simple (?) mod for JOYO Ultimate Drive for BASS?

[jewellworks]

ive got the JOYO Ultimate Drive, (JF-02) ive been using for bass, but it cuts off a lot of low end because its obviously made for guitar. even with the tone turned down, it just doesnt sound like i want it too.  then it dawned on me.  why cant i just bypass the whole pedal (in a sense) and use the level control more as a Blend with the straight signal? 
heres what i was thinking...  put a jumper from the in jack straight to the out jack, and when the pedal is turned on, you get the drive on TOP of the straight signal?  the level control would become a blend with the straight signal.  --is it that simple?? 
my next question is, i dont know where to put my jumpers exactly, to make this happen.  when i put a meter on it, i get strange continuity paths, and im not seeing any obvious reading when i engage the pedal (turn on/off).  it doesnt help that i cant seem to find a schematic for this pedal that hasnt been modified in some way, and in most schematics ive seen, the bypass (on/off) switch isnt included...
any thoughts???  is that just plain stupid??

[DavidRavenMoon]

I use mine for bass all the time. You don't need to mod anything.  It works great. I keep the gain near the low end and the tone about half way. I get as much low end as when it's bypassed.

I did mod the pedal though. I replaced the germanium diode with a jumper and soldered a 500k resistor across the gain pot to lower the pot's value.

And you can't mix the straight signal in that way. It won't work. If you want a pedal with a clean blend try a Boss ODB-3. I use one of those too, but I've replaced it with the Joyo.


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[Passaloutre]

Jumping in-out might give you the input signal at the output, but it also gives you the output signal at the input, which means feedback. If the pedal is net no inverting, the feedback is positive and quickly leads to oscillation. If the pedal inverts, then the feedback is negative, and should cancel the input.

[DavidRavenMoon]

It also won't work because of impedance problems


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[jewellworks]

i thought it was too simple to be possible.  thanx for explaining why

[nonoxxx]

if you want more bass you can modify the bass roll off in the feedback loop

considering this schematic : http://4.bp.blogspot.com/-_pwztcRxxis/Ty0xtP0wq9I/AAAAAAAAA3M/1lDrCwrPZO8/s1600/Harley_Benton_-_Ultimate_Drive_schematic.jpg

the roll off is at 1000 herz so all frequencies above 1000 herz will be boosted (it give a midrangy tone for guitar but don't work well on bass), by replacing C2 by a 1UF film cap you will have a lot more basses , all frequencies above 72 herz will be boosted and 1.5uf will be something like 50 herz.

Take care when modifying Joyo/Harley Benton stuffs the PCB is really fragile

[jewellworks]

Are you sure 1uf??  Not a .1??  Seems awfully high... 
I'm headed to my parts surplus store today.  I can get just about any value cap...

[anotherjim]

C2 at 68n is about 1kHz if the resistor is 2.2k. 680n takes it down to about 100Hz. So 1uF isn't a bad choice.

[nonoxxx]

Yes bass need to be low
Personnaly I should go for 50 herz  so the 1.5 uF.( I suggest polyester/ polypropylen Film cap ,MKT or Wima MKP2 are easy to find at this value)

50 herz are a good start for a bass because in a mix ( on an album) the guitars are filtered at 80/100 hertz and the bass has to fit under this frequency to sound like a bass

A metal overdrive of the brand protone for guitar start at 72 herz for really low tuning of 8 string stuffs. So 50 herz for a bass is not so low

[nonoxxx]

Are you sure 1uf??  Not a .1??  Seems awfully high... 
I'm headed to my parts surplus store today.  I can get just about any value cap...

Sorry it's a european retailer  but: (with film you  can find as high as 3 uf)

for 1,5 uf

https://www.musikding.de/MKS2-15uF_1

and for 1uf :

https://www.musikding.de/MKT-1uF_1

1uf  film caps is like a standard in coupling caps in bass pedals so it's not that hard to find. But avoid general electronic stores because you will only find electrolytics for these values (and only if you have some luck).

[jewellworks]

Well!  That certainly did something!!  I've got a ton of low end passing through now, which is a good thing.  I replaced the 68pf with a 1.5mf (it's C4, btw, on the JF-02 circuit-board) but now I have a new problem.  Too much distortion.   I barely turn the gain knob and it's overdriving like mad.  If I turn it up past 3 it goes into oscillation.  I don't dare turn the level or the gain up past a crack.  The hi and low switch doesn't seem to have much effect.  It's all just incredibly loud all the time.
I realize I'm pushing a lot more frequencies through the circuit than before, and low end is a much bigger wave than 1k and above, but damn! 
So how do I dial back the overdrive now??  Less gain?? 
I also realize that's what pedal freaks like is more gain, but in this case, less is definitely more...
I sure wish someone had a schematic of this 02 circuit, and not just the old one.  It would be easier to trace....

[nonoxxx]

On this picture

the cap seem to be the red/brown one on the upper left , just under the gain pot.

can you post a picture of the back of the board ? It will be easy to trace .

also you mentioned  68pf with a 1.5mf , just make sure it's an error ( the good values are 68n and 1.5uf)

[jewellworks]

Yes, that's the one.  68n (marked 683) replaced with a 1.0
(marked 105.  Sorry, I thought I put in a 1.5 /155.  I bought both.  I had to use a conversion chart from European values to American)
I traced it coming off pin 1 of the op amp, to 2 resistors (going different directions ) to the 683 cap.  It's layed out a bit different than the 01 version of the schematic,  but it's identical in circuit design.  At least that part is...
Sorry, not sure how to post pictures. ...

[nonoxxx]

The weird thing is that I modified a lot of equivalent circuits by adding  more bass this way and it never caused a problem (screamers,Boss DS1 , RAT..) . The IC can take as much bass you want and amplify it.

[jewellworks]

It's amplifying it alright, just too much.  It's uncontrollable.
I swear all I did was swap that cap... 
I might have a 683 in my parts bin somewhere...  I could put it back...
If I can figure out how to post pics, I'll attach what I could trace.  I drew a diagram.   It's not complete, but it shows what's going on in that general region.  It's slightly more involved than the 01 version

[nonoxxx]

Can you give me the values of the resistors in the feedback loop? the one that go to the ground (just after the cap) and the one in serial with the pot ?

[anotherjim]

With the cap large enough to make it full range, the gain may be at least x10. Bass is going to clip the op-amp most of the time at that. The resistor in series with the cap, R3 2k2 in the Ultimate drive scheme, needs increasing. My first instinct would be to try to fit a trimmer pot in it's place 20k to 50k value.

[nonoxxx]

For what I know , with the cap so high and the frequency so low , the gain will stay the same and it's only the curve of the sound that will be affected and boosted on the low frequencies  ,you will notice more gain only because you will get a bigger range of sound, but no clipping ,the gain is only set by the two resistors and the pot this way so the cap is not involved with the resulting gain  : 

in mV : 1  + ( ( r1 + pot) / r2)   where r2 is the resistor connected to ground

and in db : 20* log of the result

Correct me if I am wrong but  I made it on tube screamers and rats a lot of times and it was fine with all the range of the gain pot from low to max

[anotherjim]

The original capacitor had significant impedance, Cx, at most of the instruments fundamental frequencies.
68nF is worth around 4.7kohm at 500Hz! With the Rin at 2.2k making nearly 7k, so with Rf at 19k (! what an odd value !) making the Rf/Rin gain factor a little under x3 (actually x4 total). Below 500Hz, the gain gets reduced from there, but as this is a non-inverting amp, there is always at least x1 gain added to the Rf/Rin value.

So the original design values means the signal is nearly always below the high pass -3dB point, somewhere down the curve and the gain is heavily frequency dependant.

1uF is worth only 300ohm at 500Hz, which is getting down to negligible impedance in the face of the other values.

It definitely changed the basic Rf/Rin gain, making Rin the dominant factor .

[nonoxxx]

It's really interesting anotherjim,

Generally when I do this kind of mods I don't think about the impedance of the caps.