[GreenKeys] DT600 replacement op amps

Thu Jul 9 01:34:12 EDT 2020

Gil the DT600 is here:
http://rtty.com/development/tu/dt-600/dt-600.htm
with schematic here:
http://rtty.com/development/tu/dt-600/dt-600.jpg

It was designed by K4OAH and some other Navy Mars members in the mid 
70's.  The original article was in the February 1976) issue of Ham Radio 
Magazine.  The original article had a pc board layout. That board was 
single sided and had provisions for a loop supply on it but the board 
still used the 22 pin .156" (or was it .196") spacing edge connector.  
Data Technology Associates in Florida took that board and made it double 
sided and removed the look supply circuit.  I know that Garey had 
arguments with Irv Hoff over the design of the ST6 and this was Garey's 
answer to the ST6.

I have a Data Technologies version of the pc board.  I also have a built 
DT600 that has the original single sided pc board in it that I picked up 
at a hamfest last year.  I do not have a working teleprinter so the loss 
of a loop supply does not matter.  The DT600 was like the ST6 in that 
the input circuit uses the 709 while all other circuits use the 741.  
They are dual inline 8 pin sockets that I have mounted although the 709 
(TO package) is shown being hand wired into the dual inline footprint.

I just thought that with all the newer op amps that there might be a 
drop in replacement for at least the 741's.  I always thought the DT600 
was a great converter.

Thank you.

Jim Pruitt
WA7DUY

On 7/8/2020 7:29 PM, Gil Smith wrote:
> 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 <mailto:gil at baudot.net>
>
>
>  -------- Original Message --------
> Subject: [GreenKeys] DT600 replacement op amps
> From: Jim Pruitt <jpruitt67 at gmail.com <mailto:jpruitt67 at gmail.com>>
> Date: Wed, July 08, 2020 4:08 pm
> To: GreenKeys at mailman.qth.net <mailto: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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