anyone in the know about racking preamps?

  • Thread starter Thread starter Steenamaroo
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Steenamaroo

Steenamaroo

...
hey all,,,


i've started into a wee project, with limited knowledge,,but i'd say i'm capable..

i understand the immediate reaction may be "buy a pre racked preamp", but i'm doing this for a bit of fun, and i'd really appreciate a wee bit of help, so,,,here's my situation


I have an auditronics 110a input channel with 3 band eq
i have a JLM transformer and 5 rail power supply.(0v,+24v,+15v,-24v, -15v and 48v)

I've put together a test setup on the bench, and have wired the following...


psu 110


+24 to +24
-24 to 24v common
+15 to +15
-15 to -15
+48 to +50
and 0v to power supply common


XLR INPUT
xlr input hot and cold, to mic input a + and - on the 110.
and the xlr ground sleive to input common on the 110



i've run an earth wire from the panel/case of the preamp, and the mounting holes of the psu, to plug socket earth (as i figure these will ultimately connect to the rack chassis..)


OUTPUTS
i've taken program out and program common as the outputs. I read somewhere that they are the outputs to be used in conjunction with the jensen output transformer

i haven't attempted to use the jensen transformer yet,,,i just wana keep it 'simple' for now.


i've wired the output to an amplified speaker set that i don't care about, and plugged in a cheap dynamic mic, but the audio output that i'm getting is very very distorted, and has lots of harsh buzz/hum.


i'm waiting to get hold of a volt meter,,,all i have at the minute is an electric test screwdriver, but,,i'm getting a slight live reading from all three xlr terminals!??!

I've contacted Joe at JLM,,,lots of people said he was the man for these projects, but i thought i'd post up here too!




I'm a beginner with limited knowledge, and this is a steep learning curve, but if there's something obviously wrong that you can see, i'd very much appreciate your help....

if it helps, i have schematics, and can post up pics/lists if necessary.

thanks in advance all!!!
 
Check for continuity between "24v common" and "power supply common". If there is, that's your problem--don't wire -24v there.

And next time, buy the multimeter before you start a project!
 
ok, i'll check that when i get the meter,,,,thanks!


if it's wrong though, where should 24v common on the 110a be wired to?
 
Don't know, but I'm guessing that's signal ground. But I don't know what the 24V is used for. And if there are separate power and signal grounds, there might not be continuity between them on the unit (that would be done elsewhere in the device).

If there is continuity though, no question that's a ground and not a power rail.
 
ok, so you're right,,,


according to another thread, 24v common shud be grounded along with psu common.


sadly it makes little or no difference to my distorted output :(
 
ok,,back again.....


i spent a bit of time working away, and reading up, and i'm glad to say i have a nice clean preamp working like a charm!

thanks for your help in getting me there...


the only problem that i'm having is this..


when i turn the gain knob (multi stage selector rather than a smooth pot) i get a peak, like if you unplug a jack lead?


it only happens on the lowest 4/5 positions,,,so i'm thinkin maybe just an old/duff selector,,

but i thought i'd ask incase anyone knows better...


thanks again.
 
Well, you are disconnecting and reconnecting the signal or some portion thereof, so yeah, I'd expect to get a pop. Doubly so if there's a DC offset in there.

If it only is problematic at certain gain settings, that might be dirty contacts, differences in the pole spacing in the switch (make-before-break vs break-before-make), or it might be that some of the gain settings allow a significant DC offset to flow through the switch, which would make the pop much worse. If that's the case and if the DC offset is unimportant at that particular point in the circuit, you could add a capacitor inline somewhere to block the DC. That said, I'd expect that the offset is important if it is there, and I would tend to suspect dirty contacts or differences i pole spacing rather than differences in the DC offset. Just a gut feeling.
 
ok,thanks for all the info..

tbh, some of it is right over my head,,,but i've discovered that the noise only occurs if a condenser mic is in use.


At the minute, there is no 48v switch, so it's always on....but there's no popping with dynamic mics, so i guess it's only happening when the 48v is in use?

i'm pretty confident that my wiring is perfect now, but idk..i just don't trust this noise,lol.......it makes me nervous about pluggin in expensive condensers....

i'm currently testing with a samson pencil jobby so all is good,

as someone else pointed out though, there's nothing to stop me wiring up a pot as a fader, and just dropping it for gain changes...but still..
 
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