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 #113526  by Cmnaround
 Sat May 05, 2012 2:42 pm
How is the active preamp in a Carvin DC400A similar or different than a unity gain buffer? Apparently the pickups are passive and the active mode is a pre-amp, which leads to my question- do I already have a unity gain buffer with this active preamp or is that something completely different?
 #113530  by Cmnaround
 Sun May 06, 2012 6:22 am
Thanks, reason I'm asking is because it's not working - constantly shorting out so I'm about ready to rip it out and get it back to a straightforward all passive set up. But in a rare moment of sanity I thought if it is intended to work the same way as a unity gain buffer maybe I should see if I can get it back to working order and use it as intended. Right now it is just a pain as its quickly becoming non functional to the point where I can't play live with it - so I'm at the point of trying to fix it or really fix it by pulling the circuit board completely and just having normal wiring.
 #113531  by hippieguy1954
 Sun May 06, 2012 6:43 am
Either way, I would try and see whats "shorting". Unless it is something on the board such as a component, you should be able to find out what is going on that is causing it to not work. Check all the connections (if you haven't).
:smile: :smile: :smile:
 #113537  by gr8fulbluz
 Sun May 06, 2012 8:47 am
to the best of m recollection ( which aint great ). The active mode is an on-board preamp which allows bass and treble boost and cut.
So I think it is similar in that it is a preamp boosting the passive signal from the pickup. But apart from that i am unsure and i dont think it is exactly the same.

My Carvin has the piezo option so I don't have a passive mode, my actives are on all the time, no push pull pot.


Good luck. hope it all works out.
 #113539  by tcsned
 Sun May 06, 2012 10:43 am
Brad or Waldo could give you a better answer but the difference as I understand it is that the unity gain buffer doesn't boost your signal it keeps it constant through long cable runs. I think it actually keeps your gain slightly below unity. It also helps effects like the envelope filter as it provides a steady predictable signal. An onboard preamp boosts signal, it might give you some of these properties but it's a different animal.
 #113540  by JonnyBoy
 Sun May 06, 2012 11:13 am
Active means that there is a current supplied to to the circuit, ie 9 volt battery. Like a passive monitor (not powered) and an active monitor (powered). That's what I think at least.

Preamp can mean lots of different things. Essentially it means a circuit stage before the power section. What that circuit does can vary greatly. So a guitar having an "active preamp" on board means that it has a powered circuit that could be doing a number of things within the guitar. It may not be buffering the signal though. Carvin should have some info on what that circuit does. Usually the description would say, " this guitar has an active preamp that does this that or the other." if it doesn't mention buffering the signal, then it may not have a buffer in the circuit.
 #113544  by gr8fulbluz
 Sun May 06, 2012 2:32 pm
indeed these two guys said it wonderfully. I have not thought about it in a while but i have considered swapping it all out. And now i like it the way it is, these things change for me, so indecisive.

not that it will help. here is a piccy of the active module.
mine is different with the piezo but similar cavity.
Image
 #113546  by Rusty the Scoob
 Sun May 06, 2012 4:53 pm
Cmnaround wrote:How is the active preamp in a Carvin DC400A similar or different than a unity gain buffer? Apparently the pickups are passive and the active mode is a pre-amp, which leads to my question- do I already have a unity gain buffer with this active preamp or is that something completely different?
gr8fulbluz wrote:to the best of m recollection ( which aint great ). The active mode is an on-board preamp which allows bass and treble boost and cut.
So I think it is similar in that it is a preamp boosting the passive signal from the pickup.
But apart from that i am unsure and i dont think it is exactly the same.
Both of these sound exactly like the same type of onboard preamp that bass players often have installed. Even $200 basses commonly come with preamps and onboard EQs from the factory now, I'm not sure why only Carvin seems to think it's useful for guitar, it's pretty handy to have an EQ at your fingertips.

Comparing John Kahn's 77-78 tone to my Fender Precision with the preamp both bypassed and engaged with the treble and bass boost set to 0, the engaged mode is much closer to his tone, it really nails it nicely. Commercial onboard bass preamps were very rare at the time and aftermarket ones were non-existant, so I suspect he had a custom UGB installed similar to Jerry's.