LOW DISTORTION AUDIO GENERATOR.

The IG/SG-18 was a very popular product for the Heath company. The IG-18 was the kit version and the SG-18 was the assembled version which was targeted primarily to schools and colleges. Both models were identical, differing only in the paint job.

They, it from now on, is a lamp stabilized bridged-T oscillator with a schmitt trigger to provide a square wave output. A lamp stabilized oscillator has the potential to have very low distortion but this one, for a number of reasons, did not live up to those expectations. It has been about 10 years since I performed the modifications described here so I don't remember exactly what the distortion was but I think it was about 0.5%. After a series of modifications I was able to get it down to 0.015%.

Of course if you don't have a harmonic distortion analyzer you really have no need for an audio oscillator with a distortion of 0.015%. Many older vacuum tube analyzers such as those made by HP are now turning up on the benches of hobbyist and experimenters. The lowest range on these units is 1% so an oscillator with distortion of 0.1% would be considered adequate. The steps to reduce the distortion in the IG/SG-18 are given from the simplest to the hardest so when the distortion is low enough for use with your analyzer you can stop.

At the time I modified my IG-18 I didn't do it in the logical order presented here. Actually, turning down the voltmeter calibration pot was the last thing I did. That took away the last few hundredths of a percent. Some of the mods may not even be necessary. I honestly don't know

There are a number of causes for the high distortion in this unit and they are as follows.

1. The front panel voltmeter employs a bridge rectifier which places a nonlinear load on the output.
2. The harmonics from the square wave leak over to the sine wave output.
3. The output amplitude is operating a little too close to clipping.
4. The active filter in the power supply does a very poor job of filtering.
5. The amplifier which serves as the gain block for the oscillator has too much distortion.
6. The magnetic field from the power transformer induces 60 cycle hum into the sine wave output.

To clean up the signal the following things must be done.

1. Disable the front panel voltmeter.
2. Disable the square wave generator.
3. The output must be reduced from 10 volts to 8 volts.
4. The emitter follower active filter must be replaced by a 3-terminal IC voltage regulator.
5. Use one of the transistors in the square wave circuit to form a current source to linearize the gain block.
6. The power transformer must be mounted outside of the case.

Fear not, I will elaborate further on each item. The order has been very carefully chosen so you can do the simplest things first. Now we need to see the schematic diagram.

1. The Front Panel Voltmeter.

• Introduction and Modifications 1, 2, and 3. (You are here.)
• Modification 4, Power Supply Regulator.
• Modification 5, Decreasing Distortion in Amplifier.
• Modification 6, Mounting Power Transformer Outside the Case.

The day that the Heath company closed its doors
was a sad day indeed for electronics home brewers.
Before it existed there was nothing in the world like it,
It was never equaled by its wood-be competition,
And there will never again be anything like it.

Home

Or use your "Back" button to return to where you were.

This page last updated April 15, 2006.