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Cheap MP3 Player as Signal Source - Output Current

Started by Robert Miller May 4, 2017
I would like to use one of these iPod knock-offs as a portable signal 
source. For example, I could produce a complex waveform in software and 
play it as an mp3 via the device's SD card slot.

http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc

My question relates to how much current could be reliably supplied by 
the player without damaging its output stage, given the same repetitive 
full volume signal on both channels.

If I plug in a 3.5mm cord (instead of headphones) and solder left and 
right channel leads together to produce a "mono" signal, how much 
current could the output stage be expected to deliver on a constant basis?

For example, would it drive an 8R air core coil at frequencies below 200Hz?

Robert Miller
On 5/4/2017 12:08 AM, Robert Miller wrote:
> I would like to use one of these iPod knock-offs as a portable signal source. > For example, I could produce a complex waveform in software and play it as an > mp3 via the device's SD card slot. > > http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc > > > My question relates to how much current could be reliably supplied by the > player without damaging its output stage, given the same repetitive full volume > signal on both channels. > > If I plug in a 3.5mm cord (instead of headphones) and solder left and right > channel leads together to produce a "mono" signal, how much current could the > output stage be expected to deliver on a constant basis? > > For example, would it drive an 8R air core coil at frequencies below 200Hz?
ISTR most earphones/buds are 32ohm devices. You also have to ask yourself how *faithfully* you want the signal to be reproduced -- and, over what frequency range(s). E.g., there may be a small time skew between the left and right outputs. Summing them with a mono source that has been encoded in stereo may leave you with distortions in the "waveform" you are trying to synthesize. Likewise, PMP's are designed with human ears as the intended "load". So, no guarantee that they make *any* effort to have a flat response, no artifacts, distortions, etc. Why don't you try measuring the output into a high impedance source and see how much cruft comes along for the ride?
On 04/05/2017 08:08, Robert Miller wrote:

> I would like to use one of these iPod knock-offs as a portable signal > source. For example, I could produce a complex waveform in software and > play it as an mp3 via the device's SD card slot. > > http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc > > > My question relates to how much current could be reliably supplied by > the player without damaging its output stage, given the same repetitive > full volume signal on both channels.
Why not put the antiphase signal on the other channel and take the output as a bridge across left and right channels with 2x the voltage across the transducer you will get 4x the power into a given load. (assuming the output drivers can cope)
> > If I plug in a 3.5mm cord (instead of headphones) and solder left and > right channel leads together to produce a "mono" signal, how much > current could the output stage be expected to deliver on a constant basis?
Seems like a crude way to do it. I expect the output stage is designed to survive shorted to ground faults so it might work. I suspect it will only deliver a few mW of output power though.
> For example, would it drive an 8R air core coil at frequencies below 200Hz?
Only way to find out is to give it a try and see how loud it is. You might be better off with a 64R speaker. -- Regards, Martin Brown
On a sunny day (Thu, 4 May 2017 17:08:15 +1000) it happened Robert Miller
<robert.miller@teledyne.com> wrote in <BrAOA.1177025$F83.1058645@fx38.am4>:

>I would like to use one of these iPod knock-offs as a portable signal >source. For example, I could produce a complex waveform in software and >play it as an mp3 via the device's SD card slot. > >http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675. >c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D1 >00005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc > >My question relates to how much current could be reliably supplied by >the player without damaging its output stage, given the same repetitive >full volume signal on both channels. > >If I plug in a 3.5mm cord (instead of headphones) and solder left and >right channel leads together to produce a "mono" signal, how much >current could the output stage be expected to deliver on a constant basis? > >For example, would it drive an 8R air core coil at frequencies below 200Hz? > >Robert Miller
I have one like this, much cheaper: http://www.ebay.com/itm/131899318162 it is powered by a single lipo 3.7 V. I do not think that would drive an eight Ohm woofer. For that you need to add an external audio amp.
On 04/05/2017 08:08, Robert Miller wrote:
> I would like to use one of these iPod knock-offs as a portable signal > source. For example, I could produce a complex waveform in software and > play it as an mp3 via the device's SD card slot. > > http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc > > > My question relates to how much current could be reliably supplied by > the player without damaging its output stage, given the same repetitive > full volume signal on both channels. > > If I plug in a 3.5mm cord (instead of headphones) and solder left and > right channel leads together to produce a "mono" signal, how much > current could the output stage be expected to deliver on a constant basis? > > For example, would it drive an 8R air core coil at frequencies below 200Hz? > > Robert Miller
I don't think an MP3 file will reproduce your complex waveform. If your device can play a .wav (or some uncompressed format) you'd be in with a chance. Cheers -- Clive
On Thu, 04 May 2017 00:14:41 -0700, Don Y wrote:

> On 5/4/2017 12:08 AM, Robert Miller wrote: >> I would like to use one of these iPod knock-offs as a portable signal >> source. >> For example, I could produce a complex waveform in software and play it >> as an mp3 via the device's SD card slot. >> >> http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-
Screen-FM-Radio-Video-Games-Movie/371612073734? _trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE% 26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid% 3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc
>> >> >> My question relates to how much current could be reliably supplied by >> the player without damaging its output stage, given the same repetitive >> full volume signal on both channels. >> >> If I plug in a 3.5mm cord (instead of headphones) and solder left and >> right channel leads together to produce a "mono" signal, how much >> current could the output stage be expected to deliver on a constant >> basis? >> >> For example, would it drive an 8R air core coil at frequencies below >> 200Hz? > > ISTR most earphones/buds are 32ohm devices. > > You also have to ask yourself how *faithfully* you want the signal to be > reproduced -- and, over what frequency range(s). E.g., there may be a > small time skew between the left and right outputs. Summing them with a > mono source that has been encoded in stereo may leave you with > distortions in the "waveform" you are trying to synthesize. > > Likewise, PMP's are designed with human ears as the intended "load". So, > no guarantee that they make *any* effort to have a flat response, no > artifacts, distortions, etc. > > Why don't you try measuring the output into a high impedance source and > see how much cruft comes along for the ride?
And if there's such a thing as an mp3 player that'll play wav files (or an uncompressed format that fits in mp3 -- I _think_ mp3 is just a wrapper), then use that. -- www.wescottdesign.com
On 5/4/2017 8:01 AM, Tim Wescott wrote:
> On Thu, 04 May 2017 00:14:41 -0700, Don Y wrote: > >> On 5/4/2017 12:08 AM, Robert Miller wrote: >>> I would like to use one of these iPod knock-offs as a portable signal >>> source. >>> For example, I could produce a complex waveform in software and play it >>> as an mp3 via the device's SD card slot. >>> >>> http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD- > Screen-FM-Radio-Video-Games-Movie/371612073734? > _trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE% > 26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid% > 3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc >>> >>> >>> My question relates to how much current could be reliably supplied by >>> the player without damaging its output stage, given the same repetitive >>> full volume signal on both channels. >>> >>> If I plug in a 3.5mm cord (instead of headphones) and solder left and >>> right channel leads together to produce a "mono" signal, how much >>> current could the output stage be expected to deliver on a constant >>> basis? >>> >>> For example, would it drive an 8R air core coil at frequencies below >>> 200Hz? >> >> ISTR most earphones/buds are 32ohm devices. >> >> You also have to ask yourself how *faithfully* you want the signal to be >> reproduced -- and, over what frequency range(s). E.g., there may be a >> small time skew between the left and right outputs. Summing them with a >> mono source that has been encoded in stereo may leave you with >> distortions in the "waveform" you are trying to synthesize. >> >> Likewise, PMP's are designed with human ears as the intended "load". So, >> no guarantee that they make *any* effort to have a flat response, no >> artifacts, distortions, etc. >> >> Why don't you try measuring the output into a high impedance source and >> see how much cruft comes along for the ride? > > And if there's such a thing as an mp3 player that'll play wav files (or > an uncompressed format that fits in mp3 -- I _think_ mp3 is just a > wrapper), then use that.
No, MP3 is lossy, by design. It's like JPEG -- trying to fit (M)DCT's to the waveform to gain compression. You can try to improve the "quality factor" -- at the expense of the file size. I.e., the OP won't be able to create any *arbitrary* waveform but, rather, will be able to create a certain *class* of waveforms, reliably. If the PMP can handle lossless formats, that would be preferable. [RIFF (WAV) is actually a tagged container format so can cover a variety of capabilities (sins?). But, I think always lossless]
On Thu, 4 May 2017 17:08:15 +1000, Robert Miller
<robert.miller@teledyne.com> wrote:

>I would like to use one of these iPod knock-offs as a portable signal >source. For example, I could produce a complex waveform in software and >play it as an mp3 via the device's SD card slot. > >http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc > >My question relates to how much current could be reliably supplied by >the player without damaging its output stage, given the same repetitive >full volume signal on both channels.
At that price, it probably doesn't have an output stage. The headphones are probably connected between Vcc and a PWM transistor to ground, plus or minus discrete components needed to make it work. The output will be high impedance (30-50 ohms) and will be shaped to sound good. Plus it will have the crappy asian software such products are known for. Suggest you take a look at this: http://teuthis.com/daisy/ It's intended to be embedded in a larger project. The design and firmware are Open Source. Any non-linear filtering is probably implemented in software - easily removed. John
> >If I plug in a 3.5mm cord (instead of headphones) and solder left and >right channel leads together to produce a "mono" signal, how much >current could the output stage be expected to deliver on a constant basis? > >For example, would it drive an 8R air core coil at frequencies below 200Hz? > >Robert Miller
John DeArmond http://www.neon-john.com http://www.tnduction.com Tellico Plains, Occupied TN See website for email address
On Thu, 04 May 2017 17:08:15 +1000, Robert Miller wrote:

> I would like to use one of these iPod knock-offs as a portable signal > source. For example, I could produce a complex waveform in software and > play it as an mp3 via the device's SD card slot. > > http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc > > My question relates to how much current could be reliably supplied by > the player without damaging its output stage, given the same repetitive > full volume signal on both channels. > > If I plug in a 3.5mm cord (instead of headphones) and solder left and > right channel leads together to produce a "mono" signal, how much > current could the output stage be expected to deliver on a constant > basis? > > For example, would it drive an 8R air core coil at frequencies below > 200Hz? > > Robert Miller
I use an old Sansa M240, one AAA, with single-tone mp3's and some music, for a portable speaker tester at yard sale or thrift store. With 4 or 8 ohm speakers its clearly listenable above background, no damage to player.
On Fri, 05 May 2017 16:08:26 +0000, Wond wrote:

> On Thu, 04 May 2017 17:08:15 +1000, Robert Miller wrote: > >> I would like to use one of these iPod knock-offs as a portable signal >> source. For example, I could produce a complex waveform in software and >> play it as an mp3 via the device's SD card slot. >> >> http://www.ebay.com/itm/8GB-Digital-MP3-MP4-Media-Player-1-8-LCD-Screen-FM-Radio-Video-Games-Movie/371612073734?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D40130%26meid%3D27374150337b4cdcbbcb0a318da82ed6%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D132179149515&rt=nc >> >> My question relates to how much current could be reliably supplied by >> the player without damaging its output stage, given the same repetitive >> full volume signal on both channels. >> >> If I plug in a 3.5mm cord (instead of headphones) and solder left and >> right channel leads together to produce a "mono" signal, how much >> current could the output stage be expected to deliver on a constant >> basis? >> >> For example, would it drive an 8R air core coil at frequencies below >> 200Hz? >> >> Robert Miller > > I use an old Sansa M240, one AAA, with single-tone mp3's and some > music, > for a portable speaker tester at yard sale or thrift store. With 4 or 8 > ohm speakers its clearly listenable above background, no damage to > player.
I did _NOT_ connect the channels together!