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KSA-5 inspired amplifier builds


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By faster do you mean lower capacitance? Or higher fT/Current Gain - Bandwidth Product?

I ask since at 10V I think the Cob for 2SC5171 is about 16pF versus 4pF for each MPSA06. So 4x MPSA06 would be about 16pF. And 2SC5171 has higher fT.

So does it mean in this case that one 2SC5171 would not be slower than four parallel MPSA06?


I could certainly put four parallel TO-92 but I would like to understand it better before I do the layout. I have a bunch of genuine 2SC5171/2SA1930 from dead receivers that I used the chassis/heatsink and transformer to make KSA-50 monoblocks. So I am looking for the best place to use them. Otherwise I can just get some MPSA06/MPSA56.

I found a couple Spice models for 2SC5171/2SA1930 but I don't know if I can trust them. One had quite a few default looking values.


I also have some leftover TTC004B (12pF Cob) and TTA004B. I have not found the BD139/BD140 Cob yet.695373673_4xMPSA06or2SC5171.thumb.png.0e5ecd301f72e751fe48c86cfa2de912.png

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Done the first power-on. No magic smoke escaped! 

But something's not right: Bench power supplies set to 21 Volts. Current draw is 161mA for both the positive and the negative rails.

Had to adjust the bias-pot all the way to one side to get a minimum current across the test points of 109mV. I thought that there should be more play here?

The power transistors / heatsinks hardly get even warm - after waiting for roughly 10 minutes.

DC voltage at the output (with input shorted to ground and no servo OP-amp installed) is at 70mV.

Any ideas, anyone?

Maybe I will stop for today and have a good look with fresh eyes tomorrow...



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there was a long explanation on this topic somewhere at diyaudio.  in general N x smaller transistors is going to have less total silicon area than 1 bigger transistor at the same power dissiption which probably means less cob. 

edit: without the servo 70mv on the output is about right. i don't remember what the output stage bias is supposed to be, too lazy to look it up.

hook up signal generator 16 ohm load and scope and you will know whether its working right or not.

edit: edit:  your desk is way to neat. nothing good can come from something this clean. look at pictures in the megatron thread for what your desk shoud look like.

Edited by kevin gilmore
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Right - I found my mistake: swapped the emitter and collectors on the two MJE15031 PNP output transistors...

Stupid me! No matter how often you check, re-check and double check something, good old murphy is always lurking somewhere!! 

It's an easy fix, optically not really pleasing. Lucky for me to have house guests over the weekend, so no time in the workshop. 🙄

@kevin gilmore nice one with the desk! 😂 ... Believe me, my workspace looks like yours from time to time!

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@kevin gilmoreI use kicad and definitely have forward annotation in place. Funny coincidence, in this case I had two errors for the same part: I had the PNP symbol the wrong way round (Emitter and Collector swapped) AND I had a wrong assignment of the footprint (Q_PNP_BEC - should have been Q_PNP-BCE)... what are the odds??

Anyway - both errors cancel themselves out, leaving me with a layout that is correct - for that part anyway!

Then I inserted the OP amp and re-measured the DC Voltage at the output: 1,3mVDC - perfect!

Next up: signal generator, dummy load and scope. Set the generator to sine wave with 500mV pp and the load has 250 Ohms on the output.

  • 20Hz ... 2,9V pp
  • 200Hz ... 2,9V pp
  • 2kHz ... 2,9V pp
  • 20kHz ... 2,9V pp
  • 50kHz ... 2,8V pp

Seems to be ok - amplification at 5,8. Is that what is to be expected? Removing the load didn't change anything.

I still can't figure out why I can't dial in under 110mV across the output resistors. And the heatsink barely goes over 28°C after 1 hour of operation. (19°C ambient)

Any ideas what might be wrong? From what I read in the forum posts, 400mA per power rail should be drawn. I have 165mA.

Here's a picture of the (still too tidy) workbench:


Edited by AlexS
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On 11/20/2021 at 7:26 PM, kevin gilmore said:

if you use a circuit board system with integrated schematic, if the schematic is right, the layout has to be right. Believe me i have found out the hard way.

It really made my life a lot easier when I started to use integrated schematic. But if schematic is wrong, this might happen… 

Interesting project you have, AlexS.

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