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Roman 207 V3 attenuating HF in proportion to channel level?
MUFF WIGGLER Forum Index -> Buchla, EMS & Serge  
Author Roman 207 V3 attenuating HF in proportion to channel level?
Peake
EDIT: There is a mistake in the V3 BOM. Instead of six SMD resistors there are six SMD 0.1uF capacitors. In that position, between wiper to ground, they act as a variable lowpass filter. Thanks Dave Brown


Built a V3 of the 207r for a customer and noticed the high frequency content diminishes on each channel as the level is reduced from maximum. My customer also noticed it. I don't remember if earlier versions did this, might anyone know if this is expected behaviour?

The 292 Combo Mode behaved purposely in that regard, combining level and timbral control so a signal appears to recede into the physical distance as level is decreased.

Can't find a copy of the original Buchla site with individual module descriptions anymore :(

Edit: Most of it apparently occurs between 9.0 and 10.0, full volume.
mritenburg
https://web.archive.org/web/20040812085230fw_/http://www.buchla.com/hi storical/b200/207-mixerpreamp.html
tarandfeathers
It's not a deliberate feature but given the design (i.e. completely passive up to the summing amps with no pre or post fade/pan buffer amplifiers) it's not surprising either.
Peake
I installed the six surface-mount components, wonder if that might also matter. Dave Brown's page only details versions 1 and 2 and makes no mention of spectral shift.
jimfowler
I’ve got both (my v3 is not done yet) but if you wanna send me a file to use as a reference I’d be happy to try and confirm/deny what you’re experiencing as “normal behavior”.
Peake
It's not here, it's at my customer's place. Blow some white noise through each channel and see if there are any timbral changes as you reduce from maximum to say 9 then to 8 in level. Appreciated.
Synesthesia
i have a V3 Mike, i'll check it out.
Peake
Thank you! I installed the SMD components which give the sliders a log curve but according to Dave Brown's page also change the impedance, unsure if that would be causing what is going on.
b4
Any updates on the behavior that the high frequency content diminishes when reducing the volume? Can anyone demo it?
jimfowler
I'm happy to run some audio through my V3 mixers that do NOT have the caps installed.
jimfowler
Here's a quick video of my 258 being attenuated by the 207. Again, no SMD caps/resistors installed. Looks pretty linear to me w/ no HF roll off.

https://vimeo.com/350420521
Peake
Apologies, I updated my posts on this topic elsewhere but missed this one. I've put the discoveries at the top of the OP.

(BOM mistake, caps swapped for SMD resistors, and if you install caps it does form a variable LPF). Dave Brown is the man. He also says on his build page it's probably not a good idea to install the SMD resistors either.
b4
jimfowler wrote:
Here's a quick video of my 258 being attenuated by the 207. Again, no SMD caps/resistors installed. Looks pretty linear to me w/ no HF roll off.

https://vimeo.com/350420521

Thank you, really helpful.
b4
Peake wrote:
Apologies, I updated my posts on this topic elsewhere but missed this one. I've put the discoveries at the top of the OP.

(BOM mistake, caps swapped for SMD resistors, and if you install caps it does form a variable LPF). Dave Brown is the man. He also says on his build page it's probably not a good idea to install the SMD resistors either.

That was the information I was looking for. Thank you for the clarification.
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