Toft ATB16, approx. 40dB high-end noise, and phantom power

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jziggy
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Toft ATB16, approx. 40dB high-end noise, and phantom power

Post by jziggy » Wed May 13, 2009 2:06 pm

Hey guys. While I wait for Tech Support at PMI to call me back (fantastic customer service by the way), thought I might bring this issue to the tapeop community and see what possible answers might be found.

Channels 1&2 on my Toft ATB16 has an unacceptable amount of self-noise (around 40dB of high-end hiss on each). We have swapped Input ICs, the whole PCB, then resorted to shipping it off to PMI for them to look at. Got it back and the noise is still there though I did dig up some more clues while testing the channels today.

Channel 1, with nothing plugged in, fader at full ride, and the input pot all the way down (mic input or line input), exhibits approx. 32 dB of high-end noise. Channel 2, with the same conditions as before exhibits approx. 43 dB of high-end noise on the mic line & 32 dB on the line in. Here's where the phntom power comes in. If phntom power is put on chnnel 1, excercising the line input button brings loud pops with every press, the pops getting less loud with every press. Phntom on chnnel 2 removes noise for few seconds then it returns.

suggestions, impressions, everything is welcome. Thnks.

nclayton
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Post by nclayton » Thu May 14, 2009 1:59 pm

Hi. I assume you mean -32 and -43 dB noise? Your Toft doesn't have high enough voltage rails to put out 43 or even 32dBv of noise (or anything else) since you're talking about 40 or so volts peak for the lower of those two.

I'm not picking on you here....I think it's important to figure out how much noise you're really talking about. If the channels have, say, 70dB of gain then that would make input equivalent noise of -102dB and -113dB -- not really surprisingly high. The second of those levels would be pretty close to what you'd expect the 6.8K phantom power resistors to make just as thermal noise. The input noise probably goes down if you plug in like a 200 ohm mic and shunt the phantom resistors' noise out.

Rather than leaving the mic inputs open, try jumping a little piece of wire between pins 2 and 3. That will give you an input noise level that will be a little closer to what you'll hear with a mic plugged in (but without a bunch of room noise that a mic would pick up).

But if the noise is really somewhere *between* -43 and 43 dB then there might be a problem. Just got to figure out exactly how much noise you're really talking about.

Ned

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jziggy
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Post by jziggy » Tue May 19, 2009 9:47 am

Yea. My language and understanding of this is confused. Sorry for the loose description.

So I ran from the direct out of the toft in channels 1 and 2 to Pro Tools. Recorded the incoming noise. Used the PT Gain plug to measure the rms and peak levels of the noise with a minimum value of -96 dB on the plug. Channel one measured -64 dB rms, channel two measured -53 dB rms.

After speaking with the tech here is what we tried. The toft has the submaster returns normalled to their respective channels line input (i.e. submaster 1 return is normalled to the line input of channel 1 and so on through 8 ). I disconnected the returns from the submasters on channels 1 and 2. The noise dropped but not enough.

Channel 1 now measures -69.3 dB rms, Channel 2 measures -68.8 dB rms. We now think that the ribbon cable is bad. Got one arriving today along with some other parts as backup plans. I'll post with the results.

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