[Boatanchors] Audio Quality Comparison

[Richard Knoppow]

----- Original Message ----- 
From: "Al Klase" <ark at ar88.net>
To: <boatanchors at mailman.qth.net>
Sent: Monday, February 03, 2014 12:46 PM
Subject: Re: [Boatanchors] Audio Quality Comparison


Bob,

The radios are quite similar. The design goes back to the 
HQ-120. I
doubt you'll hear any significant differences in audio.

That said, the grid coupling caps on the first and second 
audio amps are
sized for "communications quality" audio, that is response 
rolls off
below 200 Hz. If you're looking for "hi-fi" audio increase 
these caps by
a factor of ten. Lack of iron in the output transformer may 
be a
limiting factor for really deep bass.

Al

    My HQ-129-X sounds pretty good although my hearing is no 
longer very good. I have not measured the response of the 
amplifier, might be an interesting project. I don't think it 
rolls of that soon but certainly does below about 100hz. The 
output transformer looks large enough but keep in mind that 
most communications receivers (and lots of others too) have 
single-ended pentode amplifiers with full plate current 
running through the transformer. The DC in the core tends to 
reduce the LF response and increase distortion.  A few 
receivers, the AR-88, GPR-90 and a few others, have some 
feedback around the amplifier which improves response and 
reduces distortion.  One reason receivers like the Super-Pro 
and SX-28 have such good sound is that they employ push-pull 
amplifiers which eliminate the DC in the transformer and 
have much better low frequency response and much lower 
distortion. The SX-28 also has enough feedback around the 
amp to help. The type of audio amplifier in even 
sophisticated receivers like the R-390/A and other Collins 
receivers and most others is fairly crude. If you compare a 
receiver with a push-pull amplifier with one with a 
single-ended amp you will find that even CW sounds better 
because the distortion from the amplifier tends to 
exaggerate noise.  Single-ended amps were used so widely 
because they are cheaper than push-pull even when feedback 
is used. They also tend to produce less heat, another reason 
for their use.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com