Two opamps for preamp balanced out - Page 1
Forum section: electronics / projects

author: AllanP
author_rank: Newbie
timestamp: December 27, 2016, 05:11:31 am
content: HelloI am trying to construct a balanced preamp via opamps. At the moment it's for single supply, using resistors as a voltage divider to create virtual ground. I am basing it on a schematic which had the input connected after a high load. I've build half of it and it seems to work just fine using a NE5534. I don't know exactly how it works and comparing the wiring of the opamp to other examples I can't figure out if it's inverting or not. Likewise I am not certain of each capacitor or resistors function.Nevertheless I've attempted to extend the circuit with another opamp to give it balanced output. The first opamp has the input connected to the non-inverting input. The next opamp just has the input coming in on the inverted input, and the feedback network moved to the positive supply. Could this work to create a balanced output signal?The preamp will be connected to an input which has the ground pin connected to chassis/wallplug earth. I guess it's not a good idea to pass the virtual ground in the cable shield?The drawing software didn't have a microphone icon, so the speaker looking thing is the input. For clarity I've removed protection diodes and decoupling capacitors from drawing.

author: BrianHG
author_rank: Super Contributor
timestamp: December 27, 2016, 06:22:35 am
content: Now I know that some may fringe at my particular mod, but, it will produce a matched balanced output so long as the input doesn't have a common GND connection with your oscilloscope. If your source is battery powered or a microphone, this circuit will perform perfectly.NOTE: R14 should be tied to the GND, not the 9v. I fixed this in later drawings. If you are using a studio balanced MIC, tie the +and- to what you see in the schematic and tie the shield to you 0v. In the case where some studio MIC connect the shield to a center-tap on a transformer, you will need to connect a series 47u cap between your circuit's GND and the shield GND. In the case where the studio's MIC is a fake balanced signal, or, connects the MIC's - to the shield, this will mess up the performance of this circuit.Fix #2 which allows a common ground coming up in the next post...

author: BrianHG
author_rank: Super Contributor
timestamp: December 27, 2016, 06:42:56 am
content: Ok, now this circuit is 0v common GND friendly and should give you a proper pre-amp 0v centered balanced friendly output. But, because of the second inverting amp's 1k feedback resistor network, the input to this pre-amp will no longer be 400k, but just below 1k.Now, if you want output 2 to mirror op-amp #1 and keep a high input impedance, without a ridiculous high impedance gain resistors on the second opamp, you can just tie the output of op-amp #1 into a 1:-1 negative input on op-amp #2. See my schematic #3.

author: BrianHG
author_rank: Super Contributor
timestamp: December 27, 2016, 06:49:45 am
content: Now this circuit has your 400K input impedance, though the - output opamp will have a little delay, since it's gain is only -1, not -10, it will be really fast.With this circuit, the common-mode rejection of the mic is lost since it is converted to un-balanced by the first opamp. This may still be fine depending on the location and wiring/wire length from the mic. Opamp #2 will create a new balanced output to feed your next piece of balanced audio equipment.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 27, 2016, 09:58:50 am
content: This last circuit is what i've often seen, for producing an opposite-polarity signal, in addition to the "normal" polarity one.That being said, you guysAREaware that the whole point of balanced-line connections, at least in the audio world, is to provide an IMPEDANCE-balanced line, regardless of whether there is any signal on the "cold" pin (pin 3, in case of XLR connections), right?I've seen small-diaphragm condensers, audio interfaces and (relatively small-format) mixers use "only" impedance-balanced outputs, with no problems at all.http://sound.whsites.net/articles/balanced-2.htm

author: AllanP
author_rank: Newbie
timestamp: December 27, 2016, 04:09:34 pm
content: Thanks a lot for the input, it is much appreciated.Quote from: BrianHG on December 27, 2016, 06:22:35 amIf output is moving around vMid, in between v+ and v-, why does it matter if R14 is connected to either pole?Mic input is unbalanced so nothing is lost. You did model it as working as a capacitor, which I forgot. Supposedly this is why the high input impedance is needed.Quote from: BrianHG on December 27, 2016, 06:49:45 amI initially thought of connecting the mic directly to both opamps as I thought any noise introduced by either would end up being cut in half in a balanced setup. The mic also picks up mains noise sometimes. But as first half of the circuit is working it might be a good idea to keep that and have a second opamp inverting the output at unity. Any noise introduced with the opamp might be the least of this circuits problems. I have included an updated schematic which seems similar to what you suggest.Quote from: KhronX on December 27, 2016, 09:58:50 amThanks for the link and info, I do not understand all of it fully. But I think you are suggesting the second opamp, to invert the output, might not help noise reduction at all? The mic is prone to pick up mains hum though.I am using an unbalanced input with the preamp, which in turn should connect to a balanced audio interface. How about connecting the audio interface positive to the first opamp output, the negative to v- and shield to any possible casing?

author: KhronX
author_rank: Frequent Contributor
timestamp: December 27, 2016, 04:16:05 pm
content: What exact microphone are you building this for? And what are you planning to connect it to?

author: AllanP
author_rank: Newbie
timestamp: December 27, 2016, 04:22:13 pm
content: Quote from: KhronX on December 27, 2016, 04:16:05 pmHomemade piezo to a Tascam US-2x2 (Using Line-In I guess). IIRC measuing the capitance of the the mic it showed 35nf.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 27, 2016, 04:30:34 pm
content: http://sound.whsites.net/articles/high-z.html

author: Audioguru
author_rank: Super Contributor
timestamp: December 27, 2016, 06:34:09 pm
content: I nearly choked when I saw the boomy undamped 8 ohm speaker used as a microphone in your schematic.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2016, 08:37:29 pm
content: Quote from: Audioguru on December 27, 2016, 06:34:09 pmQuote from: AllanP on December 27, 2016, 05:11:31 amQuote from: AllanP on December 27, 2016, 05:11:31 amThe NE5534 might be very noisy with such a high impedance input.Does it need a differential input too?I'd suggest the TL074, which has a high input impedance and low bias current.

author: AllanP
author_rank: Newbie
timestamp: December 28, 2016, 01:16:16 am
content: Quote from: Hero999 on December 27, 2016, 08:37:29 pmThe schematic you posted seems like what I've tried to create in the first place. Both have differential input to my knowledge, which seems ideal if using a piezo in an un-shielded cable. I have trouble understanding why the output from U1B and U1C wouldn't be enough to act as a balanced output. Then again I am not sure what the initial gain is. Does U1B and U1C just duplicate the voltage swing of the input, but at a lower impedance/with higher current possibility?I am using a NE5534 because of Richard M's post athttp://www.megalithia.com/sounds/tech/piezo/opamp.htmlwhich is also the source of the only thing I have working so far.:QuoteHe mentioned trying with a stereo NE5532 aswell, but found the NE5534 less noisy. I have no preference TL07x or something else, NE5534's just is what I have at hand, would they preform worse than a TL07x is the configuration you posted?

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2016, 11:03:18 am
content: Quote from: AllanP on December 28, 2016, 01:16:16 amThe circuit you originality posted, didn't work because the inverting & non-inverting connections on the second op-amp were reversed. Even if if it was corrected it still wouldn't have been a differential amplifier because any change in both the input voltages would pass through to the output voltages.Another thing to note is that, because the input impedances aren't balanced, a BJT op-amp such as the NE5532 won't sit at half the supply voltage because the input bias currents will influence the voltages on the biasing resistor networks.Yes, U1B and U1C in my circuit are just unity gain buffers with a 1M input impedance. They drive the 10k input impedance of the differential amplifier (U1A).QuoteWhether the TL07x or NE553x is better depends on the source impedance. The NE553x has the lower noise voltage but it has much higher bias currents, therefore more current noise than the TL07x. The current leaking into the input transistors of the NE553x gets converted to a noise voltage V = IR, which dominates at high source impedances. The TL07x has very low bias currents and negligible current noise but higher voltage noise than the NE553x. With a low input impedance the NE553x will be less noisy than the TL07x but with a high source impedance, the current noise will exceed the voltage noise of the TL07x. The TL07x is a better choice at input impedances greater than around 40k.http://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdfhttp://www.ti.com/lit/ds/symlink/ne5534.pdfhttp://www.ti.com/lit/ds/symlink/tl072a.pdfLooking at the link you posted:QuoteGiven this, it's not surprising the NE5534 was less noisy, than the TL072 in his experiments. Whether this will be true in the field, depends on whether he modelled the piezo mic correctly or not.Applying the above to the circuit I previously posted. For minimum noise, U1B and U1C should remain the same (JFET op-amps like the TL072 are better for 1M high impedance amplifiers) U1 and U1A should be the NE5532 and R9 & R10 could be reduced to 10k (BJT.op-amps like the NE5532 are better for low impedance amplifiers).