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Modifying input impedance of Boss OC2 to use as piezo buffer
Modifying input impedance of Boss OC2 to use as piezo buffer
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Old 28th December 2017, 11:51 AM   #1
curlewcat is offline curlewcat
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Default Modifying input impedance of Boss OC2 to use as piezo buffer

I play a violin with a piezo pickup that likes to see 5M ohm impedance. I use a 10M ohm input DI box and don't personally find a problem with it being 4 metres down a low-cap cable. I want to avoid a fet buffer on the instrument.

Problem comes when I want to use effects. The Boss OC2 octaver is the first in the chain and has an input impedance of 1M ohm. I know many guitarists use Boss pedals as buffers before other pedals as they have a FET input whether bypassed or on.

Can I simply change the 1M ohm resistor R3 for a 5M ohm to give me a 5M ohm input impedance? or is that likely to screw with something else?

The output from my pickup is also too hot for the OC2 so I will also change the value of R1 or put a trim pot in series.
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Old 28th December 2017, 12:27 PM   #2
GeorgK is offline GeorgK  Austria
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I think this should work.
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Old 28th December 2017, 05:03 PM   #3
GeorgK is offline GeorgK  Austria
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Originally Posted by GeorgK View Post
I think this should work.
ok, not completely...
putting a trimmer in series with a 5M input inpedance will not change much, unless use a 10M trimmer. I doubt the results will be fine with this values.

If you really need input attenuation, maybe leave the 1M resistor and change R1 to maybe 1M, this will give you an attenuation of roughly 2:1 and increase Zin to about 2M.
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Old 28th December 2017, 06:11 PM   #4
curlewcat is offline curlewcat
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Cheers for that GeorgK.
If I change R1 and R3 to 2.4M each I'll achieve attenuation of 2:1 with a Zin of 4.8M. Is there a downside to this?


Quote:
Originally Posted by GeorgK View Post

If you really need input attenuation, maybe leave the 1M resistor and change R1 to maybe 1M, this will give you an attenuation of roughly 2:1 and increase Zin to about 2M.
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Old 28th December 2017, 09:55 PM   #5
GeorgK is offline GeorgK  Austria
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Well, I am somewhat sceptical about dealing with such h enourmous inpedances AND longer cable runs, but it should work out that way.
Some things that came to my mind:
unless you are sure that distortion is already occurring in the buffer stage, you could do attenuation afterwards.
And I am not sure that you won't sacrifice the advantage of yout Hi-Z input with a the considerable length of unbuffered cable. But this is just an idea.
You could also add a JFET opamp buffer, this would give you some freedom in choosing input impedance and adding attenuation if necessary.
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Old 28th December 2017, 10:53 PM   #6
curlewcat is offline curlewcat
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Yeah, I know... I did listen to all the usual advice but then I did some listening tests.

I have a Headway EDB1 with switchable Zin and 3.5m of the lowest capacitance cable available (sommer spirit LLX). I have recorded playing slow scales over the instruments full range.
At 1Mohm Zin there is noticeable LF roll off compared to 5M.
20Mohm Zin makes no discernible difference to 5M with my ears.

Swapping the sommer cable with a 6m fairly standard guitar cable of unknown capacitance I can hear the HF is attenuated. The 3.5m sommer cable sounds totally fine to me with 5Mohm Zin, infact if anything I'd like to remove a bit more HF.

Basically I'm fairly sure I want 5M Zin and to lose some signal voltage (of unknown amount due to having no scope!) Sure enough to try it if I know the method will achieve those specifics and not blow my fave pedal up!

I'm also 95% sure I don't want a buffer on the instrument as it's only small and pretty hard to work out where the battery would go. It's really not so easy as a guitar. When I am using effects I am always using the OC2 first in line so it make more sense to me than having another box.

Anyway, that life story is my thinking!

So... making those changes will achieve that aim? I'll give it a go and listen, if it works, great, if it doesn't I'll have to think where I want to compromise.

Cheers
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Old 28th December 2017, 11:09 PM   #7
kevinkr is offline kevinkr  United States
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Modifying input impedance of Boss OC2 to use as piezo buffer
You can add capacitance to reduce the output level and HF extension, if you need a bit more HF extension/higher bass cutoff you can then reduce the resistor value.
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Old 29th December 2017, 02:47 AM   #8
PRR is online now PRR  United States
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Quote:
Originally Posted by curlewcat View Post
...The output from my pickup is also too hot for the OC2...
Put a few thousand pFd across your piezo pickup. This will NOT cut highs, as it would for a resistive or inductive source. It will reduce output equally at all frequencies. It will be less sensitive to resistive loading; you may have fine bass with 1Meg loading.

Or you could remove R6 in your OC, eliminate that early gain which an older magnetic pickup may need but your piezo does not.
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Old 29th December 2017, 04:54 AM   #9
GeorgK is offline GeorgK  Austria
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Originally Posted by PRR View Post
Or you could remove R6 in your OC, eliminate that early gain which an older magnetic pickup may need but your piezo does not.
Making gain of IC1 adjustable would be a perfect way for an attenuation after the buffer.
I would prefer this over a large resistor on the input.
I rather think the OC stays clean when bypassed?
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Old 29th December 2017, 05:38 PM   #10
curlewcat is offline curlewcat
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So a capacitor across the pickups reduces output impedance and signal voltage which is the same effect as changing the values of the potential divider at the OC2 input.

So does the use of a capacitor there have an advantage over the potential divider? Do they have pro's and cons or just 2 different ways to "skin the cat" (as they say)
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