IC for DC servo ?

K-amps · 5th April 2004, 01:47 PM

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!

wrenchone · 7th April 2004, 08:15 AM

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.

2002tii · 7th April 2004, 01:42 PM

According to Electrocompaniet the DC servo will affect the bass respons on the amplifier.
The OPA134 will sound better than LF411.

peranders · 7th April 2004, 03:04 PM

Quote:

Originally posted by K-amps
[B]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
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).

peranders · 7th April 2004, 03:08 PM

Quote:

Originally posted by 2002tii
According to Electrocompaniet the DC servo will affect the bass respons on the amplifier.
The OPA134 will sound better than LF411.

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!

The opamp should have decent low distortion meaning do not use LM324/358

jackinnj · 7th April 2004, 03:26 PM

OP97EP and AD745 were the devices used in the AudioXpress 2004 articles on the Adcom GFP-565 modification.

K-amps · 7th April 2004, 07:06 PM

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-

wrenchone · 8th April 2004, 03:51 AM

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....

EchoWars · 8th April 2004, 04:20 AM

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.

K-amps · 8th April 2004, 04:08 PM

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!

5th April 2004, 01:47 PM	#1
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	IC for DC servo ? 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!

7th April 2004, 01:42 PM	#3
2002tii diyAudio Member Join Date: Apr 2002 Location: Norway	According to Electrocompaniet the DC servo will affect the bass respons on the amplifier. The OPA134 will sound better than LF411. __________________ Regards Dan M.

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7th April 2004, 08:15 AM	#2
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	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.

7th April 2004, 03:26 PM	#6
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	OP97EP and AD745 were the devices used in the AudioXpress 2004 articles on the Adcom GFP-565 modification.

7th April 2004, 07:06 PM	#7
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	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-

8th April 2004, 03:51 AM	#8
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	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....

8th April 2004, 04:20 AM	#9
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	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.

8th April 2004, 04:08 PM	#10
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	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!

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