[lejaz]

Anyone know the output impedance of the LD mkV amp? For some reason the manufacturer of the mkV seems to be keeping it a secret. He was asked that question on the LD forum, but he never gave an answer. Is this amp suitable for low impedance phones? I know it's supposed to do well with the 300 ohm Sennheisers, but how about with the k70X?  I read a report that the 70X sounds a lot brighter out of the mkV than out of other amps. Would an O2 be a better match with the 70X? Any help with this would be much appreciated.

 

[lejaz]

Any AKG k70X users out there who have the mkV or tried it in the past? Curious if anyone else finds this combination overly bright. It's possible it's just the phones themselves, and nothing to do with the mkV's output impedance or anything else.

 

[GotNoRice]

Quote:

Is this amp suitable for low impedance phones?

From their site:
 

1. Power Output:
2. 187 mW @ 300 ohm
3. 250 mW @ 120 ohm
4. 500 mW @ 32 ohm

 
Looks like Low impedance headphones are fine.  If anything, it looks like it would have issues with high impedance headphones.  I noticed they didn't even provide a measurment for 600ohms... probably <100mw.

 

[lejaz]

Many thanks for the input GotNoRice. I was reading some comments about output impedance on the O2 website and they claim you need an amp with a very low output impedance for low impedance headphones. They even have some graphs that show huge frequency response deviations with low impedance phones caused by a high output impedance on the amp.
Quote:

From their site:
 

1. Power Output:
2. 187 mW @ 300 ohm
3. 250 mW @ 120 ohm
4. 500 mW @ 32 ohm

 
Looks like Low impedance headphones are fine.  If anything, it looks like it would have issues with high impedance headphones.  I noticed they didn't even provide a measurment for 600ohms... probably <100mw.

 

 

[mikeaj]

In some scenarios, yeah, there can be quite a severe shift in FR.  That's not going to happen so much with a Q701.

 
If you take the Little Dot spec literally, then you can calculate the output impedance from the given power delivery numbers.  It's 187 mW into 300 ohms and 250 mW into 120 ohms.  Here I assume the amp isn't current limited into 120 ohms, which it most probably idn't.
 
V_delivered1 = sqrt(P * Z) = sqrt(0.187 * 300) = 7.5 volts into 300 ohms
V_delivered2 = sqrt(P * Z) = sqrt(0.250 * 120) = 5.5 volts into 120 ohms
 
V_delivered1 = 7.5 = V_s * Z_L / (Z_s + Z_L) = V_s * 300 / (Z_s + 300)
V_delivered2 = 5.5 = V_s * Z_L / (Z_s + Z_L) = V_s * 120 / (Z_s + 120)
 
That implies the V_s (maximum voltage from the source) is 9.9V and the output impedance is 96 ohms or so.
 
However, this calculation means that the 500 mW into 32 ohms is impossible because that implies the max voltage a 32 ohms load would get is 9.9 * 32 / (96 + 32) = 2.475 V for like 191 mW, so there seems to be something wrong with the specs.  I would guess that the output impedance is actually significantly lower than 96 ohms.
 
Even were the output impedance like 96 ohms, you'd only be looking at a dB shift or two in the FR for the Q701 up until 10 kHz or so (slightly more than that above that, all the way to 20 kHz).

 

[WiR3D]

Quote:

In some scenarios, yeah, there can be quite a severe shift in FR.  That's not going to happen so much with a Q701.

 
If you take the Little Dot spec literally, then you can calculate the output impedance from the given power delivery numbers.  It's 187 mW into 300 ohms and 250 mW into 120 ohms.  Here I assume the amp isn't current limited into 120 ohms, which it most probably idn't.
 
V_delivered1 = sqrt(P * Z) = sqrt(0.187 * 300) = 7.5 volts into 300 ohms
V_delivered2 = sqrt(P * Z) = sqrt(0.250 * 120) = 5.5 volts into 120 ohms
 
V_delivered1 = 7.5 = V_s * Z_L / (Z_s + Z_L) = V_s * 300 / (Z_s + 300)
V_delivered2 = 5.5 = V_s * Z_L / (Z_s + Z_L) = V_s * 120 / (Z_s + 120)
 
That implies the V_s (maximum voltage from the source) is 9.9V and the output impedance is 96 ohms or so.
 
However, this calculation means that the 500 mW into 32 ohms is impossible because that implies the max voltage a 32 ohms load would get is 9.9 * 32 / (96 + 32) = 2.475 V for like 191 mW, so there seems to be something wrong with the specs.  I would guess that the output impedance is actually significantly lower than 96 ohms.
 
Even were the output impedance like 96 ohms, you'd only be looking at a dB shift or two in the FR for the Q701 up until 10 kHz or so (slightly more than that above that, all the way to 20 kHz).

Not that i know anything about this, but wouldn't the equation be alot more complex then that? Taking into account power supply, and all the caps and resistors aswell?

 

[mikeaj]

Quote:

Not that i know anything about this, but wouldn't the equation be alot more complex then that? Taking into account power supply, and all the caps and resistors aswell?

That probably wasn't so clear.  Certainly how much power can be delivered will depend on the output stage and the rest of the circuits (including the power supply, which is probably very rarely the limiting factor for that).  The equations aren't to figure out how much power the circuit can deliver based on the design and parts used.
 
I was taking the power delivery figures as a given, since they're provided.  Based on those numbers, I was trying to solve for the output impedance Z_s by taking two simultaneous equations equations and solving for the two unknowns.  Each equation just comes from knowing that the voltage across the load is the voltage from the source multiplied by the ratio of load impedance to [load impedance + source output impedance].  It's just a voltage divider:
http://en.wikipedia.org/wiki/Voltage_divider
 
The result was that their numbers look wrong and inconsistent with each other.  Either that, or I screwed something really basic up.

 

[WiR3D]

Either way back on topic i dont think it will work with low impedance phones, but in the interest of clarity it would be nice if someone could find out exactly what it is.
 
And mikeaj, I'm not challenging you, it just doesnt seem correct, which you pointed out, It just doesn't seem that simple because of the nature of amps. But i dont know that much so you could be correct

 

[stv014]

The specs are probably not accurate, and the power into 32 Ohm is most likely rounded up from some lower number. The only way to know is to actually measure it; for that, at the minimum, a sound card (even onboard is sufficient) and Y splitter are needed. If you have no resistors to simulate a load, even the headphones are OK for an approximate result.
 

 

[lejaz]

The manufacturer of the mkV won't release the specs of the output impedance ....maybe for fear of scaring off people with low impedance headphones. On the big LD mkV thread from a few years back some folks liked the amp with low impedance Denons, but a few reported hiss, I think. There was also some talk of using it with an impedance adapter, which makes me think it has a fairly high output impedance for a ss amp. 
Quote:

Either way back on topic i dont think it will work with low impedance phones, but in the interest of clarity it would be nice if someone could find out exactly what it is.
 
And mikeaj, I'm not challenging you, it just doesnt seem correct, which you pointed out, It just doesn't seem that simple because of the nature of amps. But i dont know that much so you could be correct

 

 

[WiR3D]

Quote:

The manufacturer of the mkV won't release the specs of the output impedance ....maybe for fear of scaring off people with low impedance headphones. On the big LD mkV thread from a few years back some folks liked the amp with low impedance Denons, but a few reported hiss, I think. There was also some talk of using it with an impedance adapter, which makes me think it has a fairly high output impedance for a ss amp.

Thats probably true regarding the output impedance, you weren't the first to mention it.
Impedance adapters are not as simple as you might think, increasing and decreasing impedance are quite different, and an impedance adapter in the general sense actually increases output impedance

 

[lejaz]

Thanks for clarifying about the impedance adapter. I mistakenly thought it would increase the impedance of the phones, but apparently it's the other way around.....definitely not what I'm trying to accomplish with the mkV. 
 
Quote:

Thats probably true regarding the output impedance, you weren't the first to mention it.
Impedance adapters are not as simple as you might think, increasing and decreasing impedance are quite different, and an impedance adapter in the general sense actually increases output impedance

 

 

[WiR3D]

Quote:

Thanks for clarifying about the impedance adapter. I mistakenly thought it would increase the impedance of the phones, but apparently it's the other way around.....definitely not what I'm trying to accomplish with the mkV.

Here you go Impedance Adapters/Cables | Explained & Listed
 
 

 

[GotNoRice]

Quote:

i dont think it will work with low impedance phones

So they were just kidding when they listed a 32ohm power value for the amp in their specs?

 

[lejaz]

 
Quote:

So they were just kidding when they listed a 32ohm power value for the amp in their specs?

 
I think what he meant was, it won't work well with low impedance headphones. It certainly works with my low impedance dt48/25ohms. Whether the output impedance of the amp is low enough to work well with low impedance phones is the issue. According to what I read on the O2 website, a very low output impedance on the amp will give best results with low impedance phones.