Power Supply Options

[PAiA-Scott]

I am a novice electronics hobbiest who has built a few of your kits including the RIAA Phono Preamp. I have been running this awesome amp for about 2 years on dual 9V batteries as per the build instructions. I was hoping to add a 9V wall wort for more reliable power and am a little confused where to tie that in to the circuit. I have the power supply bridged as shown in the instructions and thought I might be able to tie the wall power into the main connections at point A and B. I can take out the batteries completely but was unsure exactly where to tie in the DC 2 pin connector.

Any tips?

-0-0-0-0-0-0-0-

A single DC adapter can be used but it takes some extra circuitry, then it is worthwhile considering the 9770R-12, a dual-polarity, regulated DC power supply kit (it uses a wall-mounted transformer 12VAC output and some extra circuitry for the DC output voltages.

With the two nine-volt batteries wired in series, the plus of one to the minus of the other, connecting the mid-point to "ground"/circuit-common, or the 0V DC reference, the other ends of this series connection result in positive and negative DC supplies (+9 0 -9)

A wall-mounted DC power source can have its single polarity DC output (it is positive DC if the minus pole is connected to ground/circuit-common -- it is negative DC if the plus pole is connected to ground/circuit-common). Two could be used, wired in series as with the nine-volt batteries, but then you have two chunks to plug in somewhere (and this might be OK -- they could go on opposite sides of an extension cord and perhaps even put in a box as a "power brick"). With only one though, its output must be divided to work as a dual-polarity DC supply. Connecting two equal value, series-connected resistors across a single DC supply works to provide 1/2 V positive, zero, and 1/2V negative. It would allow you to use one DC source to power circuitry expecting a dual-polarity DC supply. It is not optimum though. The impedance of the "ground"/circuit-common/zero-volt reference is half of the resistor value used in the series-connected resistor voltage divider. The lower the better, zero is ideal. Practical is in a range of hundreds of ohms to thousands of ohms. And, the series resistance connected across the single DC supply is a load on it as current flows through the two. Say you have two 1000 ohm resistors series connected to divide the output in half, that's 2000ohms connected across, say, an 18V DC adapter. 18V divided by 2000 Ohms results in 36 mA of current flow and that's without anything such as the preamplifier connected. The pre-amp will only have about 10 to 20 mA through it, so it isn't such a problem having the combined current flow of about 56 mA from a DC Adapter -- they're usually rated at 100mA and likely much more (300mA, 500mA, 1000mA (1A)).

With the 9770R-12, circuitry (rectifiers, filter capacitors, and voltage regulators) take the AC voltage from a wall-mounted transformer and generate clean, stable DC power supply voltages of +12 0 -12. The zero volt circuit-common/"ground" is much closer to zero without the waste of power flowing through a resistor divider. Also, since the circuity works to double the output from the transformer, the 12VAC from it results in positive 12VDC, zero, and negative 12VDC -- 24VDC when measured from the negative to the positive supply.

The manual for the 9770R-12 is posted here (and included with the kit):

http://www.paia.com/talk/viewtopic.php?f=6&t=127

The product page for it is here:

http://www.paia.com/proddetail.asp?prod=9770R-12

Just in case you might be interested in housing it all with room for expansion, the FR-7 can mount a 9770 series power supply behind an FR-PWP panel in the end panel, leaving ten single spaces across the front. One could be the panel for the 9802 RIAA PhonoPreAmp.

FR-7
http://www.paia.com/proddetail.asp?prod=FR-7

FR-PWP
http://www.paia.com/proddetail.asp?prod=9802FRM

Some of the 9700 series modules (mic preamplifier, VCA, etc.) could be put in other spaces, or blank panels for custom, user-specific, or retro-fit builds/applications.

9700 series modules
http://www.paia.com/p9700s.asp