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Old 23rd July 2011, 11:51 PM   #1
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Default Pre amping these LM3886 chip amp circuits

Following on....Well i still have abit to go on this one. I think i'll need to add a pre-amp with volumn control in front of the signal, match the impedance correctly. A soft start circuit maybe, anything else?

I have a little Topping T amp TP20, if matched the impendance from the high level output to the signal input of the chip amp with a couple resistors i could use that TP20 as a preamp, right? but i'll need to measure and calculate and balance loads, and this i'm not too familiar with.

Soft start circuits, fairly straight foward i'd think, but should that be on the primary side of the transformer, or the secondary?
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Old 23rd July 2011, 11:56 PM   #2
tomchr is offline tomchr  United States
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Quote:
Originally Posted by richard.C. View Post
Following on....Well i still have abit to go on this one. I think i'll need to add a pre-amp with volumn control in front of the signal, match the impedance correctly. A soft start circuit maybe, anything else?
Soft start circuits, fairly straight foward i'd think, but should that be on the primary side of the transformer, or the secondary?
I'm not familiar with the TP20 you mention. But an LME49710 with a motorized pot in front would make a killer preamp. Just add remote control...

The soft start circuit should go on the primary side.

~Tom
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Old 24th July 2011, 12:16 AM   #3
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The Topping TP20 is class T amp, a fun little amp, it produces about 10W into 8 load. I was thinking i could use this as a pre amp, with some impendance matching to the amp.
Anyway i'll consider your LME49710 suggestion, thanks.
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Old 24th July 2011, 05:47 PM   #4
AndrewT is offline AndrewT  Scotland
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Don't use a Power Amplifier as a line level Pre-amplifier.

If you need extra gain, you can add gain in the Pre-amp or add gain in the Power Amplifier.

Maybe, all you need is a 10k log law stereo potentiometer.
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Old 25th July 2011, 03:41 AM   #5
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Ok, i was thinking it would have been alright to use the TP20 with a large value resistors on the high levels to bring it to a low level. The main reason for this idea is to add a volume control, and knowing the chip amp may need some pre gain on the signal to make full use of it's power.
Whats your theory with the 10K Log potentiometer Andrew?
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Old 25th July 2011, 03:58 AM   #6
infinia is offline infinia  United States
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determine from your source/s the max voltage swing and it's output driving impedance and calculate the chip amp gain needed to drive full swing at the chip amps output depending on its power rails. you want to keep the source level maxed to keep a good S/N ratio and use a variable pot at the chip amp input. Depending on the source impedance and pot value you may have some gain reduction to contend with. In most cases a preamp with gain is unnecessary.
example / with a source output of 2 Volts p-p, a chip amp with 32 Vdc rails will need a gain of about 20log(1+29) note 60 Vpp using 2V of overhead for each rail before clipping into very light loads. if the source is 1K into a 10K pot you will lose 20log 10K/(1K+10K) or a little less than 1dB
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Last edited by infinia; 25th July 2011 at 04:14 AM.
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Old 25th July 2011, 04:36 AM   #7
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So basicaly i may not need any pre amplification for as long as the source signal is strong enough, thats a no brainer. And a potentiometer on the input side for impendance matching and volume control) between the source and and amp. Is this correct what i'm saying?
I mean at the end of the day the source signal may well be my laptop sound card, the laptop may have the control of, and volume to drive the amp, no?
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Old 25th July 2011, 04:47 AM   #8
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Just a note; I just did a quick measurment of some pink noise thru my laptop 3.5mm headphone jack, at full volume i got 0.150VAC.
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Old 25th July 2011, 05:12 AM   #9
infinia is offline infinia  United States
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I suggest a tone within range and tolerence of your AC measurment. convert RMS to Vpp
measure open circuit then add a load and calculate the Thevenin source impedance due to drop seen
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Last edited by infinia; 25th July 2011 at 05:17 AM.
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Old 25th July 2011, 05:47 AM   #10
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Quote:
Originally Posted by infinia View Post
I suggest a tone within range and tolerence of your AC measurment. convert RMS to Vpp
measure open circuit then add a load and calculate the Thevenin source impedance due to drop seen
I must add i am a beginner to the math and building of amps, so alot of this i can interpret, but may not be correctly. So please bare with me.
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