| jweber |
Has anyone tried the simple bridging method described by
Rod Elliot?
http://sound.westhost.com/project20.htm
Nice when it works, but I have only two LM3886 chips left and don't
like to blow them up...
Any disadvantages doing it this way?
Thanks for your comments. |
|
|
| Upupa Epops |
| You can use it, but instead TL 072 give OPA 2134 and instead both 100 k resistors give there 10 k. |
|
|
| jweber |
Have a look at the drawing again - it is NOT a bridging adapter!
It seems you can bridge two LM3886 chips directly with two additional resistors. |
|
|
| pinkmouse |
Why don't you just use the circuit shown in the LM4780/1 datasheet, you can't get much simpler. |
|
|
| Upupa Epops |
| This solution is worse, have higher distortion. |
|
|
| BeanAnimal |
which solution is worse?
1) Rod Elliots
2) Rod Eliots with OPA substitute
3) The 4780 datasheet solution
The last post was ambiguous to say the least. |
|
|
| theChris |
| i'd have to assume #1. it looks like distortion from amp 1 would then get distorted by amp 2, then add to the output. |
|
|
| leadbelly |
Rod Elliot's article is being quoted out of context...he is addressing the simplest way to bridge a couple of commercial amps.
For scratch built DIY you would be better off just building one of the different designs discussed on this forum most often. |
|
|
| tschrama |
I dunno if Rod Elliots simplest bridging adaptor is worse....:confused:
The distortion products of the non-inverting amplifier are being fed to the inverting amplifier: so those non-inverting-amplifier-distorion-products will cancell out. The only distortion you are left with are the distortion products of the inverting amp (and the diustortion of the distortion products, far to low to be significant I assume).
So.. You are left with a bridged amplifier having the same distortion specs as a non-bridged amp in halve the load..
versus...
When a perfect balanced signal is obtianed from a unballanced signal and this is fed to two equal amplifiers, distrotion of each amplifier alone is like driveing halve the load but.. :att'n: the even-harmonics tend to subtract from each amplifier, while the uneven-harmonics tend to sum from each amplifier... this might even be worse...
go with Elliot, that guy never failed me once.... |
|
|
| TwoSpoons |
| quote: | Originally posted by tschrama
The distortion products of the non-inverting amplifier are being fed to the inverting amplifier: so those non-inverting-amplifier-distorion-products will cancell out. |
Nonsense. The distortion (and noise) from the first amp will be distorted by the second amp. You will also get intermod products on the distortion. Its a non-linear process, not just add/subtract. |
|
|
| jackinnj |
| the LM4780 is basically a pair of LM3886's -- the design by National Semi on the LM4780 PDF works very nicely indeed if you have to bridge -- I see no reason that a pair of LM3886's shouldnt do as nicely providing that they are properly heatsinked. |
|
|
| theChris |
| yeah, i'm not conviced the distortion would cancel as opposed to summing. if the distortion was in phase on both amplifers then yes, but as it stands, i'm not conviced this will happen. |
|
|
| pinkmouse |
| OK, this sounds like an ideal job for someone with one of those spice simulator things. Sim the two designs, (they can be any old op amps in the sim, it's the principal of the thing that matters ), and see how the two distortion figures come out. Or, of course, someone with the two amp configurations could just measure them I suppose! :) |
|
|
| arcom |
| I think that this is the common bridging technique for most chip amps. I have used it in my TDA7294 amp and it works great! |
|
|
| jackinnj |
in theory, it works perfectly -- i put it on Multisim8 and there is no distortion to speak of:
in the real world I have found distortion of 0.01 to 0.07% with the capacitor coupled, bridged LM4780 |
|
|
| pinkmouse |
| quote: | Originally posted by jackinnj
iin the real world I have found distortion of 0.01 to 0.07% with the capacitor coupled, bridged LM4780 |
I wonder if one of the differences between sim and real life comes down to tolerance in other components between the chips. Could this be reduced by more exact gain matching I wonder? |
|
|
| jackinnj |
| quote: | Originally posted by pinkmouse
I wonder if one of the differences between sim and real life comes down to tolerance in other components between the chips. Could this be reduced by more exact gain matching I wonder? |
I guess I should put up the picture -- with the resistors matched to 0.1% there is a slight difference in gain between the A and B channels -- Gain on A is 19.67dB, and on B it is 19.77dB. I am averaging 64 samples of a 1kHz signal. |
|
|
| ingrast |
| quote: | Originally posted by jackinnj
I guess I should put up the picture -- with the resistors matched to 0.1% there is a slight difference in gain between the A and B channels -- Gain on A is 19.67dB, and on B it is 19.77dB. I am averaging 64 samples of a 1kHz signal. |
Gain matching is not an issue in a bridge configuration except for efficiency (max available output at exact matching). If one side has 100% gain and the other 50%, then the output voltage will be 150% compared with the single ended case.
Below 0.01% THD, other issues like PSU and layout are much more relevant.
Rodolfo |
|
|
| pinkmouse |
| Thanks for clearing that up Rudolfo, little tips like that are often the things you miss when you are teaching yourself. |
|
|
| TwoSpoons |
| The problem with the sim is that the model generally does not include the non-linear elements that create the distortion. Op-amp macro models are pretty simplistic. |
|
|
| Cesar |
| If you are still interested in the LM3886 bridge, etc here is something from national. "Better to have it and not need it, than need it and not have it" |
|
|
|
