Noisy ART Tube MP pre-amp

[RJVB]

Following advice from here and elsewhere I bought an ART Tube MP pre-amp for a simple recording set-up using a large-diaph condenser (T.Bone SC600).

The pre-amp does do the job but as others before me I am struggling with noise. In my case there's the usual white noise hiss that seems to come from the input stage, and an even more annoying "prrrrrfffttt" of about 20Hz that comes from the output (tube) stage and becomes an obstacle when I open the output gain beyond 9 o'clock. (I thus can't get much benefit from the tube...)

Two samples, phantom power to the mic off:


Does anyone recognise this kind of parasite, and more importantly, did anyone find a simple solution? I have been considering a line filter (the tighter around 60Hz the better) but have yet to find one that doesn't require me to find an appropriate enclosure so suggestions there would be welcome (esp. if the use Euro-style sockets). One review mentioned solving the issue through an unspecified D.I. on the output, suggestions on that approach are welcome too.

Annoyingly this pre-amp is powered by a 9VAC powerbrick; finding another, better PSU is thus next to impossible (I found 1 that costs 10x the price of the pre-amp).

Thanks in advance!

[runamuck]

I'm guessing that most of that is noise from the mic.

ART micpres are on the low end but I'd be very surprised that they're that noisy unless there's something wrong with it.

[RJVB]

Can't be from the mic, it's not being powered here. Also, I hear no difference when I unplug the mic cable from the pre-amp.

[jim1960]

The first thing I'd do is check each cable by swapping them out one at a time. Cables do go bad so that's a possibility.

Next, I'd get a replacement for the 12AX7 tube because tubes can also be a source of noise.

If you know anyone with a working studio, you might bring your preamp there to see if the problem duplicates itself in a studio where there is no issue with the incoming electricity.

If you rule out the mic, the cables, the tube, and the electricity as the culprits, then the problem is the box or the power supply. At that point, the solution is probably more than what the unit is worth.

[RJVB]

I already ruled out the mic cable, and I think we can rule out the output cable. I don't see how a bad output cable could insert noise that increases (independently!) with increasing the input or output gain on the pre-amp. Cable noise from, say, bad shielding, should be constant and not proportional to the signal.

I have indeed begun playing with the idea of ordering a 2nd unit and send back my current one if the other is better. I'd need to check with Amazon if that's an option. Changing the tube on this is apparently not that easy and will probably void my warranty so not the most logical next step in this case.

[jim1960]

Originally Posted by RJVB

I already ruled out the mic cable, and I think we can rule out the output cable. I don't see how a bad output cable could insert noise that increases (independently!) with increasing the input or output gain on the pre-amp. Cable noise from, say, bad shielding, should be constant and not proportional to the signal.

I have indeed begun playing with the idea of ordering a 2nd unit and send back my current one if the other is better. I'd need to check with Amazon if that's an option. Changing the tube on this is apparently not that easy and will probably void my warranty so not the most logical next step in this case.

If it's a brand new unit, then ship it back for a replacement. Amazon is pretty good on those things and make getting replacements a fairly easy process if the unit came directly from them and not a third-party vendor. Even with a third-party vendor, however, you'll still be able to do it, it will just take a bit longer.

But I'd still make sure every cable is functioning correctly and not simply assume they are. Bad cables can create some funky problems whether you "don't see how" or not. It's an easy enough thing to check. And I'm speaking from experience on that issue. I've had more than one cable fail me back in the day. It's why I stopped buying the cheap ProCo cables from Sweetwater and started ordering Mogami/Neutrik cables from RedCo.

[keith.rogers]

I'd blame a poor power supply for the hum. I googled and see that it's AC/AC so likely just poorly made. I.e., you may not have a lot of luck seeing that change with a different unit.

But, the low frequency can be EQ'd out later. However, the overall noise seems high, and that I'd guess is worth the return hassle to see if it's typical. It would be more telling to hear what the s/n is in a mix by recording whatever it is you plan to put the mic on and posting that, too.

P.S. If you want to to measure the noise of a preamp's XLR input, you should plug a completely isolated dynamic mic in or a dummy XLR plug with a 150Ω resistor soldered across the appropriate pin connectors. https://youtu.be/2DXEBahqBz4

[MarkF_48]

What is the rest of the signal chain (interface, mixer, or other)? XLR, TRS, or TS cable to next device? Settings on the Tube MP (gain, output 'o'clock', +20db button enabled).
Any LED lighting nearby or other possible sources of electronically generated 'hash'? Power supplies/adapters for a laptop or other gear near that may have switch mode type power converters?
Do you have a dynamic mic you could try with the same settings as used with the condenser mic.

The power supply in the Tube MP appears to use DC rectification employing a voltage tripler and a pair of transistors for regulation (there no switch mode power supply or conversion that could generate noise such as heard)

[RJVB]

Originally Posted by MarkF_48

What is the rest of the signal chain (interface, mixer, or other)? XLR, TRS, or TS cable to next device? Settings on the Tube MP (gain, output 'o'clock', +20db button enabled).
Any LED lighting nearby or other possible sources of electronically generated 'hash'? Power supplies/adapters for a laptop or other gear near that may have switch mode type power converters?
Do you have a dynamic mic you could try with the same settings as used with the condenser mic.

The power supply in the Tube MP appears to use DC rectification employing a voltage tripler and a pair of transistors for regulation (there no switch mode power supply or conversion that could generate noise such as heard)

In the sample recordings there is no further signal chain: the ART outputs via an XLR->3.5TRS cable that goes into a 3.5TRRS splitter cable (phones + mic) that goes into my iPhone.

There's LED lighting in the house (including a big ceiling light in my study) but the noise is also there when it's off. My screens (Macbook Pro internal and a BenQ external) are usually on but both have backlighting on a much higher frequency (all our LED bulbs also run on a frequency >20Hz btw).

I get the same noise with or without mic plugged into the ART, so I don't really see what testing with another mic could tell me.

However, very weird: I tried the 6.35 jack output (through a 6.35->3.5 TRS adapter and then a male/male 3.5TRS extension cable...) and found the noise gone down to a negligible level (without the low freq. component). Thinking the XLR output cable was bad after all I switched back to it and found the noise equally low.

Annoyingly I don't have anything like a Radio Shack in close enough vicinity, just regular DIY shops where you're lucky if they have shrink-wrap. I'm not exactly electronically capable anyway (and certainly no soldering hero) ... but I'll keep hunting and repeating my tests trying to correlate my findings with things going on elsewhere in the house. Annoyingly it looks like my iPhone doesn't recognise the impedance of the ART's JACK output so I can't use that for my video lessons :-/

[MarkF_48]

Originally Posted by RJVB

In the sample recordings there is no further signal chain: the ART outputs via an XLR->3.5TRS cable that goes into a 3.5TRRS splitter cable (phones + mic) that goes into my iPhone.

There's LED lighting in the house (including a big ceiling light in my study) but the noise is also there when it's off. My screens (Macbook Pro internal and a BenQ external) are usually on but both have backlighting on a much higher frequency (all our LED bulbs also run on a frequency >20Hz btw).

I get the same noise with or without mic plugged into the ART, so I don't really see what testing with another mic could tell me.

However, very weird: I tried the 6.35 jack output (through a 6.35->3.5 TRS adapter and then a male/male 3.5TRS extension cable...) and found the noise gone down to a negligible level (without the low freq. component). Thinking the XLR output cable was bad after all I switched back to it and found the noise equally low.

Annoyingly I don't have anything like a Radio Shack in close enough vicinity, just regular DIY shops where you're lucky if they have shrink-wrap. I'm not exactly electronically capable anyway (and certainly no soldering hero) ... but I'll keep hunting and repeating my tests trying to correlate my findings with things going on elsewhere in the house. Annoyingly it looks like my iPhone doesn't recognise the impedance of the ART's JACK output so I can't use that for my video lessons :-/

The XLR output of the preamp is 'balanced'. The input of your phone is not going to be a 'balanced' input even though it may be trs or trrs. The cables/adapters might change balanced to unbalanced, but I'm thinking they don't (there are cables that will do this) and the hi/low signal of the balanced audio from the preamp gets mostly nulled out and the noise from somewhere becomes prevalent as gain is increased to compensate for a lower audio signal. You lose the benefit of noise rejection into the cables when a misconnection is made like this.

Leaving the preamp input open ended for a test may still allow for some noise. Connecting a dynamic mic would load it down to its normally 'sensitivity' much like the resistor connected to an XLR connector as was suggested.

[RJVB]

Originally Posted by MarkF_48

The XLR output of the preamp is 'balanced'. The input of your phone is not going to be a 'balanced' input even though it may be trs or trrs. The cables/adapters might change balanced to unbalanced, but I'm thinking they don't (there are cables that will do this) and the hi/low signal of the balanced audio from the preamp gets mostly nulled out and the noise from somewhere becomes prevalent as gain is increased to compensate for a lower audio signal.

Are you saying I should get an XLR->TRS cable that explicitly states it is unbalanced? I saw those, but not knowing what this referred to I thought it had to do with how the signal was distributed over the 3 TRS contact. But come to think of it an XLR also has 3 terminals, so I think we have to presume that the pre-amp puts out a mono signal over a stereo cable.

Leaving the preamp input open ended for a test may still allow for some noise. Connecting a dynamic mic would load it down to its normally 'sensitivity' much like the resistor connected to an XLR connector as was suggested.

I'm familiar with phantom signals on unconnected DAC pins left the mic and its cable attached even if I don't hear any difference. I'd expected that the impedance of an unpowered passive mic isn't significantly different from the same mic receiving phantom power, am I wrong? (I realise I may not have made it clear that the mic was in the circuit, just not powered so its signal is sub-threshold.)

[MarkF_48]

The audio signals are out of phase on the two wires that carry the audio in a balanced connection. If the signal in these two wires are summed (added) in a mono connection like what a TRS plug may do in the phone, the sum of the signal will be zero if they are perfectly equal. The phone connection may create a connection that sums out the audio and allows any noise on the cable to be amplified.
The other issue may be a level mis-match, as the preamp will be 'line level' and I assume the phone to some kind of mic level.

I have used a cable linked below to connect camcorder mic inputs. I don't know if it would work with a phone or not. The pin 3 of the XLR gets connected to the sleeve/ground of the TRS and creates an 'unbalanced' connection to the device it connects to. Not the best way to do it, but may work. There is a diagram in the link that shows how the XLR and TRS plug wiring is done
https://hosatech.com/products/analog...bles/xvm-100f/

https://www.amazon.com/Hosa-XVM-102F...ef_=ast_sto_dp

There maybe a better way to connect the mic preamp to the phone using an interface of some type. Others on this board could better direct you on what might be needed to do this properly, as I primarily use a computer with a USB interface.

Crude diagram of a balanced XLR to TRS and the phasing of a sine wave signal on each conductor
(Missed marking the 'T' in the diagram for the TRS plug)

[keith.rogers]

Clearly, I assumed too much...

As MarkF explains, the output (which is labeled on the back of the ART) is a *balanced* signal. Never mind it's line level, which might be a bit hot and impedance mismatched for a phone's headset mic (mono) input.

So, you need to get the balanced output down to mono, unbalanced, and actually just plugging in a 1/4" (6.35mm) instrument/guitar cable will do that. (Yes, it's not perfect, but it should work.) Then, you only need to have the right kind of splitter, and I'm guessing something like this ought to be close to what you have: https://www.amazon.com/KINGTOP-Heads.../dp/B08C7FLX2Y

Start with the OUTPUT of the ART all the way down, and just set the GAIN at 12:00. See if that works as you slowly raise the OUTPUT and can get a signal that doesn't clip.

Going into an audio interface with USB to iPhone adapter plug, or an interface that is iOS compatible would have been a more common way to do this.

[RJVB]

Interesting, in the schemas I've seen, showing the XLR head-on with the middle pin below, the upper left pin is connected to the L & R channels of the TRS plug, while the XLR middle and upper right pins are both connected to the TRS ground:



I'll check what mine does (it's a "Pro Snake" bought from Thomann).

[RJVB]

Originally Posted by keith.rogers

Start with the OUTPUT of the ART all the way down, and just set the GAIN at 12:00. See if that works as you slowly raise the OUTPUT and can get a signal that doesn't clip.

Output all the way down is labelled minus infinity and indeed there's no signal anymore then.

Going into an audio interface with USB to iPhone adapter plug, or an interface that is iOS compatible would have been a more common way to do this.

Yeah, and that would also have allow me to use that jack output because right now it fails to register - which could be because I'm using a 3.5 TRS cable. That works fine with the mic input on the Soundblaster interface though...
I already have one of those tiny µUSB-Lightning dongles, are you telling me that a simple USB-A(f)-µUSB adapter cable will allow me to connect any audio interface to an iDevice? Or indeed the standard lightning cable with a fem/fem USB-A cable?