LA3220 Application

[Evgeny]

Hello!
I have a LA3220 chip which I would like to connect to an electret mic. Right now the microphone is being powered through a classic connection (ie, power supply with a resistor and capacitor), and the rest according to the typical application circuit as found in the datasheet. The problem is, as soon as I turn the power on, if there is enough amplification to hear something on a standard 8ohm speaker, a whole bunch of internal noise is generated by the circuit, and nothing really works. The same also happened with some LAG668 and LAG665 chips I have.
Can anyone please suggest a fix to this problem?
Thanks in advance,

-Evgeny Ternovsky
Marc Garneau Collegiate Institute

 

[Roboticinfo]

reply

What are you trying to do?

I am talking about what type of project are you doing?

 

[Evgeny]

Thanks for the very quick reply!
For now, I'm just trying to connect the microphone to a set of earphones so that you can hear a sound gathered by the mic in the earphone. Eventually, I want to try some sound manipulation with the signal from the mic, but for now, I have to get the mic to work in general.
Thanks,

-Evgeny Ternovsky
Marc Garneau Collegiate Institute

 

[audioguru]

The noise you hear from the speaker might be acoustical feedback.
If the microphone can hear the speaker, the mic's output is amplified, makes an output from the speaker which is heard again by the mic and the sound goes around and around, howling or squealing.

If your earphones are completely sealed, the mic won't be able to hear them and the feedback noise won't occur.

You say the LA3220 is driving an 8 ohm speaker. It is a preamp not a power amp so has a minimum load rating of 680 ohms, not 8. It is severely overloaded by the speaker and making very bad distortion noise.

You don't describe the noise so it might be mains hum or buzz caused by not using a shielded cable for the mic.

 

[Evgeny]

The noise you hear from the speaker might be acoustical feedback.
If the microphone can hear the speaker, the mic's output is amplified, makes an output from the speaker which is heard again by the mic and the sound goes around and around, howling or squealing.

If your earphones are completely sealed, the mic won't be able to hear them and the feedback noise won't occur.

You say the LA3220 is driving an 8 ohm speaker. It is a preamp not a power amp so has a minimum load rating of 680 ohms, not 8. It is severely overloaded by the speaker and making very bad distortion noise.

You don't describe the noise so it might be mains hum or buzz caused by not using a shielded cable for the mic.

Thanks for the suggestions. I have gone through most of them:
- Feedback is not a concern as a) the mic is too far to pick up anything from the headphones and b) the noise exists even if I just have the amplifier connected to the earphones without the mics connected.
- I understand the concern in regards to driving the 8 ohm speaker, and its understandable with the 3220, but the LAG668s should not have this problem - their circuitry contains both a pre and a power amp stage, both of which I used according to the typical application schematic, and yet, the nosie precists.
- The noise I'm getting is sort of like you're underwater: you can hear whats outside, and you can make sounds out, but everything is much more muted and only mid-range frequencies are picked up.
Do you have any further suggestions based on this information?
Also, is the chip itself the problem, in terms of not being specifically designed for usage with microphones? If so, what chips can you recommend that will get the job done?
Thanks again,

-Evgeny Ternovksy
Marc Garneau Collegiate Institute

 

[audioguru]

Hi Evgeny,
The application schematics in the datasheets show tape-head preamp/equalizers (NAB), not microphone preamps. A tape head equalizer has a lot of bass-boost that might be causing your "underwater" sound.

Please post which schematic you used (there are a few in the LA3220 datasheet ) and we can tell you which parts to change to make it a mic preamp.

 

[Evgeny]

Hi Evgeny,
The application schematics in the datasheets show tape-head preamp/equalizers (NAB), not microphone preamps. A tape head equalizer has a lot of bass-boost that might be causing your "underwater" sound.

Please post which schematic you used (there are a few in the LA3220 datasheet ) and we can tell you which parts to change to make it a mic preamp.

Hello again!
For the 3220, I'm using the configuration labled "Variable Monitor System" on page 3 of https://www.alldatasheet.com/datasheet-pdf/view/SANYO/LA3220.html. For the 668s, I'm using the only application diagram (the one on page 4) of https://www.alldatasheet.com/datasheet-pdf/view/ETC/LAG668.html.
The only differences between the refernce and my schematics is the way the mics are connected (the pin numbers are in reference to the above datasheets):
**broken link removed**
The reason I chose both chips as ones to experiment with was that both were found in a circuit from an old cassette player which included a voice recorder. The mics are the original electrets, if it matters.
Thanks yet again,

-Evgeny Ternovsky
Marc Garneau Collegiate Institute

 

[audioguru]

Hi Evgeny,
You don't understand that the LA3220 has NAB tape-head equalization that is not suitable for a mic preamp. You need to modify the circuit for flat response to be a good mic preamp.

 

[audioguru]

I have modified the right channel for flat response for a microphone:

 

[ljcox]

I would remove pots P2 & P3 from your circuit. They are not necessary.

If the microphone is overloading the amp, you can reduce the gain of the amp.

I have not studied the LA3220, but the data sheet for the LAG 668 page 4 shows that the gain can be set by R1 and/or R2.

C and R3 are not necessary.

There is an error in the formula for GVL on page 4.

It should be GVL = GV + 20log{R2/(R1 + R2)}.

From the Bode plot on page 4, GV = 38 dB approx.

So if R1 = 33k and R2 = 5k, GVL = 38 + 20log{5/(33 + 5)} = 20 dB as in the Bode plot.

Len

 

[Evgeny]

Thank you both guys!
Audioguru, the circuit you provided me with did work well after some minor adjustments, in particular using 10uF instead of 1uF for a much cleaner sound. I just wonder though, my design for a mic connection was approximately the same (I did disconnect the feedback chain beforehand), so was it the resistor values I used that were off? Really weird.
Anyways, thank you both for the replies! I'll do a bit of an update once everything else is done.

-Evgeny

 

[josedad]

I`m a novice here. I found this Post after 12 years been posted. I try to make pre-amp mic for PC recording. But so far still doesn't work. Perhaps audioguru could help me out. I attach my redraw interpretation base on the schematic above. Please help me, is that correct?

 

[josedad]

go to new thread
www.electro-tech-online.com/threads/la3220-schematic-electret-pre-amp-need-help.152381/

 

[JimB]

I am locking this old thread so that replies to josedad are not posted here.

JimB