[GreenKeys] DT600 replacement op amps

Thu Jul 9 00:35:35 EDT 2020

Wow, excellent information Gil.   Perfect for a project I’m currently working on too…    I may print this out and tape it on my shop wall! ;-)

Clay

KG7LWX

From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Gil Smith
Sent: Wednesday, July 08, 2020 8:30 PM
To: Jim Pruitt; GreenKeys at mailman.qth.net
Subject: Re: [GreenKeys] DT600 replacement op amps

Ahh, the first semiconductor opamp honor goes to the legendary Bob Widlar's designs: uA702 (1964) and the popular-but-fussy uA709 (1965) from Fairchild.  

The ancient (and still available) uA741, LM741, which dates to 1968, was super popular since is was very stable and easy to use -- a dual version of the same basic design is the LM1458 and a similar quad is LM348.  Yup, still available.  Some fun facts:
https://web.stanford.edu/class/archive/ee/ee214/ee214.1042/Handouts/ho15opamp.pdf

In the early 1950s, George Philbrick made the K2-W which used two vacuum tubes -- I have one around here somewhere.
https://www.computerhistory.org/revolution/analog-computers/3/156

I don't know what a DT600 circuit looks like or if you are looking to use an original circuit board.  If you need to substitute on an original board, you need to pay attention to the "extra" pins on a single opamp.  The 709's dip-8 (and pricey TO can) have input compensation on pins 1 and 8, and output compensation on pin 5, and the 741 has offset null on pins 1 and 5 and nc on pin 8.

Newer single opamp parts may have compensation, offset null, or no-connect on those three pins.  The good news is that single opamps in dip-8 (and TO if avail) still use the same in/out/power pins as the 709/741 used.  A good choice to replace a 709/741 would be a new amp that is internally-compensated and has no null (just nc on pins 1/5/8) so you can leave the existing pcb circuitry on pins 1/5/8 alone.  However, a lot of good singles still have offset pins (usually 1/5) so you may need to lift those from your board if you are dropping into existing sockets.

Presuming you are using +/-15V supplies in an old design like that, it eliminates a lot of really nice new cmos parts. In the spec sheets you need to pay attention to power supply voltage range and current, gain-BW, slew-rate, offset-voltage, and maybe Ibias, open-loop-gain, and voltage/current noise, depending on your performance goals.  You can't often put a bipolar part in a circuit designed for a jfet-input (probably too much bias current, and if hi-z input circuitry, too much current noise and possible offset current issues), but in a circuit designed for bipolar you can substitute more.

If you want to stay bipolar, you might try a classic low-noise OP27, or pricey ultra-low-noise LT1007, LT1028, LT1115, OPA211, ADA4004-1, etc.

For a jfet-input part it is still hard to beat the classic and cheap TL071, or try a pricier OPA130, OPA132, OPA134, OPA137, ADA4625-1, or a decent-price/perf OPA1641.

The TLC071 is a nice bi-mos part.

There are a few good cmos parts that can run on high-voltage supplies like OPA171, and OPA192.  Cmos opamps have really gotten to be low-noise, low-distortion high performers these days, with TI leading the way in awesome offerings, but most are single-supply 5.5V max power supply.

The OPxxx parts were from the now-gone PMI (precision monolithics inc), not to be confused with the OPAxxx parts from the now-gone Burr-Brown (now a part of TI), and TI continues to add to the OPA line today.  The LMxxx parts were originally from now-gone Fairchild and now-gone National Semiconductor (part of TI now).  NExxx was now-gone Signetics, iirc.  MCxxx was now-gone Motorola.  Not sure where TLxxx originated.  There are oodles of super-nice expensive parts from Analog Devices (ADxxx) and now-gone Linear Tech (LTxxx) which is now part of AD.

I mostly use dual opamps, which don't apply to your above circuit I presume.  Some parts I like these days are:

  (these generally can run down to about +/-2.5V, some cmos parts down to a single supply as low as 1.8V)

- bipolar:  LM4562, LME49723, RC4580, TL972, OPA1602, OPA1662

- jfet:  OPA1642, OPA2134 (a fav of mine)

- cmos:  LMV797, OPA1652, OPA1678, OPA2376 (my current fav for 3-4V operation)

Some many opamps, so little time to play with them all...

gil

gil smith, AF7EZ
greenkeys moderator
gil at baudot.net

 -------- Original Message --------
Subject: [GreenKeys] DT600 replacement op amps
From: Jim Pruitt <jpruitt67 at gmail.com>
Date: Wed, July 08, 2020 4:08 pm
To: GreenKeys at mailman.qth.net

I wonder if anyone knows if there are better and more modern op amps to use in the DT600 (if someone was building one now...which I am) than the 741 and the single 709?  I know the original ST6 by Hoff specified the 709 in the input and not to be substituted but am wondering if the DT600 had that same requirement and whether a newer more available through hole op amp would be better.  Has anyone done that?  What were the results (better?  or same?  or worse?)?  What opamp(s) would be good replacements?

Thank you.

Jim Pruitt
WA7DUY

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