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Posts posted by audioguru

  1. Hotwaterwizard,
    TR1 and TR2 won't switch anything, with D4 and D8 connected backwards.
    Where is the positive supply for TR1 and the negative supply for TR2 going to come from?
    Also in your oscillator, when one transistor conducts, it drives the base of the other transistor negative, through the coupling capacitor, much beyond its reverse breakdown-voltage rating of about 6 or 7V, causing slow destruction. Each oscillator transistor's base needs a protection diode.

  2. MP,
    In the schematic, U2 and U5 are shown without negative feedback. Therefore, DC-wise, their outputs would be idling against a power rail since their enormous gain would amplify their offset voltage. AC-wise, any tiny signal (or noise) that is greater than their offset voltage (only a few millivolts) would therefore be rectified (causing severe distortion) and also be over-amplified.
    U2 is fixed by adding a dot on the schematic at pin6 so that it connects to C3 and R7. Then it will have feedback and will be a classic "Sallen and Key equal-capacitor (requiring its gain of 1.6) Butterworth 2nd-order low pass filter" with a cutoff frequency of about 103Hz, as others have noted and as described in the project's text.
    U5 is fixed by changing its part number in the parts list to an LM386 with its built-in feedback and biasing, and correcting the pin numbers on the schematic for its output and +9V. If U5 had feedback and its pin4 connected to -9V so that it could function as a 741 opamp, then it certainly would not be able to drive an 8 ohm earphone like the LM386 is designed to do.

  3. Cdak,
    MP's expander circuit is a good idea. But your scope picture seems to show a continuous low-frequency. Are you using shielded cable from your mic to avoid hum pick-up, and insulating the mic and its cable from the patient to avoid a ground loop?

  4. MP,
    Well, let's see: Electronics-lab posted an audio circuit which has 2 op-amps running open loop without any feedback, an input capacitor which can be calculated to severely cut the desired sound, an output stage without proper bias and many complaints that it doesn't work. The author of the circuit no longer supports it and apparently hasn't even tried it.
    I am just trying to be helpful in assisting members, getting this circuit corrected, improving it with newer parts and pruning the useless parts. Otherwise, the next guy who tries it will also be frustrated.
    The 8 changes that I recommend are quite minor, physically. The original circuit is basically the same, but corrected.
    If I had a need for this circuit then I would build it, but for now it is experience and attention to detail that is talking.

  5. Hotwaterwizard,
    1) I saw an advertisment for an ultrasonic "sound beam" about 3 months ago but have lost the source. Apparently when the modulated beam hits an object (like a statue in a museum) then the sound demodulates and appears to originate from that object. Only people near the object can hear the sound without everybody being bothered like with a PA system today. Their biggest problem is to make the ultrasonic beam powerful enough because it is absorbed by the air and tweeters are fragile and low-power.
    2) Plazma (flame) tweeters have been demonstrated for many years. They are very impractical.

  6. Hi guys,
    Sorry for this late post. This project has attracted a lot of attention but has also created much frustration and confusion, due to schematic and parts list errors in the original article. Let us stick to the original circuit using standard parts and having 9V batteries. Dpak showed a nice circuit but the LM387 is obsolete (discontinued in '98) and it cannot be powered with 9V batteries easily.
    Corrections to the original circuit follow:
    1) Connect pin6 of U2 to the junction of C3 and R7. This applies proper negative-feedback which dramatically reduces gain (and noise) and allows U2 to function as a 2nd order low-pass filter. Thanks, T_ang4 and Staigen.
    2) Use an LM386 for U5 and swap its pins: pin5 is output and pin6 is +9V. The LM386 has built-in feedback and biasing to be used with its inputs as shown, and can drive an 8 ohm earphone. Thanks, Staigen and Mozikluv.
    3) Change C2 to 4.7uF (the + lead to the mic) so that it can pass a heartbeat sound. Listen to Pink Floyd's "Dark Side Of The Moon" rock song since it starts with a 16Hz heartbeat. The original circuit's gain is 'way down at 16Hz.
    4) Get rid of U3 which doesn't do anything. U2 can easily drive U4 and the volume control.
    5) Get rid of R3 and R9 and replace them with wire . They also don't do anything.
    6) Use a TL071 (or a dual amp TL072) for U1 and U2. It is low-noise and inexpensive. Or use a quad amp TL074 for U1 to U4.
    7) Disconnect the junction of R1 and C1 from +9V and add a 1K resistor from their junction to +9V. This will filter the amplifier's input from the bouncing +9V power supply. Add an additional 1000uF capacitor fom +9V to ground. This helps the battery provide power to U5.
    8) Add a 1K resistor across the ouput jack. This will stop a loud "pop" when you plug-in your earphone if the stethoscope is already turned-on.
    If the mic is properly mounted in a stethoscope-head (jar lid or whatever) then it should reproduce a heartbeat sound well without much background sound. If breathing sounds must be heard then change R5 and R6 to 1K resistors, but background will be louder, and keep the mic away from your earphone to avoid howling. Add a switch to hear either sound properly.
    Please reply if you make these mods and let us know how it works.

  7. MP,
    If a back-EMF protection diode is built into a fan then it would be in parallel, not in series. So if you connected it backwards then the diode would be forward-conducting across the power supply which may blow-up the diode and/or the power supply.

    Maybe you misunderstood my explanation of the reason for having a protection diode across a fan or relay coil: without a protection diode then a high voltage will occur across the fan and its driver, not the power supply, when the fan is turned-off.

  8. Hi Mastrila,
    Maybe when your parts got hot they were damaged.
    The guys who are talking about modifying this power supply to get 5A of current output are having problems similar to yours with the original circuit. Their post is here:
    Those guys and I believe that the circuit depends too much on "selected" parts as follows:
    1) A transformer that has really good regulation (low resistance windings) so that its voltage doesn't sag too much at the voltage peak when it is sourcing many amps to charge the filter cap.
    2) Rectifiers that have a low voltage drop during #1 above.
    3) A filter cap. that is at its extreme max. tolerance (+100% is double its rated value) and has a very low ESR.
    4) Q1 and Q2 with more hFE than minimum or typical spec.
    5) A power utility that provides 120VAC at all times (don't your lights dim when your computer is turned-on?).
    6) Good luck.
    Such a circuit can be described as having a "minimal" design.
    A 2N3055 might be older than me. A newer PNP output transistor or power FET might be better.
    I haven't built this circuit and won't unless it is upgraded so that it can meet its own specs.

  9. That's right, MP. Please let me elaborate.
    The motor has coils of wire around a core, just like a relay. They are inductors. When you energize an inductor then a magnetic field is built-up around the core. When you remove the power source then the collapsing field induces a high voltage across the coil. That is the principle that some DC-DC step-up converters use.
    This high voltage is the reverse of the applied voltage and could damage a driver transistor. In Amp's circuit, the high voltage could spark and cause pitting of the relays' contacts or jump to adjacent circuitry causing damage there.
    A reverse-connected diode arrests the induced voltage.

  10. Hi Mayo,
    I hope that you do not want to transmit on the FM broadcast band of 88 to 108 MHz because the VCO in the 4046 has a typical max. frequency of only 0.8 to 2.4MHz (page 7 on data sheet).
    A broadcast band FM transmitter needs an 88 to 108MHz VCO which feeds a very high frequency digital divider. If you want to transmit on 100MHz then divide that down to 1MHz to feed one side of a phase comparator (you can use the 4046 phase comparator) and the reference frequency would be an accurate 1MHz crystal oscillator. Then you can frequency-modulate that oscillator a little with a varactor. The small amount of FM at the oscillator is multiplied by the divider (but also its frequency inaccuracy and drift).

  11. Amps, your revised circuit shows 12V relay coils powered by only 5V.
    Use 5V relays.
    When you turn-on the computer, it will be cold and the switch or temp.-controller will apply only 8V to 1 fan. Will it start running on this low voltage? Most fan controllers apply full voltage for a moment to get the fan started then ramp the voltage down to a lower running voltage.
    As MP stated in his 3rd reply, use anti back-EMF diodes across the fans.

  12. MP, you are absolutely correct. Some of those monster sized and heavy weight car amps produce very much power by drawing up to 100A from the battery, stepping-up the voltage and using high-power circuits with many power transistors. But others cheat the numbers by stating a non-standard 1 or 2 ohm load. They attempt to give tons of power into a dead short but much less into a standard 4 ohm speaker. Those amps are for the boom-boom cars with the trunks (bonnets?) full of woofers.
    My post referred to the factory-replacement receivers that are all rated at between 45 to 50 Peak Watts.
    Soon some day most amps will be compact class-D (switching) circuits, for those of us with tin ears.

  13. Thanks, Sec, for that excellent link. I've seen the site but not that article before.
    Maybe the advertisers should spell Watt as What.
    There is another link somewhere that examines the cheap junk that is built into a well-known and expensive home surround-sound sub-woofer and amps with satellite speakers system. Designed by a doctor (with a PHD or is he a Veterinaryist).
    You don't always get what you pay for.

  14. MP, to boost the current output of a regulator, a power PNP transistor and input sense resistor is recommended to be wired across the regulator, as shown at fig. 10 in the datasheet of the MC78XX series by ON Semi here:


    This way, their superb load regulation (the output drops only 1.3mV typ. with output changes of 5mA to 1.5A) is kept.
    If you parallel regulators as you mentioned with 0.22 ohm resistors on their outputs, the output will drop up to 330mV.

    You are correct that transistors can be paralleled this way and are equalized with minimal consequence.

  15. Hello all,
    Looking for a car receiver or amplifier and saw 200W? Well it has 4 channels so each one delivers 50W, right? Maybe, but what kind of watts?
    If you measure a car battery to be 14.15V when charging, then use it to power an amp that incorporates the very latest low-saturation FETs, then the power output is 14.15 squared divided by the 4 ohm load = 50W peak-to-peak. But REAL power (light bulbs, toasters and name-brand home amplifiers) is measured using RMS voltage, not peak-to-peak. So the RMS power per channel could be as low as 14.15V peak-to-peak divided by 2 = 7.075V peak, converted to RMS by dividing by 1.414 = 5.0V, squared, then divided by 4 = 6.25W RMS per channel.
    The manufacturer does use a good method to boost the power by using 2 amps per channel connected as a bridge which effectively doubles the RMS voltage. So each channel delivers 5V X 2= 10V squared, then divided by 4 = 25W RMS or 50W peak, as the label says.
    In the olden days, a store catalog said:" Our 240W stereo console actually provides 6W RMS continuous power per channel, with both channels operating, from 20 to 20,000 cycles per second, with less than 1% distortion, and into 8 ohms" But their marketing people saw: instantaneous (before the power supply sagged) peak-to-peak voltage (phoney double-double) with only 1 channel operating (so that the power supply is extra high), at only 1K cycle (since the amplifier could not provide much power at 20 or 20,000 Hz), with a horrible amount of severe distortion (but extra numbers), and into a very low resistance (lots of current, before it blew-up).
    And I see for sale a tiny stereo amp, made in a country that I never heard of, that has printed on it "1000 Watts". But since it has only a 1/4A fuse, I know that 1/4A X 14.15V = 3.5W input, X 0.7 amp efficiency = 2.45W for both channels. So they are using the store's marketing numbers-crunching AND they left-out the decimal point(100.0). Maybe that's why the small print says to use only 32 ohm headphones, not speakers.

  16. Hi Firefly, a heatsink is used to carry heat away from a component conducting the heat to the surrounding air, since the heatsink has a large surface. Heat is caused by the power that the component is dissipating (power = the current through it X the voltage across it).
    The 2N3055 can have up to 3A through it. If the output voltage is set low (say 1V) then the voltage across it is the difference between the rectified input voltage of about 32V and the output voltage which could be 1V, therefore in this case the voltage across it is 31V. The power dissipated is 3 X 31 = 93W. So it has a big heatsink.
    The 2N2219 would pass up to 0.15A and in this case have 30V across it so it would dissipate 0.15 X 30 = 4.5W. So it has a little heatsink.
    The rectifier diodes also pass up to 3A but have only 1V across them when conducting. They each conduct 1/2 the time so they dissipate only 3 X 1 X 1/2 = 1.5W. In a bridge 2 diodes conduct simultaneously so it dissipates 3W. For its amount of surface, that would be quite warm but not too hot.
    When building or designing you must consider power since even resistors must have their power calculated.

  17. Hello Tom and others,
    You CANNOT put stabilized power supplies in parallel since you cannot ensure that they have EXACTLY the same output voltage. The supply with the slightly higher voltage will carry the whole load while the lower voltage one(s) will tell its regulator that the voltage is high enough or is too high and therefore to shut-down.

    Although you CAN isolate parallelled supplies with a series resistor from each one's output to the load, so that the supply with the slightly higher voltage carries enough current to allow its voltage at the output of its series resistor to drop to the level of the slightly lower voltage one(s) so that this one (these ones) carry the remaining current. But those series resistors ruin the voltage stabilization since the output voltage will fluctuate with load current changes.

  18. Do not use a 2N2222 instead of a 2N2219 for Q2 since it must dissipate up to 5W and therefore is shown with a heatsink.
    The 2N2219 has a larger metal case and is designed for use with a heatsink. The 2N2222 has a small plastic case and cannot dissipate more than about 1/3W. With 5W in a 2N2222 it will smoke then burn.
    Although the two transistors have the same voltage and current ratings, the little 2N2222 is designed to SWITCH current (when switched-on it has very little voltage across it and therefore low power).

  19. Most general-purpose or audio voltage-feedback opamps have a low frequency gain of 100,000 or more and then start a 20 dB/decade roll-off at 10 Hz to 100 Hz. They have the built-in roll-off so that at a high frequency such as 1 MHz or more, where the op-amp circuit will have close to 180 degees of phase-shift, then the roll-off causes the gain to be less than one.
    When you add negative feedback then the op-amp's phase-shift will cause that feedback to become positive at a high frequency but since the roll-off reduces the gain to less than one then the circuit will not oscillate.
    Negative feedback is used:
    1) To reduce the gain to a useful number and to make that gain very accurate.
    2) To extend the bandwidth depending on gain (higher bandwidth with less gain).
    3) To reduce distortion depending on gain (less distortion with less gain).

  20. MP, let's hope that Rich can get the circuit working.
    Don't you count 4 equal RC networks on your filter? It looks like two 2-pole filters cascaded, but the first 2-pole filter isn't quite a multiple-feedback type since the feedback resistor doen't connect to the junction of the 1st resistor and cap, but instead is connected as a simple integrator.

  21. MP: Isn't your circuit a 4-pole filter?
    Rich: Russ's classic 3-pole filter will filter the bass very well but will not have enough gain for a microphone unless it is placed right up against a very-loud speaker. But don't simply increase the gain of the 1st stage becuse it will overload during loud vocals.
    Add a variable-gain stage to the output of Russ's circuit which will be the same as his 1st stage except replace the 100K feedback resistor with a 220K volume control in series with a 10K resistor. Couple into the new stage with a 10 microfarad cap.
    Of course you must cap couple your mic to the input, and bias an electret mic. This circuit probably won't work with an xtal mic.
    You must also cap couple (10 microfarads) the variable-gain stage to your bulb-driver transistor and use a 10K resistor in series with this. Put a reversed diode across the base and emitter to allow the cap to charge and discharge equally.
    Use a 1000 microfarad cap from Vcc to ground.
    Now your bulb will flash (not very loud since it receives only 1/2 wave rectification) to the bass.

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