HOWTO DIY DISTORTION METER BY ERIC RUSSELL Properly setting amplifier and processor gains can optimize the Sound Quality and/ or Sound Pressure Level of your car audio system. The best way to do this is with an oscilloscope. However, if you don’t have access to one, you can build a Do-It-Your- self Distortion Meter for around $30 with parts found at your local Radio Shack™. You’ll need a mini-amplified speaker and a couple of voltage dividers. These instruc- tions include schematics for building the voltage dividers along with diagrams on how to connect and use them with your mini-amplified speaker. NOTE: This article is a companion to “How to Properly Set Your Gains”. Search our Knowledge Base for Answer #46 or keywords “Gain Tutorial”. Parts List The below chart refers to seven items needed from your local Radio Shack™ store. The total cost for these components is around $30. Most of the parts needed are used to create two Volt- age Dividers; one to measure the source unit output voltage, and another to measure the amplifier output voltage. These are needed because the sensitivity of the mini amplified speaker is so low, it can easily be over-driven. These voltage dividers “drop” the signal down to a safe level so we can properly drive the mini amplified speaker. If you don’t have wire and heatshrink, the chart also references these part numbers. Part # Radio Shack Description Price 277-1008 Mini Amplified Speaker $14.99 274-0339 2PK Solder Type Phono Plug (RCA) $3.79 274-0288 2PK Solder Type 1/8” Mono Phone Plug $4.19 270-0356 2PK Fully Insulated 2” Alligator Clips $2.59 55049108 NTE EW110 1/8W Metal Resistor 100 Ohms 2% $1.09 55049129 NTE EW310 1/8W Metal Resistor 10K Ohms 2% $1.09 55049134 NTE EW410 1/8W Metal Resistor 100K Ohms 2% $1.09 SUB TOTAL $ 28.83 Optional Parts 55048778 HS-6 in 1/8” Heatshrink Tubing in Assorted Colors $2.20 278-1222 75-Ft. UL-Recognized Hookup Wire (20AWG) $7.99 SUB TOTAL $ 10.19 GRAND TOTAL $ 39.02 Mini Amplified Speaker Source Unit Voltage Divider Amplifier Voltage Divider Source Unit Voltage Divider The source unit voltage divider reduces the amplitude of the signal by 100:1 and can handle up to 10 Volts AC (most source units produce between 2V to 4V RMS). The resistors will be housed inside - Parts List • (1) RCA Jack • (1) 1/8” mini phone plug • (1) 10,000 Ohm, 1/8 Watt resistor • (1) 100 Ohm, 1/8 Watt resistor • Assorted 1/8” heat shrink tubing • 20” Red 20 AWG wire • 20” Black 20 AWG wire Assembly Instructions 1. Disassemble 1/8” plug and RCA jack. 3. Prepare and trim heat-shrink tubing to proper length. 4. Trim red and black 20 AWG wires to approximately 20” length. 6. Solder resistors in place and trim off excess leads. 9. Solder wire leads and reassemble RCA shell. Source Unit Voltage Divider SourcR1e Unit= 10k V Ohm,oltage 1/8 Divider Watt R2 = 100 Ohm, 1/8 Watt Sourc e Unit Voltage Divider R1 V in (+) R1 V out (+) V in (+) V out (+) R2 V in (–) R2 V out (–) V in (–) V out (–) RCA Jack 10K Ω Signal/Core 1/8” Phone Plug RCA Jack 10K Ω Signal/Core 1/8” Phone Plug Ground/Shield Ground/Shield 100 Ω 100 Ω Source Unit Voltage Divider Amplifier Voltage Divider The amplifier voltage divider reduces the amplitudeR1 of the signal by 1,000:1 and can handle up to 100 Volts AC (that’s a pair ofV Power in (+) T2500-1bdCP amplifiers Vconnected out (+) together in bd-SYNC mode). Like the previous method, the resistors will be housed inside the 1/8” phone plug shell, so careful preparation is needed. Test fitting the components priorR2 to soldering will ensure a proper fit. V in (–) V out (–) Parts List • (1) RCA Jack • (1) 1/8” mini phone plug • (1) 100,000 Ohm, 1/8 Watt resistor • (1) 100 Ohm, 1/8 Watt resistor RCA Jack 10K Ω • Assorted 1/8” heat shrink tubing Signal/Core 1/8” Phone Plug • 20” Red 20 AWG wire • 20” Black 20 AWG wire Assembly Instructions 1. Disassemble 1/8” plug and Alligator Clips. Ground/Shield 2. Test fit resistors in place (once soldered, they can be trimmed to size). 3. Prepare and trim heat-shrink tubing100 to proper Ω length. 4. Trim red and black 20 AWG wires to approximately 20” length. 5. Configure heat-shrink tubing and 1/8” phone shell in the order of assembly. 6. Solder resistors in place and trim off excess leads. 7. Position heat-shrink tubing in place and carefully heat with lighter to fit. 8. Prepare other end of harness by sliding Alligator Clip sleeves over wire ends. 9. Solder wire leads and reassemble Alligator Clips with protective sleeves. Amplifier Voltage Divider AmplierR1 = 100k Voltage Ohm, Divider 1/8 Watt SourcR2e Unit= 100 V oltageOhm, 1/8 Divider Watt R1 R1 V in (+) V out (+) V in (+) V out (+) R2 R2 V in (–) V out (–) V in (–) V out (–) 10K Ω RCA Jack Signal/Core 100K Ω 1/8” Phone Plug Signal/Core 1/8” Phone Plug (+) Alligator Clips Ground/Shield (–) Ground/Shield 100 Ω 100 Ω Amplier Voltage Divider R1 V in (+) V out (+) R2 V in (–) V out (–) 100K Ω Signal/Core 1/8” Phone Plug (+) Alligator Clips (–) Ground/Shield 100 Ω Source Units Source Unit Voltage Divider R IN L EXT RFX8350 DISP OPEN COMPACT DISC PLAYER WITH DIGITAL TUNER CD CHANGER CONTROLLER R SEAT VOL 9V COMPACT LOUD DIGITAL AUDIO AS MODE LO/DX SCAN RPT RDM DOWN DISC UP k PUSH BAND1 2 3 4 5 6 PS MUTE RadioShac MENU EAKER MINI AMPLIFIER • SP Source Unit Voltage Divider 1. Insert 1kHz @ 0dB test tone into CD Player.R IN L EXT RFX8350 DISP OPEN 2. Slowly increaseCOMPACT DISC PLAYER WITHvolume DIGITAL TUNER CD CHANGERuntil CONTROLLER “raspy” sound emits from speaker R SEAT VOL 9V COMPACT LOUD 3. Once youDIGITAL AUDI O determine the clipping point of the source unit, DO NOT adjust the source unit’s volume AS MODE LO/DX control until youSCAN RPT haveRDM DOWN completedDISC UP amplifier configuration(s)Amplifier Vbelow.oltage Divider k PUSH BAND1 2 3 4 5 6 PS MUTE RadioShac MENU I EAKER 4. Reconnect RCAs from source unit to amplifier system. N MINI AMPLIFIER • SP Source Unit Voltage Divider EXT R 9VIN Mono & Stereo Amplifiers L EXT RFX8350 DISP OPEN COMPACT DISC PLAYER WITH DIGITAL TUNER CD CHANGER CONTROLLER R SEAT VOL 9V COMPACT LOUD DIGITAL AUDIO RadioShack AS MINI AMPLIFIER • SPEAKER MODE LO/DX SCAN RPT RDM DOWN DISC UP k PUSH BAND1 2 3 4 5 6 PS MUTE RadioShac MENU Amplifier Voltage Divider MINI AMPLIFIER • SPEAKER IN EXT 9V Amplifier Voltage Divider RadioShack MINI AMPLIFIER • SPEAKER Amplifier Voltage Divider IN EXT IN 9V EXT 9V 1. Slowly increase amplifier GAIN until “raspy” sound emits from speaker. RadioShack MINI AMPLIFIER • SPEAKER 2. If testing a Stereo amplifier; repeat test on both amplifier channels. RadioShack 3. If testing a Mono amplifier; use either Speaker OutputsAmplifier “A” or V “B”oltage on amplifierDivider. MINI AMPLIFIER • SPEAKER IN EXT Amplifier In Bridged Mode 9V RadioShack Amplifier Voltage Divider MINI AMPLIFIER • SPEAKER IN EXT 9V RadioShack MINI AMPLIFIER • SPEAKER 1. Slowly increase GAIN on BOTH left/right channels equally until “raspy” sound emits from speaker. ©2012 Rockford Corporation, All Rights Reserved E.W.R. V1.1 05/18/2012.