STONES SOUND STUDIO

DC TESTING WOOFERS DOME TWEETER & RIBBON SPEAKERS

by Russell Storey  2005

DOME and RIBBON  TWEETERS (description) Large & Small diaphragms.  more ...........>>  PHASE_IT Speaker Phase tester available from Stones Sound Studio +

Ribbon Tweeter   True ribbons (or Super Tweeters with a transformer) Have an impedance coupling transformer to couple the very low impedance of the ribbon diaphragm to the crossover circuit network .The crossover circuit sees an almost constant  ac impedance of nominally 4 or 8 ohms from 2 to 40Khz (120Khz TAD Pioneer) with high quality coupling transformer design making them excellent constant loads with linear impedance and group delay.
The True Ribbon tweeters dc phase (polarity) can be  tested by applying a low voltage dc to the primary side of the transformer by switching this dc voltage from a battery on and off to the ribbons terminals like a light switch to see the ribbon diaphragm move. The primary resistance of a ribbon transformer is usually only 0.1 ohms DC which can be difficult to read with normal digital voltmeters. Ribbon diaphragms can be damage very easily with low order 6db crossovers and also by using too low a crossover frequency usually >4kHz is a reasonable starting point for any type of 12 18 24 db crossover.
Ribbon diaphragm material has traditionally been made from  wafer thin Aluminium or Aluminium alloys with various types of corrugations stamped into the material to add strength ,rigidity and dampen any unwanted resonances in the response . Now composite Alloy diaphragms are available which achieve greater damping of unwanted harmonic resonances and increased power handling of the  diaphragm  by using larger BL magnetic fields supplied by Neo Magnets.
Ribbon diaphragm lengths can vary from 40 to 160mm The larger 160mm ribbons can go to lower crossover cut off frequency's usually>900Hz is a reasonable starting point for any type of 12 18 24 db crossover given the  higher power rating of a larger ribbon.
The larger Ribbon Tweeters of (120 to 160mm in length) are suitable for low or high power 2 way speaker crossover designs for high end high definition studio monitors PA Home theatre and music speaker systems  
Because the ribbon diaphragms mass is only 8 to 25 mgs (milligrams) for Small ribbons or  35 to 80mgs  for Large ribbons then wind generated from an over head fan or open exterior door way can bend and damage the diaphragm .I recommend some form of cover over the front of the ribbon tweeter when not in use . The detail from these tweeters can be excellent to 3 dimensional holographic  with the good application and crossover design .Ribbon tweeters are usually more expensive  than  Planar Magnetic (leaf tweeters) but are usually worth the exercise.

Leaf Tweeter  Planar Magnetic (i.e. not a "true" ribbon has no transformer ) .The diaphragm is usually made from an alloy foil laminated onto Kapton base which is etched using printed circuit board techniques to create a rectangular or circular voice coil. This laminated diaphragm sits over the top of high flux density(BL)) bar magnets which generated a very strong magnetic field behind and in front of the diaphragm. The  Planar Magnetic leaf tweeter is more robust than the true ribbon tweeter but suffers from a higher diaphgram mass and ultimately less transit response and low level detail .Planar magnetic technology has improved with  smaller neo magnets and diaphgram base materials like mylar,kapton  giving it a more stable and flexible diaphgram under higher power levels than the true Ribbon tweeter. The detail from these tweeters can be excellent and are  lese expensive to purchase than the true ribbon tweeter.

Dome Tweeter   Dome tweeters are usually made from 3 common materials pvc ,metal or fabric (poly ,silk,berillium,titanium) and have very small dome movements (excurtions)with an Xmax of (+/- 0.75 to 1.5mm).Diaphram mass depends on type of material used and with special fabrics like doped and treated  the mass of the moving diaphragm  is usually in the  range from (0.22 to 1.5)gms .Because of their small surface area and stiff compliance they require a slightly higher dc voltage to move the tweeter diaphragm so you can it actually can be seen moving. A voltage of 3v is usually sufficient to see the dome move but some types of tweeters will require 4 to 6 v to see any movement. The voltage should be only applied momentariarily for a fraction of a second or else the voice coil or dome assembly will be damaged.Some manufactures actually reverse the phase of the diaphragm in accordance with normal studio microphone phasing be careful. This is why some  2 channel Cd recordings sound flat due to a reverse phasing some where in the recording studio chain like the  microphone , internal studio cabling ,or the sound mixing console .This principal also applies to  DVD movie and music sound tracks. Reversing the  speaker leads to both left and right speakers will change the top end detail and sound stage radically this is why you have to know is it my tweeter  or bass driver that is reverse phased or is it a recording phase error. Some amplifiers have a reverse speaker switch which will do the same as reversing the connections to the speakers.

TESTING PROCEDURE FOR TRUE  RIBBON TWEETERS using a DC battery source 

Ribbon Tweeter  true  (with transformer) DC  Polatity  test.

 #You can check the polarity of the true ribbon tweeter by placing a (1.5V battery ) with a 68 ohm 1watt ( protection resistor) in series  with the ribbon tweeter ( +Ve ) terminal.

ü     CORRECT IN PHASE :   If the phasing of the terminals is correct the ribbon diaphgram should move outwards
 around (1mm )or so when the #battery is applied. This should be the red terminal

ü       WRONG OUT OF PHASE :If the terminals are the wrong way round then the ribbon diaphragm will move inwards when the battery is applied.

ü       Do not apply the battery for more than a  few seconds at a time .

ü       If you put a battery across the ribbon tweeter with out the  ( protection resistor) the ribbon diaphgram will be damage

SEE _ Bass speaker battery phase test  (DC Only) (diagram below) the same principal applies to the ribbon tweeter diaphram direction of  movement. Note only use a 1.5V battery for ribbon tweete

Ribbon Tweeter  true  (with transformer)    AC Pulse  Acoustic Phase and Polarity  test.

NOTE : A 9V 25Hz single shot positive edge pulse  must be  applied to the ribbon tweeter via  4.7 uF  capacitor (6db HP filter)   in series with the +(VE ) terminal  and correct acoustic phase is monitored by a Microphone the diaphragm should move upwards if the  polarity is correct.   ( more .. see  PHASE_IT Speaker Phase tester available from Stones Sound Studio +

SUB WOOFERS   BASS/MID  MIDRANGE and WIDE-RANGE speakers  (    more ...........>>  PHASE_IT Speaker Phase tester available from Stones Sound Studio

SUB WOOFERS  Can have very low  dc voice coil resistance so usually we need to use a 2x 1.5V batteries (3V) to get enough current
to more the voice coil especially with car sound drivers with  twin 2 ohm voice coils

Single Voice coils  use a 2x 1.5V batteries (3V) see (BASS SPEAKER BATTERY PHASE TEST (dc) ONLY)diagram below

Double Voice coils Test the phase of the voice coils  one at a time to check if the ternminals are marked correctly.Then join the two subwoofer voice coils together in series (like connecting 2 batterys to get 3volts)and the cone should move in and out according to the diagram below

BASS/MID  

Single Voice coils  use a 2x 1.5V batteries (3V) see (BASS SPEAKER BATTERY PHASE TEST (dc) ONLY)diagram below

Double Voice coils Test the phase of the voice coils  one at a time to check if the ternminals are marked correctly.Then join the two bass/mid voice coils together in series (like connecting 2 batterys to get 3volts)and the cone should move in and out according to the diagram below

MIDRANGE

Single Voice coils  use a 2x 1.5V batteries (3V) see (BASS SPEAKER BATTERY PHASE TEST (dc) ONLY)diagram below

WIDERANGE

Single Voice coils  use a 2x 1.5V batteries (3V) see (BASS SPEAKER BATTERY PHASE TEST (dc) ONLY)diagram below

HORN COMPRESSION  DRIVERS  Horn compression PA drivers are usually some form of alloy with their domes similar to a tweeter diaphram but an  inverted (concave) diaphram with voice diameters from 35 to 65mm in diameter and are designed to handle higher average power than conventential dome tweeters because of there application . Materials link aluminimum,berrilium,and tiatanium are used because of the heat sink effect the metal diaphram provides for the voice coil and also provide good power bandwidth inconjuction with alloy coil formers for  commercial Pa and sound reinforcement applications . The diaphram is mounted at the  rear of the Horn Flare .The Horn Compression unit (itself) can be removed manually from the  horn flare  by unscrewing  it or by removing 4 mounting screws.                    

Single Voice coils  use a 2x 1.5V batteries (3V) see (BASS SPEAKER BATTERY PHASE TEST (dc) ONLY) diagram below

BASS SPEAKER BATTERY PHASE TEST (dc) ONLY) Diagram

   

TESTING THE CROSSOVERS IN THE (Peerless speaker Kit Boxes) 

CROSSOVER UNIT  IMPEDANCE  (dc) test only

 Attach a DVM ( Digital volt meter) on the low ohms range across the rear of the twin terminal cup  +VE & -VE terminals and measure the dc resistance see table below .(Approximate measured values) for speaker Kits

Russell  Storey

Stones Sound Studio (Aust )