Voltage doubler AM detectors.
 

G'day all, I've recently put together a simple TRF AM receiver using a simple BAT46 two schottky diode 'voltage doubler' AM detector circuit arrangement which produces absolutely superb audio quality on detection.

Way better than any other simple approach AM detector that I've used. I am aware than some kit and other receivers used voltage doubler AM detector arrangements in the past.

I wonder how common they were as based on my personal experience the potential performance as a high quality AM detector is exceptional! Regards, Felix (vk4fuq) aka catman.

Can you furnish a schematic of the circuit? Nothing fancy, hand drawn is fine.
There's only a few stations around, that play music and most newer equipment, the bandwidth is too narrow to provide good fidelity.

There is some good old equipment....The mid 50's - early 60's AM-FM simulcast tuners tried to make the AM channel sound as good as the FM channel by way of really good engineering in the AM sections....This is the exact reason why last meet I went to I grabbed a Fisher simulcast tuner, and later bought an FM MPX adapter. With those I can build a tuner that sounds really good on FM AND AM.

Could be fun to 'roll your own' homebrew HiFi AM radio though....

G'day all, it's getting a bit late in the day (over here in Oz), but I'll do up a circuit tomorrow if I get a chance. It's nothing too complex, just a two diode 'doubler' detector terminated by a 27 k load resistor and a 220 pf RF bypass capacitor working into a simple MPF102 FET source follower audio buffer stage.

I use an untuned FET RF stage to provide some RF gain into the detector, as we are 100 odd miles from our 'local' AM transmitters, and all diode detectors like a reasonable amount of RF input to work optimally. Regards, Felix (vk4fuq) aka catman.

G'day all, here is the 'circuit'. You might require a series capacitor on the output and also a series capacitor on the gate input of the second FET (1uf MKT).

I used a 1 uf MKT capacitor on the input to the detector but a smaller value might be better, so experiment if you wish! Regards, Felix (vk4fuq) aka catman.

Addition 040315. I've determined that a .1 uf capacitor is pretty well ideal as the value for the 'input' capacitor to the diode voltage doubler detector. I used an MKT capacitor. Regards, Felix (vk4fuq).

Addition: another circuit modification. The 27 k load resistor should be changed to 100 k. This increases the output level considerably without compromising any aspect of normal operation.

Cool! thanks for posting... could a second tuned circuit replace the 1.8 k resistor to make it a "2 dial" set?
Does Australia *actually* have AM stations that broadcast with decent fidelity? Most stations in the US limit the audio response to 8 kHz with many only going to 5 kHz... On top of that, a lot of compression/clipping is usually employed... the resulting sound quality is pretty awful! :(

jr

G'day mate, yes there are good and not so good stations! We have a full 9 KHz spacing over here and with that audio bandwidth the recovered quality is comparable to FM.

Having said that, AM and FM do 'sound' different, but still excellent! However very heavy compression is frequently applied with an 'Optimod' processor and the results 'can' be ok but compression is generally pretty distasteful in any case! Regards, Felix (vk4fuq) aka catman.