Has anybody used 78xx regulator for negative voltage?
Before you say "why don't you use a 79xx voltage regulator", reason for asking is that I am looking at low noise regulators that are pin compatible with 78xx devices, such as those based on LT3045 3094 devices, see below. The -ve voltage regulators are £ 10 (50%) more expensive than the +ve ones I'm so penny pinching!
My application is a DAC - a search reveals that low noise regulators have been discussed before but has anyone used one of these to replace the supply to digital section (+5V) and/or the analogue (+/- 12 v) and was the effect noticeable/worthwhile?
From Fairchild application notes, it seems that you can - but I'd like to hear of personal experience before I try it as see clouds of smoke!!! 😱
Before you say "why don't you use a 79xx voltage regulator", reason for asking is that I am looking at low noise regulators that are pin compatible with 78xx devices, such as those based on LT3045 3094 devices, see below. The -ve voltage regulators are £ 10 (50%) more expensive than the +ve ones I'm so penny pinching!
My application is a DAC - a search reveals that low noise regulators have been discussed before but has anyone used one of these to replace the supply to digital section (+5V) and/or the analogue (+/- 12 v) and was the effect noticeable/worthwhile?
From Fairchild application notes, it seems that you can - but I'd like to hear of personal experience before I try it as see clouds of smoke!!! 😱
Attachments
well spotted, correction should be like this ..... but is the 78xx going to work as negative regulator?
Yes, the configuration shown in the LT1086 schematic works very nicely.
The only caveat is each regulator must have its own transformer winding, as shown.
The only caveat is each regulator must have its own transformer winding, as shown.
79xx voltage regulators are cheap, probably less than £1, but that is not the issue.
The low noise regulators based on LT3045 are around £20 for +ve and £30+ for the +ve, that's a big difference.
The low noise regulators based on LT3045 are around £20 for +ve and £30+ for the +ve, that's a big difference.
For what it’s worth, the OnSemi MC78xx and MC79xx are superior to the common 78xx/79xx. It’s widely held they are a different (superior) design that are pin compatible instead of a clone of the original Ti. The noise specifications in the datasheet are notably better than standard.
They cost a pound or less.
They cost a pound or less.
It might make a measureable difference as you surely have read, but is it audiable and therefore worthwile?
Personally I doubt it, but that depends entirely on you and what you can hear, or what you think you can hear.
I used a couple of ESS ES9038Q2M DAC boards on recent projects and had similar questions so I followed the tweaking threads here for a while.
Gave up after I realized all the improvements were well into the area beyond human hearing, and I think the chip sounds great even in its basic implementation.
I build stuff for listening, not for the sake of numbers.
Very often when looking into DACs, CD players, Amps with regulators the capacitors between the legs of 78xx and 79xx were missing!
Just adding appropiate C's due to dratashhet made the sound much better.
Just adding appropiate C's due to dratashhet made the sound much better.
Keep in mind that these 78xx regulators will source, but not sink current.
The point of 78xx and 79xx is quick and simple, not high performance. If you want something better, then you need a hand full of transistors and maybe an op-amp. You could use a 78xx as a reference and an op-amp and PNP pass transistor or LM337 to follow the 78xx with a tracking negative supply. And have a look at LV's LM317 (etc) noise improver.
https://www.diyaudio.com/community/...grade-any-317-based-v-reg.331491/post-7207080
https://www.diyaudio.com/community/...grade-any-317-based-v-reg.331491/post-7207080
Another thing to keep in mind when using these regulators is the amount of capacitance on the input and output sides of the regulator. If there is a large amount of C on the output and a lesser amount on the input side, then there can be trouble when you power down the circuit. Discharge current can flow backwards through the regulator and clobber it. To prevent that, place a 1-amp standard rectifier diode across the input and output pins of the regulator such that the diode is not conducting (is reverse-biased) when the circuit is in operation. Upon shutdown, if reverse current flows, the diode will catch it and rout it around the regulator, protecting it from reverse current damage. The diode will not degrade the regulator's performance during normal operation.
78xx/79xx are common emitter output, their true superpower is that you can put a LOT of capacitance downstream if you like.
The diode(s) as @egellings suggests are always a good idea.
The diode(s) as @egellings suggests are always a good idea.
