IBANEZ/MAXON SD9

[bonehead1972]

hey guys!
i built the SD9 according to this schematic and it sounds good... my question is why R3 and R15 are not of the same value? it looks (to me) like they have the same purpose but R3=510K and R15=1M. why is that
thanks in advance and may The Tone be with you!

[anotherjim]

R15 makes sense at 1M so it doesn't change the taper of the 100k pot preceding it. This is following the 10 to 1 loading rule.
Why did the Japanese designers pick 512k for the input resistor is perhaps a better question. I suppose it's less sensitive to noise than 1M which is going to get amplified a lot by the opamp. That said, the transistor base also adds a load to the tune of 10k x its hFE in parallel with the resistor. As hFE varies some between parts it will make less change to the total input load with 512k than 1M would.

[bonehead1972]

thank you for your answer, Anotherjim! i would  like to ask one more question regarding those two transistors- what is the optimal emitter voltage ?

[iainpunk]

4.2v is the Vcc is exactly 9v.
If 9.6v comes from the battery, 4.5v is more ideal.
Its basically (Vcc-0.6)/2 or, most people use is half-Vcc-ish.

Cheers

[bonehead1972]

The thing that bothered me was with R15=1M the Q2 emitter was below 4v.

[antonis]

The thing that bothered me was with R15=1M the Q2 emitter was below 4v.

To flesh out what Jim said above, high bias resistor value results into "sensitive" bias point circumstances (strongly dependent on particular BJT h_FE and V_BE)

So, the actuall Emitter bias point is set by VB - (I_C/h_FE X Rbias) - V_BE = [(I_C + I_C/h_FE) X R_E] = VE

That said, to obtain Emitter follower biased ar power supply mid-point, you have to set VB higher than Vcc/2, due to voltage drop both across bias resistor and Base-Emitter junction..
(in reality, Emitter should be biased even higher for maximum symmetrical output but that's another story..)

[bonehead1972]

Thanks for your help, Antonis!
I have tried 2sc1815, 2n2222, bc109c, all around 300hfe (as suggested) with similar results.....kind of lowish emitter voltage. 2 x bc109c  was my last try and I kept them. I would be grateful if you share some of your personal suggestions on this...cheers

[antonis]

I would be grateful if you share some of your personal suggestions on this...cheers

Replace Q2 buffer with unused amp of IC.. cheers

[iainpunk]

I would be grateful if you share some of your personal suggestions on this...cheers

Replace Q2 buffer with unused amp of IC.. cheers

i would negate the input buffer, like in a MXR distortion+ or DOD250.
then use the unused opamp as a buffer after the tone and volume controls.

cheers

[antonis]

i would negate the input buffer,

Agree as far as op-amp is JFET input..

For JRC4558, bias resistor can't be of high value (without raising Vbias a bit) thus dominating input impedance..

P.S.
I know for Perfectionists have no place in distortion builds, but..

[bonehead1972]

Thanks for your help guys!