Quantify low impedance on HH Scott 340B headphone jack

[b brian]

I am looking to get a set of headphone to listen through my HH Scott 340b and the owners manual only give "low impedance" head phone jack.
Current definition of low impedance is less the 50 ohm.

I am wondering if "low impedance" at the time the 340B was built is the same as it is today?
I am looking at a pair of AKG headphones and they are rated at 62 ohm. Low'ish by today's standard, but wondering if they will match well with the 340B.

Thanks

 

[Dave451]

I'm scratching my head a little on this one. I am using my AKG's with the 340B and they are 600 ohms and work fine. Not sure what that tells you about 62 ohms, but maybe it's a data point.

 

[b brian]

Dave451
Which AKG are you using?
What is the volume level you have to use for listening?

 

[RamblinE]

So, is the manual referring to the headphone impedance? Or the jacks output impedance? Lower output impedance is optimal with a lot of newer headphones.

 

[b brian]

The manual says that the headphone jack on the 340B is for low impedance headphones.
My question is if Low impedance then means the same as low impedance now?
Will the 340B match well with a headphone that is rated at 62 Ohm? And what people feel is the high end of Impedance that the 340B will work well with.
100 Ohm?
200 Ohm?

 

[RamblinE]

It sounds like you have K701s, or 702s or 712s, or 7XXs or some other can from that family. I've used mine with portable "high"-current amps. I really really doubt they're gonna hurt your Scott. It'll have plenty of power which is what the AKGs need.

 

[b brian]

Its more asking what the upper limit might be on the Scott
Can I use headphones over 100 Ohm?

 

[Dave451]

They are old standby K240 Monitors--nothing special, but not a bad set of headphones. Volume in the phones tracked the speaker volume pretty well, so fairly loud at about 10 o'clock with FM input. Back in the day (recalling my ancient broadcasting days, and not reliable at that), 'low impedance' was 600 ohms or below. This would be 60's/70's era of my youth.

Looking at the schematic, the output for the phones and center channel output comes from the 16 ohm taps on the OPT with 150 ohm resistors installed in series with the phone jack, so 150 ohms? This is different from, say, the 299C where the phones come right off the tap for the OPT (in fact, the OPT selector tap appears to actually be run through the headphone jack). So, in this setup, the phone impedance would be the same as selected for the speakers. The 299D is wired the same as the 340B. However, as you have probably seen, the 340B operating manual does not actually describe what 'low impedance' means.

150 ohms would be my guess.
Dave

 

[BinaryMike]

62R phones would work okay with 150R source impedance, but damping factor is almost nonexistent. You might try adding a pair of resistors between about 6.8R and 15R to ground at the headphone jack, or in an outboard enclosure, or just haywired inline, to discover if the phones sound better with high damping. The volume control will have to be advanced considerably, but that's no problem as long as you're not depending on the loudness function for for frequency response adjustment, and it may improve the signal/noise ratio.

 

[I LIKE MUSIC]

It uses a 150 Ohm resistor in series with the output tape, which every one is selected.

So, for a basic example, if you are using the 16 Ohm taps on the output transformer and you set the amplifier so if a 16 ohm load was connected to it and the output power was 15 watts then...

The voltage available to the headphones will vary with their impedance.
In round numbers. First number is headphone impedance.

150 Ohms=7.5 volts=0.375 watts (Interestingly, this shows max power when impedances are matched, which it should based on theory).
75 Ohms=3.7 volts=0.33 watts
28 Ohms=1.8 volts=0.10 watts

600 Ohms = 12 volts = 0.24 watts
500 Ohms = 11.5 volts=0.26 watts
400 Ohms = 10.8 volts=0.29 watts
300 Ohms = 9.9 volts = 0.33 watts
200 Ohms =8.4 volts = 0.35 watts
100 Ohms =6.0 volts = 0.36 watts

Now if you drive you headphones directly from the 4 Ohm tap, you will get the same numbers.
The 8 Ohm tape will give you more power, as will the 16 Ohm tap.

The internal series resistors can help attenuate the noise floor of the amplifier.

None of this tell you how a certain set of headphones will sound, but it will give you an idea of how the amplifier will drive headphones of various impedances. You can look at the sensitivity rating for the headphones to get an idea of how loud they will play.

This is all just Mr. Ohm's law at work for your listening enjoyment.

The harder you drive the amplifier the more voltage and power you will have for the headphones (and Mr. Ohm can tell you then amount of power available for each impedance.

BTW, if you look at the numbers you can see a pattern.

With this example, the 600 Ohm headphones will be seeing 12 volts and the is 0.24 watts into a 600 Ohm load.

The 100 Ohm headphones will be seeing 6 volts and that is 0.36 watts into a 100 Ohm load.

The difference in power is only about 1.8 dB, not likely greatly noticeable over all difference.

In this example, one might say that the 600 Ohm headphones need a lot of power because they have to turn the volume control up higher, but it is more about needing more voltage than power. Of course the sensitivity rating of the headphones must be taken into account, because the 600 Ohm headphones and 100 Ohm headphone may play at the same level, in terms power, if they have the same sensitivity, even though the 600 Ohm headphones will need a lot more voltage than the 100 Ohm headphones.

So, one last example. If the sensitivity ratings for the 600 Ohm headphones and the 100 Ohm headphones is equal (so they both play at the same SPL for the same power input), if the 100 Ohm headphone are playing at the .36 watt level (using just 6 volts), the 600 Ohm headphones will need 14.7 volts to play at the same power level.