Headphone Amplifier

Headphone Amplifier

Discussion about the headphone interface on the 6AW8 PC Speaker amp
got me to thinking so I whipped up a little OTL circuit, just for
headphones, using the 6CY7 (essentially a 'half power' 6EM7)..

Why? Well, when researching the series resistor thing I noticed a lot
of the so called 'tube' headphone amps have solid state outputs and a
lot of the all tube ones couldn't drive 32 ohm headphones very well..
The 6CY7 should take care of both those issues, and with the IEC
specified 120 ohm source impedance

Schematic and pics of the breadboard (I only did one channel) are up
on my site * flipperhome.dyndns.org

It's not really fleshed out more than needed to breadboard and see the
circuit work, but that it did.

There's plenty of room for tweaking it. Spice indicated excellent open
loop performance, in case that's what floats your boat, but I wrapped
13dB around it anyway for the exercise. And there's room for a bit
more gain since, for the sake of simplicity, I didn't bypass the gain
triode Rk.

I figured some might get a few laughs at my sloppy breadboard so I
included a pic of it for your amusement.

Re: Headphone Amplifier

flipper wrote

> Discussion about the headphone interface on the 6AW8 PC
> Speaker amp
> got me to thinking so I whipped up a little OTL circuit,
> just for
> headphones, using the 6CY7 (essentially a 'half power'
> 6EM7)..
>
> Why? Well, when researching the series resistor thing I
> noticed a lot
> of the so called 'tube' headphone amps have solid state
> outputs and a
> lot of the all tube ones couldn't drive 32 ohm headphones
> very well..
> The 6CY7 should take care of both those issues, and with
> the IEC
> specified 120 ohm source impedance
>
> Schematic and pics of the breadboard (I only did one
> channel) are up
> on my site * flipperhome.dyndns.org
>
> It's not really fleshed out more than needed to breadboard
> and see the
> circuit work, but that it did.
>
> There's plenty of room for tweaking it. Spice indicated
> excellent open
> loop performance, in case that's what floats your boat,
> but I wrapped
> 13dB around it anyway for the exercise. And there's room
> for a bit
> more gain since, for the sake of simplicity, I didn't
> bypass the gain
> triode Rk.
>
> I figured some might get a few laughs at my sloppy
> breadboard so I
> included a pic of it for your amusement.

Exactly the same as my breadboard, except you use screws
where I use solder.

I started a thread a while ago about valve headphone amps,
and I don't think any different now, except thanks for the
detail of the IEC standard (regulation, guideline...what are
these things and what are they for?).

For what it's worth, my current design aim is for an
open-loop output impedance of 5 ohms, with feedback to
reduce that to a couple of ohms at most.

At the time of that thread I was looking into the consequent
desire to limit power output for safety reasons, and
wondered how. I will experiment once I've built the amp,
which will be some time after I've found some money to pay
for the transformers.

I think you will find, if you look at decent high fidelity
headphone amp designs, and transformers intended for decent
valved examples, they achieve an impedance ratio of 1:10 or
lower. One problem of traditional valved designs is that
they are intended for phones of 70 ohms or more. 32 ohms is
increasingly the norm, it seems. I can't see how driving a
non-linear voltage-controlled device of 32 ohms impedance
with a 120+ ohm source is ever going to be a good idea.

Perhaps if a high output impedance is enforced, high
impedance phones may become common, but surely that would
entrench a compromise to the detriment of fidelity?

cheers, Ian

Re: Headphone Amplifier

On Tue, 13 May 2008 03:58:36 +0100, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:

>flipper wrote
>
>> Discussion about the headphone interface on the 6AW8 PC
>> Speaker amp
>> got me to thinking so I whipped up a little OTL circuit,
>> just for
>> headphones, using the 6CY7 (essentially a 'half power'
>> 6EM7)..
>>
>> Why? Well, when researching the series resistor thing I
>> noticed a lot
>> of the so called 'tube' headphone amps have solid state
>> outputs and a
>> lot of the all tube ones couldn't drive 32 ohm headphones
>> very well..
>> The 6CY7 should take care of both those issues, and with
>> the IEC
>> specified 120 ohm source impedance
>>
>> Schematic and pics of the breadboard (I only did one
>> channel) are up
>> on my site * flipperhome.dyndns.org
>>
>> It's not really fleshed out more than needed to breadboard
>> and see the
>> circuit work, but that it did.
>>
>> There's plenty of room for tweaking it. Spice indicated
>> excellent open
>> loop performance, in case that's what floats your boat,
>> but I wrapped
>> 13dB around it anyway for the exercise. And there's room
>> for a bit
>> more gain since, for the sake of simplicity, I didn't
>> bypass the gain
>> triode Rk.
>>
>> I figured some might get a few laughs at my sloppy
>> breadboard so I
>> included a pic of it for your amusement.
>
>Exactly the same as my breadboard, except you use screws
>where I use solder.

You can tell from the pic I end up with a mix, especially with a
circuit like the headphone amp, and that 'two tube' board, where it's
not 'just' a pair of PP tubes (like 6BQ5s) into an OPT.


>I started a thread a while ago about valve headphone amps,
>and I don't think any different now, except thanks for the
>detail of the IEC standard (regulation, guideline...what are
>these things and what are they for?).

Well, an interconnect standard, for example, is so things
interconnect.

A dramatic example is Apollo 13. One 'simple' solution to the oxygen
problem was to swap control valves with the working unit in the Lander
but they were built by two different manufacturers and the threads
were different.

If there had been a 'standard' defined then it would have worked.

In the case of headphones, it's so if you buy a 'standard' headphone
and have a 'standard' amplifier then they should work together. At
least within the parameters defined in the standard.

>For what it's worth, my current design aim is for an
>open-loop output impedance of 5 ohms, with feedback to
>reduce that to a couple of ohms at most.
>
>At the time of that thread I was looking into the consequent
>desire to limit power output for safety reasons, and
>wondered how.

As I mentioned in the PC Speaker thread, the 120 ohm source impedance
helps that issue, over differing impedance headphones anyway. By that
I mean, if you set the volume for 1V out, driving 300 and 32 ohm
headphones directly, with 'low impedance' results in an almost 10dB Po
difference but (increasing Vo to compensate) it's less than 1dB with
120 ohm source impedance.

> I will experiment once I've built the amp,
>which will be some time after I've found some money to pay
>for the transformers.

How are you planning to solve the wide range of impedances presented
by differing headphones?


>I think you will find, if you look at decent high fidelity
>headphone amp designs, and transformers intended for decent
>valved examples, they achieve an impedance ratio of 1:10 or
>lower.

I did look and found just about 'everything' imaginable, as well as
units that meet IEC specifications. Of course, those have 120 ohm
source impedance.

> One problem of traditional valved designs is that
>they are intended for phones of 70 ohms or more.

I don't know. Before I finally found one or two with one I had thought
the 'norm' was *no* headphone output on valve amps.

> 32 ohms is
>increasingly the norm, it seems.

FWIW, my circa 1970 Sansui headphones are 8 ohm. My new ones are 32
ohms.

> I can't see how driving a
>non-linear voltage-controlled device of 32 ohms impedance
>with a 120+ ohm source is ever going to be a good idea.

Well, you have imbedded assumptions in there but a low source
impedance won't magically turn a non linear device into a linear one.


>Perhaps if a high output impedance is enforced, high
>impedance phones may become common,

I don't know how much you consider 'high impedance'. Seems to be a lot
of 300 ohm phones out there. Is that 'high'?

> but surely that would
>entrench a compromise to the detriment of fidelity?

Again you're operating from the imbedded assumption of some source to
driven impedance ratio but you don't explain the reasoning for it.

I get the impression it's an extrapolation from driving speakers but
headphones are not speakers. Mass is, by comparison, virtually
non-existent, they move almost no air, inductance is miniscule, and so
on. I don't have math to evaluate all that but they're so dramatically
different that simple extrapolations are highly suspect, at best.

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.

With low impedance headphones there's a teensy improvement in
distortion because, without the 120 ohm, output swing is less but
it's negligible with 300 ohm headphones.

Like I said, plenty of room for tweaking ;) But the basic topology
looks good and 6CY7s are rather cheap.

You felt the 6AW8s looked 'gloomy'. Well, lemme tell ya, the 6CY7s are
anything but and those suckers glow like Christmas tree lights.

Oh, something I noticed after posting everything. When I compromised
with 200V on the phase splitter I forgot to readjust the R3 current
soak resistor. It's there just to suck up the idle current V1A puts
onto the output rail, and at 200V it's less than at 300V, so R3 should
be more like 250k. Doesn't really matter all that much, though.

I kept idle current low for the wallwart but gain goes up, and
distortion goes down, a bit if you increase idle and if you use a
conventional AC main supply you can go up to 50mA, which increases
idle from 4 Watts to a little over 10 Watts (per channel). Max Po also
goes up to 200mW. That's into 32 ohm headphones but it's more with 300
ohm phones and it'll do over a half watt with those (with 300V on the
phase splitter and, perhaps, a bit of optimization).

>cheers, Ian
>

Re: Headphone Amplifier

In article <kk4i24t13io43j65n6jgmbjnpbr7juna56@4ax . com >,
flipper <flipper@fish . net > 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,
with the expected effect on source impedance?

> With low impedance headphones there's a teensy improvement in
> distortion because, without the 120 ohm, output swing is less but
> it's negligible with 300 ohm headphones.
>
> Like I said, plenty of room for tweaking ;) But the basic topology
> looks good and 6CY7s are rather cheap.
>
> You felt the 6AW8s looked 'gloomy'. Well, lemme tell ya, the 6CY7s are
> anything but and those suckers glow like Christmas tree lights.
>
> Oh, something I noticed after posting everything. When I compromised
> with 200V on the phase splitter I forgot to readjust the R3 current
> soak resistor. It's there just to suck up the idle current V1A puts
> onto the output rail, and at 200V it's less than at 300V, so R3 should
> be more like 250k. Doesn't really matter all that much, though.

I was wondering about that, I had assumed that the 120k value shown on
the schematic was for the 200 volt supply feeding the phase inverter on
the "breadboard", and wondered why you hadn't also shown the correct
value to use with the intended 300 volt supply, now I see that I had it
sort of backwards.


Regards,

John Byrns

--
Surf my web pages at, * fmamradios . com /

Re: Headphone Amplifier

On Tue, 13 May 2008 08:22:52 -0500, John Byrns <byrnsj@sbcglobal . net >
wrote:

>In article <kk4i24t13io43j65n6jgmbjnpbr7juna56@4ax . com >,
> flipper <flipper@fish . net > 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.

I liked the idea of eliminating one of the high voltage blocking caps
and there's another version with CF, put the 120 ohm in parallel with
the headphones. You get the same 'semi constant' Po vs headphone
impedance but it increases the uF needed for the output coupling cap
because the minimum load is then 120 || 32 instead of 152.

>
>> With low impedance headphones there's a teensy improvement in
>> distortion because, without the 120 ohm, output swing is less but
>> it's negligible with 300 ohm headphones.
>>
>> Like I said, plenty of room for tweaking ;) But the basic topology
>> looks good and 6CY7s are rather cheap.
>>
>> You felt the 6AW8s looked 'gloomy'. Well, lemme tell ya, the 6CY7s are
>> anything but and those suckers glow like Christmas tree lights.
>>
>> Oh, something I noticed after posting everything. When I compromised
>> with 200V on the phase splitter I forgot to readjust the R3 current
>> soak resistor. It's there just to suck up the idle current V1A puts
>> onto the output rail, and at 200V it's less than at 300V, so R3 should
>> be more like 250k. Doesn't really matter all that much, though.
>
>I was wondering about that, I had assumed that the 120k value shown on
>the schematic was for the 200 volt supply feeding the phase inverter on
>the "breadboard", and wondered why you hadn't also shown the correct
>value to use with the intended 300 volt supply, now I see that I had it
>sort of backwards.

Yeah, I screwed up. I drew it up as an exercise then, later, suddenly
got a bur under my saddle to build the thing and was 90% done when I
got to the 'connect to 300V' part and, well crap, I had forgotten the
300V was in there. LOL

But at that point, hey, what the hell? Might as well try it at 200V.

The 'saving grace', so to speak, is it doesn't have to produce a huge
voltage swing so it still works at the reduced voltage.

>Regards,
>
>John Byrns

Re: Headphone Amplifier

In article <7f8j24tbsh7grb75t8jpiomohmfo0hvhn2@4ax . com >,
flipper <flipper@fish . net > wrote:

> On Tue, 13 May 2008 08:22:52 -0500, John Byrns <byrnsj@sbcglobal . net >
> wrote:
>
> >In article <kk4i24t13io43j65n6jgmbjnpbr7juna56@4ax . com >,
> > flipper <flipper@fish . net > 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 and people
seem to feel they need to be driven from a voltage source. They talk of
"damping factor" and such. Can I assume that "damping" isn't a factor
with headphones, at least if they are driven from a 120 Ohm source?


Regards,

John Byrns

--
Surf my web pages at, * fmamradios . com /

Re: Headphone Amplifier

On Tue, 13 May 2008 13:35:45 -0500, John Byrns <byrnsj@sbcglobal . net >
wrote:

>In article <7f8j24tbsh7grb75t8jpiomohmfo0hvhn2@4ax . com >,
> flipper <flipper@fish . net > wrote:
>
>> On Tue, 13 May 2008 08:22:52 -0500, John Byrns <byrnsj@sbcglobal . net >
>> wrote:
>>
>> >In article <kk4i24t13io43j65n6jgmbjnpbr7juna56@4ax . com >,
>> > flipper <flipper@fish . net > 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

Re: Headphone Amplifier

> 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.
>

Hi RATs!

Yes indeed, current drive will be better than voltage drive,
sometimes :)

Why not use both, as a double media to really make it sound like ABBA
is a interesting digital noise shaper ;)

Just a thought ... ???


Happy Ears!
Al

Re: Headphone Amplifier

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
proportional, 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

Re: Headphone Amplifier

On Thu, 15 May 2008 01:26:18 +0100, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:

>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?

It means the cone is made to move by magnetic field and the field
generated by a coil is proportional to nI.

> They are generally
>designed to be voltage controlled.

As I said, we tend to think of voltage as 'the signal'.

>
>>> 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.

Because we design with that concept. But, for example, the 'signal'
from a photo diode is current, the output of a tube is current and
bipolar transistors are both current in and current out.

> If it is convenient, it is not uncommon to see the
>signal transferred to current within circuits.

The other way, actually. A vacuum tube's output current is converted
to voltage by the load but with bipolars that not need be as current
can be the input as well..

> 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...

Current drives the cone and your 'if it weren't for' is precisely one
of the 'problems' with using constant voltage drive.


>> 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 both oppose cone movement requiring more power aggravating the
problem when your 'signal is a voltage that gets reduced by source
impedance in series with coil impedance.


>They also have more area, and much more power available to
>overcome the mass and move the air.

Power that drops across the source impedance when your 'signal' is
constant voltage.

> The crucial point is
>that the scaling of all these parameters is not equally
>proportional, and the result favours quieter things in
>smaller spaces.

So why go through all that only to arrive at the same conclusion?

>
>>> 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.

I didn't say the 'principles' were different.

> The differences arise only
>from scale,

Well, the 'only difference of scale' is why large animals don't have
exoskeletons like insects.

'Scale' breaks a lot of things, 'same principles' notwithstanding.

> 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.

While they could all be derived from 'principles', down to quantum
mechanics to be pedantic, but 'equations' are usually reduced to
'significant' variables with the "10 to 1" damping factor we're
talking about as not necessarily applicable being an excellent example
seeing as how there's not a hint of cone mass, excursion, or anything
else even obliquely referenced in it.


>In reality, unless you are designing headphones, you don't
>need to know how they work. We just need performance data.

You do if you're trying to make a case that speaker damping factor
rules of thumb are applicable to proper headphone operation.


>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.

Why do you go through all that only to arrive at the same conclusion
you're supposedly 'debating'?


>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.

Depends on what you mean by 'high impedance drive' and the proposed
series-series FB circuit was not a constant voltage drive through
'high impedance' but a constant current drive that has high impedance.
They both have high impedance but in one case the voltage doesn't
change while it does in the other. Or vice versa, depending on which
one likes to call 'the signal'.


>> 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
>