Author Topic: Building a sound mixer within my guitar amp with line IN & mic input  (Read 13355 times)

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Offline Yansi

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #25 on: June 02, 2019, 11:00:03 pm »
Yansi:
Would general purpose PNP transistors work here?

Why wouldn't they? :)

BC557C, BC560C, or even some higher current ones provide for less noise, like BC327-25.

In the old days, the golden transistor for the preamp was a 2SB737.

Other trickery may be to use two parallel transistors to get the noise another 2dB down*.

Here is an example of a mixer input stage:



*Noise relations for parallel connected transistors: http://leachlegacy.ece.gatech.edu/papers/Parallel.pdf
If I remember right, such trickery was used in some Allen&Heath mixing desks.
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #26 on: June 03, 2019, 08:20:42 am »
Jeez. Why 12GBP part for something that can be solved with a couple of discretes?
Quote
Your differential mic amp is useless. Instead of simulating stuff, where it tells you nothing more that the simulation is worth, try breadboard it.
LTspice can simulate noise, but I've not played with it much.

I thought more of a practicality standpoint.  Noise-wise it will be horrible no doubt, but the input impedance imbalance is a real issue in practical application.  With such imbalance, you can't even design a sensible EMI protection around it that will not screw the CMRR even more.

H.
Which circuit/IC are you referring to?

Let's also not forget that there's little point in designing an amplifier with an input noise voltage much lower than the noise figure of microphone. A 200R dynamic microphone will have thermal resistance noise voltage of 1.8nV/√Hz at room temperature, plus some thermal acoustic noise, roughly matching its frequency response. The air molecules and the actual transducer itself will randomly vibrate, at temperatures above absolute zero.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #27 on: June 03, 2019, 12:09:15 pm »
I'm reading about 3 op amp instrumentation amplifiers now and I'm a bit confused. Apparently it's a differential op amp with both inputs buffered through 2 low noise op amps. But that would have a very high input impedance, didn't I need low impedance at the inputs?

Which circuit/IC are you referring to?

Pretty sure he was talking about my circuit
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #28 on: June 03, 2019, 01:55:16 pm »
I'm reading about 3 op amp instrumentation amplifiers now and I'm a bit confused. Apparently it's a differential op amp with both inputs buffered through 2 low noise op amps. But that would have a very high input impedance, didn't I need low impedance at the inputs
Yes, that's correct, it will have a high impedance input. The good thing about it is it doesn't require adding any resistors in series with the microphone, which increase noise. The impedance can easilly be reduced by adding resistors in parallel with the microphone.
 

Offline Bassman59

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #29 on: June 03, 2019, 04:17:37 pm »
Those INA163 go for 12€ on ebay

They're easily less than half that through a proper distributor. eBay? Why bother.

Also consider THAT 1510 or 1512.

 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #30 on: June 03, 2019, 09:28:19 pm »
I'm reading about 3 op amp instrumentation amplifiers now and I'm a bit confused. Apparently it's a differential op amp with both inputs buffered through 2 low noise op amps. But that would have a very high input impedance, didn't I need low impedance at the inputs
Yes, that's correct, it will have a high impedance input. The good thing about it is it doesn't require adding any resistors in series with the microphone, which increase noise. The impedance can easilly be reduced by adding resistors in parallel with the microphone.

Of course  :palm:

Those INA163 go for 12€ on ebay

They're easily less than half that through a proper distributor. eBay? Why bother.

Also consider THAT 1510 or 1512.

Because after shipping fees to Spain it's actually pricier in digikey or mouser last  time I checked.
I'm making a parts list for  the preamp that Yansi suggested, but I'm pretty sure that I have everything I need except for the pnp transistors.

How about this INA217? The plan right now is to build both, the one with instrumental opamps and Yensi's design
 

Offline Audioguru

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #31 on: June 03, 2019, 09:33:03 pm »
An instrumentation amplifier has all the resistors needed perfectly matched for excellent common mode rejection. They have a high input resistance because they are used to measure high impedance heart and brain waves. A differential circuit needs the resistors matched which is either with very expensive resistors or with a trimpot or two for you to fiddle with.
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #32 on: June 03, 2019, 10:03:49 pm »
I'm reading about 3 op amp instrumentation amplifiers now and I'm a bit confused. Apparently it's a differential op amp with both inputs buffered through 2 low noise op amps. But that would have a very high input impedance, didn't I need low impedance at the inputs
Yes, that's correct, it will have a high impedance input. The good thing about it is it doesn't require adding any resistors in series with the microphone, which increase noise. The impedance can easilly be reduced by adding resistors in parallel with the microphone.

Of course  :palm:

Those INA163 go for 12€ on ebay

They're easily less than half that through a proper distributor. eBay? Why bother.

Also consider THAT 1510 or 1512.

Because after shipping fees to Spain it's actually pricier in digikey or mouser last  time I checked.
I'm making a parts list for  the preamp that Yansi suggested, but I'm pretty sure that I have everything I need except for the pnp transistors.

How about this INA217? The plan right now is to build both, the one with instrumental opamps and Yensi's design
The INA217 looks perfectly fine to me.

It's possible you wouldn't be able to tell the difference in sound between the two designs, especially if you didn't know which one was in the circuit, assuming you set the gains the same. This isn't because there won't be any difference in performance, just that it could be imperceivable.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #33 on: June 03, 2019, 10:17:02 pm »
Thanks again guys. I'll order the parts I need to build those preamps, and while they arrive I'll see if I can get this one working with a TL082. Not sure how to set the gain in that one though



Then I can compare their performance
« Last Edit: June 03, 2019, 10:22:43 pm by dazz »
 

Offline Yansi

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #34 on: June 04, 2019, 07:23:44 am »
Look for single opamp differential amplifier at google. Gain way to easy! In this circuitabout 316. And you are supposed to trim the bottom feedback path to exact resistor ratio as the upper one for best rejection. However wit the input impedance imbalance it is pointless anyway.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #35 on: June 04, 2019, 08:24:38 am »
Look for single opamp differential amplifier at google. Gain way to easy! In this circuitabout 316. And you are supposed to trim the bottom feedback path to exact resistor ratio as the upper one for best rejection. However wit the input impedance imbalance it is pointless anyway.

Oh, OK. I thought the trimpot was meant to balance the impedance of both inputs, even though that's not what the simulation showed.
So the trimpot is there to make the ratios between R1/R4 and R2/R5 equal, to maximize common mode noise rejection, but of course, it's imposible to pick the resistors so that those ratios are equal and the input impedances are also the same.

Well, at least I think I'm starting to get it (yeah, I'm a bit slow).
I've order the components I needed to build the preamp you posted. I think I'll still try to build the differential one just because those components will take a couple of weeks to arrive.

As for the gain control, I was considering adding a non-inverting opamp with variable gain after the differential stage, because adding a pot to the differential opamp would mess up the R1/R4 and R2/R5 ratios. I increased the values of the input resistors from 1k to 10k to lower the gain of the differential stage. I get 1k64 and 1k24 ohms of input impedance. A bit low and quite a significant imbalance, isn't it?
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #36 on: June 04, 2019, 09:33:33 am »
I wouldn't bother with the potentiometer. I dare say 1% tolerance resistors will be good enough. If you're using screened twisted pair cables for the microphones, it should minimise common mode noise anyway. Another possibility is matching the resistor values by measuring them with a decent multimeter.

The problem with the TL082 is it's relatively high voltage noise, which is why the NE5534 was suggested.

What parts in the design Yansi posted, are you having difficulty sourcing? There doesn't appear to be anything non-standard or difficult to find.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #37 on: June 04, 2019, 09:57:07 am »
I just need PNP transistors. Ordered 100 bc557 already. Actually now that I think of it I might have a couple of cannibalized PNP transistors lying around
 

Offline Yansi

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #38 on: June 04, 2019, 12:54:15 pm »
NE5534 on by itself (no discrete diff amp before it) is still quite noisy (TL0xx is heck of noisy) for a general purpose MIC amp. Do not go this way.  Noise levels will get unacceptable when you will go with gains like 40dB or more.

You can make a rough calculation of output SNR: 
NE5534 has a rating of 3.5nV/sqrtHz.  At an audio bandwidth, this gets you to 0.65uVrms of equivalent input noise voltage. At a gain of 40dB, this results in 65uVrms noise.  Considering a nominal signal level of 0dBu, that is a SNR = 20log(775/.065) = 80dB.

Okay, 80dB aint that bad, but considering there will be a lot of added noise from the surrounding passive components and other culprits here and there, 80dB is the best case. In reality? 65 to 70dB I'd say.  Even if 75dB SNR. Now imagine that mic amps are often used with gains over 40dB, this will worsen the SNR even further.

So really, it is best to design for lowest noise right away. The discrete PNP stage is nothing complicated, in fact very cheap well known and robust solution to the problem. Also it is (or at least was) widely used back in the days, where engineers were engineers and not people gluing together fancy blackboxes (unnecessarily expensive ICs).


 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #39 on: June 04, 2019, 01:36:25 pm »
That's a bit of a generalisation. Whether the NE5534 is too noisy or not, depends on the application. The microphone impedance, sensitivity and how loud the sound being recorded, will all be factors. If the microphone is used in a fairly loud situation, such to record singing or a pick-up on a musical instrument, I doubt it will be an issue, as the ambient sound will always be higher than the NE5534's noise floor. If it's being used to record quiet sounds, such as insects moving around, then it certainly won't be good enough.

I'm currently designing an intercom system, using the NE5534 as a dynamic microphone pre-amplifier, simply because there's no point in using anything better. It's not a balanced system: the headsets all have single core screened cable and tests have found that noise pick-up from the cable, exceeds that in the NE5534, even with a noise gain of 60dB (a two channel mixer with a gain of 50dB). I would love to use a balanced system or headsets with a built-in pre-amplifier, but it has to be compatible with an existing system, which is unbalanced, so is a no-goer.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #40 on: June 04, 2019, 04:31:14 pm »
I believe I need much more than 40dB of gain if I want to take full advantage of the power that the amp can deliver. The power amp has a gain of 20dB, so x10 voltage gain. At 24V the voltage swing at the output of the power amp is 48V peak to peak, so at 20bB that's 4.8V at the output of the mic preamp. I read somewhere that the typical output of a dynamic microphone is 1mV, so that's 20log(4.8/.001)=73.6dB. Quite a lot of gain indeed. I understand the voltage output of the mic will depend on many factors, but does that work as a rough approximation?

The plan is to design the preamp to be used with a good mic like a shure sm58 or something like that. I guess there's no point in designing for crappy equipment.

So I couldn't find any pnp transistors, but there's a local store I can resort to. Bit of an inconvenience to drive there, but anyway. They have BC557C's in stock so I should be able to start breadboarding Yansi's preamp shortly

BTW, I'm guessing it's probably a good idea to roll a proper pcb for this to minimize noise issues, right? short & thin traces with proper separation and all that.
Oh, and I need to modify the schematic so that it works with a single dc power supply, I'll do that now
 

Offline schmitt trigger

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #41 on: June 04, 2019, 07:49:49 pm »

BTW, I'm guessing it's probably a good idea to roll a proper pcb for this to minimize noise issues, right? short & thin traces with proper separation and all that.
Oh, and I need to modify the schematic so that it works with a single dc power supply, I'll do that now

And a proper ground plane, too.
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #42 on: June 04, 2019, 08:26:57 pm »
Regarding modifying the schematic for a single supply. Awhile ago, Audioguru posted some schematics showing single and dual supply configurations for inverting an non-inverting amplifiers. here's my version, which includes the differential configuration.


The circuit Yansi posted can also be converted to single supply, fairly easily.



The -17V supply becomes 0V and +17V, 24V. R6 and R7 can be replaced with potential dividers consisting of 10k resistors going to +24V and 0V, with the tap connected to the bases of Q1 and Q2. I can upload a schematic if that's not clear, but don't have time at the moment.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #43 on: June 04, 2019, 08:56:21 pm »

BTW, I'm guessing it's probably a good idea to roll a proper pcb for this to minimize noise issues, right? short & thin traces with proper separation and all that.
Oh, and I need to modify the schematic so that it works with a single dc power supply, I'll do that now

And a proper ground plane, too.

Yeah, I always try to have a large ground plane taking up as much area as posible.

The -17V supply becomes 0V and +17V, 24V. R6 and R7 can be replaced with potential dividers consisting of 10k resistors going to +24V and 0V, with the tap connected to the bases of Q1 and Q2. I can upload a schematic if that's not clear, but don't have time at the moment.

Thanks Hero, this one was super straightforward. I'll keep those pics for future reference too

I have the preamp modeled in spice and working fine. Trying to figure out the best way to increase the gain. I get some 20dB with a 20V dc supply.
Can I change the values of the resistors in the differential opamp to achieve more gain? I can significantly increase the gain by increasing R8 but will that mess up the CMRR?
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #44 on: June 04, 2019, 09:56:56 pm »
You need to increase the value R9, so it's the same as R8, to keep the CMRR high.

You will probably need to add another stage of amplification afterwards.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #45 on: June 04, 2019, 10:06:32 pm »
You need to increase the value R9, so it's the same as R8, to keep the CMRR high.

You will probably need to add another stage of amplification afterwards.

Understood, I'll do just that  :-+
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #46 on: June 05, 2019, 08:35:57 am »
OK, I've been playing with the simulation of this preamp for a bit and there seems to be a problem.
The op amp is biased very close to the negative rail and it's distorting a lot.

Here's the output of the fourier analysis for a sweep in the gain pot

Code: [Select]
Fourier components of V(micout)
DC component:1.90883e-006

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 1.806e-04 1.000e+00    -0.69°     0.00°
    2    2.000e+03 3.648e-07 2.020e-03   -91.38°   -90.69°
    3    3.000e+03 8.163e-10 4.519e-06   177.92°   178.61°
    4    4.000e+03 2.217e-12 1.228e-08    83.24°    83.93°
    5    5.000e+03 1.890e-13 1.046e-09   -14.35°   -13.66°
    6    6.000e+03 2.190e-13 1.212e-09    -5.75°    -5.06°
    7    7.000e+03 2.814e-13 1.558e-09   -10.74°   -10.05°
    8    8.000e+03 2.764e-13 1.530e-09    -9.19°    -8.50°
    9    9.000e+03 3.352e-13 1.856e-09     0.70°     1.39°
Total Harmonic Distortion: 0.201955%(0.201950%)


.step g=10
N-Period=1
Fourier components of V(micout)
DC component:2.10847e-006

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 2.247e-04 1.000e+00    -0.68°     0.00°
    2    2.000e+03 5.644e-07 2.512e-03   -91.37°   -90.69°
    3    3.000e+03 1.571e-09 6.993e-06   177.93°   178.62°
    4    4.000e+03 5.268e-12 2.345e-08    84.87°    85.56°
    5    5.000e+03 2.031e-13 9.038e-10   -19.10°   -18.42°
    6    6.000e+03 2.822e-13 1.256e-09   -16.49°   -15.81°
    7    7.000e+03 3.063e-13 1.363e-09    -9.38°    -8.70°
    8    8.000e+03 3.310e-13 1.473e-09    -1.81°    -1.13°
    9    9.000e+03 3.624e-13 1.613e-09    -7.81°    -7.13°
Total Harmonic Distortion: 0.251209%(0.251205%)

.step g=20
N-Period=1
Fourier components of V(micout)
DC component:2.48944e-006

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 2.908e-04 1.000e+00    -0.68°     0.00°
    2    2.000e+03 9.453e-07 3.251e-03   -91.36°   -90.68°
    3    3.000e+03 3.406e-09 1.171e-05   177.95°   178.63°
    4    4.000e+03 1.474e-11 5.069e-08    86.21°    86.88°
    5    5.000e+03 3.803e-13 1.308e-09   -21.54°   -20.86°
    6    6.000e+03 3.849e-13 1.324e-09   -19.06°   -18.38°
    7    7.000e+03 4.657e-13 1.602e-09    -8.78°    -8.10°
    8    8.000e+03 4.754e-13 1.635e-09    -6.95°    -6.27°
    9    9.000e+03 5.518e-13 1.898e-09    -8.37°    -7.69°
Total Harmonic Distortion: 0.325115%(0.325112%)

.step g=30
N-Period=1
Fourier components of V(micout)
DC component:3.28645e-006

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 3.947e-04 1.000e+00    -0.67°     0.00°
    2    2.000e+03 1.742e-06 4.414e-03   -91.35°   -90.68°
    3    3.000e+03 8.522e-09 2.159e-05   177.97°   178.64°
    4    4.000e+03 5.040e-11 1.277e-07    86.94°    87.61°
    5    5.000e+03 7.158e-13 1.813e-09   -11.48°   -10.81°
    6    6.000e+03 4.867e-13 1.233e-09   -22.51°   -21.84°
    7    7.000e+03 6.092e-13 1.543e-09   -16.19°   -15.52°
    8    8.000e+03 6.413e-13 1.625e-09   -11.22°   -10.55°
    9    9.000e+03 7.496e-13 1.899e-09   -12.83°   -12.16°
Total Harmonic Distortion: 0.441366%(0.441364%)

.step g=40
N-Period=1
Fourier components of V(micout)
DC component:5.15258e-006

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 5.681e-04 1.000e+00    -0.66°     0.00°
    2    2.000e+03 3.608e-06 6.352e-03   -91.32°   -90.66°
    3    3.000e+03 2.540e-08 4.471e-05   178.00°   178.66°
    4    4.000e+03 2.164e-10 3.810e-07    87.19°    87.85°
    5    5.000e+03 2.640e-12 4.648e-09    -9.51°    -8.85°
    6    6.000e+03 8.161e-13 1.437e-09   -24.29°   -23.63°
    7    7.000e+03 9.191e-13 1.618e-09   -19.11°   -18.45°
    8    8.000e+03 9.834e-13 1.731e-09   -14.61°   -13.95°
    9    9.000e+03 1.089e-12 1.918e-09   -11.45°   -10.79°
Total Harmonic Distortion: 0.635190%(0.635188%)

.step g=50
N-Period=1
Fourier components of V(micout)
DC component:1.01764e-005

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 8.787e-04 1.000e+00    -0.64°     0.00°
    2    2.000e+03 8.633e-06 9.825e-03   -91.28°   -90.64°
    3    3.000e+03 9.399e-08 1.070e-04   178.07°   178.71°
    4    4.000e+03 1.240e-09 1.411e-06    87.37°    88.00°
    5    5.000e+03 1.825e-11 2.077e-08    -4.91°    -4.27°
    6    6.000e+03 1.496e-12 1.702e-09   -35.16°   -34.52°
    7    7.000e+03 1.426e-12 1.623e-09   -21.79°   -21.15°
    8    8.000e+03 1.627e-12 1.852e-09   -20.43°   -19.79°
    9    9.000e+03 1.763e-12 2.006e-09   -18.38°   -17.74°
Total Harmonic Distortion: 0.982516%(0.982515%)

.step g=60
N-Period=1
Fourier components of V(micout)
DC component:2.62225e-005

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 1.486e-03 1.000e+00    -0.60°     0.00°
    2    2.000e+03 2.469e-05 1.661e-02   -91.20°   -90.60°
    3    3.000e+03 4.546e-07 3.059e-04   178.19°   178.79°
    4    4.000e+03 1.014e-08 6.826e-06    87.56°    88.16°
    5    5.000e+03 2.384e-10 1.604e-07    -3.40°    -2.81°
    6    6.000e+03 7.550e-12 5.081e-09   -74.17°   -73.58°
    7    7.000e+03 2.928e-12 1.970e-09   -30.58°   -29.99°
    8    8.000e+03 3.242e-12 2.182e-09   -26.96°   -26.36°
    9    9.000e+03 3.450e-12 2.322e-09   -23.85°   -23.25°
Total Harmonic Distortion: 1.661638%(1.661637%)

.step g=70
N-Period=1
Fourier components of V(micout)
DC component:8.82526e-005

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 2.788e-03 1.000e+00    -0.51°     0.00°
    2    2.000e+03 8.684e-05 3.115e-02   -91.02°   -90.51°
    3    3.000e+03 3.000e-06 1.076e-03   178.46°   178.96°
    4    4.000e+03 1.256e-07 4.504e-05    87.93°    88.43°
    5    5.000e+03 5.484e-09 1.967e-06    -2.66°    -2.15°
    6    6.000e+03 2.547e-10 9.136e-08   -92.05°   -91.54°
    7    7.000e+03 7.848e-12 2.815e-09 -147.69° -147.18°
    8    8.000e+03 6.917e-12 2.481e-09   -37.77°   -37.26°
    9    9.000e+03 7.515e-12 2.696e-09   -37.99°   -37.48°
Total Harmonic Distortion: 3.117192%(3.117192%)

.step g=80
N-Period=1
Fourier components of V(micout)
DC component:0.000356279

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 5.657e-03 1.000e+00    -0.32°     0.00°
    2    2.000e+03 3.570e-04 6.310e-02   -90.64°   -90.32°
    3    3.000e+03 2.503e-05 4.425e-03   179.02°   179.34°
    4    4.000e+03 2.124e-06 3.754e-04    88.69°    89.01°
    5    5.000e+03 1.880e-07 3.323e-05    -1.66°    -1.35°
    6    6.000e+03 1.739e-08 3.075e-06   -91.98°   -91.67°
    7    7.000e+03 1.635e-09 2.890e-07   178.29°   178.61°
    8    8.000e+03 1.407e-10 2.487e-08    83.03°    83.35°
    9    9.000e+03 3.223e-11 5.697e-09   -35.81°   -35.49°
Total Harmonic Distortion: 6.325785%(6.325785%)

.step g=90
N-Period=1
Fourier components of V(micout)
DC component:0.00113987

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 1.023e-02 1.000e+00    -0.03°     0.00°
    2    2.000e+03 1.161e-03 1.135e-01   -90.07°   -90.04°
    3    3.000e+03 1.474e-04 1.440e-02   179.88°   179.91°
    4    4.000e+03 2.253e-05 2.202e-03    89.83°    89.86°
    5    5.000e+03 3.600e-06 3.519e-04    -0.23°    -0.20°
    6    6.000e+03 6.001e-07 5.866e-05   -90.30°   -90.27°
    7    7.000e+03 1.024e-07 1.001e-05   179.66°   179.69°
    8    8.000e+03 1.774e-08 1.734e-06    89.49°    89.52°
    9    9.000e+03 3.153e-09 3.082e-07    -1.54°    -1.51°
Total Harmonic Distortion: 11.441146%(11.441146%)

.step g=100
N-Period=1
Fourier components of V(micout)
DC component:0.00101971

Harmonic Frequency Fourier Normalized Phase  Normalized
 Number   [Hz]    Component Component [degree] Phase [deg]
    1    1.000e+03 1.253e-02 1.000e+00     0.07°     0.00°
    2    2.000e+03 1.716e-03 1.369e-01   -89.86°   -89.93°
    3    3.000e+03 2.671e-04 2.131e-02 -179.80° -179.87°
    4    4.000e+03 4.959e-05 3.957e-03    90.25°    90.17°
    5    5.000e+03 9.654e-06 7.703e-04     0.21°     0.14°
    6    6.000e+03 1.970e-06 1.572e-04   -89.65°   -89.72°
    7    7.000e+03 4.055e-07 3.235e-05 -177.85° -177.93°
    8    8.000e+03 7.317e-08 5.838e-06    89.69°    89.62°
    9    9.000e+03 2.300e-08 1.835e-06   -33.04°   -33.11°
Total Harmonic Distortion: 13.864246%(13.864246%)


The THD seems a bit out of whack, doesn't it? Here's a table with the THD for each gain level:

Code: [Select]
Pot	 Gain	THD
 0 -14dB 0.201955%
 1 -13dB 0.251209%
 2 -11dB 0.325115%
 3 -8dB 0.441366%
 4 -5dB 0.635190%
 5 -1dB 0.982516%
 6   3dB 1.661638%
 7   9dB 3.117192%
 8 15dB 6.325785%
 9 20dB 11.441146%
10 22dB 13.864246%


It gets a lot better if I reference the opamp to Vref (Vdd/2) instead of GND through R8 (R14 in my simulation below), but can I do that without ruining my CMRR again? Do I also need to add coupling caps to the opamp inputs if I do that?
« Last Edit: June 05, 2019, 08:38:22 am by dazz »
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #47 on: June 05, 2019, 09:55:39 am »
What's the power supply voltage? Try reducing Vref to 8V. That should increase the bias current through the transistors, thus the voltages across the collector resistors.

By the way, does anyone know what the diodes are for? They're continuously reverse biased and appear to do nothing.
 

Offline dazzTopic starter

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #48 on: June 05, 2019, 10:14:50 am »
What's the power supply voltage? Try reducing Vref to 8V. That should increase the bias current through the transistors, thus the voltages across the collector resistors.

How the hell do I manage to miss the simplest solution every frigging time?  |O
Supply voltage is 20V in the simulation. Ideally I'd like it to work down to 12V, or 9V if possible.
« Last Edit: June 05, 2019, 10:16:35 am by dazz »
 

Online Zero999

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Re: Building a sound mixer within my guitar amp with line IN & mic input
« Reply #49 on: June 05, 2019, 10:37:55 am »
I think you'll struggle to get that circuit to run down to 9V, because you won't be able to get the voltage across the collector resistors to fall within the TL72's common mode range, at low voltages.

You could replace the emitter resistors with constant current sources: try 1mA. See link below. Note that the diodes and resistor provide a reference voltage, which can be used for both current sources.
http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT%20Current_Source/BJT_Current_Source.htm

I hope you can see why I suggested an IC, for simplicity's sake, but you won't learn so much and it'll still work out more expensive.
 


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