KA-electronics Flat phono MC preamp.

[Pål gundersen]

It is a DB-9 pinned out like this:

Thanks

 

[KA-Electonics.com]

@KA-Electonics.com

Is it ok to leave the phono-amp on all the time or do you recommend a power switch?

My old Graham Slee RIAA didnt have a power button so it didn't cross my mind to add one when i put mine together, so now i am just wondering if i should hurry up and and add one, or do it at a more relaxed pace

I'm not sure the preamp itself really cares whether its left on or not. There are no electrolytic capacitors in the signal path that would need to be constantly-formed. So yes its OK to leave on.

If you switch the preamp on or off with the Meanwell supply only the DC connections are being switched so the Meanwell is always on anyway. In that case the preamp might as well be left on too.

I run several things continuously but living on an escarpment, where it's prone to lightning, I always switch off or disconnect things during thunderstorms.

 

[bjarnetv]

Tok en runde med DC sjekking i dag, bare for å se at alt var som det skulle (noe det naturligvis var), men la merke til at motstanden som var koblet over 5v på strømforsyningen ble ganske varm - nærmere bestemt 100 grader.

nå tviler jeg jo ikke på at den er dimensjonert for å tåle dette, men det føles jo også litt unødvendig å ha en slik varmeovn i kabinettet om den kun er der for å senke støyen fra strømforsyningen ørlite gran.

@KA-Electonics.com
How much good does the resistor across the 5v pin on the power supply do, i.e how much does it lower the noise floor of the power supply?

I am tempted to remove it just to keep the cabinet temp down if its not strictly necessary, as having a 100c heat source inside the cabinet seems a bit detrimental To the long term health of the amp.

And i guess if it does indeed do much good, and you recommend i keep it, i better whip out the drill bits and improve the airflow a bit

 

[Pål gundersen]

Tok en runde med DC sjekking i dag, bare for å se at alt var som det skulle (noe det naturligvis var), men la merke til at motstanden som var koblet over 5v på strømforsyningen ble ganske varm - nærmere bestemt 100 grader.
Vis vedlegget 896288

nå tviler jeg jo ikke på at den er dimensjonert for å tåle dette, men det føles jo også litt unødvendig å ha en slik varmeovn i kabinettet om den kun er der for å senke støyen fra strømforsyningen ørlite gran.

@KA-Electonics.com
How much good does the resistor across the 5v pin on the power supply do, i.e how much does it lower the noise floor of the power supply?

I am tempted to remove it just to keep the cabinet temp down if its not strictly necessary, as having a 100c heat source inside the cabinet seems a bit detrimental To the long term health of the amp.

And i guess if it does indeed do much good, and you recommend i keep it, i better whip out the drill bits and improve the airflow a bit

100 var mye…

 

[KA-Electonics.com]

100°C is indeed hot but not unexpected as the resistor is dissipating about 2.5W. I do think it is a bit hot for a plastic case.

I made measurements on the P25A some time ago - the measurements are of the DC rails and not the preamp.

+15.32, -14.95 measured with 5V loaded in 500 mA. (10Ω)
V+ noise -52 dBu (100kHz) -40 dBu (4MHz) 5V loaded in 500 mA.
V- noise -68 dBu (100kHz) -40 dBu (4MHz) 5V loaded in 500 mA.

(The measurement bandwidth of the HP400F meter is shown in parenthesis.)

+14.72, -14.32 measured with 5V unloaded.
V+ noise -38 dBu (100kHz) -30 dBu (4MHz) 5V unloaded.
V- noise -36 dBu (100kHz) -30 dBu (4MHz) 5V unloaded.

The preamp however has considerable power supply rejection and I sometimes test boards without the 5V load.

What you might do is perform an FFT with a 96 kHz sample rate both with and without the resistor.
I use ARTA and AudioTester for FFTs.
If you don't see much difference in the noise floor remove the pre-load.

 

[KA-Electonics.com]

I just ran some FFTs comparing a bench linear supply to the Meanwell P25A both with and without the 5V preload.

There was no discernible difference between the three so I see no harm in removing the resistor if the heat is a problem.

 

[bjarnetv]

I just ran some FFTs comparing a bench linear supply to the Meanwell P25A both with and without the 5V preload.

There was no discernible difference between the three so I see no harm in removing the resistor if the heat is a problem.

Nice!
Thanks for taking the time to do the test!

 

[cyberkul]

Hi Wayne! @KA-Electonics.com. I notice that in the building instruction it says put a shunt on J2 during soldering of the 6-pin header, and leave the shunt in place. But in the assembly picture the shunt is located on J1?