AFTER OWNING MY NEW Mustang for a year I found the radio's poor sonic qualities had become so annoying I rarely listened to it. Good car audio equipment is available in the aftermarket, but before spending $600 on a new receiver and amp I decided to try to improve the sound of my radio, using the typical upgrades that are so effective on home audio equipment.
The current generation Ford AM/FM radio incorporates some sophisticated features typical of those offered by other major auto companies. The digital display and soft touch controls enable you to tune in 0.1MHz steps or seek the next station in the band. The seek function can be restricted to only strong stations if desired. The radio has eight presets in each of two FM bands and another eight in the AM band. The separate bass and treble controls, as well as left/right and front/rear balance controls, including volume, are all rotary types with click stops. Power output is 4W for each of four channels. The radio has an active tuning feature that varies bandwidth and separation to maintain signal quality when interference is present.
Ford also offers a separate option, the Premium Sound System, which includes better quality speakers and a four-channel power amplifier with bass equalization designed to offset the noise spectrum of the moving car. This amp has the advantage of direct-coupled speaker outputs rather than the capacitor coupling used in the standard radio. Each channel is rated at 20W power output.
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ABOUT THE AUTHOR: Mr. Buschmann is a chemical engineer designing air pollution equipment, whose interest has always been divided between electronics and chemistry. He built his first crystal radio set at the age of seven, and for a time held an amateur radio license. During college the author subscribed to TAA and built stereo components from kits. Later he ‘moved up ’ to high quality commercial equipment. He has since re-subscribed and rebuilt almost all his commercially made components. Mr. Buschmann, age 35, is married and has three little girls.
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FIGURE 1: Coupling and EQ caps should be installed with 1-ohm isolation resistors.
Unfortunately, all this sophisticated hardware lacks any trace of musicality.
The bass is muddy, extreme highs are nonexistent, the midrange is peaky and no depth or detailed resolution is apparent at any frequency.
Internally Before I even began modifying the radio, I was frustrated by Ford's excellent built in anti-theft features and by the top secret nature of the circuit diagrams for the radio. It takes skill, strength and a special tool to get the radio out of the dash. I talked the local dealer into doing the job for me. The circuit diagrams were not available from any source and I was forced to trace circuits to determine their function.
The radio is a mixture of eighties and fifties technology. Most circuits are on ICs, the tuning and station presets are con trolled by a microprocessor and the power supply is well-filtered and regulated. A large number of poor quality electrolytic coupling capacitors are used in the design. In addition, the out board power amp is driven from the speaker output of the amp in the radio.
I'm not surprised the sound is so poor given the audio circuit design.
Dash Distortion
The peaky midrange sounded like a balance problem between the dash mounted speakers and those in the doors and rear seat. The tweeters in the dash have a crossover capacitor that blocks sound below about 500Hz. Adjusting the value of this capacitor did not seem to help the situation much. What did sound better was simply disconnecting them.
These 3-inch cone speakers do not have any more high end than the dual-cone main speakers. Thus when playing they merely added to the midrange response of the already fairly flat door and rear speakers.
PHOTO 1. Interior view of amplifier.
PHOTO 2. Interior view of tuner control unit.
Radio Noise
The 12V supply to the low level circuits in the radio is already regulated. I removed the poor quality 470uF bypass cap at the regulator output and replaced the small 2uF bypass cap (C812) at the audio buffer amp (IC801) with a 1,000uF Sprague 672 electrolytic type. The circuit trace that leads power from the regulator to this IC is long, thin and convoluted. To maintain low impedance at all frequencies I made a jumper of four 24-gauge wires braided together and ran it directly from the regulator output to the positive lead of the new C812.
IC801 takes the output of the FM demodulator (IC551) and, through manipulation of its feedback loop, applies volume, balance and bass/treble equalization to the signal. Coupling between IC551 and IC801 is via two 1uF electrolytic caps (C813 and 814). Two additional 2.2uF caps (C810 and 811) provide equalization. I replaced all these with polypropylene types, installing them with 1Q isolation resistors as shown in Fig. 1.
This buffer IC normally drives the power amp within the radio, which in turn drives the external amp. I bypassed the completely unnecessary coloration of the internal amp by jumpering with two 24-gauge twisted pairs from the positive leads of the buffer IC output coupling capacitors (C819 and 820) to the speaker output terminals of the radio plug connector.
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PARTS LIST
Part Description | Location
Capacitors
(2) C813, 814 (2) C810, 811
(4) power amp input
(3) C812, power amp filter
(4) C812, power amp feedback
(8) C812, power amp feedback 100 uF
el mM all capacitors 15V; pp (polypropylene), ps (polystyrene), el (Sprague type 672) 14F PP 2uF pp
0.01uF ps 1,000 uF el B,N 15uF el mM
Resistors
(4) power amp input
(28) capacitor isolation
10 all resistors 0.25-W metal film
1M-ohm
0 various interconnect (3 ft. 2P24 cable w/shield)
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Next, I removed the four large 1,000uF power amp output coupling caps and C819/820 to avoid destructive feedback around the power amp. This modification allows the buffer IC to drive the connecting cable to the external amp with the lowest possible output impedance. I have discovered through modifying other equipment that this invariably results in the best sound and the lowest noise. In this case you must bypass the front/rear fader control. If you wish to keep the fader operational, replace C819/820 with polypropylene caps and pick up the signal where the four-conductor cable connects to the small board on the back of the tone controls. Disconnect this cable after you have installed the jumper to the radio output. The fader places a 10k variable resistor in series with the IC output and significantly degrades the sound.
Premium Amp
Each of the four channels in the premium power amp has its own circuitry, sharing only a chassis, power supply and heatsinks. Thus, you must make the next modifications four times.
The power amp uses electrolytic caps, two 100uF and one 10 uF, for feed back in each channel. These are poor quality and appreciably degrade the sound. Replace them with Sprague 672 electrolytic capacitors, which have low resistance and inductance and sound comparable to polypropylene types in most applications. Sprague does not offer a 10uF so substitute a 15uF. Use isolation resistors per Fig. 1. Replace the 2,200uF power supply filter caps with Sprague 1,000uF types to assure low impedance over a wide frequency band. Numerous locations within the amp use zero ohm resistors to connect one trace with another. Replace them with wire jumpers to ensure they are not degrading the sound.
The input circuit shown in Fig. 2 is intended to accept the signal from the speaker output of the radio. You can greatly improve the circuit if it is driven only by the low level signal from the buffer IC. The revised component values are shown in parentheses. The higher load impedance at the power amp input allows the use of an excellent quality polystyrene film coupling cap rather than an electrolytic.
The disadvantage of this circuit con figuration is a voltage transient, causing a sharp pop when the amp is turned on. Because the input signal level from the radio to the amp is now much lower, it is more sensitive to noise pickup.
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PART LOCATION GUIDE
Photo 2 Location Description
A regulator output bypass cap
470uF C812 1C801 IC551 C813 c814 c810 C811 c819, c820 1,000 uF output coupling caps 4-conductor cable at fader output feedback caps (2) 100 uF and (1) 10uF 2,200 uF filter caps input circuit (Fig. 2)
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The original wire harness connecting the radio and amp is simple unshielded wire with signal and unfiltered power lines bundled together. Disconnect the signal lines from the harness at each end and reconnect with a shielded twisted pair as protection from the noisy electrical environment in the car.
Ground the shield at both ends and solder a heavy braided copper ground strap from the power amp chassis to the ground strap provided on the radio.
Results After investing about $50 in parts and many hours of work, I put the radio back together and tested it. The sound is greatly improved. The bass is clear, well defined and extends as far as can be expected given the size of the speakers.
Highs are surprisingly extended given the lack of a separate tweeter. Normally, high-frequency response is limited only by the quality of the radio broad cast. The stereo image is better defined.
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FIGURE 2: The input circuit accepts the signal from the radio's speaker output; replacement values indicated (xx).
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Much more detail is revealed in a quality broadcast and the depth is surprising. You can clearly hear the difference in sound quality between various stations depending on their source material, compression, and so on. When driving you become aware of subtle changes as the automatic tuning features of the front end work to minimize noise and multipath distortion. The quality of the incoming signal normally limits the quality of the sound.
There are probably millions of this model Ford on the road, and many other car radios that share similar design features. They will no doubt respond equally well to tweaking. Many people spend more hours listening to their car stereo than their home unit.
Why not enjoy good sound in both places?
SOURCES
Roden Electronics 205 Oklahoma Ave.
Knoxville, TN 37901 (615) 546-8755 (film resistors)
Shields Electronics 4722 Middlebrook Pike Knoxville, TN 37921 (615) 588-2421
(film resistors, polystyrene caps)
Newark Electronics 6500 Papermill Rd.
Knoxville, TN 37919 (615) 588-6493
(Sprague capacitors, polystyrene capacitors, interconnect cable)
Old Colony Sound Lab PO Box 243 Peterborough, NH 03458 (603) 924-6371
(polypropylene caps, film resistors, interconnect cable)
Madisound 8608 University Green PO Box 4283 Madison, WI 53711 (608) 831-3433
(large value polypropylene caps--greater than 1uF)
Radio Shack (interconnect cable)
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Also see:
A MULTI-TONE INTERMODULATION METER, PART 1