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    Portable headphone amp w. batteries

    Iīm testing several discrete OpAmpīs for my DIY portable headphone amplifier (with first generation NiMh 16VDC/2Ah battery pack), all with good results. And the batteries are drained to about 7VDC, then the volume will be a lot lower and if going below the OpAmpīs working voltage, it will also be distorted. Now, I wonder about some things (please see attached schematics):

    1. As the input power is directly connected to the +/-15VDC output at the voltage regulators LM317/LM337 to minimize the power draining from the battery, so also the voltage regīs and the two big power power filter capīs and of course also the rectifier diodes, will not be used - may the voltage regīs draw any mAh current backwards, altough there are no draining component at this direction other than the unused power stage. Maybe itīs best to remove the whole power section (by removing the voltage regīs or cut the PCBīs copper conductorīs for the output pinīs)?

    2. Should the two big filter capīs (with original value 4700uF swapped to 10000uF) be used altough the amp is running by batteries, or will the OpAmp be satisfied without them powered in realtime directly from the batteries?

    3. Can I safely remove all the three (or any of them) LEDīs for the amp, to minimize the battery draining?

    4. As those discrete OpAmpīs will accept 24VDC, will it be fine if raising the batteries voltage to something between 18 and 24VDC?

    5. With above in mind, should the amp be better if replacing the NiMh with Lithium, such as dynamic and transient speed etc?

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    Re: Portable headphone amp w. batteries

    1. about 13mA plus backwards leakage in the regulators (= small) will flow back to the PSU so it would be best to remove the regulators and the voltage setting resistors R305 to R308.
    2. you can remove everything left side of C309 & C310.
    3. The LEDs are only for monitoring the supply, you can safely remove them.
    4. The op-amps will be OK but you might find increased current consumption in the output stages. It might work but treat with caution.
    5. The battery technology is almost irrelevant here, as long as you have sufficient Amp hour capacity to keep it running for as long as you need.

    What worries me a little is there is no DC stabilization in the output stage and it has DC coupling to the headphones. An imbalance in supply voltages could result in a interesting hair style.

    Brian.
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    Re: Portable headphone amp w. batteries

    Thankīs betwixt, for you reply!

    Ok, I will cut the "incoming" copper conductor at the PCB for the output at both LM317 and LM337 and leave most components as is. This way they will act the same way as if they were removed. And should I NOT the C307, C309, C310 and C312?

    For DC coupling (in case there are too much mA at the input of the amp), I use two 1uF MKP capīs. The original value are 1,5uF parallelled with 0,022uF capīs- so there may be a very small amount of low level bass reduced, buth I donīt think itīs noticeable. Is that what you refer to DC stabilization?

    What I meant about swapping to Lithium, is because they will have a lot higher C, so it can deliver much more current. But not sure if this is releveant for audio, as itīs maybe more important with speed as the big filter capīs are not used - so everything will be delivered from the batteries in real time when needed.

    Also, Iīm using a TLE2426 from Texas Instr. paired with a MKP 1uF cap. to create a virtual signal ground, and NOT using the negative voltage from the battery for ground. The datasheet sayīs the Supply current for the TLE2426 are 170 micro Ampere. Is this what this component part will consume/drain?

    http://www.ti.com/lit/ds/symlink/tle2426.pdf



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    Re: Portable headphone amp w. batteries

    I noticed that the driver and output transistors are not included in the overall negative feedback loop therefore distortion is produced at high levels into low impedance headphones.
    At anywhere near maximum output level then this amplifier will cause deafness (maybe that is why it must be so loud).



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    Re: Portable headphone amp w. batteries

    Also, when I use the discrete OpAmpīs in my AK4399 DAC, the manufacturer recommend me to parallell the 240uF original electrolytic capīs with a small bipolar/unpolarized plastic cap to each power pin to nearby ground.

    He told me the standard value to use are 0,1uF, but better is 1nF or at least 0.33uF. Also 1uF is OK, but not 47nF like that. This is to avoid the Fc of that capacitor to be in the midrange or top of the audible range, either above 20kHz or below 200 Hz.

    What is he talking about? Please explain to me! However, I soldered 2,2uF (measured 2,1uF) and think the sound maybe get better with this mod.



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    Re: Portable headphone amp w. batteries

    You should keep C309 and C310 as they are but remove everything before them.

    For DC coupling (in case there are too much mA at the input of the amp), I use two 1uF MKP capīs. The original value are 1,5uF parallelled with 0,022uF capīs- so there may be a very small amount of low level bass reduced, buth I donīt think itīs noticeable. Is that what you refer to DC stabilization?
    The bass will indeed roll off at very low frequencies but I doubt you would notice it. The compromise is between good low frequency response and the settling time when something is plugged in. It takes time for the capacitors to charge if there is any DC superimposed on the input signal and the bigger the value the longer it will take. The DC stabilization isn't from the input signal, it is from the output stage. Consider that with symetrical supplies the output point should ideally be at zero volts when no signal is present. With that design, any differences in components, including transistor gain, may cause differences between the top half of the output stage and the bottom half and that would result in a DC 'offset' being permanently present across the headphone coil. Good practice is to use negative feedback from the output point back the the input so any offset is cancelled out.

    Also, Iīm using a TLE2426 from Texas Instr. paired with a MKP 1uF cap. to create a virtual signal ground, and NOT using the negative voltage from the battery for ground. The datasheet sayīs the Supply current for the TLE2426 are 170 micro Ampere. Is this what this component part will consume/drain?
    TLE2426 can only supply or sink 20mA maximum, that is far too small for an amplifier of that type, especially if you are driving low impedance (< 32 Ohms) headphones. If the TLE2426 can't cope you will get serious distortion and cross talk between the channels. 170uA battery drain can be ignored it is so small (0.00017 Amps).

    Also, when I use the discrete OpAmpīs in my AK4399 DAC, the manufacturer recommend me to parallell the 240uF original electrolytic capīs with a small bipolar/unpolarized plastic cap to each power pin to nearby ground.
    This has nothing to do with the sound quality, it is to reduce the chances of instability. Ideally, the supply pins to the DAC have only steady DC on them but in real life, there will always be some resistance and inductance in the supply and wiring (including copper PCB traces) which produces a drop due to the current flowing through them. The capacitors close to the supply pins work like a local power reserve, they charge from the supply line but being very close to the IC they can release charge if necessary at the point it is needed. The reason for using two capacitors is that high values are inevitably electrolytic types, good for holding a large reserve but not so good at releasing it quickly. Small capacitors have small reserve but can release it almost instantly. By using them in parallel you get the best of both characteristics. Technically, electrolytics tend to increase their impedance as the frequency increases and small values, which are usually ceramic do the opposite. You should use a ceramic capacitor and 100nF (=0.1uF) is a good choice but the type is more important than actual value.

    Brian.
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    Re: Portable headphone amp w. batteries

    Hi,

    There are class D headphone amplifiers. They will consume less power, which increases battery lifetime.

    Klaus
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    Re: Portable headphone amp w. batteries

    Example, how a 555 IC can be made into a simple class D amplifier. The 555 is configured as a pulse generator. Audio is fed to the 'CTL' pin, creating pulses of varying duty cycle.

    The earphone gets a few volts amplitude from an 8V DC supply (theoretically). This illustrates the advantage of PWM.

    Click image for larger version. 

Name:	555 IC iclass D amplifier 8V supply LCC fil 20 ohm load.png 
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    The LC second-order low-pass filter has an additional DC-blocking capacitor which produces AC to the earphone. (There may be more effective filter arrangements.)

    It's not easy to find correct L & C values which produce flat response of the entire audio spectrum. A handy trick is to pre-emphasize bass or treble. Or perhaps to bi-amp, that is, split the signal into bass and treble portions, and send them through separate amplifiers.



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    Re: Portable headphone amp w. batteries

    Quote Originally Posted by betwixt View Post
    You should keep C309 and C310 as they are but remove everything before them.

    TLE2426 can only supply or sink 20mA maximum, that is far too small for an amplifier of that type, especially if you are driving low impedance (< 32 Ohms) headphones. If the TLE2426 can't cope you will get serious distortion and cross talk between the channels. 170uA battery drain can be ignored it is so small (0.00017 Amps).
    Brian.
    Thankīs for your suggestions!

    Should I remove the TLE2426 totally and maybe replace it with some resistors only, about 4,7kOhm? Or will it be ok to not use any rail splitter/virtual ground at all, and only use the +16 and -16VDC from the batteries?

    And about the stabilization caps (to minimize the risk of oscillation for high speed/Slew Rate OpAmps); the headphone amp has 22nF MKS/MKP. He recommended me to use Mylar (Polyester) but I choosed Polystyrene for this amp. Do you think ceraminc will do better here, and why?

    - - - Updated - - -

    Quote Originally Posted by betwixt View Post
    You should keep C309 and C310 as they are but remove everything before them.

    The bass will indeed roll off at very low frequencies but I doubt you would notice it. The compromise is between good low frequency response and the settling time when something is plugged in. It takes time for the capacitors to charge if there is any DC superimposed on the input signal and the bigger the value the longer it will take. The DC stabilization isn't from the input signal, it is from the output stage. Consider that with symetrical supplies the output point should ideally be at zero volts when no signal is present. With that design, any differences in components, including transistor gain, may cause differences between the top half of the output stage and the bottom half and that would result in a DC 'offset' being permanently present across the headphone coil. Good practice is to use negative feedback from the output point back the the input so any offset is cancelled out.

    Brian.
    Do you have any thoughts how to use negative feedback for this amp? Should I solder any resistors or caps from output to input pins at the OpAmp, thatīs not in the schematic?



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    Re: Portable headphone amp w. batteries

    You have to understand that the output of each channel is a voltage referenced to the zero voltage (virtual ground) point. All the output current from both channels therefore has to flow to/from that point. Although using equal value resistors in a potential divider will give half the total at their center junction, in this instance any current difference in the top (+supply) or bottom half (- supply) of the amplifier will make the virtual ground voltage shift. With 4.7K resistors you would see serious shifting, possibly almost to the supply rails. To keep the virtual zero point at half rail you would have to use very low value resistors, maybe less than one Ohm so the current flowing through them would be about 16 Amps and the battery would die very quickly.

    There are two solutions: first is to make an active virtual ground circuit, like the TLE2426 but capable of much higher current, this is fairly easy but the extra circuits will draw more current from the battery.
    The other solution is to DC stabilize the output stage so it tracks at half supply voltage, that means a redesign of the output stage but it is the optimum solution if you want to stay with that kind of amplifier.

    Brad's solution is very cheap and electrically efficient but not very linear, you would get some distortion.

    Klaus has the best idea, to use a class D amplifier. These are electrically quite complicated but everything is done inside ICs for you these days. You can get excellent quality and very low power consumption and in small packages. Most run on a single supply so you don't have to worry about virtual grounds at all.

    Brian.

    [afterthought] Look at the data sheet for the PAM8403 for ideas.
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    Re: Portable headphone amp w. batteries

    Ok, thankīs!

    But if I leave the TLE2426 in itīs position, what can be the result if it can not deliver enough current to ground?

    And what will happen if I remove the virtual ground totally for this amp, by just using positive and negative voltages from the batteries? Have tried this earlier before I connected the TLE2426 and it worked, but not sure if it was better/worser.

    Will the volume be limited, bad sound or some other limitiation or risk for any of the above?

    The OpAmp will draw current between 14 and 20mAh by themselves.



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    Re: Portable headphone amp w. batteries

    The virtual ground or supply mid-rail must be there, otherwise that point in the circuit will float according to the current drawn by each half of each channel. It is the reference for all other voltages. It might still amplify but the channels will interact with each other and almost certainly cause severe distortion.

    The op-amps draw typically 4mA per channel - where are you getting your figures from?

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    Re: Portable headphone amp w. batteries

    https://www.newclassd.com/index.php?page=125&hv=1

    Just click at the Specifications tab

    SparkoS SS3602 will draw 28mAh and also this one worked great in this amp.

    It will also work in pure Class A up to ~15mAh, then it will go into Class AB.

    Maybe the TLE2426 will do fine if the current/volume are not very high. I can not hear any bad things going on. If the TLE2426 will not do the job (as you refer to), how will I notice this?

    - - - Updated - - -

    The original toroid will give dual voltage, 15 - 0 - 15 VAC but the battery only giving single voltage.

    As I only use + and - 16Volt from the battery, will it only be 8Volt to the OpAmp?

    If so, maybe I should raise the battery voltage up to ~30Volt!?



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    Re: Portable headphone amp w. batteries

    Look closely at the power supply schematic - the '0' of the 15-0-15 is connected to the ground point. It has half wave rectifiers to the positive and to the negative rails and then a 1.5A rated regulator in each output. Essentially it can provide 15V at up to 1.5A from each rail. what you propose with the TLE2426 will be a 20mA rated supply, that is 75 times less rating!

    What will happen is the virtual ground voltage will shift in time with the audio as the top and bottom halves of the amplifier change the current they draw. It will be made worse by the virtual ground being used by both channels, if one channel current changes, it will shift the virtual earth on the other.

    Consider something like this instead - it has much greater power output, runs from a single 5V DC supply and is up to 85% efficient using class D technology: https://www.ebay.co.uk/i/13201639883...CABEgKLAPD_BwE

    The only thing to watch out for is the output pins are both above ground so you need to be careful if one side of your headphones is commoned to the other side.

    Brian.
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    Re: Portable headphone amp w. batteries

    Iīm testing several discrete OpAmpīs for my DIY portable headphone amplifier (with first generation NiMh 16VDC/2Ah battery pack) detailed measurements and comparison of two battery operated portable headphone amplifiers.Portable Battery/AC Operated Headphone Amplifier: Amps ... Ultra-Portable Battery Powered Dual-Output Headphone Amplifier.



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