| Rachel |
Does anyone know how to parallel the LM3886 together to increase max. output current and improve damping factor.
I do know that Jeff Rowland DG is using this approach in their amplifiers.
Many Thx.
Rachel |
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| AudioFreak |
| yep you do this ............ |
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| AudioFreak |
| the 3875 shown is a scaled down version of the 3886 and the 3886 can be placed in this circuit without any problems. |
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| Rachel |
Many thx. AudioFreak. Are you using DC servo for the feedback loop for each Power OpAmp module? Fantisic....
Anyway, many thx. |
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| AudioFreak |
| The servos are so that you dont get a different DC offset @ each of the outputs ... you'll also probably want to add a circuit that protects the speaker from DC which is not catered for in the schematic above. |
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| paulb |
National had a conceptual schematic as well, I have the PDF but can't seem to find it on their site anymore. Email me if you'd like me to send it to you. The title is:
Bridge/parallel amplifier (BPA-200) documentation, Sept. 19, 1997. |
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| AudioFreak |
| Paul that's exactly where the schematic above came from. |
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| palesha |
Apexjr has sold parapix amps which contains two lm 3886 ic's. On his site their is lot of information available regarding lm3886. I have also used the amp. Hope this helps.
Mahendra Palesha |
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| ssanmor |
I am also interested on parallel-bridging LM3886. Has anyone tried to do that? And what if you don't use servo circuits?
Please post any schematics you have.
Thanks! |
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| arthur |
| trim each op amp individually on the -ve i/p |
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| ssanmor |
| sorry, I don't understand your notation, could you be more explicit? :-) Thanks! |
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| arthur |
apply a adjustable dc voltage to -ve i/p via a 1m resister
assume the op amp is in non-inverted mode |
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| ssanmor |
Ah, you mean applying a small (a few mV) variable voltage at the inverting pin of the LM3886's ?
That sounds good for the non-inverting stages, but what for the others?
I was thinking on building 4 identical stages, non-inverting, and adding a inverting circuit with an op-amp to excite two of them, so your idea can be suitable.
But do you think it is going to behave well as the offset changes with temperature?
Thanks! |
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| rljones |
ssanmor,
I'm now stuffing a new PCB I made up, doing what you've talked about. I've got on one board that's about 2 x 7 inches, 4-LM3886 devices, all running non-inverted (this gives better performance: you don't have the feed back 'bucking' the source that's driving the - input).
I based it on the schematic in the application link that AudioFreak listed above, but only using the top half of their schematic--that is, the non-inverted portion. I repeated the upper non-inverted portion for the 'bottom' half. Each output pair drives a resistor/inductor and then the load.
Next, I split the single input buffer op-amp into two, using one to drive the upper non-inverted pair with a non-inverted input signal, and the second to drive the bottom half with the inverted input. That is, each input polarity is running into a non-inverted AD8610.
12V regulation is provided for the input buffers devices separately from the regulated supplies for the LF411CN used in the DC servo circuit. (Instead of using two op-amps, I've also set it up to use a Jensen input transformer, with one secondary leg driving the + LM3886 pair and the negative leg the other LM3886 pair; it's always fun to compare!)
This means the whole amplifer, while bridged, is fully balanced and non-inverted. I designed the boards so that multiple units could be paralled to even further increase the low impedance drive capability. I was thinking that 2 or 3 boards, each with a two pair (4-LM3886), should drive a 2 ohm load with around 700 watts. (4 bridged units give around 150 to 200 watts into 8 ohms and 300-400 watts into 4 ohms; so 8 units should handle a 2 ohm load with 600 to 800 watts, depending upon the power supply and heat sinking.)
Hopefully, I'll have one board running by this weekend and if it tests out OK, I'll plug it into my system. |
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| ssanmor |
Thanks for the explanation. Could you send me a schematic of your whole amplifier for clarification?
I am also going to do some tests this week, I will tell you how it goes.
You can send the sch. to ssm07@tid.es or ssanmor@canal21.com
Thanks!! |
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| Opie |
rljones,
I have been working on this very same design but have had some problems. I am unfamiliar with the dc servos and their function. The amp I built also runs way too hot. Could you e-mail me your schematics and pcb if possible.
My e-mail is bopppe069@uwsp.edu
Thank you in advance,
Opie |
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| ssanmor |
My idea is this (I will try to explain it as clear as I can):
I am going to use four identical non-inverting amplifiers like the one shown in this picture. The output R in parallel with L is substituted by a 0.1Ohm/5W resistor, (all picked by-hand to minimize differences between them).
Then two of them are excited by the direct input signal from my preamplifier (with a gain=1 buffer). This signal is inverted by an inverting opamp with gain=1 and then drives the other two power stages.
The outputs of the two top stages are connected together and to the + terminal of the speaker; The outputs of the bottom stages are also connected together and to the - terminal of the speaker.
All the gain resistors are also picked by hand to match them within 0.1%
As you can note, I don't use servos. The stages are AC coupled, so they don't amplify the DC offset from the preamplifiers.
Does this set-up have any chance of working well? If you don't think so sincerely, please send suggestions.
Thanks! |
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| Nelson Pass |
If I was you, I'd insert some finite resistance in
series with the output of each device to prevent
the amps from arguing with each other. |
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| paulb |
| quote: | Originally posted by ssanmor
The stages are AC coupled, so they don't amplify the DC offset from the preamplifiers. |
But it does not help with the DC offset from the amplifiers themselves. The 0.1 ohm resistors you're proposing may be too small to keep the two amps from "arguing" as Nelson says.
Didn't that App note from National warn about trying to do this without the servo?
If the two amps have, say, a 100mV difference in offset, this would be 0.5 amps through the balancing resistors (100 mV / (0.1+0.1))! |
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| rljones |
ssanmor and Opie,
I'll be happy to send you my schematics and even by PCB layout once I've debugged it and make sure that all is OK. I use ExpressPCB (http://www.expresspcb.com/) for my small runs and I could email you the file and you could then order how ever many you want.
I powered one of the boards up this morning and found one tracing that needs changing, and I'll probably play with some other values through this week.
As for your schematic without the servos, I agree with paulb that the app note and the way I'm working up the amp is probably best done with the servos. This does require another +/- power supply for the 12 to 15V for the LF411CN opamps (I switched from 15 to 12 V because the AD8610 can only take up to +/- 13V). This isn't too difficult though, just tap off of the +/- 35V LM3886 supplies running through a couple of 3-legged regulators.
As for inversions, I'm not inverting anything in the amplifier, but I'm running it balanced. It you want it single-ended, just ground the negative inut. |
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| ssanmor |
Beware with that circuit applied to the LM3886!
What that circuit does is to use the non-inverting part as the master, its resistors set the overall gain. The output is then used as the input for the other stage, that is an inverting one with unity gain. LM3886 are not stable at unity gain.
To make it work, you can do two things:
a) Connect the output of the first stage to the second one after being attenuated. Then set the gain of the 2nd stage to the same amount. Then the amplifier has a gain (for example, 20) but the overall result is that its output has the same amplitude as the output of the 1st stage (but inverted).
b) Directly excite both stages with the same input, but one stage is connected inverting and the other non-inverting. See:
http://david.lewander.com/projects/amp/amp.htm
Suggestions or corrections? |
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| AudioFreak |
| quote: | Originally posted by ssanmor
My idea is this (I will try to explain it as clear as I can):
I am going to use four identical non-inverting amplifiers like the one shown in this picture. The output R in parallel with L is substituted by a 0.1Ohm/5W resistor, (all picked by-hand to minimize differences between them).
Then two of them are excited by the direct input signal from my preamplifier (with a gain=1 buffer). This signal is inverted by an inverting opamp with gain=1 and then drives the other two power stages.
The outputs of the two top stages are connected together and to the + terminal of the speaker; The outputs of the bottom stages are also connected together and to the - terminal of the speaker.
All the gain resistors are also picked by hand to match them within 0.1%
As you can note, I don't use servos. The stages are AC coupled, so they don't amplify the DC offset from the preamplifiers.
Does this set-up have any chance of working well? If you don't think so sincerely, please send suggestions.
Thanks! |
you cant replace the 10ohm resistor with a 0.1ohms resistor ....the 2 resistors perform 2 different task so there must be a seperate 0.1ohm resistor @ the output of each chip ... as per the schematic i posted right @ the beginning of this thread.
| quote: | Originally posted by paulb
But it does not help with the DC offset from the amplifiers themselves. The 0.1 ohm resistors you're proposing may be too small to keep the two amps from "arguing" as Nelson says.
Didn't that App note from National warn about trying to do this without the servo?
If the two amps have, say, a 100mV difference in offset, this would be 0.5 amps through the balancing resistors (100 mV / (0.1+0.1))! |
this is correct, for decent performance you should definately use the servos .... also, note that this does not guarantee that there wont be DC @ the output it just assures that all the chips in parallel create the same small DC offset....
Actually, while these amps are not unity stable, this may just work because of the clever way it is done but personally with chip amps like these, i wouldnt try to bend the rules :) just use the original method for bridging as per the schematic @ the start of this thread which was taken almost straight from the national application note.
Hope all this helps ....
AudioFreak. |
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| rljones |
the bridging technique described at http://sound.westhost.com/project20.htm is not quite true bridging; it is more of a hybrid. i believe it is more commonly referred to as a master-slave amp.
by having the output of the master amp driving the input to any number of slaves, all sorts of phase anomalies are created at the final output. i would avoid this design for high end audio (maybe its ok for a car stereo system where phasing is already chaotic, but not for a home system). this is what gave bridged amps a bad name several years ago.
the method originally described by AudioFreak at the start is better; my only suggestion was to use all non-inverted configurations and to keep the whole thing fully balanced with one 'set' for the positive input signal and another for the negative input signal. (i believe this balanced/bridged configuration, using the LM3886 devices, is essentially what is done in many of Jeff Rowland's products.)
i would also second (or third) the opinion that 0.1 (or maybe even 0.2) ohm resistors are essential for proper behavior of each output device, and that one resistor/inductor is also needed in most situations to prevent the amp from oscillating (and thereby overheating). This resistor/inductor should probably be used on each side of a bridged amp's load, but only on one side in a single ended version. |
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| ssanmor |
rljones,
I agree with you, what you say seems sensible.
But what do you mean when you say "using one set for the positive input and another for the negative input"? you mean providing adjustable gains for both phases of the signal in order to fully balance the waveform? Please make this a bit more clear.
And, do you think that the resistor + inductor in the speaker terminals would be mandatory for using a parallel-bridge setup and a 4 ohm woofer (this is my application ;-) ?
Thanks. |
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| rljones |
ssanmor,
it's not the impedance of a speaker that interacts with the amp. if all speakers were purely resistive loads, life would be easy. however, it's the inductance and capacitance of not only the speaker, but the crossover of the speaker and the cabling that's running to your speaker that affects the amp. unless you can see what's happening to your amp on a 'scope, i'd put in the resistor/inductor. many builders also include a 0.1 microfarad cap in series with a 10 ohm (or sometimes slightly smaller) resistor. this resistor-cap goes from the positive output lead to ground on a single ended amp. these go by the name Zobel or Boucherot networks.
as for the 'using one set' business, what i mean is that there is an input op-amp as a buffer (the lf411cn in audiofreak's first schematic), but running in non-inverted mode, driving a paralleled pair of lm3886 running in non-inverted mode. this is what i'm calling a 'set'. into this is put the + signal. another one of these 'sets' is used for the negative input. this is now a balanced amp. (you might want to search on the internet, or within this forum, balanced versus single ended amps.) |
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| rljones |
i'm going to continue on this same thread since it involves parallel lm3886s. i'll try to keep this as short as possible (and assume the above posts were read).
i finished my board and ran tests on it today. the results seem very good (described below). it also led me to incorporate some changes that will make it easier to run both single ended (read RCA input) and balanced (read XLR input), without having to change the output (bridged) strategy. once i'm sure the newer PCB is working well, i'll post the file for anyone who wants have it made at ExpressPCB (http://www.expresspcb.com/).
as mentioned in an earlier post, i'm using non-inverting inputs. the test circuit used a pair of opa627 (one each for the +/- input legs). each buffer amp drove a paralleled pair of lm3886. each of these pair had lf411cn servo opamps and had their output through 0.1 ohm resistor and then onto a resistor/inductor series element as described in previous posts, and then finally to the 4 or 8 ohm test load. power supply was +/- 35 VDC with 330 VA transformer. no input caps were used.
the following specs were found (all were at 1 W/8 ohms unless noted):
single ended input/single ended output (4 or 8 ohms to ground; ie, not bridged):
60 W into 8 ohms or 106 W into 4 ohms at 1kHz
freq resp: -0.2 dB at 10 Hz and -1.5 dB at 100 kHz (limits of test equip)
%THD: 0.012 at 20 Hz, 0.007 at 1 kHz, 0.017 at 20 kHz
noise (shorted input): 175 microvolts (80 microvolts with 400/80 kHz filter; note this is on test bench with open wires, aligator clips, and 2 adjacent computers)
DC offset = 2 mV no load; 4 mV at 1 W 20 kHz
balanced input with bridged output:
280 W no load
170 W at 8 ohms
230 W at 4 ohms
% THD: 0.009 at 20 Hz, 0.009 at 1 kHz, 0.030 at 20 kHz
freq respon and DC offset same as above
noise: 200 microvolts (same disclaimer as above)
single ended input with bridged output:
54 W into 8 ohms and oscillated (spike limiter kicked in) at 4 ohm load
these findings are very good. output would probabaly not droop as much with a larger transformer; i plan on using a 1 kVA per channel along with 2 of the above boards per channel (maybe 3, which would mean 12 lm3886 per channel). i did not listen to the amp. i will first make some changes described next, and then build a stereo unit for listening. one point i must mention is that in an earlier post i referred to transformer input. i tried two differen ones and was not able to adequately drive the paralleled lm3886s, so i instead turned to the buffered op amp method.
as for the main modification that i alluded to at the start of this post, it deals with the single ended input issue. as you can see from the data, a single ended input with a bridged output has lower power than when using the same input but with a single ended output (ie, not bridged). this is probably due to the two circuit halves 'bucking' one another. to eliminate this and to obtain maximum output using the whole bridged circuit, i'm going to use two separate inputs: one via RCA and one via XLR. the XLR version is completely balanced input to bridged output. this is the circuit i tested above.
for the RCA input, i'm going to add two more buffer op amps and then run them into the XLR circuitry. the two op amps will simply take the single ended input and one will forward it to the + input op amp (in the XLR input section); the other op amp will invert and pass it onto the - input op amp. the reason for using one inverting buffer is obvious: it gives the necessary out-of-phase signal for a balanced signal. the other op amp is simply used to treat the non-inverted signal as closely as possible to the inverted half. the other design change i'll make on the PCB is to allow use of either SOIC devices like the AD8610 as well as 8-in dip op amps like the opa627 i used in the prototype for the buffered input op amps.
one other area to test is to see if using a lower noise op amp instead of the lf411 would lower the noise floor. does anyone know if this would help in lowering noise? |
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| paulb |
| Your servo amp has an ultralow bandwidth, like < 1 Hz, right? I can't see how a different op amp would make any audible difference. Noise at these frequencies is termed drift. |
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| rljones |
| yes, the filter i believe is a 2nd order low pass at less than 0.5 Hz. i was wondering if the inherent noise of the op-amp accompanies the DC component and if this was potentially reducible by using another op amp. |
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| ssanmor |
rljones, please, could you post the schematics? And also a picture of the PCB so anyone can make it without Express PCB would be desirable.
Thanks! |
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| rljones |
i'd be happy to post the schematics. let me try to see how legible i can make it; none of my programs seem to do a good job (i work on macs, but the pcb layout is done on a pc).
as for posting the image of the pcb, you will honestly find it next to worthless without going through expresspcb. (i don't wish to make any money off of this, nor do i want to start selling boards.) it is a double sided board that has some very fine traces. (and i cannot imagine home-brewing a double sided board.) even if i posted the image of the layout, the image would end up being too small to see the exact details. if i post the file, you can download the application from their website for free and then you can scroll or enlarge the image at will, but only on a windows machine. but let me look into it; i suppose at the least you could get a rough idea of how to place the parts, if i only posted an image of the layout... |
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| rljones |
I rigged up the one board I built up, and previoiusly discussed on this thread, in order to get a listen to the it. I ran the one board in a parallelled, but not bridged, configuration. I did this so I could listen and compare a stereo unit in my home system. (To briefly clarify, there are 2 pairs of paralleled devices and I ran a right channel signal into one and the left into the other.)
In a nutshell, it sounded simply wonderful. I compared it to several amps I've built/bought and it is outstanding for the money invested. I think I like it slightly better than my 'Modified TA0104A Tripath Amplifier' (http://www.diyaudio.com/forums/show...14&pagenumber=1). Of course, I don't know how the bridged unit will sound, but I'm rather encouraged.
A couple of other things that I found that are worth noting. First, I used the new AD8610 in my active crossover (based on the design found at: http://sound.westhost.com/project09.htm, but using 0.1 microfarad caps and 15K resistors throughout to give a X/0 at 75 Hz.) The whole thing is SMT devices and fits on a 2.5 x 3 inch board, including power supply with dual regulation for each channel. I won't discuss it more here as it would belong in a separate thread, except to say that the AD8610 is excellent. I cannot detect it either in or out of the system. (Residual noise is 100 microvolts and distortion low.) I will now definitely use the AD8610 on the input stage of the LM3886 amp.
The second finding is the ever annoying heatsink issue. I wanted to build a low profile amp (2U) and keep the heatsinks inside. I was looking around Fry's this morning and bought a couple of CPU heatsink/fan combos. The heat sink is 2 x 3 x 1 inches (h x l x d) with fan. One covers one pair of LM3886 and costs about $9 each. I added an 8V regulator off of the amp pcb, supplying the 2 fans which I ran in series. This made the fans exceedingly quiet and yet moved sufficient air to cool the LM3886TF devices. At full power (2 parallel devices into 4 ohms at 1 kHz, giving 110 watts), the devices themselves never got over 60 deg C, with the heatsinks running around 45-50 deg C (this is in the open and not enclosed with perforated chassis). When actually listening the the amp in stereo, the heatsinks are cold, the devices don't even feel warm and I was playing the system loud. So those of you who also have trouble getting just the right heatsink might want look into these CPU coolers: they're available are all over the globe and are relavtively inexpensive.
As for the pcb, I made several changes and submitted the file tonight (v1.1). I should have the boards later this week and will built one up over the followng week. If it tests out OK, I'll post the file for those who contacted me about it. |
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| ssanmor |
I also have been making tests with LM3886's, but my results are not so satisfactory. In my first tests, I got about 80W into 4 ohms with a single chip, with a BIG heatsink and the chip got only warm. But now I have re-designed the PCB and it gets quite hot quickly. I am using the same heatsink, BUT the chips are mounted in a zone where the section of the alluminium is thicker (about 7-8mm). There is also a thermal insulator between the chip and the heatsink (SARCON from Fujipoly).
I suspect that mounting the LM3886 directly against a large mass of aluminium doesn't allow the heat to quickly go out of the chip, so perhaps it would be better to screw it on a relatively small "angle" and then firmly fix it to the big heatsink. Anyway, I would like this point to be commented by you.
I would be glad using CPU coolers, but the size of the case
doesn't allow me.
When I have this problem solved (I don't discard an error in the board so I will check with the old module), I will run two LM3886 in parallel. I haven't used servo circuits, but the gain resistors are hand-picked within 0.1% tolerance.
I have also observed a variable offset at the output of about -300mV, oscillating between 0 and -350mW approx. (running my amp from an unregulated switching power supply, ) Is this normal? |
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| rljones |
ssanmor,
I've noted a few times the temperature going up, but all were fault conditions: screwed device, not sufficiently tightened to heatsink (gap filled with grease apppeared misleadingly tight), bad servo op amp (I'd unwittingly fried it--but that's another story), and misplaced resistor on one servo op amp from a pair of LM3886. Thus, localized heatsink problem or in-operable servo were the problems. Your offset is most likely due to not using the servo; if the devices are not perfectly matched you may get offset. As I previously posted, I found the DC offset to be between 2 and 4 mV. This latter point was stressed in the App note from National. A properly operating servo in this amp is a good thing.
The sound is really good from this amp. And measuements great too: distortion around < 0.001% up to it's symmetrical clipping point; and it's even < 0.07% at 50 kHz! I re-measured the unit I listened to and it's flat from 10 Hz to 100 kHz (down perhaps 0.5dB at 100 kHz, the limit of my test equipment).
Another potential source of overheating would be oscillation. Have you connected your amp to a scope? Also, if you're trying to run in bridged mode with a single ended input, it won't work and may overheat.
As for the thermal insulator, I used the 'TF' version of the LM3886. This comes fully isolated, albeit with a slight power de-valuation, and only requires some grease (or film) to attach to the heatsink. I prefer these packages as regular screws without insulators can be used.
Your measurement of 80W into 4 ohms is about right for one device. The values I quoted were for a paralleled pair--this distributes the current load and permits 110 W into 4 ohms.
hope this helps, good luck. |
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| ssanmor |
Thanks, rljones, for you help.
Do you really think that using TF package is worth the pain for high powered amps? I mean, for a parallel-bridge configuration giving more than 200W/4 Ohms?
And, the offset you get, 2 or 4 mV, is WITH servos, right? Do you have measurements without them?
And, at last, what about tightening the device to the heatsink with some kind of metal bar so it is pressed against it over all the body, not only the screw hole? This can be good for several devices, what do you think?
Thanks again. |
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| ssanmor |
Ah, another thing I forgot.
Yes, I have looked at the output with an oscilloscope. It seems to be no oscillation, except a little in the peaks when you are about to reach the supply rails. When connecting to a real speaker, I have observed no problems.
The strange thing about the offset is that it varies a lot randomly, between 0 to say -300 mV. And with no signal there is about 50mV p-p of noise from the switching supply.
(It was only to give you more data)
Thanks |
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| rljones |
ssanmor,
You mentioned in your earlier post that the first amp worked and only got warm, then you "re-designed the pcb" and now the amp gets hot. Maybe you have some traces wrong? Are you driving it single ended or balanced? Do you invert the output devices like the app note (I don't: I send in an inverted signal on one leg, but run everything in the amp non-inverted for better linearity)? So maybe you changed your design circuitry between the two units?
If not layout or design, possibly it is the power supply. You indicate that you are using a switching supply--maybe that's a problem. I use an old fashion power supply: a toroidal transformer, rectifier bridge, and capacitor bank for my main +/-35VDC supply to the output devices (LM3886TF), which of course is also locally regulated to +/- 15VDC for the op amps. I'm no expert on digital power supplies, but you may have switching noise contaminating your signal. The power supply also may be radiating and affecting the input; maybe try adding a filter on the input (100 pF to ground?). Try rigging up a non-switching supply and see if the noise doesn't go away. Otherwise, you may need to fiilter it better, or try a greater switching frequency.
Speaking of switches, since I'm posting again, I might as well add one more thing I'm trying. As for the CPU fans that are running in series, I'm adding a thermal switch. Each fan normally runs off 12VDC. I'm supplying 8VDC (9 may be better, but I had an 8V 3-legged regulator on hand). Since both are in series, they're effectively only seeing 4VDC apiece, which is why they run nice and quiet.
The switch is Digikey item 317-1019-ND, a 70 deg C, normally-open (NO) switch that closes once it heats up to 70 deg. When it closes, the fans will start up. This is in order to leave them off until absolutely necessary. It is probably best to have one on each pair of LM3886TF. The switch can be placed in a blob of silicon sealant (very good heat conductor and glue) applied to the heat sink next to one of the devices or on one device itself. This is then in series with the fans. Another switch can be placed on the second pair of devices and run in parallel with the first switch. This way, no matter which pair in the bridged configuration gets hot, both fans will come on. If the device temperature drops, then the fans turn off again. The way things are running, the only time they got hot was on the test bench. |
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| gauthier.robert |
There is a diy article in led magazine using 8 lm3886 per channel !! I am working to get all hardware and electronics for building a 4 channels amp + xover based on this article.
Robert |
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| ssanmor |
Robert, all of us would be very happy if you could send us scans of that article you mention. Is what most of us are looking for!
Don't worry about legal issues, after some discussions in other forums I know that it is not illegal to send scans of the articles by e-mail.
Thanks in advance. |
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| ssanmor |
By the way, rljones.
There is more people using your idea of mounting the LM3886 with small CPU heatsinks + fans, and seems to work fine (the only problem I see is noise, but a little bit of experimenting with their speeds and temperature can help).
Have a look at:
http://www.pablin.com.ar/electron/circuito/audio/amp50/
Regards |
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| Paulmcv |
Hi guys (and any gals...?)
Thank you rljones for saving me a heap of trouble. I was going to embark on the suggested design in the Nat Semi app note, but it makes sense to design it the way you propose. Please add me to the list of people who want details of the circuit board you design!
The heatsink idea also blew me away. I have been grappling with finding a nice looking, decent sized heatsink that wasn't going to cost me a fortune: a cpu cooler will be far smaller, neater, and cheaper!
The one thing that hasn't been mentioned in this thread is the fact that National Semi kindly GIVE AWAY these amp chips in quantities of FIVE at a time.
Just go to http://www.national.com/pf/LM/LM3886.html, and click on the order samples button, and away you go. This is particularly nice of them, considering that our local parts supplier, Jaycar, wants us to pay AU $20 retail each for them!
Now all I need is to find a manufacturer who will give me some samples of some 10,000 uF 50V+ electros, and another manufacturer who will sample me a few 500va toroidal trannies......
Cheers from Oz!:cool: |
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| ssanmor |
Hello, Paulmcv.
What you say is not a joke. I obtained two monster 0.5F/25V and another two 0.1F/63V capacitors, as well as some "small" 3300uF/63V from BMI capacitors.
Unfortunately, I didn't manage to find a toroidal transformer to be given out as samples :-(
Have luck in your hunting! |
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| paulb |
| quote: | Originally posted by Paulmcv
The one thing that hasn't been mentioned in this thread is the fact that National Semi kindly GIVE AWAY these amp chips in quantities of FIVE at a time. |
Samples are intended for manufacturers who may be ordering production quantities. If DIYers abuse this previlege it will disappear. Please be reasonable when requesting samples. I refrain from doing it unless there is no other way I can get the parts.
Thanks,
Paul |
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| ssanmor |
I agree with paulb.
I ordered some samples before and now I work for a big company and we appreciate a lot the possibility of obtaining free samples because it accelerate the development process a lot.
I think it is not bad for companies that DIYers use free samples, as it was in my case. Now we buy a lot of the products I tried before at home.
But please be responsible, and ask for them only in case you don't have other way to find them.
Regards.
(let's continue with the issue of this thread: LM3886 amps :-) |
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| AudioFreak |
| most companies also offer samples to university students to presumably help with component costs for those studying an EE or associated major. Given that i am just starting my EE in am most appreciative of this service and would be somewhat dismayed if it disappeared due to abuse. |
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| Paulmcv |
Hey guys calm down!:(
I'm not ordering commercial quantities of the things!
If they are happy to give away samples and courier them across the globe, then thanks heaps! They have decided that they are a big enough company to support it.
I agree: abuse it and you lose it.
I have ordered the parts, I will make an amp, and I'm sure I'll need to do some tweaking with the layout etc etc. before I'm happy with it.
If a friend asks me to build one for them, no worries. A $20 amp module is a small part of the total price to build it, when you add in the cost of the toroidal transformer, filter caps, and a half decent case to put it in.
If it sounds like sh*%te, I won't order or buy the part again.
The same can be said about op amps. I've been designing a pcb for the mixer project as described in http://sound.westhost.com/project30.htm , and have also been following the threads here regarding which particular chip/fet/tranny/diode/piece of string sounds good or sounds bad.
It is interesting to come across a chip that sounds good on paper, and do the exercise of getting one. You will often hear the following conversation: "Yes sir, we can get those for you. It will take six to eight weeks, minimum order of 2500 and they cost $xxxx each...." If they don't do samples, I don't get to audition the chip.
So sorry, guys. I didn't mean to start an abuse thread.
I agree with ssanmor; let's continue with the issue of this thread: LM3886 amps.
BTW, I have built an amp using a similar chip previously, and they work really well. I can put it in bridge mode for when my wife can't hear herself singing enough, and it still cranks it out. However, when these amps clip, or go out of their SOA, they sound *bad!* Somewhat like a lightning crack..... I've yet to quantify actual power output, because I'd like to know whether it's the power supply that can't keep up, or the chip doesn't like the roughly 6ohm load I sometimes use.....
Perhaps it's my wife's singing....;) |
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| gauthier.robert |
| the price can be checked at www.national.com - i paid 3US$ quantity of 20. I think quantity of 1 is around 6$US$ |
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| nowater |
hello, did the schematic of rljones ever come to light? If it was emailed, can I have someone forward it to me please?
Also, I would rather use the isolated TF version of the LM3886 if it is only marginally worse than the T with insulating washer. Can anyone confirm if the TF version is OK in practice? (also for the 50W solo chip amps)
Grant |
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| ssanmor |
I used 4xLM3886TF chips in a parallel-bridge configuration at +/-25V and it is working well after more than 1 year. But use a proper heatsink!
Best regards |
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| capslock |
Just referring to Rachel's request in the very first post here: paralleling is not going to improve damping factor because of the series resistors. Neither will bridging help because then both internal output resistances are effeciviely in series.
Paralleling and/or bridging helps distortion though by keeping the load on the individual amp down. |
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| analog_sa |
| quote: | | Paralleling and/or bridging helps distortion though by keeping the load on the individual amp down. |
Probably true if bridging is 'ideal'. As there is no NFB to compensate for different gains in the two arms of the bridge it's very easy to end up with higher distortion. |
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| tbla |
| what if servos are used...? |
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| analog_sa |
| quote: | | what if servos are used...? |
They do nothing for the distortion. And they screw up the sound. If Jeff Rowland doesn't feel the pressure to use them nor do i :) |
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| pinkmouse |
| For a very nice sounding implementation of dual LM3886s by JBL, you might want to look at this PDF |
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| peranders |
| quote: | Originally posted by pinkmouse
For a very nice sounding implementation of dual LM3886s by JBL, you might want to look at this PDF |
Or you could check the application note AN-1192 |
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| draget |
| quote: | Originally posted by ssanmor
I used 4xLM3886TF chips in a parallel-bridge configuration at +/-25V and it is working well after more than 1 year. But use a proper heatsink!
Best regards |
Hi, do you have a copy of your final schematic?
Thanks,
draget |
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| ssanmor |
Hello...
I have to look for it, it must be hidden in some old Orcad file in my old hard disk...
As soon as I find it, I will post it here.
Best regards,
Sergio |
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| ssanmor |
Ok, I have just found it and imported it from Protel.
My car amp had 6 inverting LM3886 amplifiers, all of them wired the same way, just as in the datasheet typical app. diagram.
I combined 4 of them in parallel-bridge and other two were for the mids/highs, independently connected in stereo.
For the bridge/parallel, simply group 4 channels in two pairs. All of the channels had 0.1 series output resistors.
Join the output of the 2 channels forming the first pair, and do the same with the second pair.
Both amps in the first pair are driven by the same input signal, the amps in the other pair are both driven by an inverted version of the input signal (generated by an external inverting unity-gain opamp). Then connect the speaker between the output of the first pair and the output of the second one.
I hope you can get it clear enogh from this explanation ;-) |
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| taro28 |
hi
i sampled some LM 3886 for make a amplifier. but i only have a 50V 500VA toroid transformer, i haven't a symetric 25+25 transformer
the question is if i make a bridged amp with two LM3886, i can use single power supply with a floating GND? (virtual GND created with 2 resistors)
thanks
greetings |
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| ssanmor |
Hello.
You can not simply create a virtual GND with two resistors, as the currents involved make that impractical.
But you can feed the chips with an unipolar supply. (see Figure 2, "Single supply application circuit" in the LM3886 datasheet). That's usually avoided due to the necessity of removing the large DC component at the output, but in this case you can leave the big output capacitor out, as the Vcc/2 voltage at the output will be cancelled when wiring the amplifier in bridge mode.
You can also use a coldamp switching power supply as well ;-)
Hope this helps |
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| taro28 |
hi
ssanmor: thanks for the explanation :D
i had seen the "Single supply application circuit", but i think that these modification is because the ouput is betwen output and GND.
i seen the "aplication note 1192, picture 5.2.1 Bridged Amplifier Schematic" and i think that the GND current in these mode is really small because all power conections is only between +- power suply, GND is only for input signal and some preamp part. the current is small, because that, i think that i can use two resistors (100ohm) for create GND. or i'm wrong?
greetings |
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| Bazukaz |
Hi,
If your transfomer is a toroid , you may cut the windings in the middle and make two-secondaries trannie.
Regards,
Lukas. |
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| taro28 |
hi
isn't possible because the transformer have epoxi into center :mad:
greetings |
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| ! |
| quote: | Originally posted by ssanmor
Hello.
You can not simply create a virtual GND with two resistors, as the currents involved make that impractical. |
The currents from what? The ground current should be relatively low in a bridged configuration.
I think it's do-able though I've not done it myself. |
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| tiltedhalo |
| To create a virtual ground referenced from 50VDC+ and ground at the proper amperage would take some pretty heffty resistors,quick math around 40 watts or so per resistor to float a center and then it will suffer mid to plus sag, would offer poor performance. |
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| poobah |
Yes taro,
You can do this easily... You must use 2 resistors and each resistor must be in parallel with the lage capacitors for V+ & V-.
The resistors should draw about 10 to 20% of the total load current in order to maintain good balance between the + and - rails. It is actually the capacitors that form the virtual ground, the resistors are just there to dimnish the effects of capacitor tolerance/value. You should, of course use 2 identical capacitors from the same maker, date etc...
You will need some extra power (VA) from your transformer to waste heat across the resistors.
:) |
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| ! |
| quote: | Originally posted by tiltedhalo
To create a virtual ground referenced from 50VDC+ and ground at the proper amperage would take some pretty heffty resistors,quick math around 40 watts or so per resistor to float a center and then it will suffer mid to plus sag, would offer poor performance. |
What did your calculations use as the necessary current flow? 40W seems way too high for a bridged amp. |
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| poobah |
Say the rails are +/-35. In a bridges config this would put 70 Volts across a 7 ohm speaker load... 10 amps.
Be generous and allow for 10% of this, 1 amp, to flow through the bleeder/balancer resistors. OK 70 Ohms... 70 Watts... 2 resistors @ 35 Watts each.
Now, the bridge configuration offers the opportunity to greatly reduce the resistors because symmetric currents are drawn from the +/- rails. Maybe a 1% of total current through the bleeder would be OK... 680~700 Ohms... 3.5 (7~10) Watts.
I think I would want to use 0.1% gain setting resistors to ensure symmetric current draw.
Total capacitance from rail to rail should be about 11,000 uF for a 2 Hz break with 7 Ohms... 2 - 22,000/35V caps should do it.
:) |
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| AudioFreak |
| 35V caps on a 35V rail? I think not. |
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| poobah |
35 WVDC... 50 V surge.
SCRATCH THAT... FORGOT 10% HIGH LINE.
Yeah 50 WVDC 75 V surge...
:smash:
:clown: |
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