Beginner: Crackling and low volume on headphone amp

[Mooly]

Not familiar with the LTxxx opamps but the following points stand out a mile....

R1 and R2 can be much higher. For a PP3 every milliamp counts.

Short out R7

Make R3 and R5 equal with appropriate increase in C4

Add a cap in series with R6 of say 22uF/16volt

Use an opamp for the virtual earth that will be happy with small caps from output to each rail. In other words add some decoupling.

The TL062 is low power but also has limited slew rate, the TL082 is just a slightly higher noise version of the TLO72.

Try adding a series output resistor to the phones of say 47 to 100 ohm so that more of the available swing can be used before the opamp runs out of steam.

Any general purpose opamp will work but some are far more suited to driving low impedance loads at rail to rail swing than others. I can't just quote any from memory though.

[gatekiller]

Originally Posted by AndrewT

TL072 is not optimally suited to battery operation.
Look at TL082 and TL062 and modern equivalents.

My local supplier has TL082's so I'll get some of them next time I go.

I'm on with making the suggested changes in LTSpice so I'll post the schematic when I'm done

Thank you again for your help. Hopefully tonight I might see some results.

[AndrewT]

I said look at.
Not go out and buy.

Mooly has confirmed that tl082 is high current. That leaves you looking at equivalents to tl062. There are many old and modern opamps designed specifically for battery operation.

[Mooly]

Originally Posted by AndrewT

I said look at.
Not go out and buy.

Mooly has confirmed that tl082 is high current. That leaves you looking at equivalents to tl062. There are many old and modern opamps designed specifically for battery operation.

It's higher a noise version really rather than greater drive ability... or rather the TL072 is the low noise version.

none of these are really ideal for driving phones but they are great to experiment with. The TL072/82 etc have some of the worst drive capability of any opamp (but used within their limits are excellent performers)... just not h/phone drivers. But do try them as phones need little drive for reasonable levels.

[gatekiller]

Originally Posted by AndrewT

I said look at.
Not go out and buy.

Mooly has confirmed that tl082 is high current. That leaves you looking at equivalents to tl062. There are many old and modern opamps designed specifically for battery operation.

Originally Posted by Mooly

It's higher a noise version really rather than greater drive ability... or rather the TL072 is the low noise version.

none of these are really ideal for driving phones but they are great to experiment with. The TL072/82 etc have some of the worst drive capability of any opamp (but used within their limits are excellent performers)... just not h/phone drivers. But do try them as phones need little drive for reasonable levels.

Yeah, I'm just experimenting at the moment and I had some TL072's spare

When I start to build the circuit, I'll definitely choose the right op amp and don't mind spending a little on a good one.

[Mooly]

Go ahead and try them.

If you are using FET opamp such as these then the cap in series with R6 isn't needed. That was "catch all" for whatever opamp you might use as bjt types have significant DC offsets and for phones you want the output to be at 0.00 volts DC.

Another thought, rather than make a virtual ground and "DC" coupled circuit, why not just make an AC coupled amp. Caps get a bad press and often for no good reason.

But go ahead and try what you have and see how it performs. Remember a "typical" impedance for phones might be around 30 to 50 ohms and you need to put around 0.5 to 1 volt peak to peak across them with actual music for the loudest levels. Most opamps are OK with 600 ohms, so 30 ohms obviously limits what the opamp can deliver.

[gatekiller]

Well I made a second revision of the circuit taking into account the suggestions that you've all made - and I greatly appreciate all of them.



I'll breadboard this design tonight. For now, back to work!

[Mooly]

That looks good and should work for any opamp. If you did use a bjt as opposed to FET devices then try and make R3 and R4 equal to minimise DC offset. The FET devices are immune.

C2 and C3 in PSU. I meant for these to connect from opamp output to each rail to maintain low impedance supply at all frequencies.

It will all work though, if you can't get enough volume experiment with the series output resistor. If still not enough then you need to select a more suitable opamp but it will work OK

[gatekiller]

After a couple of days I'm just popping on to report that the headphone amp is now working

I thought the 4 power rails on the breadboard ran the full length of the board but it turns out they are all split in half. So the connections on the right hand side were open. This will teach me to test the continuity of common points.

After I got it working I noticed that the TL072 virtual ground buffer was unstable and causing crackling (I'll have to inspect the datasheet to find out why), so I swapped that out for an LM358 and it works very well.

So now it's just a case of fine tuning the components and then soldering the final circuit.

Finally I just want to thank everyone for their help and suggestions.

Cheers
Stephen

[Mooly]

Thanks for the update, pleased to hear you have it working.