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Quantec QRS/XL make sense today?
Old 3rd May 2018
  #31
Lives for gear
Thanks so much, Sean, this is a great bit of guidance on the 224xl, will definitely try this! (and love your plugs, use them constantly!)

Your insights on the quantec and sp2016 are also really helpful. So there's a single delay line bouncing around many times in these units? Can you explain some of this, or describe where someone can learn this? I've scoured tons of threads online, but they are either very basic, or far too technical, hard to figure out best way to leap from what my ear hears to why this is, how to connect the theory with the sound, without mastering multi-variable calculus.

Any advice on that?
Old 3rd May 2018
  #32
ValhallaDSP
 
seancostello's Avatar
 

Quote:
Originally Posted by fromthepuggle View Post
Thanks so much, Sean, this is a great bit of guidance on the 224xl, will definitely try this! (and love your plugs, use them constantly!)
My pleasure! I spent some quality time with the 224XL a few years back, and that Rich Plate algorithm still haunts me.

Quote:
Your insights on the quantec and sp2016 are also really helpful. So there's a single delay line bouncing around many times in these units?
Nope. Lots of delay lines. Usually several dozen or so. The Quantec and SP2016 both do things in parallel, while the Lexicons tend to use series configurations.

Quote:
Can you explain some of this, or describe where someone can learn this? I've scoured tons of threads online, but they are either very basic, or far too technical, hard to figure out best way to leap from what my ear hears to why this is, how to connect the theory with the sound, without mastering multi-variable calculus.

Any advice on that?
I wrote a presentation on this subject a few years back:

https://valhalladsp.com/wp-content/u...esentation.pdf

Read this, and let me know what sorts of questions you have. I'm giving a talk on this subject at a local university in a few days, so answering questions would be a good warmup for me!
Old 3rd May 2018
  #33
Lives for gear
Really, what university? university prof is my day job, funds my gear obsession. luckily, I am not professor of reverb studies, though, or I would do quite poorly on my own exams!
Old 3rd May 2018
  #34
Lives for gear
just read the presentation, great intro! also just read your manual to Valhalla Room, as I was told that was a great summary, and it is.

I am still a bit confused as to allpass filters, I see these mentioned in a lot of discussions of reverbs. They seem like feedback/feedforward loops that occur often in snapshots of sorts at various points in the imagined 'reverberant space.' Is that about right? I'm confused as to why they would allow all frequencies through, if at differing times. What controls the amount of feedback to feedbforward, and how do these impact the signal?
Old 5th May 2018
  #35
ValhallaDSP
 
seancostello's Avatar
 

Quote:
Originally Posted by fromthepuggle View Post
I am still a bit confused as to allpass filters, I see these mentioned in a lot of discussions of reverbs. They seem like feedback/feedforward loops that occur often in snapshots of sorts at various points in the imagined 'reverberant space.' Is that about right? I'm confused as to why they would allow all frequencies through, if at differing times. What controls the amount of feedback to feedforward, and how do these impact the signal?
Allpass filters are called "allpass" because their frequency response is flat. No notches or peaks in the frequency response, no loss. All the frequencies that come in, will end up coming out...eventually.

The feedforward coefficient of an allpass delay is always set to the negative of the feedback coefficient. The feedback coefficient value sets the ring time of the allpass. If you set the feedback coefficient really high (like, in the 0.95 to 0.999 range) the allpass will ring a LONG time, but the delayed output will be really quiet, and you will mostly hear dry signal. This is because the feedforward coefficient also has a really high value.

Allpass delays are cool, because you can cascade a ton of them, and the output will have a flat frequency response. Try doing the same thing with comb filters (i.e. standard feedback delays), and it will be amazingly metallic sounding.

Allpass delays can also sound metallic, though. Even though the frequency response is flat, this tends to be due to a single initial echo value in the output of the allpass. Once you get past that initial weird echo, the rest of the echoes look and sound identical to a comb filter. So you can't just toss them into a reverb - you need to carefully consider the delay lengths and feedback/feedforward gains.

Allpass delays are stable when embedded within longer feedback loops. So you can put an allpass delay or two or twenty inside of a larger comb filter feedback system. This is due to that "flat" frequency response. Things can still sound metallic if not set properly, but it will be stable. Avoiding that metallic sound is one of the tricky parts of reverb design.
Old 6th May 2018
  #36
Lives for gear
 
Casey's Avatar
 

Ah with dread I will pass along my thoughts. I say dread because allpass filters are simply a crutch used to do to two useful things. The first has already been nicely described. They can add echo density without using many CPU cycles. Yay! This is a good thing. The second thing is that they alter the delay between frequencies. Or in the literature “spectral smearing”. This is all good and useful stuff and is part of the sound of digital Reverb.

Now the dread. They are often used in the recirculating portion of the Reverb. Ouch! The difficulty here is that when used in the inner recirculating loop two really bad things happen.

The first is that there is no control which limits the growth of echo density. It just goes to infinity as if we just don’t care. This is why most reverb tails end in a white noise Shhhhhhh. I hate that. A great sounding barn never does that, despite many wrong minded papers that suggest that the Shhhh tail is normal. Not to my ears.

The second is that the spectral smearing is the same each time around the loop, which compounds longer and shorter delays per frequency in a very regular pattern. This is not normal. Nothing in nature does this. It sounds bad.

Blesser’s patent is very cool in that it adjusts the feedback and feed forward strengths in a way that the regular frequency smearing is tamped down each time through the loop. This is still a hack though as the initial allpass filter response is no longer allpass, no longer flat. It just doesn’t sound good.

The best course IMO is to avoid allpass filters in the recirculation loop. Use them outside of the loop instead. One time through with no buildup sounds fine with moderate feedback and feed forward gains.



-Casey
Old 22nd July 2019
  #37
Lives for gear
 
massimo's Avatar
 

Welzenbach??

Anyone got news about Sigfried Welzenbach?
The QRS/XL he sent back to me (see earlier posts in this thread), with warranty and a substantial bill, only worked two hours... I reported this to him, and he prompted me to send it back to him, which I did immediately. That was May 2018.

In the next couple of months, when I was politely asking about my machine (under warranty), he managed to make promises like "I'll ship it next Monday" for three times. That never happened.

I haven't heard back from him since September 2018. He is not responding to my emails. Never. I became anxious about his health at some point but I have heard, from some Swiss and German (Gearslut) gentlemen I had got in touch with, he is actually fine. Basically he has sequestered my Quantec.

I am writing this to warn fellow slutz. Sad to say, based on my own experience Dr. Welzenbach is no more the reliable Quantec tech he apparently used to be. And this is to put it mildly...
Old 4 weeks ago
  #38
Here for the gear
Quote:
Originally Posted by massimo View Post
Anyone got news about Sigfried Welzenbach?
The QRS/XL he sent back to me (see earlier posts in this thread), with warranty and a substantial bill, only worked two hours... I reported this to him, and he prompted me to send it back to him, which I did immediately. That was May 2018.

In the next couple of months, when I was politely asking about my machine (under warranty), he managed to make promises like "I'll ship it next Monday" for three times. That never happened.

I haven't heard back from him since September 2018. He is not responding to my emails. Never. I became anxious about his health at some point but I have heard, from some Swiss and German (Gearslut) gentlemen I had got in touch with, he is actually fine. Basically he has sequestered my Quantec.

I am writing this to warn fellow slutz. Sad to say, based on my own experience Dr. Welzenbach is no more the reliable Quantec tech he apparently used to be. And this is to put it mildly...
Really sorry to hear that you have had this problem with Mr Welzenbach. He was always prompt dealing with my unit, though I'm not 100% happy with the work, considering the cost. There are other people offering servicing on Quantec and Publison (!) gear now. One extremely thorough engineer being Jonathan Prager (Studio Piscine à Tokyo) on Facebook. His work is stunning and he's capable of amazing fault finding and repair as well as exquisite restoration too. He worked on the (sadly) late Philippe Zdar's outboard for example.
TTFN,
Ben
Old 4 weeks ago
  #39
Lives for gear
 
massimo's Avatar
 

@ bigbluewave
May I please ask when did you last hear from SW?
I would be very happy to send my QRS-XL to Jonathan Prager for repair...if only SW were so kind as to send it back to me: not only is he in fact refusing to fix a unit which is under (his) warranty, he has sequestered it! Totally unacceptable.
Old 4 weeks ago
  #40
Here for the gear
Massimo,

I'm afraid it was some time back now, around January 2017 was the last communication. I was considering sending the unit back, but decided not to in the end as I wasn't happy to pay for yet more repairs to repairs to services.
I do hope you manage to get yours back, very sorry to hear of your situation.

TTFN,
Ben
Old 4 weeks ago
  #41
Lives for gear
 
massimo's Avatar
 

Thank you very much Ben
Old 1 week ago
  #42
Lives for gear
 
massimo's Avatar
 

Has anyone here been able to get in touch with Welzenbach recently?
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