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2x LM3886 In Parallel - How To Add DC Offset Adjustment

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Ive recently changed from a home-theatre-in-a-box to a dedicated receiver, but the sub that came with the other speakers has to be powered by the receiver, and the home theatre receiver I have (as with 99% of other ones) has a sub pre-out, but not a power out. Therefore, I need to build an amp.

I have selected the popular Gainclone design, with 2 LM3886TF's in parallel. Ive found a commonly used schematic for a single chip, but I would like two in parallel, for that little bit more oomph. In some of the designs Ive seen (a lot are inverting amps aswell), people tend to have DC offset adjustment pots, to try to lower the DC offset at the speaker output. How would I add that to this board? Ill need to modify it slightly, with a 10k resistor to each stage (after R2), and a 1% 2W 0.2R resistor after each stage, before the Rz/Cz combination.



** By the way, the .1uF caps at the top and bottom on the supply lines are actually a 100uf and 0.1uF cap in parallel.

I plan on drawing up a proper schematic in Eagle, and making a PCB at school. If anyone has any tips in regards to that, Id appreciate it!

Also, for the power supply section, Im hoping to use (if I can find one) a torroid with 2x 25v secondaries (to give roughly +/-35v), between 220VA and 330VA, 8 MUR860 diodes (4 per rectifier), and among other parts, a 10,000uF 50v cap per rail.
 
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Nigel Goodwin

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You could simply bleed a little voltage to either the inverting or non-inverting inputs (resistors from each rail to a pot, slider to input via another resistor) - however, is it worth it?, the offset is probably as low as it needs to be anyway.
 

Mr RB

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You do know putting 2 in parallel wont make it any louder whatsoever... It will still make the same output voltage into the same speaker as a single amp.
 

Nigel Goodwin

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You do know putting 2 in parallel wont make it any louder whatsoever... It will still make the same output voltage into the same speaker as a single amp.
The whole point of paralleling them is to give double the power in half the impedance - where a single amp wouldn't be able to provide enough current.
 
Nigel, this is how Ive seen it done (R4 in the top-left):


As I said, the amp is going to drive a sub, so I wont need pristine quality throughout the frequency range; would you say its fine to just parallel them and not worry about the offset?
 
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crutschow

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If you need more voltage output you could possible configure the amps is a bridge configuration, each amp driving one of the speaker connections. One amp has to be configured to give an inverted signal out, of course.
 

Speakerguy

Active Member
You can use an op amp based servo to totally kill any dc at the output. Look at the additional app notes for the LM3886 for the circuit.
 

Speakerguy

Active Member
Is that a measured DC resistance of the sub? Or what is printed on the back?

If it's measured, then that is typical of a "4 ohm nominal" speaker - DCR is usually about ~3 ohms. I've measured some "4 ohm" subwoofers that dipped down to 2.7 ohms.

If it's what's printed on the back, then you should take a measurement to be sure. Actual DC resistance can be 25% less (or worse) than the stated resistance. In which case, you'd want to take that into account when designing the amp.
 
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Mr RB

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The whole point of paralleling them is to give double the power in half the impedance - where a single amp wouldn't be able to provide enough current.
I understand that. The OP clearly stated he has an existing sub woofer that needed an amp.

Are you saying that 2 paralleled amps attached to his 4 ohm sub are going to be louder than just one amp attached to his 4 ohm sub???

The LM3886 will work fine into 4 ohms;
LM3886 - High-Performance 68W Audio Power Amplifier with Mute

And like most of these new high perf amp chips it uses a lot of neg feedback and will produce a pretty accurate AC voltage output based on signal input. So 2 of them in parallel will produce the SAME accurate AC voltage out, into the SAME 4 ohm load, please explain how this will make it louder. I said "putting 2 amps in parallel wont make it louder"
:(
 

Nigel Goodwin

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I understand that. The OP clearly stated he has an existing sub woofer that needed an amp.
Sub-woofers are quite often lower than a single amp can feed, hence it's quite common to parallel.

Are you saying that 2 paralleled amps attached to his 4 ohm sub are going to be louder than just one amp attached to his 4 ohm sub???
No, I'm saying that paralleling two amps allows you to drive a speaker half the impedance of a single chip. It won't make any difference to the volume if the speaker is capable of been driven from a single chip (as I'm absolutely sure you know).

Which was why I asked what speaker he was feeding - which is still a little vague. If it is a 4 ohm speaker, then it's pretty pointless paralleling two amps.

What he could do though is use four, paralleled and bridged to give more power in four ohms :D
 
That's whats printed on the back. The measured resistance of the coil is ~3.6Ω... This is why I dont like off-brands like JVC. Not to mention the "rated power" of the subwoofer is 400w - there's no way the 22 gauge speaker cable that came with it, or the amp can drive that.

Anywho. Odds are its a 4Ω sub. From what Ive read, I wouldnt want to bridge the chips, as each chip sees half of the load, which is 2Ω. I would be best using four chips, in parallel-bridge mode, but at > $8 / chip, thats kind of expensive, and that means a more expensive PCB aswell. So whats the recommended course of action?
 
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Nigel Goodwin

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Anywho. Odds are its a 4Ω sub. From what Ive read, I wouldnt want to bridge the chips, as each chip sees half of the load, which is 2Ω. I would be best using four chips, in parallel-bridge mode, but at > $8 / chip, thats kind of expensive, and that means a more expensive PCB aswell. So whats the recommended course of action?
If it reads 3.6 ohms, it's basically a 4 ohm speaker, use just one chip to feed it. Make sure it's got a nice BIG heatsink, as sub-woofers needs lots of power.
 

bountyhunter

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Not sure how much power you are trying to get out of this thing, but paralleling seems like an uncommonly bad start to any amplifier design especially when you are driving a four ohm or even three Ohm speaker. Nearly any good quality steroe amp can drive three Ohms no problem and most put out significantly more power driving three ohms than eight for obvious reasons. Three ohm speaker impedance doesn't present any unique design challenge requiring extraordinary current drive.

Most of the audio ICs we made were good to driving 1.6 Ohms speaker loads for automotive use. There are lots of power audio ICs out there that can crank a three Ohm load, but probably limit at maybe 50 watt ballpark because of voltage contraints of IC fabrication.
 
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audioguru

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I think the JVC 400 Whats sub-woofer speaker will blow up if you feed it 68 Watts RMS.
It will probably make some odd sounds before it blows up.
Why not use a real sub-woofer speaker with real specs?
 
Im pretty sure it will be fine with 60w, I just found it laughable that they claimed it would handle 400w+.

If youre willing to hand me a nice JL sub, Ill be more than happy to use it instead, but Im trying to work with what I have on hand :)
 

bountyhunter

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Speaker power ratigs have always been pure fantasy, unrelated to actual power handling in use. I remember some woofer makers rated theirs at the power where the voice coil melts.
 

Nigel Goodwin

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Speaker power ratigs have always been pure fantasy, unrelated to actual power handling in use.
Decent quality speakers are accurately and sensibly rated - it's mostly only in-car amps and speakers where you get wildly exaggerated ratings. Oh, and computer speakers as well - with '1000W systems' working off a wallwart rated at 16V 750mA :p
 
^^ To narrow it down even further, the cheaper off-brands in car audio (JVC, Koss, Pyramid, etc) tend to really exaggerated (1500w peak, 150w rms) whereas the bigger names like JL and Rockford tend to have more sensible ratings.
 
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