powering 9v preamp circuit from XLR, AAA, wall wart w/o hum

[Mad Scientist Jr]

I built a simple preamp from this schematic -

http://www.electronicsteacher.com/circuits-and-diagrams/audio-preamp-circuits/preamp.gif

For the most part it works great - it makes guitar pickups and
microphones sound much louder.

However I do have a few issues I was hoping I could find some help
on.

The sound is pretty clean if you use a battery. However, I tried a 9v
battery eliminator wall wart and got lots of loud background hum. I
think I need to filter the power supply to clean up the sound. Does
anyone know how to do this?

Also I would like to get it to run off a AAA battery. I have a
condenser mic that uses such a battery so I am hoping the circuit can
run off of 1.5v. I think you would just have to change the two 10k
resistors to something lower. Is this correct? Can someone explain how
to calculate the values needed?

Finally, how would you wire it to be phantom powered by the XLR
connector on a mixer?

Much appreciated...

 

[BD]

I built a simple preamp from this schematic -

http://www.electronicsteacher.com/circuits-and-diagrams/audio-preamp-circuits/preamp.gif

For the most part it works great - it makes guitar pickups and
microphones sound much louder.

However I do have a few issues I was hoping I could find some help
on.

The sound is pretty clean if you use a battery. However, I tried a 9v
battery eliminator wall wart and got lots of loud background hum. I
think I need to filter the power supply to clean up the sound. Does
anyone know how to do this?

Also I would like to get it to run off a AAA battery. I have a
condenser mic that uses such a battery so I am hoping the circuit can
run off of 1.5v. I think you would just have to change the two 10k
resistors to something lower. Is this correct? Can someone explain how
to calculate the values needed?

Finally, how would you wire it to be phantom powered by the XLR
connector on a mixer?

Much appreciated...

The hum you're hearing is AC hum. There's a chance that the wall wart are
outputing AC instead of DC. You would have to put diodes in to rectify it to
DC. In that case you'd have to put in a filter (electrolyte capacitor).

Get a wall wart that fits. Be sure that the outputs are DC.
The specs are usually molded in the plastic case or on a label.

 

[Jim]

The hum you're hearing is AC hum. There's a chance that the wall wart are
outputing AC instead of DC. You would have to put diodes in to rectify it to
DC. In that case you'd have to put in a filter (electrolyte capacitor).

Get a wall wart that fits. Be sure that the outputs are DC.
The specs are usually molded in the plastic case or on a label.

I suspect it's a DC supply and he's hearing excessive ripple, 120 Hz,
from an unregulated improperly filtered supply. This is basic stuff.
I'd measure the voltage with the circuit on. If it's in the right
range, I'd add an appropriate electrolytic filter capacitor. Use on
rate at at least 15 to 20V. You could try 100 uF to start, but you
don't need a huge capacitors with small current supplies.

If that DC supply is putting out too much voltage (not uncommon when you
use a wallwart on a circuit that doesn't draw much current), either buy
a regulated supply, or put a voltage regulator on that one.

 

[sycochkn]

I suspect it's a DC supply and he's hearing excessive ripple, 120 Hz, from
an unregulated improperly filtered supply. This is basic stuff. I'd
measure the voltage with the circuit on. If it's in the right range, I'd
add an appropriate electrolytic filter capacitor. Use on rate at at least
15 to 20V. You could try 100 uF to start, but you don't need a huge
capacitors with small current supplies.

If that DC supply is putting out too much voltage (not uncommon when you
use a wallwart on a circuit that doesn't draw much current), either buy a
regulated supply, or put a voltage regulator on that one.

The circuit will run on anything from 3 to 9 volts.

Bob

 

[Gerard Bok]

I built a simple preamp from this schematic -

http://www.electronicsteacher.com/circuits-and-diagrams/audio-preamp-circuits/preamp.gif

For the most part it works great - it makes guitar pickups and
microphones sound much louder.

However I do have a few issues I was hoping I could find some help
on.

The sound is pretty clean if you use a battery. However, I tried a 9v
battery eliminator wall wart and got lots of loud background hum.

That's because this (very basic) amplifier has a flaw.

Any noise present on the powerline gets fed into the input.
Replace R1 by 2 4K7 resisors in series. And connect a capacitor,
say 10, 25 or 47 uF between the junction of those 2 resistors and
ground. (plus to the junction, minus to ground,)

You may also want to connect a capacitor, say 100 uF across the
powersupply connection points.

 

[Les Cargill]

I built a simple preamp from this schematic -

http://www.electronicsteacher.com/circuits-and-diagrams/audio-preamp-circuits/preamp.gif

For the most part it works great - it makes guitar pickups and
microphones sound much louder.

However I do have a few issues I was hoping I could find some help
on.

The sound is pretty clean if you use a battery. However, I tried a 9v
battery eliminator wall wart and got lots of loud background hum. I
think I need to filter the power supply to clean up the sound. Does
anyone know how to do this?

Try a different wall wart. Of all places, I have had extraordinary
luck with wall warts from Target in the past. If Radio Shack's
stock of warts is like in the early '90s, those are pretty clean, too.

Also I would like to get it to run off a AAA battery. I have a
condenser mic that uses such a battery so I am hoping the circuit can
run off of 1.5v. I think you would just have to change the two 10k
resistors to something lower. Is this correct? Can someone explain how
to calculate the values needed?

You really need to get the data books out for this one. Somebody
might know, though - and the schem says "3 to 9v"...

Finally, how would you wire it to be phantom powered by the XLR
connector on a mixer?

Resistor network based on the fact that you get 48VDC between
3 & 1 and 2 & 1.


From http://en.wikipedia.org/wiki/Phantom_power :


"The signal conductors are positive, both fed through resistors of equal
value (for 48 volt phantom powering the standard value is 6.81 kΩ), and
the shield is ground."

 

[Marra]

I noticed you have no power supply decoupling. maybe add 10uf and
0.1uf across battery.
Needs to be in shielded box too.

 

[Mad Scientist Jr]

I noticed you have no power supply decoupling. maybe add 10uf and
0.1uf across battery.
Needs to be in shielded box too.

Thanks everyone for your replies.

Would it help to put it in this aluminum box and soldering the box to
ground?

http://www.geocities.com/usenet_daughter/preamp_shielding_box.jpg

Thanks...

 

[Lord Garth]

Thanks everyone for your replies.

Would it help to put it in this aluminum box and soldering the box to
ground?

http://www.geocities.com/usenet_daughter/preamp_shielding_box.jpg

Thanks...

Plan on have great trouble soldering to aluminum.

 

[Mad Scientist Jr]

Thanks for your reply.

Plan on have great trouble soldering to aluminum.

I thought that too but I pulled that box out of an old mixer, and it
already has a solder glob on it that I could solder to.

BTW what kind of soldering iron do you need to solder to aluminum? How
many watts? Or can you get a good electrical connection to ground by
drilling a hole in the box & sanding the surface with steel wool and
screwing the ground wire to it?