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Mooly 17th December 2012 04:40 PM

Data sheet says -/+10v as a minimum so should be OK.

johnr66 17th December 2012 05:08 PM

I setup my TDA2050 bench amplifier on my variable PSU and adjusted each rail from 0 to 16 volts to see what happens. Normally, the output offset was 2 millivolts. When the + rail dropped below 1.5 volts the output DC offset changed to -1.2v. When the - rail dropped below -1.5v, the DC offset was .4v. Next, I put a signal on the input. In either case, the output signal stopped when voltage went below 1.5v or -1.5v. The LM1875 should behave similarly except it's turn on point is 9 volts Vs. the 2050's 3 volts.

Of course, if you had a 12 volt battery on the + rail and 6 volts on the negative rail, your output power would be limited because the lower side would be clipping earlier.

I'd rather wire up the amp for single supply use. This gives you flexibility in power supplies. I built a monoblock amp using a TDA2050 and 30 volt brick from a old HP printer. I can also use it with 19 volt laptop bricks or even 12 volt ones. Of course output power is very dependent on the supply voltage

Philfr 17th December 2012 06:37 PM

Hi Mooly and John,
Thank's for reply..
Note that the two battery I'am using are wiring in serial to give 24 volts dc.
First I saw this :
Single Chip 25W Amplifier (Project 72)
with LM1875 (or TDA2050)..
Then I'am looking about TDA7294 cause it use mosfet. I have two monoblock mosfet amps and I like this sound (for low frequencies).
I also have Trends TA10 and TA2020 chip amp powered by 12 volt dc, it's works very well but lack of power at low frequencies.
That is why I ask the question of the choice for a chip.
Sorry for these basic questions, but I'm newbie in diy amp.

Mooly 17th December 2012 06:38 PM

An AC coupled amp (single rail) as John suggests has a lot going for it... particularly in terms of "safety" for the speaker if things go wrong. Even something like a dodgy battery push on terminal thats intermittent could cause a loss of one rail and the chip to latch potentially damaging a speaker. The AC coupled design is imune from all these problems.

Mooly 17th December 2012 06:48 PM

Posted together :)

If you look at the project in your link you will see it has a PLUS 25 volt rail and a NEGATIVE 25 volt rail. Total supply, 50 volts. That is known as a "split or dual" supply and is the normal method of powering chip amps and most audio circuitry.

If you look at my quick diagram above you will see the batteries connected in series with the centre point as "ground" or zero. That is a split or -/+12 volt supply. Total 24 volts :)

The amplifier output is normally at zero volts DC but swings up or down depending on the signal. The speaker is DIRECTLY coupled to the amplifier is at risk if a fault occurs or a rail disappears.

The single rail amplifier is very similar but the speaker is AC coupled via a large capacitor to the amplifier and so is imune to any DC voltage. The amplifier is artificially biased so that the output sits at a DC voltage midway between the zero and positive rail. So for a 24 volt supply the amplifier output would arranged to be at 12 volts dc allowing an equal swing toward 24 volts or down to zero.

Mooly 17th December 2012 06:51 PM

And to answer the question of poor bass response. All the chip amps such as the TDA's and LM3886 etc all go down to DC. There is no lower limit. In practice coupling capacitors define the low frequency cutoff point and can be selected to go as low as you would ever need.

Philfr 17th December 2012 07:26 PM

Yes Mooly, It's clear now with "That is a split or -/+12 volt supply. Total 24 volts" :)
Thank's !
I'll reconsider and find a chip amp and a schematic as simple as possible build with little and good componants..

JMFahey 17th December 2012 10:02 PM

I think that with such a low supply voltage, the TO220 case chipamps are fine, don't think the larger TDA7294 will provide an advantage.
Besides, you wanted a simple design :) .
Of course, it's your project, so ....

Mooly 18th December 2012 06:30 AM

Look at the LM1875 data sheet here,

Figure 2 is the single rail version.

Philfr 18th December 2012 07:03 AM

What is the reason you said TO220 was better ?
For heat dissipation ?
@ Mooly,
Thank's for shematic in single rail, it's look fine in my case.
Today I found in France only LM1875, TDA2050 or TDA7294..
So choise is limited !
Another problem : impedance curve of my speaker show a minimum impedance at 3,2 Ohm..
A chip amp can be loaded in 2 or 3 Ohm ?

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