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Old 31st December 2012, 04:25 PM   #11
nattawa is offline nattawa  Canada
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@ gmphadte: The output stage has maximum of 5-pairs of power transistors, so it should be good for at least 300W into 8-ohm load. However, if such output power is intended, the power rails for the small current stages, including the input, VAS, and pre-driver, which is currently +/-60V should be re-sized accordingly, the TO-126 transistors used in the current design may have to be changed to higher voltage rating modles as well to suit the increeased supply rail voltage, KSA1381/KSC3503 for example.
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Old 31st December 2012, 05:23 PM   #12
nattawa is offline nattawa  Canada
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@ gmphadte, The power MOSFETs used in the solid state speaker relay in the current design have voltage rating of 80V, which is fine for an intended +/-55V rail for my first build. They should be resized too if the rails are elevated significantly.
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Old 31st December 2012, 05:25 PM   #13
khalsa is offline khalsa  India
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nattawa i want to make thise
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Old 31st December 2012, 06:07 PM   #14
CBS240 is offline CBS240  United States
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Quote:
Originally Posted by Vostro View Post
... but fine tweezer work sucks.
Tweezers would work for some of the larger SMDs. I use a small flathead screwdriver, like one for eyeglasses or slightly larger to assemble tiny SMDs for prototypes. You dip out a glob of rosin onto a lid or something, then touch the screwdriver tip to the glob of rosin and the SMD part will stick to it. Tin one of the pads with solder. Touch the SMD part to the screwdriver tip so as you can properly place it on the pads, including the one you just tinned. Then melt the tinned pad to the SMD pin so it will hold in place. Then solder the rest of the pins in place. Non-prototype SMD circuits should be soldered by reflow or wave or something. I do SMD parts as small as 0201 size resistors, 0.6mmX0.3mm. SOT-923, SOT-563 amd such is not much problem at all.
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Old 31st December 2012, 07:13 PM   #15
HighTec is offline HighTec  Croatia
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Quote:
Originally Posted by CBS240 View Post
I do SMD parts as small as 0201 size
Click the image to open in full size.
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Old 3rd January 2013, 03:37 PM   #16
nattawa is offline nattawa  Canada
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The schematic attached with the opening post has a wrong orientation. It is hardly a problem unless one views it with an ipad. So I uploaded the pdf files again. The schematic now has the correct orientation.

A slight change was made to the output sampling resistor R36. It now has a 1206 footprint, vs previous 0805, for a largest possible power rating SMD resistor for the available space, in order to reduce the distortion from the power consumption modulation to the resistor value, an effect discussed quite extensively in this forum. One may want to choose to use a thin film resistor for R36, as a thin film is not as much subject to such modulation as can be a thick film resistor. The through hole option for R36 remains unchanged at the same place. It allows the use of a 12.5mm lead spacing through hole resistor (or a stream of a few) when SMD is opted out for R36.

The prototyping of this amp may not be as easily going as may be a through hole PCB. Many SMD parts are surrounded by tall components and heat sink, demanding skills and patience when it come to troubleshooting, part changing, and signal probing. Proper soldering tools and skills will help here.

Cheers!
Attached Files
File Type: pdf Nothung_1.pdf (80.8 KB, 63 views)
File Type: pdf New_job.pdf (449.8 KB, 94 views)

Last edited by nattawa; 3rd January 2013 at 03:40 PM.
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Old 3rd January 2013, 05:20 PM   #17
rsavas is offline rsavas  Canada
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Default DC offset?

I see no provisions for an dc offset adjustment. No provision for a DC servo. Have you looked into this aspect of the design? What will be the offset voltage, what is your tolerance/limits? The input diff pair are discrete, thus matching maybe required to obtain the design goal.
How's the LTSPICE sim's going? Got your models?
I would assume that you have lots of experience with soldering SMT parts? I can help if you need some advise.
Current carrying capacity of the supply & output traces? are you planning on using 2oz copper plating? Leave some open solder mask in order to be able to deposit some extra solder to beef up the traces?
Not too sure about the optional cap underneath the SMT ecap's, such as C31/C59. Bottomside mounting of C31? will that not interfere? with the Heatsink? Not sure what side you will be mounting the O/P transistors.
Who will you get to fab the board, I can offer some sources.

Rick
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Old 3rd January 2013, 07:02 PM   #18
nattawa is offline nattawa  Canada
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Good point about the DC offset, Rick. I personally would not like more than 10 or 20 mV DC at the output. So I'll use BL grade input transistor pair that has an Hfe in the range of 350-700. This should result in a +/-16mV DC output in a worst case scenario in terms of Hfe mismatching. A simple Vbe measurement/matching would perhaps be adequate then.

The amp was designed to have an input cap to block the DC at input, and the DC gain is unity. It could be possible to implement an DC servo, but it has to be on the bottom side of the board = hard to get at. So I opted it out for now. Perhaps an off-board DC servo could be built and tested then be made into the layout afterwards.

I have zero experience with LTSpice or any other simulation s/w I hope I can pick one up quickly. No simulation has been run on this design yet. I'd appreciate any fellow forum members who are interested in this design if they go ahead run and share the simulation results.

I'll be using 2-Oz copper though the layout does allow using home-made bus bar out of straight pieces of single strand copper wire on the power rails, their decoupling ground returns, and the output node, as a possible alternative when lighter weight copper PCB is used. These wires can be easily soldered on the bottom side of PCB.

As of the PCB trace current carrying capacity, All the high current traces are doubled up on both sides of the board. Power rail trace width totals 10mm each, o/p node trace totals 20mm wide. 10mm on 2-Oz copper is capable of 15A continuous current at 5C temperature rise, and 20mm on 2-Oz has about 7-mOhm resistance over a 100-mm run at 35C.

SMT caps placed within the SMT e-cap footprints are installed when SMT e-caps are opted out and through-hole e-caps are opted-in. All SMT component except 9 that the Baker clamp is made of are on the bottom side. The o/p transistors will be on the bottom side as well.

I have used a couple Chinese PCB shops in the past, cheap in price but the shipping would cost a fortune though they are usually ok in quality for the money for simple boards like this. I'll be glad to hear about your sources when the time comes. Thanks a lot.
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Old 4th January 2013, 11:20 PM   #19
nattawa is offline nattawa  Canada
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I was able to put in the layout a Toshiba HN3A51F dual PNP as an optional replacement to the discrete input transistor pair. The two closely matched transistors should save otherwise necessary DC offset trimming or manually matching the discrete transistors.

However, I had a hard time trying to locate a vendor that stocks HN3A51F. In fact few even has the model number listed at all. I'm wondering any Diyaudio members could share a source.
Attached Files
File Type: pdf Nothung-TT_1-LTP.pdf (84.7 KB, 45 views)
File Type: pdf NOTHUNG-TT-PCB.pdf (453.0 KB, 52 views)
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Old 4th January 2013, 11:34 PM   #20
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You need to connect grounds in a star ground configuration or inputs will be modulated by output signals.
Also keep the power supply smoothing as separate as possible from the amp to again stop ground modulation.

This is what I found from designing a mixer. My first pcb hummed very badly.
Once I followed the star ground rules there was no hint of hum.

Also, sometimes it is good to add decoupling between power amp output and driver/ltp stages.
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