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Dual LM3886 Bridged Amp.


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Has anyone bumped into amplifier that is built from 2 LM 3886 Power amp IC's and can be Bridged? I'm making schematics and PCB (again) because old version got lost... actually i'm gonna make even that double (4ch. amp. 2ch. Bridged) :P Is there need for such amp for anyone? i'll put project online if someone is interested, or isn't found already from web.

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ZomBiE,
The LM3886 can be bridged to give typically 136W of low-distortion wide-band power into 8 ohms. You should build it, layout a PCB and post it in our projects section.
Do you have access to a low-distortion generator and distortion analyser? It will be important for you to analyse the amplifier's results since its PCB layout is an art at keeping the high currents from interfering with its inputs.

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Stuee,
Two LM3886 amp chips can be bridged to drive an 8 ohm load. It will current and SOA limit if it was bridge-driving a 4 ohm load.
Most car speakers are 4 ohms, so this application is not suitable for a car amp.
Besides, a car amp is lucky to provide at most 20W of real RMS, low distortion and wideband power when bridged into a 4 ohm load, without using a step-up supply voltage converter that all the big car amps use.
When bridged and driving 136W into an 8 ohm load, the LM3886s use a supply voltage of 56V, but when bridged can use a single-ended supply without using output capacitors.

Hotwaterwizard,
Bridged amps produce only slightly more distortion. The only difference is that the amp's output current is doubled, since an 8 ohm load "appears" to be 4 ohms to each amp in the bridge. See on the data sheet where typically 0.002 percent distortion at 8 ohms rises to only about 0.004 percent at 4 ohms, at 1KHz when using +, - 28V supply. Sure, the distortion doubles, but look how low those numbers are.

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Most car speakers are 4 ohms, so this application is not suitable for a car amp.


I would assume that anyone upgrading a car stereo for better sound quality or loudness is going to also install a new speaker system. In this case, you would not have to worry about what is standard in a car.

Also, I wanted to point out that there is really no longer a standard. Old cars had 4 ohm as a standard, but it seems that anything is possible with newer cars. I have seen lots of variation with ohm ratings between 4 ohm to 20 ohm, etc. I have even seen fractional ratings on some of the newer car speakers such as 12.6 ohm or similar. It depends upon who is making the sound equipment for the manufacturer. If you are upgrading, pull the speakers.

MP
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The speakers i have are sony xplod speakers, 2x 6" 2x 6"x9" and 2 12" subs.
I currently have 2x 4 chan amps only 30w rms
Is there any way to change a transistor or something to give it a little more power, like 50w rms?

also all my speakers are 8ohm, wish they was 4, would get more power from the amp :D

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Stuee,
The 8 ohm car speakers are made for people who want to drive them from car-factory radios, or to parallel them and drive them from a real amp.
I replaced the 20 ohm factory speakers (with magnets as small as a thumbnail) with 4 ohm 6X9s, and now my factory radio sounds very loud and clear. Reliability? My last car had the same setup running for 12 handed-me-down years with no problem, except that the car's lights dimmed a bit with the beat, and so did my ears.
8 ohm or 4 ohm? The difference is nearly twice the power, but it is only 3dB which sounds only a little bit louder.
I doubt if you would hear much difference btween 30W and 50W.
Your hearing is logarithmic, so 10 times the power sounds only twice as loud. Since the speaker's voice-coil heats and increases in resistance, it won't even produce twice the volume with 10 times the power.

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I think there is alot of difference between 30 watts and 50 watts. That is almost double! You can tell the difference between 5 watts and 20 watts so you can hear a big difference between 30 and 50 watts. The distortion of the amp is not the only concern when Bridging amps. They tend to run hotter as well. So the life expectancy of the IC will be Half of an Amp that has not been bridged.

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both my amps are rated 8ohm,
i have one amp running rear 6x9 300w, and the amp is already bridged ton the 2nd 2 chans for a sub, so i run my sub off that, the other amp is the same,
i would like to get a bit more power to my subs as they are in the boot and quiet, at the moment ive got the amp front set to pretty quiet so i can turn up the sub full..

Can i adjust or replace any ic or transistor in the amp just for that bit of extra power, so when i have the sub louder than the rear speakers they are bot a bit louder?
Thanks
And sorry ZomBiE for pinching your thread :D
mabey u might find this of use :D

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If the speaker is not in a box that was designed for the specs of the speaker it will not produce optimum sound or decibels. If you just buy a box off of the shelf and speakers off of the shelf they probably don't match. You need someone to do a test on the box to see what frequency it is tuned to. Also you need the tru specs of the speakers. The VAS QTS and Free Air Resonance among other specs. Then Use The Theal-Small Theroy to figure out the box. Also you need to find out if it is a Long throw or a Short Throw Cone. That will determine if the box needs to me a Ported or Air Suspension Box. It gets Real Technical Real Quick! They have Box Calculators Online For all of the Figuring if your math skills are not that good.

The Speaker Box is just as Important as the Amplifier when it comes to sound reproduction.

???

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Hotwaterwizard,
Of course you can tell the difference between 5W and 20W, it is 4 times the power and a 6dB difference. But from 30W to 50W is a very small loudness change, only a little more than 2dB.
You made a very good point about the speaker's box matching the speaker. If the match is poor or if the box leaks, then the bass response will be lacking or boomy, or any combination of the two (no low bass but boomy male voices, for example).
Another important speaker spec that affects loudness is its efficiency. A 96dB/1W/1m speaker will have the same loudness as a 90dB/1W/1m speaker that has an amplifier that has 4 times the power.

Stuee,
You can't simply replace ICs or transistors in an amp to make it a bit louder (twice the power), but box matching, speaker placement (the sub in the boot (trunk?) is blocked by the rear seats) and speaker and amp wiring may make a much bigger difference. Maybe your amps are really only 20W RMS at low distortion, but are rated at 30W with 10 percent distortion (bad clipping). If your bridged amp is feeding 40 real Watts into your sub, then it would make better loudness with a 400W amp.

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What formula are you using to figure the db difference between 30 watts and 50 watts. I figure that 50 watts PEAK is about 35 watts RMS. So 50 watts RMS would be about 70 watts PEAK. Now lets do the same with 30 watts RMS. 30 watts RMS is about 42 watts PEAK. So if we look at RMS we have a 20 watt Gain and PEAK is a 28 watt gain. So there is a sliding scale to this mess as I figure it. Now Take 30 x .666 and you get about 50 Watts. That is about a 2/3 power gain. ???I just confused myself. ??? How do you figure such a small power gain.

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Hotwaterwizard,
Never mind talking about PEAK. Peak power (also called maximum power) is just a method for marketing-guys to get a number that is doubled, since the power at the peak of a sine-wave (1.414 times its RMS voltage) is twice (1.414 squared) as much as RMS power. That's why a square-wave, or a volume control that is turned way too high, has the same peak voltage as a sine-wave and therefore has twice as much power (the distortion has the same amount of power as the fundamental).
Decibels work in ratios, not "amount gained". The power formula is 10 times the log of the ratio. So going from 30W to 50W is about a 2.4dB increase. From 300W to 500W is also only a 2.4dB increase.

The proof that doubling the power is only a bit louder is at your local high-end stereo store. Demo an amp that has a "stepped-in-dB" volume control and count 3 steps for a 3dB increase (twice the power).

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Hotwaterwizard,
Yes, the disaster damage ratings are probably logarithmic just like your hearing's response to sound levels.
Your eyes' response to brightness is also logarithmic. I made 3 triangle-wave generators to feed their linear ramps to red, green and blue LEDs, similar to the circuit in our projects section. I was shocked to see that the difference in brightness from when the ramp was half-way to when the ramp was maximum did not appear to be much more. I redesigned the wave generators so that the output current was the input current squared, then it looked fine.

Back to sound, members in our forums are trying to use equal-steps digital pots as volume controls. Since these pots are not logarithmic, the steps at the top (loud) don't make much difference to the loudness and the steps at the bottom (quiet) jump with large coarse loudness changes.

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The digital pots work fine in real world electronics.

Audioguru, you have quite a habit of starting a new topic within the thread of a different topic. This thread is about LM3886. To go further on digital pots, you need to start a new topic. Otherwise I will have to chop up the posts again.

MP

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  • 3 years later...

This topic went off, again. But i remembered this, and thought that i'd put those plans online. BUT: When i searched my archives, i found that this particular CD-ROM was damaged.  >:(  I built my own amp and it has worked for about 3 years now, almost 24/7. It doesn't work well on 4ohm speakers but 8ohm speakers it ROCKS.  ;D I built it like i said at start of this topic: 4 channels single-ended, 2 channels when switched to bridged. It uses 4 pcs. LM3886 IC's, 2 per channel when bridged.

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Hi Zombie,
The peak current in the amplifiers is doubled with a 4 ohm load (compared to an 8 ohm load) and the current is doubled again when two amps are bridged. So the protection circuit in the LM3886 will drastically limit the power when bridged into a 4 ohm load. 


Like i didn't know that???
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