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  1. audioguru2

    Stupid op-amp question

    MP, His differential amp doesnt work, that's why he started this post. It fails probably because the inverting input of it has a resistance that is too low for his sensor to drive, so I simply suggested using his spare opamp to buffer it. What is your fix for his problem?
  2. audioguru2

    Stupid op-amp question

    Hi GPG, You are absolutely right! I am the one who is stupid, because I thought that 0.05V drop in the original diff-amp was negligible, but it's not when it is amplified with a gain of 22. When I added numbers to the schematic, I realised my error:
  3. audioguru2

    College textbook question

    Hi Guys, In practice I just look at a graph that I have. It shows that a capacitor charges exponentially through a resistor from a constant voltage source to 63% of the supply voltage, in a time that equals RC. When fully charged (99% charged in 5RC), it discharges to 37% in the same single RC...
  4. audioguru2

    DIGITAL FILTERS

    Hi Shiva, A digital filter can do a lot more than just separate signals into only 3 bands. Texas Instruments took a cheap lousy-sounding speaker, and used a Digital Signal Processor IC to equalize it at many frequencies so that it sounded great! And the EQ can easily be changed with programming...
  5. audioguru2

    Stupid op-amp question

    Sorry GPG, As I said before, the output won't be 6.6V, but will actually be 6.6V above the 1.0V reference voltage, so the output voltage will be 7.6V.
  6. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Guys, I just made another discovery to improve the 5A soup-up of this project on the other thread, and it also applies here. The zener diode D8 is this project's voltage reference and must perform well. With R4 being 4.7K, D6 is operating at a current of only 1.2mA. Our project doesn't...
  7. audioguru2

    Convert 0-30V 3A PSU to 5A or more

    Hi Chester, I am amazed that your re-wound microwave oven transformer produces flat tops on its sine-wave output when the rectifiers conduct. I thought that it would be powerful enough so that its voltage wouldn't sag like that. Maybe your rectfier current is causing your electrical utility's...
  8. audioguru2

    Stupid op-amp question

    No Kevin, You must realise that because an opamp has such a high open-loop voltage gain of 100,000 or more, its inverting and non-inverting pins' voltages are virtually the same when negative feedback is applied. The input voltage of 0.7V pulls-down on the 10K input resistor, creating 0.3V...
  9. audioguru2

    Mysterious Resistor?

    Hi Shahriar, The NE5532 is different from most opamps because it has protection diodes, back-to-back, across its inputs. Opamps that don't have those diodes don't need Rx. Of course, with Rx at a certain lower value or less, the opamp's output is forced to the positive supply, and therefore...
  10. audioguru2

    Stupid op-amp question

    Hi Darrin, You don't need the high Common Mode Rejection Ratio (same interfering signal that occurs on both inputs that are driven from perhaps a long unshielded pair of wires) of that high impedance circuit. A simple follower driving your 10K inverting-input resistor will be fine and allow for...
  11. audioguru2

    Electronic Stethoscope

    Hi Doctor Emad, U5 is the LM386 audio power amp IC. It drives your earphones. U1 and U2 should be low-noise opamps such as the TL071. U4 can be just about any opamp. C3 and R7 connect together and to pin 6 of U2, the volume control and to pin 3 of U4. Please see my corrected schematic of this...
  12. audioguru2

    Stupid op-amp question

    Hi Darrin, You don't need any resistor on the non-inverting input of your difference opamp. Just connect it directly to the reference voltage. On the datasheet, only one application hinted at using a high-impedance load for the sensor, and yours is a fairly low 10K ohms. Most applications show...
  13. audioguru2

    Question on the 0-30 VDC PSU...

    Hi Bitsdoo, Welcome to our forum. A 38VAC transformer has a voltage that is much too high for our corrected project. With no load, it will probably produce 40VAC, which has a peak voltage of 56.5V. The rectified and filtered DC supply voltage will be about from 50V to 55V! Many parts will need...
  14. audioguru2

    Electronic Stethoscope

    Hi Guys, I just finished making a simple test to hear what these noises are all about: 1) 2-wire electret mic from a cell phone. It is mounted in a rubber sleeve. 2) Drilled a hole in a plastic jar lid and pushed the mic into the hole. 3) Biased the mic with a 10K resistor from a 9V battery...
  15. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Skappy, Since your transformer doesn't have a 30VAC output, your project that uses it won't be able to have a well-regulated output up to 30VDC, as I explain half-way down page 18 in the other post. The rating of your 300VA transformer is probably for the total of its 3 outputs, so maybe each...
  16. audioguru2

    Electronic Stethoscope

    Hi Doctor Emad, Use a 1K ohms resistor for R16. It prevents a loud "POP" in the earphone if you plug it in with the circuit already powered.
  17. audioguru2

    free radio frequency

    Hi Keile, I had a quick look at Google "radio frequency allocation" with the quotes and found this neat chart for the USA that looks like a bookcase until you zoom really close to see its detail. Many frequencies are called FIXED which might include unlicensed for you...
  18. audioguru2

    Question on the Plants Watering Watcher circuit

    Hi Olihou, Welcome to our forum and thanks for the compliment. My circuits have been blinking (I let my plants dry-out before watering again) for over 3 months with the original AAA battery cells and the 3.0V has dropped to about 2.7V. I'll replace the cells when they reach 2.0V. The LEDs are...
  19. audioguru2

    Stupid op-amp question

    Hi Darrins, You don't need the "common-mode balancing resistors" that are the voltage divider resistors at the non-inverting input for V2. With those resistors then the reference voltage isn't exactly 1.0V. Just connect the non-inverting input directly to the reference. I suspect that your lack...
  20. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Skappy, Welcome to our forum and greetings to France. If you want this power supply to produce 0 to 30VDC at up to 6A, then it will need modifications similar to those we dicussed in the other forum about converting it for 5A. Near the end of the other forum is a good schematic with parts'...
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