Headphone input circuitry.

I've been wanting to build up a good headphone amp for myself for awhile now, but seeing as how my Samson headphone amp is developing some, uhh..."Character," I've decided to look into making a new headphone controller/amp with hopefully a bit higher quality than what the Samson is giving me.

First up is the input bit.



I knocked that up in Tina-TI real quick.  I really just glued a couple of pieces together from the pages of Douglas Self and Rod Elliot so credit goes to them for the input and EQ sections.

First stage de-balances, second stage provides some EQ (polarity kept by the unity-gain inverter in front of the EQ), then a bit of switching and a volume pot.  After that it would go to an actual amp stage to drive the socks off of some headphones.  First stage opamps should be a 5532.  I'm wondering if the EQ stage wouldn't be better served with a TL072 or similar?  Should I have a resistor in front of mute switch (say 100r or so) for current limiting since it (and the volume pot) just shorts the opamp output to ground?

Thanks,

James

Why don't you orient the mute switch the other way round, with the Common to the output, so when you mute, the output is grounded.
It may be against your religion, but I would also fit some CR coupling in order to avoid switching clicks. install

This might turn out to be a stupid question, but I'll take the risk:
isn't the output mono all the time in this configuration?

This might turn out to be a stupid question, but I'll take the risk:
isn't the output mono all the time in this configuration?

Click to expand...

Doh!!  It is!  Hold on...I'll go fix it...

Okay...So now the "mono all the time" bit is fixed...I dropped the mute for a moment to rethink that.  You might need to do a refresh to keep your cache from pulling the old schematic up.

Thanks for catching that.

Alright...Updated once again with the mute switch relocated.  I'm assuming that the power amp stage will have coupling caps on the inputs.  With that being the case, the mono switch, mute switch and volume pot are all isolated from DC by C5/C10 and whatever coupling caps on the input of the power section.

I guess if I were really on the ball I'd go ahead and run simulations on this and see how it looks.

James

With that being the case, the mono switch, mute switch and volume pot are all isolated from DC by C5/C10 and whatever coupling caps on the input of the power section.

Click to expand...

Yes, so the pot will not scratch, but when you switch between Flat and EQ, there will probably be some click, because the offset at the output of OP3 is different than the offset at the output of OP3.
And I continue to think that the Mute switch is the wrong way round, leaving the amp's imput in a high Z state instead of being shorted, leaving it open to all sorts of interference.

Yes, so the pot will not scratch, but when you switch between Flat and EQ, there will probably be some click, because the offset at the output of OP3 is different than the offset at the output of OP3.

Click to expand...

I guess the best way to fix that is to just pop in a couple of caps before and after the EQ stage.  Or at least after the EQ stage since the simulations seemed to show that as having some offset.

And I continue to think that the Mute switch is the wrong way round, leaving the amp's imput in a high Z state instead of being shorted, leaving it open to all sorts of interference.

Click to expand...

I totally see where you're coming from now.  I wasn't thinking about the next stage and it's behavior so much and didn't take into consideration that its input would just be wide open with no connection.  Totally wrong!  So I corrected that.  Schematic updated.

RogerFoote said:

Opps wrong Forum!

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hahahaha, oh roger that was funny.

cheers