Hi guys. I'm trying to modify an amplifier. I'm definitely a beginner. So, I started by increasing the bias to 10mv across the emitter resistor. But, It doesn't make any noticeable difference in the FFT measurement. I bypassed the entire preamp board. Sending the signal to pins 5/6/8/9 on page 9. It will be actively powering 4" mids and tweeters. So, I don't need much more than 10-20w.
I've tried lowering the value of R145 as well as R121. It does seem to have excessive gain. It's a car audio amplifier and the DSP has up to 4v of output. So, I'd like to make use of it and have a low noise floor. I've also replaced the sanken 2sa1303/2sc3284 with MJL0302/MJL0281 because they were out of spec. I matched the Beta of the devices within 1 or 2. I also replaced the rail caps with MKP Nichicon and all small value caps with WIMA and all through hole resistors with 1% or less low PPM value. All electrolytic caps with polymer. Emitters with Ohmite etc.
I'm trying to reduce the high-frequency distortion. Attached a multiband measurement. It measures nearly exactly the same with 0mv across the emitter and 10mv and 15mv.
Should I experiment with local feedback? I've been told that it probably has too much gain in the front stages.
Any tips where to go from here?
I've tried lowering the value of R145 as well as R121. It does seem to have excessive gain. It's a car audio amplifier and the DSP has up to 4v of output. So, I'd like to make use of it and have a low noise floor. I've also replaced the sanken 2sa1303/2sc3284 with MJL0302/MJL0281 because they were out of spec. I matched the Beta of the devices within 1 or 2. I also replaced the rail caps with MKP Nichicon and all small value caps with WIMA and all through hole resistors with 1% or less low PPM value. All electrolytic caps with polymer. Emitters with Ohmite etc.
I'm trying to reduce the high-frequency distortion. Attached a multiband measurement. It measures nearly exactly the same with 0mv across the emitter and 10mv and 15mv.
Should I experiment with local feedback? I've been told that it probably has too much gain in the front stages.
Any tips where to go from here?
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Car amplifiers are the least suitable for extensive and meaningful improvements firstly due to limited space , thermal coupling, etc...Kenwood had higher quality amplifiers even in the car amp range so you'd better look for those amps .Other than that all big companies used to run cost down courses to make designs that were just good enough for the job.That's how planned obsolescence appeared as main marketing tool.Kenwood made the best audio amplifiers but they were sold in different packages before moving the entire design department to Accuphase.Just look for Kenwood L-02 A schematic and try to imagine how you get those components inside your car amp...In 1980 they were the kings and queens of the lowest thd arena...but that came at a price and in a big heavy package...
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Dude. I already upgraded every part inside this amplifier with high-quality components.
Just because I won't be able to match the performance of an L-02. That doesn't mean it's worthless to try and improve this design for this application. This is poor logic.
Just because I won't be able to match the performance of an L-02. That doesn't mean it's worthless to try and improve this design for this application. This is poor logic.
Instead of trying to lecture me about wasting my time. Why don’t you help me with what I ask. And I can decide whether or not it’s a waste of time?
This will help me learn.
I know car amplifiers can’t compare with home amplifiers.
But I don’t understand why it’s pointless to try to improve them.
Tiny case, ** heat sinks. By 1980, products were designed to fail in twice the warrenty period. Same market strategy as powered speakers today. , , , . Forum is leaving out the expletives this ** deserves. Hot rodding starts with adequate thermal management. Some of my house amp projects based on PA amps have half the case empty. Move this discussion over here where it belongs. https://www.diyaudio.com/community/forums/car-audio.38/
Here we get car amps for $6, based on the Chinese versions of the 4440 chip.
So modifying an old amp, with known design issues, is really a waste of time.
To increase output, or for better thermal management, bigger or more complex heat sinks are needed. They will be expensive, and / or hard to fit.
You do not have space, and the way to go will be either reduce output, or use a bigger housing, which then may or may not fit your car.
Car boot amp, with your own housing, yes.
Cooling fan, yes.
But not in the dashboard.
You can gut it, and use the parts for an indoor amplifier, that effort is also hard to justify, you can build a new one for similar effort, and less constraints of matching supply, mountings and so on.
If you need a car amp, look for known good ones, for sale on line, or at car audio shops...some may have upgraded, and the old ones may be available for a low price.
Or at car breaker yards...you might luck out.
How good are your speakers, will this upgrade be audible?
So modifying an old amp, with known design issues, is really a waste of time.
To increase output, or for better thermal management, bigger or more complex heat sinks are needed. They will be expensive, and / or hard to fit.
You do not have space, and the way to go will be either reduce output, or use a bigger housing, which then may or may not fit your car.
Car boot amp, with your own housing, yes.
Cooling fan, yes.
But not in the dashboard.
You can gut it, and use the parts for an indoor amplifier, that effort is also hard to justify, you can build a new one for similar effort, and less constraints of matching supply, mountings and so on.
If you need a car amp, look for known good ones, for sale on line, or at car audio shops...some may have upgraded, and the old ones may be available for a low price.
Or at car breaker yards...you might luck out.
How good are your speakers, will this upgrade be audible?
You can use this or another housing to build a Class D amp, they heat up less.
Audio quality will depend on so many factors, read up.
But while changing transistors and emitter resistors and so on might bring it back to spec, you are still stuck with what made it troubling in the first place.
Kenwood were decent in the first place, this model may be a bad one.
In India, Kenwood, Sony and Pioneer were the leaders in car audio, now most cars come with the system built in...only after market sales now.
Few people remove the existing units. And with integration, same screen for many functions, that is getting more difficult.
Rear view, A/c, radio, maps and so on on the same screen, or different parts of the same screen.
Audio quality will depend on so many factors, read up.
But while changing transistors and emitter resistors and so on might bring it back to spec, you are still stuck with what made it troubling in the first place.
Kenwood were decent in the first place, this model may be a bad one.
In India, Kenwood, Sony and Pioneer were the leaders in car audio, now most cars come with the system built in...only after market sales now.
Few people remove the existing units. And with integration, same screen for many functions, that is getting more difficult.
Rear view, A/c, radio, maps and so on on the same screen, or different parts of the same screen.
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Sometimes letting go is better , most of the time learning a tough lesson seems to be the best...so good luck!I was wrong.You need to work hard and harder at improving this amp.Seems there's no other way...I don't know how to improve that amp.
Oh my god you guys. Stop with the worthless replies.
I'm asking for help. With THIS amplifier. It has to be THIS amplifier.
I don't want advice about using Class D modules. Or using a home amplifier in the car.
If you look at the provided multiband measurement. Distortion is near -110db in the midrange. 1k fundamental third harmonic is 0.009%. This measures really well compared to most other car amplifiers.
My home Emotiva Class D monoblocks are only -80db throughout the entire spectrum.
I'm simply asking for advice on how to reduce the high-frequency distortion. If I can experiment with adding feedback in different stages of the amplifier to lower the gain in the front end of the amplifier. How do I do that? I've been reading about simply adding a resistor to loop the output back into the input. Can I try this without harming anything?
Why does it measure the same with zero bias and alot of bias and everywhere in between?
This does have a huge heat sink and 3 fans and it measured better before I even started on it than new $1000+ amps.
I'm asking for help. With THIS amplifier. It has to be THIS amplifier.
I don't want advice about using Class D modules. Or using a home amplifier in the car.
If you look at the provided multiband measurement. Distortion is near -110db in the midrange. 1k fundamental third harmonic is 0.009%. This measures really well compared to most other car amplifiers.
My home Emotiva Class D monoblocks are only -80db throughout the entire spectrum.
I'm simply asking for advice on how to reduce the high-frequency distortion. If I can experiment with adding feedback in different stages of the amplifier to lower the gain in the front end of the amplifier. How do I do that? I've been reading about simply adding a resistor to loop the output back into the input. Can I try this without harming anything?
Why does it measure the same with zero bias and alot of bias and everywhere in between?
This does have a huge heat sink and 3 fans and it measured better before I even started on it than new $1000+ amps.
They're Scanspeak Beryllium tweeters and Revelator mids/midbass. I also upgraded the opamps and all caps in the DSP and it's -120db of distortion with a multiband measurement.
It's a car amplifier. It's not in the dashboard???
There's nothing wrong with it. I'm trying to improve it. It has a huge heat sink and 3 fans!
Why does this belong in that section. I'm asking if anyone can offer advice on the output section of this amplifier. It's not car audio specific.
I'm asking for advice on improving this. Possibly with more feedback.
Why is this so difficult to ask?
Assuming you are reading 10mV across an emitter resistor of about 0.33ohm (I guess), you only get a bias of 30mA with is correct on the low side for a class AB amp.
What was the default bias value?
What was the default bias value?
It’s a 0.1ohm emitter resistor.
Default is only 2.5mv.
The original configuration only allowed 0mv to 3mv. Now I have significant adjustment which was needed for the MJL0302/0281 outputs.
I changed R143/133/243/244 to 420 ohm to get enough adjustment to increase the bias.
Default is only 2.5mv.
The original configuration only allowed 0mv to 3mv. Now I have significant adjustment which was needed for the MJL0302/0281 outputs.
I changed R143/133/243/244 to 420 ohm to get enough adjustment to increase the bias.
Initially I posted the wrong measurement.
Even with the preamp board completely removed. It still has significant gain.
On two channels I’ve changed the R145/R146 feedback resistor from 43k to 10k and gain is a little more manageable.
That's with a 4 ohm resistive load. I can increase the bias enough to max out my 10A current limited supply.
If I use a 1k stimulus. Increasing the bias will help slightly with high-order harmonics. But they barely appear at all.
But, Any other amplifier I've worked on. 0 bias is super bad. 10mv makes an insanely huge difference (Most car amplifiers use a 0.1ohm emitter with single pair)
I'm wondering how I can try decreasing the amount of gain at different stages of the amplifier and how to try local feedback at each stage.
It has very low distortion in the low frequencies. So, I'm wondering if because feedback comes from the main output and it has a few stages. That the feedback is too slow to be completely inverse phase from the main output. So maybe local feedback might help?
You could put a multi turn preset, and adjust for ideal settings, replace later with a 1% metal film.
And look it up, slightly different transistors substitutions can have their own issues.
If it is on your bench, try a different DSP, maybe your op amp change was not quite suitable.
Basically, you have modified all the stages, now you will have to locate the issue, and match the stages.
In my car, the FM module has to be set at 60%, otherwise it drives the amp into clipping, as I have small speakers in the dashboard. Your issue could be something like that.
And look it up, slightly different transistors substitutions can have their own issues.
If it is on your bench, try a different DSP, maybe your op amp change was not quite suitable.
Basically, you have modified all the stages, now you will have to locate the issue, and match the stages.
In my car, the FM module has to be set at 60%, otherwise it drives the amp into clipping, as I have small speakers in the dashboard. Your issue could be something like that.