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Old 21st November 2007, 03:06 AM   #11
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Quote:
Originally posted by CBS240
Hi Kanwar
I bet it has less transistors than mine. When you say grounded, you mean the bridge input is referenced to one side common since it is single supply or is it floating?
The ground reference is one of the output node, whereas the supply is floating and so is the bridge.......
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Old 26th November 2007, 07:27 AM   #12
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Interesting... kinda like one of those grounded collector amps. Mine is not that way, it has two seperate output stages that are the exact same circuit, both are complementary hexfet followers, with EC. From the results of this latest project, I don't think I will ever make an amp with GNFB again. Multiple stages and local feedback is better....IMHO. Not sure if I can post a readable schematic, I plan to draw it all on the computer only one time, in Eagle. There is a lot of components so it won't be as easy to follow as the sketch. Maybe I could try to scan it. If it is readable, I will post it.
Anyway, I finally got around to doing a few cap tests....
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Old 26th November 2007, 07:38 AM   #13
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BTW both output's are shown in each pic, 10V/Div. The two are added across the load. It is still a little ugly with significant capacitive loading, but the latest tweeks have made it a little better....

So it is moderately stable and fairly fast. Hmmm, now if I only had some real testing equipment.
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Old 26th November 2007, 09:47 AM   #14
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Now I planning something different!!

Seperate Voltage amplifier based on Common Base Cascoded topology and seperate high current buffer with unity gain using N-channel mosfets......with no overall global feedback!!
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Old 26th November 2007, 09:49 PM   #15
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Hi Kanwar

Now that's an idea that very few on this forum seem to like much. I couldn't agree with you more. It's the way to go about it, IMO. Also you may not need an output coil since there is no GNF loop to corrupt by the components of a nasty reactive load.

THIS circuit has no output coil. There are no zeners in this circuit except for mosfet Vgs protection. I don't like zeners. This is the best resolution I can get because of the 100K resolution block in the forum. At least you can get the idea. The mosfet's used as the cascode in the VAS are actually BJT's now because I don't have them yet. But in past experience, they seemed to work better. Will have to experiment to compare for sure. The DC servo circuits work on the principle that Q's 1&2 (N-Ch VAS input) have a Vgs@100uA of -0.5V and Q's 3&4 (P-Ch VAS input) have a Vgs@100uA of 1.8V. If both inputs to the VAS section are grounded (0V), then the result output is -0.6V on both channels, referenced to ground. So in order to get 0VDC for both outputs, both VAS inputs must be on the order of +0.25V. This means the output of the DC servo is always positive and can be referenced to GND instead of -15V. This makes it better for the signal reference too. The floating VAS inputs are driven by two floating outputs from the gain cells. They create balanced drive with Av of 4.8 from a single line input. VAS has an Av of 6.2 for an over-all Av of about 30. DC bias and offset is very stable. There are a few more things to tweek but it is working pretty good so far....
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Old 27th November 2007, 05:40 AM   #16
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Quote:
Originally posted by CBS240
Hi Kanwar

Now that's an idea that very few on this forum seem to like much. I couldn't agree with you more. It's the way to go about it, IMO. Also you may not need an output coil since there is no GNF loop to corrupt by the components of a nasty reactive load.

THIS circuit has no output coil. There are no zeners in this circuit except for mosfet Vgs protection. I don't like zeners. This is the best resolution I can get because of the 100K resolution block in the forum. At least you can get the idea. The mosfet's used as the cascode in the VAS are actually BJT's now because I don't have them yet. But in past experience, they seemed to work better. Will have to experiment to compare for sure. The DC servo circuits work on the principle that Q's 1&2 (N-Ch VAS input) have a Vgs@100uA of -0.5V and Q's 3&4 (P-Ch VAS input) have a Vgs@100uA of 1.8V. If both inputs to the VAS section are grounded (0V), then the result output is -0.6V on both channels, referenced to ground. So in order to get 0VDC for both outputs, both VAS inputs must be on the order of +0.25V. This means the output of the DC servo is always positive and can be referenced to GND instead of -15V. This makes it better for the signal reference too. The floating VAS inputs are driven by two floating outputs from the gain cells. They create balanced drive with Av of 4.8 from a single line input. VAS has an Av of 6.2 for an over-all Av of about 30. DC bias and offset is very stable. There are a few more things to tweek but it is working pretty good so far....
Exactly forum members usually like GNFB to get the thing work and generate good THD specs on paper......only few alike us think distinctively.....The schematic looks so wierd just because of very low resolution......and congested drwaing.....I dont want to "jam" my eyesight to view it......already getting a bit of headache

BTW: You still need to learn how to draw a neat and clean schematic and how to display it on the net........
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Old 27th November 2007, 04:15 PM   #17
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Your right, the schematic is hard to view, but it was drawn with the intent of helping me build and modify the circuit. If it was all on the computer, it would be too small to see the whole thing. Also I don't have a great electronic drafting program. That is why I just drew it out on a 76cmX127cm piece of cheap partially recycled paper. Not the best medium for posting on the net, but serves the purpose...old school style. I may start laying it out in Eagle, just to get some of it entered. Layout is important as with any amp, but with circuit 'modules' and no GNFB, there won't be a NFB loop strung all the way across the PCB. I just realized the all 4 output devices cost $5.20. There is 4 THAT340 transistor arrays at $8 a peice. 1 for each input section and 2 for the VAS amplifying transistors. Oh well, they work wonderful and are worth it IMO. I haven't been fortunate enough to try out any of thier other products. For example, this could significantly reduce the number of parts in my circuit, as I have basically built a descrete version. http://www.thatcorp.com/datashts/1600data.pdf

More... http://www.thatcorp.com/datashts/1500data.pdf on Mr. Whitlock's common mode bootstrap for this balanced line reciever chip...http://www.thatcorp.com/datashts/1200data.pdf AND some interesting apps. http://www.thatcorp.com/appnotes.html.

Maybe a second thought on using completely descete components might be prudent... but THAT would take all the fun out of it.
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Old 7th December 2007, 05:15 AM   #18
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Hi

Not that many here are interested in no global feedback designs, but I have made a significant topology change to the first gain stage/phase splitting circuit. Instead of using two separate amplifier stages, I am now using a balanced bridge. So now I have a balanced bridge driving a balanced bridge. I found the symmetry too be much better, faster slew, and fewer components. One would guess correctly that if you input a signal to just the + input, the + output will be larger than the - output, if the load impedance is the same on both sides of the bridge. This obviously will not drive both output's (VAS input's) equally and the circuit wouldn't have equal and opposite outputs. I found that it is the current, not the voltage that must be equal in the bridge. So by putting a 7.5K resistor load on the + output, and a 10K resistor load on the - output, the two output voltages are equal in magnitude. This makes the output impedances of this stage not equal, but they are driving the gates of the next gain stage input J-fets so it doesn't matter. That load is roughly equal to 2K in series with 5pF.
Transistors 1,2,3,4 are a THAT 340 matched array. Both input J-fet pairís bias @100uA per leg. The 201 has a Vgs@100uA of -0.38V. The 5460 has a Vgs@100uA of 1.8V. This means that the circuit requires a DC servo input of +V on both DC inputs, A & B, in order for both outputs 0 VDC. 'A' is also the signal input as there is an input follower stage preceding this one. I do not find the common mode bias arrangement to affect the signal; rather perhaps acting as a form of EC. Since the circuit is actively biased and DC offset is corrected via DC servo, I have dispensed with the source degeneration on the J-fet's. It doesn't seem to be a problem. Sorry for the old school drawing, it is an excerpt from a larger drawing.
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Old 7th December 2007, 05:32 AM   #19
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Now that I have installed an input filter, this is the quietest, fastest, most acurate amp I've made so far. You have to put your ear right into the tweeter to hear anything at all, and only in complete silence, when the house fan and the refridgerator compressor is off.:P This I love, it helps that all inportant first watt of power...or maybe it should be the first 1/4W of power. This is the new 100KHZ square wave 10V for each side, so 20V and over 2A RMS. 1uS/div. Other than that bit of crossover... ... that problem is in the output section... some tweeks yet needed there.
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Old 7th December 2007, 09:12 AM   #20
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Default Interesting schematic


I like cornbread and chicken too.... having some tomato and cheese going together to make spice sauce.

regards,

Carlos
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