My mic preamp doesn't work

[giralfino]

Hi all,
I just finished to build the Wallin's mic preamp but it doesn't work. I don't have a PCB so I lined up the components on a plywood board using the layout presented in the Wallin page, and connected them through wires or directly using the component's lead.

I connected also the switch and battery and tried to power the preamp, but the led didn't illuminate. So I tried to find the error and checked all the connections and component values.

The connections seem OK, the resistors also, but I found a strange behavior on the small capacitors (C2, C5 and C10 on the schematic), because they measured in the 5-10 nF range, i.e. a lot more than they should.

I subsequently found that the resistors have a capacitance in that range and since C5 and C10 are wired in parallel to a resistor this could explain my measure. Probably also the C2 measure has a similar explaination since it is wired in parallel to the mic.

Anyway I'm asking to someone that has successfully built the preamp to measure those caps and confirm that **on board** they don't measure as specs.

Sorry for the long post and thanks in advance for your help.

Ralf

[wintermute]

Hi Ralf, I'm just about to head off to work so can't measure now, but a quick question, did you use the same voltage reference as Eric did, or a different one? I used a different one and mine didn't work, I had to add in an extra resistor to get it started. I can dig up what I had to do later if you have used a different one

Tony.

[giralfino]

I hope I used the right voltage reference: I bought this LM4040 from RS Components

Thanks, Ralf

[wintermute]

Hi Ralf, from memory I used a completely different part to Eric it was a zy something I think... same specs but different manufacturer. I'll have a look tonight

Tony.

[wintermute]

Hi Ralf, I haven't had a look at the circuit yet (I'm not sure if I will be able to get accurate capacitance readings with my meter but I can give it a go), but I did find what I had to do to get the circuit to work properly.

Put a 100K resistor from +9v to the non ground side of the LM4040 (ie between R1 and the LM4040. I actually used a LT1009CZ instead of the LM4040 because at the time that was what I could get hold of, but Jason Neils original version of this preamp had the 100K resistor (and was using an LM4040).

I don't have what the symptoms were except that it didn't work, everything looked fine and "the voltage reference was out" by how much I didn't record.

Tony.

OK I found the original circuit I did, I can't get any meaningful measurement at all with my DMM. But looking at some of the big caps in close proximity to the ones you are trying to measure, separated by a few resistors, I don't think you are going to be able to work out anything meaningful from in circuit measurements. Hopefully the suggestion I have made will help, if not, taking voltage measurements at various points in the circuit for comparison would be much better

[giralfino]

Hi Tony, I will try your suggestion.
Re. the voltage measurements, unfortunately my volt meter is very basic and not accurate enough. I will try though.
And the caps can be measured better from the other side of the PCB - I was simply trying to understand if my measures were normal, I don't need a precise one.
Thanks, Ralf