LM3875 and Distortion

[Jax]

I doubt the 0.33uF was the culprit.

3886 has a ground and a mute pin. Have you connected these?

An 18 kohm resistor from V- to the mute pin is a good compromise.

[newbie1]

Originally posted by Jax
I doubt the 0.33uF was the culprit.

3886 has a ground and a mute pin. Have you connected these?

An 18 kohm resistor from V- to the mute pin is a good compromise.

This is not an LM3886 it's an LM3875 and it does not have a GND or mute pin. After playing the amp for a while (15 mins) the distortion slowly seeps back in, however, it is more faint now but still noticeable. At this point my circuit looks exactly the same as the NS typical circuit diagram with the exception of a 47uF electrolytic in the NFB loop instead of a 22uF.

[macboy]

I strongly suspect the grounding. Does your power supply truly have dual outputs (+30V and -30V in addtion to ground)?

[newbie1]

Originally posted by macboy
I strongly suspect the grounding. Does your power supply truly have dual outputs (+30V and -30V in addtion to ground)?

Well, I haven't built the PS yet, I am currently using a (rather cheap) variable bench supply. Shown here on ebay:

http://cgi.ebay.com/ws/eBayISAPI.dll...category=58286

I think I may have ground loop problems, I did bridge a the GND leads from a couple resistors together instead of making seperate ground paths back to the star ground. Could this be the problem?

[macboy]

That is a single supply! In that type of supply, the "ground" is usually connected to the ground of the AC line, and the power supply "floats", that is, it is completely isolated from any "zero" reference. Generally, the user would short the Gnd connection to the "-" connection so that ground is the zero reference. Then the "+" connection becomes some positive voltage wrt ground. But you could do it the other way (short + to Gnd) to effectively get a - supply.

Have you measured voltages at various points in your circuit both before and after the distortion begins? The place to start is to measure the voltage of each power supply rail wrt ground. Also meaure the (DC) voltage at each of the amps input pins (not the power supply pins). They should be at ~0 V. A substantial DC voltage here can definitely be the cause of the distortion (or more accurately another symptom of the same problem). You should also measure 0 V (DC) across either the .33 or 47 uF caps. I would bet that you will find a DC voltage at at least one or two of those places. It is due to the fact that your power supply is completely floating wrt your ground.

[cm961]

Definitly a single supply! I have one similiar and initially I thought if you used the green connection you could get + and - but later found out that to be untrue (to my dissapointment). You can create a voltage divider of sorts with resistors and retest at low powers, but it seems that your best choice is to go ahead and build your power supply for the amp and everything should work out fine!

Pete

[newbie1]

Yup, I am starting on the PS now. But just to clarify about the single PS I was using. Is it actually putting out +/- 30V just that there is not reference to ground? Or is it putting out +/- 15V to make 30V total?

If I understand this right you are saying I would need a dual power supply with each supply referenced to ground and capable of putting out 30V in order to get +/- 30V. And I would take the negative 30V from one supply and the positve 30V from the other. Correct?

So speaking of building my own power supply. Here is where I'm confused. I have a BL-1752 transformer I bought from Apex Jr which is rated 24VCT @ 7A. It has two primary leads labeled "0" and "230" I assume the "0" is neutral and the "230" is hot, and I would chassis ground the earth safety ground (3rd prong). However, on the other side of the transformer (secondaries) there are six terminals which are labeled the following.

28 14 0 48 24 0

So if I want to have +/- 24V off the transformer which leads do I take? I don't want to make a mistake here and potentially blow something up...which is why I have been holding off on building the PS. Any info would be helpful.