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 Forum index » DIY Hardware and Software
2SA798 matched pair in a Threeler
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Thu Jun 13, 2013 9:47 pm    Post subject: 2SA798 matched pair in a Threeler
Subject description: does the pinout matter?
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I'm working on a Threeler I built and I *believe* I may have an issue with my 2SA798 matched PNP pair. I put it in oriented to make getting the tempco in contact with the slanted surface easy, with its "back" to the trim pot. I don't think based on the datasheet I have that it should matter (1 & 5 are both bases, 2 & 4 are both collectors), but it's in japanese so I'm not 100% certain.

Any definitive understanding here?

The issue is that the output of A6a is locked at roughly -6V (it will swing down to -8 if you run the coarse pot all the way ccw but there is no effective change in the output) so low pass stages don't pass very much, high pass stages pass nearly everything, confirmed by checking the various combinations of filters and the individual outputs.

The BE/BC connections test out "good" using diode testing methods. I've swapped A6 with A1, so if the amp had an issue I'd expect to get no signal through at all now, but it's behaving the same way.

Just want to rule out the orientation as an issue before I dig further into this.

Thanks...
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Thu Jun 13, 2013 9:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Of course 5 minutes later the problem becomes obvious as I look over the underside of the circuit and see that the socket for A6 has three unsoldered pins...

Embarassed Embarassed Embarassed

Seems to be working as expected now; need to calibrate and maybe do some scope shots to be sure. Definitely sounds interesting.....
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frijitz



Joined: May 04, 2007
Posts: 1712
Location: NM USA
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PostPosted: Mon Jun 17, 2013 6:46 am    Post subject: Re: 2SA798 matched pair in a Threeler
Subject description: does the pinout matter?
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Yes, the transistor pair can be rotated to either orientation. I think most duals are this way. But anyway, glad you got it working!

Ian
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