Gain before feedback has absolutely nothing to do with S/N, if anything a microphone's pick up pattern could make a difference but that's mechanical not electrical.
JR
Microphone boosters?
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JR
Don't you know? It also cures ingrown nails and prevents hair loss!
ruffrecords
Well-known member
Not quite. Ribbons are effectively inductive sources which means their impedance rises with frequency. Over damping it will result in a loss of high frequencies, not an overall loss of gain This can be corrected with EQ. The question then becomes is this noisier or quieter than a cloudlifter?
Not quite. The benefit for certain ribbons is a high input impedance. Provided the cloudlifter is low enough noise AND contributes enough gain there will be no detriment in signal to noise ratio. However, most cloudlifter type products appear to provide about 20dB of gain. It is extremely difficult to make an amplifier with only 20dB of gain that has both a high input impedance and is quieter than a typical miic pre.
Cheers
Ian
EDIT: Well, I’ve been taken to school. Thanks for helping me learn!
Mic preamp boosters are designed exactly for when a mic needs more boost than the preamp can provide, and it’s not limited to just ribbon mics, and it has absolutely nothing to do with the preamp being junk. And yes, it’s only helpful if the booster has a lower noise floor than the amp it’s assisting. It’s almost as if you understand the use-case here… haha I’m kidding…
Anyways, tons of dynamic mics need or benefit from this extra boost, and this isn’t a new issue. Hell, the SM57 comes with a transformer built in, and the SM7B which is prime for cloudlifters and the like, is essentially an SM57 minus the transformer. The podmic from Rode is another one that doesn’t put out a very hot signal and needs a relatively large amount of gain. There are a lot out there.
I think the main thing you’re discounting here is the consumer preamp’s noise floor, which is typically impressive compared to what was out only a few years ago sure, but at what gain setting? As you approach the max of the preamp, you’re introducing a whole lot more noise, and it’s not equivalent to the amount of gain added. The purpose of a cloudlifter and similar is to provide boost with a decent noise floor to begin with, and allow that other preamp to come back down into a zone where it’s not adding a ton of extra noise.
This has been tested pretty scientifically in tons of videos and articles, and shown to help. There’s no question for me, and honestly I’m surprised that there’s any for anyone else. Anecdotally, I can tell you from first-hand experience that they do too, on my Clarett 2pre on my secondary workstation. On my API preamps that can (just) provide enough gain for the SM7B, there’s no need or benefit though.
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Which is an extremely rare case. In most cases the actual EIN of mic boosters is not documented, but their noise figure is always at least 4 to 6 dB. So there would be a benefit in terms of Brownian noise only if the mic pre has a higher noise figure; that would be an EIN of -125.7 to 123.7dBu*, which most modern preamp better significantly.
* Being european, my reference impedance is 200r
Some tube mic pre or poorly designed SS mic pre would benefit from the addition of one of the best mic boosters.
Brownian noise is not the only issue in a system, though. Interference noise in cables and connections is a problem that could be helped by the use of a mic booster, by elevating the signal high enough above the level of said interferences.
Another case is when the combination of mic impedance and cable capacitance results in significant HF attenuation; in that case the mic boosters that present a high output impedance won't help a bit, but the few mic boosters that have low output Z (emitter followers of opamps) will solve the issue.
I completely disagree with this statement. The vast majority of mic preamps use NFB to control gain, in a way that decreases the EIN as gain increases. One example among many others is attached
I'd be curious to see that.
Attachments
EDIT: I’m thinking I’m just wrong here, and I’ve learned something. Need to do some more testing disprove what I’ve been assuming for quite some time now, and that was that as you approach the max of a modern preamp, you impart more noise relative to signal. It seems the opposite might be true Thanks for helping me dispell this.
/Edit
My focusrite’s preamps DO impart more noise when maxed out. The inline preamp helps in this case. Again, with my API preamps, this is not the case. Focusrite Clarrett preamps are hardly junk or “extremely rare,” and more cheaply made pres I’m sure benefit more I’m sure. Aside from all this, total gain level is something to consider. My preamp on the clarrett simply doesn’t provide enough gain to be appropriate for relatively quiet speaking on my podmic for example.
Help me understand why a fethed is apparently useless, but Shure decided to build a transformer into their mic, other dynamic mics have similar built in, and the Cathedral Bells Durham is a circuit that is built into their ribbon mic. Surely if these didn’t have a use, manufacturers wouldn’t waste their time with them? Surely I’m missing something though.
You honestly have me doubting myself here though. Maybe I need to do some more thorough testing. Thanks for the discussion.
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Bo Deadly
Well-known member
First of all, welcome to the list.
Noise performance of both a traditional mic pre like an API circuit and the newer CFIA circuits found in pro-sumer mixers is better at higher gain. This is because, as explained in the Designing Mic Preamps presentation from THAT Corporation and as summarized in the plot of EIN vs gain:
, all else being equal, EIN of the pre will depend on the thermal noise of Rg and because Rg in these amps gets smaller as the gain goes up (Rg is the gain control pot in a an API style pre), thermal noise contribution decreases and therefore so does noise.
This plot also shows that the performance of CFIA at low gain is very good compared to the others. So noise is not better at lower gain. I don't know what the EIN of the cloud lifter circuit is but if you added that data point to the plot, it would be well above the others. It wouldn't even be close.
So regardless of the pre topology, you will get superior noise performance from one single high gain stage. Adding the cloud lifter can only decrease noise performance.
If you are experiencing otherwise, then there could very well be an issue with the environment around your pres or the pres themselves and the cloud lifter is helping because it's floating in a balanced line so it is not contributing common mode noise. Laying out gear and cables and minimizing EMI requires a lot more knowledge and care than most people think. All sorts of problems can occur that generate noise and hum. My MOTU Traveler mk3 makes a whistling noise at ~2kHz when phantom power is on presumably because of the 48V SMPS.
Thanks! This has been great already, one of my long-held assumptions has been, respectfully, obliterated by people who know better
I’m thinking you’re absolutely correct. This is one of those moments when I remember that I’ve got a lot to learn.
When you say that the cloud lifter’s noise performance wouldn’t even be close, could you briefly describe why that would be so I can get pointed in the right direction and start reading up?
I knew this video. It shows that the Fethead improves theperformance the H5. Actually, the H5 is one of the "extremely rare" cases I mentioned earlier.
Of course, all preamps increase the output noise when gain is cranked up, but so goes the signal also, and the ratio is, worst case constant, but in most cases, improves.
For me, it shows that your Claret is either poorly designed or duff.
Many dynamic mics don't have a transformer, but also many have one.
It's a design choice.
Those without transformer have a moving coil constituted of extremely fine magnet wire, very fragile, and difficult to manufacture. the moving coil is heavier, which results in poorer HF response. The benefit is less cost because no xfmr.
Those with a xfmr have a moving coil using thicker wire, easier to manufacture and less fragile, but as a consequence the level is low because less turns, so a xfmr is needed to increase the impedance and level.
Both are compromises, and have their pros and cons
One of the best dynamic microphone ever, the AKG D224, used extremely thin wire that resulted in a very light coil, resulting in extended HF response.
Unfortunately it was so fragile only a very few have survived.
Cathedral Pipes are not the only ones having built-in boosters in their ribbon mics.
The main reason is that having the electronics in the head allows using a xfmr with a higher ratio, so the booster, instead of seeing the nominal 150/200 ohms source impedance, sees about 2-5 kiloohms, which results in 10-12 dB more signal and a more favorable source impedance (you must read about OSI - Optimum Source Impedance).
You’ve convinced me. I think I may actually just be plain mistaken about the Clarett, and I’m going to do some more testing based on what you and others have outlined here. I edited my comment as you were replying.
Thanks again for your reply and sharing this info! I’ve got a lot to read up on. Now if I hadn’t just received two mic boosters in the mail haha
…and more careful testing has proven out what you described.
That being said, do you have any suggestions for the scenario where you need more low-noise gain than a preamp can provide? If the cloudlifter and the like isn’t it, are there any options you can think of?
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"Add noise in front of an existing noise floor = more noise."
No one was talking about adding noise to an existing noise floor. I was describing preventing introducing the noise to begin with. Try and make an effort to read and understand what I was saying. Though I was incorrect regarding my understanding of how using max gain effects noise floor, the concept isn't near as dumb as you're making it out to be.
Yes, I understand now, after a few people added some useful details and context, that Cloudlifters and imitations simply don't work the way I had thought they did. They did this with respect, thankfully.
I am not writing any copy, nor am I a liar. And I have no shame in having been misguided, as that is simply the world works; if you're afraid to make a mistake, you'll never learn, and the more you learn, the more there is to learn.
"It is extremely disingenuous to suggest there’s some other magic ‘clean gain’ that no maker of preamps ever sought to incorporate; it’s insulting to all makers of preamps."
No, it simply is not. No one said that no maker of preamps ever sought to incorporate anything, in fact they do incorporate similar components, as do mic manufacturers, so I'm not sure what you're on about. No one would argue that all pre-amps are made equally. Yes, we are fortunate enough today to have typically good and typically low-noise pre-amps. But they aren't all low-noise, nor do they all provide enough gain for a given use. This has nothing to do with insulting "all makers of preamps" unless of course for some strange reason you think needing more gain than a given manufacturer provides is an insult. This has everything to do with trying to find a solution to a problem.
Thank you and good day sir.
ruffrecords
Well-known member
You need to think very carefully about whether you want to do this. The laws of physics may not allow it. The thermal noise in a 150 ohm resistor is about -131dBu (this or 200 ohms is the reference by which all preamps are measured because it is typ9cal of the source resistance seen in practical mics). A completely noiseless amplifier with 60dB of gain will raise this to -71dBu. If your signal is 0dBu then you have enough gain and your signal to noise ratio will be 71dB. However, if you need 20dB more gain than this then even a noiseless preamp will raise the noise from the 150 ohm source to -51dB. If your signal is now the 0dBu you desire your signal to noise is now only 51dB.
The only ways to improve the signal to noise ratio are: 1. Get the mic closer to the source, 2. Use a mic with a higher output, 3. Use a mic with a lower source resistance. 4. Cryogenically cool the entire studio.
Cheers
Ian
I think there's some confusion about what the "this" is that I want to do, which is probably my fault. Let's scrap noise floor, and I'll re-phrase the question if that's ok: If I have a good quality pre-amp that provides 55db of gain, but I need 65db of gain in order to achieve the signal level that I need, what would you suggest?
I suspect we have some apples to oranges comparisons going on confusing things but I will answer your simple question.
If you have 55dB of gain but want another +10dB you will need to put the two gain stages in series. The question is which one to put in front?
In general you want your best s/n lowest noise stage first.... The latter gain stage will amplify signal and noise of the early stage. While almost TMI mic preamps generally deliver their best s/n at max gain, so a 10 dB gain stage may have worse s/n performance than a 55 dB gain stage. If you place the +10dB gain stage first it's output noise gets amplified 55dB, conversely if you put the 55db stage first it's output noise is only amplified 10dB.
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For my apples/oranges problem... low Z resistive source impedance (150-200 ohm) mics and preamps will behave predictably. Mics with inductive source impedance (rising impedance with increasing frequency) can interact significantly with a preamp's input impedance. Low Z mic inputs target 2k ohm input termination to be bridging to 200 ohm... alternately an inductive mic with rising source impedance might benefit from higher input impedance (like JFET buffer) stage.
I hope I didn't confuse things. Several posters here have decades of experience designing low noise mic preamps. For the record I have never messed with ribbon mic preamps, but suspect that might be the curve ball here... Loading a ribbon with higher Z than 2k may give hotter HF output and sound louder.
JR
Bo Deadly
Well-known member
5. Reduce bandwidth such as with a low-cut feature (preferable pre-gain such as by reducing the cap in the gain control network).
Reading what I wrote earlier "For me, it shows that your Claret is either poorly designed or duff.", one may think it is statistically not very likely.
Actually many mic preamps do not perform as they should because their input transistors have been subjected to zenering of their base-emitter junction. This happens when connecting an unbalanced source when phantom is on. The preamp seems to perform normally after that, except that noise has suddenly increased.
There are well known solutions, but many designers are not well aware of them, particularly because they don't understand the problem.
Now, back to your question "If I have a good quality pre-amp that provides 55db of gain, but I need 65db of gain in order to achieve the signal level that I need, what would you suggest?"
There's only one simple answer: add a line-level gain stage. I don't know of any commercial product doing just that, so you would have to build one, or use a standard mic pre that accepts a line level. It would not need to be particularly low-noise.
Alternatively you may use a 1:3 step-up xfmr inserted between the mic pre's output and the converter's input. Assuming an input impedance of 10-20k on the receiver, this would reflect as 1-2k on the primary side, which most mic-pre's can handle reasonably.
I just mention this possibility in order to leave no stone unturned, but don't advocate it.
Actually many mic preamps do not perform as they should because their input transistors have been subjected to zenering of their base-emitter junction. This happens when connecting an unbalanced source when phantom is on. The preamp seems to perform normally after that, except that noise has suddenly increased.
There are well known solutions, but many designers are not well aware of them, particularly because they don't understand the problem.
Now, back to your question "If I have a good quality pre-amp that provides 55db of gain, but I need 65db of gain in order to achieve the signal level that I need, what would you suggest?"
There's only one simple answer: add a line-level gain stage. I don't know of any commercial product doing just that, so you would have to build one, or use a standard mic pre that accepts a line level. It would not need to be particularly low-noise.
Alternatively you may use a 1:3 step-up xfmr inserted between the mic pre's output and the converter's input. Assuming an input impedance of 10-20k on the receiver, this would reflect as 1-2k on the primary side, which most mic-pre's can handle reasonably.
I just mention this possibility in order to leave no stone unturned, but don't advocate it.
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mhelin
Well-known member
You could try modding the pre-amp if possible to get the gain increase you need. Usually this means (in case of the most common two transistor followed by an opamp -circuit) decreasing the value of the gain pot's in-series resistor which sets the maximum gain (minimum negative feedback).
Newmarket
Well-known member
Out of curiosity - what pre-amp(s) are we looking at here ? I can't recall one that doesn't have minimum 60dB gain on tap but my knowledge isn't encyclopedic. Just what I've come across.
Practically I'd say just use a different preamp that does have the gain you require. Or patch the preamp output through a line gain stage to bump it up.
On the 'interference noise' front then yes I can see that it helps to have a larger signal on the cable. OTOH any circuitry running on P48 is going to be limited in design by the available current.
Obvious solution seems to be a conventionally powered preamp near to the mic. Where that is possible.
Many audio interfaces have limited gain, because they are so often used for close mic'ing. With current 24-bit converters, a weak recording can be bumped up digitally without too much harm.
What Id like to know is how do you design an op amp based output stage that takes its power and output signal over the same two (or three including ground)wires , it would also need to pass power upstream on xlr input pins etc. There seems to be a device that does this already , one of the fet head type units has power pass for condenser mic .
The limitation from 48v phantom supplies could be removed by using a dedicated power supply , it need not be 48 volts either , 24-30 volts might be preferable for op amps that use 2x15volts usually . I see on certain op amp spec sheets 'rail to rail' capable , is that the type we need ?
If the gain between the head amp and op amp driver could be adjusted remotely from the PSU over the same three wires we'd have our cake and eat it . Ive heard Bo talking about this remote idea before with control signals transmitted over the audio cable . We only need two functions , volume up/down , could the DC supply voltage itself be momentarily ramped up a couple of volts to send a signal to increase volume and down a few volts down for a decrease ? What different devices could we use to control gain and how many steps would we need ? Seeing as were likely to get fine gain control over the source at the mixer we connect it to, maybe even five steps of 10db each would suffice , going to the trouble of making a fully adjustable VCA based thing is over the top, could a simple switched resistive network with solid state relays maybe 5 or 10 positions of attenuation be adequate? .
What happens if the mic is accidently disconnected and reconnected I hear you say ?, no worries the we could just make so the power supply tells the mic the config of the relays on power up ,and bam your gain structure is preserved , the battery back up in the supply adds another layer of redundancy as gain setting is preserved even if mains power is gone all to hell , your basically bomb proof ,
So four basic building blocks , head amp ,gain control , output stage and psu . A standalone phantom power supply in the usual form factor could easily have control circuitry and an attenuator pad added , Theres a neat li-on powered phantom box on ebay , recharges via usb 5 volt power . That would run the whole show for hours without any of the usual grounding conundrums .
The limitation from 48v phantom supplies could be removed by using a dedicated power supply , it need not be 48 volts either , 24-30 volts might be preferable for op amps that use 2x15volts usually . I see on certain op amp spec sheets 'rail to rail' capable , is that the type we need ?
If the gain between the head amp and op amp driver could be adjusted remotely from the PSU over the same three wires we'd have our cake and eat it . Ive heard Bo talking about this remote idea before with control signals transmitted over the audio cable . We only need two functions , volume up/down , could the DC supply voltage itself be momentarily ramped up a couple of volts to send a signal to increase volume and down a few volts down for a decrease ? What different devices could we use to control gain and how many steps would we need ? Seeing as were likely to get fine gain control over the source at the mixer we connect it to, maybe even five steps of 10db each would suffice , going to the trouble of making a fully adjustable VCA based thing is over the top, could a simple switched resistive network with solid state relays maybe 5 or 10 positions of attenuation be adequate? .
What happens if the mic is accidently disconnected and reconnected I hear you say ?, no worries the we could just make so the power supply tells the mic the config of the relays on power up ,and bam your gain structure is preserved , the battery back up in the supply adds another layer of redundancy as gain setting is preserved even if mains power is gone all to hell , your basically bomb proof ,
So four basic building blocks , head amp ,gain control , output stage and psu . A standalone phantom power supply in the usual form factor could easily have control circuitry and an attenuator pad added , Theres a neat li-on powered phantom box on ebay , recharges via usb 5 volt power . That would run the whole show for hours without any of the usual grounding conundrums .
