flipper wrote:
>>> >> Btw, the OTL doesn't 'require' the 120 ohm resistor
>>> >> and it works just
>>> >> fine as a 'low impedance' output.
>>> >>
>>> >> You could just remove the 120 ohm, and then adjust
>>> >> the FB resistor for
>>> >> the changed Vo, but there's also a nice trick can be
>>> >> played. Instead
>>> >> of the FB R and blocking cap. as shown, put a 12ohm
>>> >> under the existing
>>> >> Rk on V2A and take the headphone return to that
>>> >> junction. That
>>> >> eliminates the DC blocking cap.
>>> >
>>> >If I am following your description of this mod
>>> >correctly, doesn't that
>>> >convert the negative feedback from voltage feedback to
>>> >current feedback,
>>>
>>> Yep
>>>
>>> >with the expected effect on source impedance?
>>>
>>> Yes, but then a moving coil device is actually current
>>> driven so I'm
>>> not sure what the 'net effect' would be.
>>
>>The common "dynamic" loudspeaker is also a moving coil
>>device
>
> Quite right, and it's current driven too.
What does current driven mean, exactly? They are generally
designed to be voltage controlled.
>> and people
>>seem to feel they need to be driven from a voltage source.
>
> It's certainly the most common means, but then that's the
> most common
> means for most things.
>
> We seem to conceptually favor voltage as the 'signal
> thing'.
The 'signal thing' *is* voltage, as we receive it, in most
cases. If it is convenient, it is not uncommon to see the
signal transferred to current within circuits. Surely
voltage is the obvious first choice for encoding a signal
intended for a speaker, because it relates directly to cone
position, or at least it would if it wasn't moving about so
quickly...
> But I've seen papers suggesting 'current drive' would be
> 'better' for
> speakers as well. Whether they're 'correct', or whether
> different coil
> techniques would/should go with it, I don't know.
>
> Interestingly, when they plotted speaker response it
> turned what was a
> response dip with constant voltage into an equal but
> opposite response
> peak with constant current. Is that worse? or better? or
> just
> different?
>
> That wasn't necessarily the only effect but it's one I
> remember.
>
>> They talk of
>>"damping factor" and such.
>
> By comparison, speakers have lots of mass and move lots of
> air, not to
> mention the coil electrical characteristics are different.
Well, moving lots of air and having lots of mass work in
opposite directions as far as the need for damping is
concerned.
They also have more area, and much more power available to
overcome the mass and move the air. The crucial point is
that the scaling of all these parameters is not equally
pro****tional, and the result favours quieter things in
smaller spaces.
>> Can I assume that "damping" isn't a factor
>>with headphones, at least if they are driven from a 120
>>Ohm source?
>
> I've seen professionals debate both sides of the issue
> but, by the
> same token, they were talking 'ultimate' and not that 120
> ohms was
> 'crap'. Unfortunately, neither side was able to put things
> like cone
> mass, etc., into numbers and, near as I can tell, the talk
> about
> damping factors is simply a direct extrapolation from
> speakers which,
> as noted above, are a whole different animal than
> headphones.
>
No, the same principles apply. The differences arise only
from scale, because some parameters are related to area,
some to volume, and others to distance. Same reason
elephants need such thick legs, and still can't
jump...er...that's not a good analogy considering I'm
arguing it's not another animal...all I mean is that you
would use the same equations for speakers and headphones,
but the results would be different because the parameters
scale in several different ways.
In reality, unless you are designing headphones, you don't
need to know how they work. We just need performance data.
Now, possibly at the same site you have been to, it is said
that a pair of Grado 32 ohm phones vary by 4 ohms over the
range 20Hz to 20kHz. It also suggests that Grados may be
well-behaved in this respect compared to other phones. Some
more data would be good.
But for the moment, how significant is that 4 ohms, when
driven by a 120 ohm source? Not very, many may argue, when
they've worked out how many dBs power variation ensues.
Considering the load they present is so nearly resistive,
there would be correspondingly little difference between
current- and voltage-derived feedback. Normally,
current-derived feedback arising from the reactive part of a
speaker load would approximate to velocity feedback rather
than the usual approximation to position feedback, for a
sine wave. For headphones, none of this applies if their
deviation from a resistive load is not significant. Neither
would damping factor be an issue, because the dynamic
behaviour of the machine is obviously not apparent to the
source, and so cannot be damped by it. Presumably mechanical
damping is sufficient.
Somewhere, last time I was looking, I remember seeing a
comparison between the ideal response for headphones (not
flat, apparently) and the response of some real phones, with
a low impedance and a high impedance drive. Can't find it
now, but such info must surely be available from somewhere?
It should be possible to find the data. The real argument
then concerns the threshold of significance.
> But I don't have the 'answer' either. I'm just relaying
> what I've read
> and discussing it.
>
> The circuit can be done either way. Well, 4 ways so far.
> Series-shunt
> or series-series FB and both either with or without the
> 120 ohm.
Cheers, Ian
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