Audio mute switch

Jon Slaughter wrote:

Audio mute switch

You can make a very good mute switch with a jfet too.

Graham

Jon Slaughter wrote:

Audio mute switch


In the real world this may generate noticeable clicks, mostly coming in
via Cgd and due to the huge input voltage swing. You might want to slow
down the gate drive a bit, for example via an RC lowpass.

--
Regards, Joerg

http://www.analogconsultants.com

"Joerg" wrote in message
. net...
Jon Slaughter wrote:

Audio mute switch

In the real world this may generate noticeable clicks, mostly coming in
via Cgd and due to the huge input voltage swing. You might want to slow
down the gate drive a bit, for example via an RC lowpass.


Thats not what I asked. I do realize that will happen but I first need to
figure out why the circuit isn't working like I want. Utlimately I'm going
to use an audio analog ic switch that takes care of all those issues but I
first want to understand the basics and I can't seem to figure out whats
wrong.

Thanks,
Jon

"Jon Slaughter"

Thats not what I asked. I do realize that will happen but I first need to
figure out why the circuit isn't working like I want. Utlimately I'm going
to use an audio analog ic switch that takes care of all those issues but I
first want to understand the basics and I can't seem to figure out whats
wrong.



** When is this trolling ****wit gonna wake up that mosfets have an inbuilt
DS diodes making them unusable for audio muting ??

Any dead horses about ??



....... Phil

Phil Allison wrote:

"Jon Slaughter"

Thats not what I asked. I do realize that will happen but I first need to
figure out why the circuit isn't working like I want. Utlimately I'm going
to use an audio analog ic switch that takes care of all those issues but I
first want to understand the basics and I can't seem to figure out whats
wrong.

** When is this trolling ****wit gonna wake up that mosfets have an inbuilt
DS diodes making them unusable for audio muting ??

Any dead horses about ??

How about a live donkey ?

Jon Slaughter is amusing. He is highly resistant to education. I don't know why
he bothers posting here since if you point out his ignorance he just gets all
defensive.

I though such switches using mos parts were all cmos (requiring n and p channel)
too.

Graham

Eeyore wrote:

Jon Slaughter is amusing. He is highly resistant to education.


And you are totally impervious to education.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Jon Slaughter wrote:

"Joerg" wrote in message
. net...

Jon Slaughter wrote:


Audio mute switch

In the real world this may generate noticeable clicks, mostly coming in
via Cgd and due to the huge input voltage swing. You might want to slow
down the gate drive a bit, for example via an RC lowpass.



Thats not what I asked. I do realize that will happen but I first need to
figure out why the circuit isn't working like I want. Utlimately I'm going
to use an audio analog ic switch that takes care of all those issues but I
first want to understand the basics and I can't seem to figure out whats
wrong.


Well, it was just a comment. There was no question in your post, if
there was then it didn't propagate to my news server. So, what don't you
like about it?

Don't count on all audio IC switches to be perfect. I had to re-design
several squelch circuits because of unacceptable popping noise and some
were using "professional" audio chips.

--
Regards, Joerg

http://www.analogconsultants.com

"Joerg" wrote in message
...
Jon Slaughter wrote:

"Joerg" wrote in message
. net...

Jon Slaughter wrote:


Audio mute switch

In the real world this may generate noticeable clicks, mostly coming in
via Cgd and due to the huge input voltage swing. You might want to slow
down the gate drive a bit, for example via an RC lowpass.



Thats not what I asked. I do realize that will happen but I first need to
figure out why the circuit isn't working like I want. Utlimately I'm
going to use an audio analog ic switch that takes care of all those
issues but I first want to understand the basics and I can't seem to
figure out whats wrong.


Well, it was just a comment. There was no question in your post, if there
was then it didn't propagate to my news server. So, what don't you like
about it?


Yes, not that it wasn't a good comment but no one has yet mentioned any
problem with my circuit ;/ This is the real issue I'm after. I want to know
why its not working like it should(or why my reasoning wrong).

In AOE its mentioned as an application for fets in the beginning of the fet
chapter(3 I think). But the issue is that either some audio is passed when
its not suppose to or there is a large amount of signal loss. (about 1/5 of
whats input) Maybe this is just the way fets work? Maybe its a bug in
mutlisim? Maybe my circuit is wrong? Hopefully someone will eventually
answer these questions

Don't count on all audio IC switches to be perfect. I had to re-design
several squelch circuits because of unacceptable popping noise and some
were using "professional" audio chips.


Yeah ;/ I think at this point anything will do. I didn't realize they were
far from ideal. The IC I was looking at only has a -80db rejection of the
signal(if I read the datasheet right). While this is very low it could
cause some problems for my circuit.

What I have now is a bunch of dip switches and I have to manually switch
them into different combinations. My goal is to replace the switches with
something much smaller and can be electrically controlled. Fets seem to
solve the problem on the surface(well, atleast in ic form as doing it
discrete makes it to physically large for my project).

I'm going to go over the datasheet again for the ic I was looking at and see
what I can learn from it.... but I'd still like to know why my circuit isn't
functioning properly. (I did implement another version that was mentioned
for an audio mute but it has even worse issues)

Thanks,
Jon

BTW, if your curious the IC I'm looking at is at

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1974

Jon Slaughter wrote:

"Joerg" wrote in message
...

Jon Slaughter wrote:


"Joerg" wrote in message
gy.net...


Jon Slaughter wrote:



Audio mute switch

In the real world this may generate noticeable clicks, mostly coming in
via Cgd and due to the huge input voltage swing. You might want to slow
down the gate drive a bit, for example via an RC lowpass.



Thats not what I asked. I do realize that will happen but I first need to
figure out why the circuit isn't working like I want. Utlimately I'm
going to use an audio analog ic switch that takes care of all those
issues but I first want to understand the basics and I can't seem to
figure out whats wrong.


Well, it was just a comment. There was no question in your post, if there
was then it didn't propagate to my news server. So, what don't you like
about it?



Yes, not that it wasn't a good comment but no one has yet mentioned any
problem with my circuit ;/ This is the real issue I'm after. I want to know
why its not working like it should(or why my reasoning wrong).

In AOE its mentioned as an application for fets in the beginning of the fet
chapter(3 I think). But the issue is that either some audio is passed when
its not suppose to or there is a large amount of signal loss. (about 1/5 of
whats input) Maybe this is just the way fets work? Maybe its a bug in
mutlisim? Maybe my circuit is wrong? Hopefully someone will eventually
answer these questions


A single FET is not going to be ideal. There will always be leakage when
it is turned off, mostly caused by the Cgd capacitance in this case.
This forms an AC-voltage divider with the 1K resistor. It'll be small
but you can run the numbers for each frequency of interest once you know
Cgd.

And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example,
Rdson is around 200 ohms then you'll lose 1/5th of the signal.


Don't count on all audio IC switches to be perfect. I had to re-design
several squelch circuits because of unacceptable popping noise and some
were using "professional" audio chips.



Yeah ;/ I think at this point anything will do. I didn't realize they were
far from ideal. The IC I was looking at only has a -80db rejection of the
signal(if I read the datasheet right). While this is very low it could
cause some problems for my circuit.


Look at a T-switch architecture. But just FYI, layout is not trivial
when you want more than 80dB over the whole audio range. And don't do
the whole T-switch in a single chip because mundane things such as bond
wire impedance can spoil the broth, stuff that doesn't show up at all in
Spice unless you make it part of your model. Also, consider differential
signal paths.


What I have now is a bunch of dip switches and I have to manually switch
them into different combinations. My goal is to replace the switches with
something much smaller and can be electrically controlled. Fets seem to
solve the problem on the surface(well, atleast in ic form as doing it
discrete makes it to physically large for my project).

I'm going to go over the datasheet again for the ic I was looking at and see
what I can learn from it.... but I'd still like to know why my circuit isn't
functioning properly. (I did implement another version that was mentioned
for an audio mute but it has even worse issues)


One FET alone might not cut the mustard here ;-)

--
Regards, Joerg

http://www.analogconsultants.com

Jon Slaughter wrote:

BTW, if your curious the IC I'm looking at is at

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1974


Just be careful, some chips have a tendency to be unobtanium after the
first few samples. I couldn't see them on Digikey and that would be a
concern for me.

--
Regards, Joerg

http://www.analogconsultants.com

Joerg wrote:

And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example,
Rdson is around 200 ohms then you'll lose 1/5th of the signal.

J108 - 8 ohms typical.

I doubt that the load needs to be as low as 1k either.

Graham

Joerg wrote:

Jon Slaughter wrote:

BTW, if your curious the IC I'm looking at is at

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1974


Just be careful, some chips have a tendency to be unobtanium after the
first few samples. I couldn't see them on Digikey and that would be a
concern for me.

The ubiquitous DG308 and DG212 etc .. seem to make regular appearances in audio
kit.

Graham

"Joerg" wrote in message
news
Jon Slaughter wrote:

BTW, if your curious the IC I'm looking at is at

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1974

Just be careful, some chips have a tendency to be unobtanium after the
first few samples. I couldn't see them on Digikey and that would be a
concern for me.

--
Regards, Joerg

http://www.analogconsultants.com

yeah, I just picked these cause they were free(samples) so I could try some
simple prototyping and maybe even some testing to see how good they really
are at audio. (If I actually implement my idea then I'll do more research on
the chips)

"Joerg" wrote in message
...
Jon Slaughter wrote:

"Joerg" wrote in message
...

Jon Slaughter wrote:


"Joerg" wrote in message
igy.net...


Jon Slaughter wrote:



Audio mute switch

In the real world this may generate noticeable clicks, mostly coming in
via Cgd and due to the huge input voltage swing. You might want to slow
down the gate drive a bit, for example via an RC lowpass.



Thats not what I asked. I do realize that will happen but I first need
to figure out why the circuit isn't working like I want. Utlimately I'm
going to use an audio analog ic switch that takes care of all those
issues but I first want to understand the basics and I can't seem to
figure out whats wrong.


Well, it was just a comment. There was no question in your post, if there
was then it didn't propagate to my news server. So, what don't you like
about it?



Yes, not that it wasn't a good comment but no one has yet mentioned any
problem with my circuit ;/ This is the real issue I'm after. I want to
know why its not working like it should(or why my reasoning wrong).

In AOE its mentioned as an application for fets in the beginning of the
fet chapter(3 I think). But the issue is that either some audio is passed
when its not suppose to or there is a large amount of signal loss. (about
1/5 of whats input) Maybe this is just the way fets work? Maybe its a bug
in mutlisim? Maybe my circuit is wrong? Hopefully someone will eventually
answer these questions


A single FET is not going to be ideal. There will always be leakage when
it is turned off, mostly caused by the Cgd capacitance in this case. This
forms an AC-voltage divider with the 1K resistor. It'll be small but you
can run the numbers for each frequency of interest once you know Cgd.

And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example, Rdson
is around 200 ohms then you'll lose 1/5th of the signal.


Well, I'm mainly worried about passing an undistorted signal as possible
with the least amount of attenuation. Unfortunately for my application I'll
be running the signal through many fets(maybe 20+ or even more depending on
how complicated I make it) and so the noise and attenuation might make the
project impractical with fets. (using dips now which I think are ok but I
haven't done any tests)


Don't count on all audio IC switches to be perfect. I had to re-design
several squelch circuits because of unacceptable popping noise and some
were using "professional" audio chips.



Yeah ;/ I think at this point anything will do. I didn't realize they
were far from ideal. The IC I was looking at only has a -80db rejection
of the signal(if I read the datasheet right). While this is very low it
could cause some problems for my circuit.


Look at a T-switch architecture. But just FYI, layout is not trivial when
you want more than 80dB over the whole audio range. And don't do the whole
T-switch in a single chip because mundane things such as bond wire
impedance can spoil the broth, stuff that doesn't show up at all in Spice
unless you make it part of your model. Also, consider differential signal
paths.

Can you find this architecture in multiple switch IC's? I'm looking for
something thats 6 or larger(the larger the better) as I'll have to use
several.

I don't think I can use differential signals as it will make it overly
complicated but I'll try to keep it in mind when working out some of the
details.



What I have now is a bunch of dip switches and I have to manually switch
them into different combinations. My goal is to replace the switches with
something much smaller and can be electrically controlled. Fets seem to
solve the problem on the surface(well, atleast in ic form as doing it
discrete makes it to physically large for my project).

I'm going to go over the datasheet again for the ic I was looking at and
see what I can learn from it.... but I'd still like to know why my
circuit isn't functioning properly. (I did implement another version that
was mentioned for an audio mute but it has even worse issues)


One FET alone might not cut the mustard here ;-)


Well, for the simulation I'm not trying to make it perfect but surely the
issue is pretty significant? I mean, its reducing the signal to 1/5 is
original amplitude... thats pretty significant and would seem to make fets
useless?

The reason though I'm so concerned with the circuit is because I've seen in
3 places already that say you can use a signal fet as an audio mute or even
volume control. I'm sure that in these circuits there is no attenuation by
anything close to what I'm getting. (I haven't implemented them exactly
though cause I'm sure they would work but wondering why mine is not(Since it
probably means I'm not understanding something)).

I'm going to try a p-jfet though as I think it will make the circuit
easier(no need for -V).

Thanks,
Jon

Jon Slaughter wrote:
"Joerg" wrote in message
...

Jon Slaughter wrote:


"Joerg" wrote in message
. net...


Jon Slaughter wrote:



"Joerg" wrote in message
digy.net...



Jon Slaughter wrote:




Audio mute switch

In the real world this may generate noticeable clicks, mostly coming in
via Cgd and due to the huge input voltage swing. You might want to slow
down the gate drive a bit, for example via an RC lowpass.



Thats not what I asked. I do realize that will happen but I first need
to figure out why the circuit isn't working like I want. Utlimately I'm
going to use an audio analog ic switch that takes care of all those
issues but I first want to understand the basics and I can't seem to
figure out whats wrong.


Well, it was just a comment. There was no question in your post, if there
was then it didn't propagate to my news server. So, what don't you like
about it?



Yes, not that it wasn't a good comment but no one has yet mentioned any
problem with my circuit ;/ This is the real issue I'm after. I want to
know why its not working like it should(or why my reasoning wrong).

In AOE its mentioned as an application for fets in the beginning of the
fet chapter(3 I think). But the issue is that either some audio is passed
when its not suppose to or there is a large amount of signal loss. (about
1/5 of whats input) Maybe this is just the way fets work? Maybe its a bug
in mutlisim? Maybe my circuit is wrong? Hopefully someone will eventually
answer these questions


A single FET is not going to be ideal. There will always be leakage when
it is turned off, mostly caused by the Cgd capacitance in this case. This
forms an AC-voltage divider with the 1K resistor. It'll be small but you
can run the numbers for each frequency of interest once you know Cgd.

And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example, Rdson
is around 200 ohms then you'll lose 1/5th of the signal.



Well, I'm mainly worried about passing an undistorted signal as possible
with the least amount of attenuation. Unfortunately for my application I'll
be running the signal through many fets(maybe 20+ or even more depending on
how complicated I make it) and so the noise and attenuation might make the
project impractical with fets. (using dips now which I think are ok but I
haven't done any tests)


20? Wow, that would warrant some thinking about the architecture.


Don't count on all audio IC switches to be perfect. I had to re-design
several squelch circuits because of unacceptable popping noise and some
were using "professional" audio chips.



Yeah ;/ I think at this point anything will do. I didn't realize they
were far from ideal. The IC I was looking at only has a -80db rejection
of the signal(if I read the datasheet right). While this is very low it
could cause some problems for my circuit.


Look at a T-switch architecture. But just FYI, layout is not trivial when
you want more than 80dB over the whole audio range. And don't do the whole
T-switch in a single chip because mundane things such as bond wire
impedance can spoil the broth, stuff that doesn't show up at all in Spice
unless you make it part of your model. Also, consider differential signal
paths.


Can you find this architecture in multiple switch IC's? I'm looking for
something thats 6 or larger(the larger the better) as I'll have to use
several.


I am an ultrasound guy when it comes to switches. Crosspoints etc.,
rather expensive or full custom. Take a look around the DG series that
Graham mentioned.


I don't think I can use differential signals as it will make it overly
complicated but I'll try to keep it in mind when working out some of the
details.



What I have now is a bunch of dip switches and I have to manually switch
them into different combinations. My goal is to replace the switches with
something much smaller and can be electrically controlled. Fets seem to
solve the problem on the surface(well, atleast in ic form as doing it
discrete makes it to physically large for my project).

I'm going to go over the datasheet again for the ic I was looking at and
see what I can learn from it.... but I'd still like to know why my
circuit isn't functioning properly. (I did implement another version that
was mentioned for an audio mute but it has even worse issues)


One FET alone might not cut the mustard here ;-)



Well, for the simulation I'm not trying to make it perfect but surely the
issue is pretty significant? I mean, its reducing the signal to 1/5 is
original amplitude... thats pretty significant and would seem to make fets
useless?


No, doesn't make them useless. You just can't operate them in an
environment with impedances as low as 1K if you want less loss. Or use
buffers.


The reason though I'm so concerned with the circuit is because I've seen in
3 places already that say you can use a signal fet as an audio mute or even
volume control. I'm sure that in these circuits there is no attenuation by
anything close to what I'm getting. (I haven't implemented them exactly
though cause I'm sure they would work but wondering why mine is not(Since it
probably means I'm not understanding something)).

I'm going to try a p-jfet though as I think it will make the circuit
easier(no need for -V).


Just watch that reverse junction breakdown. But seriously, I'd scope out
a multiplexer series. DG-series of the CD4051 through 4053, to see if
anything fits the bill here. If your voltage range is lower the latter
also comes in 74HC flavors.

--
Regards, Joerg

http://www.analogconsultants.com

Eeyore wrote:


Joerg wrote:


And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example,
Rdson is around 200 ohms then you'll lose 1/5th of the signal.


J108 - 8 ohms typical.


Those seem to be kind of hard to get these days.


I doubt that the load needs to be as low as 1k either.


Yep, got to find a compromise between loss and leakage. However, with
T-switches leakage isn't such an issue.

--
Regards, Joerg

http://www.analogconsultants.com

switch topologies.

I once made a sample-and-hold, quite good at drift, using a MOSFET (BS170,
which appears close enough to 2N7000, etc.). Of course having three pins on
the MOSFET means it has a reverse diode, so I told the diffamp to saturate
to the negative rail when holding. It made one hell of a recovery transient
(all the way to +V), but it only lasted a microsecond. ;-)

You can also use diodes in similar fashion, though leakage is worse
(1N4148's cause visible drift (for periods of seconds) on the hold cap).
Speed is of course excellent, especially if you use, say, RF mixer
schottkies...

Most things of this sort will cause an asymmetrical pop when switching, but
some can be built balanced. You can use a bridge rectifier as a switch by
forward- or reverse-biasing the DC terminals, using the AC terminals as the
switch leads. Bias current must be greater than peak signal current, and
needs to be at the same potential as the signal (obviously, being locked
within a diode drop of it!), and should present little impedance to the
signal (implying CCS's). A possible solution presents itself using +/-
power supplies with equal CCS's to forward-bias the diodes, and a pair of
open collectors to pull the reverse bias. Because the capacitance, reverse
recovery (if any), current and etc. is all equal and opposite, there is
(ideally) no offset current or charge injection. Moreover, the turn-off
slope affects a quickly-moving input signal the same way, whether rising or
falling at the instant of turn-off; there is no skew of the slew rate when
disconnected from a fast-rising input.

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

"Jon Slaughter" wrote in message
...
Audio mute switch

Jon Slaughter wrote:

Well, I'm mainly worried about passing an undistorted signal as possible
with the least amount of attenuation.

And you refuse to listen to the one person here with experience of doing just
that.

Graham

Jon Slaughter wrote:

I'm going to try a p-jfet though as I think it will make the circuit
easier(no need for -V).

But the on-resistance is higher.

Graham

Joerg wrote:

Eeyore wrote:
Joerg wrote:

And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example,
Rdson is around 200 ohms then you'll lose 1/5th of the signal.


J108 - 8 ohms typical.

Those seem to be kind of hard to get these days.

Really ? I bought 30 ex-stock from Farnell just a couple of months back. Not
*that* pricey either given the performance. About 25p each.


I doubt that the load needs to be as low as 1k either.


Yep, got to find a compromise between loss and leakage. However, with
T-switches leakage isn't such an issue.

That's for sure. It's the only way for very high performance. Mind you, single
jfets in series mode can do surprisingly well.

Graham

Eeyore wrote:


Joerg wrote:


Eeyore wrote:

Joerg wrote:


And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example,
Rdson is around 200 ohms then you'll lose 1/5th of the signal.


J108 - 8 ohms typical.

Those seem to be kind of hard to get these days.


Really ? I bought 30 ex-stock from Farnell just a couple of months back. Not
*that* pricey either given the performance. About 25p each.


Digikey stock: Zilch. That's a red flag for me when I design stuff.
Pricing is even lower but what good does that do if I can't get any?



I doubt that the load needs to be as low as 1k either.


Yep, got to find a compromise between loss and leakage. However, with
T-switches leakage isn't such an issue.


That's for sure. It's the only way for very high performance. Mind you, single
jfets in series mode can do surprisingly well.


Yep, used them in samplers a lot. But not the J108 ;-)

BTW, do you know why the BF245 fell from grace? That's a mighty fine
device. The usual marketing bloopers?

--
Regards, Joerg

http://www.analogconsultants.com

Joerg wrote:

Eeyore wrote:
Joerg wrote:
Eeyore wrote:
Joerg wrote :

And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example,
Rdson is around 200 ohms then you'll lose 1/5th of the signal.


J108 - 8 ohms typical.

Those seem to be kind of hard to get these days.

Really ? I bought 30 ex-stock from Farnell just a couple of months back. Not
*that* pricey either given the performance. About 25p each.

Digikey stock: Zilch. That's a red flag for me when I design stuff.
Pricing is even lower but what good does that do if I can't get any?

Farnell's pricing is never that keen esp for small qtys.

However they have stock available today from both Fairchild and Vishay. The
Fairchild parts are on 3 days delivery because *they come from the USA (Newark) * !
http://uk.farnell.com/jsp/search/bro...tt=j10 8&Ntx=

I imagine Digikey's stock will be based on historic demand profiles, so maybe US
designers don't like jfets any more (or don't know how to use them any more - new
graduates tend not to understand discretes - etc etc ).


I doubt that the load needs to be as low as 1k either.


Yep, got to find a compromise between loss and leakage. However, with
T-switches leakage isn't such an issue.


That's for sure. It's the only way for very high performance. Mind you, single
jfets in series mode can do surprisingly well.

Yep, used them in samplers a lot. But not the J108 ;-)

BTW, do you know why the BF245 fell from grace? That's a mighty fine
device. The usual marketing bloopers?

Never used it. Was it Philips ? They never promoted their parts much in pro-audio
over here (despite originating the 5532/4 and indeed dropping it again in the last
few years)..

Graham

Eeyore wrote:


Joerg wrote:


Eeyore wrote:

Joerg wrote:

Eeyore wrote:

Joerg wrote :


And when it is on there will always be a remaining channel resistance,
commonly called Rdson. FETs aren't ideal switches. If, for example,
Rdson is around 200 ohms then you'll lose 1/5th of the signal.


J108 - 8 ohms typical.

Those seem to be kind of hard to get these days.

Really ? I bought 30 ex-stock from Farnell just a couple of months back. Not
*that* pricey either given the performance. About 25p each.

Digikey stock: Zilch. That's a red flag for me when I design stuff.
Pricing is even lower but what good does that do if I can't get any?


Farnell's pricing is never that keen esp for small qtys.


Can't say that. Just bought a scope from Farnell UK, quantity of one.
Best price in town. Ok, not exactly in town since they are 6000 miles
from here. Now I am the proud owner of an IEC cord with a big fat UK
power plug :-)


However they have stock available today from both Fairchild and Vishay. The
Fairchild parts are on 3 days delivery because *they come from the USA (Newark) * !
http://uk.farnell.com/jsp/search/bro...tt=j10 8&Ntx=

I imagine Digikey's stock will be based on historic demand profiles, so maybe US
designers don't like jfets any more (or don't know how to use them any more - new
graduates tend not to understand discretes - etc etc ).


Oh, they do. But these days we prefer J309/310 of which Digikey has a
combined 70000 in stock. Cause they're available from ON-Semi which is a
stable source who knows who to do marketing and, ahem, how to set up a
web site that actually works. SCNR.



I doubt that the load needs to be as low as 1k either.


Yep, got to find a compromise between loss and leakage. However, with
T-switches leakage isn't such an issue.


That's for sure. It's the only way for very high performance. Mind you, single
jfets in series mode can do surprisingly well.

Yep, used them in samplers a lot. But not the J108 ;-)

BTW, do you know why the BF245 fell from grace? That's a mighty fine
device. The usual marketing bloopers?


Never used it. Was it Philips ? They never promoted their parts much in pro-audio
over here (despite originating the 5532/4 and indeed dropping it again in the last
few years)..


I don't use NXP parts anymore until I see what shakes out. But first
they have to get their web site working. It's pathetic.

--
Regards, Joerg

http://www.analogconsultants.com

Joerg...

I'm about to do my first FET audio gate. Heretofore (for the last 30 years)
I've been relying on an opamp output at some positive voltage, a decently
high value resistor off the output (say, 100K or so), then a bipolar shunt
to ground, then another decently high value resistor to a summing junction.
I'd like to reduce the package count and not have an opamp (or it's cheap
cousin the cmos ibuffer with a feedback resistor) for each audio channel.

I'm leaning towards initial testing with a 2N7000 ... cheap, readily
available, predictable Vgs turnon voltage, and all the rest of it.

Comments?

JIm

--
"If you think you can, or think you can't, you're right."
--Henry Ford

RST Engineering (jw) wrote:

Joerg...

I'm about to do my first FET audio gate. Heretofore (for the last 30 years)
I've been relying on an opamp output at some positive voltage, a decently
high value resistor off the output (say, 100K or so), then a bipolar shunt
to ground, then another decently high value resistor to a summing junction.
I'd like to reduce the package count and not have an opamp (or it's cheap
cousin the cmos ibuffer with a feedback resistor) for each audio channel.


I'd check out the CD4051 or 74HC4051 series. Cheap, pretty good off
isolation, does the job, comes in TSSOP. I absolutely try to avoid any
DC shifts upon switching because that leads to nasty audio pops.


I'm leaning towards initial testing with a 2N7000 ... cheap, readily
available, predictable Vgs turnon voltage, and all the rest of it.

Comments?


Its charge injection is high and unless the audio signal level is in the
volt range you get the multiplier effect Vswitch/AudioRMS. Discrete FET
switches is what I had to design out of squelch circuits because the
popping was annoying. Nowadays the radio mfgs seem to finally have
understood that. Wonder what took them so long. Bought a Icom R1500 a
month ago an its squelch does not pop.

--
Regards, Joerg

http://www.analogconsultants.com

I'd check out the CD4051 or 74HC4051 series. Cheap, pretty good off
isolation, does the job, comes in TSSOP. I absolutely try to avoid any DC
shifts upon switching because that leads to nasty audio pops.

The problem with the 4051 is that I have eight audio channels and can have
any combination of them on or off at one time. However, its cousin the 4066
has four independent switches in a package and might work.



I'm leaning towards initial testing with a 2N7000 ... cheap, readily
available, predictable Vgs turnon voltage, and all the rest of it.

Comments?


Its charge injection is high and unless the audio signal level is in the
volt range you get the multiplier effect Vswitch/AudioRMS. Discrete FET
switches is what I had to design out of squelch circuits because the
popping was annoying. Nowadays the radio mfgs seem to finally have
understood that. Wonder what took them so long. Bought a Icom R1500 a
month ago an its squelch does not pop.

I've solved that problem for a few years with an RC time constant on the
switching voltage (easy on, easy off) that if done right, doesn't cut the
first syllable from the audio.

Jim

RST Engineering (jw) wrote:

I'd check out the CD4051 or 74HC4051 series. Cheap, pretty good off
isolation, does the job, comes in TSSOP. I absolutely try to avoid any DC
shifts upon switching because that leads to nasty audio pops.


The problem with the 4051 is that I have eight audio channels and can have
any combination of them on or off at one time. However, its cousin the 4066
has four independent switches in a package and might work.


A mighty fine chip. Surprisingly few people use or even know about them
these days.

rant_mode
The new kids fresh out of college may be able to simulate them and even
build them on CAD. Then they have a virtual 4066. Great. But they have
no clue that you can readily buy them for pennies.
/rant_mode



I'm leaning towards initial testing with a 2N7000 ... cheap, readily
available, predictable Vgs turnon voltage, and all the rest of it.

Comments?


Its charge injection is high and unless the audio signal level is in the
volt range you get the multiplier effect Vswitch/AudioRMS. Discrete FET
switches is what I had to design out of squelch circuits because the
popping was annoying. Nowadays the radio mfgs seem to finally have
understood that. Wonder what took them so long. Bought a Icom R1500 a
month ago an its squelch does not pop.


I've solved that problem for a few years with an RC time constant on the
switching voltage (easy on, easy off) that if done right, doesn't cut the
first syllable from the audio.


That would be the "let's sweep it under the rug" fix :-)

It works for audio although it does truncate the voice a bit. But it can
fall apart on some data transmissions.

--
Regards, Joerg

http://www.analogconsultants.com