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!
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.
According to Electrocompaniet the DC servo will affect the bass respons on the amplifier.
The OPA134 will sound better than LF411.
The OPA134 will sound better than LF411.
K-amps said:
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).
Of course the servo will affect the bass response! But this can be controlled by chosing parts and dimensioning the servo parameters. A good tool for this is LTSpice!2002tii said:
The opamp should have decent low distortion meaning do not use LM324/358
OP97EP and AD745 were the devices used in the AudioXpress 2004 articles on the Adcom GFP-565 modification.
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....
The 555 uses a single supply to power the opamp, so when I replaced one, I wanted a rail-to-rail opamp capable of operating well off of a single supply, and also with tiny input bias currents.
The AD820 is the one. Perfect for the job. Too bad you already ordered. 😉
The AD820 is the one. Perfect for the job. Too bad you already ordered. 😉
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!
Since when does Digikey sell Analog Devices products?? 😕 Not in the catalog, so you have me real curious...
EchoWars said:
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).
They even have AD797 in stock. It's not in the catalog, so I presume it is new. Actually both Digikey and Mouser often have items available that are not in the catalog. However, sometimes you find these are only available as a special order with a minimum quantity. Not in this case though.
Well I'll be damned!! 😱 Why the hell don't they put 'em in the catalog?? 🙄
Been wanting to try the AD8620, now it looks like I've got an easy source...way cool!!
Been wanting to try the AD8620, now it looks like I've got an easy source...way cool!!
I took a look at the Digi-Key site, and was really surprised to see them offering AD product. At present, the actual number of devices in stock is pitifully small, but hopefully that will improve in the near future. The prices are better than Newark's, but that wouldn't take much....
Newark's base prices aren't that bad, but add their ridiculous shipping/handling charges and you've been bent over...wrenchone said:
Sorry K-Amps...didn't mean to hijack. Still, the ultimate chip for a servo is the AD820/AD822😉
By the way, the AD711 did not work for me... had a 2.98v DC (2980mV) offset on the outputs.
Turns out the original Adcom IC was ok, now I have a 2.3mV offset which is pretty good.
Thanks to all, and yes I will probably try the AD820 next time.
K-
Turns out the original Adcom IC was ok, now I have a 2.3mV offset which is pretty good.
Thanks to all, and yes I will probably try the AD820 next time.
K-
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