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PostPosted: 30 Nov 2010, 21:12 
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Joined: 28 Oct 2010, 00:11
Posts: 26
In the LM3886 circuit diagram below Cs connected to V- shows + on the ground side.
I'm a bit confused by this? Perhaps it's because I'm a tad confused
by V+ and V- to begin with. I did manage to build the power supply
circuit and it's showing about 57v DC between V+ and V- from an EI
transformer that's putting out 20v - 0 - 20v. Can someone describe what's
actually going on here?

http://diyaudioprojects.com/Chip/LM3886 ... 886_CA.htm

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PostPosted: 30 Nov 2010, 22:43 
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Joined: 24 Jun 2010, 00:48
Posts: 788
Location: Christchurch, NZ
Hi,

The connection is correct. The negative (-) supply is connected to the negative leg of Cs. If you reverse the polarity, you'll get a blown cap. :hot:

V+ and V- is a dual supply which means that half the voltage is measured when referenced to ground. Using your voltmeter under Volts reading, connect the negative probe to the ground, and positive probe to V+. You should read (57/2 = 28.5V DC).

Similarly, connect the positive voltmeter probe to the ground, and negative probe to V-. You should get 28.5V DC.

The advantage of dual power supply is that in most cases, no output capacitor is needed (as can be seen from the diagram). This can be a blessing and a curse, as if the chip fails and DC is present on the output, the speakers will be fried.

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PostPosted: 01 Dec 2010, 18:32 
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Joined: 28 Oct 2010, 00:11
Posts: 26
Thanks for the response.

I went out and double checked my pre rectified AC and post rectified DC readings.

From the EI transformer I'm seeing 39.4v between the two outer taps
and 19.7v between either of the outer taps and the center tap.

Taking readings from the taps to ground I get
Right tap: 23v, Center tap: 14.7v, Left tap: 6v.
Not sure if those values are worthwhile at all.

When I move to the rectified DC output I see 52v between V+ and V-
If I measure between V+ and ground I see 19.1v, V- and ground
shows -19.1v.

There's roughly 7v missing if I should be seeing 26v between V+ and ground.

Note that between the transformer and the DC circuit, I connected the
left and right taps to the bridge rectifier as shown. I wasn't exactly
sure what to do with the center tap as it's not exactly shown in the
diagram, so I connected it directly to the chassis ground, which is
connected to the ground pin on my mains plug.

Any suggestions on where my 7v has gone? Could the chassis/ground
be floating at 7v hence accounting for the differential when measuring V+
against ground?

Thanks in advance for all advice.


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PostPosted: 01 Dec 2010, 22:18 
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Joined: 24 Jun 2010, 00:48
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Location: Christchurch, NZ
skatefriday wrote:
Taking readings from the taps to ground I get
Right tap: 23v, Center tap: 14.7v, Left tap: 6v.
Not sure if those values are worthwhile at all.

Something isin't right. Do not proceed yet, could be risky.
Can you post a picture?

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Dinesh

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PostPosted: 02 Dec 2010, 13:34 
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Joined: 28 May 2008, 21:53
Posts: 4591
Location: Winnipeg, CANADA
Yes, something is wrong. Also, before hooking up these sorts of amplifiers to speakers make sure to check that the DC offset.
Cheers

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