Jump to content

KSA-5 inspired amplifier builds


AlexS
 Share

Recommended Posts

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

Link to comment
Share on other sites

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...

*frustrated*

?spacer.png

  • Like 1
Link to comment
Share on other sites

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
  • Like 2
  • Haha 1
Link to comment
Share on other sites

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!

Link to comment
Share on other sites

@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:

spacer.png

Edited by AlexS
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

So, over in the diyAudio forum user Algar_emi from Canada shared a commented schematic of the original clone with a few measurements he made.

https://www.diyaudio.com/forums/headphone-systems/109618-krell-ksa5-9.html#post5189211

Turns out that a single channel will draw about 180mA per rail. That's with the voltage across the 2 ohms resistor at 125mV. That works for me. All values from his notes match with my measurements.

Strange though, this only seems to work with no heatsink attached! Having only the aluminium L-profile in place, the setup settles at 44°C (100mV). As soon as I connect the big heatsink, obviously the temperature drops and the voltage across the resistor increases to 137mV. (just under 200mA on the input rails). Temperature drops below 30°C.

After some ten minutes, temperature is up to 30°C and voltage down to 130mV.

So it seems that the thermal capacity of the heatsink has a big effect on the system. Any ideas, anyone? Do away with the big heatsink? Simply live with the increased current through the resistors / transistors?

spacer.png 

Edited by AlexS
Link to comment
Share on other sites

Perhaps the range of the Vbe multiplier (resistors and/or pot) needs to be adjusted if you want to use the larger heatsink and lower temperature?

Not sure what is best in this case but sometimes a certain temperature is targeted (such as 50C in another design).

Link to comment
Share on other sites

On 11/27/2021 at 5:16 PM, AlexS said:

Having only the aluminium L-profile in place, the setup settles at 44°C (100mV). As soon as I connect the big heatsink, obviously the temperature drops and the voltage across the resistor increases to 137mV. (just under 200mA on the input rails). Temperature drops below 30°C.

After some ten minutes, temperature is up to 30°C and voltage down to 130mV.

So it seems that the thermal capacity of the heatsink has a big effect on the system. Any ideas, anyone?  

This is quite normal. As the heatsink temperature rises, the Vbe of Q14 goes down (by about 2 mV/°C), the voltage between the bases of NPN and PNP output transistors decreases, the quiescent current of the output transistors goes down, which reduces the dissipation, causing the temperature to go down. In other words, the Vbe multiplier adds a negative temperature feedback loop, which maintains not the quiescent current (which it has no way of measuring) but the temperature of Q14. Eventually the system finds an equilibrium somewhere, and the temperature at equilibrium depends, among other things, on the thermal resistance of the heatsink.

In practical terms, you adjust the bias to your liking (e.g. by the lowest distortion, or by the desired heatsink temperature, or by the power dissipated by the output transistors) and be happy.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...