I am knee deep in a project to put the Lab Series L5 preamp in a 1790NS enclosure (EHX "XO" series, or around the size of a Fulldrive)
It'll be pretty well identical to the original, except it'll be channel-switchable, with the initial gain stage shared between both channels and a footswitch to switch it from there. I am also replacing the CA3094 with a CA3080 + discrete Darlington per the original schematic in the 3904 datasheet.
Anyway: I have a couple of questions regarding power handling and biasing.
First: the supply. I don't think there is any good reason to stick with the +/-15V from the original, when compared with the difficulty of making such a supply (not to mention the danger of "rolling your own" from AC power). I am planning on using a LT1054 voltage inverter that should work with either 9V or 12V.
The LT1054 can handle up to 100mA. So, I crunched the numbers on the L5 and came up with the following power requirements:
CA3080: 1.2mA each (x2) = 2.5mA
LM741: 3mA each max (datasheet says 1.7-2.8mA) (x2) = 6mA
JRC4558D: 6mA each (3.5 - 5.7 datasheet) x12 = 72mA
LF356N: 10mA each (5-10 datasheet) x1 = 10mA (max)
Total: 90.5mA if everything was at the very top of its specified range. This is also based on their power requirements assuming +/-15v. The draw should be around 20% less at +/-12V and 40% less at +/-9V, so the full circuit should be significantly less than the LT1054's capacity.
So my first question is: Should this theoretically work OK? Or have I made a mistake in my calculations someplace?
Second question is much easier. The trimming/biasing/tuning methods for the L5 are well documented in their service manuals, and the schematic itself is loaded with test voltages at major junctions and test points. If I am using a supply lower than the standard +/-15V, should I aim for the voltages and test results they specify (since we are nowhere near the rails no matter if it's running at +/-9v, 12v or 15v) - or should I plan on everything being proportionally smaller?
It'll be pretty well identical to the original, except it'll be channel-switchable, with the initial gain stage shared between both channels and a footswitch to switch it from there. I am also replacing the CA3094 with a CA3080 + discrete Darlington per the original schematic in the 3904 datasheet.
Anyway: I have a couple of questions regarding power handling and biasing.
First: the supply. I don't think there is any good reason to stick with the +/-15V from the original, when compared with the difficulty of making such a supply (not to mention the danger of "rolling your own" from AC power). I am planning on using a LT1054 voltage inverter that should work with either 9V or 12V.
The LT1054 can handle up to 100mA. So, I crunched the numbers on the L5 and came up with the following power requirements:
CA3080: 1.2mA each (x2) = 2.5mA
LM741: 3mA each max (datasheet says 1.7-2.8mA) (x2) = 6mA
JRC4558D: 6mA each (3.5 - 5.7 datasheet) x12 = 72mA
LF356N: 10mA each (5-10 datasheet) x1 = 10mA (max)
Total: 90.5mA if everything was at the very top of its specified range. This is also based on their power requirements assuming +/-15v. The draw should be around 20% less at +/-12V and 40% less at +/-9V, so the full circuit should be significantly less than the LT1054's capacity.
So my first question is: Should this theoretically work OK? Or have I made a mistake in my calculations someplace?
Second question is much easier. The trimming/biasing/tuning methods for the L5 are well documented in their service manuals, and the schematic itself is loaded with test voltages at major junctions and test points. If I am using a supply lower than the standard +/-15V, should I aim for the voltages and test results they specify (since we are nowhere near the rails no matter if it's running at +/-9v, 12v or 15v) - or should I plan on everything being proportionally smaller?