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Old 10th September 2012, 06:31 PM   #1
maylar is offline maylar  United States
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Default 12V chip amp distortion

Typical specs I see for car receivers is 1% THD at rated power. I'm wondering what the major contributor is to that, and what they might be at higher load impedances. For instance, if the speaker outputs were used as a high level signal source (maybe 200 ohms load) with no speaker attached, would the THD still be 1% or would it be less? Anybody have an idea?
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Old 10th September 2012, 06:43 PM   #2
DF96 is offline DF96  England
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Speaker outputs are designed to produce power, and so compromise on other things, so why would you choose to take a signal from there unless you wanted power?

Distortion comes from many sources, including the BJT exponential response. Negative feedback reduces this, but it can't work magic so when the circuit is nearing its limits the feedback correction begins to run out of steam.

Using a higher impedance is likely to lead to a bit less distortion, but how much less depends on the circuit details.
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Old 10th September 2012, 10:00 PM   #3
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well.. a higher load impedance will only reduce disorsion if the amplifier is not clipping allready.
Apart from that, larger load impedance will result in less powerdemand from the amplifier and will reduce distorsion, but allso available output power. But.. it will never be even close to stuff made for line level use.
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Old 10th September 2012, 10:15 PM   #4
maylar is offline maylar  United States
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Originally Posted by Arty View Post
well.. a higher load impedance will only reduce disorsion if the amplifier is not clipping allready.
Apart from that, larger load impedance will result in less powerdemand from the amplifier and will reduce distorsion, but allso available output power. But.. it will never be even close to stuff made for line level use.
Most commercial car audio amps provide high level inputs designed to accept standard speaker level signals. It's commonly used when connecting a sub woofer amp to a stock radio that has no preamp outputs. You can also use them for hooking up an amp for the main speakers, but if 1% THD is what to expect, that might be objectionable. I'm trying to figure out how much difference there'd be between that and the RCA preouts that come off before the power amp.

Most of the modern CD receivers say they have MOSFET speaker drivers. Anybody know the typical chip amp that's used today?

Thanks
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Old 10th September 2012, 10:26 PM   #5
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What the figure really means is that at 1% THD the rated power is...

If you keep the unit from clipping and connect a high impedance load the distortion will be much lower, quite acceptable. Note though still not as good as a proper line level feed.

What you have to watch out for with car headunits, especially the modern ones with higher outputs, is that the outputs are bridged. So you cannot take one of the wires as ground or you will damage both the headunit and whatever you are plugging it in to.
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Old 10th September 2012, 10:55 PM   #6
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stick with line level.
if the unit has no rca out, then hack your way to the headunit chipamp and tap the signal there.
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Old 11th September 2012, 04:35 PM   #7
maylar is offline maylar  United States
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Originally Posted by richie00boy View Post
What the figure really means is that at 1% THD the rated power is...
Understood. It's compliance with the CES standard. Researching 12V chip amps I found one with specs like 10% THD @ 25W (bad clipping), 2.5% @ 18W and 0.15% @ 10W. So it's definitely output power related.

Quote:
If you keep the unit from clipping and connect a high impedance load the distortion will be much lower, quite acceptable. Note though still not as good as a proper line level feed.
Head unit preouts never list a distortion spec. Everyone seems to agree that it'll be less than speaker outputs but there's no emperical evidence of that. I suspect that if you push the line outputs to their limits they won't be much better. If only I had a distortion meter...

Quote:
What you have to watch out for with car headunits, especially the modern ones with higher outputs, is that the outputs are bridged. So you cannot take one of the wires as ground or you will damage both the headunit and whatever you are plugging it in to.
Being bridged has advantages - not being ground referenced eliminates the dreaded ground loop issue which can cause alternator whine. The low source impedance and high voltage (about 8V) makes them less susceptable to noise pickup. And you can use twisted shielded pairs instead of expensive RCA cables. The only down side I see is the distortion question.. which why I inquire here.

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Originally Posted by Arty View Post
stick with line level.
if the unit has no rca out, then hack your way to the headunit chipamp and tap the signal there.
Bad idea. Tapping into a high impedance low level source is begging for noise problems.
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Old 11th September 2012, 05:56 PM   #8
AndrewT is offline AndrewT  Scotland
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Originally Posted by maylar View Post
............... you can use twisted shielded pairs instead of expensive RCA cables. ............
I would expect a shielded single core to be cheaper than a shielded twisted pair.
But, if buying competent copper based cables then both are relatively cheap.
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Old 11th September 2012, 07:22 PM   #9
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Originally Posted by maylar View Post
Being bridged has advantages - not being ground referenced eliminates the dreaded ground loop issue which can cause alternator whine. The low source impedance and high voltage (about 8V) makes them less susceptable to noise pickup. And you can use twisted shielded pairs instead of expensive RCA cables. The only down side I see is the distortion question.. which why I inquire here.
Absolutely. The trouble is no car amps have a balanced input.

The reason why output stages used as line stages are not quite up to a proper line stage is that of poorer behaviour around the crossover region.

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Originally Posted by maylar View Post
Bad idea. Tapping into a high impedance low level source is begging for noise problems.
Correct.
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