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Old 25th March 2001, 12:23 PM   #1
Kanga is offline Kanga  Australia
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I'm just about to enter the world of biamping, and I'm wondering about the suitability of a small Zen amp for powering the tweeter. The woofer will be powered by my current commercial 60W amp, and with a crossover frequency of about 3200 Hz and an efficiency of the tweeter about 3dB higher than for the woofer, I figure about a 5W amp will do the tweeter.

Has anyone tried a small Zen for powering a tweeter, or have any other ideas for a suitable amp?

Another consideration is avoiding power-on thumps, which will destroy the tweeter pronto. I am planning to put a protective capacitor in series with the tweeter with a crossover frequency around the 1000 Hz mark.

Any experiences with biamping would also be of interest. Am I wasting my time?



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Old 25th March 2001, 06:00 PM   #2
grataku is offline grataku  United States
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Kanga,
I agree with, there is a real need for a one-package solid state circuit specifically designed for powering tweeters with all the accessories necessary for an absolutely tweeter friendly operation.
Once upon a time a friend of mine gave me the original article of this project to use as the tweeter amp:
http://www.sound.au.com/project24.htm#top
It was from wireless world and I don't think mods were necessary to drive a tweeter. My brother was still using that system a couple of years ago. I don't remember having major problems with the turn-on and off operation except the op-amp power-down noise from the active filter.
In my experience the capacitor is series will not give you the kind of DC protection you want during any transient event that being power-up, down, and any malfunction. I some cases I measured DC offsets up to 1V on the other side of the cap.
I think the only way to go is with careful use of relays and with a type of DC offset protection like the one Lars (LCAudio) is using in his 0 feedback amps.
I am working the bugs off my latest bi-amped project right now and I am feeling your pain!
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Old 25th March 2001, 06:04 PM   #3
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Kanga,
The rule of thumb for bi/tri/whatever-amping has always been equal power per octave. I've seen some nonsense promulgated here, there, and everywhere by well-meaning individuals who were apparently pulling numbers out of thin air. Unless you've got some special requirements (you listen to only organ pedal notes, for instance, or this boom-boom crap that people foist on you while sitting at stop lights), then you'll find that this rule will do a splendid job.
If you're running 60W below 3.2kHz, that works out to just shy of 8.6W/oct. With 5W carrying the top, you're in the vicinity of 1.6W/oct. That's getting a little light. Your tweeter amp will run out of steam trying to keep up with your woofer amp. The result will be clipping, which at the very least sounds bad and, worst case, can fry your tweeter, even if the tweeter is rated for much more than than the 5W you're putting in. (This seems to be a much-misunderstood topic, and could be the subject of an entire thread...)
There are a lot of variables here: The kind of music you listen to, how loud you want it, how efficient your speakers are, etc. Weigh the factors and choose accordingly. Personally, I feel that everyone ought to be given a bi-amped system at birth and never look back, but it is a comparatively expensive way to go (that second amp isn't free). Make your decisions wisely, so as not to get a bad taste in your mouth from a bi-amp system gone wrong.

Grey
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Old 25th March 2001, 08:09 PM   #4
grataku is offline grataku  United States
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Grey,
I think a better measure than W/oct is dB to your ears.
In a pretty typical scenario of say a 90-91 dB eff tweeter and a 87-88 dB eff midwoofer a 60w for woofer and 5-10w for tweeter works out just fine.
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Old 25th March 2001, 09:46 PM   #5
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grataku,
I agree completely--I mentioned speaker efficiency in my last paragraph. Perhaps I should have used the word driver, instead, to make it more explicit.

Grey
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Old 25th March 2001, 10:48 PM   #6
grataku is offline grataku  United States
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Grey,
ehi no problem ! I seem to recall that you are the guy with a super cool...what is it? a quad-amped system? If that is correct...how did you worked out the power-ups and downs? I seem to recall you also have valves in your system.
What kind of amps are you using for you highs and what kind of active filter?
If that is not you: I am a complete fool and I apologize!
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Old 26th March 2001, 01:40 AM   #7
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grataku,
*Sigh* Guilty as charged, I'm afraid. I have little to offer in my defense except insanity, senility, deafness, and--according to some--a surfeit of ego.
My system as it stands today (3/25/01) subject to change according to the needs of the moment:
Tweeters: Magneplanar ribbons (removed from my old Tympani IVs), currently being powered by a Threshold S-500. Now, the old Thresholds had huge banks of electrolytics...only. The highs tended strongly towards the bleached & grainy end of things. Film cap bypasses help this enormously.
>Crossover point=5kHz 6dB/oct.<
Midranges: Bohlender-Graebener RD-75s powered by a tube (valve) amp that I designed myself--130W. N.B.: I myself am guilty of breaking the rule of thumb stated above. I'll return to this below.
>Crossover point=250Hz 18dB/oct.<
Woofers: The woofer panels from the Tympani IVs, powered by another Threshold S-500.
>Crossover point=75Hz 18dB/oct.<
Subwoofers: I hesitate to even say anything, lest the magnitude of my madness become common knowledge...12-12" drivers--6/channel--the intent being a cone-motion feedback system such as found in the old Infinity IRS or the current Genesis 1.1s. This is a project that is still incomplete, although up and running. (I have this philosophy of making things modular so that I can get going, then drop in improvements as I go along.) I will eventually have (if all goes well) a dedicated amp per driver, with dedicated feedback loop, so that each driver, regardless of manufacturing variations, is corrected, and flat, and low distortion down to infrasonic regions. Currently, the amps aren't ready yet, and I have stolen the old Hafler that comprises part of my bass guitar rig to run the subs. The Hafler has a fan, and the noise is driving me crazy, so I'm in the process of building a modified Aleph 2 (water cooled...maybe) to run either the mids or highs so that I can move that Threshold down to the subs until the dedicated amps are ready.
Assorted notes & discussion: The Hafler is 250W/channel, as are the two Thresholds. That's a lotta juice. So what's that little-bitty 130W tube amp doing in there? Well, this is one of those implied loopholes in the above discussion--I have power well in excess of my listening requirements, so I don't clip (often). Yes, ideally, I'd have more power on the RD-75s, not that I want to play all that much louder, but to handle the peaks without strain. Material such as the soundtrack from Gladiator can get a bit congested. (Great soundtrack, by the way. Oddly, I had a different perspective on the music hearing it without the visuals. Whereas something like the music from, say, Star Wars can evoke scenes from the movie(s) in my mind's eye, the Gladiator music seems for me to exist separate from the movie as a purely abstract construct. Ever so what--I like it.) Ditto for my preferred classical, late nineteenth century Russian works. For jazz, which seems to comprise most of my listening these days, there's already sufficient power. What passes for rock these days bores me nearly to tears (been there, done that--so much of current rock has already been done to death that it's instant deja vu the first time I hear a new song), so I rarely play rock any more. The crossover is a crudely put together Sallen-Key using nothing more than 2N5457 (FETs) as source followers for front end buffers, and MPSA18s (bipolar) as emitter followers for the individual sections. I intend to play with the crossover more as time permits; it's already in its third iteration. Eventually, I intend to try tubes (probably 6DJ8s or 6SN7s) as well as other variations on the silicon front.
Webpages:
http://www.magnepan.com (no, they're not at all helpful about selling those tweeters to DIY folks, you'll have to buy the entire speakers and strip the tweeters yourself)
http://www.bgcorp.com (the complete opposite of Magnepan--they fall all over themselves trying to get those drivers out the door--if you're interested, call and ask to speak to Robin Ellis--tell him Grey sent you)
http://www.partsexpress.com (I'm using the Titanic 1200 woofers. There are several other woofers in this class, but a word to the wise: Beware of long-term reliability problems with foam surrounds. [We're talking on the order of ten years, here.])
Threshold still exists, I believe, but Nelson ain't there no more, so phooey. You know where to look him up now...very supportive of DIY, of course. Oh, an unsolicited editorial comment: I'm reluctant to participate in deconstruction/reverse engineering the X-series unless and until such time as he might choose to release such material to us DIY folks. He's got to make a living so as to be able to go on churning out interesting ideas for us to play with. I'd hate to **** him off...just something to think about.
The tube amp? I'd be happy to share the design if anyone's interested, but would have to figure out a way to get a clean schematic out to you. Warning, the power supply is a beast! Not for the faint of heart.
The crossover I'd be happy to share, also, with the caveat that it's very much a work in progress.
Kanga, I very much agree with grataku that relays are the best bet.

Grey
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Old 26th March 2001, 09:13 AM   #8
djk is offline djk
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Kanga, 6 watts would be adequate for a 24dB/oct crossover at 3Khz.For a 6dB/oct slope 30 watts would be more in order.An extra blocking cap is un-needed.Just reduce the output coupling cap to 20~25F and use a high quality film type.There must be a DC leakage path on the load (speaker) side of this cap for this circuit to work right.A smaller cap will charge up so quick only a small click will be heard and a relay may not be needed.
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Old 26th March 2001, 10:58 PM   #9
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Choose another design. The Zen does not go past 16kHz.
So, it's a very bad choice for a tweeter. For a mid-range, it would probably be great.

I hate to keep touting Slone's amps, but the Opti-MOS performs perfectly for highs. It's great for lows too, but I've noticed it's more difficult for amp designs to get the highs right.

BTW, Grey, adding 4.7uF film caps to the power rails didn't change the sound of the amp at all. Those suckers are expensive! I'm convinced the large 20,000-40,000uF caps do a lot for the bass of the amp, but the small film caps don't do anything. Although, the original design from Slone already had 0.1uF caps on the power rails with the large reserve caps. So, maybe they were already doing the job.

Later,
pixie
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Old 26th March 2001, 11:51 PM   #10
rljones is offline rljones  United States
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Default Zen Freq response

pixie,

As I mentioned on another thread on this forum, the Zen amp can easily be made to go flat to 50 kHz, but only with feedback. Without feedback, it is down about 5 dB at 20 kHz. Therefore, the amp, when used with the recommended feedback per NP, with the proper voltage on the power supply and running close to 2 amps on the constant current source (using 0.33 ohm resistors), will give around 15 to 20 watts into 8 ohms.

This is plenty for tri-amping, as the energy content of real music is very low above 7 kHz. Tweeters are light weight units (low moving mass) that typically require very little current to drive. (Woofers on the other hand require tremendous amounts of current, particularly for transient response and driver control.) In fact, if I correctly recall, NP says in the original article that the modified Zen amp works very well for the upper end as well as horn speakers. I can attest to the latter as I listen to Lowther Medallions (with two self-powered Sunfire subwoofers crossed over around 80 Hz).

Robert
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