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Chip Amps Amplifiers based on integrated circuits

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Old 13th April 2010, 01:04 PM   #11
AndrewT is offline AndrewT  Scotland
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that schematic is terrible.
Variable output offset sent to the headphones.
Output bias dependent on LED voltage variations.
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Old 13th April 2010, 01:07 PM   #12
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hey come on - that was my first build and I was quite proud of it!
what would you recommend as a next headphone/preamp opamp based project then?
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Old 13th April 2010, 01:10 PM   #13
AndrewT is offline AndrewT  Scotland
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look for a buffer type circuit, or at most a gain of 2times amplifier.
Since you already have opamp + discrete experience, are you ready for an all discrete build?
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Old 13th April 2010, 01:18 PM   #14
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well, my cd player is 2 vrms & 22R output, and power amp is 100k input impedance - I don't think I really need anything except a unity gain volume control in between them. every preamp I've had needed to be modified with resistors to lower the gain!!

any ideas on that front?
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Old 13th April 2010, 04:28 PM   #15
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by lordearl View Post
well, my cd player is 2 vrms & 22R output, ...................... every preamp I've had needed to be modified with resistors to lower the gain!!
yes, that is to be expected.
Most power amps will develop maximum output from a signal a lot smaller than 2Vac.
If you don't attenuate the signal, the amplifier will overload!

And 2Vac into headphones will nearly blow your eardrums.
Most are in the range of 90 to 100dB/mW.
2Vac into 100ohms is 40mW. Expected maximum SPL will be between 106dB and 116dB for 100ohm headphones.
2Vac with a 4times (+12dB) amplifier will give 118dB to 128dB into 100ohm headphones.
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Last edited by AndrewT; 13th April 2010 at 04:36 PM.
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Old 13th April 2010, 06:11 PM   #16
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by lordearl View Post
Thanks Mooly & AndrewT -
(1) How to I measure the regulation with a multimeter (bear in mind I'm a newbie).
(2) I've attached the circuit for one channel - C3 is 470uF because I'm using a wall wart - does this clarify anything (unsure whether this cap is 'decoupling' anything).
Look at PSU and see what it's specification is.
eg 10 volts output at 300 ma

1. Measure the voltage output with no load.
2. Measure the output when loaded (for this you can use resistors of suitable rating... R=V/I which means a 33 ohm would draw the rated max curent from the PSU. Use a suitable wattage too. W= I*V so this would need a 3 watt resistor.

If the PSU is regulated the volt drop will be minimal, with the wiring etc being the main cause of any drop. So if the voltage held to within say 0.3 of a volt then thats reasonable.

If the voltage off load were say 13 or 14 and it fell to 7 or 8 when loaded then that's definitely an unregulated supply.
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