By Pete Basel 9-12-2008

Classic SPICA TC-50 Loudspeaker Restoration

Part 2 - Crossover and Driver Measurements

Detailed Measurements

More detailed measurements were taken to determine what is required to realign the woofer notch filters, and to better match the sensitivities. Often, a large measured difference is caused by the accumulation of several different errors. A difference of more than 3 dB is seen between the two systems in the 2 to 3 kHz range and it was found that about 1 dB is due to the basic woofer sensitivities being different. System B's woofer crossover shows about 1 dB more baffle step compensation, accounting for 1 more dB. Finally, the woofer resonant peak has moved and is lower in amplitude indicating that the notch filter must be re-tuned in order to eliminate excessive attenuation at the upper end of the woofer passband.

Individual Driver Acoustic Output Through Crossover

The drivers were disconnected, one at a time, so that the output from each could be measured independently. The frequency response plots follow for systems A and B:

Systems A and B tweeter outputs overlaid in order to show how well they match. The crossover point is about 3 kHz, and the match is quite good from 3 to 12 kHz:

Fig. 2.1 Systems A and B Tweeter Frequency Response Through Crossover

Systems A and B woofer outputs overlaid in order to illustrate the reduced output from the B woofer previously noted. The piston range is mismatched by about .5 to 1 dB, this will be corrected by the addition of a bucking magnet on woofer B. The diminished output from woofer B is even worse from 1.5 to 3.5 kHz where it reaches a maximum of about 4 dB. This will be corrected, as much as possible by retuning the woofer notch filter:

Fig. 2.2 Systems A and B Woofer Frequency Response Through Crossover

Fig. 2.3 System A Woofer and Tweeter Frequency Response Through Crossover

Fig. 2.4 System B Woofer and Tweeter Frequency Response Through Crossover

Crossover Loaded Transfer Functions

The loaded crossover transfer functions were measured in system by bringing test leads out from the terminals of each driver. These transfer functions will provide a baseline check against the rebuilt crossovers. Note that the tweeter response dips around the resonance notch frequency as I also noted in the simulations. It can be seen that this dip appears to match a peak in the acoustic output, probably due to cabinet diffraction, that can be seen in Fig. 2.10 around 800 to 900 Hz. Frequency response plots follow for systems A and B:

Fig. 2.5 System A Tweeter Crossover Loaded Frequency Response

Marker #1 in the following curve shows the crossover notch frequency as 3.65 kHz, very close to the system service code of 3.7. However, note that the peak in woofer A has shifted, probably due to aging of the cone treatment, and is now lower, at 3.19 kHz as shown in Fig. 2.11.

Fig. 2.6 System A Woofer Crossover Loaded Frequency Response

Fig. 2.7 System B Tweeter Crossover Loaded Frequency Response

Marker #1 in the following curve shows the crossover notch frequency as 3.65 kHz, very close to the system service code of 3.7. However, note that the peak in woofer B has also shifted and is now lower, at 3.39 kHz as shown in Fig. 2.13.

Fig. 2.8 System B Woofer Crossover Loaded Frequency Response

Woofer A and B crossover frequency responses are overlaid in the following figure. The B woofer crossover provides about one dB more attenuation around 2 kHz which contributes to the diminished system output in that range. The cause of this difference will be determined and corrected when the individual crossover components are measured.

Fig. 2.9 Systems A and B Overlaid Woofer Crossover Loaded Frequency Response

Driver Acoustic Output Driven Directly

Frequency response plots follow for the tweeter and woofer outputs - systems A and B, driven directly bypassing the crossover. These measurements were taken with the felt and grille in place, on the optimum listening axis:

Fig. 2.10 System A Tweeter Frequency Response - Directly Driven

Marker #2 in the following curve shows the peak in woofer A as 3.19 kHz which does not match the woofer service code of 3.7. The peak in woofer A has shifted lower, probably due to aging of the cone treatment.

Fig. 2.11 System A Woofer Frequency Response - Directly Driven

Fig. 2.12 System B Tweeter Frequency Response - Directly Driven

Marker #2 in the following curve shows the peak in woofer B as 3.39 kHz which does not match the woofer service code of 3.8. The peak in woofer B has also shifted lower.

Fig. 2.13 System B Woofer Frequency Response - Directly Driven

The following curve shows woofer A as having a peak that is about 3 dB higher than woofer B. This indicates that the drivers have shifted significantly from their original specification since woofer B has a higher service code peak of 10 dB and woofer A of only 8 dB. The peak in woofer A should be 2 dB lower than that in woofer B however it measures 3 dB higher:

Fig. 2.14 Systems A and B Woofer Responses Overlaid - Directly Driven

Fig. 2.15 Systems A and B Tweeter Responses Overlaid - Directly Driven

Driver Thiele and Small Parameters

System A Tweeter Thiele and Small Parameters

SPICA TC-50  SYSTEM: 13593A						Sept 22, 2008
1" Audax Tweeter:
Markings: 
	TWO25D04 
	TTP0010A2
	120974W
	22304
Rdc			3.22 ohms
Fs			837.2  Hz
Re  (LAUD)		3.48  ohms	
Qe			1.03
Qm			4.04
Qts			.821
Lead in wires are solid.

Front plate thickness: 3mm, front plate bevels down about ~1.5mm
Back plate thickness: 3mm
VC wind height: 1.5 mm
VC ID: 1.0"
VC OD: 1.02"

   
Fig. 2.16  System A Tweeter Free Air Input Impedance

System A Woofer Thiele and Small Parameters

SPICA TC-50  SYSTEM: 13593A	Service Code = H 3.7-8	Sept 18, 2008
6.5" Audax Woofer:  
Markings: 
	Polydax Made in France
	REF:  HIF17JVX4(ohm
	PN:  120076P
	SN:  0330200506
	H 3.7-8 written in blue pen on magnet

Rdc			3.45 ohms
Pd			14.5 cm (manufacturer spec)
Fs			36.5  Hz
Vas			47.2   liters
Re (LAUD)		3.76  ohms	
Qe			.49
Qm			1.64
Mms			15.5  grams
no			.451  %
SPLref  		88.5  dB
Bl			5.23  T-m
Qts			.376
Cms			1.23  mm/N


   
Fig. 2.17  System A Woofer Free Air Input Impedance

System B Tweeter Thiele and Small Parameters

SPICA TC-50  SYSTEM: 13593B						Sept 22, 2008
1" Audax Tweeter:
Markings: 
	TWO25D04 
	TTP0010A2
	120974W
	22304
Rdc			3.30 ohms
Fs			838.1  Hz
Re  (LAUD)		3.71  ohms	
Qe			.98
Qm			2.71
Qts			.720


   
Fig. 2.18  System B Tweeter Free Air Input Impedance

System B Woofer Thiele and Small Parameters
Note that this is a replacement woofer that does not match the system
service code of H 3.7-8:

SPICA TC-50  SYSTEM: 13593B	Service Code = S 3.8-10		Sept 22, 2008
6.5" Audax Woofer:  
Markings: 
	Polydax Made in France
	REF:  HIF 17JVX  4* 2C  A12
	120076P
	29812
	S 3.8-10 written in red pen on magnet

Rdc			3.28 ohms
Pd			14.5 cm (manufacturer spec)
Fs			34.6  Hz
Vas			47.0   liters
Re  (LAUD) 		3.62  ohms	
Qe			.544
Qm			1.86
Mms			17.2  grams
no			.345  %
SPLref 		87.4  dB
Bl			4.99  T-m
Qts			.421
Cms			1.23  mm/N

A bucking magnet was temporarily added to woofer B, and the woofer was
retested with the following results.  The goal is to increase the drivers
sensitivity; without the magnet it is about 87.4 and with 88.2 dB -
only .3 dB less than woofer A, as compared to 1.1 dB less without the
bucking magnet.  Note that these measurements are difficult to make and
would have to be repeated with averaging to get a more accurate result.
Other expected differences include:
motor strength - Bl increases from 4.99 to 5.41	T-m	A woofer:  5.23 T-m
Basic Efficiency no increases from .345 % to .418 %	A woofer: .451 %
Driver Qts (from Qe) decreases from .421 to .365  	A woofer: .376

Fs			34.9  Hz
Vas			47.3   liters
Re (LAUD)  		3.65  ohms	
Qe			.461
Qm			1.76
Mms			16.9  grams
no			.418  %
SPLref 		88.2  dB
Bl			5.41  T-m
Qts			.365
Cms			1.23  mm/N


   
Fig. 2.19  System B Woofer Free Air Input Impedance

More to come as time permits.