Schem for a phantom powered FET circuit for a ribbon mic

[midwayfair]

I want to build a phantom-powered FET circuit for a ribbon mic, with or without a transformer is fine, but preferably something simple enough to fit in a mic shell, with a gain of 10-20dB (doesn't have to be a ton -- I just want something closer to the mic source before running it through cable to the interface). This is most likely what I'll be repurposing: http://www.musiciansfriend.com/ribbon-microphones/mxl-r40-ribbon-microphone

[PRR]

Why must it be an FET? Other types of devices have noise impedance more like dynamic/ribbon mike impedance levels. Device distortion profile will be moot at dynamic/ribbon mike impedance levels.

Why with or without a transformer? That's a MAJOR cost and size factor, and an added way for hum/buzz to get in.

Why do you think you need gain at the mike? TV studios hung ribbons in the light-truss and ran cables across the building.

Phantom into a ribbon is traditionally DON'T DO THAT!! I suppose a phantom-hack *could* change this, by eating the phantom. Also "good" Phantom power and properly balanced ribbons get along fine. Maybe I am just anal to ALWAYS kill Phantom before bring a ribbon out of the locker.

Did you do your homework? Others have used this bargain mike and not noted design shortcomings.

https://www.gearslutz.com/board/low-end-theory/597135-mxl-r40-ribbon-microphone.html
http://www.preservationsound.com/?p=3687

[midwayfair]

Did you do your homework? Others have used this bargain mike and not noted design shortcomings.

https://www.gearslutz.com/board/low-end-theory/597135-mxl-r40-ribbon-microphone.html
http://www.preservationsound.com/?p=3687

Yes, I did my homework ... It's why I picked this microphone, because I think it's useful enough without recutting a ribbon, with a bottle that should be big enough to accommodate a small active circuit.

Why must it be an FET? Other types of devices have noise impedance more like dynamic/ribbon mike impedance levels. Device distortion profile will be moot at dynamic/ribbon mike impedance levels.

It doesn't have to be a FET, but I haven't seen anything else (except tubes) used with a ribbon after looking at a lot of different mics. 99% of the ribbons I've seen are passive. Royer (on the high end) and Nady (on the low end) both have active Ribbon circuits using FETs, and Triton Audio makes the FEThead, which is made to be used with Ribbon microphones (hell, it's name is a pun on a SPECIFIC ribbon microphone). I'm open to using something else, but that was all I've seen at every level of microphone cost.

Why with or without a transformer? That's a MAJOR cost and size factor, and an added way for hum/buzz to get in.

With or without a transformer because I am capable of removing the transformer from the ribbon if needed for the active circuitry involved. I could build something outboard, but that's one more thing to remember.

Phantom into a ribbon is traditionally DON'T DO THAT!! I suppose a phantom-hack *could* change this, by eating the phantom. Also "good" Phantom power and properly balanced ribbons get along fine. Maybe I am just anal to ALWAYS kill Phantom before bring a ribbon out of the locker.

Yes, I've already had this explained to me, which is a big part of the reason I want an active circuitry in the microphone itself. The issue is that my recording interface only has two buttons for phantom power, each of which controls the phantom on two channels. I need to be able to record at least four different instruments simultaneously for a recording project coming up, and I was planning on getting a ribbon for the guitar tracks, while the other mics involved are condensers. Rather than risk destroying the ribbon, I can spend $5 on the components necessary to build an active circuit into the microphone. While I could build an outboard unit of some sort, that's one more thing to remember when using the microphone.

[karbomusic]

Yes, I've already had this explained to me, which is a big part of the reason I want an active circuitry in the microphone itself. The issue is that my recording interface only has two buttons for phantom power, each of which controls the phantom on two channels. I need to be able to record at least four different instruments simultaneously for a recording project coming up, and I was planning on getting a ribbon for the guitar tracks, while the other mics involved are condensers. Rather than risk destroying the ribbon, I can spend $5 on the components necessary to build an active circuit into the microphone. While I could build an outboard unit of some sort, that's one more thing to remember when using the microphone.

I'm sure I'm missing something but why wouldn't you just use the non-phantom channels for the ribbon(s) or is this truly as a protection mechanism only? The only other requirement for ribbons is they need more gain so I'll assume your current preamp can't supply enough and that is the motivation. That would make a good case but I'd likely find a simpler path than building a circuit into the mic to mitigate a gain problem (outside of DIY fun of course). I understand the extra component piece but it feels like you just need a preamp that can supply the gain a ribbon needs. Or you could just buy a triton fethead but that is an additional component albeit you could just leave that plugged into the mic most of the time.

FYI: From a protection perspective... Many newer ribbons won't see phantom power even if it is on. The warnings etc, stem from the risk of some wiring or patching mistake which could then allow the phantom power to reach the ribbon.

[midwayfair]

why wouldn't you just use the non-phantom channels for the ribbon(s) or is this truly as a protection mechanism only?

The switch for the phantom power is tied to two of the channels at once (the interface is a Scarlett 18i8, which I carry around to my friends' house for recording, and we're doing the main parts of the tracks live in the room together). In other words, I can't use three phantom powers and one non-phantom. Someone on Madbean did explain that modern ribbons won't be damaged in actual use by the phantom power, but I still see a potential for a momentary short killing something. And I can't use a line input without the power source and I don't want to have to build a phantom power source or use a separate power supply just for one microphone. At some point that loses the appeal of a simple recording setup.

There's got to be more than enough space in that microphone for a small gain circuit running off phantom power ... it's almost as roomy as a 1590A in there based on pictures., but even if there isn't, I definitely don't want to buy a FEThead, becuase it's not like I can't build something like that on my own. But I've never built a microphone pre before so I'm just looking for a decent circuit that's appropriate for a Ribbon mic, with or without the transformer attached.

[karbomusic]

Ribbons typically require ~15-25 more dB in gain than others just as an FYI. Thusly the challenge is usually getting enough gain while retaining an acceptable noise floor assuming you didn't already know all of this.  I'm looking for Mic pre ideas myself since I have a couple of nice Jensen transformers I've been dying to use but those won't be used in-mic but rather in a regular enclosure.

[mth5044]

I don't have anything with which to help you, just want to say that damn the 18i8 is awesome. Got one a few months ago, sounds so good and is so functional for the price.

[anotherjim]

I haven't seen inside a Ribbon mic for a very long time - but if it's totally passive (it sounds as though it is), It just has a transformer which is somewhat special. The primary is very low impedance because the ribbon generates a very low signal voltage so the ribbon current is encouraged to be high with the low impedance winding. This results in enough power transfer to a much higher impedance secondary with a step up in signal voltage. I would advise leaving the mic transformer well alone.

Yes, a fault could briefly cause a fair current surge in the ribbon - if it's a static DC fault it will be brief - but it could be intermittent, effectively AC, and cause some current flow in the ribbon, but modern workaday ribbons should be up to it - or else they would have protection in the first place. Traditional brands of Ribbon mic that have been in production since Groucho Marx was a radio star, were designed when phantom power didn't happen in the first place, so you can excuse those not liking it. But modern production (maybe not reproduction vintage) can take it.

I'm going to say you probably don't need to worry, and can better spend the time testing your cables before the session so you limit the chance of any fault.

[Gus]

A web search should give you all the information you need about ribbon microphones including active circuits

Phantom P48 should be supplied to both pins 2 and 3 by matched 6.8 k/6.82K resistors so both side of the transformer should be at the same voltage and have no or very low current in the secondary

[midwayfair]

okay, thanks guys. In that case I'll probably just stick to upgrading the transformer and consider cutting a 2.5mm ribbon for it if it looks too slack when it comes in and revisit the problem if it turns out that the Scarlett doesn't have enough gain for the mic on its own.

[PRR]

> I am capable of removing the transformer

I mis-understood.

You "MUST" use a transformer with a ribbon. The available power is not large, and the self impedance is VERY small, often under 0.1 Ohms. Trying to get decent noise-figure from a 0.1 Ohm source without a transformer is preposterous. With tubes it would be the size of a good office, with transistors it might be smaller than a breadbox.

The thing comes with a transformer, of course, to scale it to 100r-1K impedance, which ordinary mike preamps (even the $10 job) do very well with.

The one with the FET apparently uses a different transformer to scale up to the 5K-10K range where an FET is king. But _buying_ such a thing spoils the point of a low-price microphone.

I'm being obstinate, because I have designed such a booster, and I know full well it is rarely called for, and does NOT 100% eliminate Phantom thumps. Also I know that all but the rarest old RCAs stand Phantom switching fine, though they can't like Phantom *patching* (where one live wire may contact before the other).

[midwayfair]

Okay.

I'll be using a ribbon with a 1:37 transformer (or 1:35).* I'm probably just going to use Arthur Fisher's RM-5, since it's really close to what I want already. (I prefer the Shinybox after listening to everything in the $60,000 mic shootout, but $550 is steep considering this is just for home recording.) I've been reading through the entire thread on GroupDIY where PRR helped someone else make a phantom-powered booster for a ribbon, where you posted this (for a 2n4401):

http://groupdiy.com/index.php?topic=5743.80


I wasn't able to suss out if this was before or after it was established that the op was definitely using a transformer after his ribbon.

At some point during the discussion, it was decided that the voltage on the bases was identical on both bases so there was no need for the input caps and it might even be safer without them (though couldn't we use a large-ish pull-down resistor on the transformer side of those caps, just like in guitar pedals?).

This is basically all I need. It makes about 10dB of gain, and unless I'm mistaken, it can be bypassed with a 2PDT when I have access to a preamp with enough gain.

There was also this FET version further in:



The main problem there was that the FETs would have to be absolutely perfectly matched, which is impossible, and someone suggested just leaving out the 300R because "at least one commercial company does it" (but they don't say who ... !@#%!@ people are opaque with recording equipment circuits).

I also for the life of me can't understand where the +48V is coming from in that FET schematic. Are those two 6k8s just connected together and there is no actual "+48V"? Do I ignore the 6k8s under the assumption that they're already present inside the preamp that's supplying the phantom power, and they're just there as part of the sim? (I thought that the - and + pins of the XLR carried the actual phantom power voltage.) I've Googled myself silly over this and I can't find ANYTHING that explains clearly where the actual DC voltage is carried once it gets to the microphone's XLR and if anything further is needed inside the microphone.

EDIT: Just to be clear, at this point, the other reason I'm keen on using an active stage is also to lower the output impedance.

**Another note: I saw the Lundahl 1:110 transformer, which would give me way more gain and let me use just the "dual buffer" method in Royer's patent, but I can't find anywhere to buy that transformer. And in any case, I figured that going with PRR's simple two-transistor booster, which already has the amount of gain I need, lets me use a cheaper transformer AND lets me make the boost switchable since there's no way I can expect to use the Lundahl's 1:110 transformer with my Scarlett -- I can't find the actual input impedance of the Scarlett's XLR input, but I seriously doubt it's higher than a couple K.

Maybe I should move this discussion to a different forum, but I didn't expect it to be nearly so complicated to get a balanced +10dB booster circuit.

[PRR]

> "at least one commercial company does it" (but they don't say who ... !@#%!@ people are opaque with recording equipment circuits)

How does "who" matter?

> keen on using an active stage is also to lower the output impedance.

The plans above (like many others) have Zout at 300 Ohms (or at "infinity" without the resistor). It is possible your ribbon already has a Zout <300 Ohms. However if you are boosting voltage, many of the reasons for "lower impedance" become less urgent.

> where the +48V is coming from in that FET schematic. Are those two 6k8s

What is "the standard Phantom supply"?

Yes, the sim is for a complete working system with the power supply that comes from the board; otherwise it would sim as "dead". (The first BJT image may be editied.)

> I thought that the - and + pins of the XLR carried the actual phantom power voltage.

That would be Simplex. Perfectly good but less common. Muddled descriptions of Phantom are widely available on the Web, or in good books like Handbook for Sound Engineers.

> I've Googled myself silly over this and I can't find ANYTHING that explains clearly where the actual DC voltage is carried once it gets to the microphone's XLR and if anything further is needed inside the microphone.

Presumably "something" has to be done to get the power off the audio lines and into the gizmo that needs power. There are many ways to skin cats. Study *every* p-mike circuit diagram you can find.

> I didn't expect it to be nearly so complicated to get a balanced +10dB booster circuit.

Design it yourself. I think you have ample brain-power. You may need to spend time with the rudaments of balanced lines, power over lines, and then low-hiss amplifiers. But indeed it isn't rocket science.

> Maybe I should move this discussion to a different forum

That might be wise.

[bool]

2SK389 used to be a go-to matched pair for low noise applications. Perhaps too noisy for a ribbon pre-pre. Obsolete, and now expensive if you manage to find a seller.

Perhaps it would be much more convenient to just buy a cloudlifter or something similar.

ymmv.

(I have built a micpre or two, and there's mucho "other stuff" to take care with these, not just electronics.)

[patricks]

I have a shortcut for you - check out Austin microphones
High quality passive ribbon mic and dedicated preamp in kit form for between $400 and $520. Easier (and probably cheaper) than adding active electronics to a pre-made passive ribbon mic and hoping to get enough gain from it without significant noise.
If you do prefer to go the "add active electronics to a passive mic" route, it'll be worth replacing the existing ribbon with something thinner for better high frequency response. You can get sub-micron aluminium ribbon from Michael's or on Amazon. It's a bit of a pain to work with, though, so get plenty. I must've wrecked made nearly 10 "practice" ribbons before I got the hang of it.

[computerlen]

Hi. I really want to help out anyone who is searching for a wiring diagram for a preamp to boost a ribbon or otherwise dynamic microphones's output but the preamp is powered by the phantom power of the mixer used. I feel that I have searched everywhere on the web but with no luck. I finally found a circuit, wired it up and it works beautifully! I am actually excited. If you want this circuit diagram please write to me at computerlen@hotmail.com and I will reply with the diagram. It uses 4 fet transistors and little else. It blocks the dc voltage from the phantom circuit that boosts its output.

[midwayfair]

Hi. I really want to help out anyone who is searching for a wiring diagram for a preamp to boost a ribbon or otherwise dynamic microphones's output but the preamp is powered by the phantom power of the mixer used. I feel that I have searched everywhere on the web but with no luck. I finally found a circuit, wired it up and it works beautifully! I am actually excited. If you want this circuit diagram please write to me at computerlen@hotmail.com and I will reply with the diagram. It uses 4 fet transistors and little else. It blocks the dc voltage from the phantom circuit that boosts its output.

You can host your pictures on something like Imgr so you cna post them for people to see and discus them.

If you aren't posting it publically because you're selling it, then there is a buy/sell/trade area that you can post in after contributing to the forum in other ways.

[computerlen]

I am not selling the wiring diagram, I just want to write back and forth to the person who is going to build the circuit and get back to me about how good it sounds, etc. I am not always on this site or any other. I hope that I am not breaking any rules. I figure that if the person really wants the wiring then he or she will write me. There's another reason which I will let that person know about privately.

[PRR]

...I feel that I have searched everywhere on the web but with no luck....

https://groupdiy.com/index.php?topic=5743.msg73762#msg73762

[tubegeek]

...I feel that I have searched everywhere on the web but with no luck....

https://groupdiy.com/index.php?topic=5743.msg73762#msg73762

PRR, sorry to hear about your slip 'n fall: feel better ASAP!

Question: on that drawing, what do the BIAS points connect to? The in+ and in- will be +48 DC offset from the G input connection, but then the caps will block that. What am I not getting? Are the 100u and 4K7 meant to represent the built-in phantom supply on the other side of the cable, meaning the transistor bases are directly connected to the connector? Or (equivalently) is this scheme demonstrating how you'd inject +48 to a setup that didn't have it already?