[solved] xoxshop.de VCF/VCA trouble

Discuss x0x construction and related issues

Moderators: altitude, adafruit_support_bill, adafruit, phono, hamburgers

Please be positive and constructive with your questions and comments.
Locked
hi0b
 
Posts: 2
Joined: Tue Jul 22, 2014 1:49 am

[solved] xoxshop.de VCF/VCA trouble

Post by hi0b »

hello,

i have built the xoxshop.de kit - everything works but i get some strange distortion / bass boost when raise the resonance potentiometer.

here is a sound example: https://soundcloud.com/win-1/x0xb0x-trouble

compared to the Voltage reverence eagle file, i get only 6,2V at C27 (south side) and 1,2V C25 (south side)
at Q22 between Base and ground i can meassure 11,7V

I have triple checked the resistor values , capacitor & connections between parts in the VCA VCF section - i cant find the error - i need some guidance please

hi0b
Last edited by hi0b on Sun Aug 17, 2014 9:47 am, edited 1 time in total.

User avatar
antto
 
Posts: 1636
Joined: Thu Apr 15, 2010 3:21 pm

Re: xoxshop.de VCF/VCA trouble

Post by antto »

basically, you don't have resonance
when you turn the reso pot up - it feeds signal from the filter output back into the input (but inverted) and thus the overal gain drops, you get resonance
in your case, something goes wrong, the gain goes up, and there's no resonance really
also, in the waveform you can see clipping
this happens probably somewhere after the actual ladder stages, and probably on the negative (bottom) side of the waveform only

no idea what could cause this
as you've made sure that the surrounding components in the VCF/VCA sections are proper values and orientation
the only thing that i can think of is.. Q22, it's just one of these transistors in the whole x0x circuit, and it's "rare" .. if you search the forum you'll find that quite a number of problems have been due to something wrong with Q22
you could try desoldering it, and find either another such transistor or.. we've found that you can use 2 BC337 transistors in its place, it requires some bending of the legs but it works
BC337 are easier to find

if you desolder Q22 - power up the x0x and measure the voltage on those places where you get bad voltages
i will remove my Q22 from one of the x0xes and measure too, and we can compare then

hi0b
 
Posts: 2
Joined: Tue Jul 22, 2014 1:49 am

Re: xoxshop.de VCF/VCA trouble

Post by hi0b »

hello,

Q22 gently desoldered and socket inserted, base 11,7V, other Pins ~0V

i took 2 BC546 onto a breadboard and connect it to the Q22 socket - the hole thing sound way more like an acid machine - yehaaaa :-)
so, my 2SC2291 is not working - i will check to get another and will try your BC337 tip.

many many thanks for your BC337 tip - it was the right one!

User avatar
antto
 
Posts: 1636
Joined: Thu Apr 15, 2010 3:21 pm

Re: [solved] xoxshop.de VCF/VCA trouble

Post by antto »

nice!

User avatar
altitude
 
Posts: 995
Joined: Wed May 11, 2005 5:17 pm

Re: [solved] xoxshop.de VCF/VCA trouble

Post by altitude »

Matching the BC parts would help as well. Modern off the shelf parts are probably a closer match than the 2291 were back in the 80s anyway

Locked
Please be positive and constructive with your questions and comments.

Return to “Making x0x”