Basic ESSB Audio Cleanup

Minor circuitry and operational changes make a huge difference

Even if you don't use a full rack of external audio processing equipment, you can still put out a signal that is distortion free, of good bandpass, and actually sounds like you. No slop bucket here!

Many hams assume that sideband can't sound as good as AM. It's an idea generally based on the experience of tuning the bands and hearing amateur radio operators using equipment that is poorly aligned or not properly adjusted. The truth is that a properly set-up and maintained transceiver can produce fine audio on both receive antd transmit. This page will cover some of the basics for cleaning up and enhancing audio.

First is the issue of RF / IF alignment. The radio certainly should be aligned in accordance with manufacturer specifications or better. When that is done, one can expect the phantom carrier to fall exactly on those 1.0, 0.1, or 0.01 kHz increments that the rest of the ham population uses. Use one of the popular soundcard spectrum displays and look for power line hum on CW signals. If you see at least a few squeaky clean carriers, then your oscillators are clean. If each signal has the same hum sidebands, then your oscillators (at least one) are in need of filtering. More modern transcievers have one master oscillator from which all other frequencies are derived - a very good design element. Get the master on frequency and make sure it receives clean power, and you're in good shape here.

Another important element is setting the carrier at the proper offset from the IF filter bandpass. Plan on your banpass cutting off lows around 200 Hz if you have narrow filters (3.0 kHz bandwidth or less). If you have a more pleasant sounding 3.5 to 4.0 bandpass, set your offset to cut off lows around 100 Hz. Unless you are using a phasing or DSP modulator, it is wise to allow the carrier be no closer to the low cutoff of your SSB filter. Otherwise there could be carrier and opposite sideband leakage, and that is a no-no on crowded bands.

The audio stages will get the lion's share of your attention. First steps here should focus on using a good quality electret condenser microphone, and eliminating all sources of hum or other electronic noise. Make sure all sources of power to audio stages are well filtered and RF bypassed. Coupling capacitors should be increased enough to pass audio down to 10 to 20 Hz. Even if your SSB signal cuts off at 100 Hz, you want to limit the phase shifts that result from narrow audio bandpasses. On the high end, make smaller any bypass caps that limit your audio range to anything below about 5 to 7 kHz.

One factor in the audio stages is available gain. It is better to have plenty of gain available, and reduce it with negative feedback than to have insufficient gain and have to turn it up to maximum in order to reach normal output power. If necessary, add an extra stage of gain, and use some negative feedback to both widen the bandpass and reduce distortion.

The audio stage must also be capable of fully driving the modulator. In other words, if the modulator output is free of distortion until the input audio peaks reach 1.5 volts, the audio source should provide that signal level without clipping. The only clipping permissible is deliberate clipping in a processor stage!

Audio compression is a good thing, but don't use more than about 6 dB. Beyond that, there will be a lot of "background noise pumping." Limiting is important too, and should be used to prevent activation of the ALC circuit. ALC circuits may keep your final amplifier from being damaged, but it does a poor job of preventing splatter and distortion. Judicious use of compression and very light clipping will provide enough modulation without irritating your neighbors on the band.

EXAMPLE: AN OLD SWANN 350B

The ideas above have been used to greatly improve the sound of the old Swann 350B used by the author during his earlier HF hamming experience. The '350B was basically a nice sounding rig, but it was a magnitude better when fed audio from an electret microphone and solid state preamp. Light compression was added to the audio prior to going into the 12AX7 speech amp, and the whole chain received a dose of inverse feedback.

On the RF side, the stock 2.7 kHz filter was retained, but a bypass was added for AM / DSB operation. Extra filtering was added to the DC power supply to ensure very very clean DC fed the oscillators. The 350B used a TV sweep tube for a final amplifier, and it did not tolerate abuse. Therefore, keeping the RF drive within limits was vital, and helped by the compression / limiting in the audio stages.

One thing that made an immense difference in how the Swann 350B sounded was the use of a fully external audio stage (from a public address system), directly feeding line level audio to the balanced modulator. If you're not interested in changing capacitors, fiddling with feedback loops, etc., consider using an external microphone preamp / equalizer / compressor chain to feed the modulator. Attenuate as necessary to prevent overdriving the radio. Put lots of ferrite beads or other chokes on the line to keep out stray RF.

A REVIEW OF THE MAIN POINTS

To summarize making a basic old-radio ESSB set-up:

1. Filter all sources of hum and noise that could get into AF, RF, or OSC stages.
2. Accurately align oscillators and set a reasonable SSB carrier offset.
3. Use a 3.7 to 4.5 kHz IF filter for enhanced Single sideband if one is available.
4. Adjust coupling and audio bypass capacitors for a passband that is flat from 10 Hz to 10 kHz.
5. Use a quality microphone
6. Audio preamps are okay, but use inverse feedback to reduce gain and distortion.
7. Use audio gating, EQ, compression, and limiting if available
8. Direct line level audio feed to modulator is an excellent idea.
9. Always avoid tripping the transmitter ALC circuit.
10. Use plenty RF chokes and ferrite beads to keep RF out of the audio stages.

That's it, in a nutshell...now go forth and radiate some clean, amooth, and listenable enhanced single sideband!