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Chip Amps Amplifiers based on integrated circuits

Building LM3886  Amplifiers
Building LM3886  Amplifiers
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Old 2nd August 2020, 11:58 AM   #61
Sangram is online now Sangram  India
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Building LM3886  Amplifiers
Vu meters are designed to read only voltage. Maybe you should read the post again. The reference 0dBu is always computed as 0.775V into 600 ohms. That is what the vu meter is for, remember it originated in professional equipment and was adapted for use in domestic applications by the Japanese.

Because power amplifiers do not have a constant load - even 8 ohms load is not really 8 ohms - you cannot ever expect correct or close to correct reading from it.
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Old 2nd August 2020, 01:44 PM   #62
AudioTag is offline AudioTag  Portugal
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Default Calibrating

Greetings Pibull and Sangram.

First of all i want to thank you both for the efforts and the explanation for helping me calibrate the vu meters, which i find a bit less user friendly to do because i have not the adequate skills and knowledge to be comfortable enough to don’t damage the amplifier.
The way i make them all equal is finding a good amount of volume, 50 % i can handle, put a 60 Hrtz signal input and calibrate them manually do be at 90 degrees, that way, they all become equal but not accurate in db levels, I’m sure.
After all this is my first attempt of building such devices and i did not burned any component so far ;D.

Last edited by AudioTag; 2nd August 2020 at 01:49 PM.
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Old 2nd August 2020, 01:47 PM   #63
AudioTag is offline AudioTag  Portugal
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Originally Posted by pitbul View Post
Huh, I thought on direct current pass on input of your low/high signal.
As I can see it is probably DC blocked, so we can't do simple adjust by internal power supply of the amplifier with simple connection to power supply rail. If it is not DC blocked, if you connect input wire to positive rail it will show something and it will remain on some value. After that adjust the value between +3 and +6dB.
Your VU should show 0dB when you reach full RMS power. The VU meter works by input voltage, so you need to put peak voltage to the input and adjust the level accordingly.
You have your reference GND which is common for both channels (maybe best place to take single GND from power supply board for the VU board). If I understood correctly you have separate transformer for supply of the VU board, which is good, so you can't mess up with grounds.
Amplifier is capable of delivering peak power which is +3dB, when it is? when amp output reaches near maximum of your + or - V of your amplifier power supply.
Short time connection to positive or negative rail voltage should simulate pulse (or some ac signal) on your input which will be shown by VU, needle should go between +3dB and +6dB, adjust your potentiometer to show value between these numbers (better closer to +3dB).
Second option; I assume that you don't have dummy resistor of 8ohm/100W, so I don recommend adjustment by sinus signal below 400Hz and measuring RMS signal on output according to the supply voltage (when you do that way you should calculate maximum RMS voltage, positive rail voltage measured by voltmeter-approximately 2V and then divided by square root of 2, then you have RMS value which is equal your RMS power which is possible to measure by the measuring instrument-DVM). For this last task you should have audio generator (or android phone with app) and enough mV (or 1V) on output of your preamp, to achieve maximum power on output of your amp, when you achieve this, you can adjust your VU to 0dB. This measurement shouldn't be long, because amp is not generator for delivering energy, 15-30 seconds what is enough to adjust VU per channel.
Hope that first option isn't complicated, so with short connection of VU high input wire to positive or negative rail of power amp supply will help to adjust the VU. Do that channel by channel.
Applying 35V dc directly to the high input of the vu meter will not blow it up ?
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Old 3rd August 2020, 06:04 AM   #64
pitbul is offline pitbul
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I will ask you something, because I don't have that board at home, will the output of the amp with let's say full power blow it (it also have swing of voltage between +30V and -30V)? I calculated this numbers with 10% ripple and 2V dropout on each transistor in the chips.
I wrote in my previous post that you should try with (very) short time connection to + rail (or -, whatever), if it is AC coupled. If you see that needle is at full scale, reduce the pot to lowest value. If that doesn't help, put at input another divider (potentiometer, or trimmer, value about 10kohm) and try to adjust what was described.
If it is DC coupled, then it function as DC voltmeter, so you can leave connected and adjust value.
Why I am not for Sangram option, when you will listen at 88dB, and your speakers are 88dB/W sensitive, you will have only 1W at output of your amp (or maybe 0,5W per channel), so when you decide to listen louder your VU will go to max of scale...
If you like to see needle to swing on lower power, you can do some switch with another attenuator for hi-power listening. That option I have on my amp, scale with x1 and x0,1. Latter value are for listening at low power and my scale is at 20W at 8ohm load. If it is at full range then I have 200W at 8ohm load. Calculate for practice what power I have on 4ohm load.
Load on output is only for better calibrating, you can do without load but you will achieve higher value of output voltage and you will lose "accuracy", then you can adjust to let say +6dB (or close to it) on scale.
As I wrote, +3dB means that you have music power from your amp, 0dB means full RMS power reached. It is simple description, when you understand that it will not be something scientific.
This amp has about 60-70W RMS on 8 ohm load with power rail +/-35V.
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