Faulty Mic Or Preamp?

[Bob Bickerton]

I'm having trouble replicating a fault on had on a recent recording.

The rogue channel would randomly attenuate by about 6dB and on return to standard level was accompanied by a click.

I suspect the mic, but I would appreciate any views too. Unfortunately, I didn't pack out after the recording so, I don't know which mic was faulty, but I'm trying to trace it.

Here's a short recording of the fault (which reverts to full level after a few seconds): Fault

The mic was a KM183 into DAV BG2 preamp.

Thanks in advance.

Bob

[ef37a]

Bob, if I had to guess at a fault between a mic and a box of electronics I would always go for the latter.

There seems to be no distortion at either gain level so I suspect somethng in the DAV is just causing the shift, dry joint on the gain selector mayhap?

I would try setting up a mic, not the brand used, and then giving the pre amp some technical acelleration tests, i.e. thump it.

Dave.

[Hugh Robjohns]

A change in level of 6dB sounds suspiciously like one side of a symmetrical balanced line dropping out and back again -- maybe a duff cable that was going one-legged occasionally or a dodgy output driver/connector in the preamp output.

Not likely to be the KM183... but worth some careful checking all the same.

I'd do some cable checking with the 'wiggle test' and then take a close look at the preamp.

hugh

[Bob Bickerton]

Thanks Hugh and Dave.

Cables have just checked out OK. I'll take a closer look at the preamp tomorrow.

Bob

[Bob Bickerton]

I'd just like to bring this up, by way of apology to Mick Hinton at DAV electronics.

The fault mentioned above was intermittent and I could never replicate it on the bench, but it had a nasty habit of turning up in the middle of critical recordings.

Mick kindly sent spare parts and provided excellent service, but I still couldn't fully fix the problem.

However, I've finally isolated the issue today - faulty line inputs on my Focusrite 428! No wonder I couldn't replicate the problem with the mics, cables or DAV!

So I'd just like to say nothing wrong with the DAV and Mick's support was impeccable - if I wasn't so far away, I'd buy him a beer.

Bob

[ef37a]

Bob, does the 428 use the poor practice of only securing the line jacks to the PCB and not also bolting them to the plate?

I recently heard of this (here?) giving trouble with the line outs on a MOTU.Even my crappy little Wharfedale 35quid mixer bolts the jacks to the top plate!

Dave.

[Hugh Robjohns]

Bob, does the 428 use the poor practice of only securing the line jacks to the PCB and not also bolting them to the plate?

Nope. It's a properly engineered high-end product. The line input sockets have the usual hexagonal plastic nuts securing them to the rear panel.

hugh

[ef37a]

Bob, does the 428 use the poor practice of only securing the line jacks to the PCB and not also bolting them to the plate?

Nope. It's a properly engineered high-end product. The line input sockets have the usual hexagonal plastic nuts securing them to the rear panel.

hugh

Glad to hear it. So, which "end" does that put MOTU?

Dave.

[Hugh Robjohns]

So, which "end" does that put MOTU?

I think my opinion is clear from what I've already said...

hugh

[Folderol]

Being so intermittent, and affecting (probably) just one side of the input, my money is on poor soldering or a hairline fracture.

I'd just like to make the point that even with a well secured plug body, it's still possible for the board soldering to be stressed by small movements of the socket pins in the shell itself - especially if a slightly tight plug is being used. Also, even the highest of high end products can suffer the occasional duff solder joint.

Why, it's even happened to me!

[Bob Bickerton]

Obviously I've emailed Focusrite support and I'm sure they'll help out.

Apparently there have been issues with the 5532 op amps running too hot and it's happening on more than one channel.

Anyway, I just wanted to clear up any misunderstanding regarding DAV.

Bob

[ef37a]

"Apparently there have been issues with the 5532 op amps running too hot"

Ah! I wonder what the supply rails are? According to Douggy Self there is very little headroom advantage in running chips over +&-17volts and a big reliability advantage in doing so.

Dave.

[Folderol]

Now he tells us it's more than one channel

Personally, I never run OpAmps above +-15V - bit of a strange fault for the chips themselves though.

[dmills]

IIRC There is a known oddity with those parts if the supply rails don't come up together, they can go into a kind of latchup where they draw a lot of supply current.

I seem to remember mention of it in "Small Signal Audio Design".

Regards, Dan.

[Bob Bickerton]

Now he tells us it's more than one channel

That's because the second channel only started to show the fault Now!

Bob

[Hugh Robjohns]

Anyway, I just wanted to clear up any misunderstanding regarding DAV.

Before another misunderstanding risks damaging Focusrites reputation too, can I suggest everyone withholds any further speculation as to what might be the cause of the dodgy line inputs until some hard information is available.

The fact is that these forums rank very highly in google, so if someone searches for focusrite 428 there's a good chance they'll find this thread, and before you know it they'll start taking Daves mutterings as fact, and a new round of Internet misinformation starts...

Hugh

[ef37a]

Anyway, I just wanted to clear up any misunderstanding regarding DAV.

Before another misunderstanding risks damaging Focusrites reputation too, can I suggest everyone withholds any further speculation as to what might be the cause of the dodgy line inputs until some hard information is available.

The fact is that these forums rank very highly in google, so if someone searches for focusrite 428 there's a good chance they'll find this thread, and before you know it they'll start taking Daves mutterings as fact, and a new round of Internet misinformation starts...

Hugh

THAT, is the internet for you. Focusrite have the same opportunity as everyone else to come here and give the facts. And why single my name out? I merely stated the opinion of a respected engineer in the field. The original intention was to remove "blame" from DAV, all other consequences flowed from there.

Dave.

[Bob Bickerton]

Can I be very clear - the reason I even posted on this subject was because I wanted, and received, advice on what the potential fault was, as I had no idea myself and thought that our learned wise contributors may give me a lead. I am not into complaining about products in forums and indeed I'm not complaining about the 428!

However, once I finally isolated the fault, I felt it was important to make it clear that the fault did not lie with the DAV, as this could be seen to be implied from the earlier discussion.

The 428 has been the cornerstone of my studio and has been a key factor in helping me produce some very successful projects. It will continue to be a an integral part of my setup. If it is indeed faulty (and I will not know that until it has been assessed), I will have it repaired. I would replace it at the drop of a hat if I had to, and would certainly recommend it to any prospective buyers.

Bob

[Hugh Robjohns]

No problem, Bob (or Dave) -- just wanted to remind folks of the way unfounded speculations can potentially cause problems, not least to me having to placate grumpy manufacturers who might feel misinformation is being put about, damaging their reputation.



Hugh

[ef37a]

oooo! I bet you have upset a few thyself over the years Hugh.

Best bit of the old mags was the technical ding-dongs!

Dave.