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Old 28th November 2011, 07:14 PM   #1
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Question Beginner: Crackling and low volume on headphone amp

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.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.

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
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Old 28th November 2011, 09:18 PM   #2
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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.
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Old 28th November 2011, 09:46 PM   #3
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Quote:
Originally Posted by firechief View Post
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.
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?
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Old 29th November 2011, 01:16 AM   #4
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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
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Old 29th November 2011, 09:45 AM   #5
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Quote:
Originally Posted by discrete View Post
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
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Old 29th November 2011, 11:44 AM   #6
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Quote:
Originally Posted by gatekiller View Post

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...

Last edited by discrete; 29th November 2011 at 11:55 AM.
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Old 29th November 2011, 11:48 AM   #7
AndrewT is offline AndrewT  Scotland
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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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Last edited by AndrewT; 29th November 2011 at 12:00 PM.
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Old 29th November 2011, 11:50 AM   #8
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Quote:
Originally Posted by AndrewT View Post
R1 & R2 are there to discharge your battery quickly.
Could they be much higher in value?
Originally they were 100K. I've just been messing around with the values.
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Old 29th November 2011, 11:53 AM   #9
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Quote:
Originally Posted by AndrewT View Post
Is LT1001 a low quiescent current opamp?
Is LT1001 optimised for +4Vdc operation?
The LT1001 is just a generic op amp from LT Spice. In the breadboard I am using a TL072.
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Old 29th November 2011, 12:03 PM   #10
AndrewT is offline AndrewT  Scotland
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TL072 is not optimally suited to battery operation.
Look at TL082 and TL062 and modern equivalents.
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