Quote from: LOBO on April 14, 2011, 11:45:25 AMI don't mind:
I hope Mensur don't mind: http://www.megaupload.com/?d=VFD4E6KP
http://img641.imageshack.us/img641/4237/ultimaterandall.jpg
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Show posts MenuQuote from: LOBO on April 14, 2011, 11:45:25 AMI don't mind:
I hope Mensur don't mind: http://www.megaupload.com/?d=VFD4E6KP
Quote from: Alexius II on March 21, 2011, 04:55:00 PMActually it's a pretty simple stage, you see 390R is stage biasing resistor, the upper fet is current source, meaning(active load), varying potential divider i.e. 10K's you will send less dc voltage on the gate of the upper one, hence, smaller current you will get on the source,
So the only thing I have to change is to re-set the drain voltage of first two stages? That is great to hear!
I'm away from home until friday, so I won't be able to try it out until then. I will report back on how it sounds :tu:
By the way, how would one change (lower) the gain of the mu stage, if lower gain would be required? I found the "original" mu-amp here, but this does not help my understanding of this stage. I can see that ROG added 390R source resistors, somewhat changed the "bias" resistors (two 10k and a 1M instead of two 10M) and the input resistor is changed from 10M to 220k. Does someone have a simple explanation of this configuration?
Quote from: phatt on January 30, 2011, 07:31:29 AMPhil can you post the schematic of that, it;s very interesting?
Hi Steve,
Your Valve section may need some fine tuning.
i.e. add some series resistance after each triode,, you've got some ugly blocking type distortion which is destroying the sound. (but if that spitting distortion is to your taste then go for it.)
You may wish to look at the redneF Pro Junior preamp setup for some good ideas.
Or even some early Deluxe tone circuits might interest you.
Other than that It's a *Very clever idea* and good on you for having a go.
Thanks for sharing your ideas. :tu:
If it interests you I have had good results by power soaking a small PP Amp and ReAmplifiying it via a simple voltage divider setup and then into a large 120Watt SS Amp. (You get to do post power tube EQ effects that way)
Cheers, Phil.
Quote from: LOBO on November 09, 2010, 02:21:31 PMI personaly like film caps(Wima MKS4) overthere, but you can try with elcos, and see what best suites you.
Mensur, i would like to ask you again about the capacitors c5, c8 & c14. Are they normal (film, poly etc ) or eletrolitical (polarized) ? Cuz their values are 1uF...
tankx for the previous answers.
Quote from: mensur on September 25, 2010, 06:08:07 PMYou are right :tu:
Use MPF102's for both, voltage gain and as buffer.
Use 680R for RS(voltage gain) without bypass cap, BTW. Cs reduce internal resistance of the channel, hence more gain.As for Rd use 2.2K.
These are resistance values foe MPF102 FET (with -4.5V, 13.5mA Idss characteristics).
From my cacs. it will bring you to -2.12V of Vg(humbacker sweetspot) and 3.46mA Id.
Gain will be around 3 to 4x.As for buffer use same character. FET with 2.7K Rs, everything else remains the same.I don't like bipolars as buffers, you don't need much current there.
Quote from: Alexius II on September 26, 2010, 04:11:09 PM
I was a kid and bit too young to remember anything from the old Yugoslavia, but they say that were the good times
Anyway, thank you both. I went through some online theory pages and found a few other equations that helped me understand the few missing relations. I think I get it now... I hope :duh
I will now go again through the calculations for my transistor:
These are the equations I used for the gain stage:
Vgs = -Id*Rs
Vds = Vdd-Id*(Rd+Rs)
Id = Idss*(1-(Vgs/Vp))^2 (the characteristic curve is drawn from this equation)
gm0 = 2*IDSS/VP
gm = gm0*(1-(VGS/VP))
Av = (gm*Rd)/(1+gm*Rs) (if there is no Cs)
My transistor:
MPF102
Vp = -4.5V
Idss = 13.5mA
I choose Vgs to be 1/2 of Vp, so
Vgs: 0.5*(-4.5V) = -2.25V
Id = 13.5mA*(1-(2.25V/4.5V))^2 = 13.5mA * 1/4 = 3.375mA
Rs = Vgs/Id = 2.25V/3.375mA = 666ohm (closest is 680R)
gm0 = 2*13.5mA/4.5V = 0.006mS
gm = 0.006mS*(1-(2.25V/4.5V)) = 0.003mS
Then I choose Rd. I understand that with larger Rd come larger voltage gain and output impedance + lower current.
Rd = 2.2k (18V/2200R = 0.0081A = 8mA)
Av = (0.003mS*2200R)/(1+0.003mS*680R) = 2.17 (= 6.7dB)
Ok, I hope everything is OK this far.
Then comes the buffer (aka "source follower" or "common drain amplifier")
Only the Rs is necessary to bias it. I've read some theory about it, but I'm not sure. The way I understood it, you have to choose Rs in a way, that at the source you have half the voltage (Vcc). If this is correct, I then used the identical MPF102 transistor with calculated Id = 3.375mA (for Vgs = 1/2 Vp) inserted in the equation:
1/2 Vcc = 9V = 3.357mA * Rs
Rs = 9V / 0.003357A = 2680R = 2.7k
Is this ok, or am I making things up? :loco
Quote(I study something completely different, so this is just my free-time "hobby")Tell me about that, I finished Faculty of sport and physical education, on the department of sport management, and higher degree of tennis trainee