settled on a very unusual rectifier and I worked out a jfet attenuator based on a paper I got describing basically all calculations.

What I could do with now is gating... Given that, and circuit substitutions discussed, does this circuit start to look a little more usable?

 

[ericgibbs]

hi Al,
Look at this opto LDR pdf.

E.

 

[DJAE]

Very nice too, thanks.

 

[MrAl]

Hi,


Thanks for the links guys...very informative. As shown in Eric's link, one of the most useful characteristics of the analog opto (LDR+LED) is the bilateral conduction. They do state however that best performance is with signals less than 0.5 volts which is good to know too. That would mean plus or minus 0.5 volts.

DJAE:
As you can see the analog opto is probably the ideal device for your purposes, but if you'd like to experiment with an FET that's up to you. If you come up with something that looks like it works you can post it here and we'll take a look and analyze the behavior a bit to see just how good it works out in the long run. Keep in mind that if you dont bias the audio signal to some predetermined DC level then the transistor or other device may not cut the same on the positive peaks as the negative peaks and thus cause a great amount of distortion. The "analog opto" is known to work pretty well here but FET's have also been used. How good the distortion is with an FET instead of an analog opto would most likely depend to a great extent on the cleverness of the design. I think the FET designs are more popular in limiting applications where the limiting doesnt have to be super linear over the operating range, but then again there might be a way to make up for it in the design plan. Remember the analog opto is a true resistive element which is going to be quite linear for any given LED drive current (as well as bilateral).

 

[DJAE]

Brilliant, thanks... Think I am struggling a bit with distortion, will have a play today and maybe upload my schematic later for advice.

to research I read the stuff on esp site re. Peak limiter and opamp design. I also grabbed this paper https://users.ece.gatech.edu/mleach/papers/limiter.pdf which gives great info on measuring pinch off etc.

I also grabbed schematics for limiters from Veronica (aareff) and broadcast warehouse (very different breasts). I also read three papers by Dolby in the 70s about multiband compression.

My plan is to use rectifier, averaging circuit and fet with fairly slow attack and release and then a diode limiter based on opamp and zenners to grab fast transients.

Will look into my biasing thanks.

 

[MrAl]

Hi,

Ok great. When you get something together post back here and we can take a look. Sounds interesting so far.