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Old 22nd August 2008, 10:32 AM   #11
lineup is offline lineup  Sweden
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16 Ohms
earPhones

Yes, 16 Ohms takes even more current than 32 Ohm.
Now, 16 Ohms earPlugs have usually VERY high sensitivity.
( dB per milliWatt power)

So, the voltage needed across those 16 Ohm is extremely low.
From this follows that the current output will also be quite low.

I = U / R
Current (Ampere) = Volt / Ohm

Say: 0.1 Volt RMS / 16 Ohm = 6.25 mA average current.

These Op-Amps, OPA2134, have max output current like 30-40 milliAmpere.

So they should theoretically and probably even in reality be able to put out:
U = R x I
Volt = Ohm x Ampere
Volt = 16 (Ohm) x 0.035 (Ampere) same as 35 mA
Volt = 0.56


================
35 mA output means:

max 0.56 Volt average, for 16 Ohms earPlugs, Sony, Sharp, Yamaha, etc.
max 1.12 Volt average, for 32 Ohms hifi Grado
max 3.50 Volt average, 100 Ohms
max 10.50 Volt for 300 Ohms Sennhesiser hi-fi headphones

================

Now do not try to put out as much as those Voltages max output.
Because this can destroy your headPhones or earPhones
and if not,
it might DESTROY YOUR HEARING .. forever!



Lineup - has designed a number of transistor HeadPhone Amplifiers
... and so done some math & experiments with different headPhones, listening & measuring
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Old 22nd August 2008, 12:12 PM   #12
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lineup thank you very much for the explanation !

This makes me think if I should use the effort for building a nice lm317/337 regulated dual supply,(to replace the virtual ground one)instead of buildind a buffer.
Maybe both...to test them and learn from the experience...
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Old 22nd August 2008, 12:30 PM   #13
lineup is offline lineup  Sweden
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use
LM317 + LM337
and one 0 Volt GROUND reference

I not long ago helped to advice one guy
that ordered one Velleman kit for such regulated supply

Topic with some information posts by Lineup
Symmetric supply or dual supply

for this Velleman power supply kit, you get LM317/LM337 and all components
+ instruction PDF

You have to buy some suitable transformer
that is all you need
I suggest, for Op-Amp supply
you need no more than one small
5-10 VA transformer (2x12, 2x15, 2x18 Volt)
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Old 25th August 2008, 09:52 PM   #14
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Thank's for all the information/advice. I think I might leave the NE5532's in the spares box for now.

I quite like the idea of using the Szekeres driver. A single MOSFET and some carefully chosen passives. Sounds like it might do the business.

Thank's again,

Regards,

Chris.
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Old 25th August 2008, 10:54 PM   #15
lineup is offline lineup  Sweden
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Szekeres headphone Buffer (single mosfet follower)
is good.

Especially for HeadPhones 16-100 Ohm impedance.

For some HeadPhones, with Impedance 300-600 Ohm + lower sensitivity (SPL Decibel / millWatt)
you may need some Voltage gain.
Usuall Gv (gain voltage) like 5-8 wil be enough. (max + 9 dB).

All the above is for normal CD-player output level ( Max 2 Volt RMS standard ).

When comes to other soures to drive the input of your headphone amplifier you may need more voltage gain.
Or use one good Voltage preamplfier between Sound Source and one HeadPhone Buffer amplifier, Szekeres for example.


Szekeres original MOSFET Follower project is very popular & good.
His article can be found here:
http://www.headwize.com/projects/szeke1_prj.htm


Click the image to open in full size.
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Old 25th August 2008, 10:59 PM   #16
lineup is offline lineup  Sweden
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Here is one
later variation / modification to the Szekeres.
Using LM317 as one CCS, to bias MOSFET output into Class A.

Click the image to open in full size.
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Old 28th August 2008, 09:55 AM   #17
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Well,

I managed to source the MOSFET's on e-bay, I just have to wait until I can get down to the local Maplin's (90 Mile round trip) for the rest of the bits.

Just a quick thought. Would strip board do this circuit justice, or would it benifit from using a PCB with a large ground plane?

Regards,


Chris.
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Old 28th August 2008, 10:13 AM   #18
AndrewT is offline AndrewT  Scotland
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forget ground plane until you understand why it works, how it works and how to make it work properly.
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Old 28th August 2008, 03:12 PM   #19
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With regards to the power supply, could the amp be powered by a small regulated DC plug in type adaptor such as this,

http://www.maplin.co.uk/Module.aspx?...96964&DOY=28m8

Or would something like this be more appropriate?

http://www.maplin.co.uk/Module.aspx?...o=231&doy=28m8
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Old 28th August 2008, 03:26 PM   #20
lineup is offline lineup  Sweden
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forget thsecond one
it is for radio transmitters with high power output at 13.8 Volt

the first one link
the 1.5 A unit may suit,
but will only give you one positive voltage AC 24 Volt
and something like 35 Volt DC after rectification

I would not go for it.

What you would want, for most Op-Amp based headphone amplifiers
is a DUAL DC regulated voltage

for example
2 x 15 VDC = +15 Volt and -15 Volt ( total 30 Volt DC )
or
2 x 12 VDC
or
2 x 18 VDC
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