[Vintage-Audio] Speaker Connections On LK-48b

[Fred Olsen]

Duane wrote:
> The H. Scott LK-48b stereo amplifier has two terminal strips on the rear for the
> speakers to be connected to. These are screw type connectors. They are marked as
> "A" and "B", however, the "A" is the left channel and the "B" is the right
> channel. Unlike some amplifiers, this amplifier does not have a either/or
> speaker system "A" or "B" arrangement. 	
> 	
> Each terminal strip is marked: 4 ohms, 8 ohms, 16 ohms, 0, 0 The zero meaning
> ground. Both of the zero screws have identical markings and I am presuming that
> both are for negative, or ground. I am further presuming that the impedance of
> the speakers being used, 8 ohms in my case, is the positive connection for the
> speakers.

Duane, through the courtesy of Lee Shuster's H.H. Scott Archive,
http://hhscott.com/
I'm looking at the schematic for the LK-48B.  I don't have a picture of 
the rear apron available but the skiz shows that there are separate 
terminal strips for impedance selection and speaker connection.  There 
is no schematic reference to "A" or "B" terminals.

The impedance strips have three terminals, for four, eight, and 16 ohms. 
  An external connection for "0" (zero), the secondary "start" or low 
end, is not shown.  The zero end of each secondary is connected through 
the "Speakers" switch, or switches, to the 0 (zero) terminals of the 
respective right and left speaker terminal strips.  These ARE screw 
terminals and would be speaker "low".  When the speaker switch is off 
each secondary's low is returned through a 60 ohm five watt load to the 
16 ohm tap, the secondary's high end.

The speaker terminals consist of three pairs of screw terminals, marked 
on the skiz as left, center, and right.  The "H" (high) terminal of each 
of the left and right is connected to the flying lead used for impedance 
selection.  The speaker zero terminals are connected as noted above. 
The center channel is a derived difference output typical of the design 
era.  Its high terminal is connected directly to the secondary's zero 
end of the left channel output transformer.  The low terminal is 
connected directly to the 16 ohm tap of the secondary of the right 
channel output transformer.  I surmise that this works because it is not 
the "0" tap but the 4 ohm tap of each secondary which is at B- and 
chassis ground.

There is also a line level center channel output to a phono jack.  It is 
summed however, with a voltage divider driven from the 16 ohm taps of 
both channels.  The headphone jack is further divided from this point.
  	
Duane wrote:
> Does anybody out there know for sure? 	
I'm taking my best SWAG here.

Duane wrote:
> Why are there two negative, or ground, screws per speaker?
You may have to determine what's what with the terminals after we're 
done here.

Duane wrote:
> The big brother, the LK-72b, has a jumper wire that runs between the selection
> of speaker impedance. It has separate positive and negative terminals for each
> speaker. 	
Right, and remembering that H.H. Scott was big on family resemblance I 
took a look at the LK-72A and B schematics, from the same site.  The 
skiz for the 72B is as you describe here, and as I described above for 
the 48B.  The skiz for the 72A resembles what would result in your 
physical description of your 48B.  Stay with me now.  What I'm saying is 
that the 72A does not have the differential center channel output at 
speaker level, to a terminal strip.  It does still have the summed 
output to a phono jack.  Most important, the 72A takes its speaker 
outputs directly from the transformer secondary taps and does not have 
separate speaker terminal strips or the flying lead for impedance 
selection.  Its zero taps do come out to screw terminals, unlike the 
skiz of the 48B or 72B.

My conclusion is that either you have an LK-48A, repeat "A" model, or 
you have an early (I would guess pre-1963) LK-48B which is configured 
more like the A model.

If you are equipped to have Brandon take some digital pix, feel free to 
send me a closeup of the rear apron, in the vicinity of the speaker 
terminals.

I can suggest another way to help resolve this, by measurement. 
According to the schematics available to me the LK-72A has its speaker 
low or zero terminal returned to B-/chassis ground.  The LK-48B and 
LK-72B have their four ohm terminals returned to B-/chassis ground.  In 
that case it should be possible to measure some signal at the left and 
right channel zero taps, which are the speaker low or zero terminals, 
versus chassis ground.  The same left channel signal would be seen at 
the center channel high speaker terminal.

To all of you who didn't care to read my drivel, blame Duane.  It's his 
beloved "reply-to" function which shoves everything to the list.  ;<))

tired of typing Fred
and I'm not even a Scott guy!
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Outgoing checked by Norton AV