2. amps-preamps
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Is my amp the problem?

I recently decided to bi-amp my speakers so I purchased a crown xls 1002. (225w 8 ohm). I am driving a 10 woofer. It sounds horrible. I can't seem to get it to put out much power despite its rating even when I make all the right adjustments. Its a class D amp. Is that my problem? It just doesn't seem to have any Ba**s. Lol. Advice is appreciated.
jimbones
almarg's avatar
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almarg
9,670 posts
 
Thanks for the update. Although the input impedance of the Crown XLS 1002 is not specified (as Auxinput had indicated earlier), its similarly rated predecessor model, the XLS 1000, was specified as having input impedances of 10K single-ended and 20K balanced. Given also that pro-oriented equipment very often seems to have relatively low input impedances I would think it to be a good bet that the input impedance of the XLS 1002 is the same or similar. 10K happens to be the same, of course, as the specified input impedance of the miniDSP 2x4 HD.

Also, the 20K balanced input impedance probably means 10K per leg (i.e., a 10K load being applied to each of the two signals in the balanced signal pair). And the output stage of the LS100 will see that 10K load in parallel with the 15K to 23K input impedance (at various frequencies) of the KWA 100SE regardless of whether the connection to either amp is via XLR or RCA, since as I indicated earlier the LS100’s XLR output is driven with a single-ended signal.

15K in parallel with 10K is 6K. 23K in parallel with 10K is 6.97K. Both values are of course far below what Dan recommends.

The input impedance of the Pioneer SF-700 Electronic Crossover is spec’d at 100K. Much better! Although it appears that the SF-700 dates back to the early 1970s, so the possibility that it may be in less than top condition is something to keep in mind.

Good luck. Regards,
-- Al

jimbones OP
259 posts
 
Al, the pioneer is just for testing purposes. If I decide to go for an active XO I will shop for a better one. BTW, I just checked, the input impedance of the parasound is 33k!!!!! Thats great! and the sensitivity is 1.1 for full rated output. This is going to get interesting. I believe I am headed in a better direction. BTW, if you have any suggestions on XO's please suggest. Thanks again.
auxinput
2,920 posts
 

The Bryston 10B (that I mentioned above) is likely your best choice for active analog crossover to keep the resolution of your preamp. It has fully discrete analog circuits (probably running in Class A mode) and an excellent power supply. Input impedance is 15k. There’s one on audiogon for $995. They typically sell for well over a thousand.

There’s a Krell KBX for $1360 on ebay. It will probably sound more relaxed than the Bryston, but would probably need to be recapped. Also, the front dials do not show crossover frequency steps, so it will be impossible to determine actual crossover frequency without some sort of mic or line measurement device.

If you want to go cheaper, you could look at a new Ashly XR-1001, but you’d have to use some special cables (either XLR or 1/4" TRS). It is a pro audio crossover, but it is still fully analog (as opposed to digital/dsp like most of the pro audio crossovers nowadays). Input impedance is 20k ohms balanced (this is probably 10k ohms single ended as I can see a 10k ohm resistor on the input of each XLR leg). You can pick it up for $250 at many musician websites (sweetwater, parts express, etc.).

http://s298.photobucket.com/user/maryna1109/media/music/DSCN8468.jpg.html?t=1254515907

The Ashly is not going to be anywhere near the resolution/quality of the Bryston. If I did the Ashly, I would be making significant mods to it. First, Ashly likes to supply power at +/-18V, which is on the high side for normal op amp circuits, but they use normal 7818/7918 regulators. I would be doing the following:

- replace the 7818/7918 monolithic regulators with Sparkos Labs discrete regulators and drop the voltage down to +/-15V.

- Put in the largest power supply capacitors I could fit in the board/chassis (currently they are only 2 x 1000uf).

- Swap out all the op amps with AD797 on dual-channel DIP8 adapters.

- Solder 47uf Muse + 0.01uf metallized poly directly to the power pins on each op amp.

- Replace all the op amp negative feedback caps with film/foil or silver mica.

This is probably $450 worth of components (not including labor).


I’ve seen dbx, Ashly and other analog pro audio crossovers.

bdp24's avatar
bdp24
9,846 posts
 
Another good x/o is the First Watt B4. All discrete (no opamps), zero feedback. More flexible than the Bryston, with far more available frequencies, and the highly-desirable 4th order-24dB/octave slope, which the Bryston does not provide. $1250 retail, often on sale at Reno Hi-Fi for less.
auxinput
2,920 posts
 

Yep, saw that one.  There's a B4 on audiogon/reno-hi-fi for $995.  I didn't mention it because it actually has a lower input impedance of 10k ohms.  Also, I'm highly suspect about the power supply -- it looks like a basic laptop power supply, so you may have to buy/build a high-quality high-current linear DC power supply to get any sort of performance out of it.    I also read that is uses bare JFET transistors with zero feedback instead of full op-amp circuits.  The JFETS are biased into Class A, so this crossover will definitely draw some power supply current.  I can't comment on the performance of this specific device, but I have seen instances where a bare JFET will sound nice but not have enough brute force to provide enough gain for bass/midbass punch, especially without negative feedback.  JFETs in preamp circuits can sound nice - they tend to have a nice rich/lush type of sonic signature, but they are not as crisp/clear as bipolar devices (this is largely a personal preference on sound signature here).

The Bryston power supply provides +/-24V (for a total of 48 volts), so the analog circuits definitely have a lot of current to play with for good bass/midbass power.  And I confirmed that the circuits are all Class A discrete and use bipolar devices instead of FET based.

There's always the Pass XVR-1, which is probably the best of the best, but at $5k+, I suspect it is out of range here.

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