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Old 25th July 2007, 12:54 PM   #41
sandyK is offline sandyK  Australia
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Default VAS thermal tracking and distortion


Could Q15 and Q17 be getting hotter than Q16 and Q18, thus unbalancing preceeding stages ?

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Old 25th July 2007, 01:20 PM   #42
AndrewT is offline AndrewT  Scotland
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Default 2sa1381/c3503

they're really good at 100V. P(100v)=6.7W, I(100v)=67mA.
but Sanyo cheat with the Cob quoting the 30Vce value.
At 10Vce Cob=4pF & 4.6pF still pretty good and excellent for a 300Vce0 device.
Can one order hFE (10mA) F rank (160 to 320)? and judging by the graphs these hold steady (+0-10%) from 0.5mA to 30mA.
regards Andrew T.
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Old 25th July 2007, 04:51 PM   #43
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omit one stage - for better sound quality and stability.
Thank me later...
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Old 25th July 2007, 08:35 PM   #44
syn08 is offline syn08  Canada
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Default Re: 2sa1381/c3503

Originally posted by AndrewT

Can one order hFE (10mA) F rank (160 to 320)?
Unfortunately Mouser currently sells only rank E ( hFE(10mA) 100 to 200) devices. Still very good for the price.
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Old 31st July 2007, 02:18 AM   #45
GK is offline GK  Australia
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Originally posted by syn08


I've noticed that Mouser stocks and sells (for 25 cents in 10's quantities) the Fairchild KSA1381/KSC3503. These are some decent video amp devices, with Cob under 3pf. You may want to give them a shot. According to my measurements they are not quite at the performance of 2SA1407/2SC3601 but, from a price/performance perspective, they are still excellent devices for a high performance VAS.

Datasheets are available on the Fairchild web site.

I'll look them up.

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Old 13th August 2007, 03:36 AM   #46
suzyj is offline suzyj  Australia
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Default Hanging my head in shame

Now I'm really embarrassed.

In transferring parts from one of these amps to a 100W board, so I could prototype it using TO-126 VAS transistors, I found I'd used 33K load resistors in the input diffamp, rather than 18K ones.

Further investigation revealed that my 18K resistor stash was half full of 33K resistors. Of course, they're too small to read, unless you're looking under a microscope.

After putting the correct resistors in to the other one (I made a stereo pair), I'm now getting 0.0027% THD at 1KHz, and the output offset is solid as a rock.

I'm such a dork.

That said though, the single-ended to differential stage with separate SOT-23s is still way too sensitive to random air drafts and stuff for my liking.

So I'll still build the cool tiny TO-126 version, 'cos it's, well, tiny. And cool. But I'll definitely knock a version together using IMT8s and IMX4s as the second-stage diffamps, 'cos that might be even cooler.
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Old 13th August 2007, 03:55 AM   #47
anatech is offline anatech  Canada
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Hi suzyj,
Well, you found your problem with your own workmanship. It's much easier to find someone else's problems. Finding your own is much harder, so good job! No need to hang your head.

"Just because you can, doesn't mean you should" my Wife
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