TL072 - Microphone preamp (differential) - Page 1
Forum section: electronics / beginners

author: Ale
author_rank: Contributor
timestamp: December 05, 2017, 08:36:17 pm
content: Hello again,after the guitar amp this time I'm working on the microphone amplifier. I tried many schematics I found on the net, and the result is this attached. I'm using a dynamic mic, a shure SM57. My goal is to correclty transform the signal from balanced to unbalanced, pass it through some guitar effect pedal and back in balanced with a DI (not part of this project for now).1-I noted that without R7 and R11 the circuit won't work.2-Without C6 there is alot of noise, almost no noise with.3-RV2 is to control the output volume, I would like to use before of the amp stage like I did in the guitar preamp, but I don't know why since I have 2 input line here.What do you think?thank youAle

author: Zero999
author_rank: Super Contributor
timestamp: December 05, 2017, 11:28:50 pm
content: 1) Of course, the input needs to be biased to half the supply rail, for it to work.2) Yes, a capacitor is required for good supply rejection.3) Add a buffer amplifier to reduce the output impedance.To make it a truly differential amplifier,.there should be another 100k resistor between pin 5 and wheew R7, R11 and C6 join.It's also a good idea to AC couple the microphone to the amplifier, rather than connect it directly, although you'll get away with DC coupling because you're using a J-FET input op-amp.

author: danadak
author_rank: Super Contributor
timestamp: December 05, 2017, 11:32:29 pm
content: This datasheet a good ref for how to build a single OpAmp diff amp -http://www.ti.com/lit/ds/symlink/ina152.pdfhttp://www.cs.mun.ca/~paul/cs4723/material/single_supply_opamp.pdfRegards, Dana.

author: Ale
author_rank: Contributor
timestamp: December 06, 2017, 06:13:35 pm
content: Wonderful, I read the Dana pdfs and I made some modifications. It works better than before. Anyway I think that with 100k is a little too much amplification and perhaps there is a little distortion when I sing loud, but I don't have an oscilloscope for now so I can't check (it will be one of my next purchases).As far as I think there is no way to place a potentiometer between the XLR and the op-amp, so is it a bad idea to use 10k on R10 and R12 (or to use a unity gain with R8 and R9 also 10k), but place another stage after this?

author: Audioguru
author_rank: Super Contributor
timestamp: December 07, 2017, 01:39:35 am
content: Your last circuit is gone during my reply so I must go back and copy it then reply again.

author: StillTrying
author_rank: Super Contributor
timestamp: December 07, 2017, 01:48:56 am
content: The last diagram needs a ~ 470nF on the wiper of the pot so that the pot doesn't short the +Ve input's bias toGND.Quote from: Audioguru on December 07, 2017, 01:39:35 amWhen replying, I right-click anywhere over aRe: TL072 - Microphone preamp (differential)and open it in another tab so that I can see the images in the non-replying tab.

author: Audioguru
author_rank: Super Contributor
timestamp: December 07, 2017, 01:52:27 am
content: Your last circuit has a few problems:1) Its differential input impedances are not balanced: Pin 5 is 11k ohms and pin 6 is 1k.2) The 0.1uF input capacitor feeding the 1k of R9 cuts frequencies below 1600Hz.3) The pot has the DC bias current in it so it will produce scratchy noises when turned and will mess up the DC bias of the 2nd opamp. Add a coupling capacitor between the pot slider and pin 10.4) The bias of the 2nd opamp feeds half the power supply noise to the opamp input. Add a filter capacitor on the voltage divider then add a resistor from it to pin 10.Why did you add the second opamp?

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2017, 11:29:55 pm
content: The additional op-amp is required to buffer the pot, to give a low output impedance.The circuit can be simplified somewhat and will work better too.

author: Audioguru
author_rank: Super Contributor
timestamp: December 08, 2017, 01:09:39 am
content: On the latest schematic, to make the inputs perfectly balanced, R8 should be replaced with wire and R12 should be 1k.

author: vltr
author_rank: Contributor
timestamp: December 08, 2017, 04:35:40 am
content: I did this project a few months ago, but I cheated and used the instrumentation amplifier on page 48 from this book:https://books.google.com/books?id=jeKejy1Bb-MC&pg=PA29&lpg=PA29&dq=tl074+amplifier+circuit&source=bl&ots=yU-c-TEvt8&sig=VAgpv12CvwQFyzns37wbRB07RSQ&hl=en&sa=X&ved=0ahUKEwjdppyAldrTAhVEQiYKHSl5CZYQ6AEIZDAN#v=onepage&q&f=falseIt uses 3 op amps so I used a TL074.

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2017, 10:39:40 am
content: Quote from: Audioguru on December 08, 2017, 01:09:39 amNo, that would not do. Look at a schematic of a DC coupled differential amplifier.On the previous schematic R8 is equivalent to R2 and R12 is equivalent to Rgon the circuit below. For optimal common mode rejection, Rg= Rfand R1 = R2.

author: danadak
author_rank: Super Contributor
timestamp: December 08, 2017, 10:42:32 am
content: Here is a scope and sig gen and spectrum analyzer (free) that uses sound card in your PC -https://www.zeitnitz.eu/scope_enSo you can stim your design with a tone and look at its distortion/spectral response. Noteit cannot do DC measurements however. It also will allow you to do common mode analysis,eg. performance of the diff amp and matching Rs on CM response.Protect your sound card inputs against damage (make sure you do this) -http://www.pmillett.com/ATEST.htmhttp://www.daqarta.com/dw_0all.htmhttp://www.electro-tech-online.com/threads/soundcard-oscilloscope-protection.92122/And some more sound card instruments (note the RLC meter link) -http://www.radio.imradioha.org/PC_Based_Test_Gear.htmRegards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2017, 10:12:09 pm
content: Awhile ago, audioguru did schematic showing the inverting and non-inverting op-amp configurations, both DC and AC coupled. Here's my version, showing the differential configuration too. The recommended resistor values are selected to ensure the DC impedance seen by the inputs are virtually equal and is important for a bipolar op-amp, with high bias currents. In this case a J-FET op-amp is used, so it's less critical the DC input impedances match.

author: Ale
author_rank: Contributor
timestamp: December 09, 2017, 06:57:29 am
content: On Hero999 post I should set the resistors like this, because I'm in differential mode, single supply, right? so it's balanced.

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2017, 02:22:48 pm
content: Quote from: Ale on December 09, 2017, 06:57:29 amThose resistor values are fine, as long as you're happy with the gain being 100, rather than 10, as it was with the previous schematic. As I said previously, you're using a J-FET op-amp, so it's less critical to balance the DC input impedance.Some of the AC coupling capacitor value are way off. C = 1/(2pi×FR).Where R is the value of the resistor, F is the lower cut-off frequency, which can be 20Hz for audio and C is the capacitor value.For example, to calculate the value of C4:R = 100k = 100×103F = 20C = 1/(2pi×FR) = 1/(6.28×FR) = 1/(6.28×100×103×20) = 1/(6.28×2000×103) = 1/(12560×103) = 79.6×10-9F100nF is the nearest value up. I also chose the incorrect value, on the previous schematic. I probably got the decimal point wrong.

author: nfmax
author_rank: Super Contributor
timestamp: December 09, 2017, 02:28:51 pm
content: A long time ago, a friend & I designed an opamp-based balanced-input microphone amplifier for professional broadcast radio use. It ran on a single 9V radio battery. It used an LT1028 for low noise and a MAX403 to boost the common-mode rejection with a minimal increase in battery drain. It was published in Electronics World & Wireless World (UK), October 1982 pp 811-816. I can't find a copy on line, but I've attached an image of the circuit diagram.Have fun figuring out how it works!

author: Ale
author_rank: Contributor
timestamp: December 09, 2017, 03:43:11 pm
content: Well, I turn back to 10 of gain.For studio audio is ok 20hz, but for live session in quite common to use the "low cut" button on the mixer which cuts the frequencies on 80hz, with the formula:C = 1/(6.28 x 100000 x 80) = 1/50240000 = 1.99 x 10-8= 20nFI hope the math are right

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2017, 05:39:14 pm
content: Quote from: Ale on December 09, 2017, 03:43:11 pmThat looks right to me. You may find it easier to get a 22nF capacitor, as it's a standard E6 value.What's the microphone impedance? As drawn, the input impedance of the amplifier is 2k. The microphone should have a much lower impedance than that.

author: Ale
author_rank: Contributor
timestamp: December 09, 2017, 05:57:29 pm
content: Wonderful!I'm using the shure sm57 by the way:http://cdn.shure.com/specification_sheet/upload/81/sm57-specification-sheet-english.pdf. The impedance is 150ohm to 310ohm which is quite lower than 2k, is that enough?I post the updated schema with 22nF capacitor.

author: Audioguru
author_rank: Super Contributor
timestamp: December 10, 2017, 12:29:35 am
content: Since the input impedance of the (-) opamp input is R9 (1k) but the input impedance at the (+) opamp input is R8 + R12= 11k and the coupling capacitors pass different low frequency levels then isn't the microphone not properly balanced like a transformer or instrumentation amplifier would do?EDIT: Why are the values of R7 and R11 so low that they draw more current than the entire circuit and get almost smoking hot? They bias the Jfet inputs of the opamps with almost no current.

author: Ale
author_rank: Contributor
timestamp: December 10, 2017, 06:35:22 am
content: Sorry Audioguru, I'm trying to follow the schema posted by Hero999. I'm in single supply differential mode so R7 and R11 should be 1/10 of R12 and R10. Moveover R8 and R9 also should be 1/10 of R12 and R10 to get 10 of gain amplification, what am I missing?I raised the R7 R11 to 1M and the others to respect the schema balance (I think), I also changed the C8 C9 to cut under ~80Hz.thank you

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2017, 10:28:04 pm
content: Quote from: Audioguru on December 10, 2017, 12:29:35 amNo.The DC input impedances seen at the inputs are: 10k5 at the +input and 10k at the -input, a mismatch of only 5%, which won't matter much, especially as this is a JFET input op-amp, with tiny bias currents.C6 (it should be bigger than 10µF) bypasses R11 to 0V, at AC, so the AC impedance of the mic. inputs should fairly closely match, giving roughtly 1k.Sorry, for my snappy response. I see what you mean, after reading this post, years later, after searing for the attachment to use. Someone might refer back to this thread, so I through I'd edit it. The inputs will have different cut-off frequencies, when they're left floating. As you correctly stated, there's 11k on the +input and 1k on the -input, but that's no longer an issue, with the microphone connected. If the circuit is designed properly, the microphone will have a much lower impedance than the input resistors, thus connecting them together at a relatively low impedance, so any differences in cut-off frequency won't matter.QuoteYes, they should be higher, but the power dissipation is only 288mW, hardly smoking hot, unless tiny resistors are used.Quote from: Ale on December 10, 2017, 06:35:22 amThe schematic I posted shows best general practise. It's good to follow it but is far more important to understand why, then you can know when it's important and when it doesn't apply.The reason for recommending that the DC impedances seen by the op-amp inputs is equal, is because the voltages generated by the bias currents will equal one another and get cancelled out by the op-amp. This is the reason for the1/10RFrule of thumb: it ensures the DC impedance mismatch is no more than 5%. The question is, whether the bias currents are high enough to cause a significant offset? In the case of a bipolar op-amp, such as the NE5532, the input bias currents are typically 200nA, so with an amplifier with gain of 100 and a 600k input impedance at one input and only 100k at the other input, a difference of 500k, as in thesecond circuit, the input offset voltage will be 200×10-9×500×103= 0.1V. This will be multiplied by the gain of the amplifier, 100 in this case, 0.1×100 = 10V on the output which will cause it to saturate, near one of the power rails.http://www.ti.com/lit/ds/symlink/ne5532.pdfNow look at the above situation, but using a J-FET input, such as the TL072. The input bias currents are only 65pA. If the above calculation is repeated 65×10-12×500×103×100 = 3.25mV, which won't make any difference, especially as the output has a capacitor in series with it, which blocks the DC voltage.Dave made a video about this awhile ago. Please make time to watch it, as it explains everything.EDIT:I've just realised, this isn't as bad as I first thought, even with a bipolar op-amp, the DC gain of the circuit is only 1, so the offset voltages will be1/100of the previously calculated values, which shouldn't be an issue.Quote from: Ale on December 10, 2017, 06:35:22 amShould be fine. You need to decide what gain you need. This circuit has a gain of 10. Some of the other circuits had a gain of 100.

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 02:00:28 pm
content: If I need to connect an electet microhone, is it correct to add the phantom power between the xlr line 2 and C8 like this?

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 02:14:30 pm
content: Quote from: Ale on December 12, 2017, 02:00:28 pmNo, that's not right at all.Is it an electrect mic or condenser mic? There's a big difference between the two. An electrect mic doesn't need 48V. It only needs 1.5V to 5V, a 1k to 10k resistor and coupling capacitor.

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 02:15:19 pm
content: I meant condenser sorry :p

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 04:26:00 pm
content: You forgot the resistors.

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 04:30:05 pm
content: How to chose the values? Why to use another resistors between C8 and C9 and the mic?

author: Bassman59
author_rank: Super Contributor
timestamp: December 13, 2017, 04:35:12 am
content: Quote from: Ale on December 12, 2017, 02:15:19 pmStandard phantom power requires the +48 V be applied to both pin 2 and pin 3, each through 6.81k resistors.

author: Ale
author_rank: Contributor
timestamp: December 13, 2017, 09:25:54 am
content: Yes, I found a schema online, it should be like this. I have a question, is it possible to put another switch on the xlr pin 1 to implement the ground lift? I can't find an active schematic with ground lift, only with passive transformers.thanks

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2017, 09:29:19 am
content: Quote from: Ale on December 13, 2017, 09:25:54 amNo, because adding a switch, in series with pin 1 would break the circuit for the phantom power.The only other way to float the mic, is to use an isolated 48V phantom power supply, such as a battery, on the mic side of the ground lift switch.https://en.wikipedia.org/wiki/Phantom_power#Caveats

author: Ale
author_rank: Contributor
timestamp: December 14, 2017, 09:52:28 am
content: I found this schema on the net:http://www.poulpetersen.dk/xfiles/cir/gbhpdi.html, could it work? Here the ground lift is on the input side, in my case can I add it on the mic output? So I will still have the phantom power connected to gnd but disconnected from the next stage. I saw many other schematic with not only a switch, but with condenser and resistor, like this, but usually in passive DI and I don't really understand how it works. I mean... condenser and resistor didn't conduct enough current so it's like an open connection?

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2017, 11:33:40 am
content: I've edited my post to clarify the fact the isolated phantom supply, needs to be on the mic side.Those DI circuits are not designed for condenser mics.It a series resistor were connected between the 0V and mic, on a normal phantom power circuit, it would only lift the ground by around 20mV.

author: Ale
author_rank: Contributor
timestamp: December 14, 2017, 12:56:00 pm
content: What do you think of this version? the xlr pin 1 is disconnected from GND, but connected to the battery. Is it possible to do the same using a second power supply, used only for the phantom and "ground lift" (perhaps ground separation)?

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2017, 02:36:11 pm
content: Quote from: Ale on December 14, 2017, 12:56:00 pmLooks sensible to me.

author: mansaxel
author_rank: Super Contributor
timestamp: February 17, 2018, 12:40:00 pm
content: One question, why do you think you need to lift ground?In my experience (I spent the better part of the 90s in pro sound fighting the audible and sometimes unaudible artifacts of ground loops) ground lifts are the wrong solution to a problem barely understood.The reasons hum occur are several, but the most common are:* either a strong magnetic field induces hum (rare),* mains-related current flows along signal wires between mains connected pieces of equipment.In the first case, screening will help. As long as the screening enclosing the circuit is given a path to discharge the induced current; because the current is still induced, only this time in the screen, where it supposedly won't harm anything. Here, a ground lift will subvert the screening, at least at audible frequencies. Like hum.In the second case, the problem is that the signal path between the two pieces of equipment is (in comparison) the best path for equalizing the potential difference occurring from the fact that mains ground is not at exactly the same potential at all points in a local mains grid. Removing a non-audio-conductor from this set of wires (by means of said ground lift switch) will not exactly improve the situation, unless it is combined with a galvanic separation through optics or more commonly a transformer. This is the passive DI solution.Since you are proposing a microphone input, exactly how do you envision a mains connection to a SM57? Because, if you do not, the ground lift switch will only sabotage your P48 feed (I'd make a silly argument about SM57 and phantom, but I'll refrain, since you obviously think about using other microphones too) and remove the EMI screening.My advice would be:1. Connect Pin 1 of the female input XLR by means of a _very_ short and stout jumper directly to the metal chassis. DoNOTconnect it to the PCB.2. Run Pins 2 & 3 to the PCB, using a twisted wire pair.3. Make certain that the chassis and the circuit 0V are well connected, but at one point only.4. The output ideally should be balanced, but if that is impossible, and the output -ve is equal to 0V, use a metal 1/4" jack, and only connect the tip to the pcb, instead deriving the -ve from the chassis connection of the 1/4" jack.5. Save the money from not installing a ground lift.

author: Ale
author_rank: Contributor
timestamp: February 19, 2018, 08:07:28 am
content: Wow, thank you very much for your advise. I made a prototype, slightly different from this. I will post the schematic in the next days. I removed the ground lift and phantom for now.