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glitched
Joined: Mar 25, 2006
Posts: 80
Location: phila., pa USA
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 Posted: Sun Feb 20, 2011 10:19 am Post subject:
How important are these component values? |
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Hello:
I have a couple Coolaudio V2164 vca chips (functionally similar to the SSM part) and aim to build a quad-VCA or mixer. Here's the data sheet: http://www.coolaudio.com/docs/datasheet/V2164MD_DATASHEET.pdf
The implementation seems pretty simple, according to the datasheet, but I was wondering this: How important are the values for the components on the current inputs (the 500ohm resistor and 560pF cap)?
What's important here? Is it the ratio of those two components? Will more standard values (that is, values that I have, on-hand), like 1k and 1nF work? I have the same question about the 7.5k "mode" resistor.
EDIT: I have found the answer on a different site and the 500ohm/560pF cap form a lowpass filter. That makes sense. Using this site, http://sim.okawa-denshi.jp/en/CRtool.php, I plugged in values and found component values that will work in their place.
Still, the question remains about the 7.5k resistor. Does it have to be this exact value?
Thank you. |
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peng
Joined: Feb 23, 2006
Posts: 111
Location: Chicago
Audio files: 2
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| Posted: Sun Feb 27, 2011 11:34 pm Post subject:
Re: How important are these component values? |
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| glitched wrote: |
Still, the question remains about the 7.5k resistor. Does it have to be this exact value?
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Don't use the 7.5K the data sheet suggests. This influences CV feedthrough and should be much larger. There is a graph on the data sheet that shows this. I'd use a resistor in the 500K - 1M range. |
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daverj
Joined: Jul 07, 2009
Posts: 28
Location: USA
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| Posted: Tue Mar 01, 2011 8:59 am Post subject:
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You can leave that resistor off all together.
The data sheet says "open for Class AB (7.5k for class A)". What that means is leave it off for class AB operation (pretty low distortion, low noise) or put the 7.5K in for class A operation (very low distortion, more noise).
The distortion in class AB mode is still extremely low, and the noise is lower than in class A operation, so you are probably better off leaving that resistor off and using the chip in class AB mode. Just let the pin float. |
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peng
Joined: Feb 23, 2006
Posts: 111
Location: Chicago
Audio files: 2
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| Posted: Tue Mar 01, 2011 2:51 pm Post subject:
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| daverj wrote: | | Just let the pin float. |
Definitely agree that if the choice is no resistor or a 7.5K resistor I'd leave it off.
For me the concern is less about class A or AB.
It's about the CV feedthrough.
See figure 13 "Control Feedthrough vs. Rbias".
I haven't done any definitive tests so it's speculation on my part but the data sheet seems to bear this out. |
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cbm
Joined: Oct 25, 2005
Posts: 381
Location: San Francisco
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| Posted: Tue Mar 01, 2011 10:35 pm Post subject:
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For normal synthesizer VCA use, I think that class AB is the way to go.
_________________
Chris Muir
http://www.eardrill.com <– My jobby (more than a hobby, less than a job) |
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glitched
Joined: Mar 25, 2006
Posts: 80
Location: phila., pa USA
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| Posted: Tue Mar 01, 2011 11:31 pm Post subject:
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Thanks for the replies. I read about the trade-off of distortion versus noise and I don't think I'll bother with Class A operation. Low distortion isn't critical in this application.
I'll have to see if I notice any CV bleed.
Anyway, I'll be tackling this after I finish another project and I'll report back if I have any interesting findings. |
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peng
Joined: Feb 23, 2006
Posts: 111
Location: Chicago
Audio files: 2
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| Posted: Wed Mar 02, 2011 3:18 am Post subject:
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| glitched wrote: |
I'll have to see if I notice any CV bleed.
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I just clicked the link in the OP. The CA data is slightly different than the AD version. Control Feedthrough vs. Rbias is fig. 11 on the CA data sheet.
The circuit your building from there has 0.1uF caps on the CV inputs (the AD data had 1uF  ). I'm guessing this is slewing any CV input so feedthrough probably won't be noticeable in either class. |
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peng
Joined: Feb 23, 2006
Posts: 111
Location: Chicago
Audio files: 2
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| Posted: Wed Mar 02, 2011 3:20 am Post subject:
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Oh yeah, there is a long discussion about the 2164 going on at SDIY. Not related to class operation but power supply protection and other stuff.
Just a heads up. |
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synchroton
Joined: Mar 02, 2010
Posts: 20
Location: Milwaukee, WI USA
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| Posted: Wed Mar 02, 2011 5:48 am Post subject:
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| Specifically, regarding these chips tendency to fry if you leave the negative rail unpowered. |
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glitched
Joined: Mar 25, 2006
Posts: 80
Location: phila., pa USA
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| Posted: Wed Mar 02, 2011 9:47 am Post subject:
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| peng wrote: | Oh yeah, there is a long discussion about the 2164 going on at SDIY. Not related to class operation but power supply protection and other stuff.
Just a heads up. |
Peng, are you speaking of this site, specifically: http://www.sdiy.org/philgallo/mgbvca.html ?
Yes, I've been studying this schematic and I think I'll use it for the VCA implementation (I'll probably just go straight from the data sheet for the mixer; it looks simple enough).
IF you have any other tips, let me know. Thanks! |
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peng
Joined: Feb 23, 2006
Posts: 111
Location: Chicago
Audio files: 2
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glitched
Joined: Mar 25, 2006
Posts: 80
Location: phila., pa USA
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| Posted: Wed Mar 02, 2011 3:45 pm Post subject:
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Of course!! How silly of me! This is a treasure trove of information.
Thanks!
Time to pick up some more 2164s! |
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