Hi,
I've built a simple stereo amp using LM3886's. There's two LM3886's bridged on each channel. The power supply is built from a Plitron 2x22VAC 400VA transformer and 60,000 µF of capacitance for each channel.
When I input an audio source in one channel, everything is fine and the sound is pretty good. However, when both channels are hooked to the audio source, the sound looses all its bass and depth. What the hell is going on? I don't think that's the kind of problem caused by a ground loop and I strongly believe there's enough power in this amp to supply the two channels.
Below is a picture of the problem. Note that the board in the lower left corner was a regulation board...I converted it to a 'do nothing' board. It already had the bridge and connectors for the transformer so I thought it could still be useful.
Any help appreciated,
Gabriel
I've built a simple stereo amp using LM3886's. There's two LM3886's bridged on each channel. The power supply is built from a Plitron 2x22VAC 400VA transformer and 60,000 µF of capacitance for each channel.
When I input an audio source in one channel, everything is fine and the sound is pretty good. However, when both channels are hooked to the audio source, the sound looses all its bass and depth. What the hell is going on? I don't think that's the kind of problem caused by a ground loop and I strongly believe there's enough power in this amp to supply the two channels.
Below is a picture of the problem. Note that the board in the lower left corner was a regulation board...I converted it to a 'do nothing' board. It already had the bridge and connectors for the transformer so I thought it could still be useful.
Any help appreciated,
Gabriel
Attachments
More help needed
Another thing that bugs me is a little buzzzzzz when no music is played (not loud enough to be heard when music is playing).
When the volume pot is at one end or the other, the buzz is as loud as it can be. On the other hand, when the pot is centered the buzz is almost absent.
I'm sure this is not a big deal, but I'm lacking experience to solve it by myself.
Help appreciated.
regards,
Gabriel
Another thing that bugs me is a little buzzzzzz when no music is played (not loud enough to be heard when music is playing).
When the volume pot is at one end or the other, the buzz is as loud as it can be. On the other hand, when the pot is centered the buzz is almost absent.
I'm sure this is not a big deal, but I'm lacking experience to solve it by myself.
Help appreciated.
regards,
Gabriel
when i have had this problem it has been from a Ground loop or Instability in the Amp. My first recomendation is put a series resistor of about 2k between the Amp's input and the Pot's wiper, if no effect then a pair of 10 ohm resistors between each RCA jack's ground and the ground wire going to the Amp Board. make sure that the RCA jacks are insulated from the Chassis with washers. (real Importent). BTY what circuit did you use for the Bridge LM-3886 Amp?
ppl said:
I don't think you need a circuit.. I think the LM3886 has the input issolated from ground.. you can ofcorse connect it (as you would with a single amp) but when bridging I think you can just reverse the input to one of teh amps... hope this is right, or I'm posting incorrect info again..
lol
hopfully it will work good for you, also it looks like your wire routing has all the Power,Input and output wires running paralell to each other try puting the input at Right angle to the others and separating them. Good Luck
also see the assembly manual to the PM21 at http://www.marchandelec.com/articles.htm this is a pdf file on there Kit containing two LM-3886
also see the assembly manual to the PM21 at http://www.marchandelec.com/articles.htm this is a pdf file on there Kit containing two LM-3886
SkinnyBoy said:
I don't think you need a circuit.. I think the LM3886 has the input issolated from ground.. you can ofcorse connect it (as you would with a single amp) but when bridging I think you can just reverse the input to one of teh amps... hope this is right, or I'm posting incorrect info again..
I think you need to go study the LM3886 some more SkinnyBoy.
The LM3886 is just a powerful opamp.
Simply design one LM3886 as a non-inverting amp with a certain gain, and then design another LM3886 as inverting with the same gain.
The output of the non-inverting is the +ve to your speaker, the inverting is the -ve.
Easy.
Beware, you cannot power 4ohm loads with 2 bridged LM3886s as the overcurrent protection on the IC will cut in and clip the signal.
You can however, use 4 LM3886s in bridge/parallel to power 4 and 2 ohm loads.
And Gabster... yes, why so many smoothing caps?
The LM3886 has quite a high PSRR, all those caps are not needed.
If you experiment enough, bridged LM3886s can give a lot of power.
I got 220WRMS into 8ohms @ 1kHz with under 0.08% THD+N out of mine!
It had 8 channels (that 16 LM3886s) powered by 2x500VA 30-0-30 torids that was all mounted in a 3U 19" rack mount case.
This is it:
An externally hosted image should be here but it was not working when we last tested it.
MWP> Wow i am impressed with your amp are you trying to give Jeff Rowland a run for his money.. Hee... HEE.. Heck i was impressed with just using two bridged Pairs for a great 60 to 100 watt Per Ch. Amp for my PC. but heck your Beast is way Cool. Is that from the BPA 200 app note?
Skinny Boy> I was refering to whitch one of the several Bridge Circuit configuations available for this and other Power Op Amps some use the method that MWP is sugesting others tap off the Output of one and feed into the input of the Other and several others are used for this as well. The PM 21 as on the Link above uses MWP's method and is the one i also used. The Rane MA-3 also uses a LM-3886 but not in the Bridge mode. What a popular Chip National semiconductor has.
Skinny Boy> I was refering to whitch one of the several Bridge Circuit configuations available for this and other Power Op Amps some use the method that MWP is sugesting others tap off the Output of one and feed into the input of the Other and several others are used for this as well. The PM 21 as on the Link above uses MWP's method and is the one i also used. The Rane MA-3 also uses a LM-3886 but not in the Bridge mode. What a popular Chip National semiconductor has.
MWP, what would be a sufficient capcitance for each channel? I built the amp following an article in a magazine (can't remember which one) and if I remember well, I've doubled the capacitance because...well I don't really know! At that point, that was my first amp and seeing people around stuffing as much caps as they could in their amps, I thought that was the way to go for any amp.
About your amp, is it 220WRMS per set of 2 bridged LM's? Meaning a total power 1760WRMS?
If yes that's amazing and I would like to see the schematics and more inside pics.
ciao,
Gabriel
About your amp, is it 220WRMS per set of 2 bridged LM's? Meaning a total power 1760WRMS?
If yes that's amazing and I would like to see the schematics and more inside pics.ciao,
Gabriel
Yeh, thats 8 * 200W which does give 1760W.
BUT... there is no way the 2*500VA transformers in there will supply enough current for that power output.
Its a home theatre amp, not a PA amp... full power out af all the channels at the same time will never be needed.
As for cap size... do some calculations. Every different application requires a custom power supply.
BUT... there is no way the 2*500VA transformers in there will supply enough current for that power output.
Its a home theatre amp, not a PA amp... full power out af all the channels at the same time will never be needed.
As for cap size... do some calculations. Every different application requires a custom power supply.
Hi, been watching this thread for a while, and i must comment on your amp Gabster, really NICE WORK!!!
But! since I'm a very stupid person and don't understand the pdf. file ppl posted, can someone please point out to me were I can find the curcuit for bridging those LM3886 chips. I know I'ts in the data-pages from Nat. semi. but I'm to dumb to recognize it when I see it
Apparently you have to invert one chip and then connect them togheter somehow, but I never understood how that is done.
Cheers!
Chris
But! since I'm a very stupid person and don't understand the pdf. file ppl posted, can someone please point out to me were I can find the curcuit for bridging those LM3886 chips. I know I'ts in the data-pages from Nat. semi. but I'm to dumb to recognize it when I see it
Apparently you have to invert one chip and then connect them togheter somehow, but I never understood how that is done.
Cheers!
Chris
Christian said:But! since I'm a very stupid person and don't understand the pdf. file ppl posted, can someone please point out to me were I can find the curcuit for bridging those LM3886 chips. I know I'ts in the data-pages from Nat. semi. but I'm to dumb to recognize it when I see it
Why don´t you look here:New application note
Nicke
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