Transistors on regulator ground pin

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EmilFrid

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Hi!

attached is the psu diagram of an old Alice mixer. I noticed that the 7815 and 7915 has transistors attached to their ground pins, and even though I think I get what they're doing, I'm wondering if it's worth the extra cost and effort. Do they improve the performance of the regulators all that much? Was this common practice in mixers at the time?
Screenshot_20220128-153218_Drive.jpg
 

JohnRoberts

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It's just a cheat to push up the rail voltage a few volts higher. Standard op amp process technology is 36V so in theory +/-18V. IMO the extra voltage is not worth the trouble and electronics will run hotter.

JR
 

Ike Zimbel

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It's just a cheat to push up the rail voltage a few volts higher. Standard op amp process technology is 36V so in theory +/-18V. IMO the extra voltage is not worth the trouble and electronics will run hotter.

JR
Why wouldn't they have just used 7818/7918 regulators? I'm guessing because maybe they had a lot of 15 volt ones on hand (production decision), or that the 18v ones were harder to source back then...personally I don't remember, but I do remember that I had a jig for matching (or at least trying to :confused:) the +/- 15 volt ones.
 

JohnRoberts

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Why wouldn't they have just used 7818/7918 regulators? I'm guessing because maybe they had a lot of 15 volt ones on hand (production decision), or that the 18v ones were harder to source back then...personally I don't remember, but I do remember that I had a jig for matching (or at least trying to :confused:) the +/- 15 volt ones.
That schematic is dated 1987 so 18V 3 terminal regs may not have been cheap or widely available. As a design engineer I was always reluctant about brining new parts into the system. If the design really needed +/-18V the dedicated parts would be cheaper and less likely to experience unintended problems.

From my very foggy memory I recall the 7815 regs being typically +14.75V and the 7915 being -15.1 or so... the extra voltage swing may look good on paper to impress buyers but will be inconsequential to listening tests. Don't hold me to those nominal voltages all these decades later.

JR
 

Newmarket

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Why wouldn't they have just used 7818/7918 regulators? I'm guessing because maybe they had a lot of 15 volt ones on hand (production decision), or that the 18v ones were harder to source back then...personally I don't remember, but I do remember that I had a jig for matching (or at least trying to :confused:) the +/- 15 volt ones.

Out of curiosity - why were you trying to match the +/-15V regulators.
 

mjrippe

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Ike, are you a Libra like me? 😁 Always have to have pairs of things, if I get a tattoo on one arm, I need one on the other, etc...
 

Rob Flinn

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Obviously not on this particular diagram, but is it not a way that it is possible to provide current sensing control & shut down protection with adjustable regs ?
 

JohnRoberts

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Obviously not on this particular diagram, but is it not a way that it is possible to provide current sensing control & shut down protection with adjustable regs ?
Back in the 90s my senior mixer engineer designed a circuit that would insure a specific power up sequence controlling which polarity rail always comes up first. Apparently it affects the magnitude of turn on transients with some op amp families. With mixers it is too expensive to put mutes on every single output so general whole mixer turn on transient mitigation made sense.

With 3 terminal regulators there are limits to what you can control messing with the ground leg. I recall some people adding diodes in series to gain another half volt, and presumably protect against an obscure latch up scenario.

JR
 

Rob Flinn

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I recall some people adding diodes in series to gain another half volt, and presumably protect against an obscure latch up scenario.

JR
I have seen people using a diode to jack up a 12v fixed reg to 12.6v or 2 diodes to jack up a 5v fixed reg to 6.2v for valve heaters.
 

JohnRoberts

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The diodes added in series with the ground leg degrade voltage regulation slightly (the current modulates the diode forward voltage) but generally not a problem.

JR
 

JohnRoberts

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It depends on the regulator design, the ground lead current should not be very heavy or change very much with load but it will change some... 3 terminal regulators are GP ballpark devices.


JR
 

JohnRoberts

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[TMI]
For today's TMI about three terminal regulators, and Ive shared this before. I determined back in the 80s that common 3 terminal regulators had a rising output impedance with HF, no doubt due to the early IC technology that delivered 741 performance op amps for the innards. I determined with some bench measurements that I could parallel the output with a roughly 1,000uF electrolytic cap. The falling impedance of the capacitor with rising frequency nicely complemented the rising impedance of the 3 terminal regulator. At extreme high frequency the electrolytic becomes inductive/resistive so doesn't destabilize the 3 terminal regulator. This was a cheap and easy way to make a 3 terminal regulator low output impedance for a few octaves above the audio passband.

Caveat lector... I did this bench work back in the early 80s and modern capacitors have generally improved a bunch since then so maybe do your own bench experiments. I estimated the regulator source impedance by dumping an ac coupled sine wave current into the output of the regulator and look for voltage vs frequency. Note: the regulator needs to be loaded with a nominal load to be fair.

[/TMI]

JR
 

JohnRoberts

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Because I'm a big believer in symmetry? This was the early 1980's so my exact reasons may be a bit hazy...
If you like symmetry you would have liked the regulated supply i designed for my last phone preamp (ever) the P100. I actually used TL07x op amps with some discrete pass transistors. The -15v rail was literally the unity gain inverted version of the +15V rail so within a couple percent of each other. My focus in that design was for low noise and low source impedance, actual regulated voltage and symmetry were secondary concerns.

JR
 

Matt Syson

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Agreed the availability of 18 Volt regulators when this was designed is one question. The fact it says Alice Broadcast is a partial clue as to why 18 Volts was required as the clients ultimately determine the specifications of the gear they buy and in the 1980's Alice were one of the 4 or so companies all vying for the new 'local radio' contracts in the UK. I worked for another of the companies and there was of course significant rivalry. There was an article in Studio Sound magazine by Ted Fletcher (owner/co owner of Alice) with bleary eyed techs draped over an errant mixing desk, with cigarettes hanging out of the mouth and smoke from the soldering iron, in an attempt to meet IBA specifications. The use of transformers that of copurse need proper termination to achieve a frequency response that is 'flat' when some have only a few metres of cable attached but others two or three hundred metres. Oh what fun we had!
The Question of 'equal 'balanced' power rails is curious because op amps don't have a 'ground' reference as such but also most of them have assymmetric output swing capability so depending on it's operating mode (inverting/ non inverting) it is perhaps possible that greater signal voltage headroom could be achieved by running things unequally.
Matt S
 

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Paul Wolff
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Kind of like the 51X stuff. With +-16V able to hit +28 dBu balanced, will a couple of more dB in headroom actually make that record a hit? Especially when in the AtoD and DtoA it may have rails of +1.7 to 5V and then 12V or +-12V...
 

moamps

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It would be interesting to analyze whether these transistors may affect better regulation and less noise at the regulator output.
 
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