A TPA6120 headphone board used as a test pre-amp for power amplifiers.

Many of us experimenting with many different power amplifiers run into the problem of needing a pre-amp for test that can handle all power amplifiers concerning input level and impedance. With a low price and nevertheless minimum THD.

I noticed a low-cost headphone amplifier board based on the TPA6120 chip. The TPA6120 has a THD that reaches below 0.001% and can handle low impedances with a high output level. Evidently, the board can also be used with headphones.

The board was at a very modest price and included a full symmetrical power supply ready for a trafo or a symmetrical DC source. The amplifier part included only the TPA6120 chip, no other active components. I had to find out if such would do.

The power supply functions without concerns. The amplifier works but with one rather important drawback – DC offset at the output. The TPA6120 chip is configured as a non-inverting amplifier and, as recommended by TI, with low impedance resistors in the feedback path. TI recommends similar low impedance for the non-inverting input. But, this board is aiming at a high input impedance through use of a 50KOhm volume potentiometer at the non-inverting input. Such very different impedance levels seen from the two inputs easily end up in important DC offset at the output due to input bias currents. As the board was supplied as a kit, I never used the 50KOhm potentiometer but settled for a less demanding 5KOhm potentiometer. Even with 5KOhm, the DC offset at the output exceeds 50mV. With 50KOhm, the DC offset will be considerably worse.
No miracles in the amplifier part.

In the schematics below, the upper circuit is the power supply and the middle circuit the amplifier part as supplied. The values in parenthesis are the values I finally used. The lower circuit is a buffer circuit I added to overcome the DC offset problem.

A TPA6120 chip needs a low impedance input circuit, as shown by TI, such that an input buffer before the TA6120 is pretty mandatory. I made such an input buffer using an OPA2134 in non-inverting coupling and with a gain of two. With this buffer amplifier and the 5KOhm potentiometer turned to max., the DC offset at the TPA6120 output is around 2mV. A pity the board does not include such a buffer circuit before the TPA6120.

The board can be used as a pre-amp without the buffer stage if the power amplifier is not DC coupled. But for a headphone, the DC offset without a buffer stage is too much.

With the modifications I made, the board sounds great. Both as a pre-amp for a power amplifier and with a headphone. Probably as good as my Musical Fidelity and Argon headphone amplifiers. For some 15 USD, I now have a test pre-amp for (almost) any power amplifier I will make.

:wrench:
 

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Where is this "board" that you keep referring to above?

Personally, I don't think that hanging any value volume pot in front of a TPA6120(or any other current-feedback amplifier) without some type of coupling cap, or buffer op-amp, in between is a good idea.

But...if your intentions are just for testing power amps and the offset is acceptable, then it may be suitable for your needs.
 
Headphone amps make fine preamps. I like how they can drive low impedance loads. I use an OPA1680 as a Preamp sometimes.


I recently got some OPA1688 in the SOIC8 package. Fantastic specs with even less THD. I haven't had time to try them out. My only concern about those as headphone amplifier is the cooling with low impedance headphones. The other packages may be better as they have a cooling pad below. But, my OPA1688 should be very suited as the output buffer in a pre-amp. They can pull almost any load at low THD and even at only +/-5V supply.
 
Specs say this can deliver 700mA of current with +/-15v supply in current feedback Class AB topology.

Yes xrk971 it's a well know fact that the TPA6120 is and has been a very nice sounding CURRENT FEEDBACK op-amp as I noted above in my post.

After looking at the "CIRMECH" board the OP linked to above, I can now see why the price is so cheap.
A properly designed TPA6120 PCB will almost always use SMDs around a very fast op-amp like the TPA6120.
In addition...nobody in their right mind will connect some cheap(or not so cheap) volume pot connected directly to its noninverting inputs.

If you can live with the varying offset and "rustling leaves syndrome" that results from having too much DC on the the pot's wiper, then this may be the best buy around for a TPA6120 board.
The Chinese will do almost anything to sell something.:D
 
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Good layout can be done with through hole and is not less expensive than SMT. I see that metal thin film resistors are used here which are better sounding than typical thick film (metal oxide) SMT resistors. They do use SMT voltage regulators so I don’t think they were trying to save money here by going with TH on the opamp circuit. The spec sheet for the 6120 doesn’t say what the input current is, but says the impedance is 300k - which is quite high. Using a 10k pot for input volume control is probably ok for a budget application, but a voltage gain stage with JFET input is always a good idea to prevent volume knob scratchiness. However, I have made and used non JFET input stages with just a BJT based circuit and it did not have any volume knob sound. It is possible. I ordered one considering the price and we will see how it works. Having an on board regulated linear PSU is a nice feature as that can add a lot of cost and work to a plain amp PCB. This unit is basically one board ready for a case with holes. I’m sure they sell the matching case some where as well.
 
Good layout can be done with through hole and is not less expensive than SMT. However, I have made and used non JFET input stages with just a BJT based circuit and it did not have any volume knob sound. It is possible.

Yes, xrk971 anything is "possible" in your world of headphone amps...LOL.
As I stated before, nobody in their right mind would lay out a PCB for the ultra-fast TPA6120 using TH components.
Why don't you try laying out your own board for the '6120 w/ TH components without Prasi's or anyone else's help and see how it turns out??

With the TPA6120, as well as many other CFAs with high input bias current, there will certainly be DC on the volume pot's wiper whether you hear any "volume knob sound" or not.

Perhaps you should stick with your AC coupled discrete headphone designs and I certainly you do so!:rolleyes:
 
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Yes, xrk971 anything is "possible" in your world of headphone amps...LOL.
As I stated before, nobody in their right mind would lay out a PCB for the ultra-fast TPA6120 using TH components.
Why don't you try laying out your own board for the '6120 w/ TH components without Prasi's or anyone else's help and see how it turns out??

With the TPA6120, as well as many other CFAs with high input bias current, there will certainly be DC on the volume pot's wiper whether you hear any "volume knob sound" or not.

Perhaps you should stick with your AC coupled discrete headphone designs and I certainly you do so!:rolleyes:

Not sure why the condescending tone? All I said was it is possible and a bit presumptuous of you to say that *nobody* in their right mind would do a TH layout for a 6120. I have a lot of all-SMT layouts and use it when it makes sense. But if you place your resistors and bypass caps within required short distances from chip and have proper ground planes, TH can work fine in the audio band despite chip being “ultra fast”. There can be bad SMT layouts as well and it all comes down to implementation. I don’t just do AC coupled discrete amp in case you haven’t seen my TPA6188 and TPA6120. Prasi does fine work and JPS64 makes IMO, the nicest layouts in SMT or TH in DIYA. I leave the layout to professionals. I do agree that a separate voltage gain stage with low input current should be use to drive the 6120 though.
 
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Has anyone ever compared a TPA6120 to a Pass HPA1 or Pass HPA1 clone?

I am trying to wrap my head around the preference for DAC >>> OPA1612 >>> HPA1 versus DAC >>> OPA1612 >>> TPA6120 or even DAC >>> OPA1612 >>> LJM HPA2 PRO (With Parallel 4x OPA1612)

Right now I am going back and forth between the TPA6120 and the LJM HPA2 PRO (With 4x Parallel op-amps).





When the TPA6120 has a burst like RF interference I can tell them apart...