Lm3886 Pcb

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roadkill said:
Thanks for all your patient answers. I now know why you have higher frequency for your low roll-off... you have a separate sub. Kewl!
  • The badly written part on the 50-100 ohm resistor should go something like this: if the input connector to the PA falls off, then the amplifier has no ground reference against which it compares the input voltage. Thus, the output can swing to one of the rails, welding tight any speaker connected to the output. If you connect the grounds together, there *may* be hum (depending on your layout and wiring), so the compromise is to use a resistor between the two grounds (input and power). This way, there is a ground reference even if the input connector is removed. When there is a connector in place, though, the ground path through the connector, to the preamp and back to the PSU star point is much lower in resistance compared to the resistor between the planes on your amp board. Thus, no "ground loop" is formed.
Understood. What your low-value resistor does is basically act as a ground-lift resistor, used in many professional power amps. (Randy Slone uses values about one-tenth of what you've suggested, but the principle is the same.) But in my case, I am thinking of connecting the IN-GND to a common star ground point using a separate wire anyway. That way, even when there's no preamp connected at the input, the IN-GND will have a ground reference. Should be okay, don't you think?
 
DC blocking in input and the ground leg...

theChris said:
basically, if you look at what causes the DC offset problem, you'll notice that there are 2 places for the DC offset to originate: from the source, and from the amplifer. a large value cap on the input will correct the source issue, and a cap in the feedback path should help the DC offset of the amplifer. without it, any DC offset gets multiplied by the gain of the amplifier.
Got it. I had never thought of these two as alternatives till you pointed them out. I'll think some more, and when I've thunk, I'll pester my other friends and you some more, maybe. :)

Going by your reasoning, all well-designed amps (i.e. those which, left to themselves have very small output DC offset) can omit their input caps. The only time that input cap will be useful will be when I accidentally pump in a huge DC offset at the input itself, which can then travel through the power amp with a gain of 1 and still hit the speaker. But such situations will only happen due to very unusual freak accidents, right, and one can't really protect well against such accidents.... I mean, I wouldn't know how to get a high DC voltage into my amp's input even if I deliberately wanted to, other than connect some dry cells to it.

Interesting. Thanks a lot. :)
 
theChris said:
basically, if you look at what causes the DC offset problem, you'll notice that there are 2 places for the DC offset to originate: from the source, and from the amplifer. a large value cap on the input will correct the source issue, and a cap in the feedback path should help the DC offset of the amplifer. without it, any DC offset gets multiplied by the gain of the amplifier. [/B]
Sudden insight: does this mean that I can omit the (usually expensive) input cap in all non-inv amps? With these amps, I now have a means of bringing down DC gain to 0 (or is it 1?) without putting any cap in the signal path. Does this mean that I can eliminate the input cap completely in all such amps? If yes, then it's one big reason to opt for non-inv configuration, and save money and improve sonics (they say sonically the best cap is no cap at all).

For protecting the input transistors against accidental high voltages, I can always use clamping diodes between IN and IN-GND, to limit input voltage. I don't need caps for that.

What do you think, theChris? And if I'm right, then why does anyone ever use input caps on non-inv amps? (Or they actually don't, and I'm slow on the uptake? :D )
 
saltnpeppah said:
1. are you guys using unregulated power supplies or smps??
Class B power amps (this includes all chip amps I've heard of, other than the recent spate of digital amp chips) usually do not behave well with regulated power supplies of any kind, unless you have a regulated PSU which is rated for at least 10 times the current rating of the peak current drawn by the amp. This is because once the regulation circuitry begins to respond to the ebb and flow of current drawn by the power amp, it introduces one more element of non-linearity into the amp itself. This may actually degrade the stability of the amp in serious cases.

3. and would using a toroidal transformer instead of a E-I core improve the sound quality?? ( why i am asking is because the cost factor is almost 10 times)
There is no reason to use a toroidal transformer over an E+I in audio, other than (i) space constraints and (ii) minimising the electromagnetic field around the transformer. The former may or may not be an issue with you, and the latter can be addressed either with more space around the transfy, or by metal enclosures around it.

I have decided to opt for inexpensive 15-0-15V 500mA toroidals in my preamps just to keep the EMI around low-voltage high-impedance circuits as low as possible. These transfies are Rs.280 each, all inclusive, delivered from B'lore to Bombay. (An equivalent E+I in Bombay is Rs.50 each.). For power amps, I refuse to use toroids... they're just not worth the money. Constructors from other countries often use them, to be able to keep everything in small chassis, but their budgets are often higher than ours, and I wouldn't pay that kind of premium for cuteness and a slightly smaller size. Moreover, I often prefer to keep the (large) E+I power transformer in a separate metal box on the floor, so that EMI issues are pretty much eliminated anyway.... I'm sure my approach gives me lower EMI than the best toroid housed in the amp chassis. The cost of a 8"x6"x6" sheet steel powder-coated box in L.Road is Rs.80. Add some high-current connectors to wire it to the power amp chassis, and it still remains much lower than the price difference between a power toroid and E+I.

And if you still feel like opting for toroidal, I think the price differential will be much less than 10 times, if you source both transfies from the same country. A ratio of 1:2 is more likely. How did you get the 1:10 figure?
 
My PCB

Here is my non-inv LM3875 PCB, which I've yet to freeze on. Compared to Roadkill's PCB, this one is larger: it's about 3.5" by 1.5". The red rectangle around the chip is an area marked in the "tKeepout" layer of Eagle for the chip. I use that area to keep other components out of that restricted area, because I want to use a clamping bar on the chip to hold it to the heatsink, and put two bolts on two sides for the bar. Also, my PCB contains the smoothing caps (1000uF to 2200uF per rail); I don't have any off-board smoothing caps. The rectified power supply will come straight from a bridge (yes, I intend using a plain vanilla bridge of 25-35A for a set of amps) to the board.

I intend to use L-clamps to attach the PCB to the heatsink, like I've done here. In that case, only the two mounting holes at the rear edge (i.e. top of picture) will be used, the other two are there "just in case."

I have provided space for a large input cap, with lots of alternate pads to accommodate various sizes. Now, with the inputs that theChris has given, I may be able to eliminate the input cap altogether. That'll save me some money... I was planning on a good MKT cap there.

I've also put the smoothing caps to the sides of the chip, not in front, to allow me to use a screwdriver easily to tighten the bolts of the clamping bar holding the chip down. I intend to use non-isolated "T" type chips, incidentally, with mica sheets and heatsink grease.

Comments please? And the Eagle files are available in case any of you want them.
 

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Life on the edge

BrianGT said:
I would put the LM3875 closer to the edge of the board. I placed mine .1" from the back of the board, which seems to work good for mounting the board directly to the heatsink.
I too had the fear that the chip shouldn't be any further forward than I could avoid, even jutting out a bit if needed, but of course, my beloved Eagle DRC would begin hollering if I did that. :)

However, there seems to be some confusion somewhere... I've certainly put my chip closer than 100mil from the edge of the board. If you want, I can send you the .BRD file and you can take a look. From the picture, does it appear that my chip is far from the edge? Here's another picture, much magnified. See if this makes things clearer? As a data point for comparison, the centre-to-centre distances between two adjacent front-row pins, or two adjacent back-row pins for that matter, is 0.134". Visually, it seems that my chip is way closer than 0.1" from the rear edge, isn't it?

One more reason to keep the chip where it is, and not shift it any further back, was to keep the supply rail tracks at their current widths. I don't know how much I need to worry about that if I move the chip back by a few mil, but I thought this was a nice compromise.

And since I'm not using the "TF" type chip, I'll get a few mils from the mica sheet and heatsink grease.
 

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Retired diyAudio Moderator
Joined 2002
Re: Life on the edge

tcpip said:
I too had the fear that the chip shouldn't be any further forward than I could avoid, even jutting out a bit if needed, but of course, my beloved Eagle DRC would begin hollering if I did that. :)

However, there seems to be some confusion somewhere... I've certainly put my chip closer than 100mil from the edge of the board. If you want, I can send you the .BRD file and you can take a look. From the picture, does it appear that my chip is far from the edge? Here's another picture, much magnified. See if this makes things clearer? As a data point for comparison, the centre-to-centre distances between two adjacent front-row pins, or two adjacent back-row pins for that matter, is 0.134". Visually, it seems that my chip is way closer than 0.1" from the rear edge, isn't it?

I was referring to putting the top row of the pins on the LM3875 header 100mil from the edge of the board. The rows are 200 mil apart, which is enough to put the V- trace between them.

If you get DRC errors, just modify the silkscreen part of the chip.

--
Brian
 
Re: Re: Life on the edge

BrianGT said:
I was referring to putting the top row of the pins on the LM3875 header 100mil from the edge of the board. The rows are 200 mil apart, which is enough to put the V- trace between them.

If you get DRC errors, just modify the silkscreen part of the chip.
Okay. Will see how I can get this effect.

But regarding the space between the rows of pins, I seem to see a lot of the space getting eaten up by the pads. Will the gap still be enough? Can you post a magnified image of just this portion of your PCB layout, like I did? Also, there's a track going from front to back to pin 8. Since mine is a single-layer PCB, I'll have to put a jumper there. I'll find it difficult to put my feedback path components on the underside, since I have four components in the feedback path (okay, I can reduce it to three) to your one.

Thanks a lot.
 
Retired diyAudio Moderator
Joined 2002
Here it is.

If you are curious, I decided to solder the NFB resistor directly to the IC pins instead of putting it on the pcb. This cleans up the board a bit, and provides the shortest possible feedback path, which is very important to the design.

--
Brian
 

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BrianGT said:
If you are curious, I decided to solder the NFB resistor directly to the IC pins instead of putting it on the pcb. This cleans up the board a bit, and provides the shortest possible feedback path, which is very important to the design.
Actually, if I had to choose between moving the chip closer to the edge and switching to a double-sided PCB, versus keeping the board and FB resistors as they are but retaining a single-sided PCB, I think I'll choose the latter, just to keep costs low. Please try to accept my different perspective. :) I am not as advanced an audio system builder as you (and many others) are, and I'm not at all sure I'll be able to hear the sonic improvement that comes out of shortening the FB path by a few millimetres. On the other hand, I want to retain the other FB components as I've shown them, i.e. the cap and extra resistor, because others on the forum have reported instability with this amp at 300KHz and above, and I don't want to suffer that if possible.

Perhaps the only reason why I didn't opt for your excellent group-buy opportunity was because I could see that your design was geared for very high-end (and expensive) passive components, whose benefits would be lost on me. I'm trying to design this PCB to let me sample the Gainclone's sound as a first attempt with off the shelf MFR and caps.

But I'll see if I can move the chip further back anyway.
 
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Joined 2002
tcpip said:
Actually, if I had to choose between moving the chip closer to the edge and switching to a double-sided PCB, versus keeping the board and FB resistors as they are but retaining a single-sided PCB, I think I'll choose the latter, just to keep costs low. Please try to accept my different perspective. :) I am not as advanced an audio system builder as you (and many others) are, and I'm not at all sure I'll be able to hear the sonic improvement that comes out of shortening the FB path by a few millimetres. On the other hand, I want to retain the other FB components as I've shown them, i.e. the cap and extra resistor, because others on the forum have reported instability with this amp at 300KHz and above, and I don't want to suffer that if possible.

Perhaps the only reason why I didn't opt for your excellent group-buy opportunity was because I could see that your design was geared for very high-end (and expensive) passive components, whose benefits would be lost on me. I'm trying to design this PCB to let me sample the Gainclone's sound as a first attempt with off the shelf MFR and caps.

But I'll see if I can move the chip further back anyway.

I could have easily made my board single sided, but it is the same cost here in the states to get the board done double sided as single sided, as most all of the board manufacturing places use a double sided process more often.

My pcb also works with cheaper components. I am looking to offer a more budget orientated kit, in April, with cheaper resistors, knocking $20 or so off the price of a kit.

Your layout looks good so far, and it certainly is fun making your own pcb layout if you are able to do so. I hope your boards work out well for you. You could even start your own group order there in India for all your friends :)

--
Brian
 
BrianGT said:
I could have easily made my board single sided, but it is the same cost here in the states to get the board done double sided as single sided, as most all of the board manufacturing places use a double sided process more often.
Wow... I didn't know single-sided had gone so out of fashion with guys at your end of the world. :) BTW, what kind of pricing are you getting for the board manufacture, if, say, you order a few hundred boards? And are you doing the panelising or will the board maker do it himself?

My pcb also works with cheaper components. I am looking to offer a more budget orientated kit, in April, with cheaper resistors, knocking $20 or so off the price of a kit.
The price I'm hoping to do for very small quantities of my amp will be:
  • PCB: Rs.50 each, single sided, 2ounce Cu
  • Smoothing caps: Rs.40 for a pair
  • other passive components: Rs.10
That's a total of Rs.100 (a shade over USD 2.00) without the chip amp and with no input cap. If I buy the chip amp from Digikey, and use the GE metallised polyprop caps on offer at low rates from Madisound, I'll add another USD 8.00 to the total. That makes it USD 10.00. And I'll get 1% MFRs at this price, plus ordinary electrolytic Al can-type caps. Of course, I'm not putting any power resistor at the speaker output. That would have added another few cents to the total, if I'd gone for the wirewound white-coffin types available everywhere.

I don't know how this figure compares with your budget kit. I know that if I forego solder masking and legend printing, I can get the PCBs (my size, 3.5" x 1.5") done on 2-ounce Cu for Rs.20 each. And this fab-house makes good PCBs, if you see all the details I'd posted here and in subsequent posts. Of course, I'm omitting the cost of the films, which will be a one-time cost.

I want to make ten amps straight away, because I'm working on a two-way five-driver speaker system, which will be driven by one amp per driver, with active xo. I'll see whether your budget group-buy looks easy for me, or else I'll try local sources. :)

But I'd be nowhere without the help of friends on the forum, including you. You guys are the only reason I have the confidence to wing it on my own, because I'm actually not on my own. Thanks. :)
 
tcpip said:

And if you still feel like opting for toroidal, I think the price differential will be much less than 10 times, if you source both transfies from the same country. A ratio of 1:2 is more likely. How did you get the 1:10 figure?

Well, I got a quote of Rs. 2300.00 ( :bigeyes: ) for a 750VA toroidal tranformer; but on the other hand, a local manufacturer is quoting Rs 250.00 for an E-I core... its not exactly 1:10; but close to it.

By the way, what method do you guys use to fabricate PCBs?? i tried to use the toner transfer method with limited results..

I tried to use copier paper (UGH!! what a mess!!) and then with label backing sheet..I removed the labels from the sheet and printed the pattern on the glossy side; but the toner wouldn't get into the material. I tried to iron this onto the board, but again with limited results.

Please help!!!

Madhu.
 
Madhu,
Go to Om Electronics (SP Road, third shop), give him a laser print of your layout (on A4 copier paper), he will quote a price (for PCBs of the size we're talking about, it's usually less than Rs. 100), pay him, get an order slip. Come back after a few days, show him the slip, collect your PCBs. Easy :) I tried the toner transfer method, results were quite shabby. Make sure you ask for a Glass Epoxy PCB. Phenolic is not worth the 10-15 rupee savings.
Also, the price you've mentioned is normal for a 750VA toroid. A 750VA E-I will cost a LOT more than Rs. 250 (close to 1k, I guess). For 250, you'd probably get a 250-300VA transformer if you buy it readymade. A custom wound transformer (try Universal Transformers, opposite Vishal Electronics), this price will get you a 225VA transformer.

Mike,
I don't think they will ship to the US, they're a relatively small-time operation. If you like, I could try and help you out with this, although I haven't got a clue about the shipping process. The courier charges would probably be many times the cost of the board.

Tarun,
The reason I put the ground lift resistor is because a 25-paisa el-cheapo carbon film resistor can save the life of an expensive loudspeaker in the event that the wire/connector between the star ground point and the amp's input ground ever comes loose.
Nice layout, here are some points:
  • Either manually insert some space between the polygons for the ground plane and power planes, or use Eagle's "Net Classes" command to assign a clearance of more than 15 mils. The planes, if they're so close, will cause problems due to solder bridging, etc.
  • How are you going to connect the speaker wire? If you just solder it to a pad, the joint will eventually either break or cause the pad to lift off the board, since the wire used will be heavy gauge.
  • The pad dimensions you have used are more suited to a PTH board. If you use similar pads for a single sided board, the adhesion will be very poor, and the pads will tend to lift off easily. Usually, octagonal, elongated pads are used for single-sided boards.
  • I'd advise you to use large filter caps close to the bridge rectifier. It may sound like paranoia, but the charging/discharging current into the filter caps will cause EMI, esp. for bridge rectifiers without snubber caps. Keep the smaller caps (220u or 470u) on the board, for local decoupling.
  • Disable thermals for the various planes being used. They will limit the current flow very badly.
Brian,
What PCB software are you using? I hope there's a freeware version available, since it can do rounded bends and polygon edges!

I asked Om Electronics' proprietor about the PCB process. He said the laser prints are used to make films, which are then used to make a silkscreen. He then squeezes etch-resist ink onto a bare PCB and etches it. Naturally, the quality is nowhere nearly as good as a true photo-etched PCB, but it's good enough ;)
 
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