best way to handle switching in audio pre-amp

[Mac]

What is the best way to handle analog switching for line-level
high-fidelity audio? I don't think I want to use mechanical switching
because nice switches seem kind of pricey, and I might want to use a
remote control in a future revision.

So, I guess I am looking for a good analog mux or switch which won't
introduce distortion. For example, I looked at the SNL74LVC2G53, but it
seems as though it would add too much distortion for a hi-fi application.

Can I just use an N-channel MOSFET with the gate pulled to +/- 15
Volts for on and off, respectively? Any particular part recommendations?

I think I remember Win recommending using two MOSFET's in series, but I
think the idea was to float the gates at vs + 15V somehow. I don't want to
do anything that tricky if I can avoid it. ;-)

Oh, I am planning on having +/- 15 volt rails for the analog circuitry,
and I guess I'll need some 5 Volts for the digital circuitry. I'd like to
keep the 5 Volts to, say, under 10mA, if possible.

--Mac

 

[Genome]

What is the best way to handle analog switching for line-level
high-fidelity audio? I don't think I want to use mechanical switching
because nice switches seem kind of pricey, and I might want to use a
remote control in a future revision.

So, I guess I am looking for a good analog mux or switch which won't
introduce distortion. For example, I looked at the SNL74LVC2G53, but it
seems as though it would add too much distortion for a hi-fi application.

Can I just use an N-channel MOSFET with the gate pulled to +/- 15
Volts for on and off, respectively? Any particular part recommendations?

I think I remember Win recommending using two MOSFET's in series, but I
think the idea was to float the gates at vs + 15V somehow. I don't want to
do anything that tricky if I can avoid it. ;-)

Oh, I am planning on having +/- 15 volt rails for the analog circuitry,
and I guess I'll need some 5 Volts for the digital circuitry. I'd like to
keep the 5 Volts to, say, under 10mA, if possible.

--Mac

Maserati GT, Aint Got You.

Shut up and listen.

DNA

 

[Pooh Bear]

What is the best way to handle analog switching for line-level
high-fidelity audio? I don't think I want to use mechanical switching
because nice switches seem kind of pricey, and I might want to use a
remote control in a future revision.

So, I guess I am looking for a good analog mux or switch which won't
introduce distortion. For example, I looked at the SNL74LVC2G53, but it
seems as though it would add too much distortion for a hi-fi application.

Can I just use an N-channel MOSFET with the gate pulled to +/- 15
Volts for on and off, respectively? Any particular part recommendations?

I think I remember Win recommending using two MOSFET's in series, but I
think the idea was to float the gates at vs + 15V somehow. I don't want to
do anything that tricky if I can avoid it. ;-)

Oh, I am planning on having +/- 15 volt rails for the analog circuitry,
and I guess I'll need some 5 Volts for the digital circuitry. I'd like to
keep the 5 Volts to, say, under 10mA, if possible.

You can get integrated analogue switches. Siliconix's DG211 and 308 used to be
classic workhorses but nothing beats using jfets.

Check Siliconix's J111 and J174 families. As widely used in pro-audio.

http://www.fairchildsemi.com/ds/J1/J111.pdf

http://www.fairchildsemi.com/ds/J1/J174.pdf


You'll have to devise your own driver circuitry though !


Graham

 

[Ban]

What is the best way to handle analog switching for line-level
high-fidelity audio? I don't think I want to use mechanical switching
because nice switches seem kind of pricey, and I might want to use a
remote control in a future revision.

I would have recommended bistable relays, which do not consume power unless
switching.

So, I guess I am looking for a good analog mux or switch which won't
introduce distortion. For example, I looked at the SNL74LVC2G53, but
it seems as though it would add too much distortion for a hi-fi
application.

If you really want +/-15V supply and high level audio this part won't do.
Not because of distortion but of breakdown voltage.
BTW how high would be the distortion induced by an analog switch?

Can I just use an N-channel MOSFET with the gate pulled to +/- 15
Volts for on and off, respectively? Any particular part
recommendations?

There is an intrinsic diode in most mosfets which would conduct even if the
transistor is switched off.

I think I remember Win recommending using two MOSFET's in series, but
I think the idea was to float the gates at vs + 15V somehow. I don't
want to do anything that tricky if I can avoid it. ;-)

see above to avoid getting the diode limit output voltage. And with the high
levels you want, you will have to use 20V to be above Vp.

Oh, I am planning on having +/- 15 volt rails for the analog
circuitry, and I guess I'll need some 5 Volts for the digital
circuitry. I'd like to keep the 5 Volts to, say, under 10mA, if
possible.

Why the high voltage if an amplifier needs less than 1Veff. for full power?
maybe it would be better to buy a ready made preamp with your design skills?

 

[Larry Brasfield]

What is the best way to handle analog switching for line-level
high-fidelity audio? I don't think I want to use mechanical switching
because nice switches seem kind of pricey, and I might want to use a
remote control in a future revision.

So, I guess I am looking for a good analog mux or switch which won't
introduce distortion. For example, I looked at the SNL74LVC2G53, but it
seems as though it would add too much distortion for a hi-fi application.

Many modern switch IC's would do fine. As long as you drive the
input side of the switch with a low impedance source, (such as
a fed-back op-amp), and follow the switch with a high impedance
load, (such as the + input of an non-inverting op-amp stage), the
distortion added by the FETs will be negligible. The way distortion
arises in FET is that drops across the channel resistance modulate
the effective gate-to-channel voltage, in turn affecting on-resistance.
By keeping the load current thru the channel resistance small, you
keep that modulation effect small.

Can I just use an N-channel MOSFET with the gate pulled to +/- 15
Volts for on and off, respectively? Any particular part recommendations?

That would work, provided you liked the isolation it provides.

I think I remember Win recommending using two MOSFET's in series, but I
think the idea was to float the gates at vs + 15V somehow. I don't want to
do anything that tricky if I can avoid it. ;-)

Non-tricky would be to use a IC switch.

 

[martin griffith]

What is the best way to handle analog switching for line-level
high-fidelity audio? I don't think I want to use mechanical switching
because nice switches seem kind of pricey, and I might want to use a
remote control in a future revision.

So, I guess I am looking for a good analog mux or switch which won't
introduce distortion. For example, I looked at the SNL74LVC2G53, but it
seems as though it would add too much distortion for a hi-fi application.

Can I just use an N-channel MOSFET with the gate pulled to +/- 15
Volts for on and off, respectively? Any particular part recommendations?

I think I remember Win recommending using two MOSFET's in series, but I
think the idea was to float the gates at vs + 15V somehow. I don't want to
do anything that tricky if I can avoid it. ;-)

Oh, I am planning on having +/- 15 volt rails for the analog circuitry,
and I guess I'll need some 5 Volts for the digital circuitry. I'd like to
keep the 5 Volts to, say, under 10mA, if possible.

--Mac

I still use relays,
http://www.pickeringrelay.com/selectby.htm

for example
series 100
Ideal for data acquisition - Thermal EMF of 1pV or less!

or series 101, 74 logic compatable




martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

 

[Mac]

I would have recommended bistable relays, which do not consume power unless
switching.

Thanks, Ban. I kind of suspected something like that must exist because
my home thermostat uses relays (I can hear them click when the heat
goes on or off) but it runs only from a small alkaline battery, and has
been going strong for more than a year.

If you really want +/-15V supply and high level audio this part won't do.
Not because of distortion but of breakdown voltage.
BTW how high would be the distortion induced by an analog switch?

Oh, I'm sorry, my idea was to use +/- 15 Volt rails because the op-amp I
want to use (OPA2134, which I discovered in a previous thread) works best
with the high rails. I don't intend to have high level signals present on
the output. I would have only line level signals, as I said in the
original post.

There is an intrinsic diode in most mosfets which would conduct even if the
transistor is switched off.

Ah, yes. Good point. I even knew this, but somehow the significance of it
slipped my mind. Thanks Ban.

see above to avoid getting the diode limit output voltage. And with the high
levels you want, you will have to use 20V to be above Vp.


Why the high voltage if an amplifier needs less than 1Veff. for full power?
maybe it would be better to buy a ready made preamp with your design skills?

My design skills are not as weak as you think. I don't do a lot of analog
design, especially analog design with discrete transistors, but I have
designed a few op-amp circuits and lots of digital stuff.

Thanks again, Ban.

--Mac

 

[Mac]

I still use relays,
http://www.pickeringrelay.com/selectby.htm

for example
series 100
Ideal for data acquisition - Thermal EMF of 1pV or less!

or series 101, 74 logic compatable




martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

After I posted, I realized that I hadn't even considered relays. Thanks
for recommending specific ones. I will have a look.

--Mac

 

[Mac]

You can get integrated analogue switches. Siliconix's DG211 and 308 used to be
classic workhorses but nothing beats using jfets.

Check Siliconix's J111 and J174 families. As widely used in pro-audio.

http://www.fairchildsemi.com/ds/J1/J111.pdf

http://www.fairchildsemi.com/ds/J1/J174.pdf


You'll have to devise your own driver circuitry though !


Graham

Thanks. I'll take a look. I especially appreciate that you listed specific
parts.

--Mac

 

[Mac]

Mac wrote:
[snip]

So, I guess I am looking for a good analog mux or switch which won't
introduce distortion. For example, I looked at the SNL74LVC2G53, but it
seems as though it would add too much distortion for a hi-fi
application.

[snip]

BTW how high
would be the distortion induced by an analog switch?

For the part mentioned above, the datasheet says 0.01% into 10k || 50 pF.
This is at 10 kHz. Interestingly, they list the same distortion value for
1 kHz.

Now that I read it over and actually look at the test setup, I see
that they are swinging to within 0.5 Volts of the rail. If I power it at
+/- 2.5 Volts and keep the load much higher than 10 k, I'm sure I will
get better performance at realistic levels.

So maybe I can use a cheap mux afterall.

It uses so little power that I don't mind having to generate the
+/-2.5 Volt rails. And yes, I know I have to watch out that I don't drive
the inputs beyond those rails.

By the way, if you can recommend a good op-amp for line-level buffering
that will work well at +/- 5V, I would be very interested. In fact, I
would probably just change my design over to using +/- 5V instead of +/-
15. I guess I would see 4 or 5$ per op-amp (at low volume) as the upper
limit on price.

Anyway, thanks again, Ban! You seem to almost always have good advice for
people interested in audio stuff.

--Mac

 

[Ban]

Mac wrote:
]

For the part mentioned above, the datasheet says 0.01% into 10k || 50
pF. This is at 10 kHz. Interestingly, they list the same distortion
value for 1 kHz.

Now that I read it over and actually look at the test setup, I see
that they are swinging to within 0.5 Volts of the rail. If I power it
at +/- 2.5 Volts and keep the load much higher than 10 k, I'm sure I
will get better performance at realistic levels.

So maybe I can use a cheap mux afterall.

It uses so little power that I don't mind having to generate the
+/-2.5 Volt rails. And yes, I know I have to watch out that I don't
drive the inputs beyond those rails.

By the way, if you can recommend a good op-amp for line-level
buffering that will work well at +/- 5V, I would be very interested.
In fact, I would probably just change my design over to using +/- 5V
instead of +/-

If you want to buffer (no gain) I would recommend the SSM2135 opamp, which
is fully specd at +5V and can even drive 32ohm loads(headphones) with low
distortion. For gain stages I recommend the LT1124. The op you mentioned
before OPA2134 will also work on lower voltages. Just avoid the CMOS stuff,
which tends to be noisy in the bass.
To use cheap and small electrolytic capacitors (bridged with a small film
cap) instead of huge film caps it might be suitable to have only a single
supply, i.e. +12V. When switched on this gives a big bump tho, but an output
relay can disconnect or ground the amp if power is not yet ok.

15. I guess I would see 4 or 5$ per op-amp (at low volume) as the
upper limit on price.

Anyway, thanks again, Ban! You seem to almost always have good advice
for people interested in audio stuff.

THX, have a look at my loudspeakers http://www.pupazzo.page.ms/