Op amp supply-rail question
 

I'd like to build a tone control circuit using the LF353 dual op amp.
The circuit has bass, mid range and treble adjustments.

The problem:
The circuit (see
http://www.ece.ucsb.edu/courses/ECE0...remi/lf353.pdf
circuit on page 7) uses dual voltage rails of +15 and -15 volts,
though for line input levels, this will probably work down to +8 and
-8 volts. But I want to run it from a single +12 volt supply.

Usually, two 100k resistors across the supply, their junction
connected at the non-inverting input, alows an op amp to run from a
single supply, the output floating at about half supply volts. This is
fine for my use, but will it work with a dual op amp? I've searched my
documentation, but found no reference to doing this for dual op amps.
If it will work, will say two 100k resistors (one to +12v the other to
ground, junction to pin 3) work? Does it need another 100k or so
between their junction to pin 3 to isolate the input audio from the
two 100k resistor potential devider?

The first op ampin the LF353 application notes is only a buffer in
the tone control circuit, so I guess I could avoid the problem and
use a single CA3130 or similar op amp and use the 100k devider trick,
and still use the tone control components. (I doubt the LF353 has any
special properties for this particular use.)

If anyone has another three-control op amp circuit wthat runs from a
single 12v supply, I'd be interested.

Thanks...

Ian

In article , ian@morse-
code.NO.SPAM.TODAY.fsnet.co.uk says...

I'd like to build a tone control circuit using the LF353 dual op amp.
The circuit has bass, mid range and treble adjustments.

snippety

though for line input levels, this will probably work down to +8 and
-8 volts. But I want to run it from a single +12 volt supply.

snippety-two

The solution: You can easily get aftermarket DC/DC converter
modules which will take your +12 in, and provide bipolar 15V out.

Some examples:

Power-One, p/n DFC10U24D15, in stock at DigiKey for about $71. Vin
range of 9-36, Vout +/-15 at 300mA per, isolated outputs. .PDF spec
sheet is at:

http://www.power-one.com/resources/p...eet/dfc10d.pdf

A less expensive solution with slightly lower output voltages can
be found at Jameco on this link.

http://www.jameco.com/webapp/wcs/sto...play?langId=-1
&storeId=10001&catalogId=10001&productId=161728&pa =161728PS

If the link doesn't work, try searching for Mean Well part
#DKE15A-12. This one will give you +/-12V at 625mA each, isolated
output, for $35 and change.

I'm sure there are plenty of other manufacturers with products
that can address your application. Happy hunting.


--
Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
(Known to some as Bruce Lane, ARS KC7GR,
kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
"If Salvador Dali had owned a computer, would it have been equipped
with surreal ports?"

Ian Liston-Smith wrote:

: Usually, two 100k resistors across the supply, their junction
: connected at the non-inverting input, alows an op amp to run from a
: single supply, the output floating at about half supply volts. This is
: fine for my use, but will it work with a dual op amp? I've searched my


try it and see

It will work just fine with a couple of caveats. One, there needs to be a
goodly sized filter capacitor at the junction of the 100K resistors. If you
can stand a couple of seconds "warm up", a 100 uF will serve admirably.
Two, if the noninverting inputs (+ inputs) have any function other than bias
(i.e. if there is feedback to them or they are an active input) then yes,
you will need some sort of isolation resistor network. How you gin that
network up and keep voltage drop from happening is up to you.

Jim



Usually, two 100k resistors across the supply, their junction
connected at the non-inverting input, alows an op amp to run from a
single supply, the output floating at about half supply volts. This is
fine for my use, but will it work with a dual op amp?

On Sat, 20 Aug 2005 12:14:53 +0100, Ian Liston-Smith
wrote:


I'd like to build a tone control circuit using the LF353 dual op amp.
The circuit has bass, mid range and treble adjustments.

snippage

Usually, two 100k resistors across the supply, their junction
connected at the non-inverting input, alows an op amp to run from a
single supply, the output floating at about half supply volts. This is
fine for my use, but will it work with a dual op amp? I've searched my
documentation, but found no reference to doing this for dual op amps.
If it will work, will say two 100k resistors (one to +12v the other to
ground, junction to pin 3) work? Does it need another 100k or so
between their junction to pin 3 to isolate the input audio from the
two 100k resistor potential devider?

Use two 10k and at least 100uf or more of filtering . Works in
designs I've built. Also insure the supply rails are well filtered
and the input and output capacitors are suitably sized.

The first op ampin the LF353 application notes is only a buffer in
the tone control circuit, so I guess I could avoid the problem and
use a single CA3130 or similar op amp and use the 100k devider trick,
and still use the tone control components. (I doubt the LF353 has any
special properties for this particular use.)

Use the buffer, the circuit will behave better with input sources of
unknown impedence.

The 353 is a nice low noise opamp.

If anyone has another three-control op amp circuit wthat runs from a
single 12v supply, I'd be interested.

There are but the one you have with an artificial ground point will
behave well.


Allison
KB1GMX

Hello,

Use 10K resistors instead of 100K resistors and be sure to include a fat
Capacitor from the midpoint to ground. Then tie the ground end of the 10k
resistor on pin 3 and pin 5 to the junction of the resistor divider and cap.
(do not tie it to pin 3 directly). Should work with limited dynamic range.

Regards,
Pete
KB1LZH
"Ian Liston-Smith" wrote in
message ...

I'd like to build a tone control circuit using the LF353 dual op amp.
The circuit has bass, mid range and treble adjustments.

The problem:
The circuit (see
http://www.ece.ucsb.edu/courses/ECE0...remi/lf353.pdf
circuit on page 7) uses dual voltage rails of +15 and -15 volts,
though for line input levels, this will probably work down to +8 and
-8 volts. But I want to run it from a single +12 volt supply.

Usually, two 100k resistors across the supply, their junction
connected at the non-inverting input, alows an op amp to run from a
single supply, the output floating at about half supply volts. This is
fine for my use, but will it work with a dual op amp? I've searched my
documentation, but found no reference to doing this for dual op amps.
If it will work, will say two 100k resistors (one to +12v the other to
ground, junction to pin 3) work? Does it need another 100k or so
between their junction to pin 3 to isolate the input audio from the
two 100k resistor potential devider?

The first op ampin the LF353 application notes is only a buffer in
the tone control circuit, so I guess I could avoid the problem and
use a single CA3130 or similar op amp and use the 100k devider trick,
and still use the tone control components. (I doubt the LF353 has any
special properties for this particular use.)

If anyone has another three-control op amp circuit wthat runs from a
single 12v supply, I'd be interested.

Thanks...

Ian

Ian:

A friend of mine designed a bass/treble equalizer around the LF353, using a
single supply. Several people (including me) have duplicated this circuit
with good results. Perhaps you can use the DC parts of the circuit in your
3 channel equalizer: www.tleone.com/roger/mic_eq.jpg

Roger K6XQ

Ian Liston-Smith wrote:
I'd like to build a tone control circuit using the LF353 dual op amp.
The circuit has bass, mid range and treble adjustments.

The problem:
The circuit (see
http://www.ece.ucsb.edu/courses/ECE0...remi/lf353.pdf
circuit on page 7) uses dual voltage rails of +15 and -15 volts,
though for line input levels, this will probably work down to +8 and
-8 volts. But I want to run it from a single +12 volt supply.

Usually, two 100k resistors across the supply,

What is magical about 100k?

Thanks to all who have replied. I've certainly got some ideas now.

On Wed, 24 Aug 2005 23:14:02 -0700, "Dana H. Myers"
wrote:

Ian Liston-Smith wrote:
I'd like to build a tone control circuit using the LF353 dual op amp.
The circuit has bass, mid range and treble adjustments.

The problem:
The circuit (see
http://www.ece.ucsb.edu/courses/ECE0...remi/lf353.pdf
circuit on page 7) uses dual voltage rails of +15 and -15 volts,
though for line input levels, this will probably work down to +8 and
-8 volts. But I want to run it from a single +12 volt supply.

Usually, two 100k resistors across the supply,

What is magical about 100k?