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Headphone amp gain and output voltage
MagR
post May 5 2013, 22:30
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Hi

Can someone explain to me how gain operates in relation to my circumstances. I have an Epithany Acoustics EHP-O2D which incorporates the open source O2 amp and ODAC. The amp has maximum output of 7Vrms and the ODAC outputs 2Vrms. The amp is a two stage design having separate gain and output stages.

My lower gain setting is 2.5X. As the amp has a very low output impedance of about 0.5 ohm I believe this means maximum volume equates to 5V Ė within the maximum output voltage possible by 2V.

The higher gain is 5X which would equate to 10V at maximum volume with this source. However the amp can only output 7V. What happens here? Once the volume pot setting reaches 7V does it simply not get any louder or does it start to distort if you turn the volume pot any higher? Iíve seen some mention of input overload. If I use a gain of 5X which is impossible to achieve will I always get distortion no matter what the volume because Iím trying to overload the input stage of the amp?

Iím pretty confused by this. I canít really see why it doesnít just have a fixed gain of 3.5X as 2V * 3.5 = 7V. The reason why this has now become an issue is Iím thinking of buying another set of headphones which might need over the 5V available at the low gain setting to get loud enough. My existing Sennheiser HD650ís and AKG K701ís are fine at the low gain setting.

Thanks in advance

Mag
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saratoga
post May 5 2013, 23:04
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QUOTE (MagR @ May 5 2013, 17:30) *
The higher gain is 5X which would equate to 10V at maximum volume with this source. However the amp can only output 7V. What happens here? Once the volume pot setting reaches 7V does it simply not get any louder or does it start to distort if you turn the volume pot any higher?


The amp saturates and THD rapidly increases while power output gets only a tiny bit higher with increasing volume.


QUOTE (MagR @ May 5 2013, 17:30) *
Iíve seen some mention of input overload. If I use a gain of 5X which is impossible to achieve will I always get distortion no matter what the volume because Iím trying to overload the input stage of the amp?


The DAC outputs some voltage (you can look this up). If the gain times the DAC voltage exceeds the maximum input voltage supported by the amp (you can also look this up), you get clipping.

QUOTE (MagR @ May 5 2013, 17:30) *
Iím pretty confused by this. I canít really see why it doesnít just have a fixed gain of 3.5X as 2V * 3.5 = 7V.


Because different DACs have different output voltages.

QUOTE (MagR @ May 5 2013, 17:30) *
The reason why this has now become an issue is Iím thinking of buying another set of headphones which might need over the 5V available at the low gain setting to get loud enough.


20*log10(7/5) = +2.9 dB

If 5V isn't enough, you probably have some really eccentric headphones that will require specialized equipment rather than a headphone amplifier to drive. In that case you probably don't want a low output impedance amp like the O2 anyway.
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xnor
post May 5 2013, 23:28
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The 5x gain option is intended for portable sources that output about 1 V like the ipods.


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"we are having an educated and deep technical discussion"-amirm
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MagR
post May 6 2013, 13:33
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Thanks for your replies. I think I understand the output limitation now. If I turn the volume too far on high gain setting I will try to access more than the 7V available and the result is distortion.

Iím still no clearer on input distortion. Iíve read the O2 designers notes on this. He says the FiiO E9 for example will overload at input 2.1V at all volume levels. He then says the O2 can handle a source voltage up to 2.8V at 2.5X gain. As far as I can see this is the maximum voltage of 7V divided by 2.5. I canít see the difference with overloading the output. However this means that the high gain 5X setting can only take 1.4V from the source. As the ODAC outputs 2V this suggests the input stage will be permanently overloaded at 5X which is what Iím worried about. I can understand the concept of a given amp having a maximum input voltage it can handle but I canít see why this would vary with gain as I thought the whole concept of gain was output related. With the way my O2D is set up this suggests I can only ever access 5V with the ODAC using the 2.5X gain and the 5X is only relevant to low voltage line sources (which I donít use).

As an aside are ďdistortion,Ē ďclippingĒ and ďoverloadĒ all the same thing in this context.

Can anyone assist?

Regards

Mag

P.s. I tried my K701ís at high gain (but quite low volume as it goes very loud). I couldnít hear any distortion at this volume level. As I understand it if the input is overloaded it distorts at all volume levels Ė I didnít hear that.
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pdq
post May 6 2013, 13:57
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Clipping and overload are essentially synonymous and are types of distortion, but are not the only type of distortion.
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saratoga
post May 6 2013, 14:27
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The maximum source voltage is determined by the input transistors and not the output stage.
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MagR
post May 7 2013, 19:56
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QUOTE (saratoga @ May 6 2013, 14:27) *
The maximum source voltage is determined by the input transistors and not the output stage.

Thanks for that. This is what I thought. I wonder if the writer was simply warning about the importance of input voltage when selecting an appropriate level of gain. The O2 should take the ODAC's 2V input as they came as a single combined unit.

Thanks again.

Mag
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