John Curl's Blowtorch preamplifier part II - Page 1955

john curl · 23rd December 2011, 12:16 AM

It is best to have a modest amp for tweeters, to keep them from burning out.

Chris Hornbeck · 23rd December 2011, 03:44 AM

Just thinking out loud, I wonder if one of your existing amps could be rewired to reduce power supply voltages and allow higher idling current, maybe approaching class A. Heck, they're already a John Curl design, how much better do they need to be?

Thanks,
Chris

john curl · 23rd December 2011, 04:21 AM

I do have a couple of 100W/channel Class A power amps in my warehouse. Never opened the boxes. Thanks for reminding me. Actually, I was wondering more in general, rather than specifically. I don't have a specific design in mind.

ThorstenL · 23rd December 2011, 05:36 AM

Hi,

Quote:

Originally Posted by Chris Hornbeck

For comparable drivers' sensitivity how many peak dB down from midrange would you consider the music you'll typically listen to? I wouldn't figure you for a headbanger, so there'll be "some". 3dB? 6dB?

http://www.waset.org/journals/waset/v41/v41-21.pdf

From personal experience I find 6dB less maximum SPL are acceptable. However, as the Amplifier has to cover only a fraction of the spectrum, we can reduce power more than that.

If you had (say) a 100W Amplifier putting out pink noise and you used an active crossover to remove everything below 2KHz you would only need around 30Watt, but with 6dB less SPL needed it would be 7.5Watt.

Plus Tweeters are often more efficient than Woofers, so it may be possible to drop power levels even further.

HOWEVER, some music may contain peaks to nominal 0dB even at high frequencies, so it would be prudent to use an amplifier of around halve the power of the woofer section, biased for Class A operation up to a few Watt.

With enough heatsinking and CCS on the output an LM3875 Chipamp can do surprisingly well in this application.

Feed it a stabilised +/-25V and you get up to 40W for peaks into the typical (6 Ohm Impedance) tweeter, which should make sure no clipping is encountered. Add a 0.7A current source (LM317 & resistor will do) to the output and you get around 2.83V or the nominal "first watt" in SE Class A.

For the midrange section you can use pretty much the same Amp, bridged Pairs of LM3886 (one pair per nominal 8 ohm Woofer) run on +/-30V without Class A biasing can complete the woofer section for a nice 3-Way home solution that is easy to implement.

Personally I'd use Pass B1 style buffers for the crossover ahead of these chipamps. The result would likely be not bad at all if the driver selection and crossover design are competent.

Of course, we can improve the Midrange and Treble performance by using current drive and use some form of motional feedback on the Woofer in a 3-Way system where the results could be quite exceptional.

Seas has 5" or 6.5" Coaxials that would be very suited to the Job of Midrange/Treble (and which last time I looked had low enough Qm to be very usable with current drive)... Woofers can be found.

Ciao T

john curl · 23rd December 2011, 01:40 PM

Thanks for your input Thorsten. However, I will stick to discrete designs for ultimate fidelity, unless the design is truly revolutionary.

simon7000 · 23rd December 2011, 03:26 PM

T,

When woofers are designed to produce flat frequency response curves that requires reducing the efficiency. Woofers that show pistonic response are often more efficient than tweeters.

So in home use tweeters will see much less energy than woofers.

When I started doing big joints even the acoustical consultants seemed to believe that. Turns out there is 20 db or more loss at 8,000 hertz than at 150 hertz. The tweeters being horn loaded are more directional than the lower frequency devices so that appears to improve the output level. However to get the best results at long throws 200 meters or more the high frequency drivers require more power than the woofers. The midrange drivers are often horn loaded and really get to loaf. So at an American football stadium I can have 60,000 watts on the woofers, 8,000 watts midrange and 80,000 watts driving the 150 or so compression drivers!

Now the secret to using compression drivers at high power is that you can compress air almost infinitely but you have a very well defined limit on the rarefaction side! So by deliberately adding distortion you can decrease the compression time and increase the rarefaction time of the cycle. Now when you consider the distortion results of doing this realize the same air losses apply. So if I have 300% distortion at 8,000 hertz. The third harmonic would be reduced to .3% at the listeners position, just by the air loss!

The same actually holds true for a tweeter only home use amplifier. Higher order distortion products such as the 11th would have a hard time making it out of the tweeter and get some additional attenuation from the air loss! So designing a tweeter only amplifier is a bit different than just a low power HF only version of a basic amplifier. As well noted here crossover distortion would be a super - ultra - mondo - major issue. So it actually might not be unreasonable to use an (Shudder, groan, exhale) IC power amplifier with an output biased to class A! The THD just might not be as important an issue.

Of course lets talk bandwidth and layout, those would be much more important.

ThorstenL · 23rd December 2011, 03:50 PM

Ed,

Quote:

Originally Posted by simon7000

Turns out there is 20 db or more loss at 8,000 hertz than at 150 hertz. The tweeters being horn loaded are more directional than the lower frequency devices so that appears to improve the output level. However to get the best results at long throws 200 meters or more the high frequency drivers require more power than the woofers.

Well, in concert systems (my stuff) we usually dealt with such extremely long throw situations with deploying delayed towers downrange, much less headaches. This may not be possible in certain sports venues etc. for sports events, but that is a quite esotheric are of sound and somehow I feel you may not be deploying class A Amp's on these midranges and tweeters...

Quote:

Originally Posted by simon7000

Now the secret to using compression drivers at high power is that you can compress air almost infinitely but you have a very well defined limit on the rarefaction side! So by deliberately adding distortion you can decrease the compression time and increase the rarefaction time of the cycle.

Yes, Neumann designed a similar into LP cutting lathes in the 1960's and it was very widely used since. The funny bit is that this distortion cancellation system on LP only works if you have a spherical playback stylus, use anything else and you actually get added, not subtracted distortion...

I proposed a similar system to apply to domestic and studio speakers (where the problem is generally at LF) in the 1990's on the "Bass" List to an overwhelming silence...

Ciao T

scott wurcer · 23rd December 2011, 05:35 PM

Quote:

Originally Posted by ThorstenL

Yes, Neumann designed a similar into LP cutting lathes in the 1960's and it was very widely used since. The funny bit is that this distortion cancellation system on LP only works if you have a spherical playback stylus, use anything else and you actually get added, not subtracted distortion...

Ciao T

Is there any reference available actually showing this work? I would think traceing pre-distortion could be computed for any stylus geometry, but I would also think traceing distortion goes down at low frequency somewhat for any geometry. So then what is the point of line contact stylii if the records were cut for spherical ones?

My experience is that tones above a few kHz have so much distortion that it would be a challenge to see a difference.

PMA · 23rd December 2011, 05:43 PM