Looking back through the thread I am pretty sure that the offset should be adjusted to zero offset on the input of the AD844 and therefore my offset of 400mv on the output is very high. So I am again thinking about subtle adjustment to the positive or negative supply to AD844 to reduce the offset . Does this approach make sense?
Hi,
I need some advice to the Pedja Rogic AD844 stage (see schematics e.g. in post #7 of D3a IV stage)
I assembled it with 2 ADs stacked (output pins 6 connected through 68 Ohm resistors). But I'm not able to nullify the pin 5 offset and the voltage seems way off. I suspect my ADs are fakes. I bought them from China ebay because I have no other source for DIP package.
Anyway the DC voltages are: pin 1...-15.04, 2... 0.45, 3.... 0 (gnd), 4... -15.04 (supply), 5... -11.7, 6... -12.9, 7... 15.04 (supply), 8... 0
The JFET null circuit has little influence on pin 5, some 0.2 V, but no way to go to zero. Both channels are basically same. What do you think?
I need some advice to the Pedja Rogic AD844 stage (see schematics e.g. in post #7 of D3a IV stage)
I assembled it with 2 ADs stacked (output pins 6 connected through 68 Ohm resistors). But I'm not able to nullify the pin 5 offset and the voltage seems way off. I suspect my ADs are fakes. I bought them from China ebay because I have no other source for DIP package.
Anyway the DC voltages are: pin 1...-15.04, 2... 0.45, 3.... 0 (gnd), 4... -15.04 (supply), 5... -11.7, 6... -12.9, 7... 15.04 (supply), 8... 0
The JFET null circuit has little influence on pin 5, some 0.2 V, but no way to go to zero. Both channels are basically same. What do you think?
Based on your reported voltages it seems overwhelmingly likely you've been sold fakes.
The reason being the voltages you're getting on pin1 and pin8 (the offset nulling pins). My sample of AD844 shows there are ~10kohm resistors to the positive rail (pin7) at these pins yet you're seeing -15.04 and 0V there. On a real AD844 you'd be seeing ~15V on both.
The reason being the voltages you're getting on pin1 and pin8 (the offset nulling pins). My sample of AD844 shows there are ~10kohm resistors to the positive rail (pin7) at these pins yet you're seeing -15.04 and 0V there. On a real AD844 you'd be seeing ~15V on both.
The characteristics of nulling pins are perhaps the easiest way to check for fakes - you only need an ohmmeter. I seem to recall this is a good way to check OPA627s too, a widely faked chip I believe.
Thanks for the offer, but I didn't want to take a chance any more and at once placed order at Farnell. Quite costly though. If I knew they are faked so much (*), I'd rather build the Pedja discrete stage which costs almost nothing in parts.
(*) I questioned another ebay seller, now in Europe, if he is absolutely sure he has genuine parts. He cancelled the order.
(*) I questioned another ebay seller, now in Europe, if he is absolutely sure he has genuine parts. He cancelled the order.
The fake shows 1k pin 4--1 (5-1 I forgott to measure).
I installed the Farnell's AD844NZ and it of course works now. TDA1541A, only 1 AD to BUF03. Very clean dynamic peaks and choruses, probably thanks to strong buffer. If anything, it lacks some bit of immediacy, but there are still several options to try. First is 3 AD stacked.
The resistances pin 7-1, 7-8 are lower than 10k, some 3-4k and different for each pin.
I installed the Farnell's AD844NZ and it of course works now. TDA1541A, only 1 AD to BUF03. Very clean dynamic peaks and choruses, probably thanks to strong buffer. If anything, it lacks some bit of immediacy, but there are still several options to try. First is 3 AD stacked.
The resistances pin 7-1, 7-8 are lower than 10k, some 3-4k and different for each pin.