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Arvis 8th September 2012 03:59 PM

tda1562 power is not even near 70w...
I recently built an amp using tda1562 70w chip, currently feeding it from 350w old pc power supply, getting sound signal from my cellphone.
At max volume I'm getting only around 8 volt peaks, which according to formula V^2/R is like 16 watts (using 4 ohm speaker).
What could be the problem? I really want to get the full power.
Btw using 470nf input and 22000uf power capacitors.

Mooly 8th September 2012 04:37 PM

This all depends on a couple of things.

1. How much voltage does the cellphone put out. Possibly not as much as you think.

2. How much voltage gain does the amp circuit have. That can be determined easily by the value of feedback resistors.

So 70 watt into 4 ohm (RMS ? we always deal in RMS values :)) is 17 volts RMS approx. Thats 24 volts peak or 48 volts peak to peak.

Now I've just looked at the data sheet for the IC.... and see its a bridge output that can make use of a "lift" capacitor to momentarily boost the output. Presumably it adds that voltage to the main rail for transient peaks.


counter culture 8th September 2012 04:57 PM

You can't get 70W continuous sine wave power out of this chip.

You say you're getting 8V peaks, but how are you determining this? You'd need to look at the output with a scope, and capturing the peak output might not be straightforward.

Best thing really is to ensure the gain is set correctly and that the drive voltage is adequate, then if the circuit is built correctly you'll get something close to the advertised output, given that you're running it a @ 12V rather than the recommended 14.4V (something to be considered).

Otherwise you'll need to give some consideration to exactly how you're going to test it accurately.

Arvis 8th September 2012 05:03 PM

1. Well I measured during a song with a lot of bass and got around 0.32 volts max.
This probably isn't the right way to measure tho, is it?

2.Don't know what do you mean exactly but datasheet says min - 25 typ - 26 max - 27

I'm not 100% sure if it's 70w RMS or max, don't see datasheet mentioning it, but even if it was the max power, I shouldn't be getting only 8 volt peaks, should I?
It would be fine even if it was 50w rms...

Arty 8th September 2012 05:11 PM

well depends if the status pin is at high or not.
This chip is a quite sophisticated calss H amplifier, that can work in class b.

Allso notice, 70 watt at 10% thd, at 14.4 rail voltage.
The computer psu is 12 volts to begin with.
Allso, it is quite likely as mooly said, the cellphone fails to make the required input voltage.
Gain is fixed internally in these chips.
I would like to see the schematic of Your build, and the actual component values used.
These chips are -as i mentioned- quite sophisticated, if the heatsink is not big enough, and it gets hot it will switch back to low power mode.
Fancy stuff, but actualy it is a verry roboust chip.

the rail lift supply solution used in the chip IS a sensitive one on the otherhand. High ESR lift capacitors kill it. Would not be able to charge in time to discharge when in demand.

Mooly 8th September 2012 05:19 PM

You have to measure using a continuous tone and have some means of accurately measuring the value. A DVM can be used at low frequencies (up to 400hz and many will cover the audio bandwidth but don't count on that unless specifically stated).

The 25/26/27 are DB values for gain. So 26 DB is a voltage gain of around 20. So 0.1 volts RMS input would give 2 volts RMS output.

Arvis 8th September 2012 05:30 PM

It's circuit from
capacitors C4 and C5 are 22000uf, input capacitors C1 and C2 are 470nf, C8 is 4700uf and the rest is the same. E, F are speaker outputs, P is voltage source.

Btw that power supply is outputting even slightly less than 12v (11.something), could it really make that big difference in power?
Also I tried using computer as signal source (seems to have like twice higher voltage), I seem to get more power, but the led of amp starts blinking as soon as volume is more than half

Arty 8th September 2012 06:16 PM

I'm not sure its set to class -H.

theAnonymous1 8th September 2012 06:34 PM


Originally Posted by Arty (Post 3157186)
I'm not sure its set to class -H.

Yeah, looking at the PDF provided, PIN-16 Status I/O isn't connected to anything on the PCB. You might need to connect it to +VCC.

Try connecting the pad labelled "S" to the one labelled "M"... they're right next to each other on the right side of the IC.

Arty 8th September 2012 08:09 PM

if that pin is floating then its in class b.
8v output voltage at 12 v ish supply rails seems pretty decent for class b.
at class H you will get around 14 volts.
get a decent supply of DC 15 volts (hint, a computer psu can be modified with a pot to obtain that) and you will have Your 70 watts to 4 ohm load in class H, probably without clipping.

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