Headphone impedances: what the hell?
post-841457
Thread Starter
Post #1 of 18

GuineaMcPig

100+ Head-Fier
Joined
May 19, 2003
Messages
405
Reaction score
12
Joined
May 19, 2003
Posts
405
Likes
12
Hi everyone-

I just got back from a local music store where I auditioned a pair of Grado RS-2s and a pair of Sennheiser HD600s. The common wisdom on this board seems to be that the Sennheisers are much harder to drive and "require a dedicated headphone amp to shine," but my experience was the exact opposite of this.

Whenever I changed phones from the Sennss to the Grados, I would have to turn the volume up significantly. Whenever I changed back, the Senns would be too loud and I would have to turn the volume down.

I approached the store owner and asked him for an answer. Sennheiser's resistance is something like 300 ohms, and Grados are only 32 ohms, so that means that the Senns eat up much more power to produce the same volume, right? He said it was just the opposite.

He said that since the Grados have less resistance, they require more current to produce the same amount of volume. This is the complete opposite of everything that I've read here, but it was my experience in the store as well. So can someone straighten me out on this whole thing?
 
     Share This Post       
post-841462
Post #2 of 18

some1x

Headphoneus Supremus
Joined
May 20, 2003
Messages
2,186
Reaction score
58
Joined
May 20, 2003
Posts
2,186
Likes
58
power = (current)^2 * resistance

assume same sensivity (therefore same power), then lower resistance requires more current.

i don't know if this applies to audio tho
 
     Share This Post       
post-841481
Post #3 of 18

MD1032

Headphoneus Supremus
Joined
Aug 10, 2003
Messages
5,977
Reaction score
19
Joined
Aug 10, 2003
Posts
5,977
Likes
19
Grados draw A LOT of current, but have low impedance, while the Senns take less current but have a high impedance. I can't really say much more than that.
 
     Share This Post       
post-841522
Post #4 of 18

pedxing

1000+ Head-Fier
Joined
Aug 25, 2001
Messages
1,479
Reaction score
12
Joined
Aug 25, 2001
Posts
1,479
Likes
12
It seems like your amp cannot provide enough current to your grados. I usually find grados relatively easier to drive with most portable devices, but I really only tried SR-60 or SR-80. RS-2 might be a different animal, requiring more current than their cheaper brothers before its performance shines.

I know with normal amps and speakers, connecting a low ohm pair of speakers to a weak amp is a bad idea. When I went to the audio store to buy speakers, the sales people always warn me about which amp to match with low ohm speakers.

With higher impedance headphones, usually voltage becomes an issue with underpowered amps.

However, I really don't know the specifics how all this stuff comes into play with AC circuits.

edit: maybe your amp has enough power, just needs to be turned up more. Its probably not a significant problem unless you notice distortion or your amp's operational temperature is way hotter than usual.
 
     Share This Post       
post-841534
Post #5 of 18

GuineaMcPig

100+ Head-Fier
Joined
May 19, 2003
Messages
405
Reaction score
12
Joined
May 19, 2003
Posts
405
Likes
12
So here's my question in a nutshell: are Grado's efficiency with portables and Sennheiser's need for an amp both head-fi myths? Wouldn't Grados need an amp more if they require more current than Senn's? And wouldn't a pair of cans like HD580's make more sense for someone with a portable than Grados?

Another issue is that every 'phone in the Grado line is rated at 32 ohms, yet people here seem to suggest that the higher end Grados require an amp to be driven properly while the lower end ones don't. Is this just a matter of extracting more detail out of a more capable pair of phones, or do the higher end Grados actually require more current?

Thanks!
 
     Share This Post       
post-841550
Post #6 of 18

Eagle_Driver

Headphoneus Supremus
Joined
Jun 22, 2001
Messages
6,447
Reaction score
25
Joined
Jun 22, 2001
Posts
6,447
Likes
25
Which sources and/or amps were you auditioning the Senns and Grados out of?

Sure, the Senns will sound louder than the Grados - if both headphones are being driven directly from a headphone-out with a high output impedance (almost all receiver/integrated amp headphone outs have a relatively high output impedance).
 
     Share This Post       
post-841594
Post #7 of 18

fewtch

Headphoneus Supremus
Joined
Jul 23, 2003
Messages
9,559
Reaction score
28
Joined
Jul 23, 2003
Posts
9,559
Likes
28
Quote:

He said that since the Grados have less resistance, they require more current to produce the same amount of volume.


Yes, this is true. Most sources can provide enough current though (even portable sources) so it's something of a surprise if the Grados sounded quieter. Probably it is the impedance thing already mentioned. Chances are better than not that any jack that isn't a dedicated headphone amp has a high impedance (which BTW can also muddy up the presentation of Senns).
 
     Share This Post       
post-841818
Post #8 of 18

AsuAmo76

100+ Head-Fier
Joined
Sep 27, 2003
Messages
354
Reaction score
10
Joined
Sep 27, 2003
Posts
354
Likes
10
Let's do some math:

Case 1:
Amp impedance - 0; phone 1 - 30 Om, phone 2 -300 Om; 1V signal
Power1 = 1 x 1 /30; (1/30 W)
Power2 = 1 x 1 /300; (1/300 W)
Power1 / Power2 = 10;
That's why sr350 is much more sensitive than hd600 feeding thru class A solid state amp.

Case 2:
Amp impedance - 300 Om; phone 1 - 30 Om, phone 2 -300 Om; 1V signal
Power 1 = (1/10 x 1/10) /30; (1/3,000 W)
Power 2 = (1/2 x 1/2) /300; (1/1,200 W)
Power1 / Power2 = 2.5;
That's why sr350 is still more sensitive than hd600 feeding thru opamps.

Case 3:
Amp impedance - 1,000 Om; phone 1 - 30 Om, phone 2 -300 Om; 1V signal
Power 1 = (1/30 x 1/30) /30; (1/30,000 W)
Power 2 = (1/4 x 1/4) /300; (1/5,000 W)
Power1 / Power2 = 0.16;
That's why hd600 can be more sensitive than sr350 feeding thru high impedance amps.
 
     Share This Post       
post-841825
Post #9 of 18

Eagle_Driver

Headphoneus Supremus
Joined
Jun 22, 2001
Messages
6,447
Reaction score
25
Joined
Jun 22, 2001
Posts
6,447
Likes
25
Quote:

Originally Posted by AsuAmo76
Let's do some math:

Case 1:
Amp impedance - 0; phone 1 - 30 Om, phone 2 -300 Om; 1V signal
Power1 = 1 x 1 /30; (1/30 W)
Power2 = 1 x 1 /300; (1/300 W)
Power1 / Power2 = 10;
That's why sr350 is much more sensitive than hd600 feeding thru class A solid state amp.

Case 2:
Amp impedance - 300 Om; phone 1 - 30 Om, phone 2 -300 Om; 1V signal
Power 1 = (1/10 x 1/10) /30; (1/3,000 W)
Power 2 = (1/2 x 1/2) /300; (1/1,200 W)
Power1 / Power2 = 2.5;
That's why sr350 is still more sensitive than hd600 feeding thru opamps.

Case 3:
Amp impedance - 1,000 Om; phone 1 - 30 Om, phone 2 -300 Om; 1V signal
Power 1 = (1/30 x 1/30) /30; (1/30,000 W)
Power 2 = (1/4 x 1/4) /300; (1/5,000 W)
Power1 / Power2 = 0.16;
That's why hd600 can be more sensitive than sr350 feeding thru high impedance amps.



Your equations are fine, but you've messed up the results of Case 2:

Here is the real result of Case 2, based on your formula and numbers:

Case 2:
Amp impedance - 300 Om; phone 1 - 30 Om, phone 2 -300 Om; 1V signal
Power 1 = (1/10 x 1/10) /30; (1/3,000 W)
Power 2 = (1/2 x 1/2) /300; (1/1,200 W)
Power1 / Power2 = 0.4;
That's why sr350 may or may not be more sensitive than hd600 feeding thru opamps.
 
     Share This Post       
post-841849
Post #11 of 18

Eagle_Driver

Headphoneus Supremus
Joined
Jun 22, 2001
Messages
6,447
Reaction score
25
Joined
Jun 22, 2001
Posts
6,447
Likes
25
As a matter of fact, based on Headroom's measurements, the HD600 will actually be considerably more sensitive than the SR325 on a moderately high impedance (300-Ohm) headphone jack, such as that found in most HT receivers. That's because the SR325 is much less efficient than its specs would imply, while the HD600's efficiency is roughly equal to what's implied by its specs.

According to Headroom's measurements, the SR-325 requires 3.8mW in order to produce an SPL of 90dBA (equivalent to an efficiency rating of only 84 dB/mW) - while the HD600 requires only 0.75mW in order to reach that same 90dBA (equivalent to 91 dB/mW). As a result, the SR-325 would produce an SPL of only 79dBA with a 1V input from a 300-Ohm output impedance, while the HD600 would still produce 90dBA from that same jack at the same output voltage setting (or volume-control setting in this case, since most op-amps are output-voltage-constant devices with respect to the load impedance).

By the very same token, from a zero-ohm jack, the SR-325 would be louder than the HD600 at the same volume-control setting - but by nowhere near as much as what many people think: A ~3dB differential in the sensitivity from a zero-ohm jack isn't much to write home about.

Now I finally see what Grado has meant by the 98 dB/mV sensitivity rating: The "98 dB/mV" rating really means "98 dB @ 1,000 mV" or "98 dB/1 Vrms". And per Headroom measurements, with two exceptions, all of the Grado headphones are equally (in)efficient; the SR-125 is even less efficient than the rest (of the Grado line), while the RS-1 is somewhat more efficient than other Grados.
 
     Share This Post       
post-841920
Post #12 of 18

ServinginEcuador

Founder of the Head-Fi Pay-to-Post Program.
Joined
Sep 1, 2002
Messages
8,384
Reaction score
14
Joined
Sep 1, 2002
Posts
8,384
Likes
14
I didn't see this after a cursory glance, but what amp did you use? What it an OTL amp? If so, that might be your answer. The Ray Samuels Stealth does the same thing due to the loading of the tubes presented by low impedance cans. Whenever I swapped over to the Senn 600s the volume went up a bit, not down.
 
     Share This Post       
post-841942
Post #13 of 18

John Reeves

100+ Head-Fier
Joined
Apr 4, 2004
Messages
453
Reaction score
10
Joined
Apr 4, 2004
Posts
453
Likes
10
Hi,

A 3db increase in Sound Pressure Level is the equivalent of doubling the loudness. SPL curves are non-linear. The dba rating is also an averaged figure as against the SPL at octave mid band frequencies. ie. NC or NR curves. For the same power input, the SPL can vary, since the impedeance curve is not linear, and although as stated op-amps may well provide a linear output irrspective of frequency, amps can clip if the current requirement is too high due to a very low impedance at a certain frequency. For example my Isobariks drop to below 2 ohms at certain frequencies, so current availability is a major criteria in selecting an appropriate amp.
John
 
     Share This Post       
post-842043
Post #14 of 18

halcyon

Headphoneus Supremus
Joined
Oct 21, 2002
Messages
1,828
Reaction score
181
Joined
Oct 21, 2002
Posts
1,828
Likes
181
Off-topic nit-picking:

3dB rise in level corresponds to doubling of intensity, not perceived loudness.

An increase of c. 6-10 dB is required for perceived doubling of loudness (this depends on the original level/frequency).

regards,
Halcyon

Ref: Introduction to the Psychology of Hearing, 5th ed., Brian C.J. Moore, Academic Press, 2003
 
     Share This Post       
post-842142
Post #15 of 18

John Reeves

100+ Head-Fier
Joined
Apr 4, 2004
Messages
453
Reaction score
10
Joined
Apr 4, 2004
Posts
453
Likes
10
Halcyon,

Yeah, you're absolutely right, sorry about that. I was remembering when I did sound stuff for a living years ago. (A/C Systems not HiFi)
Cheers,
John
 
     Share This Post       

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top