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Amp recommendation for AKG K612 Pro headphones

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Aug 11, 2018
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#1
Hi guys, would appreciate some advice. Struggling with information overload in the audio world.

I have AKG K612 Pro headphones (and also Massdrop x Sennheiser PC37X Gaming Headset)

I was eyeing up Topping A30 but hearing a few comments saying it's not so good because of 'high internal impedance' which I don't really understand, but as far as I can tell it limits suitable headphone choices? I'd rather have an amp that's compatible with a wider range of headphones

Do you think Massdrop Objective 2 would be suitable? (https://www.massdrop.com/buy/massdrop-o2-amplifier). I can't find any technical reviews for this though

It has two options, is this how loud the amp will go?

- Standard gain (2.5x low, 6.5x high)
- Medium gain (1.0x low, 3.3x high)

I mainly just Spotify and games, and never listen very loud, so would the 'Medium' with the lower gain numbers be the best choice?

Probably will be using with Topping D10 dac

Mainly looking at what's available on Massdrop or AliExpress because they usually have low cost shipping to my country. (I looked at Schiit stuff but shipping way too expensive)
 

Grave

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#2
Your headphones are 120 ohms and 50 ohms so the A30 might not be a good idea for you.

For the O2 the 6.5x gain is for weak sources like portable audio players and the 3.3x gain is for DAC's.

I am not sure about the build quality of the massdrop O2. I owned the JDS Labs O2 and I liked that.
 

solderdude

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#3
The A30 is perfectly capable of driving the K612.
The higher output imp on the 3.5mm will actually even be better than a lower output R as it will boost the bass about an (bordering on being audible) +0.5dB and it needs every bit it can help. The K612 sounds fine on 120 Ohm output R as well.

The gaming headset has an L-PAD volume control (it seems) so the headphone may not have an impedance varying that much.
Still the 6.3mm socket is recommended.

Assuming the A30 has +/- 15V voltage rails, which is based on the output voltage swing of 26.6V (9.5V rms) which means the rail voltage = 30V

The max. output power for the 3.5mm in K612 = 7.5V = 0.5W
The max. output power for the 6.3mm in K612 = 8.7V = 0.6W
The K612 is rated for 0.2W by the way

The max. output power for the 3.5mm in PC37X = 5.8V = 0.7W
The max. output power for the 6.3mm in PC37X = 7.8V = 1.2W
You'd better dial the L-PAD a bit down !

So the A30 is way more powerful then the headphones.
As said... the output R is not a problem here.
TPA6120 amps need that extra resistor to remain stable due to their design so cannot (should not) be lower than 10 Ohm.

The max. output power for the 3.5mm in 32 Ohm = 4.8V = 0.7W
The max. output power for the 6.3mm in 32 Ohm = 7.1V = 1.6W
 

Grave

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#4
The A30 is perfectly capable of driving the K612.
The higher output imp on the 3.5mm will actually even be better than a lower output R as it will boost the bass about an (bordering on being audible) +0.5dB and it needs every bit it can help. The K612 sounds fine on 120 Ohm output R as well.
How do you know this?
 

Grave

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#6
IDK why you would recommend an amp which would change the frequency response in this case. It's great for my high impedance headphones, but I would not use it for low impedance headphones.
 

solderdude

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#7
How do you know this?
Because I owned, listened to it and measured it (K612).
This is the one: K612 on 0.1 Ohm and 120 Ohm (30 Ohm and 10 Ohm are closer to 0 Ohm)



So science, knowledge and experience.

I would not recommend it for IEM's and some low impedance headphones either.
A Magni 3 or O2 (with reservations) could be viable options as well.
 
Last edited:

solderdude

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#9
The fixed gain stage, when used incorrectly, is prone to clipping.

All the other amp designs don't have that potential issue.
 

trl

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#10
I'm using my AKG K701 with Objective2 having a gain of 3.5X paired with ODAC (1.92V RMS on outputs) and they're perfectly compatible.
However, when pairing the O2 with ASUS Essence ONE (around 2.05V RMS on outputs) there are some distortions I can hear on most songs having peaks over -1dB.

I assume a safe gain for the Objective2 would be somewhere between 2.5-3X if power adapter is used and between 1.5-2X if running on batteries. (eg.: Gain x Input source voltage < 7V RMS when running on mains and <5V RMS on batteries).
 
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