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StalfoS 20th March 2005 12:35 AM

Headphone Output to GC amp
 
What is the best way to add a headphone output to a GC amp? Has anyone had any experience with this? Can the lm3875 chip be used to drive the phones directly, or would it be better to include a separate circuit?

I wouldnt mind including a switch to turn the headphone output on/off as well as a switch for the speaker output. I sometimes like to switch between listening through the phones and speakers when I am mixing music.

sek 20th March 2005 02:00 AM

Using the chipamp to power headphones is a waste of sound quality (a headphone wants to be driven 'differently').

Technically speaking, you could add a voltage divider to the output in order to reduce the amp's output level, matching the headphones specified sensitivity (and max. power limit!). This is how it's done in most home stereos.

But this has many drawbacks, as the GC has:
- audible noise,
- output offset that could ruin the headphones,
- class A/B switching distortions that are audible via headphones,
- only average sound into good headphones, even a regular opamp could sound as good...

I've never tried connecting a headphone to a chipamp, because I always feared an otherwise unnoticed turn-on/-off thumb or the small-but-not-zero output DC offset could harm it.

Sooo, you could try it with inexpensive headphones, but to answer your actual question: The best way of switching between loudspeakers and headphones is to switch between loudspeaker amp and headphone amp. ;)

Cheers,
Sebastian.

PS: Could even be powered from the GC supply and built into the same case. ;)

Alcaid 20th March 2005 04:12 PM

A small split supply Cmoy with regulators (to get +/-15V) won't take up much space :)

StalfoS 20th March 2005 07:01 PM

Thanks for the detailed explanation... The case that I picked up for my GC amp is big enough that I should be able to fit a seperate headphone amp into it.

Any design suggestions?

Alcaid 20th March 2005 07:13 PM

Quote:

Originally posted by StalfoS
Thanks for the detailed explanation... The case that I picked up for my GC amp is big enough that I should be able to fit a seperate headphone amp into it.

Any design suggestions?

Well, the easiest one:

Quote:

Originally posted by Alcaid
A small split supply Cmoy with regulators (to get +/-15V) won't take up much space :)
Just be shure that the regulators can handle the input voltage you are feeding them.

mourip 28th March 2005 10:22 PM

Hi,

I have been driving headphones directly with my gainclone(Brian's boards) for about 6 months. It sounds absolutely great. I have used Beyerdynamic DT990's and Sennheiser HD650's with it. I simply wired a headphone jack with an ON-OFF-ON switch so that I could choose between speakers-mute-headphones. I also ended up using a voltage divider on the input to attenuate the volume somewhat. I use the amp almost entirely for headphone listening. The chipamp has lots of detail, a nice midrange, and great pace. It make me tap my foot!

I am not sure where all of the predjudice comes from against using power amps for headphone listening. If you already have one I say use it. I honestly believe that those who counsel against it have never tried it.

My project right now is a modified Sonic Impact Tripath amp which runs my AKG K-1000s. This is the best sound I have ever heard. Once you are happy with the gainclone give the SI amp a try as your next project. The Tripath chip is where the action is now :-)

Best,

Paul

StalfoS 29th March 2005 04:31 AM

Thanks for the suggestions all, I still have not decided what I will go with. I wanted to design my amp with simplicity in mind, however it occured to me that not having a headphone jack will be a problem.

If I were to include simple headphone amp circuit what would be the best way to power it? Off of the rails or with a seperate transformer?

Simply putting a voltage divider on the output is looking more and morea apealing...

joensd 29th March 2005 05:08 AM

Quote:

Simply putting a voltage divider on the output is looking more and morea apealing...
I just did exactly that in a GC for my brother.
Just put a 120Ohm resistors on the outputs and connected them + ground to the headphone jack.
I put an additional 220uF cap in series for to block DC that might occur with simplified GC.
Put in another switch for muting the main speakers as well.

Indeed itīs more noisy than a dedicated headphone amp but for occasional listening itīs more than adequate.

What are your voltage rails?
With <+-24V you could use a OPA604 or the like.

You could also buffer the input with BUF634 or discrete parts as most line signals have adequate voltage swing to power (at least low impedance <=32Ohm) headphones. Sonically an improvement for the main amp as well.
With the BUF634 and most headphone amps youīd need additional voltage regulators. If you donīt want those there are simple enough buffers that just need a fet or two and can cope with higher rails.

greets

sek 29th March 2005 06:39 AM

Hi,


Quote:

Originally posted by mourip
I am not sure where all of the predjudice comes from against using power amps for headphone listening. If you already have one I say use it. I honestly believe that those who counsel against it have never tried it.
I honestly have tried it. ;)

In class A/B amps, the transision between class A (low distortion) and class B (higher distortion) mode happens when the output swing increases beyond 'background' listening levels (if biased appropriately). The gainclone chips are set up for this transition to happen at roughly 1W into a loudspeaker.

Due to the voltage divider, you could actually have to drive the amp into class B mode without actually requiring the power (but only the voltage).

Other than that, average (commercial) amps are usually bad sounding anyway. ;)
But I have never tried it with a (good) gainclone.


Quote:

Originally posted by joensd
Indeed itīs more noisy than a dedicated headphone amp but for occasional listening itīs more than adequate.
That's what I meant ;)

Ciao,
Sebastian.

breez 29th March 2005 10:52 AM

Output impedance should be considered for headphones too, as their impedance curve is seldom a straight line through the audio range. A significant amount of op impedance would emphasize the frequencies at which the headphone has an impedance peak (many have one at the bass area, hence the usual impression of a muddied bass from speaker amp's headphone socket).


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