Questions on Car Audio Amplifier Specs

[onemilimeter]

I found this Auna 2 Channel Compact 300W In-Car Hifi Amplifier on ebay. Kindly find the specs below.

Questions:

[1] How do we correlate the "300W max." and "2 x 30W RMS" power? I thought Max. Power (Pmax) = 2 x Power in RMS (Prms)...

[2] Usually home audio amplifier do not state "low pass filter" or "high pass filter". Why do we need these specs in a car audio amplifier?

[3] Is it good to have a HIGHER or LOWER input impedance? What is the typical input impedance of an audio amplifier?

[4] Does the "Output impedance: 4 to 16 ohms" mean the output impedance of the audio amplifier or the impedance of the connected speaker?

When designing this amplifier the goal was clear, maximum power in the minimum amount of space. With 300 watts squeezed into the smallest of enclosures, this amp truly has to be heard to believed, so if you're short on space but don't want to sacrifice on sound, this is for you!

High and lowpass filters allow precise control over your sound. With robust housing and well engineered internal components, all in a brilliant compact design, this little gem wont let you down!

Highlights:
300W max. output - equivalent to 2 x 30W RMS
Adjustable high and low-pass filter
Polished silver metal case

Features:
Connections: RCA input, high input
Gold plated RCA connectors
Low Pass Filter: 40 to 250 Hz
High Pass Filter: 80 Hz to 2 kHz
Frequency response: 40 Hz to 20 kHz
Input impedance: 40 kOhm
Distortion factor: > 0.1%
Signal to Noise Ratio: > 79dB
Output impedance: 4 to 16 ohms
Adjustable Volume

[alm]

[1] How do we correlate the "300W max." and "2 x 30W RMS" power? I thought Max. Power (Pmax) = 2 x Power in RMS (Prms)...

My guess is that this is the ILS (If Lightning Strikes) power.

[2] Usually home audio amplifier do not state "low pass filter" or "high pass filter". Why do we need these specs in a car audio amplifier?

Possibly for direct connection to eg. a sub woofer without filter? It may also refer to tone controls.

[3] Is it good to have a HIGHER or LOWER input impedance? What is the typical input impedance of an audio amplifier?

Lower impedance is less susceptible to noise, but requires more current from the source. I believe something like 10-50kOhm is typical.

[4] Does the "Output impedance: 4 to 16 ohms" mean the output impedance of the audio amplifier or the impedance of the connected speaker?

Probably the speaker, the amp is usually much lower, and not that relevant (not much point in impedance matching at these frequencies).

[Alex]

[1] How do we correlate the "300W max." and "2 x 30W RMS" power? I thought Max. Power (Pmax) = 2 x Power in RMS (Prms)...
This is the Peak Music Power Output (PMPO) output power. Different manufacturers use different definitions to meet a target set by the marketing department. You cannot convert from one to another and there is no reason to even attempt to. The output power of the amplifier is 2x30 Watts (at a given distortion, load and frequency). 2x25 Watt or less will be more like it. There is no such thing as RMS Watts. This term came about to distinguish PMPO from real Watts. Watts are inherently RMS, please refer to definition of a Watt.

[2] Usually home audio amplifier do not state "low pass filter" or "high pass filter". Why do we need these specs in a car audio amplifier?
I belive this refers to adjustment knobs on the amplifier that allow you to filter some of the treble (high pass) or bass ( low pass). This is usefull if you want to drive a sub-woofer or a tweeter speaker set with this unit.

[3] Is it good to have a HIGHER or LOWER input impedance? What is the typical input impedance of an audio amplifier?
Typical input impedances range from a couple of kiloOhms to megaohms. Most are 10-100 kOhm. An audio power amplifier with higher input impedance means, in simplified terms, less signal from your audio source is wasted when you connect the two. Too high introduces excessive noise in theory but in practise the bandwidth of the system is limited by the audio amplifier so it wont make much of a difference. The stated value is great.


[4] Does the "Output impedance: 4 to 16 ohms" mean the output impedance of the audio amplifier or the impedance of the connected speaker?
It means that the amplifier can accept speakers or combinations of speakers that have a total impedance of 4-16Ohms. Lower than 4 Ohms will overload the amplifier, possibly causing a thermal shutdown or damage. Higher than 16 and you will not be utilising the available power capacity.

Looking at the rest of the specs, this does not look like a very good amplifier. First of all the power is quite low, comparable with a modern CD player source. The low frequency response is acceptable but dont connect this to a subwoofer.

A random distortion figure is given. I take it it refers to THD+N distortion? Who knows. Anyway it says >0.1% Note the unequality symbol meaning greater than 0.1%! Possibly 0.5% or 1% is what they meant? lol. This is a terrible figure from a modern transistor amplifier.


I would suggest you continue looking for a better amplifier. What system do you have in mind overall?

[onemilimeter]

If we refer to this online ebay store, it seems that the "cost per output power of car audio amplifier" is LOWER than home audio amplifier.

In terms of performance (e.g. T.H.D), do you think car audio amplifier is better than or inferior to home audio amplifier?

Can I say that the car audio amplifier is cheaper because the input power to the amplifier is DC and therefore it does not need "mains to DC" module like in an home audio amplifier?

Also, I note that car audio amplifier is lighter. Why? Is it because of the it saves the weight of a AC/DC transformer?

Thanks.

[Alex]

I am afraid there is not a single reason for any of the things you mention. Cost is affected by many other factors than just output power. It really comes down to the specific product offering.

Distortion performance...in general more can be tolerated in the car where you get trucks passing by or doors vibrating. There are car amplifiers with better performance than home ones and vice versa. Its product specific.

What you can say about its power supply is that the home one will most likely be utilising an AC-DC power supply whereas a car one might be utilising a DC-DC step up converter or, for this low power aplifier no converter at all. Construction quality, specs and marketing factors affect the price the most.

It is lighter because it happens to be lighter to the one you compared it to. I have seen home amps having blocks of lead to add weight, amplifiers with 20kg mains transformer, 10kiloW monoblock car amplifiers with toroidal ferrite cores that look like small car tyres!

This simple amplifier published years ago uses no boost converter, is small and can easily outperform the amplifier you have linked. But the ICs and capacitors (retail) will cost as much as that amplifier...

[onemilimeter]

Looking at the rest of the specs, this does not look like a very good amplifier. First of all the power is quite low, comparable with a modern CD player source. The low frequency response is acceptable but dont connect this to a subwoofer.

A random distortion figure is given. I take it it refers to THD+N distortion? Who knows. Anyway it says >0.1% Note the unequality symbol meaning greater than 0.1%! Possibly 0.5% or 1% is what they meant? lol. This is a terrible figure from a modern transistor amplifier.

I would suggest you continue looking for a better amplifier. What system do you have in mind overall?

I found another 'refurbished' amplifier on ebay. The specs look great... What do you reckon?

[Alex]

The specs are improving :-D

But your power supply is intended for a main connection (240V AC)!

You need 10-14 or thereabouts power supply voltage.

[Zero999]

Car audio equipment amplifiers are famous for lying about the specification of their equipment.

30W RMS per channel into what load? 2 Ohms? But the specification lists 4 Ohms minimum so it's not possible without a boost converter or impedance matching transformer which I doubt it has.

The maximum theoretical RMS power can be calculated using Ohm's law:

P_RMS= V²/(2R) = 12²/(4*2) = 144/8 = 18W

In reality the amplifier will loose some voltage in the driver transistors so in practise you'll get 15W at clipping (sounds terrible) and a usable power level of just 12.5W per channel.

The bandwidth is meaningless as the -dB limits aren't specified.

The distortion factor of 0.1% is probably only valid with a power output of under 10W.

I'm not saying don't buy it, just take the specification with a pinch of salt. Don't worry, you should be able to return it, if you don't like it, as it shouldn't be too hard to prove the specification is rubbish and you've been mislead.

Oh sorry it's possible to get 30W RMS into a 4 Ohm load at 12V but that's using a squarewave whcih will just sound like a loud buzzing noise, any tune or vocals will be lost in the noise.

[Alex]

This could be taken out of context as 'V' changes from amplifier to amplfiier. Take the TDA1562Q operating in class H for example. It uses two switched capacitors to boost its power supply during signal peaks.

Bandwidth is meaningful, for amplfiiers -3dB is the norm, I assumed this is the case here. But who knows seeing how the other specs are inflated.  Besides, we cant expect the audio amplifiers to have much more than 20kHz BW (-3dB). Even if we ignore the amplifier completely, the bandwidth is limited by the human ear we cant expect to perceive amplified thermal noise power from the MHz playground.

[Zero999]

This could be taken out of context as 'V' changes from amplifier to amplfiier. Take the TDA1562Q operating in class H for example. It uses two switched capacitors to boost its power supply during signal peaks.

Valid point but beware, 70W is specified at 10% distortion with a 14.4V supply, for 0.5% distortion at 14.4V it's only 55W, at 12V it'll be even lower I estimate 43W, besides that's the typical, the worst case it probably even lower.

Bandwidth is meaningful, for amplfiiers -3dB is the norm, I assumed this is the case here. But who knows seeing how the other specs are inflated.

Bandwidth is also specified at -6dB, so it's meaningless without specifying which.

[onemilimeter]

The specs are improving :-D
But your power supply is intended for a main connection (240V AC)!
You need 10-14 or thereabouts power supply voltage.

Thanks for your reply again.

I wish to purchase an audio amplifier, which, when coupled with a signal generator, will allow me to generate a controllable (amplitude and frequency) sinusoidal voltage (and hence current). Therefore, the input power supply to the audio amplifier can be mains AC or DC as I've both in my laboratory.

Frankly, I'm not able to "visualize" from the aspect of my application (as it's still under planning stage) how bad an audio amplifier with THD 1% compared to one with 0.1%.

The specs of MF450 above seems to be very good considering the THD at full load is 0.005%. I assume that the THD at full load should be the worst THD of a given audio amplifier.

By the way, in my application the output of the audio amplifier will be connected to a step-up transformer (built on a toroidal core). Since transformer is an inductive load, do you think the THD of an audio amplifier will be degraded?

[Alex]

I see. What sort of power and peak voltage do you want in your toroidal transformer? Whats connected in the secondary? If this for a step up converter you can tolerate a lot of distortion from your power amplifier.

[onemilimeter]

I see. What sort of power and peak voltage do you want in your toroidal transformer? Whats connected in the secondary? If this for a step up converter you can tolerate a lot of distortion from your power amplifier.

The load connected to the secondary winding is an inductive load with the following impedance (R=0.5ohm; L=4mH; Thus, Z = R + 2*pi*f*L, where f is the frequency of signal). My target is to supply a sinusoidal current (as pure as possible) of 1.0A peak-to-peak at 10kHz. According to this load specifications (and assume that the leakage inductance of the transformer is less than 0.5mH), the secondary output sinusoidal needs at least 90Vrms. Thus, it depends on the output voltage of the audio amplifier, the turn-ratio could 2 to 4.

I will find few more audio amplifier on ebay and hope that you will give your opinions and advices. Thanks in advance...

[onemilimeter]

Found few more as below. What do you think about them? Do you think I can use them for my application as a "variable-frequency variable-amplitude sinusoidal voltage source"? Thanks.


McHammer Mystic 5.1 Channel Car Amplifier Amp 3550W

McHammer's car amplifier provides high performance at a price that won't break the bank!
The pitch-black amplifier provides 3550W for your system on 5 channels, and ensures high quality sound even when being ran at high levels.
The McHammer Mystic 5.1 - Stylish, powerful, and durable.

Highlights:
4 x 350 W and 1 x 440 W RMS power (equivalent to 4 x 675 1 x 850 watts maximum Power)
Powerful, adjustable high and low-pass filter
Deep black steampunk design with white-lit, analog VU meters

Features:
Frequency range: 10 - 40000 Hz ± 1dB
Depth Filter control range: 50 - 250 Hz (6/12dB)
Filter height adjustment range: 60 - 1200 Hz (6/12dB)
Input sensitivity for Front + Rear adjustable separately
Input Sensitivity: 0.2 - 5 V
Signal to noise ratio (S / N):> 100 dB
THD: <0.03%
Remote control over the car radio (on / off)
LED indicator for Power & Thermal fuse
Fuse: 2 x 30 A
Power supply: 12 V - DC
Cutout for short-circuit, over voltage and over temperature

McHammer Mystic 4-Channel Car Hifi Amplifier 2700W System

At the touch of a button, McHammer amplifiers leave the competition far behind!
The jet-black Mystic 4.0 fires up your system with 2700W of 4 channel power in excellent sound quality.
As you would expect from a McHammer amp, this unit also impresses with its workmanship, handling and durability.
A car amplifier as it should be - just a little edgier!

Highlights:
4 x 350W / 2x 700W RMS power (equivalent to 4 x 675 / 2 x 1350W max. output)
Powerful, adjustable high and low pass filters (6/12dB)
Jet-black steampunk design with illuminated analogue VU meters.

Features:
Frequency range: 10 - 40.000Hz ± 1dB
Bass range: 50Hz - 250Hz
Treble range: 60Hz - 2kHz
Remote control (on/off) over the car radio
Signal to noise ratio (S / N): >100 dB
THD: <0.03%
Input Sensitivity: 0.2 - 4.5 V
Input sensitivity for front and rear separately adjustable
LED indicator for power and thermal fuses
Safety shutdown prevents short circuiting, power surges or over-heating
Fuse: 2 x 25 A

McHammer Mystic 2 Channel Car Hifi Amplifier Amp 1350W

At the touch of a button, the McHammer amplifier easily leaves the competition in the dust!
The jet-black amplifier fires up your system with 1350W of power (2 channel) and provides top of the line sound quality.
The amplifier not only impresses with its professional sound, but also with its workmanship, handling and durability.
A car amplifier as it should be - just a little edgier!

Highlights:
2 x 350 W RMS power (equivalent to 2 x 675 Watt max.)
Powerful, adjustable high-and low-pass filter
Stream jet black punk design with white-lit, analog VU meters

Features:
Connectors: RCA input, RCA output
Remote control (on / off) via the car radio
Low-pass filter 40 - 150 Hz
High-pass filter 60 - 1.2 kHz
Frequency response: 20-22000 Hz
THD <0.3%
Signal to noise ratio (S / N):> 93 dB
Input Sensitivity: 0.2 - 5 V
Input sensitivity for Front + Rear adjustable separately
Fuse: 1 x 25 A
Cutout for short-circuit, over voltage and over temperature

[onemilimeter]

Let's refer to the specs of MF450 audio amplifier shown below again. I tried to do some calculation in order to make sure I get the right power amplifier.

As mentioned earlier, I purchase it not for home audio but for laboratory testing purchase. In my application, a signal generator will be connected to the input of the audio amplifier. I wish to build a "variable-frequency variable-amplitude sinusoidal voltage source" which is expected to provide several time higher output power than a signal generator. The frequency range of interest is 10Hz~10kHz.

Based on the specifications, my calculation shows that the output voltage of the audio amplifier when it's supplying 300W RMS power to an 8-ohm load is approximately 48.99V in RMS and the output current is about 6.12A in RMS.

Kindly advise the following:

[1] Do you think the MF450 audio amplifier, when coupled with a controllable signal generator, can be used to supply "variable-frequency and variable-amplitude sinusoidal voltage"? Hopefully the output of the audio amplifier will not be in square or rectangular shape as mentioned by Hero999 earlier.

[2] Can I say that 48.99Vrms (or a sinusoidal voltage with amplitude of 69Vpk) is the highest achievable output voltage of the audio amplifier?

[3] As given, the THD at full load is 0.005%. Can I say that THD=0.005% is the worst THD of the audio amplifier for its entire load range (no load to full load)?

Thank you very much.

[Zero999]

Found few more as below. What do you think about them? Do you think I can use them for my application as a "variable-frequency variable-amplitude sinusoidal voltage source"? Thanks.

May be but they all lie about their specifications, the ones you've linked to below claim to be an overunity devices. 


McHammer Mystic 5.1 Channel Car Amplifier Amp 3550W

McHammer's car amplifier provides high performance at a price that won't break the bank!
The pitch-black amplifier provides 3550W for your system on 5 channels, and ensures high quality sound even when being ran at high levels.
The McHammer Mystic 5.1 - Stylish, powerful, and durable.

Highlights:
4 x 350 W and 1 x 440 W RMS power (equivalent to 4 x 675 1 x 850 watts maximum Power)
Powerful, adjustable high and low-pass filter
Deep black steampunk design with white-lit, analog VU meters

Features:
Frequency range: 10 - 40000 Hz ± 1dB
Depth Filter control range: 50 - 250 Hz (6/12dB)
Filter height adjustment range: 60 - 1200 Hz (6/12dB)
Input sensitivity for Front + Rear adjustable separately
Input Sensitivity: 0.2 - 5 V
Signal to noise ratio (S / N):> 100 dB
THD: <0.03%
Remote control over the car radio (on / off)
LED indicator for Power & Thermal fuse
Fuse: 2 x 30 A
Power supply: 12 V - DC
Cutout for short-circuit, over voltage and over temperature

Yeah right, assuming 2*30A fuses in parallel, it's 60A which at 12V is only 720W. All the other specifications are total bollocks, I'm not going to waste my time debunking them.

[Alex]

Got to love car power amplifiers.

1mm, how much power do you require in your transformer and at what primary voltage? Maybe you can get away with a power op-amp? Maybe a few transistors? We have no clue as to what you are trying to do.

[onemilimeter]

Yeah right, assuming 2*30A fuses in parallel, it's 60A which at 12V is only 720W. All the other specifications are total bollocks, I'm not going to waste my time debunking them.

Good observation. I will delete them from my list...

[onemilimeter]

Got to love car power amplifiers.

1mm, how much power do you require in your transformer and at what primary voltage? Maybe you can get away with a power op-amp? Maybe a few transistors? We have no clue as to what you are trying to do.

This post is related to the following posts:

Using toroidal power transformer in high-frequency application.
https://www.eevblog.com/forum/index.php?topic=2469.0

Adj. +/-60V to +/-70V DC Power Supply.
https://www.eevblog.com/forum/index.php?topic=2457.0

AC Constant Current Source (2Apk-pk, 10~20kHz) - Possible?
https://www.eevblog.com/forum/index.php?topic=2149.0


My ultimate goad is to build an AC source which is capable of supplying a 10kHz 1.0A sinusoidal current (peak-to-peak) to an inductive load (R=0.5ohm, L=4mH).

At 10kHz, the impedance is Z = 0.5 + 2*pi*10000*0.004 = 251.83ohm.

At 10kHz, a peak-to-peak voltage of 251.83V (or, 89.05Vrms) is required to push 1.0Apkpk sinusoidal current through the load.

To my knowledge, there is no power opamp or integrated power audio amplifier IC in the market which is able to provide such a high output voltage.

Thus, I plan to connect a transformer to the output of a low voltage but high power amplifier (e.g. the output of high power integrated audio amplifier IC, or the output of commercial audio amplifier unit) to step up the voltage to the desired level.

Let's say the maximum rms voltage supplied by a commercial audio amplifier unit is 45Vrms.

Thus, I need a transformer with a step-up transformation ratio of at least two (2).

For a transformation ratio of two, the power amplifier must be able to supply a current of at least 2.0Apkpk (or, 0.707Arms) to the transformer primary winding at 10kHz.

Toroidal core will be used to construct the transformer.

Kindly advise.