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Old 13th March 2004, 01:34 AM   #61
tcpip is offline tcpip  India
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Quote:
Originally posted by roadkill
Tarun,
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!
Quote:
  • 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?
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Old 13th March 2004, 01:40 AM   #62
tcpip is offline tcpip  India
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Default DC blocking in input and the ground leg...

Quote:
Originally posted by theChris
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.
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Old 13th March 2004, 03:02 AM   #63
tcpip is offline tcpip  India
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Quote:
Originally posted by theChris
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? )
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Old 13th March 2004, 03:21 AM   #64
tcpip is offline tcpip  India
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Quote:
Originally posted by saltnpeppah
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.

Quote:
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?
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Old 13th March 2004, 03:34 AM   #65
tcpip is offline tcpip  India
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Default 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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Old 13th March 2004, 03:48 AM   #66
BrianGT is offline BrianGT  United States
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Default Re: My PCB

Quote:
Originally posted by tcpip
Comments please? And the Eagle files are available in case any of you want them.
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.

--
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Old 13th March 2004, 04:03 AM   #67
tcpip is offline tcpip  India
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Default Life on the edge

Quote:
Originally posted by BrianGT
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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Old 13th March 2004, 04:10 AM   #68
BrianGT is offline BrianGT  United States
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Default Re: Life on the edge

Quote:
Originally posted by tcpip
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.

--
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Old 13th March 2004, 04:18 AM   #69
tcpip is offline tcpip  India
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Default Re: Re: Life on the edge

Quote:
Originally posted by BrianGT
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.
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Old 13th March 2004, 04:21 AM   #70
BrianGT is offline BrianGT  United States
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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.

--
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