[Boatanchors] Audio Quality Comparison

[Al Klase]

Hello Richard,

I need to take exception to some of your statements.  The audio in the 
SX-28 is a joke, IMHO.  But, rather than addressing your message point 
by point, allow me to relate my communication receiver audio adventure 
of some years ago.

I came into possession of a large ornate cabinet containing an Altec 604 
"Duplex" speaker, a 15-inch woofer with a multi-sector HF horn piercing 
it's center, plus appropriate crossover.  The speaker itself cost 
something like 150 bucks in 1940's money.  They were the darling of 
recording studio engineers for many years.  It was too large to go 
anywhere other than my living room, so I had to look at it.

After a while I hooked up the SX-28, and listened.  Receivers of this 
sort, with IF selectivity that can be opened up beyond 10 KHz, have the 
potential for good sound quality on "hi-fi" stations.  The 28 was loud, 
but not impressive.  So I lugged out the SP-200, and put the power 
supply inside the speaker cabinet.  The improvement in sound was amazing.

When you look at the Super-Pro circuit you find push-pull power 
pentodes, 42's in the original SP-10, as I recall, and 6F6's in the 
later sets.  However, the power tubes are triode-connected.  This 
dictated an additional driver transformer and a third triode connected 
6F6 as a driver to offset the lower gain of the triodes. Clearly the 
Hammarlund guys had decided that the cheaper pentode setup popular 
broadcast receivers of the day did not sound good enough for a 
professional radio.  The Super-Pro's were touted for broadcast monitors 
and program relay receivers.  In the days before feedback was fully 
understood, triodes were the only way to get clean sound.

I related this experience to a good friend of mine who also collects and 
listens to communications receivers.  He says "Now try it with your 
AR-88."  I reply "Aw Pete, it's only a single-ended 6K6.'  He replied 
"Just try it."  So out goes the Super-Pro and in comes the 100-pound 
AR-88.  Once again the improvement was like night into broad daylight.

Those guys down at RCA Camden really knew what they were about. There's 
a global negative-feedback loop from the secondary of the output 
transformer to the cathode of the first audio amp, a circuit that would 
become pretty standard in post-war high-fidelity equipment.  The audio 
amp in an AR-88 is extremely linear, both by measurement and by ear, and 
I'll bet there's extra money in the output transformer.  Among other 
things, the reduction of inter-modulation distortion makes signals 
easier to copy in the presences of noise.

So when it was time to hot rod the SP-600 that somehow failed to inherit 
the audio characteristics of the SP-200, I did this with appropriate 
results:
http://www.skywaves.ar88.net/commrx/Hammarlumd/SP-600/SP-600_Audio.html

Best regards,
Al


On 2/3/2014 5:03 PM, Richard Knoppow wrote:
> 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.
>
>

-- 
Al Klase – N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/