New Posts New Posts RSS Feed - Which impedance?
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Welcome to the Graham Slee Audio Products Owners Forum

 

Open to all owners plus those contemplating the purchase of a Graham Slee HiFi System Components audio product and wishing to use this forum's loaner program: join here (Rules on posting can be found here)

This website along with trade marks Graham Slee and HiFi System Components are owned by Cadman Enterprises Ltd


Which impedance?

 Post Reply Post Reply
Author
Ash View Drop Down
Senior Member
Senior Member
Avatar

Joined: 18 Mar 2013
Location: Dorset
Status: Offline
Points: 4334
Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Topic: Which impedance?
    Posted: 07 Jun 2018 at 11:14pm
I have a question. I am looking at possibly buying a specialist headphone (MySphere 3 and yes, I am insane) to use with Bitzie or perhaps Solo ULDE or Proprius. This headphone has two variants; 3.1 is 15 Ohm impedance and 3.2 is 110 Ohm impedance. Which one is the more sensible for use with Slee products? I am led to believe that the 15 Ohm would be driven better by the Bitzie?? The headphone sensitivity is close to that of the HD800, I am told. Any advice would be appreciated.
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Retired

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 16298
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 2018 at 9:36am
A high impedance is easier to drive but requires a larger voltage swing.

A low impedance is perfect for supply limited amps such as those which run on 5V (or even 3.3V), but such devices are no good for analogue where there is no well-defined upper limit (0dBfs).

One of the best headphone sound experiences I had was using the VAS intended for a power amplifier to drive a pair of 600 Ohm Senny's (via a DC blocking capacitor and nothing else).

But in a world where impedances range from 5 Ohms to 600 Ohms (maybe more?) I ain't going to risk making one of those.

That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
Back to Top
Ash View Drop Down
Senior Member
Senior Member
Avatar

Joined: 18 Mar 2013
Location: Dorset
Status: Offline
Points: 4334
Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 2018 at 11:24pm
In your opinion and in the real world, if 3.1 and 3.2 both have equal sensitivity, would the Bitzie be any less capable of driving a 110 Ohm load as opposed to a 15 Ohm load? The HD800 is 300 Ohm and sounded fantastic with the Bitzie so 110 Ohm should be no problem, right? Also, could I use a headphone cable to increase the impedance of the 3.1 to "convert" it to the impedance of the 3.2 if necessary or is it not as simple as this? Is it actually the coil windings in the drivers that are different?

So it's counter-intuitive. A higher impedance is an "easier" load, in the circumstance that the amp can swing the voltage faster.

The lower impedance version wouldn't be as good for vinyl if I ever wanted to use it for that?

So Bitzie output compared to Proprius output, Bitzie output is fairly high impedance whilst Proprius output is fairly low impedance? I'm still confused but trying to work through it. That "damping factor" thing you've mentioned in the past, is that what all this is about?


Edited by Ash - 08 Jun 2018 at 11:28pm
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Retired

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 16298
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jun 2018 at 4:34am
I will answer "damping factor" this reply:

When valve amps were all you could get, damping factor wasn't discussed because valve amps don't have much. Therefore damping factor didn't exist.

Early transistor amps were not much better. Fair enough, the "driving impedance" was(is) considerably less than a valve output transformer, and is made even lower by negative feedback, and can easily reach a ratio of 200 - 800 : 1 (speaker : driving/output impedance) .... but for the obligatory output DC blocking capacitor, whose impedance increases with falling frequency:

4,000uF into 8 Ohms gives a -3dB frequency of 5Hz, but looked at from the speaker back into the amp output, the pre-cap driving impedance being around 0.03 Ohms, you get a -3dB frequency of 1326Hz. So if the driving impedance is 0.03 Ohms then above 1326Hz the damping factor is 8/0.03 = 267, but at 133Hz it is 26.7, and at 5Hz just 1 (falling at 20dB/decade).

If the mass of the cone is such that it misbehaves then it will want to lag the driving signal, which is AC, and so it is reckoned that if the driving impedance can be maintained at a factor of say 100, then it will only lag by 1%.

It means that high compliance high mass bass drivers are under greater control, but the idea of high compliance was to provide more bass - never mind if it was out of control - and so "damping factor" simply reverses the trend.

When all we had was valve amplifiers, all we had was low compliance bass drive units - there wasn't a problem!

High power 60s and 70s transistor amps with blocking caps worked fine too. Used with low compliance bass drivers you got good controlled bass - sounded fine.

Then high compliance high mass bass drivers came along. Result: muddy bass. Transistor amps by that time could be made without DC blocking caps, by using dual rail power supplies and also needing servos to constantly monitor output DC level, to keep it within a few mV of 0V. A transistor amp with only 20dB NFB and 0R33 emitter resistors can easily do a damping factor of 267 right down to its -3dB bass frequency. With a bit more NFB, say double (40dB) that is increased to 2670.

Enter subjectivism and you get people saying how good the bass is using high compliance bass drive units which lag, with valve amps... (oh really?). Never mind the quality - feel the boom.

If people would only stop to consider such things, but today it is explained away by saying "I'm not technical", where people into high fidelity in the 60s and 70s would have easily understood.

But being non-technical, there is a lot of interest in specifications... !

Damping factor therefore originates in one-upmanship and is therefore a marketing term adopted by manufacturers of DC coupled transistor amplifiers.

That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Retired

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 16298
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jun 2018 at 5:42am
Headphone history:

Headphones came about because of Morse communication, and later telephony. Then came public use for private listening.

The first dynamic (magnetic) headphones were made to operate with valve preamp tubes. They would not be used in conjunction with a valve power amplifier due to output transformer matching.

An anode (plate) output could be made to swing a large voltage but into a high resistance (usually 100 kilohms), but headphone voice-coils of such resistance would not be viable (think of how thin the wire would have to be, plus the number of turns, and the inductance which would "kill" the upper frequencies).

The headphones could however be placed in series with the anode resistor, via a DC blocking capacitor, or not, depending on cost and intricasy (damping factor here would be rather negative by a very large degree...).

So as to have some signal left to be heard, the impedance would still need to be high, and often the headphone would be 2000 Ohms.

The other way of doing it would be to use a cathode follower, and a value of 2000 Ohms was highly desirable considering cathode resistors were generally in the order of 500 - 5000 Ohms.

As this was before stereo, each side could be made 1000 Ohms and wired in series to give 2000 Ohms.

There were also crystal headphones which could be used directly on the plate voltage, but fidelity was pretty awful.

Telephony standardisation brought in the 600 Ohms load. For mono this could be 300 Ohms per side, or for split feed left at 600 Ohms. A spare balanced output could be patched in for headphone feed.

The same standard was adopted for professional audio where balanced outs were via 300 Ohms per side resistors, or via a transformer, and the load being 600 Ohms to obtain the 6dB insertion loss (which made everything work within the then limits).

A 600 Ohm headphone could therefore be used to monitor a spare desk output. Voltage would be 776mV (0dBU, 1mW into 600 Ohms) and therefore the headphone would need to have good audibility if 600 Ohms, for the 1mW developed (at maximum desk output).

That covers the early valve and professional side. Now for the home audio side.

With music centres with transistor amps delivering n watts into 8 Ohms, some attenuation was felt necessary for the switched headphone socket. Usually 680 Ohm series resistors are used, and manufactuers made 8 Ohm headphones. The result was a good match in sound pressure level.

So, even up until the 90s, there were pro headphones and domestic headphones, and the choice was usually 600 Ohms or 8 Ohms.

In the late 70s Sennheiser, possibly realising a new market for their professional offerings, started to offer a plastic moulded headphone with yellow foam ear muffs. At 600 Ohms it didn't really matter whether the amp had an attenuated socket or you connected them straight across the speaker outputs (with a little DIY to make an adaptor lead). I remember they sounded absolutely fantastic (if a little bright due to tight control of the bass - they were designed for a 600 Ohm feed - here being used from around 0.03 Ohms) compared to the 8 Ohm "hi-fi" headphones around at the time.

Then came the mid 90s to the noughties, and the headphone market went strange. That's about all I can say.

We now have opinions and one-upmanship gone loony. Impedances range from 5 Ohms to (I haven't a clue anymore). Imaginary damping factor must now be foisted on everybody, with menaces. Remember that 600 Ohm headphones were driven by 600 Ohms: damping factor = 1.

If the headphone has sufficient sensitivity then it only needs 1mW to sound loud. But there are those who want their ears to bleed! Such types will find the Bitzie output too low. It can go to 5V peak to peak output, which is 1.75V rms. This is twice the voltage required for the 600 Ohm "spare desk feed" headphones. It will therefore do more than 4 times the power. It will not however sound twice as loud, you need 10 times the power for that.

As it gets harder for an amplifier to deliver a clean signal into a lower impedance, it is IMO silly to go for a low impedance headphone, where distortion is more easily heard than on loudspeakers. But a high impedance won't make your ears bleed, so for those who want that, it would be best to go for low impedance, because as your ears bleed you won't be noticing the distortion...

...but some say they can!



Edited by Graham Slee - 12 Jun 2018 at 6:04am
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
Back to Top
Ash View Drop Down
Senior Member
Senior Member
Avatar

Joined: 18 Mar 2013
Location: Dorset
Status: Offline
Points: 4334
Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jun 2018 at 4:30pm
I think I will opt for MySphere 3.2 with 110 Ohm impedance then, for more versatility with equipment. At 96dB/mW sensitivity (roughly the same as the HD800), it might still be a bit too sensitive to use from the Proprius monoblocks without any additional series resistance. But I could always request a John Cadman special in the form of a Spatia headphone cable with resistors in it to increase the impedance to 300-600 Ohms, for the right price.
Back to Top
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.01
Copyright ©2001-2018 Web Wiz Ltd.

This page was generated in 0.125 seconds.