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

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

Post by altitude »

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 9:47 pm, edited 1 time in total.

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

Post by guest »

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

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

Post by tim stinchcombe »

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
Zoom in of previous trace, plus 'c29 normal 0.85 res' curve added
Zoom in of previous trace, plus 'c29 normal 0.85 res' curve added
c29_rev_vs_less_res2.gif (13.31 KiB) Viewed 3860 times
Last edited by tim stinchcombe on Mon Dec 13, 2010 8:27 am, edited 1 time in total.

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

Post by tim stinchcombe »

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

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

Post by guest »

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

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

Post by bcbox »

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.

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

Post by aminoacid »

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!

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

Post by Brassteacher »

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.

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

Post by antto »

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 ;]

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

Post by altitude »

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?

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

Post by Brassteacher »

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 .

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

Post by guest »

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

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

Post by altitude »

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.

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

Post by antto »

Brassteacher wrote:
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 .
when recording with a PC, your audio card usually has a DC-filter (usually a 1st order HP filter at some low-ish frequency)
so the recorded waveforms from the x0x will not look right, especially the lower freqs
but there is a way to fix this, a "reverse 1st order HP filter" .. i used this with my own recordings at it works pretty good!
the only thing is: i need to know the exact frequency of your DC-filter, in my card it's around 16Hz
the "step signal" thing:
you could use some square wave for this.. even the x0x DinSync clock or RUN signal would work
record this and it'll be very clear what the frequency of your DC-filter is, because these signals have "flat" portions.. if you use the clock - be sure to use some very slow tempo ;]

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

Post by Brassteacher »

antto wrote: when recording with a PC, your audio card usually has a DC-filter (usually a 1st order HP filter at some low-ish frequency)
so the recorded waveforms from the x0x will not look right, especially the lower freqs
but there is a way to fix this, a "reverse 1st order HP filter" .. i used this with my own recordings at it works pretty good!
the only thing is: i need to know the exact frequency of your DC-filter, in my card it's around 16Hz
the "step signal" thing:
you could use some square wave for this.. even the x0x DinSync clock or RUN signal would work
record this and it'll be very clear what the frequency of your DC-filter is, because these signals have "flat" portions.. if you use the clock - be sure to use some very slow tempo ;]
Ah, Macs don't have audio cards, it's built onto the motherboard. That's why I used the 2µf cap, I wasn't going to take any chances when dealing with a motherboard. I was tempted to open the case and trace the input jack to the first resistor and cap I could find, but the processor heatsinks were in the way, and I didn't feel like shutting the machine down. I'll run the LFO of my SH-09 through the scope to see how clean its square wave is, and try to get a recording straight off the LFO test point. Luckily I already have it opened up for calibration. As an aside, if the TB-303 calibration instructions are as poorly translated and poorly written as the calibration instructions for the SH-09, there are a LOT of screwed-up TB-303s out there!

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