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problems on combining two circuits together..

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xmat

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Hello, first post into this forum, I need some help!!

I recently made a project onto a breadbroad, entailing two sections:

An electret mic,

and a (1-bit) simple analog to digital converter.

But it seems a have some load impedance problems causing signal distortion.

The input signal fed into the mic is of low frequency content (fundemantal frequency is below 100Hz)

The electret mic is used as a sensor for impulse-type signals, such ones deriving from percussion applications.

The schematic of the mic is schematic "c" into drumsens.gif..



It is powered from the supply shown into power supply for the electret mic.gif file....




For the A-to-D circuit, I have used a similar circuit to the one shown into "power supply for the electret mic.bpm" file...

...Which has at the beginnig a "full-wave rectifier/absolute value circuit" described into absolute value circuit.pdf file..

For that circuit, R=10...and the o-pamp used is LM741c...

So, doing all that I have experienced the following DIFFICULTIES:

My electret capsule seems to have somekind of built-in preamp, so it appears that when connected to the circuit it is causing signal distortion especially noticable to higher frequencies of the signal,


plus a signal level reduction when I connect all circuits together...


My guess is that everything will be solved with somekind of buffering, but I would like to know the procedure on that one..(perhaps a drawing??)

Last but not least, at the end of the analog to digital converter there is a comparator (LM311) that has to trigger a pulse from "0-to-supply voltage" pulse of variable pulse-width;
according to mic input amplitude level..Which it won't do....

..I am suspecting that it is due to low signal level fed into comparator inputs..could that be the case..??And if so, any suggestion on the solution??

(PS: all circuits are connected on a breadboard and a dual 9V voltage supply )

Many thanks in advance,

best regards,

xmat.
 

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  • power_supply_for_the_electret_mic.gif
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I started to reply, and realized I don't understand how everything is connected. You need to show one cohesive schematic, including the absolute value circuit, and all power pins for all ICs. I can see several potential problems, but I'm not sure until I see a cohesive schematic (and still may not be). :roll:
 
Ron H said:
I started to reply, and realized I don't understand how everything is connected. You need to show one cohesive schematic, including the absolute value circuit, and all power pins for all ICs. I can see several potential problems, but I'm not sure until I see a cohesive schematic (and still may not be). :roll:

Sorry for that..

Would an electronic workbench file (version 5) help??

Or I could skan the schematics..

But both can't happen until tomorrow..(I am not at home right now..)

So, if u don't mind, I will try to make a brief explanation, hoping u'll realise what's happening.

1. Suppose I place an electret capsule close to a drum membrane.
The signal has a fundemantal frequency around 87Hz, and last for around 25ms (decay envelope).

2. In file drumsens.gif,
there is diagram "c"
which is some sort of (pre)-amplification for the signal taken from the capsule.
(capsule has a built-in preamp and Rload around 1-2k)

The "ring" of the jack in this figure is connected to a 9v battery through the 10uF capacitor and 680 ohm resistor in file "power supply for the electret mic.gif"

The "tip" of the jack is connected to the absolute value circuit shown in the pdf..This circuit contains several 10k resistors, one 4.7k resistor and two LM741.

The absolute value circuit is part of a bigger circuit,
having also a fell filters and a comparator.

That circuit performs the role of an analog to digital converter; converting the audio amplitude into a pulse of variable width according to amplitude level of the audio input from the mic.

3.Problems are that the signal from the mic goes distorted to ADC which does not trigger a pulse most likely because the differential input of LM311 is too low.

Bottom line, I probably need some buffering and amplification between stages/circuits..Hope I made myself clear on that.

Thanks anyway for your help.
 
I think the old LM324 and LM741 are too slow and have slew-rate overload when fed with the signal from a close-up drum. Besides, the LM324 has pretty bad crossover distortion. Both old opamps are very noisy for audio use. I usually use TL071 audio opamps, but they need a higher supply voltage. The newer TLC071 works down to a 4.5V supply and is better suited to this application. Use a TLC072 dual opamp for the absolute value circuit.

1N400X series rectifiers in the absolute value circuit are also very slow. I would use 1N4148 or 1N914 much faster diodes.

I don't understand how this "envelope detector" project can do proper A to D conversion. Surely it just triggers a MIDI tone or recording that is making the distorted sound.
Maybe the problem is that the old opamps have a slowly ramping output that would probably make the MIDI drum sound like a pipe organ.
 
audioguru said:
I think the old LM324 and LM741 are too slow and have slew-rate overload when fed with the signal from a close-up drum. Besides, the LM324 has pretty bad crossover distortion. Both old opamps are very noisy for audio use. I usually use TL071 audio opamps, but they need a higher supply voltage. The newer TLC071 works down to a 4.5V supply and is better suited to this application. Use a TLC072 dual opamp for the absolute value circuit.

1N400X series rectifiers in the absolute value circuit are also very slow. I would use 1N4148 or 1N914 much faster diodes.

I don't understand how this "envelope detector" project can do proper A to D conversion. Surely it just triggers a MIDI tone or recording that is making the distorted sound.
Maybe the problem is that the old opamps have a slowly ramping output that would probably make the MIDI drum sound like a pipe organ.


Hi!

Thanks for the response.

Below I have a more coherent schematic (the two circuits combined together; the rest of the "1-bit ADC" I left out...they are the dots in the picture..)

1.Good point about the diodes, but I have just tried with 1N4148 and still have the same result..

2.As u can see in this schematic, I don't use an LM324 at all, neither I tend to connect the output of the "1-bit ADC" (pin 7 of LM311) to a trigger-to-midi converter.
All I want is to have a variable pulse width for every different audio amplitude signal that goes into the microphone.

3.Can u show me a drawing of TLC072 with all its pins connected to the absolute value circuit?

4.Can u see any mismatching of input/output impendances between circuits? (Circuits are seperated by the red lines; purple is ground and E1 is the electret capsule- the circuits are on a breadboard with a dual volt supply made with 2 batteries).

5.I suspect that the problem is (also..??) caused because of a built in preamp in the electret capsule.
Can u suggest a capsule with no preamp built in?

6.I thought the circuit to be a 1-bit ADC cause ultimately, what it does is an on-off situation (or 0-1 if u prefer) between "0 volt" and "Voltage supply" states. Am I wrong on that..??

7.Slowly ramping output will be avoided only with the use of TLC072..??

How about LM311 used as the comparator..??
From what I know, it has a min voltage gain of 40,000...Is there a comparator with a 200,000 voltage gain or so..??I'd sure like to know..


Your help is greatly appreciated,

xmat.
 

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Hi Xmat,
I see a few problems with your project:
1) I just realised that your 1N4148 diode is messing up your absolute value full-wave rectifier! It is causing the 100nF coupling capacitor to discharge during negative-going audio, resulting in an output voltage from the absolute value rectifier at the end of each drumbeat when there shouldn't be an output. Get rid of that diode.
2) The low input resistance (5K) of the absolute value rectifier during positive-going audio is severely loading down and attenuating the high output resistance (33K) of your transistor amplifier circuit, and the loading effect is assymetrical again causing the coupling cap to change its charge. Add a voltage-follower buffer opamp between them.
3) The low-value of the 100nF cap across the emitter resistor causes the transistor amp to roll-off sound below 727Hz. It should be a few microfarads to amplify a bass drum, I would use 10uF.
4) The transistor is biased very close to cutoff. When the battery voltage drops with use, the transistor won't work properly. Increase the value of the 15K base resistor to 47K and increase the value of the 2.2K emitter resistor to 20K. Then you could use a 1uF bypass cap across the 20K emitter resistor. Even better, replace the distorting high output resistance transistor with a simple amplifier made with a TL071 opamp now that you have dual supplies totalling 18V. Then you won't need a buffer to drive the absolute value rectifier.
5) The bypass resistor in the absolute value rectifier circuit should be exactly double the value of its 4.7K resistor. Instead of trying to find a 9.4K resistor, simply change the 10K resistors to 20K and use 10K to replace the 4.7K one.

Answers to your questions:
1) 1N4148 diodes will work quicker with faster opamps.
2) Your project will produce a variable-duration "bunch of square-waves" during each drumbeat when you fix the above problems.
3) Get the pinout of the 8-pin TL072 from its datasheet on Google.
4) Your circuits were mismatched alright!
5) Electret mics work very well for all kinds of audio applications. They all contain a FET impedance converter to match the umpteen-billions of megohms impedance of their diaphram to the rest of this planet.
The only problem is that an electret mic might overload inside a drum, most are good for 120dB. How loud is it in there?
6) Is your single-bit ADC enough? It will give you the duration of a drumbeat but not its envelope that has a changing amplitude. I don't think a drum sounds like a fog-horn, just on and off. Besides having a decaying amplitude for each beat, aren't drums played with a different intensity depending on the mood of the music or the drummer?
7) A TL072 is 11 times faster than a 741.
8) The LM311 has plenty of gain. But it goes crazy at zero-crossings without hysteresis provided by a bit of positive-feedback as discussed on its datasheet.

Whew, glad to help you. :D
 
thanx.....

Oh, dear, I must say that there are a lot of things I should have paid more attention to...

Thanks for the tips, I also have a few enquiries about your solutions..

1.First of all, can I have somekind of drawing for your final suggestion..??It seems to me that u 've stated more than one solutions and I am a little confused..


2.The whole project's idea is to extract (the maximum possible) amplitude information from an input (audio) signal.
It does not require the use of an electret mic within a drum and certainly won't be bothered by extreme mic overload problems (it involves low level sound, but of certain envelope characteristics).

The input signal is of variable amplitude and frequency, according to player's style etc..

...and I am trying to use it for a general type of sounds, that is "impulse" type sounds.

In simple words, it "analyses" the envelope/amplitude information of an acoustic signal and exctracts info related to the signals strength and time of occurence, not duration (WHEN it happened, not how much it lasted).

All the above answer your questions made at point "6",I believe..Feel free to pose anymore if u want.

3.Any other suggestions for a COMPARATOR better suited than LM311..??

For this project, a comparator should NOT have those weaknesses u've stated LM311 has.

Keep in mind that one of the inputs has a "voltage biaser" so that no triggering of pulses occurs when no input exists,

while both inputs change during time; so it is a bit hard for differential voltage to be high - so better voltage gain characteristics (i.e 200,000) (and build in hysterisis) would be essential for this project's comparator...Suggestions anyone..??

Many thanks in advance,

xmat.
 
Hi Xmat,
I am sorry but I won't be sketching a revised schematic for you. Just use a simple low-noise opamp (TL071) preamp circuit instead of the transistor.
The LM311 is a high speed comparator and is not happy with low frequencies. It is much more stable with hysteresis but then its sensitivity is ruined. An opamp used as a comparator will be better for your application.
 
thank you.

Hello, there..

It's ok that u cannot do a revised schematic.

Thank u very much for the advices.

I'll try them and I 'll bet things will get better.

Keep up the great work around here,

I may ask some more things in the future..

Xmat.
 
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