amp to amp advice por favor

[RogueGnome]

My first post on the Amp Board. (Be gentle.)

Background: I have extensive knowledge and experience with table lamps, powering them on, and almost as much, gracefully powering them back down again without incident.

I am an acoustic player with only just the occasional gig.

I am interested in connecting two amps as sort of a booster.

So Acoustic Amp A with effects to Acoustic Amp B; the output of which would include the effects signal.

How do I do this, and what are the potential electronic pitfalls to avoid?

Much obliged.

[WRGKMC]

Hum, it sounds like you want to connect the lone out form one amp to another amps input. You can do this but since I have no idea of what amps you're talking about I cant tell you if the results would be any good.

 

Things you should never ever do (because you asked) is connect one amps speaker output to anything but a speaker. It does use a 1/4" jack in many cases but just because the jack is the same doesn't mean the voltage coming out of there is safe.

 

A guitar puts out maybe 1v ac max. Many much less. An amps input jack is designed to boost 1v up many times over to 70V or more on high powered amps. What do you think youd coo connecting an AC outlet to a guitar amp input? Right, One big flash and a puff of smoke.

 

Another item to avoid is wiring the effects send of one amp to the others effects return is a criss cross manor. First off it gets you nothing to do that and can wind up having massive feedback is one amp is has a series effects loop and the other parallel. You can take a line out and feed another amps effects return if the preamp is disconnected. Sometimes you need to plug a dummy cord into the effects send to do this.

 

The jacks on the effects send loop are switched and either the send or return plug separates the preamp from the power amp to allow you to plug in an effect. If you're going to use the send on one amp to feed the return on another plug a dummy plug into that amps send and you can be sure its preamp is disconnected.

 

What you don't want to have happen is to have one amp feed an amp that doesn't have its preamp and power amp disconnected. Its OK to feed the power amp but youd be injecting a signal that would also feed the preamp backwards and anyone who knows electronics knows you can blow transistors when you feed them improperly like that. You essentially need a mixer for that kind of application which buffers the two preamp inputs to feed a single power amp.

 

Main thing is the signal doesn't flow unidirectional (one direction) to the power amp. If the second preamp isn't removed then you cant use them that way. The other item is the preamp jack on the first amp may disconnect the power amp and its waiting fro a return signal from an effects unit. Again I have to know the specific amps to give you specific details. I own amps that break the signal at the return jack and you can use the effects send as a line out to feed other amps. With others It breaks the signal at the effects send and you have no sound from the amp, only a signal send. Other amps like my Marshall has a switchable series/parallel effects sends/returns and line outs. Some of these can be used to feed another amps input, sometimes not.

 

Many times these line outs are a bit "hot" for feeding other amps input directly because the signal has already been preamplified, but its usually safe to where you wont blow anything up. If anything its just too much signal or it may distort because its too strong.

 

Headphone outputs can be a different story. There are some that can double as a line out, but many that have much lower impedances and should only be used with headphones. Unless the manual specifically states you can use it as a line out you should never use it to feed other pieces of gear.

 

 

 

Personally I'd just split my signal guitar signal and feed both amps. You could then have twice the tonal range adjusting two amps over one being a master and the second being a slave. There's likely no chance of getting it wrong and no chance of damaging anything.

 

One good option is to use a Chorus or Echo pedal with dual outputs to split the signal and feed to amps. The pseudo stereo effect this produces is allot more fun too. If the signal is weak just get a booster pedal. It will provide more gain then any daisy chaining between line outs or line sends you can cook up using two amps.

 

[RogueGnome]

WRGKMC, many thanks.

Now I have some grasp of exacly how far over my head I really am.

Jumping to near the end of your post, I think that the best scenario for me is as you say, " just split my signal guitar signal and feed both amps. You could then have twice the tonal range adjusting two amps over one being a master and the second being a slave. There's likely no chance of getting it wrong and no chance of damaging anything."

Sounds like all I need is a splitter.

You've been a big help today, WRGKMC.

 

[onelife]

You may experience the dreaded "ground loop" which occurs when there is more than one path to ground between the two amps. The power cables are grounded and the signal ground will be connected via the Y connector or, as WRGKMC suggested, a stereo chorus or other pedal.

 

 

 

You will be able to tell if you have a ground loop issue by an annoying hum when everything is connected together and the hum goes away when you unplug the input to one of the amps.

[WRGKMC]

If the two amps are grounded in the same outlet he shouldn't have a ground loop. What can cause that hum is a current flow between inputs if the input impedances are drastically different. I have that issue between my SS Marshall amp and my Fender Tube head. Connecting the two together draws voltage from the other as the impedances are combined. There are ways around it like using a buffered pedal to split the signal. On mine I have a 2 way noise filter/iso transformer. I plug the amps into each side of the filter and it eliminates the issue. Lets hope the two amps are similar designed and that's not a problem. If they're both solid state amps then they may both have similar high impedance inputs.

[RogueGnome]

One is SS, the other is tube. But they will be both grounded in the same outlet.

Man, you guys are good.

Thanks to you both.

[WRGKMC]

If you notice any additional hum when they are plugged in together or lessening when they are unlinked, let me know. Like I said I had the same issue with a Tube and SS linked and there are some ways of eliminating the problem is its an issue. If not then you're OK.

[RogueGnome]

If I can ask yet one more related question:

If I split my guitar signal and feed both amps, each amp gets the full signal, as if I was only plugged into just the one; right? Or would I have to boost the volume on either each amp or on the guitar?

 

 

[onelife]

The combined load on the guitar pickups will have an effect on the level that may be noticeable - like plugging into the number 2 input on a Fender tube amp.

[WRGKMC]

That's actually caused by the input resistors. Fender does it in such a way that you switch different resistors on and off when you use different jacks. If you use the second jack only you do get a volume dip.

 

On two-input amps, such as Fenders, when you plug into the low-level input, the 1 meg grid resistor is shorted out and the two 68K resistors act as an attenuator to cut the input signal in half. The downside of this is that the input impedance drops from approximately 1MEG down to approximately 136K, which is a rather heavy load for a high impedance guitar pickup.

 

Those resistors are not just put on the control grid for signal level attenuation purposes; rather, they also act as a very high frequency low-pass filter in conjunction with the input capacitance of the triode (which is a sum of the grid-to-cathode capacitance and the Miller capacitance).

 

This low-pass filter does a couple of things, it helps prevent high frequency parasitic oscillation in the tube itself; and it helps prevent radio frequencies from getting into the input stage, where they can be rectified and low pass filtered (AM detection) and become audible at the amplifier output; and it can limit grid current when the tube is driven into the positive grid region.

 

When you think about when that design was incorporated back in the late 60's it was well before most active guitar electronics and pedals where you'd need a high and low gain channel. There are only a few useful reasons for it from a guitarists aspect.

 

Yes you can use it to drop your signal level down and cut a high output pickup down. The more likely reason was back in the day you'd often have two guitarists using the same amp at low volume. The resistors let you share the same channel input and if another guitar or possibly a high impedance mic. You'd put the mic in low gain channel, and the guitar in the high, then roll your volume on the guitar back to get a balance between the two. With the resistors, if you turn the guitar off it doesn't completely mute a mic or another guitar because of the added resistance. In more modern times you can also connect a stereo unit and not have the two channels directly short together.

 

These are pretty lame were lame options by todays standards but they were used by musicians in many creative ways. People rarely use multiple guitars in an amp any more and most amps only have a single input per channel or a single high and low with channel switching.

[WRGKMC]

It really depends on how strong the pickups are. Very weak single coils "may" have a small dip, but in most other cases, you'll never know it.