Beginner: Crackling and low volume on headphone amp

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Hi,

The Background
I hope someone can help me by giving me suggestions or pointing out obvious errors with my circuit.

I've designed a headphone amp designed to run from a 9V battery. The circuit starts with the 9V supply followed by a virtual ground made from a voltage divider and a buffer. The line level input from the iPod goes into the op amp which has a gain of 11. I've put in a 200ohm resistor as the headphone.

An externally hosted image should be here but it was not working when we last tested it.


And these are the voltages that the simulation gives which I'm happy with. Green is the iPod input. Red is the amplified output. Turquoise is +4v. Blue is -4v.

An externally hosted image should be here but it was not working when we last tested it.


The Problem
I then build this thing on a breadboard and what do you know... it doesn't work. On low volume I get nothing. On a medium volume I hear sound sometimes but I can tell the bass is dodgy and on full volume I get very bad distortion.

The op amps I'm using on the breadboard are TL072's I had lying around. I measured all the power rails and they check out. I tried increasing the current on the voltage divider, no change. I tried decreasing the iPod input current with a 100k resistor, no change. I tried increasing the voltage +/-8V (from +/-4), no change. I've also measured the temperature of the op amps and they are at room temperature. Here is a photo of the breadboard.

An externally hosted image should be here but it was not working when we last tested it.


The only thing I cannot view/test is the output from the iPod and the output from the opamp because I don't own an oscilloscope and my DMM's don't measure AC mV very well.

So, those are all the details. Any help or suggestions you could give would be greatly appreciated. I'm still learning electronic, so please go easy :)
 
I suggest that you get another battery and make the other supply voltage with it. So + on one battery to the - on the other and that common point becomes the ground for all citcut ground connections. I checked the documentation because somtines they offer a single supply implementation example. But this time I saw none. You will have to re wire the circut but thats fun right. :rolleyes:
 
I suggest that you get another battery and make the other supply voltage with it. So + on one battery to the - on the other and that common point becomes the ground for all citcut ground connections. I checked the documentation because somtines they offer a single supply implementation example. But this time I saw none. You will have to re wire the circut but thats fun right. :rolleyes:

I like fun :)

It's late now and I'm at work tomorrow but will give it a try tomorrow evening and report back.

I'm wondering though, what is the difference between creating a virtual ground with two batteries compared with the voltage divider buffer in my circuit?
 
R7 is way too big, try 1k.

U3 also needs some local supply bypass caps.

Try to minimize the parasitic capacitance on the negative input of the op-amps. I.E. keeps leads and wires short on these pins (2 and 6).

Keep meters on the battery and the virtual ground to make sure they do not collapse under load.

Do not connect headphones if the DC output (with ipod at 0 volume or paused) is more than 20 mV.

A few others suggestions:
-put 5 to 10 ohms in series with the output to the headphones
-tie the inputs of unused op-amps to ground, otherwise they tend to go off on missions of their own
-R1, R2 should be 10k to 22k to minimize battery drain
-R2 can have a few uF across it to minimize noise
-two batteries (of same 'freshness') in series will give a more solid ground than the op-amp
-If you are serious about this, a circuit with an output buffer and a real ground is the way to go
 
R7 is way too big, try 1k.

U3 also needs some local supply bypass caps.

Try to minimize the parasitic capacitance on the negative input of the op-amps. I.E. keeps leads and wires short on these pins (2 and 6).

Keep meters on the battery and the virtual ground to make sure they do not collapse under load.

Do not connect headphones if the DC output (with ipod at 0 volume or paused) is more than 20 mV.

A few others suggestions:
-put 5 to 10 ohms in series with the output to the headphones
-tie the inputs of unused op-amps to ground, otherwise they tend to go off on missions of their own
-R1, R2 should be 10k to 22k to minimize battery drain
-R2 can have a few uF across it to minimize noise
-two batteries (of same 'freshness') in series will give a more solid ground than the op-amp
-If you are serious about this, a circuit with an output buffer and a real ground is the way to go

Thanks for those suggestions. I will most defiantly try them.

The one that caught my eye was "tie the inputs of unused op-amps to ground, otherwise they tend to go off on missions of their own". This is something I've not read before and sounds quite interesting.

I hope to replace the audio op with an OPA134 and I might consider a buffer but my supplier doesn't seem to have too many available.

I'll make the changes in the simulation and then I'll make them tonight on the breadboard. I'll also try to place all the components much closer together.

Many Thanks
 
The one that caught my eye was "tie the inputs of unused op-amps to ground, otherwise they tend to go off on missions of their own". This is something I've not read before and sounds quite interesting.

Please see page 17-28 (figure 17-21) for proper termination of unused sections:

www.ti.com/lit/an/slod006b/slod006b.pdf

EDIT: Finally got a working link...
 
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R1 & R2 are there to discharge your battery quickly.
Could they be much higher in value?
Is LT1001 a low quiescent current opamp?
Is LT1001 optimised for +-4Vdc operation?

Why attenuate the input by half (-6db) and then add gain of 11times (+20.8dB) for a net 5.5times gain (+14.8dB)?
You may find that with the digital volume control set to maximum that a gain of +3dB or +6dB will overload the amplifier and/or the headphones and/or your ears.

I suggest you use a unity gain stable opamp and set the gain to 2times (+6dB) initially. you can change that later if need be.
To compliment this change remove the -6dB attenuator by changing R7 to 1k (-0.001dB).
I further suggest you add RF attenuation to the input. Try 330pF to 1nF across R3 initially.
 
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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.
 
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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.
 
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.

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.
 
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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.
 
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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
 
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
 
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