300watts Amplifier with TDA7294 or LM3886

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mrwhy

Member
Joined 2007
TDA72983

Hi Guys
Sorry I don't know what I am doing - my first visit to you.

I have a Marshall AVT150 not working since new.
You guys sound very keen and knowledgeable: please help.

The AVT150 seems to use a bridge output stage so ANY bridge output amp schematic would help me!

It uses two TDA 7293 amp modules (wiring seems identical).

The nearest equiv I can find here in OLDE England is TDA7294V



Initial symptoms (since new) were

NO SOUND AT ALL: lights OK, fuse OK, speakers OK.

On the amp modules the collectors BOTH were at - 42 volts.



Then a test probe shorted one collector to ground (via overlap of shakeproof mounting washer)

Now running with series 60w bulb (230v mains) gives a big hum pulse modulated once per second.



The power supplies to the amp modules are (on open circuit) -50 -100 -150 (I would expect it to be -50 0 +50

By the time this power reaches the 3-pin plugs (to the modules, not connected) the volts are

-48, -150, -48

The above two -48v wires to one module are switched over - as I'd expect in a bridge. But the rest does not make sense (without a schematic)



Any advice or especially a schematic for a bridge amplifier would be much appreciated.

Many thanks

John.:)
 
M

mrwhy

Member
Joined 2007
vuki said:
hi,

I have gaincard-style amp with TDA7294 which sounds very good ...

Hi Vuki
It sounds as though you know about TDA7294 and 7293.
Maybe you can help me!
I am trying to fix a Marshall AVT150 which uses two TDA7293s each in a mini-pcb.
The only thing like that I can find here in the UK is TDA7294, which the seller describes as "Bridged Output"
Would not we need FOUR to form a bridge, or does each TDA7293 have TWO arms of the bridge. Which two arms?

Like you (!) I seem to have blown both modules (In my case by shorting the collectors of one module to ground!)

The Marshall amp never did work (Complete silence) from new.
Now at least, run from 230v mains via 60 watt lamp, it DOES emit sound! One second duration, repeated series of hum pulses.
Before the short it gave nothing at all. Such is progress!

The two mini-pcbs containing the TDA7293s seem identical.
Maybe they are in parallel and not bridged?

Any advice?
Click to expand...
 
A

AndrewT

R.I.P.
Joined 2004
rljones said:
thanks for the suggestion, but it's already in the circuit (per app. note).

I tested the units with a balanced set up. The functioning unit produced 310W into 8 ohms and blew both rail fuses when I switched to a 4 ohm load with only a slightly reduced drive. This was OK as only 5A fast blows where in the power supply and the current demand was over this rating. New fuses and this amp was running fine again. With the number of devices, it should produce around 500W into 4 ohms, and a bit more into a 2 ohm load.

At 1 W, the THD is about 0.03% from 10Hz to 30kHz, which is a good. Power output at 100W is linear over the same range, being -3dB down at 60kHz. The only unusual thing about the distortion is that it increases from 0.03% at 1 W into 8 ohms up to around 0.2% at 100 W. Typically amps have decreasing distortion with greater output. I'm not sure what to make of this; it may be a power supply issue. I have a 0.1 ohm resistor between two sets of caps and this may be a problem with greater output. Any ideas?

The other monoblock was another story. On testing it appears that 1/2 of the bridged amp self destructed, so I need to replace these 4 devices. The other half worked fine when tested in single ended mode. My friend thinks something might be fishy with the Quad speaker.
Click to expand...
This is going back a bit over old ground but this amp has a problem and it's nothing to do with fuse rating.
I got 311W into 4r0 using a pair of F3.1A rail fuses. Ipk~=12.5Apk.
And got over 450W into 2r0 with F5A rail fuses. Ipk~=21.3Apk.
These currents are well above the 8.7Apk that 300W into 8r0 outputs. If anything this indicates that his fuses were too big and closer protection may have ruptured sooner alerting the user to an oscillation problem.
 
A

AndrewT

R.I.P.
Joined 2004
Re: TDA72983

mrwhy said:
Hi Guys
Sorry I don't know what I am doing - my first visit to you.

I have a Marshall AVT150 not working since new.
You guys sound very keen and knowledgeable: please help.

The AVT150 seems to use a bridge output stage so ANY bridge output amp schematic would help me!

It uses two TDA 7293 amp modules (wiring seems identical).

The nearest equiv I can find here in OLDE England is TDA7294V



Initial symptoms (since new) were

NO SOUND AT ALL: lights OK, fuse OK, speakers OK.

On the amp modules the collectors BOTH were at - 42 volts.



Then a test probe shorted one collector to ground (via overlap of shakeproof mounting washer)

Now running with series 60w bulb (230v mains) gives a big hum pulse modulated once per second.



The power supplies to the amp modules are (on open circuit) -50 -100 -150 (I would expect it to be -50 0 +50

By the time this power reaches the 3-pin plugs (to the modules, not connected) the volts are

-48, -150, -48

The above two -48v wires to one module are switched over - as I'd expect in a bridge. But the rest does not make sense (without a schematic)
Click to expand...
Hi,
if those readings are correct then the PSU is sick.
Forget the amplifier at the moment and do not connect it until you have sorted the PSU voltage readings.
 
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