Hooking up your detector to an external audio amplifier

[W2INR]

Never fear, tapping off the receiver detector is easy! You don't need any matching transformers, etc.

    What you do is use a .1 capacitor in series with the input audio cable into the "auxillary" amplifier/stereo tuner input. You may need a pot there if you have too much drive, probably not. Control the gain with the amp's front panel gain control. If it is a tube receiver, make sure the cap is rated at 400 volts or so. Otherwise, for a solid state receiver a 50v rating is fine.

    You are looking for the junction of the detector output and first audio stage input.

    Put the .1 cap on the end of the shielded cable and poke around the detector output in the receiver. You will find a spot that gives you good drive to the amp with minimum hum or distortion. (Make sure the cable shield is grounded to the receiver chassis.

    Basically you are not using the audio circuits in the receiver. But, you can keep the audio running in the receiver as normal. Hopefully the external cable will not introduce hum.

    I usually put a female RCA jack on the back of the receiver for this connection out. Always use grounded shielded cable for all audio connections.

    (The inner conductor goes to the .1 cap then to the det/audio connection in the RX.) Mount the cap right at this connection.

    Once working, tune the AM broadcash band looking for a station that has a deep music low end. If it doesn't rumble the room, then try increasing the .1 cap to something bigger. The aux input to the amp is usually high impedance so do whatever you need to get a cap that will pass lows.

    One last thing... I found that my Hammarlund SP-600's tube detector was loading down slightly when going into the amp's aux input. I made up an FET op amp that gave a 3meg input for the tube detector and this solved the problem. The symptoms showed up when I swept the receiver and could see a slight distortion of the sine wave on the scope.

    I will talk again later about sweeping your receiver after you get this detector project working. You will be amazed at what you hear now from the Amers!!

    73,

    Tom, K1JJ

[k4kyv]

On my receiver, I found that the shielded cable to the outboard amp rolled off some of the higher audio frequencies.  I built an adaptor unit that plugs directly into the 12AT7 1st audio tube socket.  It takes the audio fed to the grid pin, and feeds a cathode follower, which then drives the outboard amp through the shielded line with no high frequency attenuation.  To get plate voltage for the cathode follower, I pulled out the 6AQ5 final audio tube, since it is not being used anyway, and replaced it with a 7-pin plug that takes +HV intended for the screen grid, and feeds it through a wire lead to the cathode follower unit.  This scheme reduces drain on the rcvr power supply and eliminates two unneeded sources of heat.

The receiver can be restored to stock simply by pulling out the two adaptor plugs and replacing them with the original tubes. In the meantime, there has been no wear on the tubes from sitting there doing nothing but drawing  current and radiating heat.