[eric343]

Alright, I just got my 2134s from Digi-Key and stuck 'em in my CHA47s. However, with one of them (that I've built with gain=6 and no Grado R5s) the opamps get pretty warm after a few minutes, and the sound starts getting distorted. Turning the amp off and then on again seems to solve the problem, but after a few seconds (or minutes, depending on how long the amp was off) the distortion comes back. Do I need to heatsink the opamps?

 

[thomas]

Under normal operating conditions, the opamps should not heat up or distort, and do not require heatsinks.

First off, make sure you are not supplying the opamps with too much voltage, that will cause them to heat up and distort.

Next, take out a multimeter, and check current draw of the opamps. It should not be more than about 15 mA. If it is more, which i suspect, then you probably have a short between one of the power supply rails and any of the pins. Check your board carefully (a continuity checker on a multimeter is very helpful) for any solder bridges...I would thing that the output of the opamp is being shorted to one of the rails (the pin next to it), if this is happening, then you will probably hear a click when you plug in your headphones.

One more thing to check is the voltage of the virtual ground in relation to the rails, it should be very close to half of the supply voltage.

 

[eric343]

Well, the opamps are getting powered off a 9V, so voltage shouldn't be an issue. I did a visual inspection and I don't see any shorts, but I haven't tried using a multimeter, since I'm not entirely sure how to check current draw with it.

 

[skippy]

when you say r5, do you mean no output resistor? if you don't have an output resistor one opamp may try to drive the low output impedance of the other opamp in parallel (i think). try adding output resistors and see if it goes away.

 

[eric343]

Umm, I'm not exactly Apheared when it comes to electronics, but how would R5 (the Grado anti-hum resistor) affect the impendance of the opamps? They aren't even connected except with the crossfeed, so I don't see how they would be trying to drive each other.

 

[skippy]

i'm sorry, i have no idea what r5 is. i've never used a hansen board. i assumed r5 was the resistor at the output of the opamp. in chu's article on the pocket amp, he calls the output resistor 'r5' and says that it would reduce noice on low impedance cans such as grados. i thought this was the resistor you were referring to.

so exactly what is r5?

 

[eric343]

Yes, that's *exactly* the resistor I'm referring to. In the documentation for the Hansen PCB it's always marked "jumper" or "wire".

Because the amp's gain/volume is *perfect* right now for my Etys, should I just heatsink the opamps?

 

[skippy]

if the outputs of the opamps are connected to each other, then that's your problem. adding the load sharing resistors will probably fix it. with the normal amp, this resistor is unnecessary and can therefore be jumpered. however, when you have opamps in parallel you should have the resistors. heatsinking won't be of much help.

 

[eric343]

Umm, I don't think the opamps are connected in any way, since this is a stock Hansen board. The opamps are dual models so there may be two stages, but that's all. The board isn't configured like a monoblock amp or anything.

 

[skippy]

you said CHA47, right? i assumed that that meant C. Hansen/Apheared 47 amp. which would be an Apheared 47 amp on a Hansen board, right? if you have a 47 amp, then you have opamps in parallel at the output. if you have parallel opamps then their outputs are connected to each other.

 

[eric343]

Yes, CHA47 is the Cmoy/Hansen/Apheared "47" amplifier. And yes, it's an Apheared 47 on a Hansen board. And I don't think the opamps are connected, since the two chips have totally separate circuitry, except for the joint at the crossfeed.

Oh, wait, these are *dual* opamps. There's two opamps per chip. Oops.

So I should just try an output resitor, then? (Keep in mind I'm driving Ety 4S, here)

 

[Apheared]

Ok, let's make sure we're talking about the same thing here... crossfeed on a CHA47... if you did a 47 implementation on a hansen board, it sorta negates the ability to have a crossfeed... There is no longer two active stages... just one...

well, you could implement a crossfeed, you just can't have it buffered and will need to adjust some values... when Chu gets HW back up and searchable, there's a few posts re: jan's crossfeed as a passive device between amp and phones - this is how you'll have to implement it. You can do a true parallel stacked output, ie using 4 dual chips, 2+2, but then it's not a 47...

I'm assuming that everyone that has EVER used the term "CHA47" takes that as CMoy/Hansen/Apheared "47"... as in the Apheared current-doubled "47" which is a variation on CMoy's pocket amp, and implemented using Carl Hansen's professionally made pcb... if this is NOT what you mean, stop using that term... cause you're confusing me.

 

[eric343]

Umm, then I'll call mine a CHA, since I followed Carl Hansen's directions to the letter and have a working crossfeed.

 

[thomas]

Check the overall current draw by disconnecting one wire going to your battery, and hooking up your ammeter in serries with the battery and the circuit.

If it is too high (>30mA) , then somthing's shorted out.