[AMRadio] 32V-2 speech amp question


John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Mon Jan 23 11:19:08 EST 2006


	The 32V driver is certainly adequate for the specifications of the
rig.  I might add that the specifications in fidelity are fine for me.  I
think it and the 75A4 receiver could do better on THD.  However, I think
Craig was looking for more fidelity.  This might be a good time for me to
inject the Woos of HiFI.  HIFI is OK for use on ground wave without
selective fading and of course when the spectrum is void of other QSOs.  The
problem that I have is the QRM/QRN that accompanies cross country QSOs when
the band is open for that type of propagation.  I narrow up the RCVR to get
a better signal to noise ratio and that's when I begin understand the
problems of having bass without treble.  I'm talking bass below 100 CPS.  It
sounds great when with a wide band pass and assuming the XMTR is passing
treble out to 15KHZ.  But when you narrow down to 6-8 KHZ you need to roll
the lows of starting at 200HZ and eliminate them below 75HZ.  I don't like
60 CPS hum so I switch to a small very desk top speaker or cheap head phones
to eliminate the bass.  The XTMR would do better in this case not
transmitting the low bass but instead putting the energy into low distortion
limited band width audio.

This takes me to the new RMs that or in place.  I can see all the sides as
having made good points.  And I just don't know what to do.  I feel that the
FCC is going to get annoyed at the whole thing and wash there hands of it
and I don't know where that will leave us.

John, WA5BXO
 

 
-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Brett gazdzinski
Sent: Sunday, January 22, 2006 7:04 PM
To: 'Discussion of AM Radio'
Subject: RE: [AMRadio] 32V-2 speech amp question

Keep in mind, the input impedance is very high, 1 meg at the mike
input stock radio. No transformer on the mike input in the 32v series.
I thought the driver transformer was adequate, its very large for a driver
transformer, the
DX100 has a postage stamp, the 32v had a fist sized driver transformer.

I use a pair of KT88 tubes as modulators and don't use the driver
transformer 
(gave it away a long time ago). If you run the high impedance mike preamp in
the 
rig and want some hifi, you have to run the mike preamp tube off DC.
I just did 1/2 wave (1 diode) and an electrolytic cap. The size of the cap
sets 
the voltage the filament runs at. 
The DC does not need to be pure, even rough DC will eliminate hum.

Neg feedback, regulated voltage on the mod tube screens, large coupling caps
will get you a long way to a good sounding 32V.
Brett
N2DTS



> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of John 
> Coleman ARS WA5BXO
> Sent: Sunday, January 22, 2006 11:03 AM
> To: 'Discussion of AM Radio'
> Subject: RE: [AMRadio] 32V-2 speech amp question
> 
> 	I have not driven it externally before Craig but have some
> experience with the circuitry and the driver XFMR is a weak 
> link in the
> circuit the XFMR barely has enough iron and coupling 
> coefficiency in it
> to pass the low frequencies that the rig is designed for.  As a matter
> of fact if the cathode resistor of the driver stage drops in 
> resistance,
> as they or known to do with heat, the driver plate current 
> will quickly
> saturate the driver XFMR causing the bass frequencies to look like a
> trapezoid instead of sine wave.  If you were going to go with and
> outboard amplifier I would include a better driver XFMR as part of the
> external circuit and go to the grids directly.
> 
> John
> Coleman
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Craig Roberts
> Sent: Saturday, January 21, 2006 10:19 PM
> To: Discussion of AM Radio
> Subject: [AMRadio] 32V-2 speech amp question
> 
> Has anyone tried driving the mod input transformer of the 
> 32V-2 (or V-3)
> 
> directly with an outboard mic preamp?  If so, what were the results,
> please?
> 
> Many thanks and 73,
> 
> Craig
> W3CRR
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