Official M2 schematic

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#581
you can do the math:

The supply voltage drops from V+ over the n-FET, R13+R14, and the p-FET to V-, total drop some 45V.
Running at a bias of 1.5A (according to ZM) R13+R14 will drop ca 1.5V.
The data sheet of the FETs will tell you the Vgs necessary for that 1.5A, and remember that the Vgs of a p-FET is negative.
R6 and P1 need to drop the voltage from V+ to Vg(n-FET), and R7 needs to drop from Vg(p-FET) to V-. Between Vg(n-FET) and Vg(p-FET) the optocoupler will drop the two Vgs you read off the data sheet and the 1.5V over R13+R14.
 
#584
djonesii said:
[from zen Mod]
or value of resistor in series with pot

If I could get some input on these values, it would help me greatly. Unlike some DIY folks, I do not have a "parts bin" So, the marginal cost of a few resistors and a different value pst is often a week in lost time, and 10 times the value of the parts in shipping!

The answers are certainly in the F5 thread, and in fact, I remember scanning them, finding them is another matter :whacko:

Thanks for the help

Dave
Click to expand...

well , if said resistor is 47K , then you need to stash few lower standard values in drawer , same as few higher

do the math

being in this hobby , it's wise to invest in resistor packages , having at least 10pcs of each in , say, E12 range (going from 1R to 1M)

it's not that expensive and it will save you few headaches in very short time

Resistor Values E6 E12 E24 E48 E96 E192

if I remember correctly - I finished having 68K in place of R6 .......

generally - if fiddling with pot can't shave positive output offset , increase R6 to first higher value (higher than sum of R6 and P1) , so 56K or 62K , or 68K in case of needing more change

same applies for R7 value , if output offset is stubbornly negative

all that referring to schm in Papa's post #1

510705d1445746020t-official-m2-schematic-m2-sch.gif
 
#585
so , going back to check some things (references) , I stumbled on this :

Zen Mod said:
count on Ugs difference between N and P mosfets , for same current

however , as with all Papa's schematics - count on fact that he's usually taking parts from pre-selected drawers** , so changing any appropriate resistor (say for + or - 10%) if needed on the fly, is trivial thing to him

(one of) purpose(s) of all these schematics thrown on all of us in past few decades is pretty much the same - to same things became trivial to us , too

** he didn't said that , but logic dictates the same, for most cases ;
however , upon completing the amp , if output offset stubbornly stays negative , decrease R6 ; if it stays positive , decrease R7 ; I believe I needed 39K for one , but can't remember which ; even though I used FQP/FQN parts , Geschenk by my älterer Bruder Generg
Click to expand...


so , it seems that I'm not so sure which values I exactly have for R6 :rofl: , but there is opposite way for doing same thing - either increase appropriate resistor or decrease opposite one

😉

regarding Iq - funny thing is pretty self-biasing ..... don't bother is it 1A2 or 1A5 or what , both channels will be the same and , as long as all parts are genuine , it'll sing as original Papa's
 
#592
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#594
With a voltage gain of 5 and rails at +-22V the M2 can use 8.8Vpp at input as explained by ZM.
My DAC does 4Vpp at -3dB, so if I want to use the M2 at full blast I need some more voltage gain on the order of 2-3.
Is there a recommendation where to get that? A particularly well suited pre-amp design (prefarably w/o NFB)?
Or is there a way to tweak the Jfet stage in the M2 to also deliver a small gain?
Thx for your input!
 
#597
Zen Mod said:
so , just imagine effect of clamping diodes (D1-D2-D3) - they are clamping output node to rails , limiting max . signal amplitude
NB that output node is in strong correlation with mosfet gates , so clamping of outout is in fact achieved with gate clamping

Q6 and Q7 are there to maintain no more than 1V25 across corresponding resistors , saving LED from nasty peaks and maintaining smooth biasing

btw. did you put second schematic , framed , on the wall ?
Click to expand...

Finally after 3 months with the schematic laying on the table, regular looking at it, I understand the finesse of the M2:cheers::cloud9:
Thanks!
 
Last edited:
#598
WalterW said:
Finally after 3 months with the schematic laying on the table, regular looking at it, I understand the finesse of the M2 ...
Click to expand...
Glad that you finally got it. :up:
Now you may want to consider the Laterals such as the Allfets used on the F7, eliminate both the optocoupler and LM385 by using 4 x 1N4148 for bias and easier to drive due to lower input capacitances.
 
#599
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