PhonoII 3.18uS correction
As standard the PhonoII corrects for the 75uS, 318uS and 3180uS RIAA equalization time constants, in addition many good quality RIAA phono designs also correct for the 3.18uS time constant. The reason for doing this is this time constant is often used during recording to protect the cutting head, it also has the benefit of compensating for roll off in the interconnects used during recording (ref: Allen Wright, 1994 The Tube Preamp Cookbook).
To introduce this correction simply insert an additional resistor between C16/17 and the junction of R31/32 and V4b/a connection 7/2. See diagram. The easiest way to do this is to lift one leg of C16/C17, the leg connected to R31/32 and solder the new resistors into position on the top side of the PCB between the lifted leg and the solder pad of R31/32.
The required value is 14K454 Ohm, absolute accuracy is not necessary or achievable so parallel either a 51K resistor with a 20K resistor to give 14K366 or an 18K resistor with a 75K to give 14K516. All resistors should be .6W 1%.
The effect is subtle but the sound should have more "air" around vocals.
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update....
The 3.18uS mod checks out well with 5751 but not quite so well for ECC83. The reason is that the extra new resistor adds the 3.18uS corner but effectively increases R output by the same amount and so shifts the 75uS corner downwards. 5751 has a lower output resistance so the added extra resistance suits it quite well,
To implement the mod for ECC83 by adding the 14k3 resistor the 180k should be reduced by a similar amount. The 14k3 value is not critical as it merely restores a little treble from around 50KHz upwards which was otherwise being allowed to roll off previously. Thus, readily available values of 165k and 15k metalfilms from RS will be fine,
6th January 2006, 09:20 PM
PhonoII 3.18uS correction
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