Fwd: multi-source audio projects looses sources when using the same power supply

[Michael Shiloh]

One of my students is building what should seem like a simple circuit,
but having issues that I can't figure out.

Suppose you build three 555 based oscillators and want to mix the sound
into one amplifier. The mixer is the basic LM741 mixer straight out of
forrest mims, with each input isolated via a 10K resistor. The LM741
provides a gain of 10 to restore signal strength, and is then fed into
an LM386 power amp.

All is well with separate power supplies, but if the the power supplies
are shared, we hear only one oscillator instead of each one.
Furthermore, if the pitch of the oscillators is adjustable, the one
oscillator that is heard is the one most recently adjusted.

I think; it's a little hard to hear exactly what's going on.

I guess we're getting some feedback through the power supply. Is there a
way to avoid this without separate power supplies?

--
Michael Shiloh
teachmetomake.com/wordpress
KA6RCQ

Educational Materials coordinator at Arduino.cc
Electronics, Robotics, Digital Fabrication, and Arduino educator
California College of the Arts
San Francisco Art Institute
San Francisco State University

[Joe]

might help to see the circuit.

by shared power supplies do you mean one shared supply?

if you disconnect the power amp does the problem persist?

[Michael Shiloh]

Thanks everyone for your suggestions. As it turns out the first
suggestion of 100uF bypass caps solved the problem. I told my student to
install them as close to the 555 square wave generators as possible, and
they did the trick.

Hi Joe,

Yes, one shared supply. Good suggestion re disconnecting the power amp.
I'll keep that in mind if we run into further problems. I have had
issues in the past with LM386 oscillating easily and so this is suspect
indeed.

M

On 11/24/2013 04:55 PM, Joe wrote:
> might help to see the circuit.
>
> by shared power supplies do you mean one shared supply?
>
> if you disconnect the power amp does the problem persist?
>
> On Saturday, November 23, 2013 5:21:45 PM UTC-8, Michael Shiloh wrote:
>>
>> One of my students is building what should seem like a simple circuit,
>> but having issues that I can't figure out.
>>
>> Suppose you build three 555 based oscillators and want to mix the sound
>> into one amplifier. The mixer is the basic LM741 mixer straight out of
>> forrest mims, with each input isolated via a 10K resistor. The LM741
>> provides a gain of 10 to restore signal strength, and is then fed into
>> an LM386 power amp.
>>
>> All is well with separate power supplies, but if the the power supplies
>> are shared, we hear only one oscillator instead of each one.
>> Furthermore, if the pitch of the oscillators is adjustable, the one
>> oscillator that is heard is the one most recently adjusted.
>>
>> I think; it's a little hard to hear exactly what's going on.
>>
>> I guess we're getting some feedback through the power supply. Is there a
>> way to avoid this without separate power supplies?
>>
>> --
>> Michael Shiloh
>> teachmetomake.com/wordpress

>> KA6RCQ<http://www.google.com/url?q=http%3A%2F%2Fteachmetomake.com%2FwordpressKA6RCQ&sa=D&sntz=1&usg=AFQjCNFnpj552GSbd3nkYVLxoj3rUjxZLw>