[AMRadio] infinite impedance detector vs diode

Brett Gazdzinski Brett.Gazdzinski at verizon.net
Thu Sep 4 22:32:14 EDT 2008

I use a circuit I found on the web someplace.
The info says:
Low distortion am detector.
A low-distortion am detector has been developed by W.T.Selsted and B.H. 
This consists of a conventional diode rectifier direct coupled to a cathode 
follower, which is in turn connected to an r-f filter.

The excellent performance of this circuit is due to two facts:

1 That the load on the diode for normal am carrier frequencies is 
essentially resistive and the normal effects of excessive shunting 
capacitance are eliminated.

2 That, since the coupling to the cathode follower is direct, there is no 
effect of/on? biasing currents which are normally developed in a diode 
loading circuit using coupling condensers.

The distortion for 100% modulation is .3% at a modulating frequency 420 cps 
and .8% at 4000 cps.

The circuit looks like this, the top of the IF transformer goes to a 
diode(plate) with a 250k resistor to ground from the cathode, the other side 
of the IF trans goes to ground. The cathode of the diode also goes to the 
grid of a triode, the plate goes to B+, the cathode of the triode has a 15k 
resistor to ground, with a 150pf cap across it, .1 uf coupling cap, 500K ohm 
to ground is the audio output.

My hearing is not good, and I never was a good judge of low levels of 
distortion, but it sounds clean and nice to me.

I wonder if this circuit has a name, and suppose, since it uses tubes, its 


----- Original Message ----- 
From: "John Coleman" <jc at pctechref.com>
To: "'Discussion of AM Radio in the Amateur Service'" 
<amradio at mailman.qth.net>
Sent: Thursday, September 04, 2008 8:47 PM
Subject: RE: [AMRadio] infinite impedance detector vs diode

Yes I know what you mean about wheels turning, I should not be doing this
but it interests me so.  I never learned how to use the spice software and
some day I have to do it.  Yes the Hi Z detector would be an advantage I
would think but I have heard others say it makes the bandpass to narrow.  Go
73, and tnx fer all the info

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