leadbreath said:

Ian

Don't know if you've looked at the schematics, I was just wondering what your thoughts are on the Hi/Lo Board Gain switches?

It seems like it was a separate board that they utilized with all the resistors preassembled?

Not sure of what value range we should be looking at, what do you think??

Mick

Both bass and treble EQs use the same basic topology. It is basically an inverting op amp. The input arm is the 20K R1 in parallel with the left hand string of resistors on the switch and the feedback arm is the right hand string of resistors on the switch. The gain of the stage is equal to the feedback resistance divided by the input resistance.

The bass circuit uses a 1,93H inductor in series with a 750 ohm resistor as the frequency selective element which will produce a shelving response. The treble circuit uses a 100mH inductor in series with a 10nF capacitor and a 1K resistor as the frequency selective element which will produce a bell shaped response centred around 5KHz.

To make things easy, let's assume that when the controls are flat the gain is unity. We can make the right hand switch resistor of the input arm arbitrarily large so we can assume when the controls are flat the input resistor is 20K. Therefore the sum of the switch resistors in the feedback path must also be 20K to achieve unity gain. From then on it is just a matter of calculating the switch resistor total at each position to get the 2dB steps required. Apart from that, all we know is that the max boost and cut is 10dB.

For 10dB boost, the input arm needs to be reduced to 6.3K. To reduce 20K to 6.3K needs a resistor in parallel of 9.2K. This 9.2K is made up of the left most switch resistor in series with the inductor dc resistance and the 750 ohms resistor. Let us assume the dc resistance of the inductor is 250 ohms which means the switch resistor is 8K2 ( a nice standard value available in those days).

Now repeat this process remembering the next switch resistor value will be whatever needs to be added to the 8K2 to get the next value. So for 8dB boost the input resistor arm needs to be 8K which means our total switch resistance needs to be 13K3. So the next resistor in the switch string need to be:

13K - 8K2 - 250 -750 = 3K8 - they probably used a 3K9 resistor

Use a similar process to calculate the remaining switch resistor values.

Cheers

Ian

Edit: I have no idea how I managed to get those paragraphs shuffled. Hopefully they are now in a sensible order.