[AMRadio] Subj change - HB designs

John Coleman jc at pctechref.com
Mon Jul 30 11:33:18 EDT 2007


First let me point out that the centertap of a XFMR does not represent 1/2
of the load resistance but only 1/4.  That is to say that if a 2500 ohm near
perfect match is made from the output of a XFMR and the load is to be moved
to the centertap using only 1/2 the winding, the load resistance must be
reduced to 625 ohms to consume the same power.  The voltage goes to half and
the current must double in order to consume the same power.

It is generally not necessary in this circuit to use "swamping resistors" as
they will only detract from the low frequency response do to the losses of
the XFMR.  If the driver stage has any inverse feed back or represents a low
Z output such as most HIFI amps do then that will stop any ringing of the

As pointed out to me by Jim (JKO), it is now possible to get fairly high
voltage MOSFET devices to use in place of the transistors.  This would allow
the total removal of the inductor device at the front end and it could be
replace with a RC coupled phase splitter/driver.  This could mean total
inductorless driver.  

But as you pointed out at what point do the returns diminish so that no one
cares.  But size is also a concern and the circuitry for all this could be
quite small. (unlike the way I build, HIHI)

John, WA5BXO    

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Brett gazdzinski
Sent: Monday, July 30, 2007 7:35 AM
To: 'Discussion of AM Radio in the Amateur Service'
Subject: RE: [AMRadio] Subj change - HB designs

Think that circuit is better then a hifi transformer
(say 2500 ohms center tapped) to 8 ohms?
8 ohm side goes to the hifi amp, the 2500 ohm
side goes to the grids.
Swamping resistors go across the 2500 ohm side
(1250 ohms to center tap)


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