[Boatanchors] Audio Amplifiers (And, no, I'm not an audiophool!)

[Michael Tauson]

Okay, here’s the situation.  I’m setting up to do some serious work in
creating animations and other stuff for Noelle, essentially creating a
digital universe where we can be together.  It's kind of like how animated
movies are made but on a much smaller scale and significantly lower budget.
The video side I have pretty well covered including creating the animations
themselves however the audio portion is still seriously lacking.  If I had
my druthers, I’d have something like a tube-type Harmon-Kardon amp and a
brace of AR-4s (AR-3s wouldn’t fit here.) but I don’t so have to go a
slightly different path.  

 

Cruising my local Goodwill & Salvation Army stores netted me no joy which is
quite different from ten years ago when I did find a pair of ugly AR-4s that
still were good inside.  I should have snared them but I was kinda sorta
homeless at the time and had nothing to connect them to.  And no cash to buy
them.  I guess that counts too.  There were also some decent stereo amps and
receivers but that was ten years ago.  Now the only things they have are
boom boxes and computer speakers which seem popular with others but have all
the audio response of a 
 hmmm 
 I can’t think of anything bad enough.  Tin
can with a slack string?  

 

Going through one of my tube boxes, I located a small collection of 25L6,
35L6 and 50L6 tubes, some 12SL7s or 12SN7s (I don’t recall which) along with
some others I can’t remember right off.  Memory serving the *L6s are all the
same save the heater voltage so mixing them shouldn’t be an issue if I have
to do so.  For the moment, let’s dismiss the heater issues and concentrate
on what’s really important – i.e., the audio.  I know that computers don’t
have the greatest audio capabilities in the world but I’d rather the machine
be the limiting factor than what comes after.  The desired end is reasonably
flat response over the 20-20k cycle range – pretty typical for a decent
stereo system back in the day – with around 5-10w out per channel.  Part of
the time I’ll be using headphones (mostly during editing) and the rest of
the time speakers.  

 

As I remember, and I could easily be wrong, the line out on a computer audio
system is looking for a 600 ohm load.  As a result, the first order of
business is to get that up to something a tube can handle more or less
gracefully.  After that come the volume controls and tone controls (bass,
mid and treble) which, last I did anything with audio were supposed to be
isolated from one another to prevent interaction.  After that, a phase
inverter and push-pull *L6s then out to irritate the neighbors.  I may add a
sub-woofer channel, tapping off each channel just before the tone controls
and pushing the audio through low pass filters to a mixer and a fairly
reasonable amp but I doubt I’ll find a sub-woofer like the old days –
response in the 10-200 cps range*.  OTOH, I may add the third channel just
in case.  It certainly can’t hurt.

 

* It seems that people are happy with 35-15k cps response and don’t care
about what they’re missing.  Even some audiophools.  Even the ones who can
hear negative frequencies and distortion that can’t be registered on any
known instruments like scopes and all.  Weird. 

 

>From what I can see, I’ll need a brace of 10kohm ct to 4 or 8 ohm output
transformers and a slew of audio taper pots.  I think I have some slide pots
that will work for that part although I haven’t measured the resistance yet.
They’re on a reject pot board from an equalizer that I have yet to
dismantle.  (The pot board, not the equalizer.  I don’t have it.  If I did,
I wouldn’t need the tone controls.)  One thing I don’t know is if metal film
resistors are good for audio systems.  The only input I have on that is from
the phools who say they sound “metallic.”  I have carbon comp resistors
although I have to dig them out and apply an ohmmeter to see how much
they’ve wandered off from the shown value but metal films are cheap and
available.  

 

Anyway 
 now that I’ve drifted into a stream of consciousness thingie, any
suggestions would be greatly appreciated.

 

Mele Kalikimaka me ka Hau'oli Makahiki Hou!

 

Michael, K3MXO, BL01hx15np24 ... or there abouts  

Note to self: Hire new henchmen.  Good ones this time.  Also start
interviews for attractive female lab assistant.

 
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Hiki Nô!