[Synchrodyne]

Well, the Sonotone data sheet kindly provided by GP49000 is quite explicit – kudos to Sonotone for providing good product literature. Whilst the network details would be cartridge-specific, the same principles would apply generally.

For direct connection, an amplifier with a 2M input impedance is required. Such high impedance inputs are/were scarce on solid-state amplifiers. For example, the radio and auxiliary inputs of the Cambridge P40 and P50 were specified as 100k, 100 mV, so unsuitable for the purpose.

In the case where an equalized magnetic cartridge input is to be used, a de-equalizing network is needed. That shown would, I think, provide an upstep curve that cancelled the equalization built into seem to have missed the cartridge. Kudos to Sonotone again for offering a plug-in adaptor that contained de-equalizing network. Clearly it anticipated the problem caused by the scarcity of high-impedance inputs on the one hand and on the other hand, in their absence, the likelihood that some users might default to direct connection of self-equalized ceramic cartridges to magnetic inputs.

The Cambridge P40 and P50 both had “ceramic cartridge” inputs of 100k, 100 mV. From the P50 schematic, it looks as if RIAA equalization was applied to this input, using the same network as for the magnetic cartridge input. Use of such an input with a self-equalized ceramic cartridge would result in double equalization, so that a de-equalization network would still be required, as with the magnetic input case. This kind of input would be right for a non-self equalized ceramic cartridge. But this was a minority type, with about one, the Connoisseur SCU1, extant in the late 1960s. All of the others were self-equalized. But the Cambridge approach was not atypical of amplifier makers of the time, many of whom seem to have missed the mark when it came to using low-impedance loading for ceramic cartridges. Quad was one of the few who attended to the detail in this regard; the C1 disc input on the 33 was tailored for self-equalized ceramic cartridges of a specified self-capacitance range (which covered most of them). One or two did offer solid-state amplifiers with high-impedance inputs, notably Tripletone, who, operating at the “budget” end of the market, had a particular need to get right the issue of ceramic cartridge matching. Another was Ferrograph, with its F307. In this case the high impedance input looked to have been a carryover from the Series 7 tape recorder, whose high impedance line input may have been motivated less by the need to match ceramic cartridges than to provide an input suitable for connecting to valve amplifiers, many of whose (unbuffered) tape outputs needed to look into impedances of at least 500k, sometimes as much as 1M.

Anyway, in the case at interest, as the amplifier is unlikely to have a high impedance “flat” input, then connection to the magnetic cartridge input via an appropriate de-equalizing network is the best answer.

Here is a sampling of several such networks, additional to the Sonotone case already covered.


Cheers,