AW_music said:
Hi Jim,
finally i've found the time to give you all the infos you nedd.
I've used a 0-220 to 0 -18V 1.39A / 0-18V , 1,39A toroid .
and this is the schemo :
I hope you can give me "the solution" ....
Many Thanks
Ok as the circuit says 2 x 20vac is the minimum transformer needed and even that is a fine line and only ok for a couple of phantom mic loads on the 48v rail.
For the 48v reg to regulate it will need about 54v at it input pin under very light load. And about 59v or more under heavy load as the pump charge values for C3 and C11 of 100uF and storage charge cap C5 of 220uF is very small so ripple will be come large and the reg will go in and out of reg every half cycle.
OK here is the simplified math (Voltages below will be up to 10% higher under load due to less losses in the transformer. But also under load the doubler value of 2 falls more to 1.5 with large ripple.)
2 x 18vac transformer voltage to the 48v reg input (18 x 1.414) -1.4v(diode loss) x 2(doubler) = 48v into the reg input pin with no load means reg is not regulating ever.
NO GOOD for anything but phantom power will still seem fine and quiet if the match of the 6k8 resistors is good but the 48v reg is really not working. Add the Fet DI load to this and you will have a DI that HUMs
2 x 20vac transformer voltage to the 48v reg input (20 x 1.414) -1.4v(diode loss) x 2(doubler) = 54v into the reg with no load.
OK. So this will be just enough for running a few phantom mics fine with a properly regulated rail.
2 x 22vac transformer voltage to the 48v reg input (22 x 1.414) -1.4v(diode loss) x 2(doubler) = 59v into the reg with no load.
2 x 24vac transformer voltage to the 48v reg input (24 x 1.414) -1.4v(diode loss) x 2(doubler) = 65v into the reg with no load.
2 x 25vac transformer voltage to the 48v reg input (25 x 1.414) -1.4v(diode loss) x 2(doubler) = 68v into the reg with no load.
All Perfect. Should be enough voltage for the 48v reg to keep regulating with an extra load of the DI's. But the small 100uF values of C3 & C11 and C5 220uF could mean the rail will sag to much under heavy load like 4 DI's maybe.
OK before you all go out and buy the correct higher secondary voltage transformer see below solutions
Solution 1 if you have a 2 x18vac transformer and don't care that the 48v isn't perfect and just want the JLM Fet DI to have no hum. Short out the 2k7 resistor on the Fet DI and wire the +V of the DI to the +18v rail. Add heatsink to +18v reg. Done
Solution 2 if you have a 2 x 18vac transformer and want a well regulated 48v for perfect 48v phantom and to run the JLM Fet DI properly. Rewire pin 3 of BR2 by cutting its track and wiring it to the + of C1 to make the doubler into a tripler like our AC/DC kit can do. Giving the 48v reg about 72v to its input to work with. Add heatsink to +48v. Done. If you are trying to run 4 x JLM FET DI's and the last one or two start the hum to come back change C3 and C11 to 1000uF 50v types to get better pump charge to the 48v reg. C5 changed to 470uF or larger would also help.
Solution 3 if you have a 2 x 18vac transformer and want a well regulated 48v for perfect 48v phantom and to run the JLM Fet DI properly. Buy a AC/DC set it to tripler and set +48v and +/-18v and you done
(Our AC/DC uses 1000uF for these 3 pump change caps so will not sag under the 4 x Fet DI load.)
stanz said:
I did try something though. Reading Joe's comments while trying to help AW_Music, I measured the voltage feeding the regulator for the 48v supply. It was measuring about 46v, and I was getting about 44v after the regulator. I disconnected power to the 2 DI boards, and then I was getting about 51v feeding the 48v regulator. I built a separate power supply for the DI to see if that was it. I had a 24v/400mA transformer laying around. Bridge rectifier with a 100uF cap gave me about 38v. Still have hum though. Now it is an octave higher though. More like a ground loop? I am not quite clear on why the 48v supply would produce hum. I am assuming it is too much for the supply, or more accurately, not enough for the DI, amperage, not voltage that is.
The higher pitched hum would seem to indicate a ground loop, where as before possibly AC inductance? I do not have the exact frequencies. The lower one was between 6th and 7th fret of a bass on the low E.
Same as above problem and your measured voltages proves it
Making a unregulated 38v rail is going to make the Fet DI hum and with only 100uF cap it is going to hum big time. It is the all important regulation bit that gives the perfectly smooth power that stops the hum being injected via the power rail.