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Weak Amp?

post #1 of 11
Thread Starter 
Hey Guys,
I've just been testing a DIY CMoy+Buffer amp I'd made a while ago now, only got round to it after getting back from university.

When driving grados the amp can drive them to incredible levels, I mean 11 'o'clock is the maximum I'd like to go before I start worrying about damaging my ears. However, plug my HD580s in there, blast the volume knob to full and although the listening volume is still higher than I'd like to go, I dont't get the same sense of power from the amp as I do from the Grados.

I can appreciate that the HD580s are way harder to drive due to their higher impedence, but is this huge difference in volume as a result of poor amplifier design or is it to be expected when driving headphones with varying impedences with the same amp??

Thanks for any help anyone could offer!
Divie

PS the amp is an OPA637 coupled with 3 paralled BUF634s inside the feedback loop per channel. Hope that helps.
post #2 of 11
High impendance headphones require more voltage, while lower impendance headphones demand current. The buffers supply mainly current, which would definitely help drive low impendance cans like the Grados, but not for your Sennheisers. Try increasing the power supply voltage and gain of the amp to allow for higher voltage swings.
post #3 of 11
Hello divie23,

What voltage are you currently using with the circuit.
What value volume pot, input resistor to ground and what source are your using. (level 2Vrms ?).

Try using a lower value volume pot (10K) and change the input resistor to ground if you have one.

This could lower your input resistance and help to transfer more singal to your amp. Try to match your input impedance of your amp with the input impedance of your source so that you will have maximum signal transfer.

What gain are you using ? A gain of larger that 10 was recommended by pmillett for high impedance headphones. (20)

I hope you find more beef in your amp.

Gavin
post #4 of 11

oops : typo

Hello,


"Try to match your input impedance of your amp with the input impedance of your source so that you will have maximum signal transfer. "

This should read :

Try to match your input impedance of your amp with the output impedance of your source so that you will have maximum signal transfer.

Sorry about that.

Gavin
post #5 of 11
Thread Starter 
Cheers for the response guys. I'm using a gain of 11 and a supply voltage of 24V. The ground resistance is 1M and the input resistance is 1K. The problem isn't really as bad as I probably made it out to be because I can also control the volume output of my CD Player and just turn it up when using the Senns.

One problem I have had is mild buzzing intermittently, which is really frustrating bacause the intermittency makes the problem harder to locate! What I did find curious though is that I can make the bzzzzzttt sound appear sometimes when turning the volume pot to maximum.

Gavin, how would you go about matching the impedences??

Thanks Again.
Divie
post #6 of 11

matching impedance (output and input)

Hello,
Use a multimeter and simply measure on the output RCA of your CD player between singal and ground.

Because there is no standard in most audio equipment, this includes amplifiers and CD players, you need to match components.

Your should most likely get a reading of 10K or 47K ohm.

Your wrote you have a input impedance of 1K is that the calculated value from your pot in parallel with your 1M of did you measure it.

Try decreasing the value of the 1M resistors. Do you have any resistors in series with your souce. If so try removing them.

I also tried a BUF634 circuit with a OPA2227. (only one buf per channel and in dip package) and from a computer sound card where you have to share your PC's ground also got noise, even when moving the mouse.

I used both 30Mhz and 180Mhz and it made no difference. The gain was approx 10 and I was used a two 12 V 7Ah lead acid batteries.

What bandwith are you using. Can anyone tell whether the high or low bandwith makes a difference. I am aware that the datasheets recommend that certain opamps are used in certain bands and in certain frequency ranges.

Any comments will be appreciated.

Gavin
post #7 of 11

Re: matching impedance (output and input)

Quote:
Originally posted by gavinbirss
Hello,
Use a multimeter and simply measure on the output RCA of your CD player between singal and ground.

Because there is no standard in most audio equipment, this includes amplifiers and CD players, you need to match components.

Your should most likely get a reading of 10K or 47K ohm.
Err - forgive me if I've mis-read what you've said here.

What I understand this to say is, "measure the resistance on the output jack of your CD player" and that this measurement somehow implies impedance.

If that is what you're trying to say, then I must respectfully disagree - impedance cannot be measured with a simple multimeter. It's actually an awfully complicated topic altogether, and the "Z" measurements slung about are a simplification.

It can be estimated with some special-purpose equipment - see http://headwize.com/projects/showpro...ongue1_prj.htm

I can't see what use measuring the resistance across the RCA jack would be, but I'm a terrible newbie at this. Perhaps you could expand on this further?

-jP

[ EDIT : Two more links on impedance measurement. Note that a signal generator is required.
http://www.mitedu.freeserve.co.uk/Theory/inzoz.htm
http://www.jensen-transformers.com/as/as048.pdf
]
post #8 of 11
Let the crossposting continue!

I answered you at the other place, sorry.
post #9 of 11

measure it

Hello,

Is this a discussion about not having enough beef in your amp or what the different meaning of the words resistance and impedance and whether it can be measured.

Just take a volmeter / ammeter that can measure resistance and measure each pair of RCA's on your CD player and see what you get.

Then measure the resistance and measure any loudspeaker (open and unused) and you will see the reading you get is the same as the nominal impedance. (even though you measure resistance). The impedance does however change with frequency when the loudspeaker is in use.

Gavin
post #10 of 11
Typo !

"Then measure the resistance and measure any loudspeaker"
should read :

Then measure the resistance of any loudspeaker
Sorry about that.


Gavin
post #11 of 11
Thread Starter 
**** guys this is real serious. I've got this permanent bzzztt kind of a sound playing through the amp now even when no inputs are connected. This has got me real worried. It's no longer intermittent its permanent!

This is really strange though, if i turn the Pot to 10 'o' clock the buzz dissapears, but anywhere below/past 10 the buzz reappears. Below 10 the buzz is lower, at 10 the buzz is non-existent and past 10 the buzz increases as you increase the Pot.

What the hell is going on??

Divie
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