ruffrecords
Well-known member
I am working on a very simple headphones amp for a mixer. The attached pdf shows my thinking so far. There is a level control before the input in each case and appropriate coupling and decoupling components are not shown
I started with a standard hi-fi type phones amp using a pair of 5532 op amps shown in the top figure. The two op amps work in parallel with 47 ohm build out/ current sharing resistors. The NFB is taken from the output so the output Z is very low and the gain is about 10dB.
The next version is one I found in the Mackie 1202 VLZ mixer sown in the middle figure. The negative feedback is now taken from the output of the first op amp and the build out resistors have been increased to 120 ohms. In this configuration the output impedance is essentially the two 120 ohm resistor in parallel which presumably gives it some advantage when working with a wide range of headphone impedances. The gain is still about 10dB.
I only need unity gain as my tube mixer internal level is +4dBu. So the third and I hope final version simply removes the two gain setting resistors and turns the first op amp into another unity gain follower.
I have built and tested the first version and as expected it has too much gain. I plan to modify it to match the other two and retest.
Comments?
Cheers
Ian
I started with a standard hi-fi type phones amp using a pair of 5532 op amps shown in the top figure. The two op amps work in parallel with 47 ohm build out/ current sharing resistors. The NFB is taken from the output so the output Z is very low and the gain is about 10dB.
The next version is one I found in the Mackie 1202 VLZ mixer sown in the middle figure. The negative feedback is now taken from the output of the first op amp and the build out resistors have been increased to 120 ohms. In this configuration the output impedance is essentially the two 120 ohm resistor in parallel which presumably gives it some advantage when working with a wide range of headphone impedances. The gain is still about 10dB.
I only need unity gain as my tube mixer internal level is +4dBu. So the third and I hope final version simply removes the two gain setting resistors and turns the first op amp into another unity gain follower.
I have built and tested the first version and as expected it has too much gain. I plan to modify it to match the other two and retest.
Comments?
Cheers
Ian