help in a mic preamp

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tried simulating it, worked fine but I didn't notice the shift of the waveform when the 100k pot's value was changed...

R5=50kohm Vout=493.302mV
**broken link removed**

R5=100kohm Vout=(*shifted downward)
**broken link removed**
 
wakeke said:
I'm a little confused on the supply voltages (+-) connected on the electrolytic caps (on the left side).
Click to expand...
The two capacitors on the left side of the sticky are the supply bypass capacitors and are C4 and C5 in your circuit.

are they the one being supplied to the mic from the positive supply?
Click to expand...
R1 supplies a positive voltage to the mic. R1 is fed the positive supply through a 1k resistor and 47µF capacitor which are a filter.

red colored voltages are the resulting biases when the high supply is applied?...
Click to expand...
The example in the sticky shows a dual polarity positive and negative 6V supply on the left and a single positive 12V supply on the right.
 
tried simulating it, worked fine but I didn't notice the shift of the waveform when the 100k pot's value was changed...

R5=50kohm Vout=493.302mV
**broken link removed**
R5=100kohm Vout=(*shifted downward)
**broken link removed**
Click to expand...

forgot to put the question: what would be causing the shift of Vout???
if R5 is zero, the waveform shifts upward and has very small amplitude (the amplitude makes sense but the shift is different/unexpected)...
 
The circuit has an output coupling capacitor that will block any DC on the output so the trace should not shift.

Your traces are wrong:
1) With the pot R5 set to zero ohms then the opamp is a follower with the output level exactly the same as the input level.
2) With the pot R5 set to 100k then the gain is 101 times and the input level must be very low to keep the output of the opamp from clipping.
 
I'll double check the wires/connections...I also reconnected the lines from the oscilloscope and still the same effect...hmm...how about grounding?? would that have an effect on it?....just a thought...
 
wakeke said:
I'll double check the wires/connections...I also reconnected the lines from the oscilloscope and still the same effect...hmm...how about grounding?? would that have an effect on it?....just a thought...
Click to expand...
Of course the ground on the 'scope must be connected to the ground of the circuit.
The negative wire of the positive supply must connect to the circuit's ground and the positive wire of the negative supply.
The ground wire from the signal generator must connect to the circuit's ground.
 

Yes, checked the traces and they are correctly placed.


Yes they all are connected...also the negative of the function gen. is connected to ground...what I mean about grounding is the grounding of the other components..e.g. resistors R2 or the 1kohm...pls check your simulation, mine's really shifting...
 
The reason for the shift could be due to the OpAmp's max input offset voltage of 4mV being amplified by 100x to 0.4V at max gain setting. Since the Thevenin resistance on the inputs of the amplifier are also unbalanced then the input offset current of the NE5532 will also come into play. How much, in volts, of an shift are you seeing when you adjust the 100K gain pot?
 
Did you connect the 'scope to the output of the opamp instead of to the output of the output capacitor C2?
Then the high bias current of the old opamp causes the output to change its DC voltage when the gain is increased by adjusting R5.

I am used to using FET input opamps like the TL071 and OPA134 which have no input bias current.

The max input bias current in an old NE5532 is 0.8uA at room temperature.
Therefore the voltage drop across R2 (100k) is 0.08V.
Therefore the voltage drop across the 1k resistor at pin 2 is 0.0008V.
Therefore the max output DC voltage is (0.08V - 0.0008V) x 101= 7.99V above 0V or 7.99V less than 0V so the output might be saturated.

Adding a capacitor in series with the 1k resistor prevents the max voltage of 0.08VDC from being amplified.
 

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thanks! it worked....a little confused about the nonpolar caps though...their values are in the category of electrolytic types... and I got a list of their ranges and they do fall in polarized caps...there were no avail. 47uF & 470uF NP in the stores....

I tried changing them n simulation into polarized caps: C3 and 22microF...an the preamp worked well as it was with NP..
 
The non-polarized capacitor has low frequency AC across it. A polarized capacitor is a rectifier when the signal polarity is reversed which causes severe distortion. Your simulation program does not know about it.

Digikey sells non-polarized electrolytic capacitors.

The 47uF and 470uF capacitors are not NP but are ordinary polarized electrolytic capacitors.
 
audioguru said:
The non-polarized capacitor has low frequency AC across it. A polarized capacitor is a rectifier when the signal polarity is reversed which causes severe distortion. Your simulation program does not know about it.
Click to expand...

In this case it would make no discernable difference, no voltage worth mentioning across the capacitor - if it bothers you, put two electrolytics in series (opposite ways round) - which is all most NP electrolytics are.
 
In this case it would make no discernable difference, no voltage worth mentioning across the capacitor - if it bothers you, put two electrolytics in series (opposite ways round) - which is all most NP electrolytics are.
Click to expand...



is this what you mean? is it safe to put it like that on the circuit e.g. 22micrF cap connected to the gnd? the capacitance of the cap at the right side would follow the rule on series capacitors?

thanks for the info...but not so sure if there's Digikey in our area...
 
You correctly show the back-to-back capacitors.
Two 47uF capacitors in series makes 23.5uF which will work fine.
 
Thanks for all your help!

there would be another mic, thus the 2 will be summed using TL074...will be trying it in actual, it worked in simulation...
 
**broken link removed**

This is the summing amp to be used for mixing...worked fine in simulation...is this fine? we're going to test in actual....
 
Your mixer opamp circuit is fine except the TL074 is a quad opamp. A TL071 is a single opamp.
They oscillate if they directly drive the capacitance of a shielded audio cable unless a 100 ohm series resistor is added from the output of the mixer circuit to the cable.
 
would try to put 100ohm resistor in series with mixer circuit output..now we got a prob in actual testing, we only hear noise..pot's working fine, grounding is common even with 2 mic preamp circuits... not sure if the out of the mic preamp is getting in the input of the summing-mixer circuit....
 
Maybe the noise you hear is acoustical feedback howling because the mics can hear the speaker and the sounds go around and around.

What are you using to hear sounds? The opamp cannot drive low impedance headphones nor a speaker.
 
we're using headphones w/ 43ohms impedance (from how we measured it)...what if we series from the mixer output a resistor that will increase the impedance? we tried using 741 w/ same summing config but diff. Vcc but had same result...both opamps worked well in simulation following the summing principle..
 
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