Specifications

In any event, any one of many, many popular amplifiers will exhibit this rising distortion with rising frequency characteristic, with

the Dynaco MK III, numerous Heathkit and Eico models, and many others all clocking 20 kHz THD figures at the specified

power levels well in excess of 2%. Of note however, this is an area where Heathkit really shined, steering very clear of such

simplified ratings, instead, choosing to offer a graph with most models, which clearly shows the rise in distortion at higher

frequencies and high power output levels -- due to the inclusion of these networks. Therefore, it is, what it is, with the distortion

levels produced in the ST-70 at 20 kHz not being indicative of any concern for proper operation.

I did test for low power THD levels, as they can actually rise in some designs from that of higher power output levels. At the

same test frequencies at 1 watt/5 watt levels (again, presented as an average between the two channels):

@20 Hz = .055% / .11%

@ 1 kHz = .035% / .043%

@ 20 kHz = .115% / .50%

This is pretty darn good performance from any vacuum tube amplifier, let alone from a stock 55 year old unit build from a kit!

SQUARE WAVE TESTING:

Fig 1: A 10 kHz square wave at 1 watt into an 8 ohm load looks identical on both channels. The somewhat slanted sides are

indicative of the frequency response figures produced, while the flat wave-top indicates a high degree of damping and stability.

Fig 2: With a capacitor only load, the amplifier is nowhere near becoming unstable.

Fig. 1 10KHz Square Wave

Fig. 2 Capacitor Only Load

For a design of the late 50s, this overall is quite enviable performance, that had other other manufactures scrambling to catch up

to. Most notable was the level of stability produced in light of the other performance levels achieved. That it is a product of David

Hafler is no real surprise, since he was a primary bandleader campaigning for improved stability margins in equipment of that

day.

So, this particular ST-70 is a healthy specimen, making a great platform for reference work, and to make various tests on when

needed.

The Power Supply

As with any stereo amplifier operating from a common power supply, compromises are made, even beyond those compromises

made in the power supplies of mono amplifiers. The question then becomes, how well were those compromises made? We

shall see.

In the ST-70, both channels are powered from the same power supply distribution points. But with channel separation

measured at 58 db at 1 watt, the 55 db Min Channel Separation spec was met handily. So there is no real compromise here, as

the separation figure achieved is very good.

The usual suspect targeted for compromise criticism in the ST-70 is the power transformer, to which could also be added the

rectifier tube, and choke.