There has been continued confusion about how to wire the transformer to the Parapix Amp so I took some closeup photos in hopes this will clear up some of the confusion. Below, I present pictures of the wiring used with an old style transformer that has 8 hookup terminals, and with the newer type with 5 terminals. Also presented below is a plot from LAUD showing the frequency response of the amp into a fixed load (5 ohms in this case) with the crossover frequency set at minimum and maximum and with the bass boost on and off.
This is the old style transformer that was originally supplied with these amps. The transformer that Apex Jr. is currently supplying has only five terminals on it. When I mention here about using terminal 1&2 together, on the new transformer that is already just one lug. The lugs on the new transformer are not numbered, but there are two lugs on one side of the unit. These correspond to 1/2 and 3/4 in my description below. There are three lugs on the other side that correlate with lug 5, lugs 6/7 and lug 8.
If you study the picture you will see that there are 8 terminals on the transformer. Terminals pairs 1 & 2, 3 & 4, and 6 & 7 are each wired together and will be treated as one terminal each. The 6-pin Molex connector is at the upper right. Just so there is no confusion, the connector pins are numbered 1 thru 6 starting at the left in this picture. To further clarify, pin #1 is closest to that round blue thing. Now for specific wire hookup details.
Transformer tab 1/2 goes to pin 3 on the connector.
Transformer tab 3/4 goes to the 120v power cord
Tranformer tab 5 goes to pin 4 on the connector
Transformer tab 6/7 goes to pin 6 on the connector
Transformer tab 8 goes to pin 5 on the connector
The first terminal on the switch should already be wired to the other side of the power cordThe remaining terminal on the switch should go to pin 2 on the connector
And finally, the first pin on the connector should be grounded. I always ground mine to the amp faceplate by scraping a clean spot around an existing screw (see the picture below) and using a small lug. Later schematics showed this wire going back to the switch. I never tried this but don't know of the advantages of one way or the other.
You may notice two resistors hanging out the back of the main circuit board. These are two 10Kohm resistors in series to bridge the output of the amp. I ran a lot of experiments with this particular amp. With an oscilloscope I was able to observe high frequency ringing with this amp terminated with a resistor, but it wasn't noticable when hooked up to a speaker and playing music. I have never actually used it in the bridged mode. Other things to note in this picture is the heatsink compound all over the faceplate. On this particular amp I removed the heatsink and faceplate, and polished them for better contact with one another. I assembled them with a mica insulator and heatsink compound.
This picture shows the new style transformer with 5 wiring tabs. I used various color wires to make the hookup easier to understand. Pins 1, 2 and 3 are hooked up with black wire. These are the "power" connections. Pins 4 and 5 are the two secondaries, and they use green wire. Pin 6 is to the center tap and I used white wire. Note that pin 1 is wired to a small lug that is attached to a screw on the faceplate. The black anodizing has been removed from the faceplate around the screw with sandpaper.
This picture shows the newer style transformer but also clearly shows the power cord and switch wiring. The two wires coming from the wall plug are separated inside the amp. One of them goes to the switch(it should have arrived like this) and the other goes to one of the primary terminals on the transformer. It doesn't matter which one!
Here is some real data on the output of the amp at the four extremes of Max and Min crossover frequency and with the bass boost on and off. To turn the bass boost on and off requires moving jumper J1. There are three pins and the jumper can short pins 1 & 2 together to have the bass boost on, or it can be moved to pins 2 & 3 and the boost is off. Alternately a single pole, double throw switch can be soldered to the three pins and then attached to the faceplate so you can control the boost with the switch. Solder a piece of #22 wire to each of the three pins at the jumper, then solder them in the same order to the three tabs on the back of a small switch.
There has been some discussion of a certain amount of hum coming from the speakers hooked up to these amps. This problem can nearly always be cured by tightening the two screws that ground the circuit board to the faceplate. These screws are in a small aluminum block that is just visable in the first picture on this page. One screw connects the circuit board to the aluminum block and the other screw connects the black faceplate to the block. It helps to remove the black coating under this, and any other, connections to the faceplate. When these connections are done right, the hum is just barely audible if you put your ear within an inch of the speaker.
Some other sources of Parapix information. They can be purchased from Apex Jr. . Schematics are currently available at Parapix Schematics. Another source of information is John Janowitz. His web page has several projects, the schematics, and a whole bunch of other interesting things.