# Audio limiter to quiet loud radio commercials

#### BobK

Jan 5, 2010
7,682
The voltage doubler is giving you a + and - output. You still get no more voltage with respect to ground.

Bob

#### Frankchie

Nov 14, 2017
149
The voltage doubler is giving you a + and - output. You still get no more voltage with respect to ground.

Bob
Bob, Here's the doubler circuit I'm using. It doubles with respect to ground.

Edit: Not sure why my above image doesn't work. The first doubler in the link below is what I am using.
https://www.electronics-tutorials.ws/blog/voltage-multiplier-circuit.html

#### BobK

Jan 5, 2010
7,682
There is no ground symbol there. In your case, the ground is one of the two input lines from radio, and it will produce voltage both above and below that ground, just as the circuit you showed produces voltages both above and below the lower transformer secondary connection.

The problem is that the source of the JFET that is turning down the audio has to be connected to the common ground between the input and the output. With respect to that ground, you can only produce either a positive or a negative voltage equal to the single sided peak voltage of the audio minus the diode drop.

You could use a transformer to get a higher voltage, but I don't think it is necessary. I will try to come up with a working 2 JFET circuit for you. I think it is possible.

Bob

#### Frankchie

Nov 14, 2017
149
There is no ground symbol there. In your case, the ground is one of the two input lines from radio, and it will produce voltage both above and below that ground, just as the circuit you showed produces voltages both above and below the lower transformer secondary connection.

The problem is that the source of the JFET that is turning down the audio has to be connected to the common ground between the input and the output. With respect to that ground, you can only produce either a positive or a negative voltage equal to the single sided peak voltage of the audio minus the diode drop.

You could use a transformer to get a higher voltage, but I don't think it is necessary. I will try to come up with a working 2 JFET circuit for you. I think it is possible.

Bob
Bob,

I think you are looking at the wrong doubler circuit in my link...The circuit I am using is the first circuit at the top of the page. It called a voltage multiplier, but it's basically a doubler.....there's no transformer in the circuit that I am talking about.

Furthermore, the doubler circuit is working, I simply reversed the diode and cap polarities of the circuit in the link to get a negative output relative to ground.

And I just got the voltage quadrupler circuit working, down further on that same link.

And the source follower Jfet is also working, although I still need a better JFET.

Frank

#### davenn

Moderator
Sep 5, 2009
14,126
I think you are looking at the wrong doubler circuit in my link...The circuit I am using is the first circuit at the top of the page. It called a voltage multiplier, but it's basically a doubler.....there's no transformer in the circuit that I am talking about.

you link DOESNT go to a circuit at the TOP of the page

it goes to another page of many links

wayyyyyyyyyyyyyy down the page there are some voltage multiplier circuits. The first circuit is for a AC mains rectifier

the second cct maybe is the circuit you are referring to ???

#### Frankchie

Nov 14, 2017
149
Yes, that is the circuit that I was talking about.

That's odd, when I click the link using Chrome it goes to the top of the page and I see my doubler circuit. When I use IE it still goes to the top of the page, but my doubler circuit does not load, just a spinning circle where my doubler should be. And the first circuit you see has a transformer. Beats me why that happens....I'll try and remember to test any links using IE before I post them here.

Anyway, the quadrupler circuit, further down on the page, works well. I have hooked up my breadboard to my Internet radio/amplifier and it is working pretty well with a bias battery. I'm hoping when I get the JFETs I ordered, that have a low gate pinch off voltage, I'm hoping it will work even better and I can also eliminate the bias battery.

As I mentioned, it's not that the commercials are extra loud it's that the main programming has a much lower volume and the commercials have a normal volume, similar to volume on the other radio stations. That's significant because initially I thought I could tolerate some distortion during a commercial, but now I know that would affect the normal volume of the other stations. So it's important that the JFET not operate in it's linear region where it has distortion, but rather snap on and off like a switch and control a resistor voltage divider to lower the volume. It does that, but occasionally (by watching the gate bias voltage) it does cross into the linear region as the audio volume occasionally naturally pauses or fluctuates, but surprisingly I don't hear much distortion. We listen mostly to news and talk radio and that sounds pretty good. I'm not sure music would be as good. If that proves troublesome I'll try increasing the caps in the doubler circuit to add some more hysteriasis to the switching function.

Thanks, again to everyone for their help.
Frank

#### Audioguru

Sep 24, 2016
3,650
A Jfet is used in many very low distortion sinewave generators to regulate the level. They use two resistors to feed half the level at the drain to the gate to reduce the distortion. If the level at the drain is 50mV peak then the distortion is very low. If the level at the drain is 100mV peak (0.1V) then the distortion is low but maybe audible. Your levels are probably way too high because you are trying to make a passive circuit without an opamp..

We use the PRT SCR button to copy a page of a document, paste it into Microsoft PAINT program, crop the schematic and adjust its size then save it. Then that schematic is easily attached to our reply here.

#### Frankchie

Nov 14, 2017
149
A Jfet is used in many very low distortion sinewave generators to regulate the level. They use two resistors to feed half the level at the drain to the gate to reduce the distortion. If the level at the drain is 50mV peak then the distortion is very low. If the level at the drain is 100mV peak (0.1V) then the distortion is low but maybe audible. Your levels are probably way too high because you are trying to make a passive circuit without an opamp..

We use the PRT SCR button to copy a page of a document, paste it into Microsoft PAINT program, crop the schematic and adjust its size then save it. Then that schematic is easily attached to our reply here.
Yes, I know that my passive operation is fairly unusual and can be challenging to get low distortion.

Thanks for the tip on attaching a picture.
Frank

Replies
9
Views
108
Replies
10
Views
157
Replies
2
Views
244
Replies
5
Views
295
Replies
5
Views
346