To Raise Impedence or Use a Transformer: Devi Ever Soda Meister

[aviherman5]

Hi all,

Storytime: I'm going away for a year and building a board. Obviously, I love dirt circuits and am building 16 of them inside of 4 1590BBs. Due to normal pedal placement, however, they are all after a buffer which is fine for 15/16 of them, but I don't think it's ok for my clone of the Devi Ever Soda Meister.

My question is a few parts:

1. Will this be an issue (buffer -> Devi Ever Soda Meister)? It's input impedence is so low!
2. If it will be, is it better to raise the impedence (without compromising tone) or to use a transformer to simulate a pickup like some Fuzz pedals do?

Thanks for all the help guys!

Schematic:

[antonis]

Hi you too..!!

Minimum Soda Meiser input impedance should be about 11k..
Feedback resistor divided by stage gain in parallel with Base reflected Emitter resistance - [2M2/130(*)] // [450(**) x 0.026/0.00045(***)]
(*) due to 2nd stage Collector loading..
(**) minimum MPSA18 h_FE..
(***) 450μA Collector current taken for mid-supply Colector bias..

A decent buffer could easily drive much lower impedance but you have to calculate the total effects parallel equivalent impedance to be sure..

P.S.
Is your buffer based on transistor or op-amp configuration..??

[aviherman5]

Is your buffer based on transistor or op-amp configuration..??

Hi! Thanks so much! It is op-amp.

Would there be any problems ala fuzzface-in-front-of-buffer with the Soda? If not, were all good, but if there would be, what's the best way to go about it?

[antonis]

Would there be any problems ala fuzzface-in-front-of-buffer with the Soda?

IMHO, No..

Soda Meiser first stage has much better input/output impedance ratio (slightly over unity) than Fuzz Face analogy (marginally 0.2)

But, if you like, you can add a 5 - 10k resistor in series with IN pot wiper to "simulate" guitar pick-up resistance..

[aviherman5]

Alrighty then, sounds good!

Will be building it as usual then!

Sneak peak of one of the pedals:

[antonis]

No SMD transistors..?? 

[aviherman5]

I have 3 BC184Ls for the powerboost and a 2N3391/2N3906 duo for my Heathkit TA-28

[soggybag]

As an amateur take my ideas lightly.

Could you dump the input pot, since this is really doing the same thing as your volume knob.

Then use a MPSA13 darlington for Q1.

[antonis]

Input pot sets the amount of signal amplitude coming into Q1, hence the amount of amplified signal (for a given stage gain)..
Although it isn't techically correct, it effectively sets first stage distortion..

MPSA13 (or any other Darlington) in place of Q1 needs defferent bias configuration or Q1 will be prermanetly saturated..
Even if it could work as it is, it shouldn't contribute much to input impedance increase..
(feedback resistor aparent value should remain the dominant part of input impedance)


P.S.
IMHO, 100nF feedback cap should be, at least, an order of magnitude smaller..
(but that's another story..)

[soggybag]

The beginners will it a second shot.

The target impedance wasn't specified, the rule of thumb seems to be 1M for guitar pedals but, a lot of pedals don't quite get there. An example would be the Boss DS-1, which has an input impedance of 470k.

If 470k was good enough you could use the DS-1 input buffer. It's just a bjt emitter follower and a few resistors.

[antonis]

use the DS-1 input buffer. It's just a bjt emitter follower and a few resistors.

Those kind of buffers suck.. (IMHO)



Input impedance is about 330k (470k // h_FE X 10k) while undistorted OUT signal is limited to low levels both for current sinking inability and low voltage Emitter bias reasons..

In breef (just for not to be classified off-topic)

Emitter is biased at about 3V, due to both voltage drop across 470k and V_BE..
This, by itself, restricts OUT undistorted swing at 2.1V_RMS (without counting neither V_BE voltage drop nor voltage gain lower than unity..)
The above only stands for next stage infinite impedance (Zin)..
With an impedance comparable to R3 value, current sinking (for input signal negative waveform) is restricted due to R3/Zin voltage divider..
So, for a Zin of 10k (for clarity), negative undistorted output is limited to half of respective headroom..
(1.5V_peak or 1V_RMS)

You see that " just a bjt emitter follower and a few resistors" might conceal pitfalls..

[soggybag]

Your feedback is invaluable for the students Atonis!

My last idea would be to use a MOSFE (SHO) or JFET (fetzer valve) in front of Q1.

[antonis]

My last idea would be to use a MOSFE (SHO) or JFET (fetzer valve) in front of Q1.

Why..??
Both are inverting amps of high output impedance (Drain resistor value) compared to any conventional buffer..

OP stated that he already uses a buffer for the rest of effects withoun any issue..
His query was about Soda Meiser low input impedance loading the buffer output..
As far as his buffer is op-amp based, 10k (or less) input impedance should create no problem..
But, as I said above, it should be better to calculate equivalent impedance of all simultaneously working effects..
(if there is such a case..)

P.S.
It should be better not to involve with FETs (either Mos or Junction) buffers pros & cons here or we are at risk of been banned..

[soggybag]

Maybe I miss understood the question. I thought the goal was to increase the input impedance.

Not sure what would get us banned in this thread. If you're referring to the reference to SHO, if I remember correctly, this was the one Zvex pedal that was allowed for discussion. I remember this from way back when the forum started up. Unless something has changed it should be okay.

[antonis]

Not sure what would get us banned in this thread.

Any babbling/garrulity/yawp classified beyond thread initial query..

[JustinFun]

Would there be any problems ala fuzzface-in-front-of-buffer with the Soda? If not, were all good, but if there would be, what's the best way to go about it?

I don't have the theory chops that others on this thread do, but it's a 15 minute job to throw it on a breadboard and find out?