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Distortion from low to high volumes

Doug_Dangger

Distortion is coming from the left channel of my receiver.  It can be heard from very low to high volume levels.  I swapped channel input and the distortion stays on the left side.  I tried all sorts of different audio sources and the sound is still there.

 

I found a video on youtube that says if there is dc voltage coming from the speaker terminals, something is wrong with the amp.  I hooked up my multi meter and I found 40mv from the offending channel.  The other is fine.  Anyone know of how to troubleshoot these?  I found recap kits on ebay and they cost anywhere from $80 - $200.  A new stereo receiver nowadays run for about $150 so I don't know if it's worth the trouble.  It's a Harman Kardon 120watt receiver from 15 years ago.

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40 mV is not going to be a problem. When I design power amplifiers I like to shoot for <10 mV, but that's as much an academic exercise as anything. Really good amps like the MC2 MC series will typically have <3mV. Some amps don't have an adjustment for it, so the DC offset is dependent upon the matching of the transistors in the input pair.

 

Even 100 mV of DC offset won't really cause a major problem, though it definitely isn't ideal.

 

A distorted output can be caused by any number of things. You didn't specify a specific model number, but if it has preamp outputs, check to see if the distortion is still present there. You want to figure out what stage of the receiver is the problem. I can tell you that if it's a fault in the power amp stage, it's probably relatively minor since a major fault would probably have put DC on the output (and the amp wouldn't come out of protect in that case). Usually a fault doesn't put 40 mV- it puts like 40 volts. 

 

The kind of distortion also affects this. Severe crossover distortion is almost always caused by something different than early clipping, for example.

 

Recapping it might fix it, or it might not. Caps are indeed a common problem, but it could also be a semiconductor issue. I would not buy a recapping kit, however. It's almost always cheaper (and you get better parts) if you make a list of what you need and buy from DigiKey. Receivers tend not to be all that hard to fix as audio equipment goes (but not if you can't get a schematic!). If you don't have an oscilloscope it's going to be a real challenge, however.

 

Lastly, check to make sure it's not your speakers. Easy to check -just swap the speaker leads- but I've seen people blame an amplifier for a problem that actually lies in the loudspeakers.

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On 2/27/2021 at 11:35 PM, H713 said:

40 mV is not going to be a problem. When I design power amplifiers I like to shoot for <10 mV, but that's as much an academic exercise as anything. Really good amps like the MC2 MC series will typically have <3mV. Some amps don't have an adjustment for it, so the DC offset is dependent upon the matching of the transistors in the input pair.

 

Even 100 mV of DC offset won't really cause a major problem, though it definitely isn't ideal.

 

A distorted output can be caused by any number of things. You didn't specify a specific model number, but if it has preamp outputs, check to see if the distortion is still present there. You want to figure out what stage of the receiver is the problem. I can tell you that if it's a fault in the power amp stage, it's probably relatively minor since a major fault would probably have put DC on the output (and the amp wouldn't come out of protect in that case). Usually a fault doesn't put 40 mV- it puts like 40 volts. 

 

The kind of distortion also affects this. Severe crossover distortion is almost always caused by something different than early clipping, for example.

 

Recapping it might fix it, or it might not. Caps are indeed a common problem, but it could also be a semiconductor issue. I would not buy a recapping kit, however. It's almost always cheaper (and you get better parts) if you make a list of what you need and buy from DigiKey. Receivers tend not to be all that hard to fix as audio equipment goes (but not if you can't get a schematic!). If you don't have an oscilloscope it's going to be a real challenge, however.

 

Lastly, check to make sure it's not your speakers. Easy to check -just swap the speaker leads- but I've seen people blame an amplifier for a problem that actually lies in the loudspeakers.

It's a Harman Kardon 4830 I think.  2 channel stereo amp.  The distortion is not from the pre amp.  I pulled the jumper plugs that bridges the pre amp with the amp and plugged in another source and the distortion is still there.

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On 2/27/2021 at 11:30 PM, Doug_Dangger said:

I found recap kits on ebay and they cost anywhere from $80 - $200.

This seems to a little bit too expensive. You will probably save at least 50% if you get the caps from a electronic component supplier. But I don't think the caps are faulty in your case.

On 2/27/2021 at 11:30 PM, Doug_Dangger said:

It can be heard from very low to high volume levels.  I swapped channel input and the distortion stays on the left side.

Finding the faulty part might be a little bit tricky. If you have an oscilloscope you could probe the different gain stages. An audio interface with a few resistors and an DC blocking capacitor could also be used. It's just guesswork, but I would probably begin with replacing the opamps, if there are some.

But don't waste too much time and effort. A used replacement might be a better option.

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On 3/2/2021 at 3:32 AM, HenrySalayne said:

This seems to a little bit too expensive. You will probably save at least 50% if you get the caps from a electronic component supplier. But I don't think the caps are faulty in your case.

Finding the faulty part might be a little bit tricky. If you have an oscilloscope you could probe the different gain stages. An audio interface with a few resistors and an DC blocking capacitor could also be used. It's just guesswork, but I would probably begin with replacing the opamps, if there are some.

But don't waste too much time and effort. A used replacement might be a better option.

 

I wouldn't even go replacing op-amps yet- it's fairly rare that they fail such that they produce a distorted output. Normally it is more catastrophic than just increased distortion. Furthermore, it sounds like the problem is in the power amp stage. There could be op-amps in it, but even if there are it's likely that the problem lies elsewhere.

 

If you don't have a schematic, you're going to have a hell of a time tracing through it, even if you do have an oscilloscope. I can be done, and I end up doing it on a semi-regular basis, but it's much, much more difficult. You need to know more about how that type of equipment typically works in order to know what to look for. 

 

If the OP is fairly new to electronics, DC coupled amplifiers are not a good place to start. They can be very tricky to troubleshoot. They also tend to be very unforgiving of mistakes- slip with a probe and it's very easy to blow every transistor in the amplifier.

 

 

 

 

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Thanks for the reply all.

I'm giving up on my receiver and I will just use it as a pre amp.  I'm getting an external amp like this.  What do you think?

 

https://www.crownaudio.com/en/products/xls-1002

 

I'm not an audiophile and I don't have $5,000 speakers.  I just want something that punches hard.  Real hard in 2 channel stereo only.

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I'm really not a big fan of the Crown XLS series amps. If you're looking to keep it cheap, then you may look at used QSC amplifiers, particularly the PowerLight series. The PLX series is a cheaper version of the PowerLight amps, but is otherwise quite similar. These things are more reliable than the Crown XLS series. They also measure and sound better.

 

I also really like some of the older Crest amplifiers, the old Pro xx01 series were some of the best amps in the world when they were new. The CA series is the same circuit design, but is a later version built in a slightly cheaper chassis. Reliability can be an issue on these, since the oldest ones date back to the early 1990s. The reliability issues do not extend to the modern Crest Pro x200 series (5200, 7200 8200, 9200), and the lab tests on them look excellent. Knowing Crest, they're probably still using the same amplifier design, just with a switching power supply to save weight.

 

If you can swing the budget, MC2 makes some of the very best power amplifiers on the market, and this is reflected by the fact that MC650s (which were released in 1994 and discontinued in the early 2000s) still fetch upwards of $800 on eBay. Reliability on MC2 amplifiers is as good as anything else and they are still made in the UK. 

 

Do keep in mind that all of the amplifiers mentioned here, including the XLS, have cooling fans. Some are quieter than others. 

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21 minutes ago, H713 said:

I'm really not a big fan of the Crown XLS series amps. If you're looking to keep it cheap, then you may look at used QSC amplifiers, particularly the PowerLight series. The PLX series is a cheaper version of the PowerLight amps, but is otherwise quite similar. These things are more reliable than the Crown XLS series. They also measure and sound better.

 

I also really like some of the older Crest amplifiers, the old Pro xx01 series were some of the best amps in the world when they were new. The CA series is the same circuit design, but is a later version built in a slightly cheaper chassis. Reliability can be an issue on these, since the oldest ones date back to the early 1990s. The reliability issues do not extend to the modern Crest Pro x200 series (5200, 7200 8200, 9200), and the lab tests on them look excellent. Knowing Crest, they're probably still using the same amplifier design, just with a switching power supply to save weight.

 

If you can swing the budget, MC2 makes some of the very best power amplifiers on the market, and this is reflected by the fact that MC650s (which were released in 1994 and discontinued in the early 2000s) still fetch upwards of $800 on eBay. Reliability on MC2 amplifiers is as good as anything else and they are still made in the UK. 

 

Do keep in mind that all of the amplifiers mentioned here, including the XLS, have cooling fans. Some are quieter than others. 

Thanks.

I looked at QSC and they're a little pricier than the Crowns.  I'd like to keep it under $400 and new as much as possible.

 

Also, I'm looking in between a class AB vs D system.  I grew up thinking that the heavier amps sound better compared to the lighter class D.  I don't know of any truth to that.

 

I know this is an odd combination but I would like to pair one of these amps with a pair of bookshelf speakers.  LOL!  They're more than enough for floor standing models but due to space and wife constraints, bookshelf speakers will have to do.  I don't want to hook it up to a sub.  I want natural sound.  I mostly listen to pop, rock, country, bluegrass and of course The Beatles.

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It depends on the implementation of the amplifier. Hypex class D modules, for example, measure and sound fantastic. Not all class D amps do. Same goes for Class A / AB / B amplifiers. 

 

If you can deal with a used amplifier, I really would look towards a used Powerlight. On the new side, the GX3 is a similar price. It's nowhere near as good as the PowerLights, but it's not a bad amplifier either and I would take it over the XLS any day of the week unless weight and efficiency was my primary concern.

 

At present, I cannot recommend any of the new Crown amplifiers. I have had very poor experiences with their reliability and average (at best) with their customer service. The QC on the newer Crown amps is not nearly what it was 20 years ago. I very rarely see QSC amplifiers in need of service- they're some of the most reliable amplifiers on the market. If you can deal with the weight, the RMX series amplifiers are some of the best things available at their price point. The RMX 850 is a little outside your budget, but I see them on the used market for very cheap.

 

The old Crown Macro-Tech series (which are COMPLETELY DIFFERENT from the Macro-Tech amps they sell now) are brutes with amazing reliability and OK sound quality, but they have the loudest fans of any amplifier I've ever seen. I have a 3600VZ (on loan to a friend right now), and it's louder than most servers. Not what you want. Still, I have forced my 3600VZ to function as a 40 kHz function generator into a heavy load, full power, for hours on end and it hasn't failed. They're probably some of the most bulletproof amps ever made. 

Edited by H713
Added some more information.
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My workhorse is a Samson Servo 120a. Something like the linked Crown XLS-1002 is completely oversized. You will get audible noise with high power amplifiers.

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20 hours ago, HenrySalayne said:

My workhorse is a Samson Servo 120a. Something like the linked Crown XLS-1002 is completely oversized. You will get audible noise with high power amplifiers.

 

That's really dependent upon the amplifier. Somer large amplifiers have a lot of gain, and that tends to result in a lot of noise. Some Crest amplifiers shipped from the factory with almost 40 dB of gain. My CA9 is like that, and I find that it's noise floor is a bit high for domestic use (though it's not awful). I suspect that this is a combination of the noise from the clip limiter (based on the NE5517- not a great way to do it IMO) and the thermal noise from the input attenuator (which always gets turned down a good amount in a domestic setting) getting amplified by 40 dB.

 

The MC2 amplifiers, however, are not insane in this regard. The MC450 has something like 30 dB of gain with a very low noise floor. The input attenuator uses a PGA based around the PMC7524 MDAC, and it is VERY quiet. The QSC PL218 has 32 dB of gain, with front-panel attenuators. Even with relatively efficient speakers you shouldn't have too much noise with amps like these. Ditto for the smaller RMX series amplifiers.

 

Now I agree that the OP does not need anywhere near this much power, but it won't hurt, and it's not unusual to need a significant amount of power to avoid clipping on transients, especially if the speakers being used are relatively inefficient. I use an MC650 with "bookshelf" speakers in a small room. Aside from the fan noise and the heat, I think it compares favorably with some very high-end home stereo amplifiers. The QSC amps aren't quite as nice, but I have seen and heard much, much worse.

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