Audio limiter to quiet loud radio commercials

[Audioguru]

Most audio sinewave generators and audio automatic level controllers use a Jfet to set the level. The Jfet on-resistance is the same with either polarity of the input signal so that the distortion is very low. I think that your rectifier is too simple to be a fullwave active rectifier.
The circuit I posted has a high impedance to feed the extremely high impedance input of a non-inverting opamp. It usually makes linear adjustments but it can turn a signal on and off if the DC voltage to the gate of the Jfet is extreme. There will be distortion if the signal level at the drain of the opamp exceeds about 40mV RMS which is why it feeds an amplifier (that has a high input impedance).
C1 and C2 in my circuit block any DC from the input or output.

 

[Frankchie]

Audioguru,
I setup my two-fet circuit onto the Internet radio and so-far I am disappointed with the performance. Under higher audio signal it does reduce the volume but sometimes it produces a little distortion. I think i can live with that. I'm still adjusting things and I'm still using my junkbox JFETs (2n5459) with a bias battery so maybe there is improvement possible. Yes, my rectifier is just a 1/2 wave, maybe I'll convert to full wave and see if that helps.

I just noticed that other stations have a significantly higher audio level for the shows and the commercials. So my description of the offending station probably should have said the commercials are normal volume and the show itself is much lower volume. The significance is that my circuit will lower the volume of the other stations much of the time, not just the commercials. Lowering the volume is no real problem, but if it causes distortion that is noticeable it could be unacceptable.

I have a feeling that your circuit is better given that you seem to be a more professional designer than my seat-of-the-pants methods. So I'll probably give it a try at some point. I'll let you know what happens. I bought some 2n5484's on ebay, but I think they are coming from China...

In the end, I may have to resort to a power supply and a more robust circuit.

Thanks,
Frank

 

[Frankchie]

Audioguru,
Looking at your N-channel circuit I'm confused by the "from rectifier" at R4 and the "positive voltage" at the JFET source. I assume you are expect a full wave, 4 diode rectifier that produces both + and - voltages. Plus goes to the JFET source, and minus goes to R4. Do I have that right?

I would have expected the source to go to ground along with the plus side of the full wave rectifier.

Frank

 

[Audioguru]

Audioguru,
Looking at your N-channel circuit I'm confused by the "from rectifier" at R4 and the "positive voltage" at the JFET source. I assume you are expect a full wave, 4 diode rectifier that produces both + and - voltages. Plus goes to the JFET source, and minus goes to R4. Do I have that right?

I would have expected the source to go to ground along with the plus side of the full wave rectifier.

Frank

4 diodes are never used to rectify audio, an active rectifier circuit with an opamp is used to cancel the diodes forward voltage drop.
The rectifier circuit produces a positive voltage when the audio level is loud and is 0V when the audio level must not be reduced.The N-channel Jfet conducts and grounds the audio when the gate and source are the same voltage. The jfet has its conduction reduced when its gate voltage is more negative than its source voltage, but a negative power supply voltage is not needed if the positive source biases the drain-source positive and the Jfet conduction is reduced when its gate voltage is at ground (more negative than its source).
C1 and C2 guarantee that the signal source and amplifier input do not change the drain DC voltage to be different from the source DC voltage.
If the source is grounded then the rectifier circuit needs an additional negative power supply.

 

[Harald Kapp]

I think your common source JFET circuit is designed to shut off the commercials completely.

Insert a resistor in the JFET'S drain connection to create a switchable resistive divider. Even better: use a potentiometer, you can then adjust the volume of the commercials in relation to the music.

 

[Frankchie]

4 diodes are never used to rectify audio, an active rectifier circuit with an opamp is used to cancel the diodes forward voltage drop.
The rectifier circuit produces a positive voltage when the audio level is loud and is 0V when the audio level must not be reduced.The N-channel Jfet conducts and grounds the audio when the gate and source are the same voltage. The jfet has its conduction reduced when its gate voltage is more negative than its source voltage, but a negative power supply voltage is not needed if the positive source biases the drain-source positive and the Jfet conduction is reduced when its gate voltage is at ground (more negative than its source).
C1 and C2 guarantee that the signal source and amplifier input do not change the drain DC voltage to be different from the source DC voltage.
If the source is grounded then the rectifier circuit needs an additional negative power supply.

Thanks, I think I got it...
I'm using a 4-schottky-diode bridge because an opamp needs a power supply that I am trying to avoid. Anyway I think your answer to my question is "yes" (plus to source, minus to R4) as that would negatively bias the JFET as you stated.

However, you also stated "the N-channel Jfet conducts and grounds the audio", but the JFET is not grounded and that was the one of my implied questions (maybe if I used an opamp it would be). There is a 50 mfd electrolytic cap filtering the output of the diode bridge, but I'm not sure there is an AC path to ground (back though the bridge?). I guess I could add a small cap to ground.

Hopefully, I'm making some sense.
Frank

 

[Frankchie]

Insert a resistor in the JFET'S drain connection to create a switchable resistive divider. Even better: use a potentiometer, you can then adjust the volume of the commercials in relation to the music.

Thanks, good idea to use a potentiometer to fine tune things.
Frank

 

[Audioguru]

Since you are using an N-channel Jfet as an on-off switch and not as a linear attenuator with automatic level control then the Jfet source can be grounded and the diode bridge creates a negative voltage to the Jfet gate to reduce the conduction of the Jfet for maximum output level of the audio.
Your simple circuit has nothing to discharge the huge 50uF filter capacitor of the rectified audio, then it will remain charged and the circuit will be reducing the audio level all day long.

 

[Frankchie]

Since you are using an N-channel Jfet as an on-off switch and not as a linear attenuator with automatic level control then the Jfet source can be grounded and the diode bridge creates a negative voltage to the Jfet gate to reduce the conduction of the Jfet for maximum output level of the audio.
Your simple circuit has nothing to discharge the huge 50uF filter capacitor of the rectified audio, then it will remain charged and the circuit will be reducing the audio level all day long.

Okay, I'll give it a try.
Good point about the 50uf cap, I realized that a few days ago and added a drain resistor.
Frank

 

[Audioguru]

Wait a minute.
With the source of the N-channel Jfet grounded and the diode bridge making a negative gate voltage for it when the input audio gets too loud then the output audio will get louder, not quieter.

 

[Frankchie]

Yes, I just saw that in my testing. I don't see an easy fix.
Frank

 

[Audioguru]

The way to fix the N-channel Jfet circuit is to bias the source to about +5V or more and have the gate voltage zero when the audio input level is low and the rectified audio makes +5V or more to turn on the Jfet which reduces the output level when the input level is loud.

 

[Frankchie]

Prior to seeing your fix, I shorted out R1 and inserted a 10k resistor from the source to ground. And then took the output from the source of the JFET. I think that's called a source follower circuit. I'm applying the rectified control voltage between the source (+) and the gate (-). I suspect I can also apply it between ground and the gate.

That seems to work but I'm seeing some distortion when the JFET is off. That is, the positive half of my 400hz test signal still gets through minimally while the negative half is completely blocked. No matter how high the bias voltage is some positive half of the sine wave gets through. Anyway I tried another JFET...same problem. My wiring is a rats nest so maybe that's the problem.

If I can't get my above fix to work I'll try yours. In the meantime if you happen to see a problem with my fix let me know.

Thanks for all your help,
Frank

 

[Frankchie]

Okay, the source follower circuit is working very well. I had to connect the output of my 4-diode rectifier between ground and the gate, rather than between the source and the gate. When connected between the source and the gate somehow some 1/2 wave ripple from the rectifier was getting through.

Anyhow, it seems to operate perfectly as a switch, and I can bypass the JFET with a pot or fixed resistor to adjust the loud commercial audio volume when the JFET is off. All this under test conditions, I have not actually connected it to the radio/Amp yet...although I'm pretty sure that will work.

The only remaining problem is the gate voltage between turn-on and turn-off. With my current JFET those levels are -4v, turn-on, and -5v turn-off. Using a bias battery the rectifier should provide the additional -1v to turn-off the JFET, but it's cutting it pretty close. Anyway, I have ordered some JFETs should solve that problem and hopefully eliminate the bias battery.

In the meantime, I'm thinking that changing the rectifier to a voltage doubler may be a temporary solution until the JFETs arrive.

Anyway, I would like to thank everyone for all their help, it was great.
Frank

 

[BobK]

I had to connect the output of my 4-diode rectifier between ground and the gate, rather than between the source and the gate.

If that means what I think it does, you no longer have a bridge. This is why a single diode is used in peak detectors, you cannot connect one of the outputs to ground. It shorts across one of the diodes, and places another diode directly across the inputs. Show us you circuit if you disagree.

Bob

 

[Frankchie]

If that means what I think it does, you no longer have a bridge. This is why a single diode is used in peak detectors, you cannot connect one of the outputs to ground. It shorts across one of the diodes, and places another diode directly across the inputs. Show us you circuit if you disagree.

Bob

Oops, of course you are correct. Grounding one of the bridge outputs shorts across one of the diodes, etc., as you said...Somehow it didn't seem to cause any problem, at-least I didn't see any problem yet. I'm so used to working with transformer connected bridges in power supplies, where you can connect one of the bridge outputs to ground, I never gave it a second thought.

Anyway, I'm now using a 2-diode voltage doubler rectifier circuit and that gives me about the same output as the bridge. And so-far I don't see any problem other than the known need for a lower gate pinch-off voltage that should be solved with new JFETs.

Thank you, I'm glad you told me that...
Frank

 

[BobK]

A single diode half wave rectifier will give you a higher output than a bridge because there is only one diode drop instesad of two. Bridges are used on power circuits to reduce the capacitance needed for filtering and the power factor on the source, not to get a higher output voltage.

Bob

 

[Audioguru]

He wants passive circuit without a power supply or opamps so it will have many problems reducing (actually muting) loud sounds.
The input signal probably has a level that is too low for a passive circuit.
Some compressor circuits use an active fullwave rectifier to sense then reduce levels.

 

[BobK]

I understand all of that and I am pointing out that a bridge will have higher losses than a half-wave rectifier in this case, as long as the smoothing capacitor is large enough.

I tried to make a 1 JFET circuit, but saw no way to get around the fact that the JFET is switched the wrong way, I think a 2 JFET circuit is quite doable, using one that has a gate threshold of about -0.5V. Given the tiny current needed for the gate supply, you could possibly even use a separate rectifier to get the operating voltage which holds the peak between commercials, in addition to a faster one that detects the commercials.

Bob

 

[Frankchie]

A single diode half wave rectifier will give you a higher output than a bridge because there is only one diode drop instesad of two. Bridges are used on power circuits to reduce the capacitance needed for filtering and the power factor on the source, not to get a higher output voltage.

Bob

Bob, you are right again, I could have sworn I got a higher voltage with the bridge, but I went back and tried it and It doesn't. However, when I ground the positive side of the bridge, like I was doing previously, the voltage almost doubles. That explains why I thought the bridge gave me a higher voltage than a single peak-detector diode.

Analyzing the schematic of a bridge with the plus side grounded, It looks like I stumbled into a standard voltage doubling circuit (Greinacher circuit-Wikipedia), although that uses two caps and my resulting circuit (with the plus grounded) only has one cap, and an extra diode that does nothing. Perhaps there is a series cap in the audio feed that acts like the missing cap.

Anyway I'm currently using the standard voltage doubling circuit I mentioned above. I'm also looking at some voltage multiplier circuits that look pretty viable (3x, 4X,etc.). Given the infinitesimal current required by the JFET gate I think these should work.

Thanks,
Frank