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Oscillating amp?

post #1 of 16
Thread Starter 
Greetings all!!

I've been scouring the archives the last three days and still need help. I've built a modified hansen amp that is giving me troubles. For my first time building one of these, I've deleted the crossfeed section and instead modified it as an Aph 47 design. Also no volume pot, instead two 15k resistors. I'm driving this with one 9v battery. I know that the 47 variant requires more juice, but I wanted to know if I could build one before I make my real one to use. The bottom of the Hansen looks clean with no apparant solder bridges. I used the RS resistor pack but didn't check each one with a meter before using them. I did check the color codes (which can be mislabled) and all are in their appropiate place on the board. I'm testing the amp with a PCDP using the volume control out.
Now for the problem. The right channel seems fine. The left is giving me of distortion and slight popping sometimes. I switched the opamps ( left to right...right to left) and the right channel now had the distortion but much reduced. The left seemed fine. So I switched them back just to make sure, and after a couple of seconds the right channel's volume output stepped down suddenly and had a little distortion like the left. I measured the idle current and got 43.7 mA (I did this by attaching only one side of the 9V battery and used my meter to measure and bridge the other terminal.) I know this is extremely high for a standard Hansen design (usually 18mA) but since I'm using the Aph 47 design, I didn't know if this was good or not.
I've tried to measure the resistors already soldered on the board but couldn't find good contacts to do so.
I've been working on this first amp for about 3 weeks off and on (more off than on) and am trying not to get frustrated with the lack of results debugging. I am new to all of this, purchased beginner books, but they didn't seem to help. I'm turning to you DIY gurus for help. Thanks.
post #2 of 16
43 mA is high, but with low impedance headphones it's possible to hit this easily. If you got that 18 mA number from one of my posts, it's because I'm a fan of high-impedance headphones, so most of my numbers come from measurements with high-impedance loads, which make for lower currents.

An oscillating op amp often makes the chip hit its full short-circuit current level. You don't say which chip you're using -- that's important. But if it's an OPA132/134 family member, they can supply 40 mA per channel. There are four op amp channels in a Hansen, so you could hit 160 mA in the worst case situation. (If you add a heat sink to the chip, you can actually make it dissipate more than that!)

Let us know what chips you're using and we can move this from the wild guessing stage to the useful advice stage.
post #3 of 16
Thread Starter 
Thanks for helping me out. Your guess is right. I am using the OPA2134PA. Also, I am using cheap koss headphones to test them (definitely low impedence.) I did try my Sony V6 and the distortion was reduced but not gone. None of the components get warm when idling or driving a signal. The 9V battery is fresh; I did put the meter on it too. I wish I was more capable at this hobby instead of a complete doofus. All the DIY posts and archives are interesting until they go a little deeper. Then I become a blindfolded deaf person walking through a mined china shop.
post #4 of 16
Blindfolded deaf person walking through a mined china shop... LOL!

Things to check:
* Did you ground the volume pot's spindle? (usually done by soldering/pinching a wire to the metal top of the pot, or pinching it between the pot and the case when you mount it)

* Is the output resistor the correct value? This can cause mondo distortion.
post #5 of 16
Thread Starter 
As per the original post, the volume pot is not connected. Instead I have a 15k ohm resister per channel. The APH47 mod for the hansen requires a 4.7k resistor from the S2 L/R out and L/R out to be connected to the headphone out along with one end of R4. I guess you mean these resistors to the out plug? I'll try measuring them tomorrow with a meter. Anything else?
post #6 of 16
Oops, I forgot about that

And yeah, R5 = output resistor.
post #7 of 16
Thread Starter 
Actually the instructions that I got when I purchased the Hansen boards was just to jumper R5. I guess I could sub a resistor to lower the total gain, but I don't know/understand how that would help my problem. Help?
post #8 of 16
Well, I reccommend a 47ohm resistor to prevent oscillation. I was having a lot of trouble with oscillation on my Hansen board before I stuck one in.
post #9 of 16
Thread Starter 
I actually ran out of 47 ohm resistors doing the APH 47 mod. The closest thing I have is a 100 ohm, would this work just as well?
post #10 of 16
Well, you'll get reduced output. It should be fine for Grados, but consider making a quick run to Radio Schack.
post #11 of 16
The APH47 mod for the hansen requires a 4.7k resistor from the S2 L/R out and L/R out to be connected to the headphone out along with one end of R4.
4.7K? If that's not a typo, there's your problem! The vertically standing resistors in the OUT and S2 OUT holes should be 47 ohms. If you don't have 47's but do have some 100's, you can parallel two of them to get 50 ohms, which is close enough.

Also, for a CHA47 you should jumper R5 -- because of the four 47 ohm resistors you're adding, there's no reason to put a resistor at R5, even with Grados. Apheared uses Grados, and you can see in the pics that R5 does have a jumper across it.
post #12 of 16
Thread Starter 

That 4.7k ohm was indeed a typo. I do have 47 ohm resistors where they should be. I wish my problem was that easy to solve.
post #13 of 16
I suppose your right channel chip (previously in the left channel) could be damaged. That's not likely if you've been careful not to "static" them and you didn't plug them in backwards, or plug/unplug them with the power applied to the circuit. The only way to be sure they're still working is to have another circuit you could drop them in.

I guess you could get a TL082 from Radio Shack and try it. It's definitely a suboptimal chip, but it's good enough for testing. If it does work, though, it doesn't necessarily tell you the OPA2134 is faulty -- it might just mean that the new chip is immune to whatever is causing problems for the old chip. You'll want to use more than 9V when testing with the TL082, though -- you'll hear distortion otherwise, which will invalidate the test.
post #14 of 16
Thread Starter 
I am sorry, "static" the chips? I don't understand that term.
post #15 of 16
He means zapping them with static electricity (like when you reach out to touch a doorknob and a spark jumps).

Basicly as long as you kept them in their static-proof bag and made sure to ground yourself before handling them you should be fine. If you soldered them in, did you use a grounded iron?
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