I'm trying to design a simple audio mute using a fet but I can't seem to get
it to function properly. (Actually this is essentially equivalent to what
I'm trying to do... So I need help on this specific method and not some
other method which might be easier(such as just a switch)). The audio signal
is is < 2V p-p.


Essentially audio signal passes through a enhanced n-mosfet connect to
source. The drain goes into a load which is what ever load the would have
been connected to it in the first place.

The base of the fet is connected to -5V. The gate is connected to a STDP
switch that selects -5V or +5V for turning on and off the fet.


The circuit does work but the signal is extremely attenuated. Its about 1/10
of what it should be. If I double the supply voltage then I can half the
attenuation. I just want the fet to pass the signal completely(with almost
no attenuation) or completely turn it off(effectively).

Is there something I'm missing? I thought that a fet could essentially act
as a voltage controlled switch? So whats wrong with my circuit? I've posted
a schematic in binaries.

Thanks,
Jon

Jon Slaughter wrote:

> I'm trying to design a simple audio mute using a fet but I can't seem to get
> it to function properly. (Actually this is essentially equivalent to what
> I'm trying to do... So I need help on this specific method and not some
> other method which might be easier(such as just a switch)). The audio signal
> is is < 2V p-p.
>
> Essentially audio signal passes through a enhanced n-mosfet connect to
> source. The drain goes into a load which is what ever load the would have
> been connected to it in the first place.
>
> The base of the fet is connected to -5V. The gate is connected to a STDP
> switch that selects -5V or +5V for turning on and off the fet.
>
> The circuit does work but the signal is extremely attenuated. Its about 1/10
> of what it should be. If I double the supply voltage then I can half the
> attenuation. I just want the fet to pass the signal completely(with almost
> no attenuation) or completely turn it off(effectively).
>
> Is there something I'm missing? I thought that a fet could essentially act
> as a voltage controlled switch? So whats wrong with my circuit? I've posted
> a schematic in binaries.

Use a jfet.

Graham

The audio signal
> is is < 2V p-p.


I always watch your posts with interest because it looks like you and I are
attempting to design similar things. Is your mute circuit for a guitar by
chance?

>
> Essentially audio signal passes through a enhanced n-mosfet connect to
> source. The drain goes into a load which is what ever load the would have
> been connected to it in the first place.


Have you tried connecting both the input and the output to the source? Then
when you switch the MOSFET on, it will conduct the signal to ground.

"tempus fugit" <toccata@quitspammingme.ciaccess.com> wrote in message
news:d0409$46c0db2e$d1d89e1e$20251@PRIMUS.CA...
>
> The audio signal
>> is is < 2V p-p.
>
>
> I always watch your posts with interest because it looks like you and I
> are
> attempting to design similar things. Is your mute circuit for a guitar by
> chance?


lol. Well, its not really for a guitar mute but it is something for a
guitar. For a guitar mute I just use a switch.

>
>>
>> Essentially audio signal passes through a enhanced n-mosfet connect to
>> source. The drain goes into a load which is what ever load the would have
>> been connected to it in the first place.
>
>
> Have you tried connecting both the input and the output to the source?
> Then
> when you switch the MOSFET on, it will conduct the signal to ground.
>
>

I'm not sure I follow.

Ultimately I'm going to use an audio analog switching ic but I want to
understand the basic idea first but can't get it to work. Well, its working
but its attenuating the signal way to much. (so its not acting like a switch
but more like a resistor)


Jon

On Sun, 12 Aug 2007 22:47:14 -0500, "Jon Slaughter"
<Jon_Slaughter@Hotmail.com> wrote:

>I'm trying to design a simple audio mute using a fet but I can't seem
to get
>it to function properly. (Actually this is essentially equivalent to
what
>I'm trying to do... So I need help on this specific method and not
some
>other method which might be easier(such as just a switch)). The audio
signal
>is is < 2V p-p.
>
>
>Essentially audio signal passes through a enhanced n-mosfet connect
to
>source. The drain goes into a load which is what ever load the would
have
>been connected to it in the first place.
>
>The base of the fet is connected to -5V. The gate is connected to a
STDP
>switch that selects -5V or +5V for turning on and off the fet.
>
>
>The circuit does work but the signal is extremely attenuated. Its
about 1/10
>of what it should be. If I double the supply voltage then I can half
the
>attenuation. I just want the fet to pass the signal completely(with
almost
>no attenuation) or completely turn it off(effectively).
>
>Is there something I'm missing? I thought that a fet could
essentially act
>as a voltage controlled switch? So whats wrong with my circuit? I've
posted
>a schematic in binaries.
>
>Thanks,
>Jon
>

Maybe you should use a circuit designed by a broadcast engineer - he
should know what he is talking about...
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2046

On Tue, 14 Aug 2007 06:52:27 GMT, Ross Herbert
<rherber1@bigpond.net.au> wrote:

SNIP

Sorry, that was the wrong link. This is the one I meant to post.
http://www.pic101.com/audiosw/index.html

"Ross Herbert" <rherber1@bigpond.net.au> wrote in message
news:jlv2c35hbddjt8p3a5pueij2vsd4a1na1a@4ax.com...
> On Tue, 14 Aug 2007 06:52:27 GMT, Ross Herbert
> <rherber1@bigpond.net.au> wrote:
>
> SNIP
>
> Sorry, that was the wrong link. This is the one I meant to post.
> http://www.pic101.com/audiosw/index.html

Thanks for that Ross. I'm still in the process of trying to implement JFETs
to bypass a string of effects and this could be a big help, although if
anyone has a way to configure the JFET switch to act as a SPDT or DPDT
relay, that'd be even better.

>
> lol. Well, its not really for a guitar mute but it is something for a
> guitar. For a guitar mute I just use a switch.
>
What are you trying to do with this circuit?


> >> Essentially audio signal passes through a enhanced n-mosfet connect to
> >> source. The drain goes into a load which is what ever load the would
have
> >> been connected to it in the first place.
> >
> >
> > Have you tried connecting both the input and the output to the source?
> > Then
> > when you switch the MOSFET on, it will conduct the signal to ground.

> >I'm not sure I follow.


I've successfully done what you're trying to do here with a CMOS switch IC.
Instead of running your signal through the MOSFET, just connect both your
signal input and output to node 1 in your diagram. When you apply voltage to
close the MOSFET switch, the signal will pass through the MOSFET to ground,
thus muting the output. When the switch is opened again, the signal will be
audible again since there is no longer a path to ground to mute it.

"tempus fugit" <toccata@quitspammingme.ciaccess.com> wrote in message
news:be081$46c1ca0c$d1d89e85$2398@PRIMUS.CA...
>
>>
>> lol. Well, its not really for a guitar mute but it is something for a
>> guitar. For a guitar mute I just use a switch.
>>
> What are you trying to do with this circuit?
>
>
>> >> Essentially audio signal passes through a enhanced n-mosfet connect to
>> >> source. The drain goes into a load which is what ever load the would
> have
>> >> been connected to it in the first place.
>> >
>> >
>> > Have you tried connecting both the input and the output to the source?
>> > Then
>> > when you switch the MOSFET on, it will conduct the signal to ground.
>
>> >I'm not sure I follow.
>
>
> I've successfully done what you're trying to do here with a CMOS switch
> IC.
> Instead of running your signal through the MOSFET, just connect both your
> signal input and output to node 1 in your diagram. When you apply voltage
> to
> close the MOSFET switch, the signal will pass through the MOSFET to
> ground,
> thus muting the output. When the switch is opened again, the signal will
> be
> audible again since there is no longer a path to ground to mute it.
>

Oh, I think I see what your saying. You mean that instead of having the
signal pass through the fet you have the fet ground the signal at that
point?

> Oh, I think I see what your saying. You mean that instead of having the
> signal pass through the fet you have the fet ground the signal at that
> point?
>

Exactly. You also don't have to worry about altering your signal since it
doesn't actually pass through any extra silicon (if that's a concern).

"tempus fugit" <toccata@quitspammingme.ciaccess.com> wrote in message
news:83d8f$46c1e71c$d1d89e85$6420@PRIMUS.CA...
>> Oh, I think I see what your saying. You mean that instead of having the
>> signal pass through the fet you have the fet ground the signal at that
>> point?
>>
>
> Exactly. You also don't have to worry about altering your signal since it
> doesn't actually pass through any extra silicon (if that's a concern).
>

Yeah. Only thing I'm worried about is that for a guitar the ground is sorta
"floating"? Else it seems like a good idea... although for my specific
application I actually need the signal to go through the fet because I need
to disconnect the audio completely from the circuit(grounding at a point
might cause some problems although maybe can be fixed other ways). The main
issue I think is that I want to try to use some ic's and not go discrete
because the application requires a ton of "switches".

I ordered some analog switches though and maybe these will work for now.

Jon