Component C29 is backwards on all TB-303's... (more inside)
Moderators: Altitude, adafruit_support_bill, adafruit, phono, mome rath

Re: Component C29 is backwards on all TB-303's... (more inside)

by Tim Stinchcombe on Sun Dec 12, 2010 6:16 pm

textile wrote:How would a bi-polar EL or a polyester type go?
Can you do a simulation of these types?
Reversing the electrolytic cap in location C29 is sometimes causing a difference in sound because the cap is polarized, and it doesn't like being used backwards - thus using a non-polarized type like a polyester or non-polarized electrolytic should not show any difference, as electrically it looks the same either way it is inserted.

Generally capacitors used in simulations are ideal, i.e. they consist purely of capacitance, whereas in reality a cap will have an effective resistance both in series and in parallel to it, and if the circuit is likely to be sensitive to such things, then you must explicitly include them in any simulation (and sometimes you need to go to the extent of including lead inductance etc. as well). There are many 'standard' models for caps that include such things, and when we start talking about modelling a reversed electrolytic, they become more complicated. On seeing guest's data, I simply decided that a resistor in parallel would suffice to show the kind of effect we were looking for - i.e. it would 'leak' lots of DC current and hence impact on the biasing. Incidentally, this means that probably the easiest way to 'replicate' this kind of effect in a x0xb0x will be to simply do as I did in the simulation, i.e. rather than reversing the cap physically, simply solder a 30kohm (or so) resistor across C29! (But my suspicions are that many owner's who might want to do this might not see that as mirroring the real thing in a TB-303, and will want to reverse the cap - the resistor method does of course have the advantage that you might not have to wait for several years - or however long it takes, I don't know! - for the capacitor to start becoming as leaky as bcbox's examples!)

The question is of course is , why did roland do it.
Probably a mistake in laying out the PCB, pure and simple, and it has never been detected. If hundreds of units had started showing up with that cap blown, then I dare say they would have spotted the error, but since it seems the units do still function with C29 reversed, no one would have realised that anything was wrong.

Also is it recommended to reverse the cap. If the TB had it .. should the x0x too? It seems logical for a true clone.
That will come down to personal taste, but so far we have little idea (well I have little idea) what it takes to get the cap into a state whereby you could say the difference in reversing it is noticeable (see above)!

Tim
Last edited by Tim Stinchcombe on Sun Dec 12, 2010 7:28 pm, edited 1 time in total.
User avatar
Tim Stinchcombe
 
Posts: 11
Joined: Wed Dec 01, 2010 5:25 pm
Location: Cheltenham, Glos UK

Re: Component C29 is backwards on all TB-303's... (more inside)

by Tim Stinchcombe on Sun Dec 12, 2010 7:16 pm

guest wrote:it makes sense that increasing the feedback
would decrease the amplitude in the passband
but there really shouldnt be much difference
besides for amplitude differences
with a different forward path gain
The simulation traces say otherwise, unless of course I have made some ghastly error (always a possibility!).

imagine you had two gain blocks
and putting them in series gave the same gain as you had before
now you merely take the output from the first gain block
you can then reduce the resonance to null out the second gain block
its the same circuit
just tapped at a lower gain point
basically you should be able to get the same frequency characteristics
with everything shifted down by a fixed amount
The key bit I'm wary of here is "just tapped at a lower gain point" - we are not moving any 'take off' point in the filter by reversing the cap, all we are doing is changing gains, and if you do some simple algebra you'll see the forward gain turns up in both the numerator and denominator, whereas the feedback gain (the resonance) only appears in the denominator, and it is likely this is what causes the difference. I did try and work out a simple example, but it gets too confusing too quickly so I abandoned it. I often find it difficult to rely on 'intuition', as it is easy to overlook some important aspect of the circuit and go astray, whereas the maths gets it right every time - when the going gets tough, resort to algebra! (search for 'obstacle'!). But if you care to 'algebracize' your example, or draw a block diagram, I'm open to being convinced that I've gotten something wrong somewhere!

Tim
User avatar
Tim Stinchcombe
 
Posts: 11
Joined: Wed Dec 01, 2010 5:25 pm
Location: Cheltenham, Glos UK

Re: Component C29 is backwards on all TB-303's... (more inside)

by Altitude on Sun Dec 12, 2010 8:16 pm

Just installed Tim's suggested 33K with a switch across C29 in my x0x and it definitely behaves similarly to Brian's sanyo demo's but with a modern cap inserted in the correct polarity. I takes away the whistlely high resonance sound in a major way, I cannot get it to chrip (which i consider a bad thing) and any res setting either with TM3 or the 10K trimmer I have installed in R96 as well. Also tried a 100K since I thought the 33k was almost too much and whistling at extreme TM3 settings is back however it is reduced substantially in amplitude so it is not over powering.
Last edited by Altitude on Sun Dec 12, 2010 8:47 pm, edited 1 time in total.
User avatar
Altitude
 
Posts: 978
Joined: Wed May 11, 2005 4:17 pm

Re: Component C29 is backwards on all TB-303's... (more inside)

by guest on Sun Dec 12, 2010 8:45 pm

tim
try a higher res setting around .85 or so
for the green plot
and im betting it will be very similar
but just shifted up by +3dB

but i do agree that algebra is better than intuition
its just not as fast
if i get a moment ill write it up
guest
 
Posts: 3155
Joined: Fri Feb 17, 2006 4:35 am

Re: Component C29 is backwards on all TB-303's... (more inside)

by Tim Stinchcombe on Mon Dec 13, 2010 7:22 am

guest wrote:try a higher res setting around .85 or so
for the green plot
and im betting it will be very similar
but just shifted up by +3dB
I think I understand what you are getting at now - see attached plot (same plot order and colouring for the first three, with the new one added, and zoomed in a bit to see the detail better...). So reducing the resonance with the cap as normal, can give (roughly) the same shape curve, albeit shifted up a little because overall it has more gain (mauve curve compared to the red!). (Which amounts to what I was saying about the gains too :) )

Tim

[Edit: I think this ties in with my abandoned example of last night, now that I know what I am really looking for (wood, trees, etc...!)]
Attachments
c29_rev_vs_less_res2.gif
Zoom in of previous trace, plus 'c29 normal 0.85 res' curve added
c29_rev_vs_less_res2.gif (13.31 KiB) Viewed 2964 times
Last edited by Tim Stinchcombe on Mon Dec 13, 2010 7:27 am, edited 1 time in total.
User avatar
Tim Stinchcombe
 
Posts: 11
Joined: Wed Dec 01, 2010 5:25 pm
Location: Cheltenham, Glos UK

Re: Component C29 is backwards on all TB-303's... (more inside)

by Tim Stinchcombe on Mon Dec 13, 2010 7:24 am

Altitude wrote:Just installed Tim's suggested 33K with a switch across C29 in my x0x and it definitely behaves similarly to Brian's sanyo demo's but with a modern cap inserted in the correct polarity. I takes away the whistlely high resonance sound in a major way, I cannot get it to chrip (which i consider a bad thing) and any res setting either with TM3 or the 10K trimmer I have installed in R96 as well. Also tried a 100K since I thought the 33k was almost too much and whistling at extreme TM3 settings is back however it is reduced substantially in amplitude so it is not over powering.
There was little doubt in my mind that that would do the trick, but it is nice to have confirmation all the same!

Tim
User avatar
Tim Stinchcombe
 
Posts: 11
Joined: Wed Dec 01, 2010 5:25 pm
Location: Cheltenham, Glos UK

Re: Component C29 is backwards on all TB-303's... (more inside)

by guest on Mon Dec 13, 2010 4:15 pm

another thing i just remembered
was that its been noted that the x0x is +3db hotter
than the 303 out of the vca
perhaps this isnt the vcas fault
guest
 
Posts: 3155
Joined: Fri Feb 17, 2006 4:35 am

Re: Component C29 is backwards on all TB-303's... (more inside)

by bcbox on Tue Dec 14, 2010 2:42 pm

in your sanyo caps recordings.. the square wave has ugly edges i've never seen before on any 303 or x0xb0x
what the hell was that?!
The recording chain is noted above in my previous post. You're seeing the effects of a square wave that's passed through a (rather steep) low pass filter with very high resonance (starting to oscillate at high freq) and then high-passed two or three times..

The question is of course is , why did roland do it.
Probably a mistake in laying out the PCB, pure and simple, and it has never been detected. If hundreds of units had started showing up with that cap blown, then I dare say they would have spotted the error, but since it seems the units do still function with C29 reversed, no one would have realised that anything was wrong.
100% agree. No question just a mistake in board layout.

At the end of the day I agree with the comments that the difference does sound like a simple offset in resonance, albeit with a corresponding offset in level.
Technically speaking this would also cause differences as it would change the signal level to the VCA, and then down the road at the mixer stage.
bcbox
 
Posts: 587
Joined: Thu May 25, 2006 12:30 am

Re: Component C29 is backwards on all TB-303's... (more inside)

by aminoacid on Tue Dec 14, 2010 4:23 pm

i just reversed the cap on a box.

and i just noticed that i have less distortion peaks on accents with volume to max and no res.

usualy i have my volume knobs on 3 oclock max to secure from distortions. i hear them very accurate in my adam monitors.

there is still distortion whith max volume but only in a soft way. no hard clips!
my machines kills fascists!
aminoacid
 
Posts: 340
Joined: Tue Jun 20, 2006 4:27 am
Location: acidburg

Re: Component C29 is backwards on all TB-303's... (more inside)

by Brassteacher on Tue Dec 14, 2010 7:57 pm

guest wrote:another thing i just remembered
was that its been noted that the x0x is +3db hotter
than the 303 out of the vca
perhaps this isnt the vcas fault


As soon as I get time, I'll be able to test this. I have my "test x0x" rigged to where I can switch back and forth from a BA6110 and a BA662. I also have multiple examples of each chip so that results can be averaged. I've also installed a socket on C29 so that I can swap the cap around. I need to make raw VCO recordings first, though.
Don Taylor, a.k.a. Brassteacher
"Jack of all trades, master of none, proficient at even fewer..."
Brassteacher
 
Posts: 248
Joined: Fri Oct 08, 2010 3:49 am
Location: Kennesaw, Georgia, USA

Re: Component C29 is backwards on all TB-303's... (more inside)

by antto on Wed Dec 15, 2010 8:23 am

i already have raw recordings from my VCO, but more are welcome
btw, record an "step" signal thru your device if you can, so i can approximate the DC-filter frequency and "undo" it, this way i'll restore the waveforms from the recordings ;]
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)
User avatar
antto
 
Posts: 1361
Joined: Thu Apr 15, 2010 2:21 pm
Location: 127.0.0.1

Re: Component C29 is backwards on all TB-303's... (more inside)

by Altitude on Wed Dec 15, 2010 11:49 am

bcbox wrote:...

At the end of the day I agree with the comments that the difference does sound like a simple offset in resonance, albeit with a corresponding offset in level.
Technically speaking this would also cause differences as it would change the signal level to the VCA, and then down the road at the mixer stage.



So if I understand that statement correctly and what Guest stated earlier is that the same effect can be achieved by adjusting TM3? Or am I missing something else?
User avatar
Altitude
 
Posts: 978
Joined: Wed May 11, 2005 4:17 pm

Re: Component C29 is backwards on all TB-303's... (more inside)

by Brassteacher on Wed Dec 15, 2010 1:54 pm

antto wrote:i already have raw recordings from my VCO, but more are welcome
btw, record an "step" signal thru your device if you can, so i can approximate the DC-filter frequency and "undo" it, this way i'll restore the waveforms from the recordings ;]


Just posted some! Not sure if Audacity can do the step signal you are talking about, and I'm not sure what that is anyway 8) . Let me know what it is, and I'll see if I can figure out a way to get it done. I'm afraid I may have to dig up a newer audio interface for my Mac, it's a 6-year-old dual G5. Great machine, but the on-board audio in is a bit limited by newer standards. You may want to check the signal polarity when you download the file, I haven't seen a DAC yet, at any price, that didn't invert the signal on playback, and very few DACs allow you to flip polarity. Either it'll need to be done in software, or through and inverting hardware buffer, or just switch the black and red connections to your speakers :D .
Don Taylor, a.k.a. Brassteacher
"Jack of all trades, master of none, proficient at even fewer..."
Brassteacher
 
Posts: 248
Joined: Fri Oct 08, 2010 3:49 am
Location: Kennesaw, Georgia, USA

Re: Component C29 is backwards on all TB-303's... (more inside)

by guest on Wed Dec 15, 2010 4:35 pm

altitude
the gain adjust refers to q21 and not tm3
it looks like people have had good luck
with a 33k resistor across c29
guest
 
Posts: 3155
Joined: Fri Feb 17, 2006 4:35 am

Re: Component C29 is backwards on all TB-303's... (more inside)

by Altitude on Wed Dec 15, 2010 5:23 pm

guest wrote:altitude
the gain adjust refers to q21 and not tm3
it looks like people have had good luck
with a 33k resistor across c29


Got it. So it is a particular gain stage, not the overall amount

I actually stuck a pot with a 10 K in series to play around with what sounds best and found 33K a little to low.
User avatar
Altitude
 
Posts: 978
Joined: Wed May 11, 2005 4:17 pm