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|8/25/1999 10:29 PM|
|Smitty||RC4739 Dual op-amp|
Does anyone know of a source for the dual op amps Anderton uses in his projects, RC4739, XR4739. I have been able to cross it to a NTE725, but they are way expensive at over $7.00 a piece. I have found a source for the CLM6000 opto-isolators he uses, but never find the 4739's listed. Thanks if anyone can help....
|8/25/1999 11:34 PM|
There isn't any magic about the 4739. It was just a low noise opamp when opamps in general were noisy.
You can replace it with a TL072 (it's dual, isn't it?) if you note which pins on the 4739 have the (+), (-) and output designations on them, and connect those points to the corresponding *symbol* on the TL07x parts; this will be on a different pin number. Also, you have to connect the most positive voltage to the + power pin on the TL07x and the most negative voltage to the - power pin on the TL. The easiest way to do this is to glue the replacement part upside down, legs up (a process known as "dead bugging") and run wire wrapping or other fine wires to the pins from the appropriate holes on the PCB. This is a simple way to rescramble pinouts.
I guess I really ought to just draw up diagrams for this and put it in the FAQ. Someone asks about once a month.
|8/26/1999 1:05 AM|
The 4739 is a dual op-amp but may not be pin compatable. It's in a 14-pin DIP with the supply pins on 4 and 10 as I recall. Also, it's a non-compensated device and so has pins across which a capacitor would be connected to set the maximum gain bandwidth (in order to reduce oscillations).
I have a Heathkit AA1214 stereo amp that uses the 4739 and over the years is has developed some "popcorn" noise. I attempted to replace it with a NE532(?) to no avail. I think the 4739's compensation caps where part of the RIAA playback curve circuitry as this was the phono cartridge preamp.
I put the noisy 4739 back and left it...
|8/26/1999 2:14 AM|
Oh, it's guaranteed not to be pin compatible. That's what the dead-bug mounting was for.
|8/26/1999 7:43 PM|
...oops, well, um, ahhh...guess I didn't read all of your message.
Sheepish in Los Gatos,
|8/26/1999 6:04 PM|
The 4739 comes in two flavours: internally compensated (XR, RC) and noncompensated (uA). I have a bunch of the noncompensated Fairchild chips and never really figured out how to compensate appropriately. Any advice would be appreciated.
As far as I know, there is a chip numbered LM1303 or something like that, which is an identical pre-amp chip, or rather, I have seen numerous schematics for RIAA phono pre-amps that list the 1303 and 4739 and use the identical pinout.
Is there something special about the 4739? Yes and no. In an article in DEVICE, Craig compared a variety of op-amps for noise characteristics under different loads. BiFets (TL072) and MosFets (CA3140) came up strong under high impedance loads and weak under low impedance, as you can imagine. 5534's came up strongest when all the resistors tacked onto the circuit were low values. 4739's were pretty much the best all round chip, though. Not superlative, but always a contender under any load. They slew at 1V/uS, which is certainly better than garden variety 4558's, though not as good as BiFet's or MosFet's. I suspect that the non-functional pins in the middle of each side (output and input pins are separated by a few) also lent the 4739 to easier PC layout, back in the days when autorouting was not widely available. Right now, I'd say the most distinctive thing about them is their lack of availability and non-competitive pricing.
|8/26/1999 7:44 PM|
"Right now, I'd say the most distinctive thing about them is their lack of availability and non-competitive pricing."
To the point - I love it!
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