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Old 12th July 2007, 04:21 AM   #21
GK is offline GK  Australia
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How about this?
This is how Sansui did it in their old "DC-DC" "low TIM" amps marketed in the early 80's (late 70's??).
The Sansui circuit was very similar to DT's AEM6000.
The complementary differential pairs share a common tail current.

Cheers,
Glen
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Old 12th July 2007, 05:43 AM   #22
Bonsai is offline Bonsai  Taiwan
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Analog_guy/Ben, here is the link to my amp that uses a symmetrical vas and cascodes.

Ovation Amp

I have since changed the vas, and drive the cascodes via a darlington rather than a single transistors. This gives about 20dB more OLG and I get close to that in simulated distortion reduction at 20Khz.

It was on this amp (spider wired) that I tried the fully balanced differential topology driving the cascodes and got the latch-up. There are some proposals earlier in the thread to use a potentiameter to bring the vas into balance on the fully differntial topology, but I have reservations about this approach wrt to temperature changes, load changes etc. I'll have to think about th e problem some more to come up with a foolproof apporach - ideally, no pots.

With the present topology on my amp, there are no problems and the amp sounds very good (I'm driving B&W 703's). My Tokyo neighbours are not impressed, but what the hell.
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Old 12th July 2007, 10:35 AM   #23
GK is offline GK  Australia
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Hi Suzy

If you are considering a second shot at getting your 50 W board working well, I reckon it might be a good experiment to try soldering in a pair of these (dead bug style) to replace your current individual SOT-23 devices.
They’re BC857BS (PNP) and BC846BS (NPN) dual transistor pairs with tightly matched Hfe, housed in a 6 lead version of the SOT-23 package. Being duals, each transistor shares a common chip, so they are tightly thermally bonded – and they cost next to nothing!

http://au.farnell.com/jsp/Semiconduc...sp?sku=1081234

http://au.farnell.com/jsp/search/pro...sp?sku=1081247

Cheers,
Glen
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Old 12th July 2007, 12:13 PM   #24
suzyj is offline suzyj  Australia-Aboriginal
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Actually I'm having a fair degree of success redoing the layout using the MJE340/350 devices, as used in the 100W version. I started by lining all ten transistors in two vertical rows of five, such that they can be bolted to a short strip of metal, and all end up at the same temperature. I took the large electrolytics off the board, and moved a whole pile of the SMD bits to the bottom of the board. The original had almost all the parts on the top.

The result (so far!) is a titchy board just 3.2 by 2.1 inches. Component heights are 0.5 inches on the top side, and 0.2 inches on the bottom (for a total module thickness of 0.8 inches or so):

Click the image to open in full size.

As you can see I'm still working on it. There's a fair few nets still to be routed, plus pouring planes etc. I'm actually kinda enthused by this one, as it really does end up pretty small. FWIW, the original was 4.4 by 3.4 inches, and (due to the large electros) 1.4 inches thick.

Just the ticket for bi/tri amping, methinks.
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Old 12th July 2007, 01:44 PM   #25
PB2 is offline PB2  United States
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I suggested the larger devices and heatsinking as a way to test your theory, as I'm not convinced that the problem is a thermal issue. However, you don't seem to offer all the results of your debugging so it is hard to know exactly what is going on. I have a feeling you do. I have to wonder if the smaller and faster devices have introduced an oscillation, but I'd guess that you looked for this already.

It is a complex design, overly complex in my opinon but that is just my opinion.

The second dual comp-diff stage, and the rest of the design look like a more conventional amp. It looks as if the FET front end was just tacked on, if you know what I mean.

Seems it would make sense to contrast this design with Bob Cordell's classic.

Just my opinion, I'm looking at the old Tiger amps just because I think they have potential, and I'd like to fully understand the instabilities so I understand being interested in the classics.

Pete B.
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Old 12th July 2007, 01:50 PM   #26
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Just how much bias current in the MOSFET output stage? While it is a different driver-horse entirely, you might want to read the Nat Semi application note AN-1645 which has an extensive discussion of the thermal issues related to driving MOSFETs.
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Old 12th July 2007, 02:06 PM   #27
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Quote:
Originally posted by PB2
Just my opinion, I'm looking at the old Tiger amps just because I think they have potential, and I'd like to fully understand the instabilities so I understand being interested in the classics.

Pete B.
Now there's a series of amplifiers which had no thermal issues whatsoever.
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Old 12th July 2007, 02:24 PM   #28
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Quote:
Originally posted by suzyj
Actually I'm having a fair degree of success redoing the layout using the MJE340/350 devices, as used in the 100W version. I started by lining all ten transistors in two vertical rows of five, such that they can be bolted to a short strip of metal, and all end up at the same temperature. I took the large electrolytics off the board, and moved a whole pile of the SMD bits to the bottom of the board. The original had almost all the parts on the top.

The result (so far!) is a titchy board just 3.2 by 2.1 inches. Component heights are 0.5 inches on the top side, and 0.2 inches on the bottom (for a total module thickness of 0.8 inches or so):

Click the image to open in full size.

As you can see I'm still working on it. There's a fair few nets still to be routed, plus pouring planes etc. I'm actually kinda enthused by this one, as it really does end up pretty small. FWIW, the original was 4.4 by 3.4 inches, and (due to the large electros) 1.4 inches thick.

Just the ticket for bi/tri amping, methinks.

Suzy,

I have had a bit better look at your schematic.

The circuit appears to be highly sensitive to VAS thermal VBE
modulation and 1st stage CM variation.

Starting at IP dif pair, you are running each device at about 0.7mA.
This, through the 1k2 collector load R's, will set the VAS current at
approx 1.5mA / side.

This is way too low for the size of R33 and R34.
Consider a thermally induced VBE change of just 0.05V in the VAS
transistor(s) which will result in a huge change in that small 1.5mA
/device.

It's worth trying the following:

- Delete ground conn between R17 / R18. This will allow dif pairs
to track more closely
- Run IP dif pair at 2.0mA / device.
- Change R27/28/29/30 to 860 ohms.
- Increase R33/34 to75R
- Increase R51/52/53/54 to 20 or 30R
- Thermally join (or keep very close) VAS pairs.

This should set up VAS at around 6mA / device which, in conjunction
with added degeneration, will keep them in a much more linear operating region.

cheers

Terry
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Old 12th July 2007, 10:43 PM   #29
PB2 is offline PB2  United States
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Quote:
Originally posted by jackinnj


Now there's a series of amplifiers which had no thermal issues whatsoever.

It has more than just thermal issues, I like a challenge.

Pete B.
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Old 13th July 2007, 01:35 PM   #30
PB2 is offline PB2  United States
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Looking at this again, it seems fairly clear that, roughly, D3 should be thermally coupled to Q9, and D1 and D2 to Q17 and Q18, for example. Similarly with the other side.

I agree that the design is too sensitive to thermal drift and component matching in general. I would not be surprised if the design works well when the diff pair devices (Q15-16, Q17-18)come from the same lot, or wafer, and very poorly when not. A prototype does not prove out a design unless mismatched semis are deliberately used to represent the expected allowable spread in tolerance, when selected/matched devices are not used. Simulation and/or analysis should be done with mismatched devices representing the allowable spread in the design. Really, we should step back and consider if the diff amp VAS makes sense? I think not since the design cannot afford significant drop in the emitter path to provide a good current source, and it is far too sensitive to device matching as Terry pointed out.

IMO, when diff pairs are used an effort should be made to have them come from the same lot in any critical application. I also wonder why dual devices are not used more often in diff pair designs.

Harman Kardon has been using duals since the 1970s with their old Citation 12, makes a lot of sense. There's really no excuse not to use duals, when a suitable one is available, in high performance designs such as these.

Pete B.
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