UNIVOX UNIDRIVE build report/rant !!

[PharaohAmps]

Looking at the input section of the UniDrive, it seems that it was designed to accommodate a variety of different inputs (keyboard / bass / guitar, etc.) and thus the stepped attenuator.  The coupling caps are also large because some of those sources have very low impedance inputs, and the lower the impedance, the bigger your coupling caps have to be in order to pass the same low frequencies.

For guitar and other high-impedance sources, smaller caps are OK.  10uf is WAY overkill, I'd suggest 1uf at most.  0.1uf would work just as well, but I'd stay away from polarized electros on the input stage because IME they can be "farty" sounding.  An 0.1uf film or poly cap would be a great choice.

As for voltage ratings, the easiest way to determine it is to figure out what voltage should be applied to each end of the cap and then add some safety margin.  In this case, one end is at somewhat less than 1V (transistor base,) and the other end is floating (but we can assume no DC potential.)   So 1V across the cap, you'd be okay with a 6.3 or 5V cap.  Since most caps you're likely to meet are higher-rated than that, 10V or 16V is probably easier to obtain.  Anything would work for this purpose.

For power supply caps, it's the same story, just figure out the voltage difference across the cap (for a battery powered circuit, it's typically 9V and a bit,) and add some safety margin.  I use 25V caps for most of my pedals, but some of my stock is still 16V caps.  I allow a 20% safety margin ALWAYS, and more is better if you can fit it on the board.

Matt Farrow

[aron]

you submit a new fuzz build report every three days....how many fuzz pedals does one man need?

What?? :shock:

There can never be enough fuzz/distortion pedals....

:roll:

[petemoore]

All of them. Until he finds the few he really is going to use, then the rest for 'diversity'...lol, that' IMCase means like 20.
 Between buying [earli in my 'career'] and building [once I started] I must have tried 100+ fuzzes., OD's...

[Doug_H]

All of them. Until he finds the few he really is going to use, then the rest for 'diversity'...lol, that' IMCase means like 20.

It is so important in this day & age that we maintain an atmosphere of multi-fuzzal diversity.

Doug

[MartyMart]

All of them. Until he finds the few he really is going to use, then the rest for 'diversity'...lol, that' IMCase means like 20.

It is so important in this day & age that we maintain an atmosphere of multi-fuzzal diversity.

Doug

 
It also gives me great pleasure to "build" something that makes a good
useful/musical noise .... WITH MINE OWN HAND !!

.... Plus I DO make my living from music .......
and I love building a "classic" that sounds like "POO" then watching as
someone pays £500 on Ebay for the same thing ... minus the "tweaks"!!!
:shock:  :shock:

M

[Eb7+9]

I ran some bias and TRANsient sims on the stock circuit and (regardless of models) it showed onset of clipping with just under 30mV of input voltage swing  and no input resistance to the circuit ...  

Univox Unidrive -- Headroom Test without Input-Source Resistance

Vcc 99 0 9
Vin 11 0 0 sin(0 0.03 1e3) ac 1

Rin 11 0 220k
Cin 11 1 10u

Rb1 1 0 150k
Rc1 99 2 100k
Re1a 3 33 7.5k
Ce1a 3 33 50u
Re1b 33 0 100
q1 2 1 3 2N5089

Cbe1 1 3 .001u

Rc2 99 4 10k
Re2 5 0 4.1k
Ce3 5 0 0.2u
q2 4 2 5 2N5089
Cbc2 2 4 40p

Rf1 1 5 100k

Re1 6 0 3.3k
q3 99 4 6 2N5089

Co1 6 44 10u
Rf2 44 3 8.2k
Co2 44 55 10u
Ro 55 0 10k

.model 2N5089   NPN(Is=5.911f Xti=3 Eg=1.11 Vaf=62.37 Bf=1.434K Ne=1.421
+               Ise=5.911f Ikf=15.4m Xtb=1.5 Br=1.262 Nc=2 Isc=0 Ikr=0 Rc=1.61
+               Cjc=4.017p Mjc=.3174 Vjc=.75 Fc=.5 Cje=4.973p Mje=.4146 Vje=.75
+               Tr=4.671n Tf=822.3p Itf=.35 Vtf=4 Xtf=7 Rb=10)

.model 2N5210   NPN(Is=5.911f Xti=3 Eg=1.11 Vaf=62.37 Bf=809.9 Ne=1.358
+               Ise=5.911f Ikf=14.26m Xtb=1.5 Br=1.287 Nc=2 Isc=0 Ikr=0 Rc=1.61
+               Cjc=4.017p Mjc=.3174 Vjc=.75 Fc=.5 Cje=4.973p Mje=.4146 Vje=.75
+               Tr=4.68n Tf=820.9p Itf=.35 Vtf=4 Xtf=7 Rb=10)

.model Q2N2222  NPN(Is=3.108f Xti=3 Eg=1.11 Vaf=131.5 Bf=217.5 Ne=1.541
+          Ise=190.7f Ikf=1.296 Xtb=1.5 Br=6.18 Nc=2 Isc=0 Ikr=0 Rc=1
+          Cjc=14.57p Vjc=.75 Mjc=.3333 Fc=.5 Cje=26.08p Vje=.75
+          Mje=.3333 Tr=51.35n Tf=451p Itf=.1 Vtf=10 Xtf=2)

.MODEL 2N3904   NPN(Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259
+               Ise=6.734f Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1
+               Cjc=3.638p Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75
+               Tr=239.5n Tf=301.2p Itf=.4 Vtf=4 Xtf=2 Rb=10)

Here's some changes I did to run it smoother and with a higher clipping threshold:

(i) 10k (PAD) pot in series with input
(ii) replace 100 ohm in first stage emitter circuit by 1k  (GAIN) pot
(iii) all caps replaced by metal film caps - no electrolytics :

input cap replaced by 0.033 poly
47u aross 7k5 replaced by 0.66u (0.22 x3)
10u across 4k replaced by 0.22u
10u output caps replaced by 0.22u each

I think some input resistance helps make the circuit feel less hyper and the idea of using smaller non-electro caps a real good idea - the 47u electro sucked ... also, as I drop the 1k GAIN pot (increasing gain) I get slightly enhanced brightness  - so there's some sort of pocket thing happening there ... great circuit for getting edgy/squeaky sound with tube amps ...  thx!

~jc

[MartyMart]

JC that's great !!
Thanks for the input, I'll build a version tomorrow with that setup and
see how it sounds

marty.