I know some of you are "bored" with DC servo's 
but I need some help replacing an Adcom part. Seems like the DC servo is bad, I need some generic that is robust and protected...
Any ideas guys?
This will go in a GFA-555. The part number is Adcom 3A (I have seen 1A in there too). I bet it's some generic re-labeled as Adcom.
Thanks!
but I need some help replacing an Adcom part. Seems like the DC servo is bad, I need some generic that is robust and protected...Any ideas guys?
This will go in a GFA-555. The part number is Adcom 3A (I have seen 1A in there too). I bet it's some generic re-labeled as Adcom.
Thanks!
If the device is a single opamp, an LF411 is a good, relatively inexpensive choice for a servo amp. It's JFET input, so the servo capacitors can be relatively small. The offset is also low. The LF412 will work in an application needeing a dual device. The OPA134 works nicely for a single device, but it's about twice as expensice as a LF411.
K-amps said:I know some of you are "bored" with DC servo's
B]
Doesn't the servo work or what is bad really?
Any low distortion opamp will work. It's a plus if the input bias currents are low, so you can use high resistance values (1 Mohms).
Thanks Guys,
I think I will try the AD711 and the AD745's. Basically I was working on the amp late----late-------- LATE night and re-cableing the whole thing and did the dumbest thing.... I interchanged the main B+ and B- cables....
I had the presence of mind to power up the amp via a fuse lamp (100 watt bulb) so I did not blow the outputs or drivers or anything ... I checked all Diodes and transistors... all checked ok with reference to the good channel.
The amp was dumping DC into the outputs.... Only after removing one input differential did the DC dump cease.... since the transistor checked ok (ie. the differential pair was ok) I conclude the only thing left was the DC servo and that could have gotten killed with the reversed polarity DC rails....
I was disraught and left the amp on the bench after that dumb mistake.... I have ordered some AD745's and a 8pin DIP socket. I will try in again with various opamps and see what works..
Any additional ideas (apart from not reversing B+ and B- ) are welcome.
K-
I think I will try the AD711 and the AD745's. Basically I was working on the amp late----late-------- LATE night and re-cableing the whole thing and did the dumbest thing.... I interchanged the main B+ and B- cables....
I had the presence of mind to power up the amp via a fuse lamp (100 watt bulb) so I did not blow the outputs or drivers or anything ... I checked all Diodes and transistors... all checked ok with reference to the good channel.
The amp was dumping DC into the outputs.... Only after removing one input differential did the DC dump cease.... since the transistor checked ok (ie. the differential pair was ok) I conclude the only thing left was the DC servo and that could have gotten killed with the reversed polarity DC rails....
I was disraught and left the amp on the bench after that dumb mistake.... I have ordered some AD745's and a 8pin DIP socket. I will try in again with various opamps and see what works..
Any additional ideas (apart from not reversing B+ and B-
) are welcome. K-
Both the AD745 ot the AD711 will work in the application. The 745 is way, way overpriced for the job. So is the 711, but it is quite a bit cheaper. You need an amp with a reasonably high input impedance and very low offset. The bandwidth doesn't matter at all, as you are limiting the response to about 0.1 Hz. Distortion (for a good opamp) shouldn't matter too much either, as you are operating very close to DC, where you have close to full open loop bandwidth. An exception of course is the LM324/358 mentioned by Per Anders, as it has a lousy class B output stage, and its offset voltage isn't too good, anyway... I have used LF411/412 and OP07 for servo amplifiers, and have never noticed any problems. The more expensive chips will work just fine, but I consider it money stuffed down a rat hole....
Thanks,
I actually ordered the AD711 not the AD745 from digikey at about $2.89 a pop. Adcom was selling their part for $11.50 !
By the way, anyone know what their ADC-55 and ADC-42 output devices are? They are supposed replacements for the no longer made Toshiba pair of 2sb554/ 2ds424 which Adcom used during the production runs of the 555/565/585's.
The devices look like ON/ Motorola TO-3 metal can casing.
I bet they are MJ15022/23 or 24/25's relabelled as Adcom parts. (Oh by the way, Adcom sells those for $10.50 a piece too )
Cheers!
I actually ordered the AD711 not the AD745 from digikey at about $2.89 a pop. Adcom was selling their part for $11.50 !
By the way, anyone know what their ADC-55 and ADC-42 output devices are? They are supposed replacements for the no longer made Toshiba pair of 2sb554/ 2ds424 which Adcom used during the production runs of the 555/565/585's.
The devices look like ON/ Motorola TO-3 metal can casing.
I bet they are MJ15022/23 or 24/25's relabelled as Adcom parts. (Oh by the way, Adcom sells those for $10.50 a piece too
)Cheers!
EchoWars said:Since when does Digikey sell Analog Devices products??
go online -- they do carry Analog -- usually you get an email from them when they announce a new line -- so it must be pretty recent.
You can now order Analog parts directly from ADI, (just like Linear Tech).
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