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Introduction:
Tom Z started a thread where he identified this product. That it seemed ideal for small active, 2.1 systems.
Gordy had previously bought two of these units. He perceived it difficult to dial in a specific LP XO point:” The pot that controls the freq. does not seem to do much like it is adjusting nothing. Almost like it is too sensitive and only controlling a portion of the freq. scale it is supposed to.”
Then he shipped his last unit to me for analysis, thanks Gordy.
Summary:
The PE Subwoofer amp suffered from two major issues. First, the component values chosen for the LP filter created a variable cut off frequency (F3) between 9 Hz and 107 Hz. Second, as the cut-off frequency was lowered, the module’s overall gain would be drastically reduced.
Taken together, the frequency adjustment pot had a extremely narrow adjustment range.
Several components were replaced to correct these issues. The modified unit has an F3 adjustment range of 43 to 228 Hz. Gain reduction has been eliminated over that range. And the entire module performs near an ideal subwoofer LP pre-amp.
Overview:
The subwoofer pre-amp performs three functions: (1) It sums a L-R stereo signal to provide a combined subwoofer output signal; (2) It provides an adjustable gain for the subwoofer output; and (3) It supports an adjustable LP frequency.
One of two potentiometers controls gain that can be varied between zero gain (i.e., no signal out) and ~10.5x gain. This potentiometer works as expected. An audio frequency, 1st order bandpass filter is integrated into this circuit. The HP F3 is fixed at 3 Hz. The LP F3 decreases with increasing gain but never gets below ~10 KHz @ maximum gain. So it doesn't affect the subwoofer out signal.
The second potentiometer controls the sub's LP F3 frequency and this is where some problems exist.
Analysis:
Original Module: 9 Hz to 107 Hz Adjustment Range
The sub's LP filter is a 2nd order Sallen-Key topology as are most active, analog 2nd order filters. The original filter's Q factor is 0.73; very close to .707, aka Butterworth dampening. After modifications, the Q factor is virtually unchanged at 0.74.
In stock form, the values chosen for the filter’s two capacitors and two resistors, in conjunction with the variable resistance of the potentiometer adjustment range, vary the frequency from a high F3 of 107 Hz down to 9 Hz. If we assume the desired adjustment range is between 60 Hz and 107 Hz, then only ~25% of the pot's rotation is useful in the desired range.
The remainder of potentiometer's rotation, ~75%, adjusts the F3 frequency from 60 Hz to 9 Hz, fairly useless especially for smaller systems where this low cost sub pre-amp would likely be used.
Initial Testing:
Measurements confirmed the adjustment range limitation. Testing also uncovered a peculiar problem. As expected, increasing the LP potentiometer’s CCW rotation lowers the F3 cutoff frequency. But in addition, the overall system gain was also significantly reduced
Further analysis revealed that the filter’s bias resistor impedance was too low @ 10K ohms. The increasing impedance of the 50K pot was allowing a significant portion of the AC signal to bleed through the filter's 10K bias resistor. With the two gangs of the 50K pot in series and in series with the two fixed 4.7K resistors, almost 90% of the signal was bleeding through the 10K bias resistor to ½ Vcc, nominal ac ground.
In a single supply op-amp circuit, signal bias resistors are needed to shift the 0 value of an ac coupled signal to half the supply voltage. This allows the op amp to reproduce the positive and negative portions of the signal (i.e., instead of V- to V+, the signal has a DC offset and spans 0V to 2V.
Solution:
First, the capacitors in the Sallen-Key filter were reduced to allow a higher F3 frequency. The 0.22 uf capacitor, C5 (EDIT: not C6), was replaced with a 0.1 uf capacitor. The 0.47 uf capacitor, C6 (EDIT: not C5), was replaced with a .22 uf capacitor (i.e., EDIT move C5 to C6 position and insert a 0.1 uf cap into C5 postion). This modification expands the F3 adjustment range up to 228 Hz.
Next, the 50K LP frequency potentiometer was replaced with a 20K “A” taper potentiometer. The reduction in overall resistance limits the minimum F3 frequency to 43 Hz. In addition the total adjustment range from 43 Hz to 228 Hz becomes more linear over the pots full 300 degree rotation.
The solution for the gain issue was to remove the filter’s bias resistor (R7). The overall gain is set by the gain potentiometer and is now independent of the LP potentiometer resistance.
Normally, a much higher value for bias resistors are chosen (e.g., 100K ohms) to avoid this issue. However, the op-amp gain stage has a bias resistor raising the ac coupled input signal to ½ Vcc. Since the op-amp gain stage is DC coupled to the filter op-amp stage, there was no need to add bias at the filter op-amp. The signal entering the filter stage was already biased to ½ Vcc by the gain stage.
Final Testing:
Post modification, the measured results coincide with the expected performance of the modified module.
The 20K "A Taper potentiometer can be procured from Digi-Key (TT Electronics # P092NQC15AR20K).The part cost ~$2.40 and the shipping cost was reasonable at ~ $3.50. Most any film / polyester capacitor can be used for the 0.1 uf replacement.
Side Note:
The measurement graphs show the effect of the 1st order HP at the input stage (F3 = 3 hz). If a higher HP point is needed to limit sub Xmax, replacing the two input signal decoupling capacitors can achieve a higher HP F3 (aka PLXXO). With the existing 4.7uf capacitors, the 1st order HP F3 is 3 Hz. Replacing those two capacitors with lower values yields the following: (1) 1.0 uf, F3 = 16 Hz; (2) 0.68 uf, F3 23 Hz; (3) 0.47 uf, F3 = 34 Hz and (4) 0.22 uf, F3 = 48 Hz. The effect of the HP stage on subwoofer performance can readily be modeled in WinISD.
Introduction:
Tom Z started a thread where he identified this product. That it seemed ideal for small active, 2.1 systems.
Gordy had previously bought two of these units. He perceived it difficult to dial in a specific LP XO point:” The pot that controls the freq. does not seem to do much like it is adjusting nothing. Almost like it is too sensitive and only controlling a portion of the freq. scale it is supposed to.”
Then he shipped his last unit to me for analysis, thanks Gordy.
Summary:
The PE Subwoofer amp suffered from two major issues. First, the component values chosen for the LP filter created a variable cut off frequency (F3) between 9 Hz and 107 Hz. Second, as the cut-off frequency was lowered, the module’s overall gain would be drastically reduced.
Taken together, the frequency adjustment pot had a extremely narrow adjustment range.
Several components were replaced to correct these issues. The modified unit has an F3 adjustment range of 43 to 228 Hz. Gain reduction has been eliminated over that range. And the entire module performs near an ideal subwoofer LP pre-amp.
Overview:
The subwoofer pre-amp performs three functions: (1) It sums a L-R stereo signal to provide a combined subwoofer output signal; (2) It provides an adjustable gain for the subwoofer output; and (3) It supports an adjustable LP frequency.
One of two potentiometers controls gain that can be varied between zero gain (i.e., no signal out) and ~10.5x gain. This potentiometer works as expected. An audio frequency, 1st order bandpass filter is integrated into this circuit. The HP F3 is fixed at 3 Hz. The LP F3 decreases with increasing gain but never gets below ~10 KHz @ maximum gain. So it doesn't affect the subwoofer out signal.
The second potentiometer controls the sub's LP F3 frequency and this is where some problems exist.
Analysis:
Original Module: 9 Hz to 107 Hz Adjustment Range
The sub's LP filter is a 2nd order Sallen-Key topology as are most active, analog 2nd order filters. The original filter's Q factor is 0.73; very close to .707, aka Butterworth dampening. After modifications, the Q factor is virtually unchanged at 0.74.
In stock form, the values chosen for the filter’s two capacitors and two resistors, in conjunction with the variable resistance of the potentiometer adjustment range, vary the frequency from a high F3 of 107 Hz down to 9 Hz. If we assume the desired adjustment range is between 60 Hz and 107 Hz, then only ~25% of the pot's rotation is useful in the desired range.
The remainder of potentiometer's rotation, ~75%, adjusts the F3 frequency from 60 Hz to 9 Hz, fairly useless especially for smaller systems where this low cost sub pre-amp would likely be used.
Initial Testing:
Measurements confirmed the adjustment range limitation. Testing also uncovered a peculiar problem. As expected, increasing the LP potentiometer’s CCW rotation lowers the F3 cutoff frequency. But in addition, the overall system gain was also significantly reduced
Further analysis revealed that the filter’s bias resistor impedance was too low @ 10K ohms. The increasing impedance of the 50K pot was allowing a significant portion of the AC signal to bleed through the filter's 10K bias resistor. With the two gangs of the 50K pot in series and in series with the two fixed 4.7K resistors, almost 90% of the signal was bleeding through the 10K bias resistor to ½ Vcc, nominal ac ground.
In a single supply op-amp circuit, signal bias resistors are needed to shift the 0 value of an ac coupled signal to half the supply voltage. This allows the op amp to reproduce the positive and negative portions of the signal (i.e., instead of V- to V+, the signal has a DC offset and spans 0V to 2V.
Solution:
First, the capacitors in the Sallen-Key filter were reduced to allow a higher F3 frequency. The 0.22 uf capacitor, C5 (EDIT: not C6), was replaced with a 0.1 uf capacitor. The 0.47 uf capacitor, C6 (EDIT: not C5), was replaced with a .22 uf capacitor (i.e., EDIT move C5 to C6 position and insert a 0.1 uf cap into C5 postion). This modification expands the F3 adjustment range up to 228 Hz.
Next, the 50K LP frequency potentiometer was replaced with a 20K “A” taper potentiometer. The reduction in overall resistance limits the minimum F3 frequency to 43 Hz. In addition the total adjustment range from 43 Hz to 228 Hz becomes more linear over the pots full 300 degree rotation.
The solution for the gain issue was to remove the filter’s bias resistor (R7). The overall gain is set by the gain potentiometer and is now independent of the LP potentiometer resistance.
Normally, a much higher value for bias resistors are chosen (e.g., 100K ohms) to avoid this issue. However, the op-amp gain stage has a bias resistor raising the ac coupled input signal to ½ Vcc. Since the op-amp gain stage is DC coupled to the filter op-amp stage, there was no need to add bias at the filter op-amp. The signal entering the filter stage was already biased to ½ Vcc by the gain stage.
Final Testing:
Post modification, the measured results coincide with the expected performance of the modified module.
The 20K "A Taper potentiometer can be procured from Digi-Key (TT Electronics # P092NQC15AR20K).The part cost ~$2.40 and the shipping cost was reasonable at ~ $3.50. Most any film / polyester capacitor can be used for the 0.1 uf replacement.
Side Note:
The measurement graphs show the effect of the 1st order HP at the input stage (F3 = 3 hz). If a higher HP point is needed to limit sub Xmax, replacing the two input signal decoupling capacitors can achieve a higher HP F3 (aka PLXXO). With the existing 4.7uf capacitors, the 1st order HP F3 is 3 Hz. Replacing those two capacitors with lower values yields the following: (1) 1.0 uf, F3 = 16 Hz; (2) 0.68 uf, F3 23 Hz; (3) 0.47 uf, F3 = 34 Hz and (4) 0.22 uf, F3 = 48 Hz. The effect of the HP stage on subwoofer performance can readily be modeled in WinISD.
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