AMPLIFIER EFFECTIVE DAMPING FACTOR (EDF)
The acoustic responses comparing no attenuation response to maximum attenuated response demonstrates no significant effect of added resistance on the voltage output of the amplifier across the frequency range the horn assembly is expected to operate. Here, we measure the effect directly. A relatively low cost, basic, solid state amplifier is used as the voltage source. The output of the amplifier is set to read 208mV (0.2V) under no-load conditions. After a time to allow the electronics to stabilize thermally, the voltage across the amplifier terminals is measured across a range of frequencies comparing both the voltage with the attenuator set at maximum attenuation (-12dB) and without the attenuator active (0dB). At -12dB readings, sufficient time must also be provided to allow the resistors to reach thermal equilibrium as evidence by asymptotic drift. The results of the test, provided below, shows that the difference between the -12dB and 0dB loads are within the uncertainty of the measurement technique estimated to be about +/- 2mV.
Based on our analysis and listening tests, the L-pad design utilized here is ideal for precise, sound pressure level adjustments. Finally, we refer the reader to Dr. Toole's discussion related to the topic of damping factor.
The acoustic responses comparing no attenuation response to maximum attenuated response demonstrates no significant effect of added resistance on the voltage output of the amplifier across the frequency range the horn assembly is expected to operate. Here, we measure the effect directly. A relatively low cost, basic, solid state amplifier is used as the voltage source. The output of the amplifier is set to read 208mV (0.2V) under no-load conditions. After a time to allow the electronics to stabilize thermally, the voltage across the amplifier terminals is measured across a range of frequencies comparing both the voltage with the attenuator set at maximum attenuation (-12dB) and without the attenuator active (0dB). At -12dB readings, sufficient time must also be provided to allow the resistors to reach thermal equilibrium as evidence by asymptotic drift. The results of the test, provided below, shows that the difference between the -12dB and 0dB loads are within the uncertainty of the measurement technique estimated to be about +/- 2mV.
Based on our analysis and listening tests, the L-pad design utilized here is ideal for precise, sound pressure level adjustments. Finally, we refer the reader to Dr. Toole's discussion related to the topic of damping factor.
If you wish to discuss this project, receive additional
information related to the design or learn more about the
performance then become a member of the
North
Reading Engineering forum and ask the questions you would like
answered.
ATTENUATOR SCHEMATIC
The step attenuator schematic is shown below (click on image). Resistor pairs are designated by (POLE, POSITION) as (R21, R11), (R22, R12), etc. The desired magnitude of step attenuation in this design is 1.2dB/step. Rotary switch is Grayhill series 42. Wiper is represented by connection between POLE 1 and POLE 2.
The step attenuator schematic is shown below (click on image). Resistor pairs are designated by (POLE, POSITION) as (R21, R11), (R22, R12), etc. The desired magnitude of step attenuation in this design is 1.2dB/step. Rotary switch is Grayhill series 42. Wiper is represented by connection between POLE 1 and POLE 2.
ADDITIONAL PHOTOS
ATTENUATION MEASUREMENTS
The plot below shows the 5 output levels of the midrange horn as the rotary switch is turned though the 5 step positions. The PURPLE response on the graph is -12dB with respect to the horn assembly without attenuation (i.e. 0dB). As is evident in the plot it is possible to establish precise, linear changes in the output of the horn assembly using this approach. The analyzer microphone was placed on-axis, near mouth location.
The plot below shows the 5 output levels of the midrange horn as the rotary switch is turned though the 5 step positions. The PURPLE response on the graph is -12dB with respect to the horn assembly without attenuation (i.e. 0dB). As is evident in the plot it is possible to establish precise, linear changes in the output of the horn assembly using this approach. The analyzer microphone was placed on-axis, near mouth location.
DESIGN CONCEPT AND COMPONENTS
The step attenuator described was developed to provide approximately 5 levels of mid-range attenuation, ranging from -12 to
-6dB relative to the output of the driver with no attenuation. The design consists of (1) 5 position, two pole, single wafer, break-before-make rotary switch, (10) wire wound, 10W resistors, (2) 10 position solder terminal strips. Hook-up wire is #16 GA. The switch we've selected for the attenuator (shown in the thumbnails above) is the extremely durable Grayhill 42 series sealed rotary switch. Although this particular design uses 5 levels of attenuation, the concept can be extended to many more.
LINEARITY MEASUREMENTS
Estimated values for the series and shunt resistor pairs for the 5 attenuation values were first approximated using the SPICE circuit simulator. Loads for both the intended network and selected horn assembly were developed, resistor pairs examined iteratively and predicted volume velocities obtained. A prototype was then built and the actual magnitudes of attenuation measured and accessed against the predictions. The SPICE model was then calibrated to provide accurate predictive capability.
The linearity of attenuation across the pass-band was easily accessed by direct super-position of the attenuated frequency response of the driver to the response of the driver with no attenuation. For example, the graph below shows three plots. The PURPLE plot is the frequency response of the horn assembly connected directly to analyzer amplifier with no network and no attenuator, the GREEN plot is the response of the horn assembly with the attenuator operating and set to the -12dB attenuation position. The BLUE plot is the -12dB response simply shifted to coincide with the PURPLE no attenuation response. As shown in the plot, a comparison between the attenuated and no attenuation responses demonstrates that the output linearity of the L-pad response is essentially identical to the response with no attenuation.
The next graph below also shows three response plots. The RED plot is the frequency response of the horn assembly connected directly to a 4th order band-pass filter and with no attenuation, the GREEN plot is the response of the horn assembly with the rotary switch set to the -12dB attenuation position. The BLUE plot is the -12dB response plot shifted to coincide with the no attenuation response. As shown in the plot, the linearity of the attenuated response is within about 2dB of the no attenuation response.
The step attenuator described was developed to provide approximately 5 levels of mid-range attenuation, ranging from -12 to
-6dB relative to the output of the driver with no attenuation. The design consists of (1) 5 position, two pole, single wafer, break-before-make rotary switch, (10) wire wound, 10W resistors, (2) 10 position solder terminal strips. Hook-up wire is #16 GA. The switch we've selected for the attenuator (shown in the thumbnails above) is the extremely durable Grayhill 42 series sealed rotary switch. Although this particular design uses 5 levels of attenuation, the concept can be extended to many more.
LINEARITY MEASUREMENTS
Estimated values for the series and shunt resistor pairs for the 5 attenuation values were first approximated using the SPICE circuit simulator. Loads for both the intended network and selected horn assembly were developed, resistor pairs examined iteratively and predicted volume velocities obtained. A prototype was then built and the actual magnitudes of attenuation measured and accessed against the predictions. The SPICE model was then calibrated to provide accurate predictive capability.
The linearity of attenuation across the pass-band was easily accessed by direct super-position of the attenuated frequency response of the driver to the response of the driver with no attenuation. For example, the graph below shows three plots. The PURPLE plot is the frequency response of the horn assembly connected directly to analyzer amplifier with no network and no attenuator, the GREEN plot is the response of the horn assembly with the attenuator operating and set to the -12dB attenuation position. The BLUE plot is the -12dB response simply shifted to coincide with the PURPLE no attenuation response. As shown in the plot, a comparison between the attenuated and no attenuation responses demonstrates that the output linearity of the L-pad response is essentially identical to the response with no attenuation.
The next graph below also shows three response plots. The RED plot is the frequency response of the horn assembly connected directly to a 4th order band-pass filter and with no attenuation, the GREEN plot is the response of the horn assembly with the rotary switch set to the -12dB attenuation position. The BLUE plot is the -12dB response plot shifted to coincide with the no attenuation response. As shown in the plot, the linearity of the attenuated response is within about 2dB of the no attenuation response.
LOUDSPEAKER PRECISION STEP ATTENUATOR
We have developed a precision step attenuator for adjusting either mid-range or high frequency drivers. Although the assembly described here is designed specifically for the Volti VTrac midrange horn driven by the BMS ND4592 MID neodymium, 2" throat, polyester diaphragm compression driver, the assembly can be modified for any transducer (horn or direct radiator).
Our analysis and analyzer measurements shown below demonstrate that resistive L-pad attenuators are quite acceptable for even the most demanding audio enthusiasts.
The near-mouth and far-field impulse, frequency and phase data files are generated using CLIO (v. 10.31) acoustic analyzer.
We have developed a precision step attenuator for adjusting either mid-range or high frequency drivers. Although the assembly described here is designed specifically for the Volti VTrac midrange horn driven by the BMS ND4592 MID neodymium, 2" throat, polyester diaphragm compression driver, the assembly can be modified for any transducer (horn or direct radiator).
Our analysis and analyzer measurements shown below demonstrate that resistive L-pad attenuators are quite acceptable for even the most demanding audio enthusiasts.
The near-mouth and far-field impulse, frequency and phase data files are generated using CLIO (v. 10.31) acoustic analyzer.