Re: [sfanalog] Digest for sfanalogcircuits@googlegroups.com - 1 Message in 1 Topic

[Mark Alexander]

Hi Michael,

If it IS feedback through the power supply, that would mean the power supply is drooping, in which case decoupling capacitors could be the answer.  This should be an easy thing to try, and it may just work.  

I'm assuming you're running the oscillators at audio frequencies.  If the droop is happening in that frequency range, you'll need some large-ish caps to handle it (100uF electrolytic should do it; something in the 10uF to 47uF range would probably work fine too).  If instead the droop is happening in the frequency range of the edges of the 555 square waves, a handful of 1uF and 0.1uF ceramics will do the job.  For the small caps, make an effort to keep lead lengths short.

Let me know how it goes!

-mark

On Sun, Nov 24, 2013 at 10:43 AM, <sfanalog...@googlegroups.com> wrote:

  Today's Topic Summary

Group: http://groups.google.com/group/sfanalogcircuits/topics

• multi-source audio projects looses sources when using the same power supply [1 Update]

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

Michael Shiloh <michaels...@gmail.com> Nov 23 05:21PM -0800  

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


 

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[Michael Shiloh]

Great idea. I'll have the student try that and see what helps. Also I'll
stick a scope on the power lines and see what I find.

On 11/24/2013 12:05 PM, Mark Alexander wrote:
> Hi Michael,
>
> If it IS feedback through the power supply, that would mean the power
> supply is drooping, in which case decoupling capacitors could be the
> answer. This should be an easy thing to try, and it may just work.
>
> I'm assuming you're running the oscillators at audio frequencies. If the
> droop is happening in that frequency range, you'll need some large-ish caps
> to handle it (100uF electrolytic should do it; something in the 10uF to
> 47uF range would probably work fine too). If instead the droop is
> happening in the frequency range of the edges of the 555 square waves, a
> handful of 1uF and 0.1uF ceramics will do the job. For the small caps,
> make an effort to keep lead lengths short.
>
> Let me know how it goes!
>
> -mark
>
>
> On Sun, Nov 24, 2013 at 10:43 AM, <sfanalog...@googlegroups.com> wrote:
>
>> Today's Topic Summary
>>
>> Group: http://groups.google.com/group/sfanalogcircuits/topics
>>

>> - multi-source audio projects looses sources when using the same power
>> supply <#1428b6cc17516c49_group_thread_0> [1 Update]

>>
>> multi-source audio projects looses sources when using the same power

>> supply<http://groups.google.com/group/sfanalogcircuits/t/7f4959583838dc5e>

[Andy Water]

How about 1uF DC-blocking caps on all the inputs?

Most mixer schematics don't show those caps but thats cuz they are assuming AC inputs. A square wave generator going from 0-5v (for example) would have a DC component proportional to the duty cycle.


Andy.

[Michael Shiloh]

Good point. I forgot about this.

Would be an easy experiment.