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Old 3rd August 2012, 01:22 PM   #21
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Quote:
Originally Posted by MagicBox View Post
Dear Mirror, dear Mirror, who's the most gorgeous man...
Now if you were an Italian.....
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Old 3rd August 2012, 03:31 PM   #22
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LOL gotta give those credits to them yes

Those BS170's are much closer together in differences, they're much easier to match. I've also found a way to now autiobias by outputcurrent, rather than by inputcurrent, which means that any minor drifting in the mirrors is compensated for which means you'll get a rock solid VAS DC current to bias the output stage with
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Old 19th August 2012, 09:44 AM   #23
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Well, finally got around to order a reasonable number of assorted transistors. J111, J112, J113, BF245B, 2SK170GR, IRF610, IRF9610, BC547C, BC557C. That should give me enough to play with and continue the design

I already had it somewhat stable, but the stability issue came from the biassing circuit which was not worked out very well in the initial design. Now that I am measuring the VAS output current I had to change around the biascontroller and a week ago I made that part into a final version that works great. It exhibits a controlled startup. No chicken/egg problems or otherwise potential latching. Once I get those ordered transistors I'll be able to build and test it.

From that moment and on I should be able to get going the VAS properly
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Old 15th December 2012, 09:54 PM   #24
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Any updates on your amplifier. Success?
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Old 17th January 2013, 08:53 AM   #25
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Hi guys

It's been a bit silent around my project but recently I had time to casually build the final schematic and PCB design. It's been designed for mounting it to a 25cm (Height) x 30cm (Depth) at a minimum. I will be using 25cm x 40cm heat sinks. There's still an open space at the upper part which is reserved for the DC error detector. The bits that already sit up there is a clip detector.

Everything's been sorted out, from the autobiassing VAS to a DC offset servo. The LTP current can also be finetuned. Further the design contains an integrated pre-input stage that uses an INA137 to take a balanced input and buffer it to drive the low impedance LTP inputs which are as low as 100 Ohms. I think the PCB turned out really nice. The ground plane functions as the ideal star for well.. the ground. There are very few lines drawn at the bottom layer so there's no obstruction of major ground returns.

Some of these days I'll start collecting the components required before I have the PCB made, but for now you can see the project is not dead
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Old 17th January 2013, 07:34 PM   #26
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I'm surprised there are no comments, be them good or bad =)

P.S. The final schematic has all the appropriate components that might be missing in individual simulation schematics. I expect this PCB to work for 95% with some 5% fine tuning left (ugly rewires/removals of components etc).
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Last edited by MagicBox; 17th January 2013 at 07:43 PM.
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Old 17th January 2013, 07:49 PM   #27
GoatGuy is offline GoatGuy  United States
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OK - what a sweet circuit board.
There.

A comment.

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Old 17th January 2013, 09:50 PM   #28
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lol thanks! It may look a bit odd, no degens for the output transistors, but it'll be ECX10N20 and ECX10P20 being parallelled and used, they can do without them and should improve output impedance and overall output accuracy.

What would one consider for a DC protection, keeping SQ in mind:
-Regular relais that disconnects, or..
-TRIAC crowbar to blow fuses?
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Old 18th January 2013, 11:13 AM   #29
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Maybe relays, disconnecting power lines after PSU (after capacitors)? If possible to mount on your board... The trick will be to found appropriate relays with sufficient current capability.
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Old 18th January 2013, 11:18 AM   #30
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And regardding yor PCB, maybe try to route power paths in the middle of the board and then 90 degree branches to power transistors. The same with small signal boards. Make no loops. In this design, with so low distortions this will have relevant impact.
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