Audioguru's sound level indicator and gain control

Thread Starter

niconoe

Joined Oct 2, 2012
6
Hi,

I've just breadboarded the famous Sound Level Indicator from Audioguru (schematic here). This circuit provides automatic gain control.
I have a few questions about it:

1) All is working very well with quiet sounds, but I find it a little less helpful with louder signals (The 6-7 first leds are always on, so only 3 of them are really responding to the sound fluctuations). I'm not sure my circuit is 100% correct, especially in this "auto gain control" part.

When I play very quiet sounds, pin 6 of the 3915 is set to 0.51v, which is correct (I think). But when I play louder sounds, it seems to float around 2v. Shouldn't it be more, something like 5.1v ? That may explain this poor response with bigger signals.

2) As this circuit will be used in a disco (where sounds are probably way bigger than in my living room), I was thinking that it may be safer in my case to adapt the circuit to remove this automatic gain control and add some sensitivity potentiometer instead. How should do that on audioguru's schematic ? Can I simply drop Q3 and R14 and add a pot near R15 ? What value should I try here ? Should I also remove C9 ?

Thanks a lot in advance for helping noobs like me :)

Nicolas
 

Thread Starter

niconoe

Joined Oct 2, 2012
6
Also, this circuit used a dual opamp MC33172, which is rather hard to find where I live. Is there any equivalent (I currently use a LM358, but as Audioguru explain in the article, it doesn't work well above 4Khz).

Thanks !
 

Audioguru

Joined Dec 20, 2007
11,249
Simply remove R14, R15, C9 and Q3. Connect pin 6 to pin 7. Then the circuit will always be at maximum sensitivity.
I don't know if the electret mic will be overloaded in a disco.

Remove R6 and C2 then the 10th LED will light when the sound pressure is about 100dB.
But years ago I measured 120dB in a disco. Then you need to attenuate the signal to 1/10th by reducing the value of R8 to 10k.

I am just guessing about the levels because I don't have an accurate sound level meter anymore for calibration.

The MC33172 is made by ON Semi of United States of America and by ST Micro of Italy.

Farnell/Element14 sell electronic parts in many countries. Go to www.farnell.com and click on the flag of your country. For example, in New Zealand there are 7 DIP ICs in stock and 2506 more can be delivered in 6 - 7 working days.
 

Thread Starter

niconoe

Joined Oct 2, 2012
6
Thanks a lot for your answer, that's simply great and very helpful to have you here answering my noob questions !

Simply remove R14, R15, C9 and Q3. Connect pin 6 to pin 7. Then the circuit will always be at maximum sensitivity.
I don't know if the electret mic will be overloaded in a disco.
Thanks, will do !
Remove R6 and C2 then the 10th LED will light when the sound pressure is about 100dB.
I don't understand this in detail! To me, your original circuit is a non-inverting amplifier with a gain of 101, and some frequency filtering thanks to C2. But if I remove R6, am I not asking for an (almost) infinite gain (gain = 1 + (100k / 0)). Also why do we remove C6 ?

But years ago I measured 120dB in a disco. Then you need to attenuate the signal to 1/10th by reducing the value of R8 to 10k.
Thanks ! I plan to replace R8 by a 10K resistor in series with a 100k pot, I guess that would allow to configure the max signal between 100-120db, which is probably a good range here.

I am just guessing about the levels because I don't have an accurate sound level meter anymore for calibration.

The MC33172 is made by ON Semi of United States of America and by ST Micro of Italy.

Farnell/Element14 sell electronic parts in many countries. Go to www.farnell.com and click on the flag of your country. For example, in New Zealand there are 7 DIP ICs in stock and 2506 more can be delivered in 6 - 7 working days.
Thanks, I've been able to order this IC !
 

Audioguru

Joined Dec 20, 2007
11,249
I don't understand this in detail! To me, your original circuit is a non-inverting amplifier with a gain of 101, and some frequency filtering thanks to C2. But if I remove R6, am I not asking for an (almost) infinite gain (gain = 1 + (100k / 0)).
Without R6 then the opamp is a follower with a gain of 1. Replace R5 with a piece of wire if you want.

I plan to replace R8 by a 10K resistor in series with a 100k pot, I guess that would allow to configure the max signal between 100-120db, which is probably a good range here.
Good.

Thanks, I've been able to order this IC !
Good. Where are you in the world?
 

Thread Starter

niconoe

Joined Oct 2, 2012
6
Without R6 then the opamp is a follower with a gain of 1. Replace R5 with a piece of wire if you want.
Thanks, I have it now !


About the sensitivity pot., maybe I can have a little more flexibility here by keeping the second stage as is (1.8 gain), but by placing the potentiometer on the first preamp stage (in place of R5), to allow a wide range, such as 1-100 gain ? Or is it a bad idea for some reason ?


Good. Where are you in the world?
I'm in Belgium. It is available at Farnell, but the shipping costs are huge if I order this alone. So I found it simpler to buy it on Ebay !

Thanks again for your availability !
 

Audioguru

Joined Dec 20, 2007
11,249
You can adjust the gain with a pot for R5 if you want. It will be very difficult to adjust low gains if it goes as high as 100.
 

Thread Starter

niconoe

Joined Oct 2, 2012
6
You can adjust the gain with a pot for R5 if you want. It will be very difficult to adjust low gains if it goes as high as 100.
You're right, I'll set the max gain a little lower then, maybe with a 22K (or 47K) pot. Then I guess the best way to ensure the circuit is adapted to the sound pressure range there is to test it in conditions !
 
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