# Problem with noise and clipping

Discussion in 'The Projects Forum' started by Aish_Zaleha, Apr 4, 2012.

1. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
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So, currently we were able to make the circuit work. i think i've shown the thesis plan on my first thread. it is a very simply prototype of a speech(language laboratory. now our problem is that there is clipping and noise. although the voice is still audible. we used four input microphone line in jacks and one coming from the output of the headphone jack of a laptop (3.5 mm stereo trs connector) and and five outputs(to the headphone 3.5 mm mono trs connector). so the summing (mixing) amplifier circuit we used is http://www.zen22142.zen.co.uk/Circuits/Audio/6ipmix.htm.
but note that the output is divided into 5, so we had to adjust the variable resistors so it can be heard. if we don't divide the output into 5, it becomes too loud which is not good.
we plan on adding a distribution amplifier after the output of the first ciruit shown on the link. so that we would need to maximize the volume and clipping would be avoided. our question is, will this help with minimizinig the noise?
we are going to use a circuit like this but we're going to use LM324.

will it work?

2. ### #12 Expert

Nov 30, 2010
16,343
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Your bias circuit looks wrong. The Vcc should go to two 100k resistors in series, the second one to ground, and the center of them goes to the input pin. and the link from the feedback path to the Vcc divider doesn't need to be there.

Either that or I'm not understanding the circuit.

3. ### MrChips Moderator

Oct 2, 2009
12,449
3,365
If you are driving high impedance loads there is no reason to provide separate buffers for each channel.

4. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
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looking at the specs of the headset... it says "impedance: 32/Hz"
it doesn't seem really high?
if i don't need to use separate buffers, what can i do to minimize the noise?

5. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
0
well, i saw that over the internet. i wasn't sure about it so i asked other people's opinion.
i just want to ask, can i automatically use the buffers after every output on the circuit on the link i provided, one on each output towards the headphone?

6. ### #12 Expert

Nov 30, 2010
16,343
6,828
Adding unnecessary amplifier (buffer) circuits might add noise, depending on the voltage level of the signal at that point, but you can use all the buffers you want.

Aish_Zaleha likes this.
7. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
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ok.. if buffers aren't necessary. i have another question and i hope you don't mind. hehe
the out of the circuit shown below is outputted in 5 different headphones so i connect the headphones in parallel. and i noticed. headphones farther from the output source decreases in amplitude (volume). the ones near the output has too much that it causes distortion. how can i make all the outputs equal in amplitude? supposing we can't vary the volume of the headphones?

8. ### MrChips Moderator

Oct 2, 2009
12,449
3,365
The TL064 are not meant to deliver power and cannot drive headphones.
You need to separate your noise/clipping problems from mixer/distribution problems.

Firstly, concentrate on driving one set of headphones.
Get the mixer working. Use an audio power amp to drive the head phones.
Make sure the volume level is adjustable and adequate with no noise and clipping.
If the output from the mixer is causing the clipping, attenuate it.
Do not use that TL064 circuit shown.

After you get clean sound into one audio amp/headphones, add additional audio amp/headphones. You do not need buffers. The input impedance of audio amps are high enough and will not load the signal.

9. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
0
we actually connected it in parallel which caused the other headphone outputs to attenuate. that is, the headphone closest to the output node produces loud sounds on the speaker. the ones that come later in the parallel connection has much lesser loudness. how do i add a another audio am.. in which specific node ? is it at the node where the 47kΩ resistors meet? like this one? the circuit that comes before the 47k, i didn't include since it already was provided before.

10. ### Audioguru New Member

Dec 20, 2007
9,411
896
Your noise (hiss?) comes from the lousy old opamps you are using. You should use LOW NOISE AUDIO OPAMPS instead.
The total gain for a mic channel is 1000 so of course there is severe clipping.

Opamps do not have enough output current to drive 32 ohm headsets so they clip when the output is not even loud enough.

You are mixing apples and oranges.
The preamp/mixer has a dual-polarity supply and the headset amplifier has a single polarity supply.

Yout heaset amplifier is wrong, I corrected it.

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11. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
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i'm not quite familiar with op-amps. hehe. anyway.. i'm using an lm324. is that still a bad audio amp?
and yes the noise is a hiss.

also, when i try turn off a microphone, the noise multiplies like 10 times. (this problem occurs for the first circuit i showed on the link. i haven't really made the preamp for the headphone yet.
thank you for the circuit.

12. ### Audioguru New Member

Dec 20, 2007
9,411
896
The LM324 is 38 years old and was the first "low power" opamp.
Because it is old and low power then its hiss level is very high and it has horrible crossover distortion (look at it in Google). Also its high frequencies go only to 1kHz or 2kHz at high levels.
Most audio opamps are low noise, have extremely low distortion and work well to 100kHz.

You do not show how you "turn off" a mic. Do you disconnect it or do you short it?

13. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
0
well, we were intending to use tl071 but unfortunately it's not available anywhere here in our place. i'll just try to find other low noise op-amp available here. most good audio amps are not available here.
how we turn off the microphone is kind of complicated. we have a separate circuit where we connect the microphone the from that circuit we then pass it to the amp.. we turn off the power of that circuit.
we actually pass the microphone to a switch so we can turn it on and off using a PC. when we disconnect the switch, the noise increases alittle but it does turn off.. but when we totally turn off the power supply, a loud noise is produce.. please know that each microphone has a different switch and power supply..

14. ### Audioguru New Member

Dec 20, 2007
9,411
896
Go to www.farnell.com and see that they have an office in many countries including the Philippines. Order a modern audio opamp from them.

15. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
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oh.. i would if i have the time. but now, i really don't have that. it feels like i can't even catch my breathe.
anyway, may i just ask why those are 330μF?
is it because you divided 1000 by 3 ( number of outputs)?
if i have 5 outputs, what should the capacitor be just before the output?

16. ### Audioguru New Member

Dec 20, 2007
9,411
896
Then you are stuck with hiss and distortion.

Didn't you learn the simple equation to calculate the value of a coupling capacitor? 1 divided by (2 x pi x R x f).
The output impedance of the opamp is extremely low so R= 32 ohms.
Audio goes down to 20Hz so I selected a -3dB (half-power) cutoff frequency of 15Hz. Use 100uF for a 50Hz cutoff frequency.

Don't you have an opamp and coupling capacitor for each output?
But if you use a power amplifier then it can drive five headsets in parallel.
Then the total load is 32/5= 6.4 ohms and you can simply calculate the value of a single output coupling capacitor.

17. ### Aish_Zaleha Thread Starter New Member

Mar 21, 2012
14
0
well, i did learn it, a few years back. but because we weren't able to apply it on an actual circuit, i easily forgot. i know, it's a silly reason. sorry about that.
thank you very much for everything. you helped a lot..