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Old 20th December 2018, 01:25 PM   #221
Rick NL is offline Rick NL  Netherlands
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Hi Juma,
I still have 4 pairs of the original 50/135 laying around and am intregued by your Cubie2 design.
Also I do like the Malcolm Hawksford enhanced cascode that you use.
Only I am wondering why you place the C2 capacitor parrallel to the (constant-)current resistor R14. I think this will degrade the PSRR as it could modulate the basis of Q6/7.
Would it not be better to place a cap over the LED's as originally done in the M-H design?
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Old 21st December 2018, 10:07 AM   #222
juma is offline juma  Serbia
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Quote:
Originally Posted by Rick NL View Post
...why you place the C2 ....
Hi Rick,

C2 equalizes eventual small HF fluctuations over R14 and since the amp is symmetrical it should improve PSRR.

Anyway, it's easy to tests and measure your ideas and see what you get...
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Old 21st December 2018, 10:20 PM   #223
Rick NL is offline Rick NL  Netherlands
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Early in the new year I will build a testversion and try. I am not yet convinced Juma, may be for symmetrical disturbances, but certainly not for asymetrical disturbances.
May be that's why you advice such a big and very well suppressed power supply.
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Old 26th March 2020, 09:24 AM   #224
kenev is offline kenev  Greece
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So, with the moving restrictions and obligatory stay at home due to the Covid-19 pandemic, it's a great opportunity to get my hands on projects left aside for a long time now. Hence, I'm bringing back to life this thread to ask juma a couple of questions regarding Cubie2.
Since two years now, I had gathered all the components, but never had the chance to go further. First of all, what I did was an extensive testing and matching of the input jfets: I have a lot of K170 and enough J74 jfets, so I managed to get two decent pairs. Next, I had to test a bunch of BCs (around 30 from each type), in order to pick-up some nice triplets. Following this, I tested 6 pairs of lateral MOSFETs (namely the EXC10N20 and EXC10P20 Exicons) in order to check the Vgs voltage at an Id of 0.7 A. The EXC10N20s read around 1.4 V and the EXC10P20s read around 1.9 V.
So my questions to juma are the following:
1. The jfets I chose have an Idss of 7.8 mA. How can I calculate the value of R11 and R13 resistors in order to have a current o 10 mA flowing through Rbias? Can I leave the original 68 Ohm in place and adjust Rbias so that I have a current of 0.7 A through the output MOSFETs?
2. Have you any experience regarding the increase in output current? I mean, if I raise the output current to 0.8 or 1.0 A, what is the acoustic difference? I understand (from what Nelson Pass has told) that, for each output MOSFET, there is a "sweet spot", where it sounds best.
3. I would like to try a 0FB version. For that, I intend to build a FB and a non-FB board and try them listening just one channel. In your description, your instructions are clear regarding the changes needed. What is not mentioned, though, is where the wiper of the offset trimmer (P1) should be connected.
That's all for the moment.
Stay well, healthy and safe,
Vagelis
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Old 27th March 2020, 07:26 PM   #225
juma is offline juma  Serbia
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Giasou Vageli,

1. Yes, keep original values and yes, you can adjust R bias to get the output stage bias you want.

2. Exicons have total Vgs of 1.4V + 1.9V=3.3V - the same value I got with 2sk1058/2sj162.
Acoustic difference is subjective so you'll have to try different values, listen and decide what you like the best.

3.See schm. below and be patient when setting DC offset, it is more thermally sensitive without the feedback loop.
P1 still sets the DC offset. Relation Ra/Rb roughly sets the gain so for Ra=120R and Rb=1k you'll have about 20db gain.
Output bias current is set with Rx,Ry i.e. Rx=Ry=150R-220R (higher resistor value means higher current so start with lower values). Rx and Ry don't have to be equal - you can change the DC offset that way too, but it will change the bias current at the same time so you can experiment with that freely (but carefully).


Best regards and sorry for my brevity - I'm extremely busy these days
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Old 27th March 2020, 08:20 PM   #226
kenev is offline kenev  Greece
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Geia sou Juma,
Thank you very much for your response.
While waiting for your reply, I tried to calculate the current through the Rbias string the hard way: I breadboarded the whole input/current mirror stage using the actual parts and found out that the current was a little lower (9.2 mA), so I had to put a Rbias resistor of 360 Ohm in order to get a voltage of 3.3 V across Rbias. A little trivial task, but it worked just fine.
Regarding the zero feedback version, I thought that the wiper of the offset trimmer should be referenced to ground, but my knowledge wasn't that specialized. So, your help was great in this matter.
Thank you again for you time and I will keep you informed as soon as I draw my impressions (unless something else comes up).
It's very good that you are busy at these hard times. On the contrary, I'm out of work for two weeks now and I'm trying to keep up with all those things that I had left behind.
Best regards,
Vagelis
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