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Combined Topology - Thoughts?
Combined Topology - Thoughts?
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Old 10th September 2010, 09:36 AM   #31
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
I am particularly aware of the driver current limit.
6mA CCS feeding the diamond limits the base current to the output device.

What hFE would be needed to drive 20Apk into a speaker load with Ib=6mA?

Now look at the Ic vs hFE graphs that the output device manufacturers publish.

A normal two stage EF works differently.
I have not used a diamond in the output so I'm guessing at it's high current operation.
Find corroboration that your proposal will work. That's what I said way back.

I also see a coincidence in selecting the two CCSs at the same 4mA current to feed the LTP and the VAS. This seems somewhat random. I cannot say it is wrong. It will work as you propose but are you selecting optimum operational currents to obtain good performance from your selected devices in their particular duties?
regards Andrew T.
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Old 11th September 2010, 08:13 PM   #32
buzz1167 is offline buzz1167  United States
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Join Date: Jun 2010
I'm not sure why you are thinking that the max current that can be driven with the drivers is 6ma... I hope I have not drawn it wrong...
I have tried to setp the circuit such that the ccs's are driving a biasing current, the drivers can and should output more than 6ma, up to whatever is required and the difference in current will be what is output to the base of the output transistors. I think this could be lowered but I also expect to add another output device.

How would I go about finding the optimum current for the vas and input stage?

Selfs article that I read seems to make it seem like bigger is better for the transconductance stage (input stage) mostly because it allows linearization by degeneration. Does this not hold true?

Last edited by buzz1167; 11th September 2010 at 08:15 PM.
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Old 19th September 2010, 07:40 AM   #33
buzz1167 is offline buzz1167  United States
diyAudio Member
Join Date: Jun 2010
Success! Or at least it works...

I figured I'd post some pictures of my final setup. I ran the amp topology that I previously posted, with some slight modifications. Changed the Input Degeneration resistors from 100 to 200ohms. Changed the current mirror resistors from 47 to 22 ohms. Used a 20k by 600ohm-220uF NFB.

As far as performance goes, it looks like the ccs's deal better with the high frequency stuff than the simple resistor/cap setup. When I had the old setup, I could subtract the signals and get an error with a little spike in it and it wouldn't go away (out of sight) unless I over biased it a bit and then the transistors would try to runaway. No fun... This one can be "underbiased" for lack of better terms and not show that spike. I've tried to induce runaway by running it full power which is currently 100W RMS into 4 for 10 or 20 minutes but I will test again with the full rail at 200W into 8. This 100W test did cause the static current to rise from 15ma to 40ma but it dropped back in a reasonable time, and since this was not even actively cooled I figure I'll be ok.

I will build the dc offset detector sometime. Maybe next weekend, testing it might prove interesting... I'm gonna need to rig up some resistors.

For sure I am going to get some production boards made based on this design but I'll need to learn to use somebody's software to make the trace drawing. So we'll see how that goes. I'll need 4 of them, but i've been looking at prices and I'll probably end up buying a few more and selling them on ebay or here if anyone's interested?

Below I have pictures of the entire setup, then a closeup of the top and messy bottom. The random wires are for jumping power to the transistors and current sources in the middle, as well as all the separate grounds I have. I don't know what I'm going to do with them all on the prod board yet, I'll have to look into it. I plan to fix all the jumper wires by simply using a 2 sided board.
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