[Tim Farrant]

OK, well, the Juggernaut uses an input transformer like many preamps, but with a difference. Most transformer input preamps use a step up type transformer typically 1:4 or 1:5 or even higher at 1:10 ratio in order to raise the source impedance (as seen by the amplifier input looking back at the mic). This makes it easier to design the amplifier and keep the noise down, and you get some free voltage gain in the process.

The Juggernaut input transformers are 1:1 ratio, it merely converts the signal from balanced to unbalanced and isolates the phantom. This means we do not need an input pad before the transformer for high levels because it does not have any voltage gain. Drawback of this is we need a amplifier that can work as if it was in a transformerless configuration (like the Elixir preamp). So I have designed a single stage amp that provides good noise spec at low source impedance as well as enough gain to reach +70dB if required.

The phantom power is fed via a centre tap on the transformer, rather than use the the usual 6.8kohm feed resistors. This eliminates the loading these resistors apply to the input, so we can have a the Mic Load pot which varies the input secondary load smoothly from 300 to 10k ohms. The two different input transformers have quite different distortion specs and this also varies with the Mic Load control.

The amp stage output is capable of quite large output currents, and this drives a 1:3 step up output transformer in order to get sufficient headroom. When the gain control is set high, there is minimal negative feedback in play, which in theory opens out the amp "sound", the +THD switch applies approx 10dB attenuation on the output after the output transformer so you can take advantage of this sound.

We have PIC controlling the relays etc, so the output mutes when you change transformers and turn on/off the +48V to avoid large bangs. The +48V switch has a delay before switching to avoid accidental use, ie, you gotta hold it down for a few seconds before it switches.

Tim.