11-03-2017, 07:37 AM
Here are some examples of the de-equalizing networks required when piezo cartridges are fed into RIAA-equalized magnetic cartridge inputs. Essentially they nullify the built-in equalization, allowing it to be reapplied in the amplifier, so avoiding “double correction”, as it were. It would appear that each cartridge requires its own network.
An apparently unusual case is that of the Hacker GP42, which has recently been mentioned elsewhere.
It has a piezo cartridge with both high impedance loading and what looks to be an equalization network, namely R1 and C1. Basically the cartridge is looking into the 1 M volume control, albeit that there will be some shunting of that – dependent upon volume control position - by what follows, namely the treble control network and the grid resistor of the first valve.
R1, C1 and RV1 probably form a step-filter network. That is, the resultant audio response is flat to the first turnover, then rising at 6 dB/8ve to the second turnover, then flat thereafter. This is the form of curve required for RIAA equalization of an amplitude-sensitive cartridge that does not have any self-equalization. Possibly the cartridge used by Hacker was not self-equalized, or if so, its equalization fell short of what was required (according to Linsley Hood this was not an unlikely event) and needed some assistance.
Note that this network does more-or-less the opposite of the de-equalizing network required for the Decca Deram cartridge when “low loaded”. This would provide a 6 dB/8ve downslope between the two turnover points.
Cheers,
An apparently unusual case is that of the Hacker GP42, which has recently been mentioned elsewhere.
It has a piezo cartridge with both high impedance loading and what looks to be an equalization network, namely R1 and C1. Basically the cartridge is looking into the 1 M volume control, albeit that there will be some shunting of that – dependent upon volume control position - by what follows, namely the treble control network and the grid resistor of the first valve.
R1, C1 and RV1 probably form a step-filter network. That is, the resultant audio response is flat to the first turnover, then rising at 6 dB/8ve to the second turnover, then flat thereafter. This is the form of curve required for RIAA equalization of an amplitude-sensitive cartridge that does not have any self-equalization. Possibly the cartridge used by Hacker was not self-equalized, or if so, its equalization fell short of what was required (according to Linsley Hood this was not an unlikely event) and needed some assistance.
Note that this network does more-or-less the opposite of the de-equalizing network required for the Decca Deram cartridge when “low loaded”. This would provide a 6 dB/8ve downslope between the two turnover points.
Cheers,
