Alright so this adventure started over a year ago when I built an EZ1073 from Colin. It worked perfectly and sounded insanely good so we decided that we wanted to invest in 3 more (to have two pairs). Everything was built with good parts (Vishay CMF55 / WIMA caps / BC184 transistors) but when building the first one I lost one 12 ohm resistor (R23) so I just used a 10 ohm and 2.2 ohm in series and no problems.
So we fast forward by a few months and then I'm building the other three. For some weird reason I again lose the bag with the 12 ohm resistors after building the first one and then I have some 15 ohm resistors that I decided to use instead assuming it would do no harm. For the sake of beauty I decide to change the 10&2.2 ohms for a 15 ohm because I hate the look of resistors in series.
I turn the guys on and test them and they are all working perfectly. I was testing with a SM58 (my test mic cause it's cheap and impossible to kill). But when we were testing them out at the studio we found that three of them were making a crazy feedback but only when mic gain is set to 40-60dB and only with a condenser mic plugged in. Since we had already tested the first one and made some recordings with it I assumed that the feedbacking trio would be the new ones. So I started looking at my mouser parts orders and found that the only difference in parts was some cap that was from a different wima series (still both wima). So just for the sake of testing I ordered that caps I used for the old one and started swapping them out. But while I was at it I found that one of the feedbacking ones actually was the oldest one.
Alright so again I started looking for differences and found that the only difference between the one working perfectly and the others was this R23 12 ohm resistor and I tried swapping it out but it made no difference. I have now tried swapping all the transistors in amp stage 3 (the only ones supposed to be affected by R23 right?) but nothing works so I'm starting to think if it might possibly have fried some other transistors by feedbacking?
Other parts I don't get is that it doesn't happen on gains 65,70 dB and below 40, only with a condenser plugged in and not on the line input.
So I decided to ask you guys if you have the slightest idea what this could be or some tests I could make to try to figure it out?
Schematics can be found here: http://www.audiomaintenance.com/downloads/ez1073_colourbook.pdf (scroll all the way down)
Thanks !
-Bubbi
So we fast forward by a few months and then I'm building the other three. For some weird reason I again lose the bag with the 12 ohm resistors after building the first one and then I have some 15 ohm resistors that I decided to use instead assuming it would do no harm. For the sake of beauty I decide to change the 10&2.2 ohms for a 15 ohm because I hate the look of resistors in series.
I turn the guys on and test them and they are all working perfectly. I was testing with a SM58 (my test mic cause it's cheap and impossible to kill). But when we were testing them out at the studio we found that three of them were making a crazy feedback but only when mic gain is set to 40-60dB and only with a condenser mic plugged in. Since we had already tested the first one and made some recordings with it I assumed that the feedbacking trio would be the new ones. So I started looking at my mouser parts orders and found that the only difference in parts was some cap that was from a different wima series (still both wima). So just for the sake of testing I ordered that caps I used for the old one and started swapping them out. But while I was at it I found that one of the feedbacking ones actually was the oldest one.
Alright so again I started looking for differences and found that the only difference between the one working perfectly and the others was this R23 12 ohm resistor and I tried swapping it out but it made no difference. I have now tried swapping all the transistors in amp stage 3 (the only ones supposed to be affected by R23 right?) but nothing works so I'm starting to think if it might possibly have fried some other transistors by feedbacking?
Other parts I don't get is that it doesn't happen on gains 65,70 dB and below 40, only with a condenser plugged in and not on the line input.
So I decided to ask you guys if you have the slightest idea what this could be or some tests I could make to try to figure it out?
Schematics can be found here: http://www.audiomaintenance.com/downloads/ez1073_colourbook.pdf (scroll all the way down)
Thanks !
-Bubbi
