How much gain setting is too much ?

Thread Starter

Externet

Joined Nov 29, 2005
1,519
Hi all.
Trying to feed a dynamic microphone on differential mode with no preamplification to a 8002 power amplifier chip, I have doubts which gain setting resistors are to be tailored and the proper way to manage the chip gain.

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I can only guess the 32Ω microphone level inputted differentially at 'Audio in', perhaps 1mV.
Goal is to get maximum stable gain to output ~ 0.5W for a RL = 4Ω. Supply Vdd is 3V. Voice range only.

The internal power amplifier for Vout2 has a fixed gain of 1; only inverts the Vout1 signal.
Do both input resistors at INN and INP have to be same value ?
Does the resistor at BP and the Rf at top have to be the same value ?
Any considerations I should be aware of ?


1593341459818.png
 

crutschow

Joined Mar 14, 2008
25,132
Do both input resistors at INN and INP have to be same value ?
Does the resistor at BP and the Rf at top have to be the same value ?
Yes and yes.
That's necessary to give input common-mode noise rejection.
I can only guess the 32Ω microphone level inputted differentially at 'Audio in', perhaps 1mV.
To give 2Vrms output for the desired 0.5W into 4Ω with a 1mVrms signals means you would need a gain of 2000, which requires that ratio between the feedback and input resistors.
I suspect the amp will have difficulty generating that much gain.
You may need to add a mic amp.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,519
Thank you, crutschow. Very helpful.
Being that chip an output power amplifier, seems very limited in its top gain setting to about 6 but saw no figures on its data sheet.

If you can suggest/point to a 3V balanced or unbalanced dynamic microphone circuit with a gain that can perform feeding a 8002 chip, it will be very appreciated. The preamplifier circuit I built is still too low in gain.
 

MisterBill2

Joined Jan 23, 2018
6,359
The limits on usable gain, if getting into the non-linear operation of any stage of an amplifier, are mostly noise, feedback, and DC offset. DC offset is the simple problem to avoid by using AC coupling. Noise is inherent in most components and wiring, and the problem there is that noise gets amplified along with the desired signal. Feedback becomes a problem when there is enough gain to amplify the reverse transfer signal to the level where it interferes with operation. In this application it seems that both noise and feedback may be problems.
The amplifier IC is intended to work with some range of input signal at some maximum amount of gain. Operation outside of the area may not be similar to what is claimed for operation within the specified limits. There are very valid reasons why manufacturers publish limits and only claim performance within those limits.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,519

Audioguru again

Joined Oct 21, 2019
1,602
The datasheet for the 8002 Chinese low power amplifier shows pretty bad distortion of 1% with a 1.5W output into 8 ohms when its gain is only 1. If you increase the gain then you increase the distortion.

The 1.5W into 8 ohms is a voltage of 3.5V RMS. A microphone 10cm away from your mouth produces 10mV when to talk at a conversation level. Then you need a preamp with a gain of 3.5V/10mV= 350 times.

Many preamps have a gain of only 100 times so you can increase the gain of the 8002 amplifier to 3.5 times but with much higher distortion. You also need some extra gain that is needed when sounds at the mic are at lower levels than normal.

Hee, hee. The preamp circuit posted uses an antique uA739 opamp that has not been made for many years.
The 8002 Chinese low power amplifier IC is also probably very old.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,519
Am happy with 1% distortion for the intended application. Unaware of a modern 8002 equivalent, that I can find with no problem in a 'breakout' board. The dynamic microphone will be 2cm from lips.
Is your "350 times " gain correct ?
 

Audioguru again

Joined Oct 21, 2019
1,602
When you "eat" the mic then it is much louder than at a distance of 10cm. Then if the gain of the 8002 amplifier is 1 the preamp gain can be 78 times but it is best to have more gain and a volume control.
 

MisterBill2

Joined Jan 23, 2018
6,359
Good information, thank you.
I ordered these, will find out how much I goofed :

----> https://www.ebay.com/itm/SSM2167-DC-3V-5V-Microphone-Preamplifier-Board-Low-Noise-COMP-Compression-Module/153515828610?hash=item23be41a582:g:JeEAAOSwEYJc-IJY

With tired eyes and unsteady hands, having it modular makes a lot of difference. Unsure if my soldering tip is gaining weight daily or the components are getting way too small...
The one construction project that I have seen that uses this IC has about 5 adjustments, each of them sort of important. The board in the listing shows NO ADJUSTMENTS at all. So I question the value of it.
 

Papabravo

Joined Feb 24, 2006
13,967
Rule of thumb for gain setting on a single stage is about 20-30 dB. Outside that range there are a couple of problems including power supply voltages, stability, distortion and so forth. You don't always get the best results with maximum gain - even with weak signals.
 

MisterBill2

Joined Jan 23, 2018
6,359
Rule of thumb for gain setting on a single stage is about 20-30 dB. Outside that range there are a couple of problems including power supply voltages, stability, distortion and so forth. You don't always get the best results with maximum gain - even with weak signals.
The IC type shown is also a high performance level compressor with a number of special functions.
 
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