Mixer - Schematic  - Page 1
Forum section: electronics / projects

author: Jbliss
author_rank: Regular Contributor
timestamp: December 16, 2018, 11:59:40 am
content: Hi All,I have designed a keyboard mixer for a friend, just looking for another pair of eyes to look over my design.Mixer has 3 Stereo inputs with an active summing stage followed by a very basic EQ (treble and bass) which feeds a balanced line driverreally appreciate any feedback(EQ has a bypass switch, unsure if I have implemented it the best way?)CheersJBliss

author: technix
author_rank: Super Contributor
timestamp: December 16, 2018, 03:28:48 pm
content: Quote from: Jbliss on December 16, 2018, 11:59:40 amI would suggest bypassing EQ on the output side, this way you don't short the input of the EQ to its output, creating an oscillation hazard.

author: Jbliss
author_rank: Regular Contributor
timestamp: December 16, 2018, 09:39:00 pm
content: Great idea! Shall do

author: Jbliss
author_rank: Regular Contributor
timestamp: December 17, 2018, 12:23:08 pm
content: Hi All, I have moved further into the design process,Now looking at possibly having the +/- 15v linear power supply on the same PCB as the Mixer? would this be a bad idea.The Mixer will be going in to a sealed aluminium project box with 15v-0-15v toroidal transformer (20VA). I am worried that heat may become aproblem in the sealed box. I can mount the linear regulators to the project box as a heat sink ?Thoughts?ThanksJBliss

author: technix
author_rank: Super Contributor
timestamp: December 17, 2018, 02:43:43 pm
content: Quote from: Jbliss on December 17, 2018, 12:23:08 pmYou can have the linear power supply inside the box, and mounting the power components to the metal chassis is a common way of cooling them, be it linear regulators or output power transistors. However you will need some electrically insulating thermal interface material though, as you will not want to have a short in the circuit.Alternatively you can just use a switch-mode supply for the voltage rails. They can be built to produce low ripple outputs with little heat output, and if you are still worried about the ripple you can add a stage of LC filter to further smooth the rails out. Put the switch mode supply in a metal can of its own or use an external switch mode power brick if you are worried about the EMI.

author: nick_d
author_rank: Regular Contributor
timestamp: December 17, 2018, 10:59:47 pm
content: I would put a low value capacitor in parallel with R3 and R23, or at least provide space for it on the board even if you don't intend to fit it. Altough the op-amp should be unity-gain stable, there is no sense in allowing frequencies through that you don't intend to use, since stray 30 kHz (say) interference could be superimposed on the input, and while it would be inaudible on the output it could cause weird clipping problems etc. I suggest that the input stage, and possibly also the internal and output stages, be band-limited to say 20 Hz..20 kHz just to be on the safe side. If you are 3dB down at these frequencies it really will not be noticeable and is much safer. If the 3dB is unacceptable then perhaps increase 20 kHz to 50 kHz just to give some extra margin.There are also some other things I would do differently, although in a technical sense they wouldn't affect much:1. I would buffer the inputs before changing the volume.2. I would do step 1 with an inverting configuration and virtual Earth like you have, and put the band limiting here.3. I would consider putting the volume control in the feedback loop of the input buffer, although since this could interfere with the 20 kHz low pass cutoff I might just put it how you have it (voltage divider from output to ground).4. I would use single-supply 5V peration with a 2.5V ref. A voltage divider 100k/100k with bypass can generate the ref and be connected to the positive inputs of all inverting op-amps. I would set up my 20 Hz high-pass input filter to lift up the DC level of the input and sit it on the 2.5V virtual Earth.4. I would have 4 inverting input buffer amplifiers (one for each channel of each input) and 2 inverting summing amplifiers (one for each channel of the outpit). These latter COULD be a different type with higher current sinking.5. I would not bother with the output drivers, it depends on the application but I feel that a standard op-amp sinking 30-40mA is adequate to drive a line out, which should have an impedance of at least a few k ohms and only needs to be driven at 20 kHz. So if the cable capacitance is say 1nF (a really large value) then even through a 1k resistor you can drive it at 1 MHz. So high current drive is unnecessary.6. I would not bother with a differential output. All this really does is to double your voltage swing when operating from low voltage supplies. What you do is simply put your single-ended output onto the + of the XLR and your ground onto the - and ground. This may seem like a waste of the 3 wires but it is not, the - and ground serve different purposes. If they were connected together in the cable or at the receiving device you would have a problem. If they are comnected at the output device as I suggest, you have noise immunity.7. I would carefully check specs to see if the output can sit on 2.5V DC without problems (i.e. is the receiving box guaranteed to contain a high pass filter). If so I would output the single-ended output to + and ground to the -. If not I would have 2 options. I could either output a buffered 2.5V reference to the - (bringing op-amp count to 7), or else I could drop it by 2.5V using a polar tantalum or electrolytic in series. If I did this I could set the cutoff to 20 Hz or even go lower since the polar caps are available in large values.I like your design already, just wanted to give you some food for thought and alternative viewpoints to shake things up a bit. I can draw a schematic for the above if requested to.cheers, Nick

author: Jr460
author_rank: Regular Contributor
timestamp: December 17, 2018, 11:32:13 pm
content: On concern 6.No, please keep an XLR output, properly balanced. Live situations run into ground loops a lot of times, being able to run XLR and lift the ground fixes many many problems. In fact I would put in a ground lift switch.(My side hobby is is either playing keys, or running sound for bands. You should up with just 1/4 output or unbalanced, and I'm not going to be happy. Since I don't know what you have, I'll break out a stereo direct box DI, that takes phantom power. However if your little mixer has all the right bits, just two XLR cables and I'm done. Also test the setup that if he output gets hit with phantom power it doesn't fry the driver...... known to happen.)If you can, I'd also add a Mono output, you never know if the they say, "Sorry your drummer is trying to be Neil Pert and has used some channels we have just one left for keys."Also test for high level inputs with no clipping. Low end keys might put out -10dBm, pro models +4 or higher.

author: Jbliss
author_rank: Regular Contributor
timestamp: December 18, 2018, 01:07:41 am
content: Quote from: nick_d on December 17, 2018, 10:59:47 pmHi NickThank you Enormously for your help and time! This is Great.These are all great ideas.1. I think Band filtering is a good idea with the buffered inputs and shall implement this!2. Regarding power I have ran into a few issues. I need to save space inside the case. A Large linear Supply is not going to cut it. I like the idea of a single supply rail, however I would like it higher that 5v. Thinking of using an external +24v dc Wall Wart then deriving my +/-15 or +/-12v from this. Just don't know what the best way to do this would be? There are Dual output DC-DC Converters that do this however they are expensive. The other option would be to use a virtual earth configuration as you suggested. What would you recommend ? Any other ideas?3. I would like to keep the balanced outputs as this box is going to be used in a Live situation where the receiving mixer will be quite far away from the stage.

author: Jbliss
author_rank: Regular Contributor
timestamp: December 18, 2018, 01:12:47 am
content: Quote from: Jr460 on December 17, 2018, 11:32:13 pmHey Jr460, I agree, keeping XLR outputs is a must for live work. Also thanks for reminding me, 48v Phantom !! I will definitely need to implement some Blocking capacitors on the output as the chances of phantom being accidentally sent is 100%Thanks JBliss

author: Sylvi
author_rank: Regular Contributor
timestamp: December 18, 2018, 02:06:31 am
content: HiI haven't read the whole thread but looked at the schematic.It might be a mistake to not have AC coupling between the input pots and the virtual-earth node. Either add a single cap between the junction of the input weighting resistors and the opamp input, or add individual caps for each wiper. otherwise, there might be a scratchy pot issue.I would leave the EQ stage intact to correct for the inversion of the mixing stage. You can make the EQ "bypassable" simply by adding two equal-value resistors give the stage unity-gain (inverting). The EQ network of the pots, Rs and Cs is still connected at the input and output of the circuit, but a switch is added between the opamp -in and the wiper resistors from the controls. This works very well and is not noisy if there is a high-value resistance across the switch.The schemo does not show what the output driver chip is. Going by the functional names for the outputs, I suspect that "sense" should be tied to the output side of the cap rather than to the chip-side of the cap. I assume the chip is supposed to provide a "zero z-out" this way - technique I a always suspicious of as it contradicts the purpose of the cap.

author: nick_d
author_rank: Regular Contributor
timestamp: December 18, 2018, 03:33:47 am
content: Since there are some objections in regard to the XLR output, I decided to sketch out the earlier described ideas to explain better what I meant. Again, these ideas are ONLY for discussion, I don't necessarily recommend changing to this, since OP's circuit is really good as far as I can see.I agree that 5V is too low, I checked and XLR equipment uses much higher levels, I didn't know that. I sketched it with 10V, it can be increased.These are SINGLE-SUPPLY circuits, I have never built anything with dual supplies and I have never found it necessary to do so, with modern parts. At worst you might use an extra op-amp to generate a reference, here 5V. By the way, "virtual earth" is not the correct term for the 5V reference, "virtual earth" is the -ve terminal of an op-amp in inverting configuration, because the op-amp maintains it equal to the +ve terminal.The first file shows some pieces: an input stage with 20Hz..20kHz band limiting and volume control, a buffered reference, and a "true" XLR output stage which drives the XLR connector differentially. The second file shows a more controversial approach, the "fake" XLR output stage which drives the XLR connector single-endedly, and provides a dummy -ve output. To show why the "fake" XLR output stage works, and works the same as the "true" XLR output stage, I've drawn also a sample of an input stage from another piece of equipment (a differential amplifier).I personally cannot see any reason to prefer the "true" XLR output stage over the "fake" output stage which uses one fewer op-amp, except that the "true" XLR output stage will be twice as loud. If you want to reduce the supply to the minimum possible (about 5V) then use the "true" XLR. Although, I am keen to be educated if there is something I have missed in my analysis. I have built these circuits several times, but I would by no means call myself an audio or electronics expert, and I also don't have any XLR equipment apart from a collection of dynamic microphones.As has been noted in this thread, creating an output that can be connected to an XLR microphone input with phantom power is a bit of a hassle. The output absolutely must be high-pass filtered, which is a shame because otherwise the output filter can be omitted. The output filter is bulky and expensive, due to the need for a low-value resistor for decent output drive, and corresponding high-value capacitor to get an RC time-constant of 50ms, for the 20 Hz high-pass cutoff that I have chosen (although you could get away with a higher cutoff in some applications).As a compromise, I've specified a 50uF BP electrolytic with a 1k ohm resistor. When the 1k ohm feeds the 6.8k ohm resistor in the 48V phantom power circuit, the result will be a 1 / (50u * 7.8k) = 2.6 Hz high-pass cutoff and a signal level of 6.8k / 7.8k = 0.87 what it should be. On the other hand, it could rarely be 12V phantom power with a 680 ohm resistor, giving 11.9 Hz cutoff and a signal level of only 0.40. Hmm. The 50uF is conservative, but I want the cutoff well below 20 Hz, because the other device also has a high-pass and I don't want to be 6dB down at 20 Hz.The volume control is also slightly tricky. I've specified a 1k ohm pot, because it's feeding a summing junction via a 50k ohm resistor, and I don't want the 50k ohm to load down the volume control pot. A factor of 50 is probably acceptable. You can also do interesting things here, I did the calculations a while ago and I cannot exactly recall how, but you can use the loading-down of the volume control pot to get an almost logarithmic response. If anybody wants me to re-design that circuit (I was doing it with a 50k digital pot) I will try to do so. Or, a larger value logarithmic pot can be used with a further op-amp as a buffer. The added buffer can either be a follower, or it can have the volume control in its feedback loop.The volume control requires a buffered reference for the "ground" end of the pot. So I used the buffered reference everywhere, in the first diagram. The second diagram is only an output stage, so I used a dedicated 100k/100k divider instead of a shared, buffered, reference. Because of this, and because the 50k resistors used everywhere must be exactly half of the 100k used in the reference divider, I've specified 50k not 47k.cheers, Nick

author: Jbliss
author_rank: Regular Contributor
timestamp: December 18, 2018, 11:07:40 am
content: Quote from: nick_d on December 18, 2018, 03:33:47 amHi Nick, Once again thank you! I am revising my schematic as I type this, implementing changes. Thinking of raising the low pass filter on the input to above 20KHz I know it realistically makes no audible difference would just like to keep the box transparent as possible. What comment values would raise the filter to 30KHz

author: Jbliss
author_rank: Regular Contributor
timestamp: December 18, 2018, 11:36:41 am
content: Quote from: Sylvi on December 18, 2018, 02:06:31 amHi Sylvi, Thanks for you help. Great idea regarding bypass Switch would you be able to draw a quick schematic. Would like to make sure I understood.Thanks JBliss

author: Jbliss
author_rank: Regular Contributor
timestamp: December 18, 2018, 12:29:13 pm
content: Quote from: nick_d on December 18, 2018, 03:33:47 amHi All,Here is my latest working schematic with a bunch of changes implemented Let me know what you think!This schematic does not include output filtering, forgot was late last night.(I have deiced to keep the DRV-134 (line Driver) in the output as I have a bunch lying round)

author: Jbliss
author_rank: Regular Contributor
timestamp: December 19, 2018, 07:10:27 am
content: Also Let me know what you think of the new PS circuit ?and input section.

author: Jbliss
author_rank: Regular Contributor
timestamp: December 22, 2018, 08:24:03 am
content: Finally got around to implementing all the changes and fix upsLet me know what you think !!Thanks AllJBliss

author: nick_d
author_rank: Regular Contributor
timestamp: December 22, 2018, 02:22:10 pm
content: Looking pretty sweetI have some concerns about the GNDREF. What is the part number of the amp that generates GNDREF? Op-amps generally don't like their outputs to be capacitively loaded, though it does depend on the part (a 7805 for example is an amp which doesn't mind driving a capacitive load). I would omit the capacitors on GNDREF. Perhaps you can get away with say 1nF to VEE only, if you're actually concerned about noise on the reference.Then I would make sure that GNDREF is used only internally to the circuit and no significant power is taken from it. Particularly the supply bypassing of all the op-amps currently uses a split bypass with GNDREF in the middle, change this to a single bypass from VCC to VEE.The XLR connectors should use the VEE not the GNDREF on pin 1. That pin really isn't used for anything except for getting the two amplifiers to some kind of a ballpark similar potential. The relative potential does not really matter provided it is within the voltage rating of the capacitors that are in series with the output (I would use 100V there). If the receiving equipment is single supply it will be connected to the VEE which will be exactly what you want.Check what the GND pin is used for on the output amplifiers, if the audio is referenced to this then the current connection to GNDREF is correct, otherwise it may need adjusting.Is there a particular reason to use separate single op-amps everywhere, or is it simply that you wanted to finalize the circuit before counting up packages? Looks like you can do it with 2 quad op-amps. If you're worried about noise and cross-talk, don't be, since the audio frequencies are not really that prone to noise.cheers, Nick

author: Jbliss
author_rank: Regular Contributor
timestamp: December 23, 2018, 04:06:59 am
content: Quote from: nick_d on December 22, 2018, 02:22:10 pmHi Nick_dThanks Once agin!1. So the Part generating the split rails is BUF634, there is some info in its data sheet regarding is use in this configuration attached below. I was also concerned about the capacitors on the output and was going to leave the footprints there for safety.2. Will Change Bypassing of Opamps to single bypass.3. Ahhh Excellent. I was wondering about that !4. Ok so the DRV 134 I believe is just using the GND pin as a audio ref I have attached some info from the data sheet below.5. Exactly, wanted to finalise circuit and see what parts I had in stock here and what is readily available!CheersJBliss

author: Jbliss
author_rank: Regular Contributor
timestamp: December 23, 2018, 07:21:21 am
content: Updated Schematic

author: Jbliss
author_rank: Regular Contributor
timestamp: January 04, 2019, 03:54:40 am
content: back after a short break anyone had a chance to look at the latest schematic ?Cheers

author: Sylvi
author_rank: Regular Contributor
timestamp: January 04, 2019, 05:48:06 am
content: Hi JblissI looked up the data sheet for the DRV134 and see that it is a self-balancing paraphase driver - if one output is shorted the other output rises so the overall gain is always +6dB. The chip has internal output resistors, so R33-36 are not needed. neither are the sense caps since the DC levels of the outputs will be very matched by the chip itself, and any DC on the actual XLR is blocked by the coupling caps.Whoever said above that the "real XLR is twice as loud as the fake XLR" is incorrect. having twice the voltage is only a few dB difference in loudness (6dB, which is noticeable but not a factor of 2) loudness-wise).The input buffers are going to add noise even though the inverting mode is lower THD than non-inverting. It is simpler just to have a 50k leak resistor and a 1k gate-stop into the chip to have high-z input without the noise penalty of 50k series resistance. This change reduces noise by about seven times.The summing stage needs a feedback cap from output to virtual-earth node to stabilise the stage, as does the EQ.The supply splitter should have a cap across one of its input divider Rs so the opamp input is at AC ground, and thus the output will be similar.You say the 24V comes from a SMPS in which case you will need some serious filtering at the DC-IN jack: a series 10uH choke follwed by 22uF ceramic to ground in parallel with 220uF electrolytic, feeding a ferrite bead to to a second 22uF C to ground into a linear reg with 22uF C to ground, another ferrite bead feeding another 22uF C and 470uF E to ground. The linear reg could be a low-drop-out type or simply a standard type, or a discrete circuit. It seems like a lot of circuitry but SMPSs are truly horrible noisy devices and issue forth a tsunami of high-frequency noise. They are not good for supporting linear circuits directly and we've found that an interface circuit is needed to keep the SMPS happy and to provide a low, constant impedance over the audio band for the audio circuits.I'll doodle that EQ lift and post it shortly.Have fun

author: TheNewLab
author_rank: Frequent Contributor
timestamp: January 04, 2019, 08:40:30 am
content: Little note:XLR is a must for live. plus any professional equipment uses XLR. 46volts is the standard for Phantom power. (why is 75v keep bouncing around in my head, dunno)Also phono jacks are mono.stereo phono are OK for monitor output or recording to a tape player..or today a digital recorder,except even Zoom recorders use XLR jacksThe schematic otherwise seems fine.oh, always create with more inputs than musician guys requests may be a good idea

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: January 04, 2019, 09:05:39 am
content: hi joshua, i like the straight-forwardness of your design. one remark here (that maybe others have mentioned already, didn't bother to go through all the replies in detail):is your friend using instruments like, say, fender rhodes stage pianos or similar? these instruments depend sound-wise heavily on the type of instrument cable used to connect them to the amplifier, as well as the amplifier input impedance itself. a lowish (less than 500 KOhm .. 1MegOhm) input impedance make those instruments sound muffled. same holds true for non-active electric guitars. so maybe you want to consider at least one of the instrument inputs to be of the high impedance variety. that would call for a non-inverting op-amp design though, because an inverting op-amp pre-amp stage with that high input impedance (say, R3 = 1 MegOhm and thus R9 = 1 MegOhm) will be more noisy than a non-inverting design.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 07, 2019, 08:16:54 am
content: Quote from: Sylvi on January 04, 2019, 05:48:06 amHi Sylvi,Thanks for your reply.1. Regarding Drv134 I shall Implement those changes !2. So would you recommend removing the input buffer stage and implementing a 50k leak resistor and a 1k gate-stop. if you have time would you mind further explaining this.All good got it ! drafting it up !3. This is great! Excellent I shall add this ! would be great if you could have a look over it once I get around to drawing it up !ThankJBliss

author: nick_d
author_rank: Regular Contributor
timestamp: January 12, 2019, 10:52:14 am
content: QuoteThat's good, I'm not an expert on DRV134 and had recommended using an ordinary op-amp, so I didn't look at the DRV134 closely. What is the value of the built-in output resistor? Because the coupling cap might have to be adjusted to give the designed high pass threshold of 20Hz.QuoteYes, I agree, I was typing quite quickly so I didn't realize I'd referred to loudness instead of voltage. Oops.QuoteIn my view the noise added by any modern op-amp is negligible (orders of magnitude lower than what could be detected by the human ear), so the choice of how many stages to have is governed by convenience rather than noise considerations.The reason he's got the inverting input buffer stage is to provide a low impedance to the volume control stage. Since the volume control is a pot to GNDREF (I recommended a 1k log pot, not sure what he has now), if this was fed directly from the external input then the resistance of the externally connected source would affect the volume level.Another reason he's got the inverting buffer stage is to bandpass the external input to reasonable audio frequencies, he has something like 20Hz..30kHz at the moment. While this isn't strictly necessary, I think it is a good idea to be cautious. And the 20Hz lower limit also serves the purpose of shifting the signal up to half the supply, as it's a single-supply circuit.An alternative way would be to use a high-value pot for the volume, fed directly from the external input, and assume the resistance of the external source is much less than the volume control pot. Doing it this way makes it significantly harder to bandpass the input, and also doesn't provide a low impedance output to drive the summing stage. If you buffer it after the volume control you haven't saved any op-amps/stages.Anyway, I would be interested to know if there's a better way to do it, so I will check back later for any developments.QuoteI definitely think it is a good idea to put space on the board to fit this should it be needed, but I don't think it will be usually, because the op-amps are compensated for unity-gain stability, and he's only using them at unity-gain.And he's already filtered out undesirable high frequencies at the input stage, so there is no need to do this again. (Each time filtering with a 30kHz cutoff leaves you 3dB down at 30kHz and this is undesirable if you do it too many times, you'd need to increase the cutoff in such case).QuoteThis is another interesting chip which like the DRV134 I'm not that familiar with, and I recommended to use an ordinary op-amp for this. So I didn't look at it closely. I originally suggested to omit one of the capacitors, but he is using the recommended circuit so I think that's OK.QuoteYes, that's a good idea. I can't help wondering if the linear regulator might be overkill though, could you get away with just the 2-stage LC filtering? And could you use ordinary inductors here? Or is there a particular reason why the ferrite bead is the right thing to use in this case?I would certainly use a low dropout regulator, and try to set the operating voltage of the circuit just under the 24VDC output of the SMPS, to avoid lots of voltage drop and heating in the regulator. But obviously there needs to be a bit of margin in case the SMPS is outputting a bit low.By the way, Jbliss: I noticed that you've got the GND connection of the incoming audio going to GNDREF. I have the feeling that it is dangerous to put such a signal to the outside world, unprotected. Since static on it could damage your supply splitter, etc. Also, in the originally sketched design that I suggested, there was no supply splitter and the 20Hz high-pass had a dual function of level-shifting the input so that the GND connection of the incoming audio could go to GND and not the half-supply reference. Since you have a supply splitter, it might change things.cheers, Nick