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Old 27th May 2012, 01:15 PM   #11
qusp is offline qusp  Australia
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sorry I meant are you hand soldering the 0201, not 0204 0402 resistor networks hey? vishay? I love their networks too, i'm looking at some of their bulk metal foil ones for the instrumentation input of a headphone amp i'm planning.
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Old 27th May 2012, 04:14 PM   #12
CBS240 is offline CBS240  United States
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This is the first build of the first proto-type, so yes I hand soldered every part including 0201's. The iron I used for this is a 25W iron with a small finishing nail as the soldering tip, along with a tiny flathead screwdriver. Tin one of the pads with solder, then stick the component to the screwdriver tip with solder flux. Place the iron on the pad with the solder and then stick one of the ends of the resistor to the melted solder and adjust the position properly with the screwdriver tip and while holding it in place, remove the iron. Then solder the other side. Those 0402 resistor arrays are not too bad to do either. SOT-923 parts are sort of a pain if the PCB pad layout is not good. You do not want to bury any pads inside large copper spaces because it makes it difficult to heat with a small iron tip. This goes for 0201 parts as well. Learned this the hard way with previous circuits. It is very tedious to do this way, but I plan to try to do a reflow process but I have to get all the component values worked out first. There is a mistake in the PCB that I had to correct for, fortunately it was not a show stopper. I discovered it just after I sent in the Gerber files, and it was too late. There are other minor changes I need to make on the PCB. For a first proto with all that circuitry, it came out pretty good. I would like to eventually make this into a kit, fully assembled but I will need to do a prototype version 2 to get any final bugs worked out of the PCB. I still have 3 PCBs left from this run and enough parts to build 1 more without ordering more, I may try my hand at a reflow trial.

It appears my previous post got cut off some how.

WRT Dimitri's last question, the IPS is cascode complementary common source J-fet with the feedback to the sources ala Borbely. It is preceeded by a buffer that sums the input signal with the output of the DC servo. The VAS is a complementary common gate J-fet stage. It provides 25Vp of signal swing plus the +/-10V spread for the output stage. The VAS bias is controled by a complementary common mode amplifer that modulates the source currents to the IPS.

I used typical miller compensation for the minor loops, and the feedback loop is compensated using TMC. The voltage gain is determined by this loop and also the common mode amplifiers as they too are part of the transfer function, and this is where it gets complex I am slowly but surely working on a paper to provide a better discription of how it works.
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Last edited by CBS240; 27th May 2012 at 04:30 PM.
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Old 21st June 2012, 02:24 AM   #13
Jay is offline Jay  Indonesia
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Hi CBS240, could you share your output NMOS model? I used a simplified version of the Fairchild subckt (as shown in the datasheet), by removing the effect of the temperature. It uses a single mosfet (MOSMOD in the model) with a few parameter included into the mosfet and some temperature dependent components removed from subckt. I'm not sure if my model is accurate.

Actually, I'm using P50N06 from Wisdom Semiconductor (WFP50N06). Their datasheet is not complete (so are Fairchilds and other), but the parameters seem to be better than the Fairchild or Intersil ones. I have break the mosfet bodies and it seems Wisdom's are better made.
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File Type: gif WFP50N06.GIF (15.5 KB, 434 views)
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Old 21st June 2012, 05:05 AM   #14
Jay is offline Jay  Indonesia
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Default EDIT

Actually Fairchild mosfet is faster and charge more current, even with higher capacitance. Here is my more simplified model based on Fairchild subckt and PDF:

.model WFP50N06 VDMOS(
+ Rg=0.690 ;RGATE
+ Vto=3.65 ;MOSMOD
+ Rd=10m ;RDRAIN
+ Rs=12m ;RSOURCE
+ Rb=11m ;assumed body diode R = Half Ron
+ Kp=18 ;assumed transconductance param. Fairchild is 35 with twice Qg
+ Cgdmax=1.365n ;PDF max Ciss
+ Cgdmin=1.05n ;PDF typical Ciss
+ Cgs=1.98p ;CIN. why so low
+ Cjo=2.05p ;DBDMOD. why so low
+ Is=1E-30 ;MOSMOD. why so low
+ mfg=Wisdom
+ Vds=60 ;PDF
+ Ron=22m ;PDF
+ Qg=33n) ;PDF
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Old 1st March 2013, 12:12 AM   #15
CBS240 is offline CBS240  United States
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Cool thanks
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Old 1st March 2013, 01:45 AM   #16
CBS240 is offline CBS240  United States
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Hi

I have completed the next prototype of this unique stereo amplifier module. I lowered the Av to 40 because I usually use the high def audio output card on my laptop as the source so I donít need much more but it can easily be set up to 60-70 so that an I-pod Nano can be used as the signal source and still clip the output. This PCB is cheap, but is intended to be a prototype. You get what you pay for. If I were to make a production, the PCB would certainly be of better quality. But, this one works for the purpose.

Now I realize many folks donít like tiny SMD parts and it looks like there is a lot of components crammed onto this PCB, but they each have purpose, from the 0201 size resistors to the large PS caps. There is a lot going on within this PCB than just a couple of amplifiers. In fact most of the circuitry is not part of the amplifiers.

This stereo module is complete with power supply requiring only 18VAC - 0V - 18VAC transformer, voltage doubler & voltage regulator circuits for each amplifier channelís VAS. It has circuit and speaker protection that is more involved than just a set of relays. The momentary push button switches the amp from stand-by mode (blue) to active mode (green), as indicated by the RGB LED. When the amp is in stand-by, the high current output transistor rails powering the output transistors and gate drive stage are shut off via the two Trench type TO-220 mosfets, the only TO-220 without a heatsink. In this mode, all the brass blocks are de-energized. The output stage for the amplifiers is described in more detail HERE. When the momentary switch is pressed, the LED turns from blue to green and the output stages are energized. There is a delay of about 3 seconds before the relays engage the speakers to the amplifier. The DC detection circuit will be tripped if a DC is output from either channel of more than +/-2.4V for more than about 2 seconds. If for some reason the amp does not settle correctly, the DC output triggers fault before the 3 second delay and the speaker will never engage the amplifier. The switched power rails are fused on both B+ and B- (+/-24V). If either of the fuses break due to a shorted output or any other reason, this is detected and fed back to the control logic resulting in the amp returning to stand-by and will not switch back to active mode. When the amp returns to standby for any reason, there is no delay in the action of the relays.
If you turn up the gain to where the signal begins to clip, then the RGB LED switches from green to red for ~1/3 seconds and then back to green. Nobody likes a clipped signal, so this is a handy indication. The heatsink is a tunnel that I made using some old scrap aluminum signage hardware and a fan is mounted on one side. This fan is controlled by a circuit that senses the heatsink temperature via the small thermistor mounted on the heatsink bracket. The fan is only on when the heatsink becomes hot, but this will not take place unless you are cranking it out so the noise is not an issue. The turn-off limit for the fan is lower than the turn on limit so it will not go --on, off, on, off, ect. The orange LED mounted at the control switch indicates when the fan is running. If for some reason the fan has a fault, and the heatsink gets really hot, the amp will switch back into stand-by mode until the temperature comes down, and then will automatically switch back to active when the temperature returns back below this set point.
The SOT 14 chip on the bottom of the PCB is a TS556 timer chip. The TS is a low current version of a typical 556 timer. The Micro 8 package on the bottom of the PCB is a 339 duel comparator acting as the limit detector for the fan control circuit. There is also a SC 70-5 single comparator and a SC 70-6 D-flip-flop. There are no other ICís on the board, just tiny TTL, nothing special. The noise floor is so low, I could barely hear a tiny bit of white noise from the tweeter but then my refrigerator in the other room, 30ft away turned on and with that small amount of ambient noise, the white noise is indistinguishable.











Number of transistors? Many
All those tiny SMDís!!! Here is a list of all the SMD semiconductors used in this circuit:

SOT-14
Micro-8
SC 70-5pin
SC 70-6pin
SOT 89
SOT 23
SOT 323
SOT 723
SOT 923
SOT 563
SOD 923 (diodes)
SOD 523 (diodes)

The first waveform photo is 50KHz square wave from each output, 1Vp @ 6R. The second is 10Vp, same load. The third photo is 10KHz square wave, 10Vp. The fourth is 20Hz square wave, 10Vp @ 6R.
Attached Images
File Type: jpg DSCF7744.JPG (193.3 KB, 325 views)
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File Type: jpg DSCF7754.JPG (253.4 KB, 58 views)
File Type: jpg DSCF7755.JPG (265.9 KB, 61 views)
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Old 1st March 2013, 01:45 AM   #17
CBS240 is offline CBS240  United States
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The PCB and some more photos.....
Attached Images
File Type: jpg DSCF7769.JPG (342.5 KB, 135 views)
File Type: jpg DSCF7756.JPG (311.4 KB, 107 views)
File Type: jpg DSCF7757.JPG (343.9 KB, 52 views)
File Type: jpg DSCF7758.JPG (343.7 KB, 53 views)
File Type: jpg DSCF7759.JPG (278.2 KB, 33 views)
File Type: jpg DSCF7760.JPG (271.3 KB, 23 views)
File Type: jpg DSCF7761.JPG (246.3 KB, 20 views)
File Type: jpg DSCF7762.JPG (254.5 KB, 22 views)
File Type: jpg DSCF7763.JPG (266.0 KB, 33 views)
File Type: jpg DSCF7764.JPG (281.0 KB, 44 views)
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Old 1st March 2013, 02:18 AM   #18
CBS240 is offline CBS240  United States
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EDIT regarding post 16: The only thing required is the transformer, the voltage doubler ect is all on the PCB.


The amplifier stages use a DC servo to set a virtual GND reference for the input transistor so as to zero the output DC. This signal is summed with the audio and fed to a very linear buffer. The audio input capacitors are those red squares under the vero board mounting addition in the center of the PCB. They are 1uf polypropylene film caps from WIMA. These and some small COG type caps used for compensation throughout the amp are the only caps that are directly in the signal path. The IPS and VAS use a common mode differential amplifier, to bias the differential mode amplifier’s common mode current. This common mode error correction scheme seems to allow an enormous linear open loop gain with what seems to be an excellent S/N ratio, when used with the complementary common gate J-fet VAS. While the output Z of this circuit may be quite high, the output stage circuit forms a buffer that only requires a few uA of current to drive. The combination works quite well.

The damping is plenty high enough that it is no problem to drive left and right channels along with left and right rear surround speakers. The output acts more like a voltage source so the effect of the speakers to each other is very minimal if at all. These would be connected in series at opposite phase and placed between the two outputs. Playing only the difference between the left and right channels like this gives you the perception of the sound stage making it very easy to pick out each instrument in the music separately.

Although it looks fragile with the output wires (the large red and black wires on top) connected to the output node by the screws in the daughter boards. This is quite a sturdy connection. It would take a bit of force to break it. On the final design, I plan to expand the space between the through hole parts just a little bit, but the construction should be quite sound using the proper SMD construction methods of course. Have you ever tried to hand solder 0402 X 4 resistor arrays? Man these are tough!


BTW, despite the perceived size of the output devices (being directly mounted to the brass aides in Pd derating greatly) I can run ~65Wrms @ 4R X 2 channels, plus the surround speakers to full volume. This is plenty for for my home stereo, who wants it much louder than that? I can drive full signal across 2R resistive. The outputs do get warm but didn't fail!

The last photo shows the Zobel filters being close to the output stages. The source ballast resistors are 50mOhm, metal film power resistors from Caddock, TO-126 package.
Attached Images
File Type: jpg DSCF7766.JPG (42.9 KB, 51 views)
File Type: jpg DSCF7768.JPG (246.4 KB, 48 views)
File Type: jpg DSCF7772.JPG (50.0 KB, 142 views)
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Last edited by CBS240; 1st March 2013 at 02:47 AM.
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Old 1st March 2013, 08:54 AM   #19
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Congratulations for the majestic work, complex PCB with enormous count of parts.

Only one minor remark. I would be a little concerned regarding squares measurements from post #16, since simple LM3886 amp can do much better.
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Old 1st March 2013, 01:41 PM   #20
CBS240 is offline CBS240  United States
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Lots of parts, but it does lots of things. I forgot to mention there is an AC detection circuit that disengages the relays imediately when AC power is switched off eliminating turn off thump.

Yes, it seems to be a bit overcompensated. I adjusted the compensation a bit and it is better with the 50KHz square wave, but I think I can get it to be better yet. The top is what it was, and the bottom is now, still a little slew limited. Need to poke and prod some more.
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File Type: jpg DSCF7774a.jpg (18.8 KB, 88 views)
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