# Class d amplifier output dumping

#### PaPiャSly

Joined Dec 25, 2022
43
Im planning on buying this class d amp (zk-ht21) because i want to have the 220w subwoofer channel but this amp is rated for 160w*2 for left and right and i dont want to replace my bookshelf speakers rated @30w rms, so im hoping there's a cheap way to dump the extra wattage from the amplifier

#### Ian0

Joined Aug 7, 2020
7,444
Not quite sure about those specs.
TDA7498 is a dual bridge-tied-load 100W amplifier, so I’m not at all sure how it can possible have two 160W outputs and one 220W output.
The datasheet claims 80W from a 34V supply but my calculations give (34^2)/(2*8) = 72W into 8Ω.
Assuming you are using 24V supply then the output can’t be more than 36W into an 8Ω load, so you have nothing to worry about apart form who is fibbing about the specs.

#### PaPiャSly

Joined Dec 25, 2022
43
Not quite sure about those specs.
TDA7498 is a dual bridge-tied-load 100W amplifier, so I’m not at all sure how it can possible have two 160W outputs and one 220W output.
The datasheet claims 80W from a 34V supply but my calculations give (34^2)/(2*8) = 72W into 8Ω.
Assuming you are using 24V supply then the output can’t be more than 36W into an 8Ω load, so you have nothing to worry about apart form who is fibbing about the specs.
Oh nice thanks for this i fo

#### crutschow

Joined Mar 14, 2008
32,054
The datasheet claims 80W from a 34V supply but my calculations give (34^2)/(2*8) = 72W into 8Ω.
I calculate (34*0.707)² / 8 = 85W.

#### Ian0

Joined Aug 7, 2020
7,444
I calculate (34*0.707)² / 8 = 85W.
Perhaps your slide rule needs recalibrating.
34^2 = 1156
1156/16 = 72.25

#### Audioguru again

Joined Oct 21, 2019
5,819
They exaggerate (lie about) the output power.
Amazon sells the amplifier and rate it at 160W + 160W + 220W and since they know nothing about electronics they say MAX or PEAK power that is simply double the REAL very distorted power.

The max allowed power supply is 36VDC. The datasheet for the TDA7498E IC says 80W + 80W into 8 ohms with 10% of horrible clipping distortion or 40W + 40W or 50W + 50W into 8 ohms at low distortion.
It can produce a little more power into 6 ohms and with a 36V supply but it might overheat.

How do they get one sub-woofer output of 220W? They do not say but I think they use a second TDA7498E IC with its two channels in parallel and driving a 4 ohms woofer. The power might actually be "only" 80W into 4 ohms at low distortion.

How do they get about 60W of distorted output into 8 ohms then the supply must be 62V? Because each amplifier is two amplifiers (bridge-tied load) driving each speaker wire with opposite phase which effectively doubles the output voltage swing which also doubles the output current swing (then almost 4 times the power of a single amplifier driving one speaker wire).

To avoid burning your little 30W speakers, modify the amplifier so that the stereo channels are powered with only 16VDC or 18VDC (if the speakers are not less than 8 ohms) but the subwoofer channel has 34VDC or 36VDC.

Won't the subwoofer be too loud compared to the stereo 30W each?

#### crutschow

Joined Mar 14, 2008
32,054
Perhaps your slide rule needs recalibrating.
Apparently it does.

I think the 80W rating comes from the 10% distortion at that power due to output clipping .
In my simulation (below), clipping the sinewave at 34V peak to get 10% harmonic distortion generates over 90Wrns into an 8 ohm load.
I imagine that would sound terrible.

#### crutschow

Joined Mar 14, 2008
32,054
How do they get one sub-woofer output of 220W?
The spec says:
• 1 x 220 W output power mono parallel BTL at THD = 10% with RL = 3 Ω and VCC = 36 V
Won't the subwoofer be too loud compared to the stereo 30W each?
Isn't that just a simple matter of turning down the subwoofer amp gain to match the stereo speaker outputs?

#### crutschow

Joined Mar 14, 2008
32,054
im hoping there's a cheap way to dump the extra wattage from the amplifier
If you need to operate the amp at 34V to get the maximum power to the subwoofer, then you could add an attenuator at the stereo inputs to limit the maximum output.

Otherwise, just don't turn the volume way up and you should be okay.
A speaker will generally distort noticeably when the applied power reaches its maximum limit.

#### Audioguru again

Joined Oct 21, 2019
5,819
Audio amplifier outputs are not "rail to rail". There is some output voltage loss that heats the amplifier.
An amplifier that is clipping produces more power than when not clipping.

Is the 30W for the stereo speakers the REAL maximum power? My Radio Shack Minimus 7 speakers were rated for 40W but when they blew, the woofers and replacements were stamped "5W".

#### Ian0

Joined Aug 7, 2020
7,444
The satellite speakers are likely to be rather more efficient than the subwoofer: reducing the bass resonance frequency by an octave reduces the efficiency by a factor of 8 (given the same magnet)

#### MisterBill2

Joined Jan 23, 2018
14,697
The way they get that 220 watts to the subwoofer channel is through a good imagination, and not being afraid to exaggerate a bit. Sort of like the old "IHF" peak power rating, which was what the amplifier could deliver with a power supply that was able to hold the supply voltage at the no-signal value for the test, And all of those ratings at 10% harmonic distortion, which if that amount does not bother you then any amplifier with any speaker in any condition is OK. And the way I read that spec is that the 220 watts is both channels in parallel, which is suddenly not stereo.
OR is the intention to use the amp only for the subwoofer??

#### Audioguru again

Joined Oct 21, 2019
5,819
The amplifier has three channels, Left, Right and Subwoofer. They claim 160W + 160W + 220W of PEAK power into 8 ohms for the stereo channels and into 3 or 4 ohms for the subwoofer at a horrible 10% clopping distortion when the supply is 34V or 36V. The distortion is not audible if the amplifiers are turned down to avoid clipping. The amplifiers are bridged then their output voltage and current swings are almost doubled.

The amplifier has TWO TDA7498E power amplifier ICs, one for the stereo channels and one with its two channels in parallel for the mono subwoofer.

#### crutschow

Joined Mar 14, 2008
32,054
Audio amplifier outputs are not "rail to rail".
A Class-D switching amp can get pretty close to the rails.

#### MisterBill2

Joined Jan 23, 2018
14,697
Really then, the simple way to reduce the power is to reduce the supply voltage to about 30 volts. Simple and easy and not prone to failure.

#### Audioguru again

Joined Oct 21, 2019
5,819
Really then, the simple way to reduce the power is to reduce the supply voltage to about 30 volts. Simple and easy and not prone to failure.
The datasheet of the TDA7498 amplifier IC shows an output of 67W into 8 ohms when the supply is 30VDC and the distortion is 10%. Each "30W" speaker will not survive.

#### MisterBill2

Joined Jan 23, 2018
14,697
The datasheet of the TDA7498 amplifier IC shows an output of 67W into 8 ohms when the supply is 30VDC and the distortion is 10%. Each "30W" speaker will not survive.
Is it possible to operate the different amplifiers at a lower supply voltage??Is it possible to provide resistors in parallel to take half of the power??
I have tested a 55 watt amplifier with a ten watt speaker and not destroyed or damaged it. There are other settings besides full power. In a normal car consider that normal driving does not use the rated engine horsepower. (NHRA drivers excepted)

#### crutschow

Joined Mar 14, 2008
32,054
The datasheet of the TDA7498 amplifier IC shows an output of 67W into 8 ohms when the supply is 30VDC and the distortion is 10%. Each "30W" speaker will not survive.
Don't understand why everyone is so concerned about the amp having a higher power rating than the speakers.(?)
If you keep the volume for music below the point of obvious distortion, then the power being applied to the speaker is not likely to blow it, since the average RMS music power is well below the peak power where distortion is first heard.
Is that not true?

#### MisterBill2

Joined Jan 23, 2018
14,697
Certainly it is true that not running an amplifier at beyond what the connected speakers are rated for will allow reasonable reproduction of the program in a satisfactory manner.
HOWEVER, I have seen a set of somewhat lower power rated speakers shredded by a couple of really stupid people who did not follow instructions to not turn up the volume. There exists a realm of individuals who believe that rap must be played at the maximum available power level. Thus totally shredded speakers, the cones both torn loose from the suspension and mostly ripped loose from the voice coils.

For the amplifier in the discussion, possibly reducing the supply voltage to 24 volts will allow a safe output level.
an alternative will be a series resistor in the speaker line, having an adequate power rating and a value of probably ten ohms.

#### PaPiャSly

Joined Dec 25, 2022
43
Don't understand why everyone is so concerned about the amp having a higher power rating than the speakers.(?)
If you keep the volume for music below the point of obvious distortion, then the power being applied to the speaker is not likely to blow it, since the average RMS music power is well below the peak power where distortion is first heard.
Is that not true?
Cause i forget to lower the volume sometimes
Certainly it is true that not running an amplifier at beyond what the connected speakers are rated for will allow reasonable reproduction of the program in a satisfactory manner.
HOWEVER, I have seen a set of somewhat lower power rated speakers shredded by a couple of really stupid people who did not follow instructions to not turn up the volume. There exists a realm of individuals who believe that rap must be played at the maximum available power level. Thus totally shredded speakers, the cones both torn loose from the suspension and mostly ripped loose from the voice coils.

For the amplifier in the discussion, possibly reducing the supply voltage to 24 volts will allow a safe output level.
an alternative will be a series resistor in the speaker line, having an adequate power rating and a value of probably ten ohms.
Il just do this to save my speakers, thanks a lot everyone