XJ8 Audio Prep - Class D amp
#1
06-01-2016, 05:24 PM
XJ8 Audio Prep - Class D amp
As I prep components for an upgrade of the audio system on my XJ, I wanted to expound a little on another professional hobby I have had for decades, and that is audio. One of the weaker links in commercially available amplifiers, is the components used in the signal chain. Virtually every "brand name", and certainly those "other brands" all suffer from some very real and improvable upgrades to take and make them some very impressive sounding audio systems.
My system starts with a 5 channel Class D amplifier, (NVX) which internal design and boards are shared by a number of companies, both big name, as well as private and not-so-big vendors. Upon receipt, I opened the case to examine components and solder quality, and found what is ever present in auto amps, and that is signal components, like op-amps and electrolytic caps of poor quality and inexpensive price, seriously degrading the ability of the amp to produce true high-quality sound. There is a HUGE difference between noise, and being-there type of reproduction. Loud isn't better. It's just noisy.
One of these in this amp, was the use of TI's TL072 op amps for all buffering and signal conditioning. This device, produced since the 70's was one of the first JFET op-amps widely used for high-impedance input circuits, and was quickly overshadowed by the NE5532 for audio usage. This $0.37 part is best used to control a variable speed motor, and not audio, since there are numerous selections of much better audio quality. With 18nV of noise, it produces a constant hiss that is audible in the output section. And while it's slew speed is 13V/us, which is considered tabletop radio quality, the most important factor of the TL072, is the phase shift over the audio spectrum, which changes more than 120 degrees over just a few octaves. In fact, when driven at modest voltages, it can change polarity, causing 180 degree cancellation of frequencies. (phase reversal) This is very bad for your audio, in fact, it's unacceptable. Needless to say, the bean-counters, who opted for least cost, gave it to us with no regard to what is given up in audio quality. This is indicative of all amps on the market today, even in the home audio arena. (My high-end Yamaha AVENTAGE RX-A2030 even suffered from this cost cutting oversight in using low-cost poor-performing op-amps)
So what I've done is to use TI's recommended replacement for input designs that have utilized JFET op-amps, like the TL072, using the Burr-Brown OPA-1642. This op-amp brings with it a number of really impressive performance upgrades. One, comparing it to the TL072, who's THD was 0.003%, with the new, gains me a very impressive 0.00005% total THD, with less than 1/4 of the noise. It's slew rate is almost double of that of the TL072, and gets into the 20V/us slew rate, which can handle complex audio without losing detail. It takes less current to run it, and makes a good drop in replacement without having to redesign the circuit. The real advantage to this change, comes from the most important factor, and that is phase. Your ears are incredibly sensitive to phase, and audio phase is one of the most important things in creating sinusoidal waves that sound "real". The OPA-1642 is phase stable (no shift) throughout the entire audio spectrum all the way up into the MHz range. The soundstage with the new op-amps is nothing short of incredible, and places it in a performance category with some of the finest home audio out there. Channel separation went from around 90dB of separation to almost 130dB, and CMRR went from -86dB to -128dB. Talk about now having an extremely quiet, and low distortion amp. Mono blocks don't perform better than this folks.
All of the signal electrolytic caps were replaced with ELNA's SILMIC II silks, which has become the defacto-standard of audio signal caps these days, and bring with them a very warm and inviting sound.
All of this was done with a little rework time and less than $50. Now, this $250 investment can rival performance of amps 10 times it's cost. The finest investment one can make in their car audio systems, is the attention to detail in the input section of their amplifier, and since all amps suffer this, is the first place to spend attention to really create a soundstage in your vehicle that will rival any out there. It's all about making a correctly phased and uncolored sinusoidal wave for your ear. The manufacturer's bean-counter ripped you off.
Next time, I'll post on the speaker selection and prep...
My system starts with a 5 channel Class D amplifier, (NVX) which internal design and boards are shared by a number of companies, both big name, as well as private and not-so-big vendors. Upon receipt, I opened the case to examine components and solder quality, and found what is ever present in auto amps, and that is signal components, like op-amps and electrolytic caps of poor quality and inexpensive price, seriously degrading the ability of the amp to produce true high-quality sound. There is a HUGE difference between noise, and being-there type of reproduction. Loud isn't better. It's just noisy.
One of these in this amp, was the use of TI's TL072 op amps for all buffering and signal conditioning. This device, produced since the 70's was one of the first JFET op-amps widely used for high-impedance input circuits, and was quickly overshadowed by the NE5532 for audio usage. This $0.37 part is best used to control a variable speed motor, and not audio, since there are numerous selections of much better audio quality. With 18nV of noise, it produces a constant hiss that is audible in the output section. And while it's slew speed is 13V/us, which is considered tabletop radio quality, the most important factor of the TL072, is the phase shift over the audio spectrum, which changes more than 120 degrees over just a few octaves. In fact, when driven at modest voltages, it can change polarity, causing 180 degree cancellation of frequencies. (phase reversal) This is very bad for your audio, in fact, it's unacceptable. Needless to say, the bean-counters, who opted for least cost, gave it to us with no regard to what is given up in audio quality. This is indicative of all amps on the market today, even in the home audio arena. (My high-end Yamaha AVENTAGE RX-A2030 even suffered from this cost cutting oversight in using low-cost poor-performing op-amps)
So what I've done is to use TI's recommended replacement for input designs that have utilized JFET op-amps, like the TL072, using the Burr-Brown OPA-1642. This op-amp brings with it a number of really impressive performance upgrades. One, comparing it to the TL072, who's THD was 0.003%, with the new, gains me a very impressive 0.00005% total THD, with less than 1/4 of the noise. It's slew rate is almost double of that of the TL072, and gets into the 20V/us slew rate, which can handle complex audio without losing detail. It takes less current to run it, and makes a good drop in replacement without having to redesign the circuit. The real advantage to this change, comes from the most important factor, and that is phase. Your ears are incredibly sensitive to phase, and audio phase is one of the most important things in creating sinusoidal waves that sound "real". The OPA-1642 is phase stable (no shift) throughout the entire audio spectrum all the way up into the MHz range. The soundstage with the new op-amps is nothing short of incredible, and places it in a performance category with some of the finest home audio out there. Channel separation went from around 90dB of separation to almost 130dB, and CMRR went from -86dB to -128dB. Talk about now having an extremely quiet, and low distortion amp. Mono blocks don't perform better than this folks.
All of the signal electrolytic caps were replaced with ELNA's SILMIC II silks, which has become the defacto-standard of audio signal caps these days, and bring with them a very warm and inviting sound.
All of this was done with a little rework time and less than $50. Now, this $250 investment can rival performance of amps 10 times it's cost. The finest investment one can make in their car audio systems, is the attention to detail in the input section of their amplifier, and since all amps suffer this, is the first place to spend attention to really create a soundstage in your vehicle that will rival any out there. It's all about making a correctly phased and uncolored sinusoidal wave for your ear. The manufacturer's bean-counter ripped you off.
Next time, I'll post on the speaker selection and prep...
Last edited by Box; 06-02-2016 at 05:32 AM.