another AVM20 question

[Cincy Dad]

After reading a post in another section from Michael Mohrmann, I have a concern about the bass management of the AVM20. I have already installed 2 Clark Synthesis transducers in the floor of my theater room. I will also have 7.1 sound. I am considering Paradigm speakers:
Studio 100s - mains
Studio/CC - center channel
Studio/ADP - side surrounds
Studio 20 - rear surrounds
Servo-15 - subwoofer

However, due to cost issues, I may have to get less expensive speakers, i.e., 60s for the mains and less expensive sub. My understanding is that the lower frequencies can be sent to the sub. Thus, the difference is sound (for movies) would not be noticable between the Studio 100s and the 60s (or even 40s).

My question is - Does the AVM20 have the capability to do what I expect it to do? Will it handle LFE being routed to two Clark Synthesis transducers as well as a sub? I am not clear about what it means to the sound when you have bass overlap or loss of bass due to improperly summing the bass signals.

If the AVM20 (I am also considering the PVA7 amp) will not do what I expect it to do, what pre/pro and amp (with similar features and in the same price range) should I consider?

My wants include THX Ultra 2 certification and 7.1 channel sound.

I would appreciate any help.

[Michael Mohrmann]

Hi Cincy,

Sorry about my other post, but I wanted you to be aware of how the AVM-20 works currently before committing to it. The separate crossovers "feature" is a bit misleading on the AVM-20. There are a few processors that do separate crossovers "properly", but most are expensive, like the Lexicon MC-12/12B (and the just released MC-8/8B) and the Theta Casablanca II. I believe that the Outlaw 950 will perform low-pass bass summing, but I don't believe it is THX Ultra 2.

Now for some good news! If you are willing to use the same crossover for all speakers set to "small" (no problem for speakers set to "large"), then the AVM-20 performs as well as any other processor or receiver that has a global crossover setting (whether fixed or user-selectable). And as long as that crossover is sufficiently high, like 70Hz or higher, then there is no real issue with the loss of the LFE (.1) channel.

I am assuming with your transducers that they will be receiving the same signal as your sub. If so, I don't see a problem. Do the transducers have their own crossovers built in to low-pass the bass even further (just curious)?

I will make a post later describing the issue of the separate crossovers within the AVM-20.

Michael

[Michael Mohrmann]

Cincy,

For additional information about separate crossovers and subwoofer output, check out this link at Secrets:

http://www.hometheaterhifi.com/volume_9_3/feature-article-multiple-crossovers-9-2002.html

The article itself now states at the bottom that some processors perform differently than described in the text, but most with a single fixed or user-selectable crossover work this way. The AVM-20 currently works as described in the article.

An interesting part of the article is about 4/5 of way into it, which starts with the following question:

"Why not low-pass a copy of each main channel at the various frequencies I want and sum that with the full LFE channel?"

The answer given is:

"Phase issues become monumental! Bass is often common to the front three channels and even more often common between the LFE channel and the fronts. Summing different low-passed copies of the same material would by definition result in a messy frequency response."

It doesn't mean that it can't be done, but that it is not a simple task to do this properly. Another reason to avoid low-pass summing is stated in the article:

"When mixing channels digitally, S/N is lost (approximately 6 dB when two channels are added for example), because after the summing, the combined level has to be attenuated to the original level."

If this is correct, it would seem that if you added several low-passed channels for the subwoofer output, a great deal of S/N for the bass would be lost.

Michael

[Cincy Dad]

Michael,

The weekend was filled to overflowing with activities for the kids so I am just now getting back to you. Thanks so much for your time in explaining bass management. I printed the referenced article you mentioned and have read it but need to read it again. My take away from it is that with the AVM20, I should set the same crossover for any speakers for which I set a bass crossover. Furthermore, I should set the crossover at about 80 Hz. For the system I am close to ordering, I should set the side and rear surrounds as well as the center channel at a crossover of 80 Hz. For the mains, I can include the entire frequency range.

This seems to be OK to me as I think the speakers I am considering will sound good under those circumstances. Or is my understanding still messed up?

Thanks again for your advice and comments. I want to understand this type of technical information as it will help me make a confident decision on what to purchase.

[Michael Mohrmann]

Cincy,

If you plan to have the mains set to "large" and all other crossovers (center, surrounds, sub) set to 80 Hz, then you will not have a problem.

Some will argue that the Studio 100s, while very good speakers, are not "full-range" and should also be set to "small". If you do decide to set them to "small", use the same 80 Hz crossover as the other speakers.

Whether you set the Studio 100s to "large" or "small" is up to you. If you do purchase the 100s for your mains, try the setup both ways and see which one you like best.

If you opt for a "smaller" main speaker, like the Studio 40s or 60s, my suggestion would be to set those mains to "small" and crossover to 80 Hz.

Michael