To get the audio curve flat and near perfect, your 75A4 may or may not require the same time and experimental effort it took me to get my restored Hammarlund SP-600 sounding good using the detector output.
Here's what I had to do to get exceptional in-house audio AND mini-disc recording playbacks. It took me a month of playing around before the guys finally gave it the OK on the air.
Here's the steps I took:
1) Tap off the detector
2) Build up an FET op amp as a buffer stage from the detector output set at five megohms input.
3) Use this buffer to drive two channels of a standard 12 band (2 channel) EQ. (the buffer gets split into two paths using a simple resistive divider) One channel of the EQ goes directly into the mini disc recorder - the other EQ channel goes into the stereo amp aux input which drives four large speakers in the ceiling/wall four corners. This gives SEPARATE in-house or recorder playback audio curve versatility using the two EQ channels..
4) The final mini disc playback goes directly into the UNPROCESSED transmitter audio input.
6) Add a phase reversal switch after the buffer to obtain the correct phase for on the air playbacks. (This way you will have the same microphone phase and recording playback phase)
Originally, without the 5 meg buffer, the detector was being loaded down by the low impedance (1K) of the stereo amplifier and/or minidisc recorder. I could see a slightly distorted waveform and peaks on the scope caused by this loading when swept with an audio modulated RF generator into the RX.
In addition, the extreme high frequency response was down due to the roll off caused by long audio cables coming off the detector to the audio amp.
If that's not enough, after the buffer was in place, the guys then complained that the high end was down from the original during playbacks. I attributed it to the I.F. rolloff of the SP-600 even at 8kc or 16kc bandwidth..
After adding a receiver EQ that took the bumps out, adding the buffer, phase reversal switch and experimenting with the EQ curve, I was told the playbacks sounded quite transparent. In house audio was fabulous using four speakers mounted on the far corners of the wall/ceiling. The "in-house" EQ channel can be tailored to room acoustics, if desired..
The final mini-disc test is to sweep the receiver and then play back the recording thru the transmitter to insure the whole system is flat on the scope when transmitted back.
I'd be glad to add any more details if anyone needs them -
Questions for Hammarlund SP-600:
: : 1) Tap off the detector......DO YOU TAP OFF THE DETECTOR DIODE OUTPUT OR OFF ONE OF THE PINS OF THE DETECTOR TUBE??
: : 2) Build up an FET op amp as a buffer stage from the detector output set at five megohms input...........COULD YOU PROVIDE DETAILS AND SCHEMATIC FOR THE OP-AMP BUFFER....???
:IS THE PHASE REVERSAL SWITCH SIMPLY A DPDT.....???
It's been over a year since I did it and did not document the specific pick off point. I really should tear it down and look inside sometime - and trace out the op-amp FET buffer circuit too.
But, for now, what I did originally was to go in there with a scope probe and poke around the detector tube looking for the best pick off point. I put a 10K resistor load across the probe, tuned to a strong broadcash station and then looked for the strongest, cleanest audio. That point was obvious when found. It's probably just the detector tube output, why not?
The FET op-amp buffer is a standard Radio Shack BiFet chip. I used a unity gain circuit. (zero gain) Use a standard handbook circuit for that...nothing special about the circuit other than having unity gain. Make it a gain of X2 or X3 if ya need it, but try to maintain a VERY high input impedance for the detector to work into.
The phase reversal switch is a DPDT type with two jumpers going across the outside end pins in an "X" configuration. The input and output connect to the remaining center pins. Draw it out on paper to see how it reverses phase.
Hopefully you can figure it out with this info, OM.