In need of a stable 12V-14V source with wall connection.

[KB3VIS]

I have been trying to deal with a noise issue that my TenTec 1320 kit is experiencing.  I have found the source of the issue to be its extreme sensitivity to an unregulated and unfiltered power supply.  I have tried three wall-warts and a PC power supply's 12V rail to no avail.  It is so sensitive, that even 5 mV noise makes it unusable.  I have tried it sourced from a 12V battery, and it has been excellent then.  I am looking for a permanent power source with comparable regulation and filtering to that of a battery.  How stable and well filtered are the bench-top power supplies? Does anyone have any permanent QRP power supplies that works for them?

[W4OP]

Are you certain the problem is ripple? Many if not all of today's regulated wall warts are switchers- and they generate enormous amounts of broadband noise. I found this out when looking for a small supply for my Lowe HF-150. I put a loop of wire near the supply and connected up to my spectrum analyzer. The resultant broadband spectrum noise was unbelievable.

Dale W4OP

[WB6BYU]

Many of the wall warts are unfiltered, or minimally filtered.  I had to add an extra 10uF
(100uF might be better) across one to run my keyer, otherwise performance was erratic.

Many switching power supplies don't regulate properly without some minimum amount of
current draw, which may be more than the rig draws on receive.  Some computer supplies
are designed for constant current draw and can have a lot of transients on SSB or CW
because they aren't designed for fast changes in the load.

A simple approach is to use a wall-wart rated 18V or so (AC or DC) and follow it with
a filter cap and a 7812 three-terminal regulator.  It's pretty simple and straightforward,
and I haven't had any issues with noise on such a circuit.  Just make sure that the
wall wart and the regulator have sufficient current rating, and enough heat sinking for
the regulator.  There are some telephone transformers that are rated 24V @ 2A or
so that are good for this, though the higher output voltage means more dissipation
in the regulator.

[KQ6Q]

Try using a small Gel Cell - 4Ah will be plenty, and charge it with a wall wart when you're NOT operating. That will give you noise and ripple free operating, with least expense. You can use the same battery for outdoor portable, and when you feel flush, get a folding solar panel to go with it!

[KB3VIS]

So I tried using a L7812CV with two 10uF caps on each side, and it didn't reduce the noise significantly.

[WB6BYU]

Oh, yes...  that sounds like the the hum that is typical when direct conversion receivers
are run off of an AC supply.  If so, it really doesn't have anything to do with the ripple
on the power supply - instead it is due to local oscillator radiation from the receiver.
This signal is picked up by the power supply wiring, modulated in the power supply
rectifier diodes, and re-radiated, where it is picked up by the antenna and fed into
the receiver.  Since it is zero beat with the VFO, only the demodulated sidebands are
heard.  They may be rather raspy due to harmonics of the AC line frequency generated
in the process.

KK7B has a description of this problem in Experimental Methods in RF Design.
There is a further discussion in section 4 of this web page: 
http://www.qrp.pops.net/hf-ragbag.asp

Basically the solution is to shield your VFO so the signal can't be picked up in other
parts of the circuit, and especially by equipment outside the receiver itself.  This
also reduces microphonics.


But here is a test:  if the hum is getting into the audio stages through the DC line,
then you should still hear it if you disable the oscillator.  Try it and see.  If the hum
goes away, then the problem isn't the audio stages, because they shouldn't be
affected.  Similarly, if the receiver is picking up a modulated VFO signal via the
antenna, then disconnecting the antenna should reduce the hum.  (But if the
receiver isn't shielded, it might be picking it up directly into the circuitry.)  Try
shorting out or disconnecting the antenna input to see if that affects the hum.

Note that this problem is unique to direct conversion receivers:  superhet
receivers don't radiate on the signal frequency, so the same power supply used
with a different receiver might not show any hum at all.

[KB3VIS]

I tested multiple Wall Warts with the radio, A and B.  One in particular had a decreased amount of noise when used with the receiver, which is A.  I compared A and B at the same time on my oscilloscope and A had less than half the noise amplitude of B.  I appreciate the idea of the feedback, but I feel like this shows its just a one way noise issue.  If I were to throw a 10,000 uF cap between the leads, do you think that would absorb the noise?

[KB3VIS]

I think I'm just going to make my own power supply with a transformer, rectifier, cap, and L7812.

[W4OP]

The Ten Tec 1320 is not a direct conversion receiver- it is a single conversion superhet according to Ten Tec.
Again, I would ask you if the power supplies you are using are switchers- most wall warts today are. Find  a simple full wave rectifier and filter wall wart and give it a try. The cheap switchers radiate broad band RF noise. There is no way to stop it as even with toroids in and out, the noise radiates through the plastic case.

Dale W4OP

[WB6BYU]

I think I'm just going to make my own power supply with a transformer, rectifier, cap, and L7812.

While the 7812 and other three-terminal regulators are quite convenient, there
are some design considerations that might not be apparent.

1) the minimum voltage drop is about 3.5V through the regulator.   That means it needs to
see at least 15.5V on the input pin throughout the AC cycle.  This is a function of the
transformer voltage, input capacitor size, and current draw.  (And the output voltage of
may small transformers varies with load.)  This is why such power supplies often start
with a 18V transformer:  a 12VAC transformer may allow the regulator to drop out of
regulation between cycles of the AC waveform.

2) you probably need more capacitance.  I'd stick in 2200uF or so, depending on what
I can find in my junkbox.  The ARRL handbook has a discussion on choosing a minimum
capacitor size in the Power Supplies section.

3) with the higher transformer voltage comes higher heat dissipation in the regulator, but
if you aren't drawing more than about 100mA this shouldn't be a big problem.  If it gets
too warm during use, bolt it to the chassis or a scrap of sheet metal for a heat sink.

3) Make sure you use sufficient capacitance wired right across the input and output
pins to prevent oscillation - a 10uF tantalum capacitor is often used, though smaller caps
will work at low output currents.  Consult the datasheet.

(Can a voltage regulator oscillate at HF?  SM0VPO built a 160m transmitter using an LM317.)

4) You can usually tell the switching wall-warts from the transformer type by weight

[KB1GMX]

the 1320 is indeed a single conversion superhet, it also has a hot receiver so if you connected to a wall wart that shares power with other warts or CF lamp you WILL hear the switching noise via the PS.  If it's bad you may hear it via the antenna.

This assumes correct assembly.

Having worked on a few they do indeed work well, when assembled right.

As to power a 2.3AH gell cell with run it for a very long time on receive and many hours transmitting.

A stack of AA cells will tuns the RX for about 40 -50 hours and TX for a few hours. 

Allison

[KB3VIS]

So I created my own power supply with a transformer, full-wave rectifier, capacitors, and a regulator.  This home-made power supply worked very well with very little noise (couldn't detect above ambient interference).  It worked perfectly for receiving, but the 0.85 amp draw on transmit was too much for the 10VA transformer I got.  I'll just have to find a better transformer and then I should be golden.  Thanks everyone for their ideas and I highly recommend building your own.  Just remember that the rated voltage of the transformer is RMS so you have to multiply by 1.414 ( sqrt 2 ) to get the voltage heading into the regulator and caps.

[N0SOY]

What about a deep cycle battery with a trickle charger.  I use one for many signal experiments when I need a clean power suppy.  Unplug the charger when using and plug in when done.