dave
Guest
Mon Oct 24, 2011 5:25 pm
I have a digital recoder similar to this.
http://www.alesis.com/protrack
It works fine on - batteries. Then I build my own -3V DC power unit
for it. It still works fine with that but there is hum *if I hold the
recorder while recordin*. If I don't actually touch the recorder the
hum is not there. I guess it's mains hum as there is no hum when run
on batteries..
The psu I built is spot-on the correct voltage and easily provides the
current demands of the recoder (500mA at most). The transformer I used
is screened and the screen to tied to ground. One side (+) of the DC
output is also grounded. It uses a LM337 regulator chip an has the
usual configuration. There is a 1u tant capacitor across the output
(as per design notes for this chip).
Any ideas how to cure(!) this hum problem appreciated.
krw@att.bizzzzzzzzzzzz
Guest
Mon Oct 24, 2011 7:56 pm
On Mon, 24 Oct 2011 18:25:42 +0100, dave <dave_at_127.0.0.1> wrote:
Quote:
I have a digital recoder similar to this.
http://www.alesis.com/protrack
It works fine on - batteries. Then I build my own -3V DC power unit
for it. It still works fine with that but there is hum *if I hold the
recorder while recordin*. If I don't actually touch the recorder the
hum is not there. I guess it's mains hum as there is no hum when run
on batteries..
The psu I built is spot-on the correct voltage and easily provides the
current demands of the recoder (500mA at most). The transformer I used
is screened and the screen to tied to ground. One side (+) of the DC
output is also grounded. It uses a LM337 regulator chip an has the
usual configuration. There is a 1u tant capacitor across the output
(as per design notes for this chip).
Quote:
Any ideas how to cure(!) this hum problem appreciated.
This is usually caused by a "ground loop" between the power supply, the unit,
and you. Often, this is caused by poor isolation in the power supply. Try
using an isolation transformer or a better power supply.
Phil Allison
Guest
Mon Oct 24, 2011 10:25 pm
"dave"
Quote:
I have a digital recoder similar to this.
http://www.alesis.com/protrack
It works fine on - batteries. Then I build my own -3V DC power unit
for it. It still works fine with that but there is hum *if I hold the
recorder while recordin*. If I don't actually touch the recorder the
hum is not there. I guess it's mains hum as there is no hum when run
on batteries..
** Your body is the source of the hum and the recorder is built in a poorly
shielded plastic case.
In any room where there is AC wiring installed, human bodies have an AC
potential of up to several volts. When you hold the recorder, some of this
voltage is injected into the electronics - probably the internal mic wiring
or mic preamps.
When the recorder is powered by batteries, there is no injection when held
because it is all at the same AC potential as you.
If you attach a metal plate to the back of the recorder and connect that to
the DC supply common, then long as you hold onto that plate with your
fingers it should fix the hum.
.... Phil
Charles
Guest
Mon Oct 24, 2011 11:03 pm
"dave" wrote in message news:ra7ba7l16iahhpmgon2gu96rctrn51vq92_at_4ax.com...
I have a digital recoder similar to this.
http://www.alesis.com/protrack
It works fine on - batteries. Then I build my own -3V DC power unit
for it. It still works fine with that but there is hum *if I hold the
recorder while recordin*. If I don't actually touch the recorder the
hum is not there. I guess it's mains hum as there is no hum when run
on batteries..
The psu I built is spot-on the correct voltage and easily provides the
current demands of the recoder (500mA at most). The transformer I used
is screened and the screen to tied to ground. One side (+) of the DC
output is also grounded. It uses a LM337 regulator chip an has the
usual configuration. There is a 1u tant capacitor across the output
(as per design notes for this chip).
Any ideas how to cure(!) this hum problem appreciated.
Your body, when touching the recorder, acts as a capacitor to ground and it
will support the flow of alternating current (hum). That induced voltage is
joining the desired signal.
Kill the ground loop, if you can. A ground loop is the path where ac
voltages return to ground. Might be as simple as changing the connection to
the ac receptacle.
Phil Allison
Guest
Mon Oct 24, 2011 11:27 pm
"Charles"
** The only difference between the "Charles" lunatic and parrot is parrots
are smarter.
.... Phil
dave
Guest
Tue Oct 25, 2011 10:27 am
On Tue, 25 Oct 2011 09:25:57 +1100, "Phil Allison" <phil_a_at_tpg.com.au>
wrote:
Quote:
"dave"
I have a digital recoder similar to this.
http://www.alesis.com/protrack
It works fine on - batteries. Then I build my own -3V DC power unit
for it. It still works fine with that but there is hum *if I hold the
recorder while recordin*. If I don't actually touch the recorder the
hum is not there. I guess it's mains hum as there is no hum when run
on batteries..
** Your body is the source of the hum and the recorder is built in a poorly
shielded plastic case.
In any room where there is AC wiring installed, human bodies have an AC
potential of up to several volts. When you hold the recorder, some of this
voltage is injected into the electronics - probably the internal mic wiring
or mic preamps.
When the recorder is powered by batteries, there is no injection when held
because it is all at the same AC potential as you.
If you attach a metal plate to the back of the recorder and connect that to
the DC supply common, then long as you hold onto that plate with your
fingers it should fix the hum.
... Phil
Thanks - will give it a try. I did think it was not a good idea to
make such a sensitive device in a plastic box - I guess alumin. one
would send the proce right up.
