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Old 10th February 2014, 09:45 PM   #1
danny92 is offline danny92  Portugal
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Default Preamplifier with aux and mic inputs

Hi everyone,

I'm projecting a simple preamplifier to use in an university project, that is the first module of an AM radio modulator, this preamp stage should have a computer microphone input, a aux input (for MP3, computer, etc.) and should be able to have volume adjustment for each input, and an output global volume adjustment, the output level should be 3-4 V. This circuit should have an output for the next stage (the frequency mixer), and a line output to connect audio devices (for example an audio amplifier, this one is lacking in our circuit).

I've made this circuit with my collegues, we don't know how to use the computer microphone, do you have any ideas?
This circuit could have problems?


PS: Pot1 and Pot2 (mic pot), R18 and Pot4 (aux pot), R8 and R13 (main pot).

Best regards,
Daniel
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File Type: png amp2_2.png (25.2 KB, 140 views)

Last edited by danny92; 10th February 2014 at 10:06 PM.
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Old 11th February 2014, 11:29 AM   #2
Calvin is offline Calvin  Germany
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Hi,

the circuit doesnīt do what You described.
Your decription doesnīt specify a mixing of the inputs, but a choice of a single source by switching between several sources.
If mixing of the inputs is required, rather use inverting OPAmp stages for U2 and U5.
Also, the Mic Input will probabely be single ended not balanced, hence Mic- would have to be connected to gnd.
As drawn, the Input stage is not balanced/symmetrical anyway.
Also AuxL and AuxR would be mixed together, for a single Mono-output.
Is Mono-Mode intended or is Stereo required?
Your simulation specs for the Mic show 70mVrms and 700mV of input voltage for Aux.
Seems a bit high for the MIC input and a bit low for the Aux-Inputs.
The gain for he Mic-Input is set to ~22times -->+27dB.
The Volume Poti should rather follow the Mic-Gain stage than preceeding it.
The choice of the OPamps may be thought over again.
The LT1022 is an rather old and not specifically lownoise type for lowimpedance sources.
Besides the possibilty of reducing the number count of OPAmps, it might also be sensible to use Dual or Quad OPamps to keep size and cost down.
The OPA134/2134/4134 (Single/Dual/Quad by TI) come to mind, or Duals like NE5532 (several sources), LM4562 (TI), LME49880 (TI), or Quads like the LME 49740 (TI) or similar.
Itīd be helpful to precisely list the specifications/requirements and to add a Block-diagram.

jauu
Calvin
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Old 11th February 2014, 10:35 PM   #3
danny92 is offline danny92  Portugal
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Hi Calvin and thank you for your help,

The mixing of the inputs is intentional, why I should use an inverting stage? The noninverting mixer doesn't work?
The circuit is intended to work in mono mode.
Why the MIC input is not balanced?
I'm using TL084 opamps, because this circuit is for a low cost application.

PS: The MIC circuit doesn't work with a computer microphone, I've tested it and I can only hear strange noises, that are not caused by the microphone.

Best regards,
Daniel
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Old 13th February 2014, 12:55 AM   #4
danny92 is offline danny92  Portugal
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Hi everyone,

The circuit of figure 25 of the attached file (from Texas Instruments TL08x datasheet) can work as intended? The circuit seems to behave like a regenerative comparator. V+ and V- are swaped?


Best regards,
Daniel
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Old 14th February 2014, 11:57 AM   #5
Calvin is offline Calvin  Germany
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Hi,

attached is a circuit that could do what You require.
The different subassemblies are marked on the schematic and may be variied depending on Your specificartions (for example if different gain is needed for the Mic-channel).

Both channels Mic and Aux start the same with a Input filter with bandwidth limits of ~1.6Hz to 160kHz.
It prevents DC and HF to enter the circit.
Input impedance of both Inputs is 50kOhms.
Then linear gain stages follow.
For Mic the gain is set to A1= 1+R3/R2 = 20.6x = +26dB
For Aux the gain is set to A2= 1+R9/R10 = 2x " +6dB
Small compensation caps C3 and C6 prevent oscillation.
In both channels Preset-Potis follow, allowing to dial in on the desired individual Volume level.
If more input channels are required simply add similar stages.

Then follows the Mixer in the classic inverting architecture that adds both channels together.
The gain for each channel is -R14/R12 and -R14/R13.
This is followed by the Master Volume Poti.
A inverting Buffer guarantees sufficient drive capability and low output impedance.
A series Resistor is added to enable the OPamp to drive capacitive loads.
Finally a DC-blocking cap with bleeder resistor keeps the output free of any DC.

The OPamps could be Duals like I mentioned a few before.
One of the cheapest and still best would be the NE5532 (a bipolar type).
A JFET-Input equivalent would be the OPA2134.
Donīt use the TL0xx types.
They are simply outdated, grossly inferior and not cheaper than a NE5532.
Not shown in the schematic are the power supplies and associated decoupling caps.

jauu
Calvin
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File Type: gif Mic - Aux - Input stages Mixer.gif (50.1 KB, 103 views)

Last edited by Calvin; 14th February 2014 at 12:00 PM.
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Old 14th February 2014, 04:29 PM   #6
danny92 is offline danny92  Portugal
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Hi Calvin, thank you very much for your great help,

I don't understand why the adder should be an inverter adder, why I can't use a non inverter adder?

The 100pF capacitors in parallel with the feedback resistor are really necessary?
This resistors are used to provide unity gain at higher frequencies, to decrease the 0dB point adding more phase/gain margin, but they are really necessary in the circuit with such an high value?
I've seen only values between 1pF-10pF.
Why you don't use a decoupling capacitor at the non inverting input after the gain setting resistor, this capacitor reduces the DC gain to unity, but at the expense of some added distortion, because it has to be electrolytic.
Why do you use a 2.7kohm resistor in the feedback path of the buffer?


PS: I've already made the PCB and I have to show my work at university, in my country TL08xx is very easy to find. Other ampops are very rare and have to be ordered.

Best regards,
Daniel

Last edited by danny92; 14th February 2014 at 04:40 PM.
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Old 14th February 2014, 07:42 PM   #7
Calvin is offline Calvin  Germany
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Hi,

one can use both the inverting and the noninverting Mixer.
There are distinct differences though.
With the noninverting Mixer one channel 'sees' the other channels resistor as load. its own resistor and the other channels resistor form a voltage divider.
If both resistors are of same value, the signal voltage is divided by two.
At the mixer-OPamps input the voltage is the sum of both input voltages divided by 2.
With 3 channels it woud be the sum of the three inputs divided by 3.
The loss in signal voltage requires that the mixer OPamp has a gain of 2, or 3, depending on the number of channels.

The inverting Mixer sums up the currents flowing into the inverting input node. The summed up current then flowing through the feedback resistor, thereby creating the output voltage.
Due to the current summation, each input is independent from the other.
One can even set different gain factors for each channel.
Also the voltage swing at both inputs is close to 0V.
The so called common mode voltage remains small, which keeps distortions small too.

The small caps in parallel to the feedback resistors prevent oscillation.
The lower the gain of the stage, the more prone it becomes to oscillation.
The caps also reduce the Bandwidth of the stage, which improves noise.
The caps may be omitted with, but the behaviour of the circuit should be tested for stability.
A cap betwwen the second feedback resistor to gnd reduces the DC gain to 1.
This is a good measure if DC offsets may become a problem for the circuit.
In my schematic the complete circuit is AC coupled in each input and the output.
Offsets created in between remain so low, that the additional caps are not needed.
The 2k7 resistors serve the purpose of reducing input bias current related offsets.
The input bias current of each OPamp input create a voltage drop over the associated impedance.
The two buffers see the Potis impedance at one input and the 2k7 at their other input. The poti's impedance seen from the OPamps input varies between
0Ohm (either end) and 2k5 (2x 5k in parallel). The 2k7 is close to the 2k5 which generates the largest input offset voltage, but its still high enough as minimum load of the OPamps output (quite often 2k is specified as minimum load).

The NE5532 is such a long lived common OPamp, that I doubt You can't easily and cheaply source it.
The TLxxx are by no means good OPamps after modern standards and they are not cheaper.

jauu
calvin
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Old 15th February 2014, 08:24 PM   #8
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Calvin,

I was looking at your schematic since I needed something similar in my '3 channel quad mixer' thread, and I have a few questions:

1. C1 surely should be 1uF for a ~1.6Hz HPF?

2. a normal HPF has the capacitor near the input [1], your schematic has it on the other side. any reason for that?

3. C1 + R1 are the HPF and C2 + R2 are the LPF. what does R3 do in there, and won't it distrupt the LPF?

thanks,
peter


[1] http://sim.okawa-denshi.jp/en/CRhikeisan.htm

Last edited by petertux; 15th February 2014 at 08:31 PM.
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Old 15th February 2014, 10:22 PM   #9
danny92 is offline danny92  Portugal
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Thank you very much for your help, Calvin
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Old 15th February 2014, 11:05 PM   #10
danny92 is offline danny92  Portugal
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Hi, I think that the LPF are C1+R3, and R1 doesn't have influence in the LPF. R1 lowers the input impedance to 50 kohm approximately.
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