Hi!
I have recently designed and built a TPA6120A2 based headphone amp. It is designed for both unbalanced and balanced inputs, selectable through a switch. Another Opamp then does the balanced->unbalanced conversion.
I initially had DC problems when working in unbalanced mode with no source connected (as the datasheet/evm manual warned me of). However I did not have these extreme DC problems (250mv+) when using the amp in balanced mode (with the other OPamp before the TPA). So I made both inputs use the 2nd Opamp as a buffer by placing the switch in front of it. My volume pot is between the two opamps. Now I do not have the high DC offset when in unbalanced mode anymore, but unfortunately I still have a rather high, gain-dependent (and Pot-dependent) offset left.
I made the TPAs gain selectable with a jumper, so one can easily switch between 3.2 and 6.4. When working with a gain of 3.2 I have only little DC offset of mostly below 20mV. But when I turn my pot from 12 to about 2 o clock the offset quickly rises to ~50-60mV and lowers again to >20 once i pass 4 o clock. With the gain set to 6.4 all DC values double giving me a dangerous offset of max ~110mV.
As a workaraound for this I placed switchable Caps before the headphone output; this works but it is only a bad fix, since I wanted to keep the signal path cap-free :/
What I thought of now is increasing the gain of the buffer stage and locking the TPA gain at 3.2, but if there are any nicer solutions for this problem, I would be very thankful to read them
Thanks for all help!
Schematic:
I have recently designed and built a TPA6120A2 based headphone amp. It is designed for both unbalanced and balanced inputs, selectable through a switch. Another Opamp then does the balanced->unbalanced conversion.
I initially had DC problems when working in unbalanced mode with no source connected (as the datasheet/evm manual warned me of). However I did not have these extreme DC problems (250mv+) when using the amp in balanced mode (with the other OPamp before the TPA). So I made both inputs use the 2nd Opamp as a buffer by placing the switch in front of it. My volume pot is between the two opamps. Now I do not have the high DC offset when in unbalanced mode anymore, but unfortunately I still have a rather high, gain-dependent (and Pot-dependent) offset left.
I made the TPAs gain selectable with a jumper, so one can easily switch between 3.2 and 6.4. When working with a gain of 3.2 I have only little DC offset of mostly below 20mV. But when I turn my pot from 12 to about 2 o clock the offset quickly rises to ~50-60mV and lowers again to >20 once i pass 4 o clock. With the gain set to 6.4 all DC values double giving me a dangerous offset of max ~110mV.
As a workaraound for this I placed switchable Caps before the headphone output; this works but it is only a bad fix, since I wanted to keep the signal path cap-free :/
What I thought of now is increasing the gain of the buffer stage and locking the TPA gain at 3.2, but if there are any nicer solutions for this problem, I would be very thankful to read them
Thanks for all help!
Schematic:
An externally hosted image should be here but it was not working when we last tested it.
for an indication of likely dc specs, see the THS6012 datasheet - I believe the TPA6120 is the same chip, talk to a TI application engineer to see if the TPA have better/worse DC specs due to sorting
1st I wouldn't have put the jumper gain select option in - the datasheets and eval board layouts are very emphatic about keeping parasitic C at a minimum on the -in node
the THS6012 DC specs are not great: 5 mVos, 10 uA +in bias could give the numbers you observe
since the +in bias current works into the varying R of the volume pot you cannot easily trim
2 trim pots per channel on the TPA (one injecting a bias canceling current into the +in through a high Ohm R and the other injecting a Vos into the -in would be my approach) and possibly another Vos trim for each input buffer would be necessary to completely trim output Vos - and then you would have to see if temp stability was adequate
I use the TPA in a multiloop arrangement where the higher DC spec outer loop op amp controls the Vos
1st I wouldn't have put the jumper gain select option in - the datasheets and eval board layouts are very emphatic about keeping parasitic C at a minimum on the -in node
the THS6012 DC specs are not great: 5 mVos, 10 uA +in bias could give the numbers you observe
since the +in bias current works into the varying R of the volume pot you cannot easily trim
2 trim pots per channel on the TPA (one injecting a bias canceling current into the +in through a high Ohm R and the other injecting a Vos into the -in would be my approach) and possibly another Vos trim for each input buffer would be necessary to completely trim output Vos - and then you would have to see if temp stability was adequate
I use the TPA in a multiloop arrangement where the higher DC spec outer loop op amp controls the Vos
[continued from above - 30 min edit limit is too short!]
also with Vos adj described above: if you run from both battery power and a different V wall adapter then the Vos adjust will vary, and again as the battery discharges the V varies - as much as 30% depending on chemistry
also your active ground splitter is very likely inadequate - the ground has to "soak" up as much current as the both channels of the TPA can put out when driving your heaviest load: remember all currents flow in closed loops, identify them, design and layout with them in mind and you will be way ahead in the signal quality department
the classic 3 term TLE2426 has very asymmetric and small current capability: less than 30 mA on one side as I recall - you would be much better off with those 4x 2200 uF caps AC bypassing the active ground - and isolating the TLE2426 from the lo Z AC ground with ~10 Ohm R relying on the bulk C between battery +,- terminals and the AC gnd to carry the load current (calculate smallest R that keeps TLE current <20 mA with AC gnd ripple of lowest audio frequency, lowest Ohm load, max amplitude output, both channels simultaneously)
also with Vos adj described above: if you run from both battery power and a different V wall adapter then the Vos adjust will vary, and again as the battery discharges the V varies - as much as 30% depending on chemistry
also your active ground splitter is very likely inadequate - the ground has to "soak" up as much current as the both channels of the TPA can put out when driving your heaviest load: remember all currents flow in closed loops, identify them, design and layout with them in mind and you will be way ahead in the signal quality department
the classic 3 term TLE2426 has very asymmetric and small current capability: less than 30 mA on one side as I recall - you would be much better off with those 4x 2200 uF caps AC bypassing the active ground - and isolating the TLE2426 from the lo Z AC ground with ~10 Ohm R relying on the bulk C between battery +,- terminals and the AC gnd to carry the load current (calculate smallest R that keeps TLE current <20 mA with AC gnd ripple of lowest audio frequency, lowest Ohm load, max amplitude output, both channels simultaneously)
thank you very much, jcx!
as for the offset i have decided to remove the gain jumpers - a gain of 3.2 is more than enough to drive all my headphones at very high levels - and accept the resulting volume-pot dependend offset of upto 55mV. Also I noticed that I can not hear any difference at all, when using the Caps at the output.
as for the ground circuity: I do understand the problem of the TLE2426; however I have been a little unsure about this.
I have read somewhere (cant find the link right now) that when using the TLE2426 in an headphone amp it would be better to place big Cs only between + and - and not 'behind' the TLE beacause of the then halved capacity. However I just found a very interesting site about all this (http://www.tangentsoft.net/elec/vgrounds.html) which suggests exactly the same thing as you. So I will now start with placing 2 of the caps between +/- and gnd (with my current board, i luckily can do this very easy).
I will go next week and measure my +/- levels at my university while driving a heavy load at high volume to see, if the TLE gets imbalanced at some point.
Thanks again!
as for the offset i have decided to remove the gain jumpers - a gain of 3.2 is more than enough to drive all my headphones at very high levels - and accept the resulting volume-pot dependend offset of upto 55mV. Also I noticed that I can not hear any difference at all, when using the Caps at the output.
as for the ground circuity: I do understand the problem of the TLE2426; however I have been a little unsure about this.
I have read somewhere (cant find the link right now) that when using the TLE2426 in an headphone amp it would be better to place big Cs only between + and - and not 'behind' the TLE beacause of the then halved capacity. However I just found a very interesting site about all this (http://www.tangentsoft.net/elec/vgrounds.html) which suggests exactly the same thing as you. So I will now start with placing 2 of the caps between +/- and gnd (with my current board, i luckily can do this very easy).
I will go next week and measure my +/- levels at my university while driving a heavy load at high volume to see, if the TLE gets imbalanced at some point.
Thanks again!
just to keep this uptodate:
Bypassing the ground with 2 of the 2200µ caps lowered my distortion factor at low R loads by about 4 (at 0.0031 with 30R, <0.0012 above 150R), so this is really a plus! Thanks a lot for the tip, jcx! Additionally I placed two more TLE2426s into my circuit and placed them next to each other, giving me the possibility to tripple the possible GND current (just using one atm).
Also I tried building a DC servo for the offset, but I didn't quite succeed; I will continue working on this after my exams (~2 weeks). I built a 2nd order multiple feedback lowpass from out to +in, but yet I do a positive output voltage from a positive input (since multiple feedback inverts phase, there must be some bug in my circuit)... however, i will care about this in some time..
I did now recalculate resistor values to increase the buffer's gain and thus reduced the TPAs gain to 2, limiting my theoretical offset to a max. of ~60mV (real values should be much lower). I can change the resistors easily with my current prototype, which I will do once I have some spare time again.
Would it be possible to reduce the offset by inducing the 12µA via a large resistor (500k @ +-6V) from negative supply? I tried this at home (with an old, unconnected board) and it seemed to work - however I am not sure, if this has any negative effects...
These are my current circuit and layout; should anybody have any more comments or suggestions about them, please don't hesitate
Thanks!
Bypassing the ground with 2 of the 2200µ caps lowered my distortion factor at low R loads by about 4 (at 0.0031 with 30R, <0.0012 above 150R), so this is really a plus! Thanks a lot for the tip, jcx! Additionally I placed two more TLE2426s into my circuit and placed them next to each other, giving me the possibility to tripple the possible GND current (just using one atm).
Also I tried building a DC servo for the offset, but I didn't quite succeed; I will continue working on this after my exams (~2 weeks). I built a 2nd order multiple feedback lowpass from out to +in, but yet I do a positive output voltage from a positive input (since multiple feedback inverts phase, there must be some bug in my circuit)... however, i will care about this in some time..
I did now recalculate resistor values to increase the buffer's gain and thus reduced the TPAs gain to 2, limiting my theoretical offset to a max. of ~60mV (real values should be much lower). I can change the resistors easily with my current prototype, which I will do once I have some spare time again.
Would it be possible to reduce the offset by inducing the 12µA via a large resistor (500k @ +-6V) from negative supply? I tried this at home (with an old, unconnected board) and it seemed to work - however I am not sure, if this has any negative effects...
These are my current circuit and layout; should anybody have any more comments or suggestions about them, please don't hesitate
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Thanks!
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