Hi. I'm going to build the headphone amp QRV-07 and have a question about it. I want to be able to adjust the volume on the headphones that I connect to it. I wonder if anyone can advise me on where to put the potentiometer and which value it should have? I have thought on a stereo potentiometer from ALPS with 10kOhms resistance. Would it work to put it in R5s place and increase R4 to a value between 5k-10k? Or in R7s place and increase R6?
Or should I go for a potentiometer with a value of 50kOhm and replace R1 with it and "break" up the circuit in front of C1 and connect C1 to the middle connection of the potentiometer? Is there a value of amplification that you not should go over? Thanks in advance for your help.
You should connect the pot at the input, nowhere else. A 10 k pot would be very nice. If you want (or need) the C1 etc., it's your choice.
Hi PerAnders. Thanks for the fast reply. I have some further questions regarding the volume control.
The first resistance in the signalpath is a 470k. If I replace it with a pot of 10k, will it affect the signal somewhat? Have I got it right when I think that the first capacitor is for no letting lowfrequences through, and the second one, just before the OP is for leading high frequences to the ground? Will this filtering be affected?
R1 is only a pull down resistor. A 10 k pot will affect lowpass filter R3 and C2 a bit. The corner frequency will be 31 kHz when the pot is at - 6 dB. Less attenuation and more will decrease this affect. Nothing to worry about really.
C1 and R2 is a highpass filter.
Now I have mounted the finished board in a box and put a 10k pot at the input, thanks for the help :-D
I don't think it was especially hard to mount the smdparts, but you need a solderingiron with really thin point, a good pair of tweezers and a magnifyingglass. The hardest part was to fit it all in a box and make room for all the cables. This is my second day listening to the amplifier and I think it sounds very clean and nice. Thanks for a real nice project! :-D
Don't foget to come back with a small review later, only if you want of course.