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Will I blow my preamp up attaching cans ? Studio Headphones
Old 16th August 2007
Gear Head

Question Will I blow my preamp up attaching cans ?


I have a hifi preamp that has, according to the marketing spiel and hifi-mag review a unity gain output buffer with over 20V rms headroom and a max power of 3W.

this seems plenty to power a pair or hd600 cans ? they are 300ohms.

i think from the speil that the output impledance is 50 ohms.

Obviously the ouput stage is designed to be driving power amplifier inputs with 10+ kOhm impedance not headphones

SO will I blow up my amp if i attach the headphones ?

will I cause problems when the jack in inserted/removed as there is a posibility of a momentary short across the connections ?

the amp is a musical fidelity A3CR - see page 2 col 1 of this pdf :

All advice greatfully received !


-specs for my amp and another that is very similar I think
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Old 16th August 2007
Lives for gear
frans's Avatar
The output impedance is not expecting a load like your headphones, don't do it.
In terms of input/output compare it to a truck and a trailer: if the truck is expecting a heavy load and the trailer is lightwight, it will jump - if the truck expects a light load and the trailer is heavy, it will choke. In short: truck and trailer have to fit each other, like output and input impedance have to.
Preamps/poweramps are connected so the input impedance is 5 or 10 times higher than the output impedance of your preamp. There are some details and deviations, but that should serve as a starting point.
Old 16th August 2007
Gear Head


I believe that the output impedance is 50 ohms ( 47R resistor) and the headphones are 300 ohms - 6 times more. this preamp can spitout 3W supposedly. BUT it was designed to expect a minimum say 10K I guess.

I think the bigger problem will be that there is a (probably quite large, i'll check tonight) coupling capacitor which probably forms a high pass filter with the load ? so I might end up with no bass response ?

I know the answer is to buy a proper can amp ! (with a properly low ouput impedance !)

Old 16th August 2007
Registered User
Rick Sutton's Avatar

I plug headphones (300-600 ohm) into preamps all the time for various reasons. Haven't harmed one yet. The listenability varies but it shouldn't be a danger to the preamp.
Old 16th August 2007
Lives for gear
ulysses's Avatar
Try it and see. If you get no bass, or horrible distortion, you're not going to continue doing it. You're not going to hurt anything.

I have a Benchmark +4/-10 "bump box" that I use as a headphone amp all the time. It works great.
Old 17th August 2007
Gear Head

Originally Posted by ulysses View Post
Try it and see. If you get no bass, or horrible distortion, you're not going to continue doing it. You're not going to hurt anything.
i've got my hands on a qouple of schematics for a musical fidelity dac and one of thier headphone amps - they all have the same output buffer curcuit topology - push-pull darlington power devices in each.

I've just now had a look under the hood of my amp - the power darlingtons in my amp have a higher power rating that those they have used in the headphone amp ! (mospec bdx33, bdx34)

BIG difference is the coupling capacitor, which in my preamp is 10uF - so fine with a 10kOhm load but the highpass point is probably 50ish hertz with a 300 Ohm load. Their dac has a 470uF cap - which seems far too big ? esp. compared to the headphone amp which has 220uF.

Looks like I will be OK driving the cans BUT i'll need to add maybe a 100uF coupling cap to get the bass back.

apart from my dodgy soldering, It seems to me that this should be a fairly safe modification, I can't think I'll make the amp unstable ??

shoul I bother with bypassing these big NP electrolytics with poly caps to try to smooth the treble ?

Thanks for all the advice (so far)

Old 17th August 2007
Registered User
Rick Sutton's Avatar

Originally Posted by fatgaz View Post
oul I bother with bypassing these big NP electrolytics with poly caps to try to smooth the treble ?
My advice is to take a listen before you fire up your soldering iron. Theory is nice but it is no match for empirical evidence in a situation like this. Or to put it in a less "scientific" tone.......don't pull the trigger 'til 'ya know what 'ya got on 'yer hands (in this case ears)!
Old 20th August 2007
Gear Head

Smile amp seems to work OK. hd600 > hd650.


managed to loan cans from the local hi-fi shop - (not as useless as i remember them).

hd600 definately flatter than the hd650's - the midrange & top seems to be too far back - maybe the result of too much bass added by sennheiser ? So i vote hd600 over hd650. the 650 cable is fatter, and possibly a bit better soundwise - not alot in it. (also tried some grado 125s - very bright, to the point of being unsuitable with some test tracks.) hd600 not muddy, hd650 possibly a little.

Anyhow, I have connected them to the pre-amp - plently of drive for them. I did need to bypass the stock output capacitor to get the bass back. Only had some cheap 220uF non-polar eletolytics to hand - seem OK. the bass seems a tiny bit muddy at the moment or the top is a bit recessed - might be that the caps need time to settle down ? I tried to bypass them with some poly caps I have lying around (.022 uF i think) from tube amp building days- didn't really notice a lot of difference so far. would like a touch more 'air' I suppose. A 20-20k sweep was fairly smooth, bit lumpy above 8ish k but nothing bad. My tinitus confuses matters above 16k, but there was output at the top.

amp seems happy with the new caps - I don't have a scope so I can't check for any oscilations etc. but i detected no obvious symptoms.

soundstage with hd600's was a bit odd - extreme panned tracks seemed to be coming from behind my shoulders ?? this may be caused by the enclosing-the-ear ear pads and the shape of my head ? my etymotics don;t do this, nor the old 'on-the-ear' sennheisers I have. maybe i need to try some kind of cross feed. also very comfy - don't press on the arms of my specs.

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