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New EQ Board

By Heinz Weierhorst [21010276]

Why an new EQ-board?

Well, most mixers, FX devices and so on nowadays have balanced inputs/outputs. So why shouldn't such an old baby like the Chroma get that feature too? On the other hand the nearly obsolete CEM3360 VCAs on the EQ-board should better used as a replacement part on a faulty DVB as substituting them isn't an easy attempt for some reasons. The circuit is simple: a quad VCA SSM2164 (Z1) replaces the two dual CEM3360 VCAs. It's a current-in current-out device with 33mV/dB controls. Note: This not an OTA! The core looks more like a multiplier (see Figure 1). The (audio) voltages 0-3 coming from the CMB are converted into currents by the 30K resistors. The 510Ohms 470pF series pad from input to ground prevents oscillation.

Figure 1: Simplified Schematic (One Channel). Source: Datasheet Catalog.

The current outputs are converted to voltages by Z1A&B, Z2A&B and the 15K feedback resistors. The output level is -6dB in respect to the input level. It's because the balanced drivers Z4, Z5, Z6, Z7 and Z9 have +6dB gain. Those so called crosscoupled Opamps act like transformers (Figure 2).

Figure 2: Instrumentation

It doesn't matter if balanced or single ended - the output level remains the same. The 10UF bipolar electrolytic caps prevent offset voltage. The EQ section is similar to the old one. The only exception is all resistors are metal-film 1% and the caps are 22K 5%, 4K7 1%, 1K2 1% and 1k 1% - but that isn't a must.

The volume control is hooked up around Z10. A TLE2021 was choosen because it's low offset voltage and price. With the given values the control range is -60 to 0dB (Figure 3). For those who want a different control range a simplified formula will help: RATT = RFB*12/ATT*33mV RG = RFB*12/ATT+GN*33mV

Figure 3: VCA Control

Note: A positive voltage attenuates, a negative voltage amplifies the gain of the SSM2164.

The mute circiut is around Q2 and a relay. This way was choosen because it needs ten "dry switching contacts" if the relays were in the signal way. Those relays are hard to find and expensive and occupy much space. When the Chroma is switched on the reset signal is low thus Q2 and the relay are off.



When reset goes high there is a delay is added due the 1UF gate cap and the 1Meg resistor. After that Q2 is conducting and the relay switches on and Z2 – Z9 were powered up. When switching off reset goes low again thus Q2 is switched off very fast by the 5K6 resistor and the diode. The relay contacts flips to the opposite direction discharging all supply caps via the 120 Ohms resistors to ground. In the meatime I've changed that resistors to 6.8 Ohms because the relay can surge currents up to 2A and the switch off click was to loud.

The 2N7000 N-Fet is a low treshhold type and recommended. The relay has a high sensivity coil and is drawing low current.