problem with Wallin preamp 2

[wintermute]

Hi,

I made a wallin preamp 2, http://www.gti.net/wallin/audio/preamp2/preamp2.html but unfortunately it has some problems.

it basically works but has very low gain and only seems to amplify high frequencies (maybe over 1Khz) .

If I touch the negative or the case of the WM60AY mic capsule it picks up a lot of hum, but the gain increases dramatically (and the freq response seems to be a bit better too). I suspect this is a clue to the problem, but I don't know how to interpret it.

I made it on an experimenter board, had to do my own layout, I have tripple checked my layout and checked all my component values etc, but must be missing something, or have a solder bridge or something which I just can't see.

If anyone has any idea where I should be looking (I thought C4 R9 and R10 was a good starting point) and could give me some pointers that would be great.

The circuit has 4.1 V across the mic input with no mic attached, and it drops to about 3.7V with the mic attached so I'm figuring that the problem isn't in the voltage reference part of the circuit.

I also figure it is unlikely to be in the clipping indicator part of the circuit, (which seems to work properly).

Tony.

[SY]

My first guess would be a wiring error somewhere around the input circuit. I would also suspect you've got an oscillation. The two problems may be inter-related. Do you have a scope or access to one?

No matter how many times you trace the wiring, you can still overlook something. Have a friend double check your work with fresh eyes.

[wintermute]

Thanks SY,

I have a pc based scope which is only good for audio frequencies. I've used quite long wires (probably about 10cm) going to the gain switch, so I guess that could be causing oscillations, I'll try disconnecting them (It should run at full gain without the switch).

I know what you mean about no matter how many times you trace the wiring, bit like programming, can be staring you in the face and no matter how much time you spend looking you can't see it, someone else will look at it and see the problem in a few seconds.

Only problem is none of my friends are into eleotronics, so I don't really have someone to show it to I'd left it for about 6 months and came back for a fresh look, hoping I'd see it this time!!!

Tony.

[wintermute]

I took out the gain switch tonight and no change. I noticed that if I touched the mic capsule with one hand and the output jack with the other the gain increased even more than if I just touched the mic capsule.

thinking this was rather odd, I got out the trusty DMM and checked the resistance between the ground on the input jack and the ground on the output jack, should have been 0 ohms but was 147K!!!!!

turned out it was one of the jacks. They both have chasis insulation washers on them but the ground lugs were between the insulation (I haven't mounted in its box since it wasn't working), It was the input jack with the high resistance so you were right SY . put the lugs in the right spot and low and behold the preamp works perfectly.

I suspected it was going to be something simple, but I kept concentrating on looking for an error in my wiring or a solder bridge, when it was not even on the circuit board doh....

In the box with it now, and onto some speaker testing

Tony.

[SY]

Great work- troubleshooting is always a matter of keeping your eyes open and following logical hunches. Now, off to do something useful, eh?

[wintermute]

Well it taught me a valuable lesson look at the whole circuit!!!!!!!!

as far as something useful.... Hopefully! I got to the point with my speaker project where I need to measure the FR of my drivers.

I now need to find the box that the preamp is supposed to go in, and also need to get some brass tube for my mic wand. Shouldn't be too far off

Tony.

[wintermute]

This was the culprit. Notice the lug is between the insulating washers rather than between the back one and the nut. doh.

Tony.

[wintermute]

This is the preamp mounted in the box ready to get the lid screwed on.

[wintermute]

and here is the final fully assembled and working preamp

Tony.

[Sherman]

Originally posted by wintermute
...turned out it was one of the jacks. They both have chasis insulation washers on them but the ground lugs were between the insulation ...

I suspected it was going to be something simple, but I kept concentrating on looking for an error in my wiring or...

Tony.

Good job with trouble-shooting! Those "simple" things are very often the hardest to track down!

It always amazes me that even though the things we build are relatively complex when we have problems they are
usually something simple (and therefore overlooked).

I spent hours trouble shooting a new amp with low output and a clicking noise sent to the speakers. When I finally found the problem (after re-soldering several connections and replacing a switch) it was a speaker binding post that was grounded to the chassis! (It was supposed to be insulated but the plastic sleeve had apparently slipped off before I installed it and I never noticed.) Replaced the sleeve and all was perfect! OK, as perfect as something I make is going to be.