Driving headphones from Hi-Fi amp

[shockingcandy]

Not sure if this is the right place to ask this question but...

I am trying to improve the headphone monitoring at the studio I have just become the engineer at.

Basically, the wall box in the live room has 8 jacks wired in parallel (each in series with a 100ohm resistor presumably to prevent damage in the event of a short circuit). The headphones we have are mainly the 400ohm DT100s. Previously, these were being driven from a single output of a headphone amp. Predictably things were pretty "quiet" so I have rigged up a Sony Hi-Fi amp we had lying around instead.

Now, the speaker outputs feed the wallbox and the amp gets it's signal straight from the console monitor outs. This works for a minute or so then the amp trips out only to return 10 or 20 seconds later and the cycle is repeated.

Anyone know what the cause might be?

 

[Harvey Gerst]

What's the DC resistance at the box input jack with all the headphones plugged in?

 

[shockingcandy]

What's the DC resistance at the box input jack with all the headphones plugged in?

I don't have a multimeter to check but if recalling me physics theory it should be...

1/R= (1/R)*8 which works out at 62.5ohms

That said it trips out with only 1 set connected i.e. 500ohm load

 

[Harvey Gerst]

I don't have a multimeter to check but if recalling me physics theory it should be...

1/R= (1/R)*8 which works out at 62.5ohms

That said it trips out with only 1 set connected i.e. 500ohm load

And that's why I said to measure the DC resistance. It sounds like you have a short someplace in the box. That's the only reasonable explanation as to why the amp keep tripping the circuit breaker. Does the amp cut out with no load plugged in, or with just the box plugged in (without any headphones plugged into the box)?

 

[witzendoz]

Hi,

I used to use a hi fi amp to run my headphones before a got a proper headphone amp. I had a little adaptor box thing (not very technical I know) that got from Tandy (Radio Shack) that connected to the amp speaker outputs and converted it to a high power headphone output. I never looked inside to see what it did but I found this site on the net showing a circuit of how to build one, this design has a bypass switch but you don't need to have that. The reason you need an adaptor like this is so the amp sees the correct impedance, it would have been designed for 8 ohm, and with multiple headphones you could end up with anything and damage the amp.

The 100ohm resistors that you have on your headphone bus could be there to prevent amp damage if the amp is on with no headphones plugged in, open circuit not resistance, this can blow up an amp very quickly. You may need to remove them if you use the adaptor on the link.

I also used to wire the head phone send in mono and use the right side of the amp for one send and the left for another so I could create 2 different mono mixes.

You could just buy one of these (yes I know its Behringer but it is for headphones) I replaced all my old headphone amps with one of these and no ones ever complained about the quality and volume. If you want a different brand Presonus, Furman, ART, and many more all make headphone amp systems. Sometimes buying something is easier than messing around.

Cheers

Alan.

 

[shockingcandy]

And that's why I said to measure the DC resistance. It sounds like you have a short someplace in the box. That's the only reasonable explanation as to why the amp keep tripping the circuit breaker. Does the amp cut out with no load plugged in, or with just the box plugged in (without any headphones plugged into the box)?

Unfortunetly without anything plugged in I can't actually tell if there is sound coming out of tha amp or not

I suspect I may not have explained this very well. When I said tripping out I didn't mean at the mains circuit breaker. The amp remains powered at all times but the sound is temporarlly muted and there is a faint sound of what I presume is a relay within the amp.

In a ideal world I would replace all the crappy DT100s with lower impedance modern headphones and drive them from individual headphone outputs but we can't afford to do this at the moment so have to make the best of what we have.

 

[SouthSIDE Glen]

Now, the speaker outputs feed the wallbox...Anyone know what the cause might be?

An I reading this right, that you're running the amplified *speaker outs* from the amp into the ins on a headphone distribution box? That'll trip many amp circuit breakers almost for sure if your amp outs can't handle anything outside of a 4-8ohm load. You're plugging a firehose into a lawn sprinkler .

G.

 

[Brian Oshman]

Impedance is only a problem if it goes below 4 ohms on a standard receiver type amp.

The 62.5 ohms is fine and actually desired that it is higher than 8 ohms since it will reduce the total wattage output at the speaker of the rated 8 ohm rating since the speaker is right against your ear and pretty much 1 watt of power is more than plenty.

You need to look at a couple of things.

1) Does the amp have a common or floating ground for the speaker outputs?
If it is a floating ground then you cannot use that to power a common ground speaker system which is what a headphone is since it shares the ground. However, most receiver type amps are common ground even if they have separate connections for speaker ground just for convenience to not have to stuff more than one conductor in the connector.

2) Is the headphone breakout box wired in stereo or mono? You cannot combine output channels.

3) Is the amp blown? Did you test it with a stand alone speaker that you know works?

4) Are the jacks of a shorting type when the plug is not inserted? It shouldn't be anyway in this application. The resistors are in there for impedance matching of a single headphone jack on an amp so multiple headphones will not load the amp. They are not needed in this application since the amp will handle as low as a 4 ohm load but I would leave them in there for ear protection. A little added attenuation.

5) Have you rung out the wires with an ohmmeter to check for continuity or for shorts?

 

[shockingcandy]

2) Is the headphone breakout box wired in stereo or mono? You cannot combine output channels.

Ahh I've got a feeling that although the connection at the control room end is Stereo, the wallbox is in mono. Damn. Personally, I prefer to monitor in stereo anyway but whoever orginally wired the studio had different ideas.

Thanks for your advice.

Glen, I think your analogy applies when the load has smaller impedance than the source. Nice analogy anyway - I might have to remember that one!

 

[shockingcandy]

UPDATE. Yep the wall box is at fault. Plugged a set of headphones directly into the speaker outputs (using a Y cable) and ran the amp at decent volume for 2 hours without a problem. Time to get the soldering iron out!