Making best use of a 14.8V battery supply for Headphone Amp

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robjordan

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Joined 2020
#1
I'm a relative novice building a TPA6120A2-based headphone amp for fun and for personal use.

Suppose I have a 4S Lithium Ion battery pack, giving a nominal 14.8V, though varying between 16.8V and 12V dependent on state of charge. Which of the following power supply approaches would you favour:

(a) Simple resistor-based rail-splitter to give an unregulated +/- 7.2V (approx) to the opamp.
(b) Simple rail-splitter followed by low drop-out regulators to +/-5V.
(c) Use an isolated DC-DC converter to make a +/-15V supply and regulate down to +/-12V

It seems to me that (a) has advantages of simplicity and best headroom for a linear power solution. I'm not sure whether the regulated supply of (b) compensates for loss of headroom. The switching noise of (c) might degrade the signal, but gives maximum headroom.

My headphones of choice are Sennheiser HD 580 @ 300 ohm, and a calculator suggests I need the following voltage swing to achieve various power levels:
110 dB, 6.3V peak
105 dB, 3.5V peak
100 dB, 2.0V peak
etc.

Thoughts?
 
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#2
Since you are a novice, I would suggest to start with a simpler project. Save the 6120 for a challenge.

Rail splitters not recommended because there is no way to make them robust. There is a risk of headphone damage. Also, modern opamps can be configured to work with a single supply if you have that constraint.
 
#4
fubar3 said:
Since you are a novice, I would suggest to start with a simpler project. Save the 6120 for a challenge.
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I take your point, but the commonly available cheap TPA6120 boards do a pretty amazing job by just implementing the single-ended, non-inverting example design from the datasheet. My thought was just to replace their dual supply with a battery-powered solution.

rayma said:
Surely you don't want to listen to headphones at over 100dB?
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I certainly don't want to listen at over 100dB, but most design guidance for headphone amps seems to suggest 110 dB as a design point. Why? I'm not sure, but "headroom" seems to be the magic word.

Use two battery packs, and have a real ground. Or (c) but you'll have noise problems to deal with.
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I could treat the cells as 2+2 and use the mid-point as a real ground, though there are complications with the typical lithium charging & protection boards which treat a 4 cell pack as a unit.
I guess the unaddressed question is whether +/-7V is enough supply to optimally feed headphones that likely need less than +/- 2V output for normal listening.
Also whether regulation provides any benefit for battery supply. The simplistic approach of +/-7V unregulated from 2+2 cells is appealing... just not sure if it compromises performance.

I guess I will measure different approaches and see how they compare.

Rob
 
#5
Try this:

1729141494463.png

I first encountered the virtual ground in Quad303, if the bass is to weak for you increase capacitors to 470uF. Transistors can basically be anything that can handle say 100mA like MSA56 and MPSA06
 
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#8
I found that NwAvGuy, the designer of the O2 (Objective2) battery-powered headphone amp, addresses this question on his website.
2-20 BATTERY POWER SUPPLY: The O2 only needs less than 200 mA DC clipping a sine wave in both channels into 15 ohms. In real world use with music the current is under 60 mA DC total under even difficult conditions. Just considering battery operation for the moment there are several choices:


  • Bipolar DC-DC Converter – This allows using a relatively low voltage battery like a single 3.7 V li-Ion cell, pair of AAs, etc. A DC-DC converter generates dual bipolar supply rails at the desired voltage (+/- 12 volts for the O2). If the battery becomes too low the converter shuts down and the amp shuts off gracefully. But DC-DC converters are expensive and are typically only 50% - 80% efficient when not fully loaded so 20+% of the battery capacity is wasted. They also create substantial electrical and magnetic noise (EMI) that will find its way into the audio circuitry no matter how hard you try to keep it out. You can see an example of this noise on the blue square wave at the end of the FiiO E7 review.
  • Single DC-DC Converter Or Charge Pump – This is a lopsided version of the above solution. Only the negative rail is generated and the battery is used “raw” for the positive rail. For the O2 that would mean at least a 9 volt battery. The FiiO E5 and E7 use a single charge pump built into the output chip amp. The supply has asymmetrical impedances which can degrade performance. So you would be lucky to get 3 hours instead of 7 – 9 hours. Battery life would be rather poor with only 50% of the total battery capacity (watt/hours) plus the losses in the converter. And it may have unpredictable behavior when the battery gets low. The plus is the amp could be smaller.
  • Dual Batteries – A battery is quieter than any power supply. Which helps explain megabuck battery powered phono preamps like the Nova Phonomena. Two batteries are also 100% efficient—all the battery power goes to the amplifier rather having some wasted in a power converter or even more wasted in a virtual ground/third channel. The main downside is the cost and space of two batteries and, more important, possible headphone damage if one battery becomes disconnected or dies first. This issue is addressed in 2-23 below.
Click to expand...
Also, as his schematic shows, he doesn't put the battery supply through regulators. He has a strong opinion against virtual grounds.

Seems the best way to go with Lithium cells is 2+2, or better perhaps 3+3, with a real ground between. The issue still arises what to do when battery power expires on one side only. NwAvGuy has a sophisticated power management solution using mosfets. Lithium protection boards already have something similar for a pack as a whole, but I suspect if I tap off between cells, the over-discharge protection of the complete pack would not prevent a voltage still emerging at the centre point. Seems to call for a relay or mosfet approach that cuts both sides of the supply as soon as the protection board cuts the end-to-end pack voltage.
 
#9
The advantage of using the virtual ground is of course that it remains halfway between the supply rails, forever regardless the unequal discharge of the batteries and would not have the amplifier clip in one direction or the other thus never cause any dc offset to damage the phones. I guess you did not consider this.
 
#10
There is no real ground as you or Navguy implies, it is a reference only around which the output swings. If there is no reference the output will not symmetrically swing around the reference point and could clip in one or the other direction. Besides with a virtual ground, the battery discharges linearly unlike battery pack split made with equal number of batteries for each rail, you cannot guarantee that one battery discharges faster or slower than another, that is just pure chemistry.
 
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