Bipolar op-amp volume pot wiper issue...

Discussion in 'General Electronics Chat' started by foxfire3, Jan 29, 2013.

  1. foxfire3

    Thread Starter Member

    Jan 29, 2013
    34
    0
    I've been changing out the op-amps in a old Hafler D-100 preamp.

    Most of the ones I've tried have been jfets and all have worked well with the existing circuit.

    But when I inserted a LT1229 bipolar op-amp, I get all kinds of funny noises when turning the volume control toward its minimum position.
    After the volume is up slightly, it works as it should from that point on.
    After doing some research, my assumption is probably a high DC offset issue that may be increasing as I decrease the volume.

    The circuit is a simple non inverting one with Rp(from input to ground) being 274K ohms, R1(from -input to ground) is 1.1K ohms and R2(from output to -input) is 10K ohms.

    I tried adding 2.2uF poly caps to the outputs, but that didn't help.

    Can someone please tell me how I can get this particular op-amp to work in the circuit I described?

    Here's a link the LT1229 data sheet:
    http://cds.linear.com/docs/en/datasheet/122930fa.pdf

    Thank you...
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    The LT1229 is not an appropriate op amp for you application. It's a current-feedback amp designed for high frequency (100MHz), low impedance applications. If you had JFET amps that worked properly why did you select such a different amp? :confused:

    The noise may be due to the high input bias current (±3μA for the plus input) which can cause noise through the varying contact resistance of the pot wiper as it moves.

    In conclusion, don't try to get that amp to work. Instead select a low-noise JFET replacement op amp and you should have no problems.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    Putting a 2.2μF on the output of an op amp is a cruel thing to do.
    Why are you using a current feedback op amp? This is a video amp.
    Use a TL072 low noise op amp.
     
  4. foxfire3

    Thread Starter Member

    Jan 29, 2013
    34
    0
    Why am I using it?
    Because it sounds WAY better than most of the jfets I've tried.

    I was hoping there was a way I could make it work since I don't mind a challenge.

    I don't like using caps. on the output either.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,029
    3,790
    If yo want a nice upgrade of the tl072, you can try the updated brother OPA2134.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    If the LT1229 sounds better then a good JFET amp such as the TL072, then something is wrong with the circuit.

    How does it sound "WAY better"?
     
  7. foxfire3

    Thread Starter Member

    Jan 29, 2013
    34
    0
    The TL072 sounds ok. I have several of them along with many other dual and single op-amps.

    Asking me how something sounds better than something else is somewhat of a loaded question.
    I'm not going to try to use adjectives to describe the differences.

    I actually do blind listening tests and there's no contest between a 072 and the 1229.

    Anyway, I'm not trying to start another "one op-amp sounds better than another op-amp" thread here.

    I've read there are ways to reduce the problem using different value resistors and installing a pot on the inverted input of the op-amp.
    I don't know the specifics though, that's why I'm asking here.

    Anyone can just stick a jfet op-amp in there and call it a day.
     
  8. #12

    Expert

    Nov 30, 2010
    16,278
    6,791
    Personally, I almost never ask people why they want to try a circuit...so I'm not asking. Now I will speculate for a while.

    The chart labeled, "Maximum Capacitance Load Vs Feedback Resistor" doesn't go anywhere near 10k ohms. I believe you've set yourself up with a circuit that is extremely sensitive to stray capacitance.

    Speaking of stray capacitance, there is some in the potentiometer and moving the wiper changes the capacitance. If there is any voltage at all in there, changing the capacitance moves the charge and a moving charge (often called current) is what this chip works with. The datasheet says there are 3 pfs on each input pin of the chip and they are definitely at a DC voltage level. In addition, there are stray pfs in the wiring, especially if you're using a plug-in prototyping board.

    I'd say, start thinking in terms of sneaky picofarads and rearrange the circuit to minimize their effects. Then there is the idea that I dreamed up this circuit from very little information and this schematic might be very inappropriate. Read my signature line.
     
  9. foxfire3

    Thread Starter Member

    Jan 29, 2013
    34
    0
    Thanks for the circuit.
    Without knowing the resistor values, cap. values, the number of the transistor and what "some voltage" is...I'm afraid I don't know how to construct it.

    You mention that it's inappropriate?
    Does that mean it's likely not to work and I would probably be wasting my time building it?
     
  10. #12

    Expert

    Nov 30, 2010
    16,278
    6,791
    I mean that I completely guessed about where you have put the volume control, what voltage you will use to set a bias point, what the bias point will be, how much gain you want, what is the original schematic of the Hafler pre-amp, what voltages you will give the chip for a power supply, where you have put the coupling capacitor, etc. I worked with what you gave me to work with, and it wasn't a lot.

    As far as I can tell, this amp circuit will work, but not without you supplying some more information and adjusting values to get the chip to amplify. You said you get funny noise when adjusting the potentiometer. That means you have built a circuit and turned the potentiometer. What you did not do was give me even a hint about how you built the circuit.
     
  11. foxfire3

    Thread Starter Member

    Jan 29, 2013
    34
    0
    I didn't build the circuit. It's a Hafler design.
    It's very easy to change things out the way it's made.

    Power supply is + and - 15 volts.

    IC3 is the one I've been swapping out.

    C8...1uF
    C35...47pF is NOT used in this version of the DH-100.
    R14...274K ohms
    R15...10K ohms
    R16...3.57K ohms

    If you need to know anything else, just let me know.

    Thanks...

    Here's the schematic:
     
  12. foxfire3

    Thread Starter Member

    Jan 29, 2013
    34
    0
    #12...are you still around?
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    If it functions, but you only hear the wiper noise as you change volume, and you like the sound better, just leave it as is and avoid changing the volume. You could use a digital pot to avoid a physical wiper, but that gets more complex.

    There's a reason the engineers who built the amp used JFET input op amps, which are typically better in most aspect to bi-polar op amps for audio. Wiper noise was often just a fact of life prior to the newer circuits.

    This is a case of "If it isn't broke, fix it until it is."
     
Loading...