J E M box information

[Will C]

Well Geoff,

The JEM box turns out to be a buffer with a pair of BC109's, the first as an amplifier with a passive treble cut control on the input, and the second as an emitter follower, for low Zout, with a volume control across the output.

[Jif]

Yeah, pretty much of a pattern on these type of boxes Will. Don't know why they would put a treble cut in? I prefer to use a FET as it has a higher input impedence than a bipolar tranny.

Just to clarify a unity voltage gain, current amplifier is commonly known as a unity gain amplifier in the electronics world. The current gain does have a beneficial effect in 2 areas, first off it reduces the signal to noise ratio in the cable, and it also reduces the high frequency losses in the cable, the cable having it's own inherent resistance and capacitance.

20-30 years ago I could have put up a proof re the frequency response of a co-axial cable and it's relationship to the amount of current passing through it, these days my brain unfortunately is a bit like a Swiss cheese, (full of holes!)

[Will C]

Yes Geoff, same here. But this JEM box is pretty old, and FETs are somewhat more recent arrivals - they started showing up in the late 60's but there were few at the time. As you say, the important thing is to have a low output impedance so it will not suffer ill-effects from the cable, although to be honest the cable is what it is, and anything after the buffer will still degrade the signal if it's not top quality.