AML Ez1081 bias/calibration

[Tom snich]

looking for some insight from seasoned builders.

I built a AML EZ1081 mic kit this week everything went well with the build. http://www.audiomaintenance.com/downloads/ez1081pre_colourbook.pdf
I started the calibration procedure turning bias trimmer ten turns clockwise, fired it up and all good, no smoke
I measured the DC bias on pins TP6 and TP7 and got 47-48 mv. (It should typically measure about 50-60 mV) A tad low, but thought this will be fine so I proceeded with the rest of the calibration procedure
Applying a signal of -40dBu (7.75mV RMS) ( I am applying a 1khz sine to the input) I then turned the gain control up to +60dB
I Measured the output signal, and adjusted the gain trimmer R28 until the output measured +20dBu (7.75V RMS) Everything measured fine.

I then Measured the DC voltage between bias test points TP6 & TP7 and tried to adjust the bias trimmer R51 to get 90mV, but here the trimpot did nothing and the bias DC was still on 47-48, I have turned the trimpot a good 4 turns either way but nothing happens to the bias voltage.

I have tested the preamp with a mic and the signal is good with no distortion, very clean, but I'm a bit confused, is it broken?
I have emailed Colin at AML but no answer, so hoping someone could help me here.

Does anyone have any advice as to what could be causing this issue? wrong value resistor or a damaged transistor maybe.

Any help would be appreciated!

Cheers
Ben

 

[Bonsaimaster]

I have built these and have not had your issues. Although I have nothing specific for your issue I would do two things. Measure the resistors to make sure they were placed correctly - I have done this wrong many times. Check the similar size capacitors (diameter) to see if they are misplaced. Finally resolder ALL the joints to see if their is a bad joint. - Sometimes the easy things fix the issue. Good Luck

 

[Tom snich]

Thanks for the encouragement!
I was being a bit of a whimp and did not turn the trim pot far enough I turned it a bit more and it started to trim the voltage up to 90 mv . I have a tendency to be a bit too careful with soldering, it is sometimes to my advantage and sometimes not!
I did resoldered everything and checked every component and everything is correct of course!
Nice sounding preamps!

Cheers
Ben

 

[andrewvait]

Hi folks of this fine thread: I've recently built an ez1081 and am having an issue that I can't figure out for the life of me. If it's helpful to know - I built a second unit and it works perfectly*

I initially installed the input transformer upside down - don't ask me how or why, but upon plugging it in, the only sound it would make is a clip level popping noise every 1.5-2 seconds (regular intervals). I bought a new input transformer and every transistor for the 1081 build from AML and replaced everything. Upon plugging it in this round, it delivers the same clip-level pop but it also passes signal - an improvement!

I noticed that the bias points spike - they read a normal 80-ish mV before spiking to 140+ and into overload on my multimeter. I ordered new volume and bias trimmers and swapped them out, but the issue persists.

*Interestingly, the unit I built that works "perfectly" does pop/click once or twice upon powering up before settling into functional mode. I've compared the two units side by side, point by point, but I can't find any discrepancies, which leads me to believe something is blown or continues to blow, the circuit board is fried (it was my first preamp build and I did a good amount of messy desoldering), or it's just beyond my ability to fathom.

Any pointers would be greatly appreciated!

 

[noneissafe]

I am building one at the moment and i have to say that some of the leads on the components are a bit short. I don't have enough to bend the lead to keep the components in place when i turn it upside down to solder. Could I use tape to hold it in place? or anything do anything else?

 

[Ricardus]

Could I use tape to hold it in place? or anything do anything else?

Sure. I use tape all the time. I did it today, in fact.

 

[noneissafe]

Sure. I use tape all the time. I did it today, in fact.what

What kind of tape did you use @Ricardus?? Or will certain types of tape ruin the PCB?

 

[Ricardus]

Just regular yellow masking (painters) tape is fine.

 

[jmhdsn]

What kind of tape did you use @Ricardus?? Or will certain types of tape ruin the PCB?

Just regular yellow masking (painters) tape is fine.

What kind of tape did you use @Ricardus?? Or will certain types of tape ruin the PCB?

Masking tape, my friend. I’ve built two of these and the 1073s.

I’m very deliberate when soldering because I really suck at desoldering, so these were comparatively slow builds as I would only tape things down page by page when reading the instructions, but masking tape is your best bud here.

 

[noneissafe]

I am stuck at the moment on step 4. How did you measure the output signal? I think I might be measuring the wrong thing.