Prototype 604 Crossover - Chicago 604 Owners?

[Seadweller]

Greetings!

Well, after a year now, I'm still working to salvage my GPA 604 project, and as many of you may remember, I have been struggling with excessive midrange output that made the system literally unlistenable. I sold an excellent, high-end SS power amplifier and tube pre-amplifier (which I now regret), and moved to a tube integrated (Cary SLI-80) to see if that might help with my struggle, and while the bass of the system improved, the excessive midrange remained.

I began the project with the stock GPA XO?s, and after communicating with Stephaen Harrel, decided to try a 4th order XO that he designed for his own 604 project, which is similar to mine. The 4th order XO is very nice indeed, but as Stephaen indicated, it is a very ?warm? sounding XO. Some like it warm, but I like it hot! Disheartened, I had gotten to the point that I was about to sell the system and give up hope, until??..

The kind gentleman that built the 4th order XO for me (which is a hand-made, point-to-point wired piece of art) has been working on an XO of his own, and he sent me a pair of his prototypes recently, which I just installed in my 604 project. A UREI 809 was used to model the prototype, and I have to say, the initial results are nothing short of remarkable. While the mids are still slightly forward, it is a huge improvement over the other designs, and I have yet to experiment with the two controls on each XO. More encouraging is that the mid-bass and high end remain intact, which allows the liveliness of the driver to come through. I?ve always felt the magic in this driver came from its dynamics and detail, which gives a realistic presentation to the music, and that remains in spades!!

In order to fine tune the design, it will need to be measured with a GPA 604 driver, which he does not currently have access to. Is there anyone in the Chicago area that would be interested in collaborating on this project? If the crossover is this good based on a UREI 809 driver, I can?t imagine how good it might be if it were designed around a GPA 604 driver. I don?t want to overstate this, but this XO could be a dream come true for those that are considering, are in the process of building, or have already built a 604 based system.



Michael

[selmerdave]

I commend your efforts, but I really think you're nuts trying to figure this out with no measurements (of what you're hearing). A simple $20 SPL meter and some test tones made up on your computer would tell you quite a bit.

While designing the crossover with the correct driver will help with issues in the crossover region, it's not going to say anything about how the driver behaves in your cabinets. Your problem may just as likely be with the bass output in the cabinets, which will not change with another crossover.

Dave

[bfish]

...Your problem may just as likely be with the bass output in the cabinets, which will not change with another crossover.

That's what I've been saying all along. Can't tell from the pic if it's a box or just a baffle, but either way it isn't big enough to support decent LF from a 604.

If you invert the grin in that smiley there, you'll have a simple plot of a system that could be described 2 ways; forward midrange, or weak LF. Glass half empty/glass half full. Sure, you could build an XO to squish the mids down, at the expense of dynamics and efficiency. Or, you could put the driver in a BR cab of appropriate size and let it sound like it should (and does for everyone else with a std cab and XO).

[Seadweller]

Thanks guys....

My next step is to get an SPL meter and use my Stereophile test CD, which I assume will be sufficient, for general measurements.

Will the test CD be sufficient, or should I generate test tones via another method?

Are there specific areas of the room that should be tested, or is testing at the listening position preferred?

The cabinet is a mass-loaded-transmission-line, with around 10 cubic feet of internal volume and a vent firing down towards the floor. The system is certainly not bass shy by any means.

Thanks again! Constructive criticism and advice is MOST appreciated!

[bfish]

If the test CD tells you the various tone frequencies, and there's enough of them, it could suffice to draw a basic plot. Or, you can download some tones or generate them with free software, and burn your own test CD. For basic tests, tones at 1/3 octave intervals will get you started.

In-room tests for LF can be sketchy at best, so try several locations for the <200Hz tests and average the results. Still not accurate, but ~close enough for this purpose.

What are the dims of your cab and port? My eyes are old, but I ain't seeing 10 cu ft.

[selmerdave]

The system is certainly not bass shy by any means.

I could believe it, the point is that frequency response is relative. Any speakers will produce bass at most frequencies, and with enough power a decent amount of it. The question is at those levels how loud are the mids relative to the lows? How loud are the highs relative to the mids and lows? Again, I'm not saying the issue is necessarily cabinet or whatever, but with issues as objectionable (and unusual) as you are describing measurements would go a long way to explaining what you are hearing and possibly pointing to the source of the issue. Without knowing it could be anything, and guessing that it is this aspect of that crossover or whatever is really like throwing darts in the dark.

I would think the Stereophile test cd would get you started, I don't know what's on it but I'm sure there are some test tones or sweeps of some kind, but if you needed more obviously you could burn a cd with some specific tones.

Dave

[Seadweller]

Thanks gentlemen....

Please understand that when I began this project, I knew absolutely nothing about speaker design, XO design, impedance curves, cabinet/driver interaction, etc. I was a complete novice that had done hours and hours of research, but had no practical experience. This was meant to be not only a quest for audio nirvana, but a learning experience as well. The more I learn, the more I realize how brilliant some loudspeaker designers are!!! So, if I sound somewhat ignorant, I am!

Regarding the cabinets, they only look small in the pictures, believe me. When the cabinet maker sent me pictures of the cabinets to show his progress, I thought they were quite reasonably sized. It wasn't until I got them home that I realized (and was reminded by my wife) how big they actually are.

On the base, they stand 55" tall. The INTERNAL dimensions are 23.125" W, 18.4375 D, and 42.5" H. They have a CSA of 426.3672. The port is 6" round, located in the bottom of the cabinet. The overall volume of the cabinet is reduced by cross bracing and insulation, giving it a volume of approximately 10 cubic feet.

Out of curiosity, how do I make my own test tone CD?

Thanks again!

[604man]

Seadweller, how much dampening material do you have in your cabs ??
you need it to reduce internal reflections midrange frequencies......GC