LM3886 2 x mono to 1 x bridged Wiring

Moze

Member
2012-12-02 11:40 pm
I am new to Chip Amps and to get started I purchased an Amazon kit that includes a rectifier board (no transformer) and 2 x mono LM3886 amp boards. I would like to configure them to run in a bridged config and am wondering if this can be done without modifying the boards. The description states they can be run in parallel Here is an attachment of the wiring diagram and boards.
 

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tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
The LM3886 is not the best candidate for a bridged amp as its output current is limited to 7 A. This means it can drive an 8 Ω load, bridged, from a ±28 V supply. If you need to drive a 4 Ω load, you need to reduce the supply voltage to ±14 V to stay within the 7 A limit and any advantage of bridging quickly goes out the window.

Now, some will argue that an LM3886 under typical conditions (so an average IC at room temperature) can deliver up to 11 A, but it is only specified to deliver 7 A under worst-case conditions.

Some again will argue that they run their LM3886es bridged and they "work just fine", but they're probably only running a few W into the speakers, so why bother bridging in the first place? :)

You need to invert the phase of one channel to bridge two amp channels. You can't do that with the circuit you have without making some changes to the circuit.

In theory, you should be able to run two channels in parallel, however, you may find the performance somewhat lacking. The two LM3886es tend to fight (i.e. a standing current is developed through the two 0.1 Ω ballasting resistors). Under worst case conditions, you can get up to 100 mA flowing. This reduces the amount of current available for the load and also results in a significant degradation of the THD. You get better results by running the two channels of an LM4780 in parallel as the matching between channels is tighter for the LM4780 than for two LM3886 samples.

In your case, I'd build a ±28 V power supply and use one board per channel. No bridging. No parallel.

Tom
 
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Moze

Member
2012-12-02 11:40 pm
The LM3886 is not the best candidate for a bridged amp as its output current is limited to 7 A. This means it can drive an 8 Ω load, bridged, from a ±28 V supply. If you need to drive a 4 Ω load, you need to reduce the supply voltage to ±14 V to stay within the 7 A limit and any advantage of bridging quickly goes out the window.

Now, some will argue that an LM3886 under typical conditions (so an average IC at room temperature) can deliver up to 11 A, but it is only specified to deliver 7 A under worst-case conditions.

Some again will argue that they run their LM3886es bridged and they "work just fine", but they're probably only running a few W into the speakers, so why bother bridging in the first place? :)

You need to invert the phase of one channel to bridge two amp channels. You can't do that with the circuit you have without making some changes to the circuit.

In theory, you should be able to run two channels in parallel, however, you may find the performance somewhat lacking. The two LM3886es tend to fight (i.e. a standing current is developed through the two 0.1 Ω ballasting resistors). Under worst case conditions, you can get up to 100 mA flowing. This reduces the amount of current available for the load and also results in a significant degradation of the THD. You get better results by running the two channels of an LM4780 in parallel as the matching between channels is tighter for the LM4780 than for two LM3886 samples.

In your case, I'd build a ±28 V power supply and use one board per channel. No bridging. No parallel.

Tom

Thanjs Tom as a temporary solution is this the propr way to parallell my boards:

Preamp out to amplifier signal in to both boards, wire the positive speaker terminal to the lm3886 speaker out and the negative speaker terminal to both amps ground terminal? Obviously also send my VCC and VEE signals to each board...I am temporarily using a centre tapped 25v transformer.

Also I see many kits for 150w LM3886 kits with 3 x LMs but do you know anyone in the states who sells 2 x LMs for mono output boards?
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
If you want to put your boards in parallel, the inputs go together in parallel and the outputs go together in parallel. Plus to plus. Minus to minus. That's all.

What you're describing above sounds more like an attempt to bridge the two boards. An attempt that'll end in blown fuses and spectacular fireworks, may I add... :)

2x25 V with a centre tap is perfect for a paralleled set of LM3886es. That should give you about ±35 V rectified.

Tom
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
Also I see many kits for 150w LM3886 kits with 3 x LMs but do you know anyone in the states who sells 2 x LMs for mono output boards?

I sell a Parallel-86 which uses the two channels of an LM4780 in parallel. It also uses a precision opamp to provide error correction on the power amp, thereby achieving vanishingly low THD. I'm currently out of boards but will have more available by the end of the month. I will also shortly have a supply of LM4780 ICs as these are getting harder to find. I'll be happy to ship to the US. The shipping cost is about $8 and takes about a week in most cases.

You can read more about the Parallel-86 here: Neurochrome: Parallel-86.

Tom
 

Moze

Member
2012-12-02 11:40 pm
If you want to put your boards in parallel, the inputs go together in parallel and the outputs go together in parallel. Plus to plus. Minus to minus. That's all.

What you're describing above sounds more like an attempt to bridge the two boards. An attempt that'll end in blown fuses and spectacular fireworks, may I add... :)

2x25 V with a centre tap is perfect for a paralleled set of LM3886es. That should give you about ±35 V rectified.

Tom

I may take you up on the boards...in your first response you said you think that it is possible to parallel the boards I have but it would not be optimal...to clarify, the boards that I have, so I understand would just be plus to plus minus to minus to put them in parallel? Thanks for your help
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
Lemme clarify using the terminology from the schematic. To put two of these boards in parallel, connect the following:

PW-1 (board 1) to PW-1 (board 2). These both go to +28 V.
PW-2 (board 1) to PW-2 (board 2). These both go to GND.
PW-3 (board 1) to PW-3 (board 2). These both go to -28 V.

IN-1 (board 1) to IN-1 (board 2). These both go to the input signal (hot).
IN-2 (board 1) to IN-2 (board 2). These both go to the input signal (GND).

OUT-1 (board 1) to OUT-1 (board 2). These both go to speaker (+).
OUT-2 (board 1) to OUT-2 (board 2). These both go to speaker (-).

The vendor of these boards should be answer these questions just as easily as I just did, by the way... :)

One note: That circuit will have issues with oscillation as the output signal approaches the power supply rails. It also needs a snubber/Zobel network from the output to GND and a Thiele network (L || R) from the output to the speaker connector. In particular for parallel operation, I would definitely include these components. You can find the proper values in the LM3886 data sheet.

Tom
 
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