Bridged/Parallel LM3886

[cloud85]

Hi all,

I have a question on paralleling LM3886.

I bought a Kit from a local store. It comes with 4 LM3886. There is no schematic, just the board and parts(Caps, resistors and enclosure) . I was told by the vendor that the chips are configured in bridged mode(One LM3886 for L+, one LM3886 for L- and the same for R channel). Hence its pushing 68W x 2 = 136W into each Speaker.

I find that the system tends to cause distortion when i crank up the volume too much. I suspect that this is due to insufficient power. My speakers are a pair of 4ohms speakers.

As such i plan to buy another set of the circuit from the vendor and run it in bridge/parallel mode. My proposed setup will be one enclosure per channel.

I have attached a drawn schematic. Can the experts here point out to me if what i proposed is ok to go?

Regards,

[panson_hk]

The app note AN-1192 is a must read before you take action. It describes bridge, parallel and bridge/parallel configurations. I guess your kit follows the BA100 configuration. You should be able to verify it.

When you say distortion occurring in high volume, is it not caused by overloading the speaker? Insufficient power can be due to the power supply. Do you know how big the transformer is? How much reservoir cap is there? You can measure the supply rails to see how much sag occurs at high volume. If you can take a picture, we can estimate whether your power supply is sufficient or not.

[cloud85]

Thanks for the reply!

I have read through the AN-1192 and i am quite sure that the KIT is in BA100 configuration.
As such, i am asking if i can make use of my existing setup to create BPA200 by using another kit.

I don't think i am overloading my speaker as my dynaudio audience 82 power handling is 300W@ 4 ohms.

My reservoir caps is 60,000uF for each channel. Trans is 300VA

[panson_hk]

If your kit is BA100, it should be possible to convert it to BPA200 with an additional board.

Is DC servo employed in your kit? To do parallel, you need to make sure the gains of 3886s closely matched. I do it with multi-turn trim pot instead of expensive 0.1% resistors. DC offset of each 3886 is also trimmed if no DC servo is used.

[cloud85]

Thanks Panson!

my kit is this Lite Audio kit,
LM3886 IC???|????|LITE AUDIO ???? ????????????? - Powered by MvM mall

There is no DC servo employed.

Maybe i can hand match the resistors to ensure close gains.
Can I employ a pair of air gaped audio trans to parallel it?

[panson_hk]

The board of the link is a stereo, one 3886 per channel board. This is not the four 3886 board you mentioned in your first post. The page mentions bridge mode is possible by using two such boards.

Can you illustrate how do you parallel them by a transformer? Two 3886 on primarily side and secondary side as output?

[alexcp]

You're getting distortion because in bridged mode, each 3886 sees only a 2 ohm load, for which 3886 is not specified. Most likely, internal protection shuts down the chip at signal peaks.

When paralleling, make sure that gains are matched (as panson_hk noted), dc offsets are minimal, and each 3886's output is connected to the load via a separate resistor; outputs must not connect directly to each other.

Have a look: PA100 DIY 2x LM3886 in parallel gainclone audio amplifier.

[AndrewT]

Originally Posted by cloud85

..... that the chips are configured in bridged mode.......................Hence its pushing 68W x 2 = 136W......... I suspect that this is due to insufficient power. My speakers are a pair of 4ohms..............

I suspect it is not lack of power nor too much power.
I expect that your amplifier is suffering from lack of current.
Chipamps are crippled by a poor current capability. Some will argue against my point of view, as did a Member just a couple of days ago.
This current non-capability gets even worse if the amp operates @ Tc>25degC.

I find that the system tends to cause distortion when i crank up the volume too much.

No, it is the operator that is the cause of the distortion.