On Thu, 9 Sep 2010 11:57:28 -0700 (PDT), Andre Jute
wrote:
On Sep 9, 6:59*am, "Iain Churches" wrote:
"Andre Jute" wrote in message
......[snip!].....
In a modest-sized room (by horn standards anyway) I want to rig up a
singleton of my modded semi-Fidelio bicor horns for Lowther PM6A. I
thought I'd do the monoing in a specially made 1 metre cable between
the CD player and the power amp.
The QUAD CD expects to see at least 10K impedance on its outputs.
Output of the CD player is 2Vrms and about 0.5Vrms of that is required
to drive the amp to enough power to handle the horns, so high value
resistors can be used in the monomaker if there is any advantage to be
gained.
However, the sound is NOT intended to be reprocessed as stereo
downstream (home installation, not studio); if I want stereo again,
i'll just plug in a stereo cable.
The pot on the integrated amp is probably (the board won't be here
until Tuesday and i can't read the spec on the piccie) 50K Alps but I
can replace it with a DACT of which I have a good selection of values.
So what value resistors would you use for monoing?
How many? Only two, or three?
With your choice of values, what can I expect to lose from the monoing
in bandwidth as distinct from gain?
Andre Jute
Visit Jute on Amps at
http://www.audio-talk.co.uk/fiultra/
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Hello Andre.
Good to see you posting again.
I would try three R's *arranged as a Y, with the
top two arms (R=10K each) connected at the sources,
and the bottom leg (R=2k2) to ground,
with the mono output taken from the junction of
the three resistors.
I don't get this, Iain. You and David both want a low value resistor
shunted to ground. But it seems to me you're looking backward to the
series resistors whereas the important direction is forwards to the
unknown pot, with which the shunt resistor will be in parallel. If the
shunt resistor is low, the parallel combination will be grotesquely
low.
In the prototype you could make the 2k2 a
trimmer, *to allow you to adjust the attenuation
precisely, and then replace it with the equivalent
fixed value resistor.
Nice touch. However, precision isn't all that important; we just want
to be in the ballpark of not overloading the speaker.
Andre Jute
http://www.audio-talk.co.uk/fiultra/Andre%20Jute's%20Utopia%20Kranich.pdf
The value seems rather low, but let's figure it out with a few
assumptions:
1- impedance (looking back into CD player) is low - quite valid if
it has considerable feedback.
2- Both channels are going full out (2V), and both have identical
signals (worst case scenario for highest mono output level)
3- 10K (or so) are recommended resistors that "Y" the two CD player
outputs.
4- the impedance of the amp is very high
Effectively, your signal source appears as 2v in series with 5K. To
attenuate the signal to 0.5 volts, Vout/Vin for your attenuator is
0.25, and the resistor to ground works out to about 1.66K .
But.... assumption 1 is probably not valid, and 4 isn't either
(input pot). So with a bit of juggling of values, I'd say that 2-2.5k
is quite reasonable, as suggested above.
The other consideration is that you may get recordings that will not
put out the maximum 2V (they don't always max out just shy of the
maximum possible from the DAC circuits). In such cases you won't be
able to run your system full out, whereas if you had more of a gain
margin you could get it. That would suggest even higher values for the
resistor to ground, or even no resistor to ground.
On the other hand, since it takes a factor or 10 times the power to
sound twice as loud, I don't think it's going to make a big difference
to the sound level if the resistor to ground varies a bit. If I were
you, my primary concern would be if someone (like the kids) turn
everything up full out, then you could burn out your speakers. In that
case, I'd use the 1.66K (or thereabouts) to be on the safe side.
Are your horns rated for continuous RMS (rare for home systems) or
some other standard where there is a peak level for a short duration?
If the latter, I would be much more worried if I were you, especially
if they are expensive or "one of a kind". If your speakers are rated
for short peak levels, the above calculations would not be
conservative enough.
Getting somewhat off topic, what is the best way to protect a
speaker from destruction if the amp is sufficiently powerful, and you
have no control over volume? It happens.... the kids have fun with
your stereo, the grounds come loose on the inputs (huge buzz), the
system oscillates, etc.
Paul G.
Making a mono-from-stereo cable
Linear Mode
