How to use OPA549 and OPA541?

[JojoD818]

miguel2,

I personally haven't tried an inverted version for the OPA549, but I believe someone has already done that in another thread. I have successfully built several gc amps but never had an oscillation problem. Strange though...

JojoD

 

[miguel2]

I built a few IGC with OPA541, OPA549 and LM3875. This was the first non-inverted.

I just dont know what was wrong with it and, more important, I dont know what fixed it . It was getting way hot and with voltage rail at ouput. sudenly it went t normal..

So I am a bit caucious to connect it to my sub driver... I guess I will make it with another chip (that is the only reason I find - crazy chips )

Miguel

 

[JojoD818]

miguel2,

I think you had some oscillation problems during that time. Anyways, try to keep wire runs short and make a good distance between signal carrying wires and the output. Had that problem/mistake before on a SS amp and fried a couple of 2SC5200/2SA1943!

JojoD

 

[miguel2]

Changed the chip and everything is ok! It is driving a subwoofer and does not even get warm. So I dont know what hapened, but its solved .

The problem now is huummmmm.

I have 2 LM3875 for the mains speakers and this OPA549 for the sub. All are driven by the same (MUR860) bridge (please dont hit me yet, because I have to say too that the only 1 tx is a rotel I got from a 20W integrated...). So I have to route all earth cables.

Which is the best choice: to have a star ground for each chip (including the speaker ground) and then star the 3 to the tx? Or to star wire all grounds to one star? What about the difference between the ground of the speaker/psu caps and the grounds of the resistors(input to ground and - to ground).

Miguel

 

[GregGC]

If I understan your question correctly, IMHO: Star at the powersupply side. Star at each amp. Separate wires from Powersupply star to each amp-star. This way no GND wire is shared between amps.

 

[JojoD818]

I agree...

 

[miguel2]

I agree...

Me too!

I made it last night and it reduced the hum BUT it is not quite silent yet. I also placed a 100R/0.22uF to chassis but it didnt help. I'll try some re-routing og the wires...

I tried to keep the tube preamp ground separate (it is in the same case, making an intgrated), with the only common part being the chassis (by 100R/0.22u), but it hummed a lot and volume control could not make the system silent when playing music. So I had to link the pre and power grounds, which I didnt expect...

Miguel

 

[GregGC]

Miguel,

I think the preamp shoud have a star gnd and then the output-gnd of the preamp (not the star-gnd) should get connected to the signal-gnd (input-gnd) of the power amp. This should be the only gnd connection between the preamp and pwramp. Also, I don't think that you should have more than one connection to chassy (only one, from one of the star-gnds of the power supply through 100/.22.). The Earth terminal of the power outlet to chassy. O, and the input connectors of the preamp have to be isolated from chassy.

I hope I got it right.

 

[miguel2]

what is the output ground of the preamp?

Now I am using a star ground in the preamp and I have to connect it to the star ground of the power amp to get lower hum.

So you say to disconnect the star ground of the preamp to power amp and link only the output ground. Please tell me what are you considering as output ground.

Miguel

 

[GregGC]

To be perfectly honest, I'm not 120% sure here, but that's how I think it should be done, so that you minimize potential differences between the preamp-out and power amp-in. I think it should work better that way. Again I could be wrong and I'm sure it depends on the gnd and decoupling cap configuration of the preamp. Analog's AN-202 talks about it.

 

[miguel2]

I use a tube preamp, but managed to work things out. But it still humms a litlle

Anyway, it drives the speakers crazy on power up and shut down, due to the tube, so I am going SS all the way. It's settled.

Miguel