The Altec 9069B High Pass Filter (HPF) has been used a lot in the 70’s and later. It is a passive “T” type filter that is composed of capacitors and inductors. It was not an “innovative” filter at the time because technically since the 30’s (at least) there were other filter models with the same circuit.

Our choice and preference for the Altec 9069B is due to the creative use that King Tubby made of it. He had an Altec unit built into his MCI box, known as the “Big Knob”.

The Altec 9069B is a 3rd-order filter, it has an attenuation slope of 18dB/octave. It provides, in each of the ten filter passes, a T-type filter adjusted to the frequency indicated by the manufacturer.  We could say that it is a “network” of filters. The 9069B is designed and built to interact with impedances (both input and output) of 600Ω. These impedances are a reference to the old standard of power transfer that worked at 600Ω at both the source and the receiver.

By default the T-type filter has a certain sensitivity to the variation of impedances that are connected to it, both at the input and output, and responds differently depending on these.

The most notable effect that we observe when connecting a 9069B filter, unmodified, to an actual system is the appearance of resonance in the cutoff frequency region. We proceed to simulate several possible situations:

These simulations are made assuming “ideal” components, pure capacitances and inductances. Figure 1

Figure 1 illustrates the 70Hz frequency position with a 600Ω system. The device connected to the input of the 9069B has an impedance of 600Ω at its output and the device connected to the output of the 9069B has an impedance of 600Ω at its input. As Figure 1 shows, the filter adequately responds: it shows an amplitude drop below 70Hz of 18dB per octave. It can also be seen that the signal shows an attenuation of 6dB in the signal above 70Hz. The 6dB attenuation is to be expected in a 600Ω power transfer system. The old power transfer system implied that to obtain maximum power transfer only half of the power was transferred. 6dB implies that the signal has half the amplitude coming out of the 9069B as it does going in. 6dB is an approximation. Figure 2

In Figure 2, we see a simulation of the 9069B connected to a typical current system in which the device connected to the input has an impedance of 100Ω at the output and the device connected to the output has an impedance of 10kΩ at the input. As can be seen a strong resonance of 10dB appears and the signal above the resonance zone is transferred practically at 0dB, with no signal loss. The attenuation slope is no longer 18db/octave, it is about 13dB/octave and we also observe a displacement of the height point that places the resonance peak above 100Hz. Figure 3

Figure 3 depicts Figure 1 and Figure 2 superposed.

Now let’s see what happens when we have a 600Ω device at the input and a 10kΩ device at the output Figura 4

Figure 4 shows that the resonance practically doesn’t exist as in the case of the filter connected to a 600Ω system. We have practically the same signal level above 70Hz at the input and output (only 0.5dB of loss). The slope is such as that of the connection of a modern system 13dB/octave. In this case, there is a frequency shift from high to 94Hz. Figure 5

In Figure 5 depicts the response with a 100Ω output device connected to the input of the 9069B and a 600Ω input device connected to the output. It is obtained the correct slope (-18dB/oct) and there is not a significant signal loss (-1.3dB). On the other hand, the resonance bell is attenuated, getting the maximum peak around 85Hz with 2dB of magnitude. Figure 6

Figure 6 compares the response of the system all at 600Ω (blue line) with a system that at the output loads 9069b with 100Ω (red line), that is not very common, it is hard to find 100Ω-input impedance in devices that work with line signal.

Figure 4 and Figure 5 represent situations combining “old” equipment at 600Ω with “modern” equipment with more actual impedances. These are the conclusions:

– The attenuation slope depends directly on the impedance that we couple to the output of the 9069B. When 600Ω is used, it is obtained 18dB/octave. That’s why the circuit was designed to work with that impedance value.

– When some of the connected devices is “modern”, the signal loss effect of 6dB is severely limited.

– To get the original frequency response, the filter should interact with impedances of 600Ω both at the input and at the output.

We can say that this filter, in conjunction with a 600Ω system (or compensating the impedance to simulate such a system) is very useful for editing or recording. It was traditionally used to remove unwanted noise or sometimes to simulate a “radiophonic” effect.

The combination of the Altec 9069B and modern equipment generates a resonance effect around the cutoff frequency, which might be exploited artistically or creatively. In this conditions, it might be used to get a remarkable tone, that emphasises the frequencies close to the cutoff whereas eliminates the part below the boost.
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