[boredboy94]

Hey all,

I've been wondering about possible damage to my headphone amp with the set up I have. I have an old Sears Roebuck stereo system hooked up to my amp via 2 channel RCA switch. Have a 6.3mm to RCA cable from the 6.3mm headphone out on the stereo system going into the RCA switch (Schiit Modi DAC is in the other RCA input on the switch). Obviously, the headphone out of the stereo system is outputting some power, and for the volume control on the stereo I never have it past 9 o'clock. The equipment I have is below. Am I in any risk of damaging my headphone amp since the signal it is getting from the stereo system is already an "amplified"/a hot signal? Let me know!

Headphone amp: Schiit Magni Heresy

RCA Switch: Duttek RCA Switch Box, 2 Port AV Switch Box https://www.amazon.com/gp/product/B0753CQ668/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

"Audio: Input level Resistance: 2V(RMS)/47KΩ
Output level Resistance: 2V(RMS)/1KΩ
Frequency: 10Hz-20KHz，0db
Input Connector:2 (R+L), Output Connector:1 (R+L)"

Sears Stereo: (NOT MY VIDEO but shows the same stereo I have)

 

[Paul Mohr]

As long as you don't turn it up too high I would say no but I am not a technician nor do I have a degree in electrical engineering. My advice would be to play some music using the modi dac and listen to the sound level, or better yet measure it if you have a db meter. You can get an app for most phones for this. Then switch it to the input for your old stereo system and adjust the volume on the sears stereo until it matches the sound level the Modi was putting out. This should keep you pretty safe I would thing. What you don't want to do is crank up the volume knob on the sears system and over drive the inputs on the other amp and distort it. This might damage something. So basically, be very conservative on the input volume until someone with more knowledge than me comes along. I will tell you in my 50 some years of screwing with janky audio system I have done this before and I never blew something up unles I was being stupid with it. Especially considering you are using the headphone input. As for how good will it sound? Probably not great.

 

[rfernand]

Be careful!

That’s a 10x difference in voltage right there. RCA inputs are not expecting the power of speaker outs, which is what I think you’re doing here.

Will it ruin your Schiit? I don’t know how it’s constructed and what femtocrystalfuse would trip (if any) before you fry something. But why risk it?

 

[Paul Mohr]

I don't think he is running it off the speaker outputs. If I read it correctly he is using headphone out on the old sears stereo and using a 1/4 inch jack to rca adapter/splitter/cable thing and running that into the amp rca inputs. This kind of cable used to be fairly common with computers so you could run the audio out to powered speakers or a stereo amp from the speaker out jack on the motherboard. You just have to be careful how loud you turn up the original source or it will have too much gain and cause distortion or mess up your input stage.

Now hooking up the speaker outs from an amp into the rca inputs on another amp is just asking for trouble. You might even let the magic smoke out doing that.

 

[rfernand]

I don't think he is running it off the speaker outputs. If I read it correctly he is using headphone out on the old sears stereo and using a 1/4 inch jack to rca adapter/splitter/cable thing and running that into the amp rca inputs. This kind of cable used to be fairly common with computers so you could run the audio out to powered speakers or a stereo amp from the speaker out jack on the motherboard. You just have to be careful how loud you turn up the original source or it will have too much gain and cause distortion or mess up your input stage.

Ah, *wheh*. Well then yeah, it may be OK. But why?

Now hooking up the speaker outs from an amp into the rca inputs on another amp is just asking for trouble. You might even let the magic smoke out doing that.

And we all know electronics need their smoke to stay inside to keep working.

 

[Paul Mohr]

Ah, *wheh*. Well then yeah, it may be OK. But why?

My guess is he is trying to use the old sears unit to use the tuner/tape deck or turntable or all three without having to actually buy a new device to plug into it. My guess is it isn't going to sound as good though. He might as well just plug the headphones into the output of the sears unit and use it. Its not like daisy chaining all that is going to improve the sound any. It might make it louder though. If you input a crap signal into a Heresy its just going to output a louder crap signal.

 

[boredboy94]

Thanks @Paul Mohr and @rfernand
I think I'll just use the headphone out on the stereo instead of trying to run it through my Amp. I thought too that the Amp would somehow help the sound but you are right, if there's crap in the stereo it will only be crap through the Amp. Guess I'll keep the cords and switch and use them for another setup. Thanks again!

 

[Paul Mohr]

Hey you can try it at least if you have the stuff. Nothing is ever for sure. Try it both ways and see which sounds best to you. I would be interesting in knowing as well if you try it. If it doesn't work at least you have a cool switcher and the cord and adapters will for running sound out of a computer jack or phone or whatever if you need to at some point. That is how I used to run the sound from my computer to my AV receiver back in the day before HDMI was a thing. I actually have one those cords sitting here somewhere. I would say whatever it is you are trying to use the sears system for just get a dedicated component that does that. You might be able to find one used cheap.

 

[tomb]

Thanks @Paul Mohr and @rfernand
I think I'll just use the headphone out on the stereo instead of trying to run it through my Amp. I thought too that the Amp would somehow help the sound but you are right, if there's crap in the stereo it will only be crap through the Amp. Guess I'll keep the cords and switch and use them for another setup. Thanks again!

There's no way the output from the Sears will hurt your amp(s), unless there's a dead short in the Sears' headphone output. (If that was the case, the Sears equipment would probably burn up/stop working.) What will happen to an amp if there is too much voltage, or an offset voltage, is that the amp will clip when driven beyond its limits. That in itself is not a damaging condition to an amplifier, but it can be seriously damaging to the end device (headphones). That's the thing you have to worry about - whether your headphones will get over-driven or damaged from a voltage offset.

Your use of the headphone output should be self-limiting in terms of current availability, but sometimes older equipment run headphone outputs from a resistor dropping down the voltage from the speaker outputs. Many older receivers work this way. The problem is that old equipment "drifts" from their original adjustments. Many will have voltage offset on the speaker outputs, meaning there's some DC present. That can blow any transducer from headphones to speakers, under the right conditions. Good practice when using an old receiver or other stereo component is to measure the outputs with a DMM to see if there's a DC voltage present.

Again, as I said - the headphone output of that Sears stereo should be limiting the current available, but it's still possible some DC voltage can get through. It might be in the tenths of a volt, but your amplifier will amplify that up to whole digits of DC voltage, which will still not hurt the amp, but could fry any transducer connected to the output.

 

[boredboy94]

Yes, I can't tell if my ears have gone worse but I did notice my 58x doesn't quite sound as good as before. I'm talking 5% reduction in overall sound quality at most. For the most part the stereo was in excellent condition and an absolute steal at $30. I just had to grease the phono motor with some wd40 and clean the heads on the 8 track and cassette players and it works great. The headphone out actually gets quite loud so I think the circuitry and everything is working as it should.

 

[Paul Mohr]

Just as a heads up, that Heresy and those headphones will probably make a cheap cassette, 8track and turntable sound really bad. It is just going to help bring out all the bad flaws those mediums have. That is a crystal clear revealing set up. Good sounding music always starts at the source. How well was it recorded and mixed and what sort of medium are you playing it back on. Cheap amps and speakers tend to mask some of the flaws and make it more listenable. Better amps and speakers can bring out more details, which isn't always a good thing lol.

If you decide to try it just do what we suggested. Start with the volume on the sears unit very low and bring it up to level match the volume. You should be safe.