Hi there,
Steve
My life was going along OK, when WHAM I was in a "minor" car accident - which resulted in my and my passenger's significant high-frequency hearing loss when all of the explosive air bags and pretentioners went off. Seeing the dreadful state of hearing aid "technology", I set about to design a MUCH better unit. My concept is NOT something you put in or wear on your ear, but rather a small unit you carry in your pocket or put on a table, to "fill in" for sounds you and those around you are unlikely to be able to hear.
At this moment in future history, I just got a breadboard prototype "working", well sort of, if you ignore the unintended crap my oscilloscope shows is going on inside. Since this is fundamentally an audio project now incorporating an LM1875 to drive one inch speakers (talk about overkill), and some of you guys have experience with such things, I figure it is time for others to enjoy some of the fun I have been having.
I don't come empty handed, as I have had to deep dive into various issues that have continued to vex audiophiles, like why compressors tend to put additional glitches on tall waveform peaks (because most compressors adjust to have less than unity gain for strong signals, for which their op amps have NOT been adequately compensated), and why op amps tend to ring when driving comparators and CMOS (because spec sheets lie when they say 1pF/input. Many devices have considerable additional "virtual capacitance" on their inputs when they are powered up, like an additional 30pF or more, which can be enough to make an already loaded op amp ring).
I am retired now, but my former "day job" was as a physics trained high-tech consultant mostly employed to figure out how to make computer and electronic things work where others had previously failed. I have had >100 such projects.
So, please, join me on my adventure to recapture my full hearing abilities. Someday you may need one of these hearing aids, and with your help the unit you purchase might be a little better than it otherwise would have been. The market for this is truly astronomical, as most of the population of Ukraine and around half of the elderly U.S. population really needs this.
At this moment in future history, I just got a breadboard prototype "working", well sort of, if you ignore the unintended crap my oscilloscope shows is going on inside. Since this is fundamentally an audio project now incorporating an LM1875 to drive one inch speakers (talk about overkill), and some of you guys have experience with such things, I figure it is time for others to enjoy some of the fun I have been having.
I don't come empty handed, as I have had to deep dive into various issues that have continued to vex audiophiles, like why compressors tend to put additional glitches on tall waveform peaks (because most compressors adjust to have less than unity gain for strong signals, for which their op amps have NOT been adequately compensated), and why op amps tend to ring when driving comparators and CMOS (because spec sheets lie when they say 1pF/input. Many devices have considerable additional "virtual capacitance" on their inputs when they are powered up, like an additional 30pF or more, which can be enough to make an already loaded op amp ring).
I am retired now, but my former "day job" was as a physics trained high-tech consultant mostly employed to figure out how to make computer and electronic things work where others had previously failed. I have had >100 such projects.
So, please, join me on my adventure to recapture my full hearing abilities. Someday you may need one of these hearing aids, and with your help the unit you purchase might be a little better than it otherwise would have been. The market for this is truly astronomical, as most of the population of Ukraine and around half of the elderly U.S. population really needs this.
Steve
I'm sorry to hear that. I've learned that there's no such thing as a minor car accident. Some douche decided to run a red light and plough into the driver's side of my car while I was driving it. Working through the concussion and now whiplash has been pretty insightful. But at least I didn't lose any of my hearing (that I know of). That would have compounded an already sucky situation.
Anyway. If you're going for something that plugs into your ears, the LM1875 wouldn't be my first choice. I'd probably look at the OPA1622. But that's a topic for a different thread.
Welcome to diyAudio.
Tom
Tom,
Steve
As I explained in my original posting, this goes in your pocket or on a table, so the OPA1622 probably won't make enough noise from there to do the job - which is to make noises, like a "sh" sound about as loud as you can possibly make it with your mouth. My design doesn't put out anything below 400Hz, and little below 800Hz, so it won't take much of a speaker to do the job. I am now experimenting with two 28mm diameter speakers. With little cone movement at these frequencies, the primary limitation of speakers will be cone deformation and heating.
A theoretical problem that concerns me is that naturally occurring sounds won't drive a speaker to its limits, but this is making artificial sounds that likely could drive a speaker to its limits. I guess I will just have to complete this project and find out if this is even a real-world issue.
However, my primary present problem is designing a stable audio output stage that incorporates basics, like upper and lower bandpass limitations. It appears that anything I do in the feedback of common audio output chips like the LM1875 to limit the bandpass reduces the gain below 20 at the bandpass limits, and throws the chip into oscillation. Apparently, I am supposed to do this with passive components BEFORE the signal hits the LM1875?
Also, I can't find much of anything on the electrodynamics of speakers - especially tiny Chinese 28mm speakers. How can anyone simulate an audio output without a SPICE model of their speakers.
Addressing other related threads about LM1875 chips, people have complained about a "thunk" on turn on. Wouldn't this be solved by connecting the audio output chips directly to the speakers, and connecting the other side of the speakers to two capacitors - one to Vss and the other to Vdd? This would form a capacitive voltage divider that would remain at about the same average voltage as the chip's output, and would eliminate the turn on and turn off glitches. Right?
Anyway, like others here, my LM1875 project "works", despite oscillating at ~170kHz. I am about to rip up my breadboard and put bandpass limitations in BEFORE the chip instead of in the feedback. You'd think adding a compensation cap would solve this problem, but I tried one and it didn't work.
Why doesn't SOMEONE put out an audio amp that works just like an op amp that is compensated all the way down to unity gain? This would save SO many designers from baldness due to pulling out their hair.
Part of the problem with this project is that there is SO much that is unknown about the real world in which it is to interact. How much power is needed? Where are the upper boundaries of voiced speech and the lower boundaries of unvoiced speech? As a result, I have done lots of experimentation, e.g. making noises with a spectrum analyzer on my left and a storage oscilloscope on my right.
A theoretical problem that concerns me is that naturally occurring sounds won't drive a speaker to its limits, but this is making artificial sounds that likely could drive a speaker to its limits. I guess I will just have to complete this project and find out if this is even a real-world issue.
However, my primary present problem is designing a stable audio output stage that incorporates basics, like upper and lower bandpass limitations. It appears that anything I do in the feedback of common audio output chips like the LM1875 to limit the bandpass reduces the gain below 20 at the bandpass limits, and throws the chip into oscillation. Apparently, I am supposed to do this with passive components BEFORE the signal hits the LM1875?
Also, I can't find much of anything on the electrodynamics of speakers - especially tiny Chinese 28mm speakers. How can anyone simulate an audio output without a SPICE model of their speakers.
Addressing other related threads about LM1875 chips, people have complained about a "thunk" on turn on. Wouldn't this be solved by connecting the audio output chips directly to the speakers, and connecting the other side of the speakers to two capacitors - one to Vss and the other to Vdd? This would form a capacitive voltage divider that would remain at about the same average voltage as the chip's output, and would eliminate the turn on and turn off glitches. Right?
Anyway, like others here, my LM1875 project "works", despite oscillating at ~170kHz. I am about to rip up my breadboard and put bandpass limitations in BEFORE the chip instead of in the feedback. You'd think adding a compensation cap would solve this problem, but I tried one and it didn't work.
Why doesn't SOMEONE put out an audio amp that works just like an op amp that is compensated all the way down to unity gain? This would save SO many designers from baldness due to pulling out their hair.
Part of the problem with this project is that there is SO much that is unknown about the real world in which it is to interact. How much power is needed? Where are the upper boundaries of voiced speech and the lower boundaries of unvoiced speech? As a result, I have done lots of experimentation, e.g. making noises with a spectrum analyzer on my left and a storage oscilloscope on my right.
Steve