In the first article about the variable EQ phonostage I wrote about some initial thoughts. In the meantime things progressed considerably. In fact the phonostage is almost completed. So it is time to continue this series with the second part. As I wrote in the first part, the same basic circuit topology as in my EC8020 LCR RIAA phonostage was chosen. A two stage circuit: First gain stage with low output impedance driving a LCR EQ network, followed by a second gain and output stage. Only this time the EQ should be selectable. This is the block diagram of the circuit:
Two switches for independent selection of low frequency turn over and high frequency roll off. Since the EQ should be done with LCR networks, a completely separate LCR circuit needs to be built for each turnover and roll off. LCR networks are quite large, so a good compromise had to be selected to cover as many EQ curves as possible but limit the amount of networks. The given chassis size would allow for 8 separate networks. In addition a flat setting can be easily implemented for the roll off, by just selecting a feed through.
This is the list of turn over and high frequency roll off points which was finally selected:
Turnover frequency / low frequency cut:
400N : 400Hz / -
500N : 500Hz / -
500R : 500Hz / 50Hz (RIAA)
500C : 500Hz / 100Hz
It is actually not possible to make a LCR network without low frequency cut. This would require infinitely large inductances. Instead the cut was moved to a very low frequency, around 20Hz.
For the treble EQ, 4 settings were chosen, plus flat. Listed as the attenuation at 10kHz: