B&K 2230 (SPL meter) calibration.

[Tubetec]

I saw you mentioned deriving balanced output with an op amp which could be usefull for long cable runs , but then again if your mounting the ADC very close by it might not matter as much , of course that limits you to 5 meters via usb instead of a hundred over two core screened audio line if the need arrises . I was thinking about unbalanced to balanced input transformer to Ivans Cosmos ADC from the filter in ' of the B&K , whats the golden number for the transformer ratio and core size though ? Cosmos ADC presents between 300 and around 3k ohms depending on gain setting/mono/stereo opperation , B&K pre out wants to see at least 600 ohms with 316mV for 0db deflection of the meter , how do we position core size/saturation relative to 0dbfs digital , how could series parallel switching of the windings be used to best advantage to match the sensitivity and impedence of the Cosmos? maybe a question for one of the higher mathmatical powers .

I had a chance to open up the new unit , a handfull of Mullard OC44 on the power and filter boards , I didnt bother disturbing to further invesitigate the head amp or main board , but I think its solid state in any case , seems like its inductor based eq . powers up and registers signal with capsule attached ,although I only did the basic 'ref' calibration both units are in the same ball park as far as SPL readings are concerned , the one capsule I have is noisey so no point in listening yet unfortunately .

 

[NOON]

Take the output from after the first gain stage where it sends to the optional filter unit. You don't need the extra gain and it bumps the noise floor up significantly to attenuate then boost again. You'll hear what I mean when you get a working capsule. It also leaves you with a bonus gain stage that might be able to be used as a DI or line in or another mic pre, handy if you're fitting a stereo A/D into the back.

 

[Tubetec]

I had a chance to run through some of the calibrations , I have the service document for both models and line up levels are slightly different which makes it a bit like comparing apples and oranges , the later one lined up perfectly with both internal ref , externally applied known signals and level checks with the multimeter at various points in the signal path all agreed within mV's to the spec'd values , only a tweek of the external ADJ pot was needed .

Second unit showed a small deviation in level a db or two out up or down depending on ref or test tone source , I did the meter sensitivity cal and things line up better ,but I had the meter on its side to get at the preset in the back so I think the weight of the needle itself scewed the results a bit , I'll try that again later . I did the checks on the filter roll off settings and again the older serial of the two was within spec but a little sloppy compared to the later op amp based unit . Nothing that cant be corrected though. . I tried a basic noise measurement with the multimeter and it seems a bit noisey, I'll need to have a proper look at the noise floor in REW later .

Ive been using a 5.5mm(2.1mm center terminal) DC jack for the audio connection , the external diameter is wrong so it flops about the place and doesnt make a good contact , 6mm OD might fit but 3mm bore may miss the centre terminal , screened metal DC jacks are hard to find also, the poor conectivity makes any kind of testing a pain . I'll have to shell out for more few accessories ,although the JJ2614 adapter comes in very useful for testing and cal .