Can anyone recommend a replacement for OPA2134 (hence, dual - browndogs won't fit my unit) that has excellent PSRR? My PSU is a wall wart (says regulated, but I doubt it), so trying to hunt down an opamp with the best PSRR.
Additionally, the headphone amp this is for uses a circuit which biases the output heavily into class A - any chips out there which would benefit from this?
Additionally, the headphone amp this is for uses a circuit which biases the output heavily into class A - any chips out there which would benefit from this?
Just add some Bypass caps close to the IC Pins , It will filter out some of the PSU noise ...... Or built a well filtered Regulated supply and drop the wall mart ....
The OPA2134 doesn"t have Bad PSSR and getting an opamp with better PSSR still doesn"t get rid of the problem of a Noisy PSU .....
Cheers
The OPA2134 doesn"t have Bad PSSR and getting an opamp with better PSSR still doesn"t get rid of the problem of a Noisy PSU .....
Cheers
Best PSRR I know of is National's LME49990, though I suspect it'll be some time before a dual version's available. The LME49720/49860 offer a slight gain over the 2134 (but their main advantage is they sound a bit more precise, probably due to the higher GBP). If you want more rejection you'd be better off throwing together a basic 7815/7915 type supply; that'll give you 60-80dB versus the 10-30dB from an op amp upgrade. Chips are cheaper too. Seeing as the 2134s are ~110dB I'd be surprised if PSRR is really an issue; have you measured the ripple or checked for bad bypass caps?
In other words, what Minion said.
In other words, what Minion said.
With typical decoupling arrangements, your performance is probably not going to be PSRR limited. Rather it will most likely be limited by power supply hash coming down onto the signal ground through the decoupling capacitors. Better to regulate the wall wart (relatively easy) or rejig the grounding (more difficult).
So long as those caps are between the rails, it could be a good idea. If you tie them to the signal ground, all that happens is you dump power supply rubbish on your ground. The second option is sounder.
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Thanks people - that's a great start. I'm going to build a better PSU. My headamp pcb already has 470uF caps on it, so you are probably correct simply swapping the opamp may not make too much differece. I notice that OPA2134 gives a nice, well defined bass, but the top end is a little shrill and not as tightly defined as the bass - how would the LME49990 compare with this?
So measure the output off and on load to see what the regulation is.
Scope the output under load to see what the ripple/noise is.
is the decoupling as recommended by the opamp manufacturer? Have you seen any reliable references to alternative decoupling that improves on the manufacturer's recommendation.
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).
(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).
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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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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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