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Old 23rd May 2008, 06:41 AM   #11
Mooly is offline Mooly  United Kingdom
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There is a TDAxxxx I/C for this very use, and I can't remember the number !
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Old 23rd May 2008, 11:04 AM   #12
Mooly is offline Mooly  United Kingdom
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It's the TDA7050, this is as simple as it gets, works down to 1.6 volts as well. You need series resistors to the 'phones if you use this.
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File Type: jpg tda7050.jpg (30.6 KB, 666 views)
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Old 24th May 2008, 07:00 AM   #13
Dan2 is offline Dan2  South Africa
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well all i could get are LA4570 chips. i have not checked the datasheet for them yet but they should work.
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Old 24th May 2008, 07:09 AM   #14
Mooly is offline Mooly  United Kingdom
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Have you a circuit to use one of these. These are a dual pre and power in one package I think.
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Old 24th May 2008, 09:28 AM   #15
lineup is offline lineup  Sweden
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Default Re: low voltage headphone amp

Quote:
Originally posted by Dan2

what i really want to do is build it so it can
get its power from one of the usb plugs on the xbox,
so no batteries will be required.

i also want it to sound quite decent
- as i am sure the usb has enough power to do (i think usb has 5v, 200mA output???)

i was thinking of using an opamp, but if you have a better idea please share.

One thing you may want to checkout, is how much 'PC-noise' is on USB power jacks.
Computer circuits generate a lot of noise. From high frequencies and also lower.
Any audio amplifier benefits from a CLEAN Supply.
For example those PC speakers we use, mostly use OWN Power supply adaptor.
You may have to filter the USB power a bit. Using some capacitor/resistor.

I am sure you can use USB, as you only need a few Volts to get enough power.
I guess we are talking 16 / 32 Ohm freestyle or light type of headphones.
The sensitivity of such can be very high: 100-105 dB per mW is not unusual.
And they can be powered good from only 2 x 1.5V batteries. = 3.0 Volt supply.

If you could find Specifications on headphones in question
it would be a good help for advice you.
As AndrewT has mentioned, there are 2 things we would want to know:
- 1. Impedance, in Ohm. Can be 16, 32
(Some modern hifi big headphones has got 150, 300, 600 Ohm impedance)
- 2. Sensitivity. In dB SPL = Sound Pressure Level
Usually how loud in dB at 1 mW input power.


Regards, Lineup

---------------------
fotenote:
There is new standard for sensitivity, used by e.g. Sennheiser.
SPL at 1 Volt RMS.
See my attachment:
Specifications for Sennheiser HD 650 ( Top of the line expensive HeadPhones)
Quote:
Nominal impedance .................................................. ............... 300 Ω
Sound pressure level................................115 dB (at 1 kHz, 1 Vrms)
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File Type: png sennheiser-hd650_specs.png (8.1 KB, 584 views)
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Old 24th May 2008, 11:48 AM   #16
Dan2 is offline Dan2  South Africa
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i didn't even think of noise on a usb power supply!! i will have to check that out.

as for the headphones - well - i want it to be able to run with any type as i may be building a couple for other guys. i seriosly doubt any of them will have top of the range type headphones. for example, the headphones i am building this circuit for are (almost) bottom of the range - the type that has vibration bass (little motors that shake with the bass) and although quality is ok, its not anywhere near the quality of expensive headphones.
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Old 24th May 2008, 08:56 PM   #17
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If you care about sound quality, why not just steal others success? I think the very well renowned portable headphone amp Xin Reference use AD8615 as L/R amps and AD8531 as ground channel amp. They are specified for 2.7 - 6 V supply and are good for 150/250 mA output current. AD8397 is also a very popular opamp for headphones, 3 - 24 V supply, 310 mA peak output current.

For your convenience, use a board from Mini3, or why not the whole amp, it uses the AD8397 mentioned above for left/right and OPA690 for ground (specified for 4.5 - 12 V supply and can deliver plenty of current):
http://www.amb.org/audio/mini3/
This is a true hifi headphone amp. You can even buy complete kits from
http://www.glassjaraudio.com/main.sc

If you have "normal" = lowish impedance headphones, you don't have to worry about not having enough sound pressure. 1 V into phones like this is very loud, 5 V is crazy. It's simply not true that monolithic opamps can't drive headphones.
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Old 25th May 2008, 07:35 AM   #18
Dan2 is offline Dan2  South Africa
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well i already got these LA 4570 chips - but i cant find the datasheet for them!!! i will keep on looking.....
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Old 25th May 2008, 08:17 AM   #19
lineup is offline lineup  Sweden
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Quote:
Originally posted by Dan2
well i already got these LA 4570 chips
- but i cant find the datasheet for them!!!
i will keep on looking.....

Hard to find a datasheet.
Here is a Romanian schematic, with data on LA4570.
Max voltage is 4.5 Volt. And typically it is used with 3 Volt DC.
This means 2 x 1.5 Volt battery.

2 x 0.02 Watt = 20 mW is more than you probably ever need.

We can also see a bit about the PIN configuration and figure out howto use it:

Click the image to open in full size.
From this page:
http://www.electronica.ro/audio/LA4570.shtml

Sanyo LA4571 may be a very similar headphone IC.
Here is one link to this PDF:
http://www.ddrservice.net/66c507f/In.../la/la4571.pdf




Regards, Lineup
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Old 25th May 2008, 01:55 PM   #20
Dan2 is offline Dan2  South Africa
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thanks for finding that!!

just to double check - the odd looking symbol on pin 2 and 15 is the inputs??? and the 10k variable pots are the volume control (or gain?)??

if i use a usb and just run a diode in series that should give me the right voltage (or even a 3v regulator - that should help with the noise)
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