#1
Hey guys. I've been searching google for answers and have yet to find one. Maybe I'm not using the correct keywords, but I'm hoping you can help.

BACKGROUND: (You can skip this, but it may help you help me) I've been playing for 8.5 years but am only now getting into the half-stack world. I play in a lot of churches and for a recording artists that doesn't like his guitarists having half stacks / amps on stage. We use pedal boards going into DI boxes. I currently use amp simulation on a Digitech RP250 and use the 4x12 cab simulation on my GI-100 direct box. It's okay, but I really want to start playing with heads since you can really dial in the tone you want. I just bought a used head (crate GX-1200H) yesterday and am buying a used Sonic 4x12 cab this Sunday. They aren't the best quality, I know, but I don't have a lot of money to spend on gear at the moment, and having a half stack for $220 sounds good to me.

INQUIRY: Is there some sort of product that you know of that supplies 4+ Ohm impedance that I can plug a 1/4 into that isn't a speaker? What I want to do is plug my head directly into my direct box (which mine is designed to be able to do. It has up to -40dB pad on it). The problem is that the direct box manual tells you that you still need to plug the speakers in through the parallel port that's on it if you don't want to damage your head. This makes sense since you need a proper impedance load for it.

You may ask why I don't just plug it into the cab? Well churches I play at don't really want a cab blaring in the church. They want to be able to do all the mixing. For the band I'm also forming, I'll be using the cab so I feel the investment's worth while already, but I'd also like to use it in churches.

My amp head says it needs 4 Ohm minimum of impedance. I'd like something that gives that much impedance without making sound that I could plug a 1/4" into. ALTERNATIVELY, if I developed a circuit with a jack that gave this result, would this work?
#2
Isn't a Crate GX-1200H solid state? You wouldn't need a dummy load. Solid state amps don't care. They make a constant voltage output regardless of the load attached (or lack thereof) down to their rated impedance limits.
The problem is that the direct box manual tells you that you still need to plug the speakers in through the parallel port that's on it if you don't want to damage your head. This makes sense since you need a proper impedance load for it.

If it was a tube amp this would be correct.
Last edited by _LoveFuzz_ at Aug 8, 2013,
#3
It is a solid state. Are you sure, though? I'd think the circuit is designed to need a minimum impedance. I don't want to fry the board/transformer. Otherwise, it wouldn't have a minimum of 4ohms written on the output, right? I could be wrong, though.

When I turned it on for a short time (I was testing it for side note below), it smelled funny, kinda like something was burning. (remember, no speaker load. This was when I first thought, "hey, this probably isn't good to have no speakers connected".)

On a side note, I got it cheap because it didn't have a fuse, so I didn't know exactly which fuse to put in since all the manual and the back of the amp says is "To reduce risk of fire, replace fuse with only the same size and type." Very helpful if you don't have the original to work off... I took it to the hardware store, the guy looked at the writing in the back and said I could use a 3A or 5A. I bought both, figuring I'd be conservative with the 3 first and move up to the 5 if it just blows. I blew a 3A when I tried it (again, with no speakers). It took probably like 10 seconds for that to happen. I think not having that impedance (remember impedance is pretty much a representation of the collective resistance in an AC circuit) made the current go too high, thus blowing thing fuse. The amp may want a 3A if speakers are connected.

Again, I may be wrong. I am an engineering student, but a mechanical major; my electrical classes were my worst, so it's definitely possible that I'm wrong.
#4
It should do as long as the input impedance on the di box is higher than the amp output's.

Though, you probably won't hear much because the amp has a 4 or 8 ohm output, and the input impedance of di boxes is usually high.
Over 9000. Seriously.
So, out of those 70w @ 8ohm (which are probably about 50w if the thing is actually made to spit out 100w @ 4ohm), the output at 47000 ohm would be a 5875th of the one rated at 8ohm.
You could always raise the volume after the di, but you'll be better off sticking to the pedalbord instead of triying to play with a now 0.012, but probably actually 0.0085w amp.
Name's Luca.

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Quote by Cajundaddy
Clue: amplifiers amplify so don't turn it on if you need quiet.
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#5
Quote by Spambot_2
It should do as long as the input impedance on the di box is higher than the amp output's.

Though, you probably won't hear much because the amp has a 4 or 8 ohm output, and the input impedance of di boxes is usually high.
Over 9000. Seriously.
So, out of those 70w @ 8ohm (which are probably about 50w if the thing is actually made to spit out 100w @ 4ohm), the output at 47000 ohm would be a 5875th of the one rated at 8ohm.
You could always raise the volume after the di, but you'll be better off sticking to the pedalbord instead of triying to play with a now 0.012, but probably actually 0.0085w amp.


hmmm. I could see that making sense, although the point of direct boxes is taking a high impedance signal to low impedance, isn't it? That's why on all the boxes, it says Hi Z --> Low Z.

Although, is that also what an amp does? What is considered high impedance and low impedance?
#6
Quote by stephen.avery
It is a solid state. Are you sure, though? I'd think the circuit is designed to need a minimum impedance. I don't want to fry the board/transformer. Otherwise, it wouldn't have a minimum of 4ohms written on the output, right? I could be wrong, though.
4 ohms is the minimum impedance. An open circuit is a high impedance. So it does not violate the minimum rule. You don't need a load on a SS amp.

Tube amps have an output transformer to convert low current/high voltage to high current/low voltage. Without a load on a tube amp the internal voltages can build up high enough to start arcing and burning up the internal circuits.

SS amps operate at low voltage/high current and not having a load does not affect anything.
#7
Quote by stephen.avery
It is a solid state. Are you sure, though? I'd think the circuit is designed to need a minimum impedance. I don't want to fry the board/transformer. Otherwise, it wouldn't have a minimum of 4ohms written on the output, right? I could be wrong, though.


solid state amps don't use an output transformer to match the output section to the speakers, evidently the output section on SS amps is a low enough impedance where the speakers provide enough impedance to load the circuit.

you are right though, the head expects to see a min impedance (usually 4 or 8 ohms for guitar heads), but when you leave the head unplugged the amp 'sees' an infinite impedance because the circuit it broken and this greater than the min required impedance.

edit: nevermind, arby already got this.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
Last edited by gumbilicious at Aug 8, 2013,
#8
Yes, DI Boxes are used to change the impedance of a signal from high to low.

An amp also does that, but it primarly amplifies a signal.

Guitar amps are integrated amplifiers, which means that they contain both a preamp, the part of the circuit that is fed with a low level and usually high impedance signal and makes it a lower impedance signal, making it actually "amplifiable".
Done that, the power section, which is called power amp if it's a standalone unit, kicks in and amplifies the signal to a much higher level.
Then, in solid state amplifier you have an output of an Xohm impedance, usually 2 in standalone units, which is used to carry the signal from there to the speaker cabinet.

Impedance in amplifiers outputs is low because you can send a signal from a Xohm output to a Y times Xohm (with Y >= 1), thus reducing your power output by half everytime you double the input impedance.
Don't do this with tube amps though, since the result would be the same as stated below.

Sending a signal from a Xohm output to a Y times Xohm input (this time with Y < 1), would instead result in blowing up a fuse, or the entire output section if there's no safety system.

Then.
A Guitar output is veeeeeeeeery high impedance.
Not sure about how much exactly, though it's veeeeeeeeery high.

Good devices made to handle a guitar output have an input impedance of about 470kohm.
Better devices have an input impedance of about 1Mohm.

Your everyday DI Box has a line level output impedance, which is about 10kohm.
You would actually be better off putting your Amp directly into a mixer's line level input.
Name's Luca.

Quote by OliOsbourne
I don't know anything about this topic, but I just clicked on this thread because of your username :O
Quote by Cajundaddy
Clue: amplifiers amplify so don't turn it on if you need quiet.
Quote by chrismendiola
I guess spambots are now capable of reading minds.
#9
Thank you guys very much! So much awesome information. I learned A LOT!

Quote by Spambot_2

Your everyday DI Box has a line level output impedance, which is about 10kohm.
You would actually be better off putting your Amp directly into a mixer's line level input.


That makes a lot of sense. And since it's at low impedance, there shouldn't be [a large amount] of buzz as if you just plugged a pedal board into a line jack, right? Granted, if there's also the 60Hz ground buzz, I wouldn't know how to get rid of that since I usually use the ground button on my DI box to cancel that.

Would anyone know what that burning smell might have been when I turned on the amp, then? It wasn't super bad, but noticeable withing a couple feet of the head. Is that normal for SS heads?

Also, if the manual/amp doesn't say what the right fuse to use is, how do you know which to use. Like I said above, I'm using a 5A fuse now, but I'm paranoid it's the wrong one. Should I be making a new thread for this? You guys have been so helpful and I'm hoping it's a quick, easy answer for someone to give. Once again, Google hasn't helped me much.