guitar signal into 4 practice amps

[christobean]

ok, so i have been searching through the archives and at other sites for the past hour, and i think i am searching for the wrong term or something, i cant imagine being teh first one to ever run into this situation.

my friend has 3 or 4 small, 1x8 ish practice amps and we usually crank one for a jam in his basement.  however, we just aquired a drum set, and now one amp is not cutting it.  so i am thinking of making a box that will have one input and 4 outputs, to send to the different amps.  preferably, no transformers as i will need to order one specially for this project. however, anything with opamps and stuff is cool.


what i do want: a cheap, simple, quick solution to be able to send one guitar into 4 amps.

what i do not want: a high performance project, any parts that i dont have in my stash already, anything with extra features or frills.

noise is not really a concern, as they are just ~10 watt practice amps.  i know i may be asking a lot, but any info anyone can share on this subject is great since i know almost nothing about signal splitting, etc.

thanks!

[Shepherd]

Three Y-cables ought to do it

[Gladmarr]

You may need to lift the grounds on three of the four amps to cut down on the 60 cycle hum.  As long as they're all connected by signal ground, you're still safe from shock.

[R.G.]

You may need to lift the grounds on three of the four amps to cut down on the 60 cycle hum.  As long as they're all connected by signal ground, you're still safe from shock.

... as long as no one disturbs a cable! Especially in what basements are often like - damp.

Bad idea. Never lift a ground unless you hard-wire a safety ground to the piece of equipment that you're leaving grounded. Cables are not safety grounds.

It's not worth dying for. Does it matter that you did it and got away with it 437 times if on #438 you get dead? Here's an odd factiod for you: all living Russian Roulette players have an unbroken string of wins - so far. 

Instead, figure out a way to force the existing grounds to work. First, plug all the amps into the same outlet strip. If that doesn't do it, use the minor-hums eliminator at GEO: http://geofex.com/FX_images/oaspltr.gif, from 6/7/2000. This one should take care of any cases where the hum is from normal amps plugged into the same outlet strip. You only need one buffer opamp and then one differential opamp per isolated output, so the setup with four amps can be done with two dual opamps or one quad. The power switching is to let you turn on both polarities at once.

[Rafa]

Mhhh
Maybe a premap and then the

Three Y-cables

Cheers
Rafa

[Roobin]

Can I ask what 'lifting the ground' is? I've seen it on the back of amps occasionally, and here, but have no idea what it is.

[d95err]

Can I ask what 'lifting the ground' is? I've seen it on the back of amps occasionally, and here, but have no idea what it is.

It means disconnecting the ground somehow. It can be used in different contexts. One is lifting safety ground from an amp (bad idea) e.g. by taping off the safety ground connectors on the plug.

Another context is with DI-boxes. DI-boxes usually have a ground lift switch that is used to isolate the signal ground from one device to another in order to eliminate ground loops. This is safe (as long as the devices have a proper safety ground).

[Meanderthal]

 Hmmm... the Superbuffer ought to be fine for this, just hook up 4 outs to it.

[petemoore]

  "Don't know anything's" in cotrol too often.
  I pointed out that the ground lug was yanked out like a bad tooth, a Soldano/Yamaha 100w tube amp...nice amp, except for the excessive ground hum...and the POTENTIALLY LETHAL VOLTAGES.
  I even offered to install a FREE service job, to replace the power cord end or whatever...and GET A GROUND on it !
  I was referred to someone else when I told the owner of the club, where this amp threatens the life of any unsuspecting jammers...[one room has lotsa 3'' conduit, 1'' thick wires routed all over the place...18' PA Towers etc.]..a more than adequate supply for electrocution.
  I referred my concerns to 'someone else', the current soundman, and he referred to 'that guy': "I told that guy to put a ground on that amp".
  Would be funny, and it is, but stands the chance of being as serious as a heart attack, literally...there aughtta be a law...probably is, I guess by 'proxy of knowing', I'm held responsible for enforcing it...?...
  Say 'fix it or be charged'?..I would guess a health inspector or some such dept. would be interested in such L or D matters.

[christobean]

You may need to lift the grounds on three of the four amps to cut down on the 60 cycle hum.  As long as they're all connected by signal ground, you're still safe from shock.

... as long as no one disturbs a cable! Especially in what basements are often like - damp.

Bad idea. Never lift a ground unless you hard-wire a safety ground to the piece of equipment that you're leaving grounded. Cables are not safety grounds.

It's not worth dying for. Does it matter that you did it and got away with it 437 times if on #438 you get dead? Here's an odd factiod for you: all living Russian Roulette players have an unbroken string of wins - so far. 

Instead, figure out a way to force the existing grounds to work. First, plug all the amps into the same outlet strip. If that doesn't do it, use the minor-hums eliminator at GEO: http://geofex.com/FX_images/oaspltr.gif, from 6/7/2000. This one should take care of any cases where the hum is from normal amps plugged into the same outlet strip. You only need one buffer opamp and then one differential opamp per isolated output, so the setup with four amps can be done with two dual opamps or one quad. The power switching is to let you turn on both polarities at once.

i think this is what i am looking for.  all the amps will be plugged into teh same powerstrip, so the grounding thing you mentioned should be taken care of. my only problem now is figuring out how to set up the system you linked the picture to, i have no idea what is going on in that, or how i will add 2 more outputs.

[R.G.]

What the heck, it's Christmas.

Go look at the link again. I modified it to fit what you need - four semi isolated outputs. I did a cheap and dirty on the power supply to make it easy. You can power the four-way from a single battery, but you may NOT use the input-jack switching trick to turn power on and off. You'll have to use a switch or plug the battery in to power it. I just split the power supply with two resistors, then grounded the split to the signal. The power supply floats around the signal ground of the input buffer stage. I think it will work OK, and it avoids the use of two batteries. But it also has only one battery worth of voltage, so trying to use boosters and overdrives ahead of this thing will likely sound ugly.

Use 47K for all the "R" resistors. The actual value isn't very important, just that they are all the same. 5% resistors work OK, but 1% metal films will give better rejection of any hum. The three auxilliary outputs are differential amplifiers which "subtract" any hum from the amp they're feeding from the signal going in so that there is no apparent hum into the amp being fed. Diffamps work better with matched resistors.

Do not let the three isolated output jacks touch signal ground - isolate them from the metal box you put this in; otherwise, they will hum in spite of your efforts.

Yell with questions.

[christobean]

What the heck, it's Christmas.

Go look at the link again. I modified it to fit what you need - four semi isolated outputs. I did a cheap and dirty on the power supply to make it easy. You can power the four-way from a single battery, but you may NOT use the input-jack switching trick to turn power on and off. You'll have to use a switch or plug the battery in to power it. I just split the power supply with two resistors, then grounded the split to the signal. The power supply floats around the signal ground of the input buffer stage. I think it will work OK, and it avoids the use of two batteries. But it also has only one battery worth of voltage, so trying to use boosters and overdrives ahead of this thing will likely sound ugly.

Use 47K for all the "R" resistors. The actual value isn't very important, just that they are all the same. 5% resistors work OK, but 1% metal films will give better rejection of any hum. The three auxilliary outputs are differential amplifiers which "subtract" any hum from the amp they're feeding from the signal going in so that there is no apparent hum into the amp being fed. Diffamps work better with matched resistors.

Do not let the three isolated output jacks touch signal ground - isolate them from the metal box you put this in; otherwise, they will hum in spite of your efforts.

Yell with questions.

wow, thanks a lot!
clears things up a lot as far as me understanding whats going on in there. thanks again

[vanessa]

A small Beringer mixer (around $30). Line in to out L/R and also use Tape out L/R (or headphone take your pick). Gets you 4 outputs and saves you the time and money of building a unit to do this. You can use it for other stuff afterwards.

http://www.behringer.com//UB502/UB-502_r.jpg

[alextheian-alex]

Couldn't it be done a bit simpler with a pair of MAX435?  

[R.G.]

A small Beringer mixer (around $30). Line in to out L/R and also use Tape out L/R (or headphone take your pick). Gets you 4 outputs and saves you the time and money of building a unit to do this. You can use it for other stuff afterwards.

Not a bad idea - except that the mixers I've seen all have a common ground and so lay you open to hum from multiple amplifiers.

However, a DEAD four channel mixer would make a dynamite enclosure for something like this.

The issue is the ground loops.

Couldn't it be done a bit simpler with a pair of MAX435? 

A dual 250MHz transconductance amplifier that's been discontinued? ? ? ? ? ? ? Instead of two TL072s?

I don't know how. Is MAX435 the chip you meant?

[alextheian-alex]

Couldn't it be done a bit simpler with a pair of MAX435? 

A dual 250MHz transconductance amplifier that's been discontinued? ? ? ? ? ? ? Instead of two TL072s?

I don't know how. Is MAX435 the chip you meant?

HAHAHA!  CRUD!  I was looking over some old schematics and accidently typed 'MAX435'.  I suppose you *could* use that chip, I have used them for phase splitters before.  i don't remember what specific part I was thinking, but i meant a differential output buffer.

[JimRayden]

Well well, isn't that Alex from AX84... what on earth made you leave the steamin hot bottles of glass alone and pay us, worthless silicon-tinkerers, a visit?

Naah just teasin'. Glad to see you here.

----------
Jimbo

[R.G.]

i don't remember what specific part I was thinking, but i meant a differential output buffer.

No problem. I was just baffled.

I'd go with something like the AD8132 differential in/out amplifiers. That chip looks like it may have been designed with guitar hum loop breaking in mind. Neat chip. The LT1994 would be OK, but it seems to be optimized for lower impedances and cable driving. Max power supply on the AD8132 is 10V and only 5V on the LT1994.

But I'm guessing both may be a bit more expensive than a TL072.

Does anybody besides me lament that the first guitar amp makers were too cheap to use one whole dual triode as a differential input amp? Or that we're stuck with it because you'd never, ever be able to get the world to change the input jack and cables for guitars now?

[alextheian-alex]

Does anybody besides me lament that the first guitar amp makers were too cheap to use one whole dual triode as a differential input amp? Or that we're stuck with it because you'd never, ever be able to get the world to change the input jack and cables for guitars now?

LOL!  i was just thinking that today... as i drew up an uneccessarily complicated RLC input network for a pedal to surpress the staggering amount of RFI/EMI here in the middle of the city. Honestly, I run EMG's anyway: I should just wire up a differential buffer in my guitar and then a tube differential input stage in my amp, plug in an XLR and call it a day.  Heck with that,  screw the buffer and instead drop a small microphone tranny into the geetar.

[alextheian-alex]

Well well, isn't that Alex from AX84... what on earth made you leave the steamin hot bottles of glass alone and pay us, worthless silicon-tinkerers, a visit?

Naah just teasin'. Glad to see you here.

----------
Jimbo

CRAP!  I've been nabbed!  Hey Jim.  I have not posted on this board for years, but i drop in every now and again.  I have just been in a chatty mood lately, and I'm wiring up a few pedals as a break from the 'high voltage cascoded pentode ccs tube differential mu stage blah blah blah' to just build some fun little boxes.  Besides... what makes you think I don't shove tubes in my stompboxes!