Pre amp voltage regulators help.

[AMV8]

Hi

I would like help with voltage regulators that I am building ( trying to build ) for a couple of preamps. I am having trouble finding the right location and value for the capacitors that go close to the regulators.

Both pre amps have remote power supply transformers rectified to provide 24-0-24 to the preamps via low reistance cable. The pre amps work off 18-0-18 volt supplies and I am planning to put the voltage regulators in the pre amp cases. The pre amps supplies are for line stage sections only so I was not making a complicated voltage regulator. I was planning to use Lm317/LM337 or LM7818/LM7918 although I am aware that some would use more sophisticated regulators.

I found that without a capacitor between the input and ground of the regulator there is a big loss of treble and some confusion to the sound. In fact the sound is better without the regulator. I read the national semiconductor datasheets. They indicate that a capacitor will help between input and ground but are vague on the value and location. I experimented by adding different capacitors.

It seems that my best sound is with 1000uf soldered across the input and ground pins of the regulator. Is this normal or am I doing something wrong.

By the way I already have a capacitor between the output and ground pin but that seemed easy to fix in size and location and does not seem to have any effect on the capacitor across ground and input.

All help appreciated.

Don

[AndrewT]

Hi,
input and output caps are compulsory for 78xx, 79xx, 317 & 337 regulators.

The ST data sheet specifies 330nF in to ground and 100nF out to ground for the fixed regs.

Nat semi specifies the adjustable to need caps from in to ground and out to ground and their performance is significantly inferior to the fixed if the adjust to ground cap is omitted. They also mention stability issues if the wrong type of cap is used.

BTW,
I interpret their instructions to mean caps directly on or adjacent to the relevant regulator pins.

[AMV8]

Andrew

Thanks for your reply. You confirm what I am finding.

I am struggling to find the best value and location for thr caps though. To my surprise even moving the caps 2cm for the regulator legs seems to make a big difference. Am I doing something wrong?

Don

[jackinnj]

in any case, the adjustable regulators are quieter than the fixed voltage units, much better still if you bypass the adjust pin with an electrolytic.

when Audio Amateur (or was it AX?) had an article on the Adcom GFP565 modification all the respondents stated that cleaning up the power supply with a high quality regulator had the most dramatic improvative effect on the sound. the circuit boards are still available from Old Colony Sound and are easy to implement if you have the real estate.

[AndrewT]

You confirm what I am finding.

no!!
I am telling you to read and implement the datasheet.

[AMV8]

Andrew

I did read the data sheet as my first post states. ( I always read what manufactures say ) However the data sheet does not say how close to the regs the capacitors need to be - that was my question.

Don

[lumanauw]

PSRR is one important parameter for audio reproduction. 78xx-79xx are low valued in "ripple rejection", the 79xx don't have the graph for this data (National Semiconductor datasheet). LM2990 - LM2940 has better ripple rejection. Making discrete might result in better ripple rejection.

[AndrewT]

I interpret their instructions to mean caps directly on or adjacent to the relevant regulator pins.

[AMV8]

Hi Andrew

Yes; I find that soldered right on to the pin seems to work best. It just looks such a mess.

However, I am surprised that if it is so important to solder onto the pins I would have expected the datasheet to say that. I just assumed that I was doing something else wrong that made it necessary to solder the caps directly onto the pins . I am probably still doing something wrong.

Everyone who replied

Thanks everyone for the comments and help.

Don

[Minion]

This is a standard useage of the LM7xxx regulators....