Preamp EQ Simulation

I use a small signal AC simulator running on DOS called Analyser III by Number One Systems.

It comprises a 50kHz filter to compensate for the rolloff applied to the cutter head. It is based on Allen Wright's work in this area. The sim runs the low pass filter into the inverse RIAA EQ which is the input to the sim of the preamp.

The inverse EQ and an adapted 50kHz filter are based on an article in Audio Amateur 1/91 "An Active Inverse RIAA Circuit" by Dmitri Danyk and George Pilko. The accuracy of the inverse EQ is very good when compared to the RIAA standard. They use a LH0002 so I sim'd that from the National Semiconductor Datasheet.

Models in the sim for valves are the basic FET model with Ra as a resistor across drain and source. gm and Ra are entered into the model for the operating point of the valve. Miller is an extra capacitor across grid and cathode external to the valve model as it depends on stage gain. I usually double the manufacturers claims on the datasheet as they always seem to be optimistic. In this case for 5687, have assumed 100pF Miller.

As my preamp is a TVC, the output of the preamp will react with the primary inductance of the TVC to form a resonant circuit. A basic transformer model is used in the sim to gauge the effect of the "lossy parafeed" output capacitor combo.

Simulator Models

Sim Results

Note the Y scale is not the actual circuit gain as there is attenuation in the TVC. What should be noticed is the scale units. The final response is flat within 0.25dB 20 to 20kHz which is good enough considering I don't alter the capacitor values in the EQ. If they were closer to ideal values, the sim and circuit would be even more accurate. One day I must try the results of that.

Output at anode of second stage

Output without Lossy Parafeed

Note the resonance bump at LF caused by the TVC in conjusctio with 1uF coupling capacitor.

Corrected Output

The Future

If I was to lower the EQ Z by an order of magnitude, Miller of the second stage would become largely irrelevant. There are many that say lower Z EQ sounds better too. The sim would say I can get the circuit much flatter and lower the insertion loss by 1.7dB. Something for the future.