JLM AUDIO

Hey, I'm having some trouble with my FET DIs. Everything with the Go Betweens works fine, but when I add the FET DI I lose 6 volts on my 48V rail, and with a 4 channel pre I'm ending up around 28V or so, did I do something wrong?

Also, the search function for the forum doesn't allow me to search "go between"

benlindell wrote:Hey, I'm having some trouble with my FET DIs. Everything with the Go Betweens works fine, but when I add the FET DI I lose 6 volts on my 48V rail, and with a 4 channel pre I'm ending up around 28V or so, did I do something wrong?

No just sounds like your 48v rail cannot handle the current load of the Fet DI's (20 to 25mA each).

What power supply is running the 48v rail?

Is it heatsinked if it is a linear LM317 type so it doesn't shut down from overheating?

Yea it's a heatsinked LM317

That should be able to handle it right? Using this power supply but with a 25-0-25 transformer and an LM7824 instead of 7818, and left out the 7918 as well

benlindell wrote:Yea it's a heatsinked LM317

OK then what is the power supply circuit and secondary transformer winding voltages and VA or current as it doesn't sound like there is enough current from the transformer or high enough unreg input volts to the LM317 input to hold the current needed for the load. Also when you say heatsink a small U fin bolt on it not usually enough heatsink.

benlindell wrote:That should be able to handle it right? Using this power supply but with a 25-0-25 transformer and an LM7824 instead of 7818, and left out the 7918 as well

OK does it use a pump charge circuit to get the 48v unreg rail? Schematic of the power supply is what I need see to work out why the 48v is sagging so badly.

I don't think it uses a pump charge, here's the schem : http://www.mentol.dk/diy/upload/DIY_9k_preamp.pdf Page 5.

The power transformer is a 50 VA transformer.

benlindell wrote:I don't think it uses a pump charge, here's the schem : http://www.mentol.dk/diy/upload/DIY_9k_preamp.pdf Page 5.

The power transformer is a 50 VA transformer.

Yes it uses a pump charge with only small 100uF pump C3 & C4 and 220uF C5 storage caps. These do not allow for much current draw before the volts to the 48v reg will sag to much.
Also if drawing a lot of current from the 24v supply you will need to make C1 much larger in value as well. 2200uF to 4700uF plus bolt the 24v and 48v regs to a large metal heatsink or metal case.

On our AC/DC kit we use 1000uF 50v for C3 & C4 and 1000uF 100v for C5 to give about 200mA current draw from the 48v.

Also check that the FET DI relay is working when one is plugged in. You should here the relay click on and off when a jack is plugged in. If not check you have the diode the correct way around on the PCB otherwise the DI will draw a lot of current and not switch properly.