LM3886 VS. Discrete

SteveG

Account Disabled
2002-01-07 7:20 pm
Newton Falls, Ohio
I have read through a lot of the threads on this topic... my point in asking is to find out if there is a difference in the sound. The specs are very good for this type of design. Is there really a downside to using an integrated over discretes in this case? I guess the only thing I can think of is that you can't really regulate the front end supply, as it's combined with the output stage.
 
Althought the distortion curves may look great, the clipping characteristics of these chips are nasty, generally a spray of high order harmonics. Also there is an increased low frequency distortion (less than 50 Hz) due to thermal cycling of the substrate.

On the other hand, they are real easy to use, and almost indestructable. If this is your first amp project go for it... I'm sure you can find some boards for them real cheap too..

-Amps-

SteveG said:
I have read through a lot of the threads on this topic... my point in asking is to find out if there is a difference in the sound. The specs are very good for this type of design. Is there really a downside to using an integrated over discretes in this case? I guess the only thing I can think of is that you can't really regulate the front end supply, as it's combined with the output stage.
 

SteveG

Account Disabled
2002-01-07 7:20 pm
Newton Falls, Ohio
Thanks for the info

I had a feeling that these things weren't exactly as clean as they looked on paper. I will be building 6 amp modules for a home theater amp, and it seems like it would be a really easy way to go. I have built other stuff in the past, and I have been an electronic tech/designer for 10 years, so I have assembly experience. I will be making my own pcb's.
I think I will go the chip direction anyway- using the bridged/paralleled config. I don't think I will run into clipping, considering each amp will put out about 200w/8ohms, and I have fairly efficient speakers. For this application, I'm not too worried about absolute quality.
Thanks for the info.
Steve
 
Re: Thanks for the info

SteveG said:
I had a feeling that these things weren't exactly as clean as they looked on paper. I will be building 6 amp modules for a home theater amp, and it seems like it would be a really easy way to go. I have built other stuff in the past, and I have been an electronic tech/designer for 10 years, so I have assembly experience. I will be making my own pcb's.
I think I will go the chip direction anyway- using the bridged/paralleled config. I don't think I will run into clipping, considering each amp will put out about 200w/8ohms, and I have fairly efficient speakers. For this application, I'm not too worried about absolute quality.
Thanks for the info.
Steve

As it sounds like your hands are not turned completly upside down. Maybe try go for..

http://sound.westhost.com/index.html

There is a project called "3a".

I know it is not a chipdesign but it had some good powerbjt's and is a simple construction.

Sonny
 

SteveG

Account Disabled
2002-01-07 7:20 pm
Newton Falls, Ohio
Thanks Sonny,
I was thinking about the chips so I could keep the parts count down- simplify cramming all 6 amps into one big enclosure. I don't think that 60 watts will be enough (probably more than enough, really, but I want to be in the 100-200watt range per channel).
I think I may look into a MOSFET design.
Steve
 
SteveG,

If you design a PCB for the LM3886 bridged/parallel connection, will you make the design available here or elsewhere? I've just obtained the chips to make one channel 200W for a sub amp. I looked at the BPA amp that National has on their web site. It doesn't include all of the extra components that they show in their app note on the part, just the extra parts needed for the bridged/parallel configuration.

Siegfried Linkwitz recommends these chips to power his Phoenix loudspeaker. I think that as long as you stay away from clipping them, they should be very good. The thermal safety feature is nice too.

RonS
 

SteveG

Account Disabled
2002-01-07 7:20 pm
Newton Falls, Ohio
Ron,
If I come up with anything worth using, I will make the plans available to anyone who wants them.
I assume that the best way would be to create a jpeg or pdf of the artwork.
BTW, what is everyone using to make their PCB's? Has anyone ever tried that iron-on stuff that you print with a laser printer?
Steve
 
Thanks Steve.

I design my boards on my Mac, save it as an EPS, have a service bureau print it out on film (emulsion side down) then contact it with copper. Expose with flourescent light, then develop and etch. Works great, but I did have to buy a proper flourescent light fixture. The time for exposure is very precise with this fixture, and worked much better than using my photo enlarger.

The other idea I had was to mount the 4 chips onto the heatsink, then hardwire everything, using a piece of unetched copper PCB as a ground plane. Anything that needs to go to ground just gets soldered to the copper sheet, which would be just below the chips. Could even use wire wrap wire IC sockets tacked to the copper for the servo chips. Use a socket with more pins than the opamp, extra pins get soldered to the copper for support, the rest get bent up for making your connections. Not as elegant as an etched board, but quicker and easier.

RonS
 
chip amps

Hi,

I have built chip amps using OPA548, similar to LM3886, a bit less power. I am very satisfied with them although I took no special care assembling them - point to point air wiring, standard external components (no fancy high level caps/resistors/etc.). I think the advantage of these chip power amps is that you can stick to the recommended circuit with quite some confidence and it will work reliably - you won't have to tweak a design with great potential but a lot of finicky issues until the cows come home. The clipping shouldn't become a problem in the first place if you do your calculations right - adjust the gain in such a way that your pre voltage output isn't capable of driving it into clipping . Consider the necessary current as well in your calculations, however: the trickiest and most expensive part of such an amp IMHO is the power supply. For example I have a total of 4 OPA548 chips (bi-amped system). the chips are capable of 3A continuous and 5A peak, each! for +-24 V rails that means 12A x 48 = 576 VA *continuous* power output capability and the necessary transformer power plus losses etc. . For peak demand you might get away with large reservoir caps though.

All in all - can't fault sound quality of chip amps. Then again I never tried big buck high end amps either; my reference is Rotel quality level :)

MBK