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Old 16th May 2012, 07:32 AM   #1
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Default Dual voltage from a battery

Ok, I have built a cMoy style headphone amp using a single 9v PO3 battery as the power supply, which I have to sst sounds great. I was looking back over the schematic and I don't quite understand it.

I have 2 resistors of 4.7k connected in series from pos to neg of the battery with 220u caps accross them giving me +-4.5v and a virtual ground. Obviously the op Amps run off the +-4.5 and can draw whatever current they need, but I don't understand how the current is drawn through the headphones. For example, when the output of the opamp is on the positive part of the wave, you have the current going to the speaker, through it's cool and the to virtual ground, surely from here it had to get back to the negative of the battery, and the only way is through a 4.7k resistor, hence limminiting the current.

An I missing something obvious?
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Old 16th May 2012, 07:46 AM   #2
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Hi - no I don't think you're missing something important in what you are saying. If you want to supply the amplifier through a low impedance PS you can increase the capacitor values (which will help for "shorter time" current boosts) or make a power supply which has a lower output impedance at + gnd & -.

Also, unless your battery is rechargeable the way the resistors is connected means that there's a constant draw of ~ 1 mA whether you play or not. Don't know if this is feasible in your context ...?

There are many approaches to power supplies, however, my own preference is for simple, yet careful designs with selected components. If you make a search here on DIYaudio for power supplies you can find many entries to this.

Best regards,

Jesper
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Old 16th May 2012, 08:05 AM   #3
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Hi. The battery is re-chargeable, so the setup I have is Ok.

But surely the maximum current flow I can get, even with a short between positive and virtual ground is 1.9 mA (9/4.7k). Surely my headphones are drawing more current than this. Or are the 220uF caps passing the current?
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Old 16th May 2012, 08:21 AM   #4
qusp is offline qusp  Australia
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buy another 9v battery and make a center tapped supply, get rid of the resistors and just add some lowZ caps
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Old 16th May 2012, 08:33 AM   #5
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I am happy with my single battery setup and havn't enough room in the enclosure for another. Just wanted to understand how the current actually returns from virtual ground back to the battery.
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Old 16th May 2012, 10:04 AM   #6
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Your caps are providing the AC current. On average sound waveforms have zero DC because they have come through at least one coupling capacitor somewhere. On positive parts you slightly charge the caps one way, then the negative part slightly charges them the other way. It averages to zero.
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Old 16th May 2012, 10:24 AM   #7
qusp is offline qusp  Australia
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ok, but you will always get sub-par performance with a single battery with divider. the impedance on the 'ground' reference is considerably higher so low impedance headphones may struggle
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Old 16th May 2012, 10:31 AM   #8
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440uF coupling capacitance ought to be enough for all except the very lowest impedance phones. For 20Hz LF rolloff you can go down to 18 ohms phones.
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Old 16th May 2012, 11:40 AM   #9
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yes, but many IEM's these days are multidriver and thus even models with nominal 28ohms can present less than 10ohms at low frequency. just something to be aware of, these type of power supplies driving headphones with virtual ground have been thoroughly debunked over the last few years.
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Old 16th May 2012, 12:16 PM   #10
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Also you can create a virtual ground using one more op amp wiring voltage divide r to the +inp and giving 100% NFB, with -inp to output, and then, using the op amp out as a virtual gnd. The opamp can present lower AC resistance than the caps.

RGRDS.
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