[Premium-Rx] The "Sound" Of Receiver IF Amplifiers

[Bob Milne]

Hi All,

For years I have been intrigued (and mystified) by the very noticeable
differences in the "sound" of various receivers. Much of this I
chalked up to differences in the audio sections and tube vs.
solid-state amplifiers. But that ain't necessarily so.

Having recently acquired a Sherwood SE-3 Synchronous Detector on eBay
my eyes (and ears) have been opened. Besides performing superbly as a
synchronous detector, the SE-3 is a marvelous tool for evaluating
receiver IF sections (since it bypasses the on-board detector and
amplifier circuits).

Using an HP 8640B and a Mini-Circiuits ZAD-1H mixer to match the SE-3
to IF's ranging from 5 MHz to 50 kHz, I started comparing various
receivers. Here's a few quick observations:

To my ears, the Mackay MSR 5050A definitely sounds smoother and less
harsh than the Racal RA6790/GM. But the SE-3 revealed that the
occasional bursts of high-frequency distortion the 5050A has on some
signals (mostly AM) originates in the IF section--and not in the audio
section as I had suspected.

The Drake R7A sounds superb on the SE-3, as does the Collins 51S-1.
And because the 51S-1 uses offset 2.75-kHz mechanical filters, you can
switch from AM to USB to LSB without losing lock on the SE-3. This is
the first time I've been able to take advantage of the 51S-1
mechanical filters on AM signals since the non-synthesized VFO just
isn't stable enough for normal ECSS tuning. And the SE-3 preserves at
least 90% of the crystal-clear audio the 51S-1 is capable of.

Another thing that surprised me is how well-matched the 51S-1's
mechanical filters are. Using the SE-3 it's hard to detect any change
in tonal quality switching between the USB and LSB filters. In
contrast, the MSR5050A's USB filter definitely has more treble
emphasis than the LSB filter. 

Anyway, what these few quick comparisons have proven to me is that
many of our Premium Receivers have wonderful features which are hard
to do without once you've used them; stable and precise VFOs, memory
channels, keyboard frequency entry, etc. But as for producing the
best, most-listenable audio, some non-premium receivers have them
beat.

Any comments or observations from the group?

Regards....
....Bob