[AMRadio] 32V-2 speech amp question


Gary Schafer garyschafer at comcast.net
Mon Jan 23 12:09:27 EST 2006


Hi John,

I think this "hi-fi" thing gets carried a little far with a lot of people. I
agree with you that most sound "not so good" with narrow receiver filters.
Like you say unless it is a pretty much local qso you can't use wide
bandwidth on the receiver. 

There is a lot of energy in the low frequencies. They do little for
intelligibility. But if they are present they take away modulation power
from the more usable frequencies in the audio bandwidth. The low frequencies
will over modulate the transmitter before the mid range frequencies will so
that limits the amount of energy that you can apply to the more useful
mid-range.

73
Gary  K4FMX


> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of John E. Coleman (ARS WA5BXO)
> Sent: Monday, January 23, 2006 11:19 AM
> To: 'Discussion of AM Radio'
> Subject: RE: [AMRadio] 32V-2 speech amp question
> 
> 	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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