S. Hawk Ltd Hawk II Tonal Expander and Hawk III Bass Expander

S. Hawk Ltd. was a company that lasted very briefly in the mid '70s. The man behind S. Hawk Ltd. was Harry Kolbe. Kolbe is a well known NYC tech. Kolbe's website, www.soundsmith.com, says the company opened in 1974 and closed in 1974. http://www.soundsmith.com/next.html

There are three Hawks: the Hawk I is a distortion unit built around a 741 with a single gain slider and clipping diodes. The Hawk II is the one with 3 sliders and called the "Tonal Expander." The Hawk II has a treble boost switch identified by a backwards "L" symbol. The Hawk III is the one with 2 sliders and is called the "Bass Expander." The Hawk III has a bass boost switch identified by a "L" symbol. So when looking at the photos, note the number of sliders and whether the middle switch symbol is a "L" or a backwards "L." In the photo above, the Hawk III is on the left, and the Hawk II is on the right.

The Hawk II Tonal Expander and the Hawk III Bass Expander are simple op amp filter circuits that actually use old fashioned REAL inductors (as opposed to op amp "gyrator" simulated inductors that we all use now). These are battery powered only devices and they are designed to run on 2x 9V in series for a 18V power supply. The unit needs the headroom - the preamp boost gives +20 dB and the treble boost gives +30 dB!

For a long time I posted here that I had lost my notes on the circuits of the first two units I worked on. Finally I was sent another Hawk II Tonal Expander, and this time I made a proper schematic to share with everyone. I regret I don't have the notes for what is different on the Hawk III Bass Expander. Frequency centers might be different, and how he chose to implement the bass boost would be interesting to see.

The inductors are unmarked except for simple blue and red dots on the bass and middle components respectively. I measured them for both inductance and dc resistance and noted this info on the schematic. Using the measured inductance values and the labelled capacitor values, I calculated the frequency centers of the sliders to be 162 Hz, 581 Hz, and 2.9 kHz. My initial tests of the unit with pink noise and a RTA showed slightly different numbers: 140 Hz, 440 Hz, and 2.7 kHz. The 2.2uF and 500nF (electrolytic!) capacitors were to blame for the disparity: the 2.2uF measured 3.5uF out of circuit, and the 500nF measured as 1000nF. After replacing the caps (including the treble cap because the lead was breaking), the measured set was: 170 Hz bass boost, 160 Hz bass cut (not sure why the shift there), 580 Hz, 2.8 kHz.

The pics of the RTA curves will be posted soon to check out the "Q" of the filters.

In the Hawk II, the treble boost has an interesting design. The way I would explain its operation is like this: normally the preamp gain is governed by the R4/5/6/7 and C2 feedback network around U1A. This is a negative feedback path. Without the path, the gain goes up and up, i.e. making R6/7 higher in value makes the gain go up. SW2, when closed, completes a RLC filter that removes any signal that can pass through the RLC circuit from the negative feedback path. It's similar to making R6/7 appear to be huge at the frequencies that can pass through the network. This causes these frequencies to receive less negative feedback, and more preamp gain. I calculated the RLC center frequency near 1 kHz, although when I tested the unit with a RTA, the overall effect was a strong peak at 3.4 kHz.

(This schematic may contain errors - I seem to recall it definitely has at least one error I never got around to fixing...)