Roman 207 V3 attenuating HF in proportion to channel level?

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Peake
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Roman 207 V3 attenuating HF in proportion to channel level?

Post by Peake » Thu Mar 28, 2019 4:21 pm

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.
Last edited by Peake on Sat Jul 27, 2019 5:33 pm, edited 2 times in total.
I just can't help imagining that this would be better with vactrols

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Post by tarandfeathers » Thu Mar 28, 2019 6:47 pm

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.
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Peake
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Post by Peake » Thu Mar 28, 2019 7:03 pm

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.
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Post by jimfowler » Thu Mar 28, 2019 7:22 pm

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”.

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Post by Peake » Thu Mar 28, 2019 7:54 pm

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.
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Post by Synesthesia » Fri Mar 29, 2019 4:56 am

i have a V3 Mike, i'll check it out.

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Post by Peake » Fri Mar 29, 2019 12:33 pm

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.
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Post by b4 » Fri Jul 26, 2019 5:05 pm

Any updates on the behavior that the high frequency content diminishes when reducing the volume? Can anyone demo it?

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Post by jimfowler » Fri Jul 26, 2019 6:42 pm

I'm happy to run some audio through my V3 mixers that do NOT have the caps installed.

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Post by jimfowler » Fri Jul 26, 2019 6:53 pm

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.

Last edited by jimfowler on Fri Jul 26, 2019 10:36 pm, edited 1 time in total.

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Post by Peake » Fri Jul 26, 2019 9:03 pm

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.
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Post by b4 » Tue Aug 06, 2019 4:47 pm

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.

Thank you, really helpful.

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Post by b4 » Tue Aug 06, 2019 4:50 pm

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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