Proper wiring to avoid ground loops
- Thread starter mrtnasty
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drtechno
Well-known member
That's Correct. That is why I said the resistor is optional and can be omitted when used just for this mic, I land it in installs because its not always a guarantee that all the time the studio is going to use the input for a passive mic. Broadcasts , station its not much of an issue because the other device they would connect, a telephone hybrid, would have self terminating low z connection.
Btw this is only unbalanced to the gain circuit and not the transmission line (mic cord) that would fallow balanced operation.
drtechno
Well-known member
1K reflected, 200 ohm output. 1K reflective rating is for a capacitor coupled preamplifier. 200 ohm output is the transformer's output impedance that is matched the same on a transformer input.
moamps
Well-known member
Neumann clearly said that the output impedance of the microphone is 200 ohms and that it should be optimally loaded with 1k. If we load it with 200 ohms, we will get a 6dB smaller signal than when it is not loaded, and we will probably not get a guaranteed frequency characteristic and we will get somewhat larger non-linear distortions.
The load resistance can be an actual resistor (1k) at the input of the mic preamplifier or a reflected load from the input microphone transformer from the secondary to the primary, or combination of two.
BTW, I've seen in a few cases that users say the best load for the U87 is 600ohm, but I didn't notice that on my vintage example.
moamps
Well-known member
The microphone user manual clearly suggests that if a balanced input is used, which is correct in your example, that pin 7 should be connected to ground. Normally this is not necessary, but I have noticed that with some older transformers that have that CT lead at the input, connecting to ground gives better results. So, if that's better for you, leave pin 7 connected to ground. You can also test the performance of the system by changing the load of the microphone by connecting a resistor across pins 1 and 6 (trimmer of 2k5 or similar) of the mic transformer.
drtechno
Well-known member
But in all they were created for the 200-250 transformer convention and its terminated as such going into a transformer without any resistor (because its in the circuit). U87 and a U47 could be modified for 600 ohms output, that the only thing involved would be moving where the output terminals out of their attenuator circuit they have in their power supply. That away you are across the whole secondary at 600 ohms pre terminated at 600 ohms.
drtechno
Well-known member
Another question is why all these off topic things in a ground loop thread?
U87 and U47 can be modified for 50 or 200 ohms operation, not 600. This mod is done in the mic, not in the PSU.
OT is welcome here, as long as it makes sense.
drtechno
Well-known member
Then I'm getting this old garbage mixed up
This garbage mic the telefunken ELM is another one of these garbage mics They are all old garbage.
There are $35 mics that win in blind test over them now.
That is why they are overpriced garbage.
Strange wording in these period documents.
I came to know that an "unloaded input transformer" means one which secondary is directly connected to the grid of the input tube, without grid resistor (or one of very high value), so the reflected impedance at the primary is mainly governed by the primary inductnace at LF and by the reflected Miller capacitance at HF. With a pentode as input tube, Miller capacitance is quite small (only a few pF), which reflects as only about 1nF, resulting in a HF -3dB point above 50kHz.
All this results in an input impedance that doesn't go below 2 kohm in the audio range.
Clearly, the Ancients knew the benefits of low-loading mics, low load being high impedance.
Because the starting title of the thread included the phrase "ground loop," but it really is about low noise connection to a mic preamp. The term "ground loop" as commonly used is poorly defined, but I would be hard pressed to concede that a passive microphone on the end of a cable could form a literal "loop" with anything. The original description did describe microphone, pre-amp, and compressor, so started with the expectation it could have been a current loop through the two line powered devices, but seems to have slowly shifted during the course of the thread to concentrating on the microphone interface, which is presumably more about pickup of radiated noise and not line power supply related interference currents between chassis.
mrtnasty
Well-known member
Thanks everyone for the all the suggestions, thanks for not letting it go and making sure it all made sense and making me think, sorry if I didn't properly describe the problem initially, I just didn't know what was wrong and ground loop was my best guess, just happy it's not buzzing anymore!
drtechno
Well-known member
there is only some answers that work for case purposes which is separate from symptoms, because of the type of noise it could be.
I can tell you the cases of when a dynamic mic can ground loop. One type can be made from connecting more mic cable to extend the run, and the metal connectors that link the two cables together touch a grounded surface, Which could be a floor, steel beam, pipe, even wood. This is why you will see mic cables extensions wrapped with gaff tape in live settings and as an old stage tech I've been there and done that. Certain boom arms can do it in a booth when they are affixed on conductive surface that outlet strips are attached and a mic like an SM7b is used and its ground from the body ground loops through the metal boom arm to the grounded metal surface the metal outlet strip is mounted. Which that mic patch, lifting pin 1 and shield fixes it. Those are the instances in the past 40 years I know and the only one I see come up often I tell studio techs is the XLR links because they never learned that unless they worked under a live sound tech. The boom arm press box instance with the SM7b, I've only seen it twice ever.
The only other one with the mic end would be the case where a microphone snake or sub snake box don't have its rubber feet and it grounds out to the floor. For multiple channels of ground loop. I've seen that a few times in recording studio environments they hired me to fix it. I remembered a corporate record label people pay me once to train their studio techs about ground loops and interfacing equipment.
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drtechno
Well-known member
You have to put it in context that is all. Unloaded input transformer is un-terminated that is usually terminated because its a power load to a another device. The 600 ohm rack gear suppose to have this load resistor across the primary. Say that we want to look at the trs balanced out of an Ampex tape deck. Internally there is a 600 ohm resistor across the output, but connecting a trs cable to a capacitor coupled interface lifts this resistor because the gear expects the destination input have this 600 ohm resistor, The result is the Vu meter on the tape deck not showing the correct signal level, and noise until a resistor is put in across to load it.
Direct transformer connected to a grid I call an unprotected input. As the grid stop is used to prevent excessive current flow that can cause the grid winding to rise in magnetic field that causes sensitivity to RFI to rise. That is why I find most of these mic circuits wrong in the sense of someone who build tube gear from formulas. I find it interesting that the knowledge of grid resistors is out there and no one seems to be utilizing that but ones that went through school. I posted that on a diyaudio forum just before they banned me for "Conduct unsuited to forum culture" because their hive mind couldn't take that someone from out of the blue knew so much more than they did. Including the chinese builders there making boutique phono preamps, but don't know how to design it and wanted to fix their crappy design. They wanted the awnser, that I didn't give them, because they need to learn the theory behind what they were doing. When they get me to that level of designing the circuit for them, then I turn it into a lesson which they didn't appreciate it.
The effects of loading on an output transformer and on a mic are somewhat different.
Particularly in the case of said Ampex machines, loading the xfmr had two roles, damping the HF response rise and ensuring the VU-meter accuracy, the latter being the most significant effect. Low impedance transformers do not have much HF resonance in the audio range.
Microphones react differently to loading.
Most active mics react to loading with just a little decrease in output level, sometimes with slightly increased distortion and reduction in max SPL, but no change in frequency response.
Dynamic and ribbon mics react to loading with a decrease in level AND changes in frequency response.
There is a change at the intrinsic resonance frequency of the diaphragm, where loading damps the resonance hump, and a change at HF, where the inductance creates a low-pass filter.
The global output decreases when such a mic meets a low impedance, by the mere effect of voltage divider.
Most vintage mic preamps took care of these issues by unloading the secondary, making the input impedance inductive, which resulted in quite high impedance at mid and high frequencies.
Most of solid-state preamps designed in the 70's and on concentrated on bench performance with dummy loads, which resulted in fairly constant but low-ish input impedance.
Cf Neve preamps with 600/2400 switchable input impedance.
It posed no problems with condenser mics, and dynamic mics were often considered inferior anyway. Ribbon mics were commonly accepted as "dark".
It's only quite recently (2000's) that the subject has been revisited, with in particular AEA that came with high input impedance that allowed them to demonstrate that ribbon mics were not as dark as commonly admitted.
Loading a mic with a resistor of value close to the nominal impedance brings no benefit in terms of EMI/RFI rejection, because whatever reduction in noise is accompanied with a similar reduction of global level, requiring increasing the gain.
With very few exceptions, mic manufacturers don't publish the effects of loading on their mics, but these effects are real, and significant enough to be heard even by untrained listeners, with a large majority voting for the highset impedance load..
Particularly in the case of said Ampex machines, loading the xfmr had two roles, damping the HF response rise and ensuring the VU-meter accuracy, the latter being the most significant effect. Low impedance transformers do not have much HF resonance in the audio range.
Microphones react differently to loading.
Most active mics react to loading with just a little decrease in output level, sometimes with slightly increased distortion and reduction in max SPL, but no change in frequency response.
Dynamic and ribbon mics react to loading with a decrease in level AND changes in frequency response.
There is a change at the intrinsic resonance frequency of the diaphragm, where loading damps the resonance hump, and a change at HF, where the inductance creates a low-pass filter.
The global output decreases when such a mic meets a low impedance, by the mere effect of voltage divider.
Most vintage mic preamps took care of these issues by unloading the secondary, making the input impedance inductive, which resulted in quite high impedance at mid and high frequencies.
Most of solid-state preamps designed in the 70's and on concentrated on bench performance with dummy loads, which resulted in fairly constant but low-ish input impedance.
Cf Neve preamps with 600/2400 switchable input impedance.
It posed no problems with condenser mics, and dynamic mics were often considered inferior anyway. Ribbon mics were commonly accepted as "dark".
It's only quite recently (2000's) that the subject has been revisited, with in particular AEA that came with high input impedance that allowed them to demonstrate that ribbon mics were not as dark as commonly admitted.
Loading a mic with a resistor of value close to the nominal impedance brings no benefit in terms of EMI/RFI rejection, because whatever reduction in noise is accompanied with a similar reduction of global level, requiring increasing the gain.
With very few exceptions, mic manufacturers don't publish the effects of loading on their mics, but these effects are real, and significant enough to be heard even by untrained listeners, with a large majority voting for the highset impedance load..
radardoug
Well-known member
Which Ampex?
drtechno
Well-known member
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Brian Roth
Well-known member
Ampex 440 and ATR-100 (maybe others?) have a "term" switch on the rear panel for each channel that drops a 620 Ohm resistor across the output transformer (and hence between pins 2 and 3 of the output XLR).
Bri
Bri
drtechno
Well-known member
correct, and you suppose to use them to load the output transformers and the vu meter correctly on those old tube units. But you also need it if you are going into something not transformer input (transformer inputs would have its own termination by default).
