Quote:
I've got a question. Is it normal that I've got a DC offset of max. 5mV at a gain of 20dB while I've got 11mV at a gain of 0 ?
Should the DC offset stay constant within the burn in time ? Is it normal when it rises or is this a bad sign ?
Hi,
Knusperfisch!
I will answer to you, but first I would like to try to explain something.
Let say in this way. DC offset depends (mostly) on your transistors. But let say that you cannot change your transistors (because that demands soldernig another transistors and "guess and error" game play), you can replace regulator resistors with potentiometer (trim pot). That is easier. And you can change an opamp. Without any tool.
DC offset depends (mainly) on those two things mentioned before. (regulators and opamp). I tried five (5) opamps. OPA2134, 2132, 2107, 2x627, 2xAD797. All this opamps had different DC offset. Some of them had (AD797) very high offset (11mV), and some of them too god to be true (OPA 2107 - 1mV). Situation becomes more complicated if you try to put opamp into A class. And it changes if you change gain (0, 10, 18, 20 dB).
So it depends on lots of things.
You can improve DC offset if you do fine tuning of regulator voltages. Because LC have exactly the same voltage on both channels you can raise or lover positive or negative voltage on LM317 or LM337 regulators. That will make small "drift" (push pull) but only what matters is situation on headphone output. So it does not matter if you have for example +15.1V and -14.95V after regulators . DC gain (as I said before) changes with gain, so you will have to choose gain that suits your headphones the best, and then tune DC offset.
DC offset of 11 mV only matters if you have low impedance headphones like Denon, Grado, ATH. In that case 5 mV or below, will be better because two things. First is that low impedance headphones need small voltage to be driven, and
small DC offset involved in entire signal will make sound bad. Second is rule U2/R=P. U - DC offset, R - impedance, P - power dissipated into your headphones because DC offset.
I
f you have high impedance headphones like Sennhesier (300 Ohm) or Beyerdinamic (600 Ohm) then DC offset can be as high as 20-30 mV. The reason is the same, like stated before. High impedance headphones need more voltage to be driven, so
DC offset is less involved into entire signal. And power dissipated into headphone coil is less, because bigger resistance.
Also, you have to look at technical specification of you headphones, especially in stated power (in mW) where you have to divide that stated power with number two (2) (because of some other things...).
But power does not tells you all. Some headphones are easy to drive, and some not. For example ATH W1000X is rated about 500-1000 mW AND EASLY DRIVEN. AKG K701, for example, is rated to 100 mW and
NOT EASLY DRIVEN. So, with AKG you have to be very careful and have
flawless amplifier - if you want to raise your volume. You will need a good amount of power (closer to headphones rated power, than with ATH, for the same amount of volume)
WITHOUT DC offset. AKG K701 coils do not forgive if they are overpowered with wasted power (DC), but in the same time they need a lot of power. ATH forgive much more wasted power (DC), and need less power to be properly driven.
That is reason why I always talk about
VERY GOOD amplifier, when someone says: "AKG K701". You should try different amplifiers. You would be amazed how power sensitive (quality dependent) AKG K701 are.
Sorry for bad english. I tried to explain as much as I could.