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Old 11th March 2013, 09:18 PM   #1
jcm1963 is offline jcm1963  United States
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Default effects send and/or preamp out jack on guitar amps

Hello Everyone,

I'm trying to figure out how to do the following:

allow the preamp output of my guitar amp head (a solid-state head, i should mention) to be plugged into the front end of another guitar amplifier.

i'm obviously able to plug that preamp out into the effects return of another guitar amp, or into a power amp. but i would like the option of plugging it into, for example, an older fender amp that does not have an effects loop.

I used to own a Mesa Boogie Studio Preamp. On the back it has "main outputs" and "recording outputs". In the manual it states that the recording outputs are buffered and eq'd in a way that allows them to either be plugged directly to a mixing console, OR, directly into the FRONT END of another guitar amp (which would be kept on it's clean channel). I actually did this quite often and it sounded really good.

anyone have a guess as to what mesa boogie did to those recording outputs to get them to work in that fashion?

any thoughts on this are welcomed. thanks in advance!

- Jeff
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Old 11th March 2013, 09:34 PM   #2
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1) This belongs in the MI forum; a mod will probably move it there.

2) Just google the preamp schematic. It clearly shows the circuit M/B uses. It's basically a 12AX7 CC stage, one output dry, and 1 output with passive EQ.
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Old 12th March 2013, 01:17 AM   #3
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Sure, you can connect the preamp out into the input of another guitar amp, but the signal level is probably going to be way too much, resulting in heavy clipping. The whole purpose of the 'first' preamp is to amplify the small guitar signal by a certain amount. The second preamp is expecting that same small signal and not the already amplified one. You could turn down the first preamp's gain, but that would influence the tone and the signal that's being fed to the effects in the FX loop in a negative way. Your best bet would be to reduce the signal comming out of the first preamp with an additional control (passive or active).

The reason the Mesa Boogie worked is because a 'recording output' level is much much smaller than the signal level inside a preamp. A recording output is basically nothing more than a reduced signal level with a certain buffered (isolated) output impedance. Maybe there's some filtering going on, but probably not that much. Perhaps it's referring to the fact it's pre EQ(?)
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Old 12th March 2013, 07:46 AM   #4
jcm1963 is offline jcm1963  United States
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Thank you for the info. I do remember the recording outs on that boogie to be much quieter than the main outs. I found a schematic of the mesa boogie, I'll see what I can do about trying to copy their recording outs.
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