Where to put the volume control.

[BeBuLamar]

I build a simple headphones amp using the TL084 op amp. Each channel are 2 stage of inverting op amp. It works fine but I need to put a volume control when use with source that has no variable output. Where should I put the pot? Before the first stage?

[TimFox]

That's the usual place, in order to avoid over-driving any of the stages.
Once the first stage overloads, there is no way to avoid the distortion in the signal.
The only reason not to put it before the first stage is if the signal into the volume control is so low that the noise contributed by that stage would degrade the signal/noise ratio.

[CaptDon]

I would suggest that prior to the first stage would be proper as it would allow you the ability to not overdrive the input stage which could happen if the control was after the first stage. Will a TL084 have enough current drive for your headphones?? You may need something with a bit more 'Moxie'.

[BeBuLamar]

Well the TL084 has enough current. I tested on the breadboard without the pot and it works fine.

[magic]

Isn't it the case that if the negative supply goes missing for any reason, the output jumps to the positive?
Are the headphones AC coupled?

[BeBuLamar]

I would suggest that prior to the first stage would be proper as it would allow you the ability to not overdrive the input stage which could happen if the control was after the first stage. Will a TL084 have enough current drive for your headphones?? You may need something with a bit more 'Moxie'.

The TL084 is OK for current because my headphones are 600 Ohms and I only needs 18mA to drive it to the maximum power of 200mW. One of the reason I want to build the amp because most other amps put out too low voltage to drive a 600 Ohm headphones.

[Zero999]

I would suggest that prior to the first stage would be proper as it would allow you the ability to not overdrive the input stage which could happen if the control was after the first stage. Will a TL084 have enough current drive for your headphones?? You may need something with a bit more 'Moxie'.

The TL084 is OK for current because my headphones are 600 Ohms and I only needs 18mA to drive it to the maximum power of 200mW. One of the reason I want to build the amp because most other amps put out too low voltage to drive a 600 Ohm headphones.

Crazy idea, but have you tried a couple of mains transformers?

I've found them to be quite linear, if de-rated significantly. A couple of 3VA 120V:24V mains transformers, with the 120V going to the headphones, might do the trick.

[magic]

I see no point.

±15V is supposedly good enough for 600Ω headphones on the market (but I haven't tried), and there are cheap opamps specifically designed to run on such rails and drive 600Ω like NE5532 (which also has more output current than TL084 and no phase reversal).

[BeBuLamar]

Mine has no phase reversal because each channel has 2 inverting stages so the output is in phase with the input.

[magic]

Disconnect temporarily the negative supply and see what happens to the output.
I would advise trying it with no HPs connected, unless they are AC coupled, and maybe even then just don't.

[Zero999]

Mine has no phase reversal because each channel has 2 inverting stages so the output is in phase with the input.

No, that's not what is meant by phase reversal, in the context of op-amps. The problem with the TL082 is the inputs exchange functions i.e. the -input becomes the +input and +input, then -input, when the common mode range is exceeded. In other words the phase of the output is reversed, when one of the inputs is taken to within 2V or so of the negative rail. This makes the TL082 prone to latch-up, under certain conditions, as negative feedback, becomes positive, the output voltage will keep moving towards one of the supply rails, until it saturates and stays there. It can be a problem when one of the supplies is lost, or at turn-on as AC coupling capacitors charge.

Magic is right. The NE5532 is a better choice.

I still think you should try transformers. No gain is required, just a higher voltage. If the amplifier is designed to drive low impedance, 8 to 32 Ohm headphones, then it will have no problem driving your 600 Ohm headphones, via a suitable transformer.