Auto amplifier diagnostics, output very low

[peikis]

Hi,
I'm testing an amplifier by connecting it to an ATX power supply. My audio source is an iPhone and the load is a 50W car speaker (small, usually mounted in door). The problem is, that the output volume is very low (unconnected iPhone plays louder). Besides that the sound is distorted, especially low frequencies. I tried changing source to laptop, but the result is the same. I would appreciate any thoughts on troubleshooting this device. I also may add more info if needed.

Photos:

 

[(*steve*)]

First thing to do is to check the output voltage from the ATX power supply.

They are designed to have a significant load on the lower voltage rails and can behave strangely without it.

 

[Six_Shooter]

If you're doing this for educational reasons, have at 'er.

If you're wanting to repair this amp to actually use, I'd not bother. It's an off brand, that is not worth the effort, and can likely be replaced for less or the same as what the repair may cost (accounting for time and parts). I've also found that sometimes the parts used in these amplifiers are hard/impossible to get to find.

You may find that you need something other than an ATX power supply to test the amplifier as well, while rating indicate that it should at least turn on and play at low volume, some amplifiers require a lot of stand by current that eats up the output of the ATX supply, and leaves no head room to actually play anything.

Once you are satisfied with a good power source, you need to start by checking internal rail voltages of the amplifier, to make sure the power supply side is working, which is a usual suspect in car audio amplifiers.

 

[peikis]

Thank you for the tips. I will come up with the results when I come home from work.

 

[jcurrie]

what is the out put voltage of the amp could be of a higher voltasge than the 50w speaker is rated at as you are driving a 50w speaker with a 200/400 watt amp over driving the speaker would account for distortion of sound.

 

[Six_Shooter]

what is the out put voltage of the amp could be of a higher voltasge than the 50w speaker is rated at as you are driving a 50w speaker with a 200/400 watt amp over driving the speaker would account for distortion of sound.

No it wouldn't.

IF the amplifier were at full power, which I doubt is anywhere near 400 W or even 200 W, then there might be a bit of audible mechanical distortion.

I've driven speakers with much more than their rated power, and sounded great, because the amplifier driving those speakers was kept away from clipping.

In the case of the OP, the amplifier is hardly putting out any power, so thermal and mechanical power handling of the speaker is not a concern.

This does bring up one question though, is the speaker a known good speaker?

 

[peikis]

I did some measuring:
Voltage between GND and +12V contacts:
Remote off (amp off) - 12.3 V, remote on, no speakers - 12.2 V, speakers connected, volume minimum - 12.2V, volume maximum - 12.2 V.
Current:
volume minimum - 0.3 A, volume maximum - 0.3 A.
Rated current on power supply - 18 A. Tried using a MeanWell 12 V 2.2 Ah battery instead, but the current and even the voltages are the same. The speakers were tested on a car radio and played well. I am sure I did not overdrive the speakers. When i started testing this amp (thought that it is working) volume was down and i was careful turning it up. I also noticed that one of the channels is playing a bit louder. But i think that the main problem is low current usage.

 

[Six_Shooter]

How are you measuring negative voltage on the remote terminal?

 

[shrtrnd]

Impedance matching issue?

 

[peikis]

I guess my punctuation was wrong in the last post. My dashes were understood as minuses. All voltages were positive and measured between GND and +12V terminals during different states.
I will be able to try different impendance speakers next week.

 

[Arouse1973]

Do you think you might not have enough drive from the iPhone? Car amps plug into the back of car stereos don't they. The output of a common car stereo would be something like 25W per channel max. You might find you need to amplify the signal first. Driving a set of 32 Ohm headphones is miles away from driving a 400W amplifier.
Adam

 

[Six_Shooter]

*sigh*

Please people, if you don't understand automotive audio systems, don't speculate.

I've worked as a professional car audio install tech for 20 years, longer as a hobby.

The impedance of the speaker will be fine.

Car audio amplifiers are designed to drive a range of loads between 2 and 8 ohms nominal, some will go as low as .5 ohm and most will continue to work fine up to about 16 ohms, just the output will be greatly reduced over the typical 4 ohm load that most full range car audio speakers are designed to have.

The iPhone has more than enough output to drive the high Z inputs (AKA "Low-level" or "RCA") of the amplifier. For many years the output of a typical car audio head unit on the RCAs was .5 volt or less, and started to be increased in the mid '90s to upwards of 4 volts with a few exceptions being 8 volts, but the amplifier had to be able to accept such high inputs. The iPhone is capable of upwards of 2 volts on the headphone outputs into a 1k ohm load IIRC, I might be a bit off on the load impedance but suffice it to say the iPhone is capable of driving the input. The output rating of the amplifier has ZERO to do with what is required to drive the inputs.

That's the problem with dashes, they can get confused for indication negative. I will tend to use a colon ":" if I need to have some punctuation there.

 

[(*steve*)]

Driving a set of 32 Ohm headphones is miles away from driving a 400W amplifier.

Indeed it is.

The 32 ohm headset would require more power.

 

[Arouse1973]

*sigh*

Please people, if you don't understand automotive audio systems, don't speculate.

I've worked as a professional car audio install tech for 20 years, longer as a hobby.

The impedance of the speaker will be fine.

Car audio amplifiers are designed to drive a range of loads between 2 and 8 ohms nominal, some will go as low as .5 ohm and most will continue to work fine up to about 16 ohms, just the output will be greatly reduced over the typical 4 ohm load that most full range car audio speakers are designed to have.

The iPhone has more than enough output to drive the high Z inputs (AKA "Low-level" or "RCA") of the amplifier. For many years the output of a typical car audio head unit on the RCAs was .5 volt or less, and started to be increased in the mid '90s to upwards of 4 volts with a few exceptions being 8 volts, but the amplifier had to be able to accept such high inputs. The iPhone is capable of upwards of 2 volts on the headphone outputs into a 1k ohm load IIRC, I might be a bit off on the load impedance but suffice it to say the iPhone is capable of driving the input. The output rating of the amplifier has ZERO to do with what is required to drive the inputs.

That's the problem with dashes, they can get confused for indication negative. I will tend to use a colon ":" if I need to have some punctuation there.

Please don't sigh, it's very rude. You may have been installing car audio for twenty years or so but a lot of us have been designing electronic circuits for a lot longer. Without detailed information on the exact amplifier we can only throw things into the pot as suggestions, they are called ideas. Audio amplifiers have a large range of audio inputs requirements which may effect what you can plug into it. OK car amps might take 250mV to 6V input range but if there is not enough current to drive the input then it won't work. We don't know everything about all electronics but if we didn't suggest something to try and help people then a lot of posts would go unanswered.
Regards
Adam

 

[Six_Shooter]

I would say that people guessing at possible problems and sending the OP on goose chases is worse than no answers at all.

If you really have been designing amplifiers for over 20 years, then you would understand how they work a lot clearer than some of the guessing going on in this thread is suggesting. I have seen a SMALL range of requirements for the inputs to amplifiers, especially in the unbalanced high Z input segment. Small current, and a little voltage has been all that has been needed in most cases, the only exception is record players that tend to need pre-amplifiers. Any device that will drive headphones will drive the high Z inputs of an audio amplifier.

To the OP, when I said measuring the voltage rails, I meant internally, not just the power supply. This is where you're going to need to get to know amplifier design and understand which voltages are going to need to be where, or should be where in the amplifier, which parts are power supply, which are pre-amp, and which are the power amplifier sections. Being an off brand I can't see finding schematics to be very easy, though the design does look simplistic enough.
It would also help you to understand how some of the individual devices work, and understand what you should be reading when probing the circuits with a DMM and/or oscilloscope. Some devices will fail without any physical indication that they are failed.

 

[Arouse1973]

I disagree Six_Shooter. If the amplifier has an input low pass filter which many do and this has become faulty then if he checked the input signal and found it to be low and this was the cause of the fault. What wild goose chase is this. Fault finding is a logical process starting at the input and working your way through the circuit so if one of our comments just made him check the input and he finds the fault the job done.
Regards
Adam
P.S I didn't say I had been designing audio amplifiers for twenty years, read the post again.

 

[(*steve*)]

Given that this amplifier is known faulty, I'd be suspecting an output or power supply fault before an input fault.

Perhaps the OP can try connecting the ipod to the input in parallel with some headphones. If the sound through the headphones is not significantly reduced in volume or distorted then you can at least be satisfied that the input is not overloading the ipod with the distortion coming from that.

 

[Arouse1973]

Good helpful advice steve. I wish others would follow suit. This is why we are here to offer advice which may sometimes be incorrect, we are all here to learn.
Regards Adam

 

[Six_Shooter]

Steve said the same thing I have, other than creating a check for the input that in my experience has never been an issue. There's not much to go wrong in the input section.

Your previous post shows even more that you don't know how an amplifier works, or what goes wrong in them, proving my point, sending the OP on goose chases instead of pointing him to areas that are known to be faulty, and instructing him on ways to diagnose. Amplifier circuits are the basics of electronics design, well besides the simple timer and LED control circuits (many of which use the principle of amplification to work).


To the OP what tools do you have to use?

 

[Arouse1973]

Given that this amplifier is known faulty, I'd be suspecting an output or power supply fault before an input fault.

Perhaps the OP can try connecting the ipod to the input in parallel with some headphones. If the sound through the headphones is not significantly reduced in volume or distorted then you can at least be satisfied that the input is not overloading the ipod with the distortion coming from that.

Why do you say that Steve. It could be anything surely.
Regards Adam