Members ashivraj Posted November 11, 2003 Members Share Posted November 11, 2003 as you've probably discovered by now, im studying "audio technology", and we're doing a bunch of fun stuff like basic electronics, and math for acoustics (dB and such). so i've got the little knowledge enough to make it a dangerous thing, but that doesnt stop me from asking questions! i have a shure BG1.0 dynamic mic that i took into the electronics lab to use for testing purposes (we're building a mic preamp - not really designing it as such, but more like translating a circuit diagram into a veroboard layout and actually soldering stuff etc). just to make sure, i checked the outputs of the mic on the oscilloscope, expecting to see pins 2 and 3 to give me similar signals but in opposite polarity. instead, i got pin 2 giving me the expected wavy line thing, but pin 3 gave me just a lower-level version of the same signal, NOT inverted. so it was basically like pin 3 was a padded version of pin 2. here's the two possibilities i've come up with so far:- could be a faulty microphone. FWIW, i pulled the bottom out and made sure the wires etc. were done up right, couldnt see any obvious fault. pin 1 was definitely ground. - is this what "impedance balanced" means? like, instead of true out of phase signals, the (-) is actually just the same as the (+) but in-line with some kind of resistance? any suggestions/ideas/explanations? i apologize for all the questions. it's just that im excited about studying something i actually have a passion for, and so i want to milk it as much as i can. thanks a lot! AS Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 12, 2003 Members Share Posted November 12, 2003 Abhi, this is a great question... which needs more than a simple answer. (Did you expect less;) ) Ok, I'll bet that you referenced your ground to pin 1 which is called "ground". The problem with this is that on most microphones (dynamic at least) this pin is totally floating and only connected to the mic housing, allowing the shield to extend from the mic cable to the mic housing and grill to shield the element. Since we are talking about a balanced signal, all that means is that both lines (pin 2 & 3) carry signal, but it is with respect to each other, not to ground. The mic element acts like a transformer (it is actually a mechanical transformer) and some mics also have a transformer inside to affect impedence adjustment. Now you will need to use either a differential amplifier (look up in your text book) or you can alternately ground one end of the "transformer" (pin 2 or 3) to the scope ground and look at the other signal carrying pin to see the ground referenced version of the balanced (floating) voltage. You should see about 10mV when talking into it. Your scope can be used as a differential amp by using chan a on pin 2 and chan b on pin 3, inverting chan b, then using the chan a+b function on the vertical input amp section on your scope. By adding the -b signal you will see a-b which is the difference between a and b which is exactly what the mic pre's input stage does. Hope this helps! Link to comment Share on other sites More sharing options...
Members ashivraj Posted November 12, 2003 Author Members Share Posted November 12, 2003 Originally posted by agedhorse Abhi, this is a great question... which needs more than a simple answer. (Did you expect less;) ) go for it. im in my knowledge-sponge mode right now, so im ready to absorb anything! Ok, I'll bet that you referenced your ground to pin 1 which is called "ground". yep, exactly like this. The problem with this is that on most microphones (dynamic at least) this pin is totally floating and only connected to the mic housing, allowing the shield to extend from the mic cable to the mic housing and grill to shield the element. yep, i sorta knew this, but confirmed it yesterday. Now you will need to use either a differential amplifier (look up in your text book) we dont have a textbook (rather, i havent bought it yet ) but i have some clue - we're using differential op-amps on the pres we're building. something like a 741 for testing, to be replaced by an xxx (cant remember what) when it's all done or you can alternately ground one end of the "transformer" (pin 2 or 3) to the scope ground and look at the other signal carrying pin to see the ground referenced version of the balanced (floating) voltage. You should see about 10mV when talking into it. yep, i tried this as well, and got the exact result you just described. though i wasnt particularly looking at the voltage level, i was too fascinated by the waveforms:D Your scope can be used as a differential amp by using chan a on pin 2 and chan b on pin 3, inverting chan b, then using the chan a+b function on the vertical input amp section on your scope. By adding the -b signal you will see a-b which is the difference between a and b which is exactly what the mic pre's input stage does. this is exactly what i did. i ran pin 2 to channel A, pin 3 to channel b, and grounded them both to pin 1 (but now you're saying this isnt actually grounding?). when i added them, i was expecting two signals of opposite polarity, but i saw what i described earlier - a small hill under a big hill, both pointing in the same direction. then i inverted ch b (pin 3) ,and then i saw a waveform in which channel a was positive (let's say a peak of 10mV) and channel b was negative (with a peak of -3mV). now im not sure if this is what is expected, especially after you brought up the floating ground, but i'll check it out next week (tuesday) in the lab. im also gonna try to get my hands on a 57/58 and see if it gives me the same result. Hope this helps! immensely! thanks for taking the time... it's awesome to be ahead of the class and to have all the answers, but little do they know my secret... HC Live Sound for LIFE!! AS Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 12, 2003 Members Share Posted November 12, 2003 The reason that you do not see a truly symmetrical signal around ground is that the ground is floating and can not be used as a reference point! Keep learning, it can only lead to better things. Link to comment Share on other sites More sharing options...
Members ashivraj Posted November 13, 2003 Author Members Share Posted November 13, 2003 Originally posted by agedhorse The reason that you do not see a truly symmetrical signal around ground is that the ground is floating and can not be used as a reference point! andy, sorry if i seem really thick and "why doesnt he get it?" but... will i see a truly symmetrical signal around ground if ground was something else? i.e. if both the ground clips on the ch.A/ch.B leads were hooked up to the ground of something else? in our pre, the audio and voltage grounds are the same. two HUGE batteries (PP9s) in series, with the wire between them being referenced as ground. will this help me any? Keep learning, it can only lead to better things. except that im learning more here than in class - because i cant wait an entire week to get to class, and by then i've already got the answers from here! thanks a lot... AS Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 13, 2003 Members Share Posted November 13, 2003 But the mic coil is not referenced to your ground, it would need to be a center tapped coil, which you never do because of phantom power problems. Link to comment Share on other sites More sharing options...
Members ashivraj Posted November 13, 2003 Author Members Share Posted November 13, 2003 right. makes a bit more sense to me now. anyway, it had the professor stumped as well, so now i'll go back and talk about center-tapped transformers and such. ooh, that will be a kick! thanks a LOT! AS Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 14, 2003 Members Share Posted November 14, 2003 You couls also center-tap and thus ground-reference a mic diaphrap's voice coil too, in the same way. Link to comment Share on other sites More sharing options...
Members RickJ Posted November 14, 2003 Members Share Posted November 14, 2003 If you're building a simple preamp, won't you be biasing it with DC voltage? The AC waveform then "rides" on that voltage. If the DC voltage is 12, ground is at 0 volts DC. But the zero reference for the AC signal is 6 volts DC. One half of the wave is above 6 volts; the other is below. There is no actual ground for the AC signal. It's been well over a decade since I've messed with op amps, but I recall using DC-blocking caps at the output to strip the DC bias voltage off of the AC signal. Correct? Disclaimer: I don't build 'em. I just use 'em. (Which is no doubt a very good thing!)-- RJ Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 14, 2003 Members Share Posted November 14, 2003 Originally posted by RickJ If you're building a simple preamp, won't you be biasing it with DC voltage? The AC waveform then "rides" on that voltage. If the DC voltage is 12, ground is at 0 volts DC. But the zero reference for the AC signal is 6 volts DC. One half of the wave is above 6 volts; the other is below. There is no actual ground for the AC signal.It's been well over a decade since I've messed with op amps, but I recall using DC-blocking caps at the output to strip the DC bias voltage off of the AC signal. Correct?Disclaimer: I don't build 'em. I just use 'em. (Which is no doubt a very good thing!)-- RJ Depends on if you are using a bi-polar power supply or a single ended supply. Single ended required biasing the op-amp but not the mic. Link to comment Share on other sites More sharing options...
Members ashivraj Posted November 17, 2003 Author Members Share Posted November 17, 2003 two 9V batteries (let's call them A and B) connected such:A+ and B- are voltage supply to the board; A- and B+ are connected to each other, with that being used as the grounding point as well. does this mean they're just connected serially, and so supply+ is 18V, supply- is 0V, and supply "ground" is actually 9V?? there are caps on the board that connect both supply+ and supply- to ground. so the power supply to the vero-board looks something like this: (+)---|-----------------........C....................(G)---|------|-----------.................C...........(-)-----------|----------- what do these caps do? are they converting DC to AC? as for the signal output, it doesnt pass thru any caps. though i think there's some between the mic input and the input on the op-amp. i guess some of my statements are blatantly incorrect, and some of my questions are annoyingly stupid, but please bear with me... one day, i'll be really good at this, and i'll thank you guys when you're 95 and still kickin. thanks! AS Link to comment Share on other sites More sharing options...
Members RickJ Posted November 17, 2003 Members Share Posted November 17, 2003 Batt A + Batt B in series = 18vDC. By using the center rail as ground, you're essentially creating a +9/-9 bi-polar supply. The parallel-wired caps are power filters. Technically, they are removing AC remnants from the DC supply. But since you're using batteries rather than stepped-down and rectified AC power, it's not as critical in your simple circuit. Smaller value caps are also used for this purpose at the component level, usually positioned as close to the supply pins of the chips as possible. Series-wired caps are HPFs used to block the DC, which in this situation can loosely be considered to be an AC signal with a frequency of zero. At the inputs, they insure against DC entering the circuit from the previous component in the signal path. At the outputs, they protect the next component. Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 17, 2003 Members Share Posted November 17, 2003 The caps are used to stabilize the voltages on the rail that might tend to fluctuate a little with loading on the op-amp. This can be a problem at high frequencies where oscillations can occur. They are called power supply decoupling or bypass caps. Link to comment Share on other sites More sharing options...
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