Amp Related: Going directly into return IN, bypassing all preamp stuff

[nordine]

Hya guys,

Maybe this could be a very obvious topic, but amp stuff, the basics, is a territory i've recently been exploring 

case is:
I have an IbanezSW35, nice bass amp, but not the tone im after
toneshaping seems to favor mellow jazz stuff (honky mids) and slap (has a tweeter), but i think for rock/hardcore it has to much of "nice" sound

I want it raw and dirty.... Theoretically, i want to build a nice preamp with the clanky sound i love, BUT, i want to know what might be needed to drive that power amp stage

Maybe a Jfet-Opamp hybrid, with semi-para eq, then what?

To equal the current loudness of this amp

Do i need to feed the circuit with more than 9 volts?
Do i need a booster in the out?
what is it? Thanks in advance 

[newfish]

Hi - this can be done.

A friend of mine runs a 'POD XT' thing into the Effects return of his Behringer combo with no trouble.

The only thing I'd be cautious of would be that some amps ground the input if there's no jack plugged into the 'Input' socket - so you may need a 'dummy' jack plugged in.

Am I right in thinking the Peak-to-Peak of a post-pre-amp signal is roughly 1v - as in, +/- 0.5v?

Cheers!

[boogietone]

For most amps with a effects loop, this is not a problem. Search "four cable method" to see how it is possible to have the possibility to keep your amp's pre as an option.

[defaced]

The input being grounded is favorable since it will decrease noise from the amps preamp. If you power your new preamp from the wall (not a wall wart, directly from the wall with a safety ground on each device), you will want to make sure the amps and preamps wall sockets are wired properly. You can turn either one inside out if they're wire out of phase. Otherwise, doing this is a nonissue.

[familyortiz]

If the amp input remains grounded, wouldn't the two inputs to the power amp wrestle each other unless summed?

[boogietone]

If the amp input remains grounded, wouldn't the two inputs to the power amp wrestle each other unless summed?

It is not always the case, but the effects loop of many amps are serial meaning that inserting a cable into the return jack of the amp breaks the connection to the "on-board" preamp. Now, this is not always the case. Some amps have parallel effects loops and a blend control. This, however, are designed to sum the preamp and the signal coming in from the return jack. In this case, phase relations are important. In general, the power amp has know way to know whether the signal it receives from the return jack originated from it's preamp or something else. So long as the signal is within limits, the return jack can sometimes be looked at as simply the input to the power amp.

Disclaimer: In most cases, guitar amps are pretty fault tolerant and can handle all kinds of abuse. But, this is not always the case. If you do not understand what you are doing ask your local tech to confirm it for you. Get to know your amp. Check the schematic to be sure.

[defaced]

Wrestle? Eh, not really. More like mix the noise from the amps preamp with the signal from the outboard preamp. Also, the design details of the FX loop count. It may disengage the amps preamp signal when something is inserted into the return jack.

[petemoore]

   If no limit to the options:
   Try a different speaker arrangement.
   Perhaps just some treble-boost = bass rolloff at right before the input [enter: perhaps inside the amp? questions here].
   An input requires ground, signal input / nothing else, a schematic makes this easier to read, taking into account every type of possible wiring method and ferreting out the exact signal path is another way to get input be only the source, if these conditions are met, whether or not the 'preamp' satisfies what the amp/you expect in terms of performance/matching would be another priority.
   Focus on the speaker alone is worth a mention, then input basscutting methods might be a good order to get it more guitar sounding, but the woofer with the tweeter disconnected might be a good sound too, especially for distortion and Fuzz tones. Possible caveats being that the woofer rolls off too much highs, combined with basscutting a bassy speaker might reproduce less than an extended frequency range.
   Putting a switch on a tweeter wire is probably enough to get rid of unwanted high-highs, that and a bass knob tweek might be enough.
   It definitely depends on the voicing of the speaker..what the amp will sound like.
     
   

[familyortiz]

Wrestle? Eh, not really. More like mix the noise from the amps preamp with the signal from the outboard preamp. Also, the design details of the FX loop count. It may disengage the amps preamp signal when something is inserted into the return jack.

Actually, unless the connection is broken from the amp, when something is inserted into the return or a summing juntion is used, they will wrestle with each other because one output wants to be at ground and another wants to be at another level, usually. Not only wrestle but wrestle destructively in many cases unless short circuit protection is onboard.

[defaced]

Huh? I'm definitely not understanding what you're describing. Do you have a schematic you can reference that would behave this way?

[boogietone]

Wrestle? Eh, not really. More like mix the noise from the amps preamp with the signal from the outboard preamp. Also, the design details of the FX loop count. It may disengage the amps preamp signal when something is inserted into the return jack.

Actually, unless the connection is broken from the amp, when something is inserted into the return or a summing juntion is used, they will wrestle with each other because one output wants to be at ground and another wants to be at another level, usually. Not only wrestle but wrestle destructively in many cases unless short circuit protection is onboard.

Any amp that "works" like this and has two signals "wrestling" with each other is either broken or very badly designed.