On Tue, 13 May 2008 13:35:45 -0500, John Byrns <byrnsj@[EMAIL PROTECTED]
>
wrote:
>In article <7f8j24tbsh7grb75t8jpiomohmfo0hvhn2@[EMAIL PROTECTED]
>,
> flipper <flipper@[EMAIL PROTECTED]
> wrote:
>
>> On Tue, 13 May 2008 08:22:52 -0500, John Byrns <byrnsj@[EMAIL PROTECTED]
>
>> wrote:
>>
>> >In article <kk4i24t13io43j65n6jgmbjnpbr7juna56@[EMAIL PROTECTED]
>,
>> > flipper <flipper@[EMAIL PROTECTED]
> 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.
> 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'.
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.
> 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.
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.
>
>
>Regards,
>
>John Byrns
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