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Serenadi
Joined: Jul 03, 2007
Posts: 89
Location: Germany
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 Posted: Thu Sep 25, 2008 12:55 pm Post subject:
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Great, nice suggestion too.
Thanks for explanation, sounds logical.
On the other hand, it will be easier to add a resistor to my (already) existing board layout instead of redesigning several pads for the switch.
I have to take a look at it.  |
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BananaPlug
Joined: Jul 04, 2007
Posts: 307
Location: Philly
Audio files: 5
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| Posted: Thu Sep 25, 2008 2:12 pm Post subject:
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| Quote: | | I mostly use VTL5c1, 5c2 and 5c4, and if you are very, very lucky you can find a good pair in a batch of 10 vactrols. |
Can you suggest an optimal screening procedure that's fussy enough to be meaningful but simple enough to be practical? Seems a ton of people will be buying Vactrols for this project maybe we should group buy and sort them into matched sets or at least have a common system for testing them. |
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djthomaswhite
Joined: Nov 22, 2007
Posts: 140
Location: Orange, CA
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| Posted: Thu Sep 25, 2008 9:33 pm Post subject:
Latest Test Results! |
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Hey guys,
Just did some testing of the initial PCB run I made for protos. I added the 100K resistor from pin 12 of IC1 to ground. I change the wiring of W6 to the right connection per the original schematic. Like Seranadi said before it simply makes a *very small* difference in the brightness of the filter. On my "slowpass VTL5C4" version I think I will now need to change the value of the 47K Deep Switch resistor a bit. I will wait to test this potential *need* on my dual VTL5C3 version (waiting for jacks from Bridechamber).
Thanks to Seranadi and Chris Sugar for the quality control!!!
So, I'll add a spot for the 100K to ground on Pin 12. I'll switch the wire to match the Buchla schematic. And this sould clear the new design for Prototyping. Thanks for the help guys! If my mixer sections work on the proto I should be good. Can someone take a look at those? They are simply a summing stage with 100K resistors at the inverting input to the 1st section of the opamp with the second section serving as an inverter. I am pretty certain I got it right but second set of eyes on the circuit would help me to feel more at ease. Nothing has changed with this part of the circuit that has been posted earlier in the Zip file of 1.7 Docs. Any changes between now and release will be implemented and the public will get to purchase a Revision 2 board after all this testing.
Also, I have been meaning to mention that the MOTM Spacing of the pots is MOTM "VERTICLE" spacing. The Offset Pot is meant to be above the Resonance Pot on MOTM designs. If you want to go side-by-side you'll need to use 1.625 inches spacing instead of the MOTM horizontal standard of 1.720 inches.
Thanks!
Thomas
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Thomas
www.naturalrhythmmusic.com/diy.html |
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bugfight
Joined: Aug 02, 2007
Posts: 188
Location: Arlington, TX USA
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| Posted: Fri Sep 26, 2008 9:28 am Post subject:
Re: Latest Test Results! |
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| djthomaswhite wrote: |
... If my mixer sections work on the proto I should be good. Can someone take a look at those?...
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i'm no expert, but it looks to me like feedback resistor R31 should be connected to U2:Pin8
(similarly R25)
not a big dif though...
otherwise looks all good to me |
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fonik
Joined: Jun 07, 2006
Posts: 3950
Location: Germany
Audio files: 23
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| Posted: Fri Sep 26, 2008 11:30 pm Post subject:
Re: Latest Test Results! |
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| bugfight wrote: | | djthomaswhite wrote: |
... If my mixer sections work on the proto I should be good. Can someone take a look at those?...
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i'm no expert, but it looks to me like feedback resistor R31 should be connected to U2:Pin8
(similarly R25)
not a big dif though...
otherwise looks all good to me |
i'm no expert, too, but i can see that you are absolutely right, bugfight.
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matthias
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chrissugar
Joined: Sep 23, 2008
Posts: 34
Location: ROMANIA
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| Posted: Sat Sep 27, 2008 12:31 am Post subject:
Re: Latest Test Results! |
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| djthomaswhite wrote: | Hey guys,
Thanks to Seranadi and Chris Sugar for the quality control!!!
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You are welcome.
I had a quick look at your new documents (rev 1.7) and it looks like I found some errors.
Look at output 1 of IC1. You connect that (S1) point to the ground (S2) when the switch is closed. You missed a second 15k resistor that goes to the inverting pin.
Also R25 and R31 in the summing stage should go direct to the outputs before the 1k resistors.
And a typo, R13 is 4.7 Mohm not nano.
Hey Thomas, redraw the schematic and I will check it very carefully.
chrissugar |
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fonik
Joined: Jun 07, 2006
Posts: 3950
Location: Germany
Audio files: 23
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| Posted: Sat Sep 27, 2008 1:18 am Post subject:
Re: Latest Test Results! |
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| chrissugar wrote: | I had a quick look at your new documents (rev 1.7) and it looks like I found some errors.
Look at output 1 of IC1. You connect that (S1) point to the ground (S2) when the switch is closed. You missed a second 15k resistor that goes to the inverting pin. |
yep.
| Quote: | | Also R25 and R31 in the summing stage should go direct to the outputs before the 1k resistors. |
that does not matter at all. in an ideal world it would alter the unity gain of the inverting opamp stage, in real world with resistor values of 100k/1k it does not do anything. the calculated gain would be (100k+1K)/100k=1.01, this is covered by the parts tolerances anyways.
with these resistor values we could leave it as is.
| Quote: | | And a typo, R13 is 4.7 Mohm not nano. |
it is always good to have several people to look at it...
thomas, did you mind posting a consolidated version of the schematic?
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matthias
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djthomaswhite
Joined: Nov 22, 2007
Posts: 140
Location: Orange, CA
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chrissugar
Joined: Sep 23, 2008
Posts: 34
Location: ROMANIA
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| Posted: Sat Sep 27, 2008 1:47 am Post subject:
Re: Latest Test Results! |
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| fonik wrote: | | Quote: | | Also R25 and R31 in the summing stage should go direct to the outputs before the 1k resistors. |
that does not matter at all. in an ideal world it would alter the unity gain of the inverting opamp stage, in real world with resistor values of 100k/1k it does not do anything. the calculated gain would be (100k+1K)/100k=1.01, this is covered by the parts tolerances anyways.
with these resistor values we could leave it as is.
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Hi Matthias
The reason why this kind of connection is good to avoid is not because the non significant gain change.
In most opamp applications that connect to the real world, that 1k resistor has many functions. One is to add a resistance at the output of the opamp if the output (a jack connector) is accidentally shorted to ground. The opamp will not die.
Another function (and this is what I had in my mind) is isolation. If you connect a patch cable to the opamp out, it will work as an antenna and all the RF noise collected will manifest in the audio as distortion. That 1k resistor will isolate the output and the feedback connection from the RF noise collected by the patch cable.
Also there is another function for that resistor, isolation from the capacitive load of the cable.
chrissugar |
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fonik
Joined: Jun 07, 2006
Posts: 3950
Location: Germany
Audio files: 23
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| Posted: Sat Sep 27, 2008 1:57 am Post subject:
Re: Latest Test Results! |
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| chrissugar wrote: | | Another function (and this is what I had in my mind) is isolation. If you connect a patch cable to the opamp out, it will work as an antenna and all the RF noise collected will manifest in the audio as distortion. That 1k resistor will isolate the output and the feedback connection from the RF noise collected by the patch cable. |
ah, good info/explanation. 
would not have thought of this...
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matthias
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Serenadi
Joined: Jul 03, 2007
Posts: 89
Location: Germany
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| Posted: Sat Sep 27, 2008 2:01 am Post subject:
Re: Latest Test Results! |
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| chrissugar wrote: | Another function (and this is what I had in my mind) is isolation. If you connect a patch cable to the opamp out, it will work as an antenna and all the RF noise collected will manifest in the audio as distortion. That 1k resistor will isolate the output and the feedback connection from the RF noise collected by the patch cable.
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You're absolutely right.
But here, the outputs of the summing stages are connected to further internal stages, so shorting or other effects may not occur.
I would say, just omit R26 and R32 and connect R25 and R31 directly to the OpAmps outputs, as said above.
Another word to the CV-summing stage.
You really don't need such a stage, because IC1 (pins 5/6/7) is already the summing stage for CV (offset pot and CV-In).
To add further CV-Ins, all you have to do is to add further resistors (like R21-23) to the summing node (pin 6) of IC1.
Don't worry about the filter components C5/R4. These have only an effect of increasing the amount of the CV-input very slightly at high (regarding to modulation) frequencies, say, 4 kHz or so.
I described this effect in the other LPG thread.
See http://electro-music.com/radio/forum/post-202112.html#202112
But If you like, you can also add this filter network to every additional CV-Input of course.
IMHO it is not worth the effort.
Instead, I find it useful to increase the sensitivity of one additional CV-input for all frequencies by changing its input resistor from 100k to 47k. It depends a bit on the modulation signals and their levels you use.
(I like heavy modulating sometimes )
The audio summing stage is IMHO useful.
At least I think it may be useful for people who use Synthesizer.com power connections, if you use their 6pin MTA100 connector standard instead of a 4pin MTA100. Just my 2 cents. |
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fonik
Joined: Jun 07, 2006
Posts: 3950
Location: Germany
Audio files: 23
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| Posted: Sat Sep 27, 2008 2:15 am Post subject:
Re: Latest Test Results! |
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| Serenadi wrote: | But here, the outputs of the summing stages are connected to further internal stages, so shorting or other effects may not occur.
I would say, just omit R26 and R32 and connect R25 and R31 directly to the OpAmps outputs, as said above.
Another word to the CV-summing stage.
You really don't need such a stage, because IC1 (pins 5/6/7) is already the summing stage for CV (offset pot and CV-In).
To add further CV-Ins, all you have to do is to add further resistors (like R21-23) to the summing node (pin 6) of IC1. |
i thought the idea was to provide two independend mixer sections on the PCB, not matter what one would do with them. so i would be okay with it, just like it is... i did not take closer look to the layout, though. IIRC there are pads for the mixer outputs.
[edit: spelling]
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matthias
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Last edited by fonik on Sat Sep 27, 2008 4:02 am; edited 1 time in total |
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Serenadi
Joined: Jul 03, 2007
Posts: 89
Location: Germany
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| Posted: Sat Sep 27, 2008 3:16 am Post subject:
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I see.
Maybe it would be nice to have additional access to the inverted CV signal (pin 1, U2 over 1k resistor).
I've done this in my layout because it's always nice to choose between original and inverted CV. Could be done with an attenuverting amount pot or via a switch. |
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chrissugar
Joined: Sep 23, 2008
Posts: 34
Location: ROMANIA
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| Posted: Sat Sep 27, 2008 9:13 am Post subject:
Re: Latest Test Results! |
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| fonik wrote: |
i thought the idea was to provide two independend mixer sections on the PCB, not matter what one would do with them. |
Exactly my thought. The summing section is not hardwired, so I thought it will have the option to be connected to the outer world (the reason why I sugested the R25, R31 correction). If not, the whole two opamp summing circuit is redundant because you can do it simple with the original schematic.
chrissugar |
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bugfight
Joined: Aug 02, 2007
Posts: 188
Location: Arlington, TX USA
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| Posted: Sat Sep 27, 2008 9:16 am Post subject:
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another thing i notice is that IC1:Pin12 should be grounded when not used.
hehe, maybe enough cooks are looking now that i shouldn't try to add to the broth...
but i do like the idea of having pads at the inverted outputs, since these summers can be multipurpose |
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chrissugar
Joined: Sep 23, 2008
Posts: 34
Location: ROMANIA
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| Posted: Sat Sep 27, 2008 9:36 am Post subject:
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| bugfight wrote: | another thing i notice is that IC1:Pin12 should be grounded when not used.
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That was already mentioned.
The right solution would be to move the whole "resonance" opamp circuit on the other side of the switch. Pin 12 would always be connected to the output, and when the switch is in NONE or GATE position C8 would not be connected to anything like in the original schematic.
chrissugar |
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Serenadi
Joined: Jul 03, 2007
Posts: 89
Location: Germany
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| Posted: Sat Sep 27, 2008 11:56 pm Post subject:
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| chrissugar wrote: | | The right solution would be to move the whole "resonance" opamp circuit on the other side of the switch. |
I tried this yesterday and it works very well.
Not only the sensitivity for hum is eliminated, also the offset plops while switching the modes aren't a problem furthermore.
So, there is no need of the 100k resistor at pin12 I suggested.
Another advantage is that there is no more influence of the resonance circuit while in "gate" mode (otherwise there was a very slight filter effect if the offset pot wasn't cranked up).
So, there's no need for a switch "original LPG -> resonance mod", too.
Thank you, chrissugar. |
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djthomaswhite
Joined: Nov 22, 2007
Posts: 140
Location: Orange, CA
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| Posted: Sun Sep 28, 2008 2:08 pm Post subject:
Sunday News Edition!!! |
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Thanks for the continued feedback guys:
I moved the Resonance section after the switch as mentioned. In this process I discovered I had not remembered to move the trace on the OCB per the previously mentioned connection to pin 8 of the output stage instead of pin 9.
I removed the 100K resistor from Pin 12 of U1 as a result of moving the resonance section.
I added an extra pad to the output of both mixer sections right by the IC in the event one would want to grab the signal from there.
I am leaving R26 and R32 as is. You can use jumpers there if you really don't like them in the circuit.
I will also leave C5 and R4. They can simply be left out if needed. The same can be said for the mod mentioned about the CV input change to 47K on R5 (100K) to increase the range of CV being patched in.
I can add pads on the inverting inputs of the mixer sections for those who want them. Please specify where exactly you'd like these to connect to and I'll put them there.
Final change was to add the synthesizers.com MTA-100 power connector instead of the 4-pin version. Thanks for the reminder on this. I don't have synth.com stuff and was trying to satisfy those who have mentioned MTA-100 for other projects before. I should have checked... thanks for the reminder.
I could use a second set of eyes in verifying the traces of the schematic versus the board layout. In the attached image, the red color is the copper on the top of the board and the green is the bottom copper. The black are component and silkscreen outlines. We are getting closer to the best possible circuit yes?
Thanks for the continued help guys!
Thomas
PS. I want to credit Chris Sugar and Serenadi on the PCB for helping with the final design. What are your last names guys?
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bugfight
Joined: Aug 02, 2007
Posts: 188
Location: Arlington, TX USA
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| Posted: Sun Sep 28, 2008 3:47 pm Post subject:
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i think that JP1B to JP2B should always be connected
with the choice of J1A to JP2A for "Resonance Mod" or
JP1A to JP1B for NO "Resonance Mod"
this avoids the floating input at IC1:Pin12
for the inverted outs, you would add a 1k resistor on each
connected to the opamp output on one side and a pad
on the other
also, to be clear, i wasn't suggesting removing output
resistors R26 and R32... not trying to debate it, especially
since a difference that makes no difference is no difference.
but i would like to know if there really is some benefit to
connecting the feedback resistor this way...
as far as i can tell the difference is a tiny decrease in headroom
and a tiny decrease in output impedence
(what would the output impedence be btw? Ro in parallel to Rf?)
anyways, big thanks for the hard work... |
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djthomaswhite
Joined: Nov 22, 2007
Posts: 140
Location: Orange, CA
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| Posted: Sun Sep 28, 2008 4:21 pm Post subject:
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| bugfight wrote: | i think that JP1B to JP2B should always be connected
with the choice of J1A to JP2A for "Resonance Mod" or
JP1A to JP1B for NO "Resonance Mod"
this avoids the floating input at IC1:Pin12 |
Since I am no designer, and more of a modder I don't quite understand this. My apologies here. But, with both jumpers in tact, the 4th section of the TL084 can be completely removed from the circuit instead of having pin 12 connected if the resonance mod is not used. Am I missing something here? Thanks for teaching me here,
Thomas
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Thomas
www.naturalrhythmmusic.com/diy.html |
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bugfight
Joined: Aug 02, 2007
Posts: 188
Location: Arlington, TX USA
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| Posted: Sun Sep 28, 2008 5:56 pm Post subject:
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| djthomaswhite wrote: | | ...My apologies here. But, with both jumpers in tact, the 4th section of the TL084 can be completely removed from the circuit instead of having pin 12 connected if the resonance mod is not used. Am I missing something here? Thanks for teaching me here... |
actually it was chrissugar's idea i think.
hehe, i'm no teacher and learning this stuff myself but i'll take a stab...
the input of the opamp presents essentially no load and there should be
no difference to the rest of the circuit with it connected or not.
but if not, it should be grounded.
i don't know for sure if this particular opamp has a problem with floating
inputs, but it's a general practice to ground unused inputs because floating
inputs can cause issues with the other circuits on the same chip... |
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Serenadi
Joined: Jul 03, 2007
Posts: 89
Location: Germany
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| Posted: Sun Sep 28, 2008 11:03 pm Post subject:
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Took a first (and quick) look at your pcb layout:
To be mechanically compatible with Synthesizer.COM you should choose a 6-pin MTA 100 connector.
(even though pin 6 is not connected)
See http://www.synthesizers.com/technical.html
(scroll down to "Internal Connectors")
For the inverted outputs of the summing stages connect a resistor 1k to pin 1 and another one to pin 14 of U2.
The other side of these resistors go to pads.
I will take a closer look this evening.
Btw, my name is Bernd Schilling. |
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djthomaswhite
Joined: Nov 22, 2007
Posts: 140
Location: Orange, CA
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fonik
Joined: Jun 07, 2006
Posts: 3950
Location: Germany
Audio files: 23
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| Posted: Mon Sep 29, 2008 12:16 am Post subject:
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| djthomaswhite wrote: | | Now here is a question... I have the large bottom copper filled plane there. It is not tied to ground. Instead there is a separate ground trace that runs around two edges of the PCB to cover all the necessary ground spots. The question is should the filled plane be tied to ground?If so, can I eliminate the 2-edge ground trace and simply have all ground points run to the filled plane? Or, should I leave the plane unconnected like a piece of shielding that won't serve as an antennae to the ground connections? |
IMO you should leave it as is. a ground plane is good if it has it's own layer (that would increase the cost of the PCB).
if you connected the filled plane to GND you might create closed circles that probably will pic up noise AFAIK.
concerning the layout. it might be good to get rid of the vias. i am not sure about the quality of vias at expressPCB. they probably are a source of errors.
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matthias
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djthomaswhite
Joined: Nov 22, 2007
Posts: 140
Location: Orange, CA
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| Posted: Mon Sep 29, 2008 12:24 am Post subject:
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Yeah, I was thinking leaving it unconnected would be the better idea.
The via's at Express PCB are high quality. You will see this on the prototype I am sending you. The whole PCB would need a significant redesign in order to have no via's. I hope the boards we end up testing don't have any issues that I haven't witnessed on any of the proto PCB's I have ordered in the past.
Thomas
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www.naturalrhythmmusic.com/diy.html |
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