problem with LM's and power supply

[KostasTheGreat]

Hello,
i have one transformer 2x24V and i want to supply one LM3875 chip and one LM1875 the same time.
because a 2x24V trafo gives us around +-35V and lm1875 can go max +-30V i am using regulator LM350..

When i power up each channel alone everything is ok.. but when i power them up together there is a terrible huge very loud noise comes out from both channels..

i suspect some ground loop because input grounds are connected together but power grounds one goes inside regulators the other is from the rectifiers directly from transformer..

i have draw the schematic i am using trying to explain my connections...

what am i doing wrong? any suggestions?

thank you very much!

 

[Netlist]

I see the power grounds connected to the input ground.

/Hugo

 

[KostasTheGreat]

if you talk about the green line its just to notice that the preamp connects the input grounds together....

on the amps pcb there is a star ground of input ground with power ground... but they are also connected inside the preamp.

i think that there is something to do with the regulators..

any clue?

 

[moamps]

Hi Kostas,

I see two mistakes in your schematic. First, you can't mix secondaries and their respective bridges. In other words, if you use one secondary to get positive voltage for one amplifier, the same secondary must be used to get positive voltage for the other amp (not negative). Second, in your configuration, you can't use a positive regulator (LM338) to get negative regulating voltage but a regulator that's normally used for getting negative voltage (with common pin connected to ground).

Regards,
Milan

 

[KostasTheGreat]

i have put the positive with the positives the negatives with the negatives.... the same problem..


i have done another test.. i powered both amps .. and i connect only the one input at a time and it is working... the problem starts when both inputs are connected.

i remind you that the preamp connects grounds together from inside. so i can think that this is the problem..
the common input ground of the preamp..

what can i do to fix it?

the regulator with LM350 works fine my friend.. this is a schematic i found here in the forum from another great guy.

what can i do :-( im so sad :-(

 

[shusha]

Second, in your configuration, you can't use a positive regulator (LM338) to get negative regulating voltage but a regulator that's normally used for getting negative voltage (with common pin connected to ground).

Why not zz, he can use positive regulators for both sides as long as he have two secondaries (if this two secs. are from same transformer as you mentioned). I am sure you know that. His schematic with 338 looks like Pedja's. I have built several regulated PSU's by it's schematics.

http://users.verat.net/~pedjarogic/audio/index.html

 

[Frank Berry]

Look at your power supply schematic very carefully. On the -35 volt supply, the cathode (+) end of the bridge is going directly to common ground. On the -25 volt supply, the cathode (+) end of the bridge is going to the input of the regulator. This will not work because you are shorting out the regulator.

You must use a negative regulator if you expect the two amplifiers to work from common windings.

 

[moamps]

...Why not zz, he can use positive regulators for both sides as long as he have two secondaries......[/B]

Bok, Hi,

Frank je dao odgovor.
Look at Frank's post.

Pozdrav, Regards,
Milan

 

[peranders]

... so if you want to use only positive regulators you must have at least 3 totally separate windings and if you fix a true negative regulator you can get away with only two secondary windings.

 

[moamps]

....or (wierd solution) keep amps grounds separated and use ground-lifted line transformers in inputs..

 

[KostasTheGreat]

but the amps is working with these schematic and these regulators thats not my problem...

i tried something else... i put both lm3875 and lm1875 on the regulator side..

and again... it is working correct if i connect one input at a time but when i connect both inputs i get a huge buuuzzz...

with no inputs connected i get no noise at all.


maybe its a problem that the one has a 56K resistor on the input and the other 1M ? or maybe beacause the 3875 is inverted input and the other is normal?

 

[runebivrin]

OK, take a hint: Put a 24V light bulb between the two grounds, and see what happens to it. Then just follow the electrons to get an explanation, OK?

Rune

 

[peranders]

but the amps is working with these schematic and these regulators thats not my problem...

... since you don't listen to us... measure the resistance between you power ground and your signal ground. Should be in milliohms => 000.0 ohms. Do you get that?

You can't use more tan one rectifier bridge for each winding.

The only way you can make thinks right is to use two rectifier bridges and change the negative regulator.

Just wondering, why aren't you using only LM3875?

 

[KostasTheGreat]

its a multichannel amp (six chans) 3 x lm3875 and 3x lm1875
but i have only two transformers of 2x24V 200W each

i put front 3875 and rear center 1875 on one transformer
and two rear 1875 and one center 3875 on the other.

ok got it... i will go to buy a new small transformer for all the 1875 channels and i will leave the two others for the 3875's

so i will throw away the regulators and then just use a single rectifier bridge for each transformer


120W 2x18V (+-25Vdc) toroidal for two lm1875 channels is OK?

 

[shusha]

Look at Frank's post.

Pozdrav, Regards,
Milan

I deliberatly dissobayed 35V psu part. I was just reffering to the imposibility of having +/- reg. PSU with two positive regulators. And yes I overlooked that you wrote "in your configuration". My mistake.