| soundNERD |
I've been working quite hard on my quad LM3886 bridged amplifier. I've got one board done, which are two chips running in standard non-inverted mode. They will be, once that board is done, controlled by a DRV134 for the bridging. With the two chips done I fired it up in standard non-brigded mode, and it sounded great - I don't think I've ever seen any manufacturer call something as powerful as this 68W, because it sounds more like 300 Sony-Watts. Since I've got the case and boards designed for bridging, I'm still going to do it that way, and I'm sure I'll be blown away-literally-by the power.
Anyway, it sounds great, but I find it quite harsh with higher frequencies. Bass and mid-range is great, and so is treble, but if the song contains any very high-pitched voices, I notice it's very harsh and not natural sounding.
The only thing I can think of is either my PSU or the choice of parts on the amp itself. I used all metal film resistors in the amplifier, plus one 22uf non-polarized tantalum Kemet brand capacitor from feedback to ground. All of the resistors are NTE brand 1% tolerance.
In the PSU I have a single 2200uf capacitor per rail, as I've been told countless times that the lower the capacitance, the better the treble/mid, and the higher the capacitance, the better the bass. The goal of this amp is the best overall quality I could do, so I made a comprimise between them by using 2200s. Maybe I need more? Maybe I need less? And also how would adding some 0.1uf caps on the PSU board and/or the amplifier boards help as far as my harsh sound issue goes?
The setup I'm running now is:
Phillips portable CD player (Also tried my Creative Zen Xtra with some 320k full stereo MP3's, same issue)
B&W DM580 floorstanding speakers
~34-0-34 VDC at 400-500VA (not 100% sure)
10K input impedence (if it matters)
Can anybody provide any suggestions as to what I can do to calm down the amplifier? It would sound amazing if not for that slight issue.
BTW I'm using the schematic from the first page of National Semiconductor's PDF for the 3886.
Thanks,
Mike |
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| aletheian |
Do you have a filter network on the output? If not, give it a try, and then tweak it for your speakers. I've had good results without one, but it is just an option to try.
I don't like tantalums though... I doubt it makes a huge difference, but you might want to ditch that cap for a film. |
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| sam9 |
| quote: | | I don't think I've ever seen any manufacturer call something as powerful as this 68W, because it sounds more like 300 Sony-Watts |
Before proceeding to the real question, I feel compellede that based on my Sony experience, that ain't saying much.
Now to be more helpful. Do you have an oscilloscope? Can you borrow. There is a fair chance that in bridged mode your amplifier may be clipping. You could verify this pretty quickly with a 'scope. The common observation seems to be that LM3886's sound pretty nasty when clipping.
LM3886 sounds un-reasonably (for cost and size) good when used with 8 ohm speakers where all you need is 50W. In fact, if your needs are limited to a good 50W ampl for 8-ohm speakers, I think it is hard to justify anything more complex. (I notice that Siegfried Linkwitz is now suggesting 3 LM3886's for his dipoles -- that should say something right there!) When pushed beyond this, I think you may be asking a bit more from the poor thing than is reasonable. |
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| MBK |
Hi,
I second Aletheian's suggestion. Don't omit the filters suggested ("optional" on the data sheet. In some configurations, the amp may go into bursts of oscillation without them.
One question of course is the impedance plot of your speakers and their sensitivity. If this is a difficult and inefficient load, you possibly do have a clipping issue right there. |
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| soundNERD |
Currently I only have the board that will become the left channel done, and since I don't have the DRV134 board done as well, I've got the amps running in standard non-bridged mode.
The speakers are 8-ohm and I belive are 87-89dB sensativity. Nothing that should be too hard for the chips to handle, especially since I'm not even close to pushing it to it's limits.
These aren't the speakers I will be driving with it in the end. I will be driving two in-wall 8-ohm speakers. I figured I'd bridge it in case those in-walls are abandoned and larger floor-standers are put in. Also the 3886 is, in a way, designed for 4-ohm, I figured I'd run it bridged to give each amp the 4-ohm impedence it wanted.
I do have a scope, and at one time had it hooked up when I was driving some old, cheap 4-ohm speakers. Sure, it would clip when the volume was cranked, but I couldn't detect anything before that. Then again the tweeters were trash compared to the B&W speakers. I couldn't detect anything unusual on it, though, when it was hooked up to those speakers. I can plug it in with the new speakers connected if needed.
I've always found the B&W's to be very bright, so I was expecting bright sound, so that may be part of it, but it just seems excessively harsh.
It could also be possible that the amp needs a few hours to break in? I've heard of drastic changes to sound quality once the chips break in.
Thanks again for the great, prompt help,
Mike |
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| I_Forgot |
| quote: | [i]
In the PSU I have a single 2200uf capacitor per rail, as I've been told countless times that the lower the capacitance, the better the treble/mid, and the higher the capacitance, the better the bass. The goal of this amp is the best overall quality I could do, so I made a comprimise between them by using 2200s. Maybe I need more?
Can anybody provide any suggestions as to what I can do to calm down the amplifier? It would sound amazing if not for that slight issue.
Mike [/B] |
Ignore the nonsense you've heard and try putting a decent amount of capacitance in the power supply. That Gain-clown BS about using 500uF or 1000 uF in the power supply is just silly. There's no way the chip can provide decent low frequency output when the power supply rails are sagging and the ripple is high. Bypass the big caps with smaller caps, as is standard practice when using big caps, and your troubles will disappear.
I_F |
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| soundNERD |
Thanks.
What would you suggest as being "decent"? |
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| sam9 |
| quote: | | What would you suggest as being "decent"? |
One common rule of thumb is 10kuF per 100W per rail. I.e., a 100W amp would have a pair of 10kuF caps. Double that for two channels. I doubt more than that would serve any purpose and personally, I've used less (6.8kuF rather than 10kuF) without hearing any difference.
Re my earlier comment, since you have eliminated clipping as a culprit, I suspect the other comments offered may com closer. The anti-zobel, anti-output inductor school may have a point but I think there iare so many pemutaions of speaker and cable reactiance that are possible, it is best to assume they are required until proven to your satifaction that they are not required in circumstance you have. |
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| aletheian |
| Anything around or over 10KuF should be fine. Switching from 1000uF to 6800uF is a big difference in sound, and from 6800 to 10000, it is less so. I have a stash of 15,000uF caps that I use in these things, but I would be happy with 10000uF if I didn't have those. |
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| lgreen |
Try some different speakers and see if it still sounds that way; I don't think its break-in or the capacitance, mine had only 2,000 uF per channel when I made the 1st version and it was fine. Might want to bypass the electrolytics with some small value films. Might want to look at the scope, look at a square wave and see if there is excessive ringing. Try a zobel (will take about 10 minutes) on the output.
If your ultimate implemention is bridged, you might get less noise, because if the problem you are having is common mode, it will be eliminated by the balanced design. So hope for the best.
Hey, I remember when you first posted on this, congratulations on getting this far! |
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| soundNERD |
Thanks again for the helpful suggestions.
The problem I'm going to run into with adding capacitance is the physical size of the case and the size of the capacitor board. I think I can fit at least one or two more sets of 2200uf caps plus a 0.1uf bypass cap on it and hope for the best.
I'll also try the zobel network on the output tomorrow afternoon and see what happens. I've never messed with zobels before so could you suggest how to go about adding one? Also, how do I add it? From + to ground on both chips, or from + to + since they will be running bridged later on?
Thanks! |
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| pinkmouse |
| I concur. HF ringing or even oscillation would give exactly the problems you describe. But then again, it could also be RF interference. Zobel the output, check your lead dress, and have a look at 10K square waves. |
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| head_spaz |
The harshness you are hearing could easily be the MP3 source you're listening to.
MP3's are compressed... and I hate the sound because of the high-end sizzle.
Good luck. |
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| soundNERD |
I have tried MP3 and regular uncompressed CD-DA audio sources. The problem is there no matter what.
how can I generate 10k square waves? |
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| lgreen |
| quote: | Originally posted by soundNERD
I have tried MP3 and regular uncompressed CD-DA audio sources. The problem is there no matter what.
how can I generate 10k square waves? |
You can use a signal generator or download a .wav file from me, see my www link below. NOTE- these are square waves which do not perfectly convert to the digital domain, so this will already have some ringing in it due to the A/D converter, so its better to use a sig. generator, but if you don't have one, you can use these or make some kind of circ. up.
Zobel should be 2-5 ohm resistor (2W or more) in series with .1 uF cap or so to ground from both outputs. |
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| BobEllis |
| Although I concur with the suggestions to add capacitance and a zoebel, another possibility is heat sinking. How much do you have? Not enough and the chip will go into its self preservation SPIKE mode, which sounds horrible. |
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| soundNERD |
I've got plenty of heatsinking - one large heatsink per chip. I've checked heat as an option. The amp is slightly warm after an hour of normal listening, but very cool, though. I can comfortably touch the heatsinks and even the chips.
It may make the problem worst if the chips are overheated, but I know for sure that during none of my testing were they even close to overheated.
I'll get a picture tomorrow and post it. |
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| BobEllis |
| Glad to hear that heat sinks aren't a problem, but since nobody brought it up I thought I'd mention it. Good luck with your mods. |
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| coredump |
If the problem still persists, try fitting a 220pF Silver Mica between + and - of the signal input. This is also mentioned in the data sheet and brought a huge performance gain in my case (dirty mains, I suspect). Please note that a quality cap is needed here, bod standard ceramics won't do (at least not without compromising the sound).
It will also attenuate the mains' clicks'n'pops nicely (if you have any).
Best,
Oliver |
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| rossl |
Change out that tantalum for a 100uF non-polarized electrolytic.
Bypass the 3886 power supply pins with .1uf as close to the pins as possible.
You might be loading your input source. What kind of input coupling are you using? |
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| sam9 |
For a different way to use an LM3886 look here:
http://www.marchandelec.com/ftp/pm21man.pdf
Some years ago I built one of the PCBs and found it works (an sounds) very well. He seems to be the only one around who does anything that is not just a variation of the app notes in the datasheet. |
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| soundNERD |
Well, the first thing I did was add more capacitance. I added another set of 2200uf caps and a set of 0.1uf caps to the PSU board.
Didn't try it with the B&W's yet, but I can definately say it made a huge difference with the other speakers. It really opened up the sound from the amp. On top of that, it (oddly) literally cooled down the amp. It ran quite warm before when I began pushing it, a little too warm for me. Now it's much cooler, especially considering I now also have it in bridge mode. I guess this is because theres less AC in the power rails the amp has to deal with.
Anyway, like I said, the DRV134 bridge board is complete. I've got two Texas Instruments LM7812 and LM7912 regulators and two DRV134 chips. Both are powered up, but only one chip is driving an amp since only one amp board is completed. This brought about some new problems. Theres a lot of static/hiss being sent into the amp. Although it cancels out in bridge mode, if I try and run it single-ended, you really hear the static/hiss. On top of that, there is a fairly loud buzz being created by the DRV134. Finally, the chips begin clipping the input signals way before the amps are close to clipping. Do you think this is due to not having any capacitors immediately before or after the regulator chips? There are currently no caps on the board. I didn't think it would matter that much, so I didn't put any on. If they are needed I can filter the regulators.
Also, would a circuit such as the following: http://www.diyaudio.com/forums/show...1075#post641075 work better than the basic no caps no resistors circuit? I wonder because I already have 10k to ground resistors on the amp. Also, what purpose does R3 in that schematic serve? If I used this schematic, the only thing I would add is the R3 resistor. I don't have room for the whole DC offset adjustment circuit.
Thanks again for the great help,
Mike |
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| lgreen |
Wow a lot of issues there.
- warning! adding caps usually increases the temp of your chip, not decreases. this is because you have a higher average voltage across the chip as there is a lesser AC drop with more caps. I found this to be true with both increased caps and a regulated supply.
You might want to check your DC values and connections again if you chip is cooler.
- Now lets talk about bridge mode. In bridge mode it should be much hotter than in non-bridge mode since each chip is outputting twice the current.
-OK, maybe you had some type of oscillation before (causing heat) that the bigger caps fixed, so now everything is working as it should be.
- I've used DRV134's with as little as 6V and as high as 15V DC supplies and never had any issue (noise) like you report. It might be grounding or might be the power supplies that you built.
Don't be shy! I'd like to see your schematic (and some pics) and I'm sure others would like to see it too! it would certainly help with these questions. |
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| soundNERD |
I'll throw together some schematics later today and post them. I'll also take some pics later today and put them on here.
I do think it was oscillating. That was probably the cause of the HF clipping. With the more caps, it's running cooler and the HF clipping is gone. |
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| soundNERD |
Schematics and pictures still coming, but I've got an update.
First of all, it's too dark to take a picture, as we all know digital cameras suck enough in the daytime, let alone at night. Tomorrow I'll take a couple of pictures.
I'm going to work on the schematic after im done with this post.
Anyway, I gave up on bridging. I decided it was a stupid idea considering the LM3875 amp could only handle 8-ohm and really struggled with the in-walls. Instead, it's paralleled now. This way it's stable into 8/4/2-ohm loads.
So I ditched the DRV134 board and couldn't be happier. The amp is dead silent now and sounds amazing. I connected it to my B&W speaker (only have one channel/2 chips done still) and am extremely satisfied. I can't wait to get the other channel done, add the volume control and finish up the enclosure. |
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| soundNERD |
| Heres the schematic of one channel how it's configured now. |
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| soundNERD |
Had a slight setback today...
Being cheap, I used the 35V caps I had rather than 50V caps in my PSU, which runs around +/- 33-34VDC. The small 0.1uf tantalums blew up today and took one of the 2200uf caps. So I tore off the blown caps and one more 2200uf capacitor to balence each rail out, so I'm back to 2200uf until I can get some new capacitors. Until then I'm not going to run the amp too much because I don't need any more capacitors blowing up.
Hopefully I can get some 50V caps, and I'll go for larger values as well, soon. |
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| EWorkshop1708 |
Tantalums suck with any type of voltage over 12V IMO
It's so easy to make them smoke and shoot red glowing stuff. :hot:
Since you are filtering powersupply, use some HV ceramic or plastic caps instead.
And how did the tantalum take out an electrolytic cap? Did the 2200uf cap vent smoke?
I bet even with only 2200 per rail, you can still keep away your oscillation, just add some more 0.1uf caps. It's the High Frequency stuff you have to worry about, and the smaller caps take care of that. Or you can get 0.22uf or larger ceramic or plastic and they work good too.
I'm using 500V 0.18uf ceramic on my amp's rails, they worked good and totally killed any oscillation I had before adding them. |
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| Nordic |
| With those rails go for 63V caps |
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| soundNERD |
I didn't really pay attention to the fireworks. I was more interested in pulling the plug so my house didn't turn to fireworks.
The two tantalums were completely exploded, the tops were burnt and opened. Even the wires directly above them got grilled too. It melted it, but didn't melt through to the wire so its OK.
I don't know how the tantalum blowing destroyed the 2200. One of them right by the 2200s was bulged on the top and the bottom was black and the blue plastic coating was slightly warped. The bottom was very likely caused by heat and black junk from the tantalums blowing, but I didn't like how the top was bulging. So I took it off.
I'm going to be on the lookout for some new caps. When I'm near it I'll look at Amercan Science and Surplus. Thats where I got those 2200uf/35V caps for only 25 cents each. I literally bought out the store (they only had above 20-25 left, but none after I was done there). Hopefully they will have gotten in some 50V or 63V caps.
Also, Nordic, why do I need 63V caps with a 35V power supply? 50V still gives a 15V buffer?
Finally, for the 0.1uf caps, I've got a bunch of brand new surface mount WIMA capacitors. I'll solder some leads to two of the 0.1uf/630V caps and put them into my power supply. |
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| Leolabs |
| Hi guys,I think you all forget about the source is just a Philips portable CD player and a MP3.To me,the harsh sound is not the LM3886's problem. |
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| soundNERD |
I've already figured out that isn't the problem.
It's playing regular CDs, not MP3 CD's.
Adding the capacitance to the PSU completely fixed the harshness. |
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| soundNERD |
Here are the pictures I've been promising.
Cables still need to be neatened up, and will once I'm done with the rest of it.
But here's what I got so far:
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