Hi,
i recently bought a cheap TDA7850 amp board, however i have some issues with it that i can‘t seem to figure out.
General performance is excellent, measured distortion and noise floor is well within spec and i sounds great even on high sensitivity horns.
(I bought it to power a two way speaker with limmer 042 horn. They are dead quiet on this amp, )
Now come the issues: first of all, i needed to increase the input capacitance to > 1uf, as the lf cutoff with the factory .1uf ceramics was around 300hz, as opposed to 16hz in the datasheet.
In addition to that the output offset voltage sits around 2.9v instead of supposedly 6v with a 12v supply. Thats costing me 6db of headroom, which is more or less what i‘d like to have more for raised home listening levels (the accompanying 15" needs quite some boost in the low range, as it‘s just a pseuso full range box, requiring dsp to get there, as usual im pa world).
Does anyone have an idea what could cause this? Could it have something to do with the surrounding circuit? I looks more or less similar to the reference design in the datasheet. Or might it be because of a fake or factory reject chip? Unfortunately it is the upside down version, so it‘s hard to check the label. I slightly lifted it up once and i didn‘t find the obvious signs of sanding down/ reengraving the chip...
i recently bought a cheap TDA7850 amp board, however i have some issues with it that i can‘t seem to figure out.
General performance is excellent, measured distortion and noise floor is well within spec and i sounds great even on high sensitivity horns.
(I bought it to power a two way speaker with limmer 042 horn. They are dead quiet on this amp, )
Now come the issues: first of all, i needed to increase the input capacitance to > 1uf, as the lf cutoff with the factory .1uf ceramics was around 300hz, as opposed to 16hz in the datasheet.
In addition to that the output offset voltage sits around 2.9v instead of supposedly 6v with a 12v supply. Thats costing me 6db of headroom, which is more or less what i‘d like to have more for raised home listening levels (the accompanying 15" needs quite some boost in the low range, as it‘s just a pseuso full range box, requiring dsp to get there, as usual im pa world).
Does anyone have an idea what could cause this? Could it have something to do with the surrounding circuit? I looks more or less similar to the reference design in the datasheet. Or might it be because of a fake or factory reject chip? Unfortunately it is the upside down version, so it‘s hard to check the label. I slightly lifted it up once and i didn‘t find the obvious signs of sanding down/ reengraving the chip...
Well, that is the big question. Unfortunately it‘s not so easy for me to get a new genuine one and i want to rule out the board first.
I can‘t really imagine such a failure mode, shifting one operating point and decreasing the input impedance by so mu… calculating with a simple rc filter it would seem like should be 100k and is around 5k now..
I can‘t really imagine such a failure mode, shifting one operating point and decreasing the input impedance by so mu… calculating with a simple rc filter it would seem like should be 100k and is around 5k now..
Once I saw a L4974 fake STM SMPS chip that has a powerfull shorcircuit inside the chip, between input pins. Fortunately I messured it prior to put in the PCB and gone directly to thrash can.