Buffered Multi

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Dude on the Street
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Buffered Multi

Post by Dude on the Street » Mon Aug 06, 2012 11:06 am

Am I right in thinking that a buffered multi is mainly useful for pitch? so that there is no drop in power between the outputs?

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rasseru
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Post by rasseru » Mon Aug 06, 2012 11:16 am

Yeah no drop in voltage.

Dude on the Street
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Post by Dude on the Street » Mon Aug 06, 2012 11:30 am

So what about using an unbuffered multi to divide up pitch cvs to send to your VCOs? I've only ever used my Doepfer passive multi but have never noticed a problem

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kimu
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Post by kimu » Mon Aug 06, 2012 12:25 pm

i think that the loss on an unbuffered multi is not so high to be a frequency divider... there are some modules out there used to create sub-oscillation from a audio signal.

starting from cv maybe only a quantizer may provide with a second cv, e.g., a 5th apart from the original one.
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sduck
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Post by sduck » Mon Aug 06, 2012 12:36 pm

Dude on the Street wrote:So what about using an unbuffered multi to divide up pitch cvs to send to your VCOs? I've only ever used my Doepfer passive multi but have never noticed a problem
Not a problem most of the time. Also depends on how picky you are about things. Start sending to 4 or more vcos at a time from one mult and you may start hearing some problems - octaves may not be quite in tune and such. Also depends on your source cv - some are more successful at driving multiple vcos than others.

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Post by Dude on the Street » Mon Aug 06, 2012 1:23 pm

I did think it may depend on the amount of VCOs alright. I've only been using two, (soon to be three) so should be fine for the moment.

Thanks for the replies guys :tu:

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synthomaniac
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Post by synthomaniac » Mon Aug 06, 2012 1:35 pm

Dude on the Street wrote:So what about using an unbuffered multi to divide up pitch cvs to send to your VCOs? I've only ever used my Doepfer passive multi but have never noticed a problem
It really depends on two factors, output and input impedance. If the output impedance of the CV source is low and the input impedance of the VCOs is high then the error is small and may not be noticeable.

Most modules that are meant to output pitch accurate CVs (MIDI/CV, quantizers etc.) have very low output impedance, 100 Ohms or less, and most VCOs will have fairly high input impedance, say 100k Ohms. The error in that instance is 100k / (100 + 100k) = 0.999 or -0.1%.

The problem comes when you drive a number of VCOs (or VCFs too for that matter) from a passive mult which puts all the input impedances in parallel; so if you have 4 VCOs being driven then their 100k Ohms combine to look like 25k Ohms. So the error becomes 25k / (100 + 25k) = 0.996 or -0.4%. Over 1 octave it's not too bad as that is just 4mV or about 5 cents low but over 8 octaves then it is 32mV low is over a third of a semitone flat.

It's still not huge assuming 100k inputs over smallish ranges, I have a Roland 100m with 2 VCOs and 2 VCFs that look like 12k when paralleled which does cause issues if not buffered.

All the above assumes that you have VCOs that can track accurately over wide CV ranges and not all do :doh:

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cv slime 800
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Post by cv slime 800 » Mon Aug 06, 2012 6:11 pm

Great post Tony, I always wonder how much voltage droop we are talking about with buffered multi or stackables and this really helps put it in perspective.

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kimu
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Post by kimu » Tue Aug 07, 2012 1:25 am

thanks tony very clear.

nice new design for your brand!
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Michaud
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Post by Michaud » Wed Aug 08, 2012 9:39 pm

Sometime, I don't really need a buff mult, but I always need it when using a slew limiter.
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sempervirent
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Post by sempervirent » Sat Aug 11, 2012 8:07 pm

I've been curious about this recently as well, since I was looking for a 4hp mult and wasn't sure if I really needed a buffered mult. I understand the difference in principle but I wasn't sure if it would actually be a problem.

I ended up getting two mults, the Doepfer A180 and the WMD Buffered Mult. I only have 4hp to spare so I'll probably end up selling one or the other depending on whether or not I can perceive a voltage drop in actual practice. From the math that synthomaniac outlined it doesn't seem like it will really be a problem in most cases.

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