Tascam 414 input noise, only on tape

[granto]

Hello,

If I record to tape with no inputs plugged in, the resulting recording is quiet. But as soon as I plug in the outputs of my PC's audio interface to the inputs on the 414, even with no signal, each respective track of the recording becomes noisy. However, I do not hear this noise in my monitor during recording (monitoring L-R). I can also plug in a mic and record just fine with no added noise.

Seems like a ground loop kind of thing, right? Everything is on the same power circuit. However, it's more of a white noise than a hum. I also don't understand from looking at the signal path why I wouldn't hear it in my monitor and then all the sudden it's on tape at pretty high levels.

Any suggestions to avoid this? What about trying to plug into a different circuit? Would trying that help diagnose things? I'd have to get a super long extension cord to even test this! If it matters, I don't use a power conditioner because I've never run into an issue.

Also, I have a cheapo replacement switched mode power supply for the 414-- would I benefit from using a linear power supply? I found an unregulated linear PS that meets the requirements. My simple understanding is that a linear PS may reject mains hum better than an SMPS.

 

[elnaldo]

Is the noise simply a high level hiss? Can you record some music at near 0dB and check the signal to noise ratio? Perhaps you are sending a high level hiss from the computer to the 414 . please check the levels recording some music.

If you have a battery powered computer, use that computer to check if the noise is generated at the computer PS.

 

[granto]

Yep it's a high hiss, that's a better word for it. I don't have a SNR for you but I can tell you that the hiss is quite noticeable when it's riding along with a signal. My guess is a SNR between 5 and 10. I also haven't spec'd it yet so I don't know what frequency it's at.

If you have a battery powered computer, use that computer to check if the noise is generated at the computer PS.

Yeah that's a good test. Even just decoupling my interface from the PC would tell me whether it's from the PC's PSU or not. I'll do some more testing and get back in a week or two. In the interim I did find a true linear power supply for the tascam so maybe that will just magically fix things. It has to do with the (short) cables going into the tascam being unbalanced, and noise coming from some power source, I'm almost sure of that. I do have one of those hum destroyer boxes that I haven't tested. If it works I might just use two of those rather than drive myself crazy trying to find the source.

 

[elnaldo]

Hum is not hiss. Ground loops create a low freq hum.

Try to locate the source of the noise before buying spares that you don't need.

I've fixed noisy installations just cleaning the connectors, so start narrowing the source of the hiss. Identify the device that's introducing the problem.

 

[granto]

Hum is not hiss. Ground loops create a low freq hum.

I totally appreciate this and it was part of my initial confusion! OK, I was just able to characterize this a little better:

First, this is not from my PC. This is coming from my audio interface, when nothing else on the circuit is turned on except the Tascam and the interface. The noise only happens when the interface is turned on. It also doesn't matter if I plug them into the same power strip.

Second, I do hear a mains hum in my monitors for sure. But it's not crazy loud. I also hear this if I send my interface's out to an external Amp. And if I turn the Tascam monitor all the way up, I also hear a hiss in the monitor, but again, not too loud. Nothing that doesn't match my memory of Tascam being generally noisy.

However, what I hear in the recording is not just a hiss but a loud whooshing sound (and not really the mains hum sound). It is definitely as present as any recorded signal is. So it's waaay louder than what I hear in the monitor and what's coming out of my interface. That comes back to my question-- it seems like it's taking a pretty benign hum+hiss and amplifying the living heck out of the hiss. And this is where I wonder about some problem in the signal/recording path. Because again, other signals like Mics and instruments are fine. Maybe that's just the frequency response of the unit and how it responds to input hum?

Again I will see if the linear power supply solves this, but I do wonder if I'll end up needing some hum eliminator for my audio interface. Again I have no power conditioning of anything, so maybe I've finally found the situation where that matters?

 

[elnaldo]

If your audio interface is generating the noise, take it out of the equation by now. Unplug and disconnect it, and record something directly out of the computer built in sound card. Or record something else without a computer, some output from some mixer or preamp.

Perhaps the problem is a poor circuit design in that interface.

Regarding the hum in the monitors, you could try lifting the ground in the monitors, using a 2 prong adaptor. It should cure or break any ground loop you have.

 

[granto]

If your audio interface is generating the noise, take it out of the equation by now. Unplug and disconnect it, and record something directly out of the computer built in sound card. Or record something else without a computer, some output from some mixer or preamp.

Yes it records other signals from other sources just fine! No issues with noise from a variety of sources.

I tested and this also happens when the interface and the tascam are each powered by different circuits in my house. One thing I wonder is that my outputs are TRS and I believe focusrite recommends using TS-TS when going to unhbalanced inputs but I wonder if a TRS-TS cable would help. There is a small distinction but I don't know if it would apply here.

Regarding the hum in the monitors, you could try lifting the ground in the monitors, using a 2 prong adaptor. It should cure or break any ground loop you have.

The hum I hear in the monitors of the Tascam is in the headphones when monitoring the L/R mix and also if I use the L/R out to go to an amp. I can create a similar hum using a very long TS cable and just picking up interference. When I record that, it simply records the hum onto tape, without the whooshing sound.

Perhaps the problem is a poor circuit design in that interface.

This is beginning to seem to be the case. I don't understand how I could not hear wooshing in the input monitor and then it's loud as hell on the tape. Like it's coming in some other way (?)

 

[granto]

One thing I wonder is that my outputs are TRS and I believe focusrite recommends using TS-TS when going to unhbalanced inputs but I wonder if a TRS-TS cable would help.

EDIT! Solved! Yes, the cable has to be TRS to TS or else a whole bunch of secret noise rides along. I don't quite understand this but anyway I found a TS to TRS adapter laying around and it completely solved the issue. Good grief.

Thank you elnaldo, Lunatic Member, for your help!

 

[elnaldo]

Hello. Check that you are not shorting the balanced output to ground. The proper way to connect a balanced output to an unbalanced input is to connect tip to tip, shield to shield, leaving the Ring disconnected (not connected to ground). In many devices you can short the Ring to the Shield, but in others you can't, it will damage the output amplifier. So it's a good practice to avoid shorting the output of balanced devices.

EDITED: I see you mentioned Focusrite telling to use TS cables, so it seems ok in that unit to short the ring to ground. But not all the devices allow that, so keep it in mind when connecting balanced to unbalanced

 

[granto]

Hello. Check that you are not shorting the balanced output to ground. The proper way to connect a balanced output to an unbalanced input is to connect tip to tip, shield to shield, leaving the Ring disconnected (not connected to ground). In many devices you can short the Ring to the Shield, but in others you can't, it will damage the output amplifier. So it's a good practice to avoid shorting the output of balanced devices.

I must say this has always been very confusing to me. My understanding is that when I plug a TS cable into my TRS out, the ring is left floating, so it just treats it as regular TS. And indeed, using a TS-TS for my outputs in many other scenarios has worked without issue. Assuming the adapter I linked does short the ring to ground, is there something particular about the Tascam's inputs that prefers this? I guess I'm just not clear on how you intuited this:

EDITED: I see you mentioned Focusrite telling to use TS cables, so it seems ok in that unit to short the ring to ground.

 

[elnaldo]

When you plug a TS (mono) plug into a TRS jack, you are connecting the Ring and Sleeve together with the longer sleeve of the TS plug. So, shorting the Ring and Sleeve. Some devices shouldn't be shorted that way at the output (it's usually OK to short the input). But it seems yours is fine and leaving the Ring floating caused problems.

 

[granto]

Hello! Well, my solution with the TRS adapters works to eliminate the whooshing noise, but I do get a pretty good drop in signal levels, so that my S/N is fairly high and I have to have the Mic gain all the way up on my Tascam for what should be line level signals. Is there any simpler way to do this that you could think of that doesn't involve TRS to TS conversion at the outputs?

Edit: my hum destroyer did the trick with TS cables only, maybe I'll just use a couple of these...