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creekree
Joined: Mar 30, 2006
Posts: 192
Location: Morgenland Neukölln
Audio files: 1
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 Posted: Mon Aug 06, 2007 11:29 pm Post subject:
MFOS single bus keyboard controller capacitor C19 question |
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hi,
i am stuffing the pcb for rays single bus keyboard controller right now.
by mere accident, looking at the photo of a populated pcb on rays site i noticed that his C19 is HUGE.
its supposed to be a 0.01uf ceramic... well i have those here, but they are much much much smaller than his.
what did mr wilson use - and why?
is there an error in the BOM (after all, there are two .01uf caps listed, C1 and C19, but they are not in the same row listed, and he usually lists groups of components...)
thanks! |
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Pehr
Joined: Aug 14, 2005
Posts: 1307
Location: Björkvik, Sweden
Audio files: 2
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| Posted: Tue Aug 07, 2007 1:29 am Post subject:
Re: MFOS single bus keyboard controller capacitor C19 question |
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| creekree wrote: | hi,
i am stuffing the pcb for rays single bus keyboard controller right now.
by mere accident, looking at the photo of a populated pcb on rays site i noticed that his C19 is HUGE.
its supposed to be a 0.01uf ceramic... well i have those here, but they are much much much smaller than his.
what did mr wilson use - and why?
is there an error in the BOM (after all, there are two .01uf caps listed, C1 and C19, but they are not in the same row listed, and he usually lists groups of components...)
thanks! |
I guess that is the sample & hold cap. It is a Polystyrene Capacitor (low leakage) that also is used in the sample & hold module. I guess it will work with a ceramic but the pitch will drop faster when holding a key down, I guess.  or after it is released... hmmm...
| Ray Wilson wrote: | | The trigger/sample pulse is fed to the gate and source of NFET Q2. Q2's drain is connected to the gate of NFET Q3. The gate of Q3 is pulled low via R14 200K to -12V. Thus the voltage on the source of Q3 only charges the sample cap (.01 uF polystyrene cap C19) when the sample pulse is high. Q2 acts like a super low leakage diode which only conducts when the sample pulse is high. The buss voltage is presented to the source of Q3 and charges C19 during the sample pulse. When Q3 shuts off the voltage on the cap has no path for current to discharge it and so it stays charged (thus the term sample and hold). OK... I'll quit saying thus. The voltage on C19 is presented to the input of the high impedance non-inverting input of U3C. Since U3C's input is such a high impedance and JFET type op amps have such low input leakage the cap stays at the sampled voltage until another sample is taken (a key is pressed or the current note is changed). The output of U3C feeds follower U3D. The outputs U3C and U3D respectively feed CV1 and CV2 outputs. The output of U3D feeds the non-inverting inputs of U3B and U3A via R8 and the glide adjust pot R11. The glide adjust pot changes the time voltage changes take to charge or discharge C20 and so a glide or portamento effect is produced on the voltage output at CV3 and CV4. |
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etaoin
Joined: Jun 30, 2005
Posts: 761
Location: Utrecht, NL
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| Posted: Tue Aug 07, 2007 12:42 pm Post subject:
Re: MFOS single bus keyboard controller capacitor C19 questi |
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| Quote: | | It is a Polystyrene Capacitor (low leakage) that also is used in the sample & hold module. I guess it will work with a ceramic but the pitch will drop faster when holding a key down, I guess. or after it is released... hmmm... |
Yes, that's the polystyrene one. And a 10nF one is about as big as it appears on the silkscreen. And yes, pitch stability will probably be terrible with a common ceramic cap.
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creekree
Joined: Mar 30, 2006
Posts: 192
Location: Morgenland Neukölln
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| Posted: Tue Aug 07, 2007 1:14 pm Post subject:
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| thanks guys! |
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etaoin
Joined: Jun 30, 2005
Posts: 761
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crashlander42
Joined: Oct 21, 2006
Posts: 292
Location: Orlando, FL (US)
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| Posted: Wed Aug 08, 2007 3:50 pm Post subject:
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That threw me to when i was building the same board. It must be a typo. On the BOM for c19 it says ".01uf ceramic capacitor", but you'll notice on the schematic for c19 it says ".01uf polystyrene or polycarbonate".
When I was building it I put a mylar one there until my polystyrene caps arrived and it worked ok. When I had a ceramic cap there it barely worked at all.
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creekree
Joined: Mar 30, 2006
Posts: 192
Location: Morgenland Neukölln
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| Posted: Thu Aug 09, 2007 5:07 pm Post subject:
Re: MFOS single bus keyboard controller capacitor C19 questi |
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| Pehr wrote: |
I guess that is the sample & hold cap. It is a Polystyrene Capacitor (low leakage) that also is used in the sample & hold module. |
hm. interesting. checking the page for the S/H module i couldnt find ANY reference to polystyrene caps at all - neither in the BOM nor in the schematics, and not in the notes.
which ones should be polystyrene?
maybe etaoin can supply another pic? pretty please??
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crashlander42
Joined: Oct 21, 2006
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creekree
Joined: Mar 30, 2006
Posts: 192
Location: Morgenland Neukölln
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| Posted: Thu Aug 09, 2007 8:05 pm Post subject:
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thanks crashlander,
but i was asking for polystyrene caps in the sample & hold module, because pehr mentioned them in his first post.
you showed the schematics or the single bus keyboard.
i admit, this is what the topic of this thread says and the reason why i started it, but i just jumped at pehrs mention of the s/h module. i just didnt want to start another thread. |
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crashlander42
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Pehr
Joined: Aug 14, 2005
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etaoin
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creekree
Joined: Mar 30, 2006
Posts: 192
Location: Morgenland Neukölln
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| Posted: Fri Aug 10, 2007 12:05 am Post subject:
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ah guys, there seems to be a misunderstanding.
i was talking about the "single chip s/h" for the soundlab.
(i double checked that there is no reference to any polystyrene caps there.)
sorry for not pointing that out.
my mind was so revolving around the SL that i completely forgot that ray has LOTS of modules on his site and not only the SL.
that probably is the reason why pehrs posting confused me.
to settle this once and for all: is there any polystyrene cap in the soundlabs s/h?
i promise to shut up afterwards  |
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etaoin
Joined: Jun 30, 2005
Posts: 761
Location: Utrecht, NL
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| Posted: Fri Aug 10, 2007 12:17 am Post subject:
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| Quote: | | is there any polystyrene cap in the soundlabs s/h? |
Indeed, there isn't in the text, but I myself would use something better than ceramic for C3, just to be sure.
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Pehr
Joined: Aug 14, 2005
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etaoin
Joined: Jun 30, 2005
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Location: Utrecht, NL
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| Posted: Fri Aug 10, 2007 1:04 am Post subject:
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| Quote: | | So Ceramic is 0k for samples 1 second or faster. If longer, the cv drop will be noticeable. |
Yes, I'm surprised Ray says that. Ceramic is usually crap for S&H. I wonder if this is yet another mix-up in the BOM, like on the modular S&H page. There's plenty of room on the PCB for C3, which suggests something larger.
But whatever the case, we've been talking about it for far longer than it takes to actually try both and see which is better...
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stefano
Joined: Nov 23, 2006
Posts: 33
Location: ITALY
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| Posted: Thu Sep 06, 2007 1:31 am Post subject:
Where to buy ? |
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Does anybody know a supplier on the web for C19 Polystyrene Capacitor ? I usually buy from www.Banzaieffects.com and they haven't..
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etaoin
Joined: Jun 30, 2005
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stefano
Joined: Nov 23, 2006
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| Posted: Thu Sep 06, 2007 1:40 am Post subject:
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Thank-you Etaoin!!
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