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April 2006
Posts: 2491
Location: Copenhagen Denmark | NORTH :
I hope that ye do n`t mind for me to answer FLY and NUMBY first , as it DOES pertain to yer queree of raising the speaker from it`s Baffle.
FLY and NUMBY :
Let `s start from the beginning , a Piezo-Tweeter is Capacitive , so what is happening , when a Sinosoidal-Wave ( Sine-Wave ) passes thru it.
Imagine how that Sine-Wave looks like , at the positive half-period...it Rises in a slight arch , rolls-over forming a smooth round top , and slopes down in an arch , at the negative half-period it does the same , just in opposite way.
However , when that Sine-W. passes thru a capacitor , it will ...rise in an arch , forms the FIRST Half of it`s Top , then FALLS Down Abruptly !!...causing a " Spike " ,..it will do so at both half periods, it is exactly that ( Spike ), what human hearing perceives as Very Annoying , certainly Not musical.
That is How a Capacitor works.
We can n`t " fill-up " the missing part of the Sine-W. , BUT....we can SHAVE some of that spike off , by coupling a small RESISTOR placed in PARALLEL , by ...connecting one end of Resistor to one end of Capacitor , other end of Resistor to other end of Capacitor.
This is called a " Bleeder-Resistor " , actually ,it causes a small " leak " at the Capacitor , preventing the Cap. to be filled to the max .
Imagine :
A water tank connected to a powerful pump , that fills it with the speed of lightning , when full , a valve opens , and that water gushes out with lightning-speed ,
when ,
a hole gets applied close to the top of the tank , and gets filled-up , when water reaches the hole , it will seep-out thru that hole , while that tank gets filled up further , albeit at a SLOWER Rate !!...and some water has allready passed thru` that hole.
And that is what happens when placing that small resistor in Parallel ( across ) with that Cap.
PARALLEL on Piezo-Tweeter , would be connecting one end of Resistor to PLUS on Piezo ... other end of Resistor to MINUS on Piezo.
Since very little power passes thru resistor ( most power passes thru Cap. ) a quart watt Resistor will be suitable for most applications .
When in doubt , drop me a line.
A Resistor placed in SERIE ( in front ) with a Cap. , will not influence a Cap. ( Piezo-Tweeter ) to behave differently , However , that would change the Impedance of a Circuit , in this case , it would change the Impedance of that Piezo-Tw.
As a " Rule-of-Thumb " , I usually base the Value of said Resistor at ....
100 x ( onehundred times ) nominal Impedance...
when Piezo-Tw. Imp. = 8 Ohms , x 100 = Resistor at 800 ( EightHundred ) Ohms.
Closest standard value would be 1 Meg.Ohm ( OneThousand Ohms )
Shaved-off top is sharper when INCREASING resistor Impedance ...
2 Meg.Ohm ( TwoThousand Ohms ) will give a
" sharper " peak. ,
and
DECREASING Imp. ,for ex. ...
500 Ohms ( fivehundred ) will give a " Flatter " top.
Feel free to X-periment , decide for yerself.
FLY :
Electronic components allways consist-of ...
Voltage
Ampere
Resistance
Capacitance
Induction
The circuit of a given loudspeaker ( drawn as a schematic , there are Caps. , Inductors , Resistors ) is rather complex , especially so , since they are Frequecy-dependant , all those components change their value according to different frequencies , but that`s okay , just use a Stable Amp.
However , to use a capacitor in serie with a Piezo is not advisable , why would ya , I mean , 2 Caps. in serie = 2 spikes in succession !!
A Resistor in serie with a Piezo would only change the cross-over roll-off.
That NORTH was informed of the possibility to use a Tweeter with his Speaker-Cab.s , is due to his Celestion`s rapid roll-off at 4 KHz. ,since most Piezo`s roll-off at about there , his speakers could benefit using a Piezo , in conjunction with the afore mentioned measures.
But...mostly ,usually only when an Acoustic is being employed !!
Since most Piezo`s are horn drivers , reaching a sensitivity of around 105 db , they will need to be matched according to the big speaker`s sensitivity.
Using a Mid-range in conjunction with a bass-driver , the same care needs to be taken , I.E..
1 ) Matching sensitivity ( speakers in between )
2) bleeder resistor on Cap. ( in Cross-Over Circuit )
Yes , bleeder resistors on Serially placed Caps. ( those in signal-path ) in Cross-Overs are recommended .
NORTH:
An Enclosed Cab. will act as a Pnuematic-spring , causing strong damping , ...Raising the speaker from its Baffle will cause a small leak , allowing the speaker-membrane to move freely , hence , more Clarity , yet , the Cab. itself , will add Warmth .
What foam is that ye have ?
foam like in hospital mattress ?
like that used in furniture ?
Lead-foam ? ( very dense..very costly )
Can n`t think of anything else...
( maybe some ramblings about comparing a speaker-cab. to a Capacitor , and the small leak comparing to a bleeder-resistor... naah , ..only when interest is expressed )
Vic |
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Joined:
February 2004
Posts: 2487
| I do love this site. There seems to be a wide variety of people who are experts in their fields. And then there are some who are just "outstanding in them" :cool:
Anyway yes I understand the resoning behind raising the speakers I suppose it is the same sort of principle as using a tuned port. It lets the speaker cone move slightly easier and keeps it from getting bogged down in it's own positive and negative pressure within the cab.
Question..........I will assume raising these speakers 1/4" will replace using ports? If so this might help me trim the cab size slightly.
I will not be using these cabs for acoustic at all, I have another acoustic amp for that so I am trying to leave the tweeters out of this, I have had lots of top end but never the low driving punch from my electrics that I like.
The foam is from those memory foam matress top pads. Very spongey, very dense and should do well to run around the corners and sides. I plan to leave the back plywood of these cabs bare. I think that this will help with sending the sound forward alomst like an echo and the speakers will play off that surface nicely.
Thanks for these posts Vic. Your a very knowledgable man when it comes to this stuff. I hope the rest of these guy's all picked up something they can use as well. |
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October 2005
Posts: 4044
Location: Utah | Vic, thanks for the info on bleeder resistors. It will be a big help. |
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January 2006
Posts: 1118
Location: NW Washington State | Vic, do you have any favorite model/brand of tweeter, and do you like any that are not piezos? My impression is that the CTS (ex Motorola) piezos are worth the extra money. CTS tweeters.
A small parallel bleeder resistor to tune the piezo is an interesting idea, I'll have to try it.
A series resistor (say 20 ohms @ 10 watts) is sometimes added for a different reason. It will keep the amplifier from seeing a purely capacitive load and breaking into oscillation at ultrasonic frequencies. Maybe this is more important with a solid-state amp with extended frequency response.
-Steve W. |
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Joined:
December 2006
Posts: 6268
Location: Florida Central Gulf Coast | Originally posted by FlicKreno aka Solid Top:
Closest standard value would be 1 Meg.Ohm ( OneThousand Ohms ) Vic, excellent analogy! Water is a great 'substitute' for electronic theory. I used that analogy quite a bit when I was an instructor (way back when!).
Just to clarify your value above... Shouldn't that be 1 K ohm, 800 ohms ~ 1000 ohms? One meg would be 1,000,000...
Great topic and info! |
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April 2006
Posts: 2491
Location: Copenhagen Denmark | Thank ya all for the roses , and ofcourse Iffy had to keep the thorns on the stem..
Vic
..ouch.. |
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Joined:
April 2006
Posts: 2491
Location: Copenhagen Denmark | Iffy...Iffy...Iffy..
INDEED !!
Onethousand Ohms = 1 KOhm
OneMillion Ohms = 1 MegOhm
I bow my head in shame , and humbly hope to come out of the corner soon and take the donkey hat off..
Vic
..be nice to the guy who has just received a pair of OLD monitors , loaded with FANE`s .. ( yummy ) |
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Joined:
December 2006
Posts: 6268
Location: Florida Central Gulf Coast | Vic,
You had written "(One Thousand Ohms)", so I knew the "Meg' was a brain fart. ;)
The last part of my post way for those that didn't know.
Besides, you're WAY beyond me in 'tronics!  |
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February 2004
Posts: 2487
| Funny, have not heard the name Fane in many years. We had 18" Fane's in a pair of Down Firing Sub Cabs we built back in 79. Those monster speakers actually took most of the bottles off the top shelf one crazy night at the bar. I think where ever they are they still work. You couldn't kill them no matter how many crown's you had behind them. |
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Joined:
April 2006
Posts: 2491
Location: Copenhagen Denmark | NORTH :
Raising the speaker from its baffle will NOT have the same effect as a bass-duct , its a different principle , and yet...
There are many formula`s calculating bass-ducts..
..none of them is " ultimately " correct , wittness that they all end their rethory with :
" Tuning the ducts ( vents ) for optimum results by experimentation "..
Let us examine yer speaker-cabinet:
The CELESTION G12T-75 has a main-resonance at app. 85 Hz. , tuning ( enhancing ) to that freq. will cause " boominess " , drowning yer Sound considerabely , Yes , that`s right , that particular freq. WILL be Louder than the rest of yer sound .
Furthermore , when ye want to keep the back-plate " naked " , do realise that it will reflect sound right thru that Very thin speaker-membrane , a resonance that ye migt be comfortable with..or..?..
We can bicker back and forth , and I`d like nothing better , but , lemme ask ya..
Are ye familiar with ; ( I can n`t remember title nor performer )
.." I can feel , the earth , move under my feet " ...
If that`s what yer after , than perhaps ye could do the following :
Build the Cab. that YOU like to make ,
and then stuff sound-damping material at will , try out Spacers of different Thicknesses ,
and mount the Lowest speaker flush to the Baffle , and ,
the top-speaker resting on spacers , this will :
1 ) Cause a phase-shift. ( phase-shift WILL cause a " Thin , Detailed sound " ).
2 ) Flush-mounted speaker induces Cab. to come into "play " ( cabinet-rumble ) .
3 ) Top-speaker ( freely-moving ) takes care of Clarity.
The Foam that ya describe I`m not familiar with , is it like ;
Furniture foam , used for sofa`s and chairs ?..or...?
At any rate , try lining the walls ( panels , back-plate ) ye may want avoid lining the baffle at the inside.
The above will , at one point induce :
Bass injection
Fleeting Highs
Much like the Ovation model 1617
which has also warm articulate Lows.
Tilted back mid-range.
Fleeting Clear Highs.
That way , Rumble is the name of the game ( Bass-Injection ) ,
Calm Intent Voicing ( Mellow-Mids ) ,
HERE it COMES !! , overwhelming Highs.
( I do realise , that this IS a Guitar-Site , and the Topic being part of an Instrument )
No tweeters , No Bass-reflex..
That`s about it , if there`s anything , drop me a line..
Vic
FLY is building a Valve-amp. at 5 watts rms ,...?..is he building a Valve-driven HeadPhone amp. at 50 MilliWatts...?..with bass-ducts situated at the back of those head-phone shells..?
COOL !! |
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April 2006
Posts: 2491
Location: Copenhagen Denmark | NUMBY :
I am not in favor of Piezo tweeters , however , overhere them MOTOROLA Piezo`s carry a good reputation , the KSN 1005 is widely used in f.ex. guitar amps. ( I have a handful in my parts-bin ) , and even in low-cost P.A systems .
My Fav. Tweeter... choosing between ALTEC , JBL , FANE , B&C , I guess that would be the JBL 2404 Bi-Radial UHF transducer , my All Time Fav. would be off course the ALTEC 511 and 811 SECTORAL Horns driven by the 802-8G Driver ( equipped with TANGERINE Phase-plug ), unfortunately ALTEC is now defunct ( scour 2hnd market ) .
I am a PROUD Owner of a pair of SECTORAL 811A `s , albeit with non-ALTEC drivers , even so , dispersion is PHENOMENAL , I hear `em on a daily basis ( coupled to 2 x ALTEC 416-B Bass-drivers ).
I am in the process of rebuilding , but can n`t make up my mind , main idea is to build a look-a-like of ALTEC`s model 19 , but with GOODMANS 18HP ( 18" Bass-Drivers ) , on the other hand , in a corner , a pair of ALTEC 421-8LF are beckoning , and next to those , a pair of 421-8H II , what to do ..?..
The Piezo marks an Imp. of 8 Ohms , so the amp. would see a load , I`ve not tried the 20 Ohm R in serie , never deemed it necessary , allways had stable amp.s at my disposal , but , next time I use an amp. with a skimpy transformer I`ll give it a try .
Vic
Thanks. |
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October 2005
Posts: 4044
Location: Utah | Originally posted by FlicKreno aka Solid Top:
FLY is building a Valve-amp. at 5 watts rms ,...?..is he building a Valve-driven HeadPhone amp. at 50 MilliWatts...?..with bass-ducts situated at the back of those head-phone shells..?
COOL !! With any luck (regulations, taxes, and ordinances permitting) it will be a line of thoroughly modern tube amps for the modern guitarist. Low wattage for sane sound levels, portable for us old guys with bad backs, simple operation for the technically challenged, a few features for modern performing venues, and GREAT looks. At an affordable price, and built in the USA.
The 50 mW headphone amp? Yeah, it's in there too! But you'll have to provide your own bass-ducted headphones. |
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Speakers Wattage Matching Amp
