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Old 2nd May 2013, 01:06 PM   #11
Bigun is offline Bigun  Canada
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Wow, looks good, you have a nice bench set up there and you've built a good looking amplifier. I hope you are pleased with the sound too.
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Old 2nd May 2013, 04:15 PM   #12
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Quote:
Originally Posted by AndrewT View Post
I have been using 200mV to 300mV as the Vdrop of the degeneration resistor, in LTP, & VAS & Current Mirror.
What is wrong with doing it that way? excess added noise? or many other avoidable problems?
If you refer back to my post I linked to earlier, a lot of degeneration really isn't necessary and isn't really helpful unless your degeneration is matched, and even then it's questionable. LTPs benefit from matching but not THAT much, especially if the rest of your circuit produces significant H2. If you really want that level of matching, you will not get it by obsessing over matching; you should use trimmers instead.

Furthermore, large degeneration bootstraps Cbe and can slow down the transistor. The BC5x0C have reasonably small Cbe, but not the 2N5401's that people always try to use here.

I think a distinction could be made between common-mode degeneration and differential-mode degeneration. LTP degeneration is differential. CM degeneration is common. The difference is that LTP degeneration will influence harmonic profile while nominally, CM degeneration will not.

Different purposes can require different standards of degeneration. Slone chose the 50mV figure specifically for current mirrors, because is it large enough to swamp the differences in BJTs, but not larger than it needs to be.

For other degenerations, such as an EF output stage or an LTP, the needs are different. For an EF, there is the Oliver null point, above or below which you get more Gm distortion. For LTPs, it's usually a question of stability and slew rate. Degeneration also changes the harmonic profile, and I've heard 200mV is preferred by some designers in places where it affects the sound.

For a sense of scale, BJTs tend to have an emitter impedance of .033/Iq. So if we set degeneration to 33mV, our resistor matches the emitter impedance. At 200mV, the resistor is 6 times the emitter impedance. Whether degeneration is above or below 33mV determines whether the stage's Gm is mostly logarithmic or linear.

Last edited by keantoken; 2nd May 2013 at 04:19 PM.
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Old 2nd May 2013, 04:39 PM   #13
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Quote:
Originally Posted by AndrewT View Post
I have been using 200mV to 300mV as the Vdrop of the degeneration resistor, in LTP, & VAS & Current Mirror.
What is wrong with doing it that way? excess added noise? or many other avoidable problems?
In contrast noise will be much reduced in the LTP which is good, one shouldnt use low degeneration as Self or Slone suggest. Larger degeneration will lower the gain a little but in terms of THD you might be looking at 1ppm for a change from 22R to 220R. Noise can be lowered by as much as 15db. The lowering of the gain is also good as all current mirrors show some peaking in their response and this atenuates it.
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Old 2nd May 2013, 05:03 PM   #14
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I haven't learned about noise, so that is not in my ballpark. Do you have references for the peaking issue? I've never had problems with this except possibly when using more advanced current mirrors.
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Old 2nd May 2013, 05:45 PM   #15
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Noise is the parameter most affected by degeneration. In most IC opamp design literature youll find references to this. Its true though that other mirrors have much higher peaking. Usually one should include a small capacitance accross the BE junction of the helper transistors to tame the peaking although using higher degeneration is enough for the basic mirror. So having high Cbe is not always such a bad thing.

Just thought of a good reference :Analogue Ic Design: The Current-Mode Approach by Christofer Toumazou, F. John Lidgey, David G.. Haigh.
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Old 2nd May 2013, 07:46 PM   #16
astx is offline astx  Austria
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Originally Posted by kasey197 View Post
slobber slobber Congratulations on this beautiful piece of engineering.
It looks looks really professional to me - you must do this for work too and not just as a DIY hobby!

Thanks for sharing this btw - really very inspiring stuff
Thank you. I am very happy to have such a beautiful hobby.
Don't forget to visit this thread from time to time!

BR,

Toni
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Old 2nd May 2013, 07:51 PM   #17
astx is offline astx  Austria
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Originally Posted by Bigun View Post
Wow, looks good, you have a nice bench set up there and you've built a good looking amplifier. I hope you are pleased with the sound too.
Thank you. It measures very good and it sounds very good in mono (the second/other channel is assembled and mounted on heatsink since yesterday, but not yet ready for listening - needs the burn in test 1 hour full load 20kHz@200W8R).

BR,

Toni
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Old 2nd May 2013, 08:25 PM   #18
astx is offline astx  Austria
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Dear keantoken, manso, AndrewT

interesting discussion about current mirrors. During learning LTSPice I have simulated "simple current mirror", "EFA current mirror", "wilson current mirror" and "4 transistor improved current mirror". Douglas Self has made a comparison between these 4 CM types (APAD Handbook, Fifth Edition, page 85) .
One of the conclusion I made, that if using bjt's with beta > 200 the error of the simple current mirror between current in/out is very low in comparison to the "better CM" variants. So I decided to use a simple current mirror as the simulation showed only a few ppm THD difference.
As there are many empty input stage pcb's left (got it cheap from overproduction) I will try to test different bjt's and degeneration values during the next days and will give a feedback
  • if 2N5401 is really a mistake in this case or
  • more or less degeneration will improve or worsen distortion measurements
  • produces more or less harmonics. (currently only H2 and H3 are visible above 100dB / measurement noise floor of HP3585A)
Keep on going to discuss the 2stageEF amplifier so one day an improved schematic/pcb revision will see the real world.
Btw.: the same input/vas stage exists assembled with matched 2SK170 and about 150R degeneration. Will add ltspice asc file the next days.

Note: one can never have enough good amplifiers!

BR,

Toni
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Old 2nd May 2013, 10:03 PM   #19
Tiroler is offline Tiroler  Austria
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I feel truly honored to be a frequent visitor and guest in the Astx Lab. Toni is the tech-guru of our DIY group. He learns and develops his stuff at an extraordinary fast pace. I cannot think of someone to keep up... So my guess: We do not have to wait long for the finished result I like that he did a discrete amp this time. The board layout is very nice!
BTW, I am in the process of building a 4-channel amp on Toni's schematics and boards. That one is based on the LME 49830 and I am going to start a thread about that build in the chip amps section when the amp is finished.
Cheers, Martin
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Old 2nd May 2013, 10:35 PM   #20
astx is offline astx  Austria
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I feel truly honored to be a frequent visitor and guest in the Astx Lab.
...
Many thx martin! Going to buy extra beer for our next lab meeting...

BR,
Toni
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