Motu M4 headphone amp dissapoints with Sennheiser HD650

[CheesyMoo]

I bought a MOTU M4 as its feature set was sufficient for my needs (recording, up to 2 inputs at once, good ADC, clean pre-amp, independent phantom power on channel)

When I plug in my Sennheiser HD650s to the headphone jack I can't hear anything until the knob is turned to about 95%. This gives me a very small range of motion to dial in the volume. It essential feels like an on/off switch.

Compare this to my Focusrite Saffire Pro 40 (12 years old) which is able to drive all of my headphones with no issue across the full range of the headphone gain knob. (In fact I would still be using the same Saffire Pro if my new machine supported Firewire audio)

questions:
- Is this expected or is there potentially an issue with my unit?
- Does the M4 use a different taper on the headphone potentiometer?
- Would replacing the M4 with an UltraLite MK5 resolve this issue?
- Do you know another unit with similar specs that actually has a useable headphone amp?
- Is this because the M4 is bus powered?

 

[EasyC]

I have an M4 which drives my Beyer 250 Ohm just fine...

 

[AnalogSteph]

- Is this expected or is there potentially an issue with my unit?

That does not sound right.

- Does the M4 use a different taper on the headphone potentiometer?

Oh, you bet. It's not even a pot in the signal path itself, it's one of these "pot on reference voltage feeding ADC" kind of deals that controls either digital volume or a digitally controlled PGA (like for the input amplifiers). Single gang and linear, I think.

Bad solder joint or bad pot, I'd say. Some quality issues are not entirely unheard of with these MOTUs. If this is your only complaint, I'd initiate a return or direct exchange for a properly working sample.

 

[Evgenioni]

Sennheiser HD650S Demanding headphones. The best to use them with an external amplifier like Topping L30. I use this amplifier with Ultralite mk5 and 650

 

[unpluggged]

Sennheiser HD650S

No such model.

Demanding headphones.

No, they are not. I hardly ever need to switch to the higher ref level from the lo-power (+7 dBu, 1.7 V) ref level — only for very quietly mastered content, such as older classical music albums. That's including the headroom for EQ with 5 dB of bass boost.

Topping L30

The one that is notorious for killing headphones? No, thanks.

 

[solderdude]

HD650s indeed does not exist.
The HD650 exists as well as the HD560S.

The L30 does not kill headphones. Only the first ones could potentially do that when some static discharge on the volume control happened but most of this problematic first runs with a serial number starting with a number below 2012 (2020 December)have been replaced with fixed ones. It is still possible some folks may sell 'potentially faulty' ones though unknowingly or deliberately. Above a certain serial number all L30's are safe and so are L30-II. (see the Topping L30 thread)

HD650 are not demanding... they just require an amplifier that can deliver a bit higher voltage to play impressively loud but require very little power.
The M4 can drive the HD650 to a pretty loud 110dB peak level.

 

[Evgenioni]

yeap, hd650.
The MOTU M2 amplifier is distorted at a high volume level (100+ dB SPL). The amplifier of MOTU UL mk5 works much better.
I have a topping l30 and a lot of free line outputs on the MOTU...

 

[CheesyMoo]

Thanks for all the replies.

Indeed my headphones are HD650 as stated in the title and contrary to the typo in the initial post.

Having some more time to play with this device in better listening conditions I need to amend my original description of the issue.
The headphone output works as expected when the INPUT MONITOR MIX is all the way to the right.

I only need to crank it to hear hardware input monitoring, even if the preamps are so hot that it is clipping the headphone output is inaudible for most of the knob's range.