Active ribbon-mic

[bcarso]

That's an interesting thread, one I hadn't looked at (I've only been on board since Feb. 2005). It's also interesting in that it was one of the ones giving rise to the Drawing Room, it appears, the rationale for which has been a recent topic.

The problem of achieving lowest noise at low gains is another interesting one ;-), and not at all trivial. I did a switched attenuator design with buffers ahead and after which managed nearly real 24 bit signal-to-noise, over a relatively small gain range, but it was a biatch. It was for introducing preset line-level gains to power amps driving loudspeakers, part of an elaborate subjective evaluation facility. The required gains were determined with pink noise measurements---the purpose being to minimize SPL variations among presented loudspeakers.

Alas, the client got parsimonious at the end, and the complete system was abandoned.

 

[PRR]

> The ultimate use of those bad boys would be in direct ribbon mic preamps and moving coil cartridge pre-preamps.

Low noise is a goal in phono amps, but the real noise-floor is the phono-disk surface noise. It appears to be "easy" to get enough efficiency in the needle-pickup to output surface noise at higher power than coil thermal noise. No doubt it helps that we already have the source concentrated, unlike microphones where we have to catch thin air.

In many-many real-world situations, mike and mike-amp noise vanishes in room noise. In my rehearsal room, quite hissy gear is not audible above HVAC racket. In my concert hall, electric bass-noise is never an issue because blower rumble is 20dB higher than even mediocre gear, though the blower mufflers and sorta-massive walls give such low room noise 2KHz-4KHz that no recording rig can touch it. (We had to use instrumentation mikes and long-term spectral analysis to plot the noise: it dips below 0dB SPL for a mid-high octave or two, while being over 50dB SPL at 50Hz.)

> I think a direct transformerless ribbon preamp would be a great experiment.

I think it is impractical. Typical ribbons are orders of magnitude lower-Z than typical mike lines. We can just about do very well at 100Ω or maybe 10Ω. But 0.1Ω is 100-1,000 times lower. Do we parallel hundreds of devices and run Amps of current? I think if there is any hope of using a transformer, that is a better answer. (And note that it is not too impractical to modify a higher-Z transformer. You don't need a winding machine, just heat-gun and hacksaw to open the case and then hand-wind a couple turns around the outside of the winding.)

> no compromise preamp that would perfom exceptionally well with normal transformer ribbon mics. I always liked the preamp PRR developed

That one has serious practical compromises. Noise at low gain settings is lower than many, but you can do better. CMRR vanishes at the lowest gains, to the point that you can "lose" the signal if the mike is really single-ended (many are) and you take the output unbalanced. And it would run HOT, possibly hot enough to affect audio quality (either the heat or the high currents).

Didn't I post a follow-on thread? I know I took it to Darlington outputs and ~500pFd compensation, but that was about the time I noticed the excess noise at low gain and smacked myself for not seeing the problem sooner.

 

[bcarso]

"I think a direct transformerless ribbon preamp would be a great experiment."

For me, I want to do this because it's there---like climbing a mountain. There's no issue about the trafo being a fine and simple solution.

Just to show that it is possible would be fun.

Of course the preamp would have to live in the mic or very close to it. The power dissipation can be kept reasonable if the voltages are small enough, so that you won't have substantial thermal air currents whooshing by ;-). About a dozen 2SC3329's and/or 2SA1316's will do the job.

Note added about 8PM PDT 18 May: Just ran across a provocative article by Hawksford in an old (Oct. '83) AES journal about granularity in bipolar transistors at low levels. I suspect he is slightly off his onion but it's fun to read. If he is right then maybe trafos will always trump active pre's, although magnetics are not without their low level effects too, like induced and spontaneous Barkhausen noise.

 

[mfdu]

listed here is the Rode D-power and the D-powerplug.

Input impedance:
100 kO balanced per arm.
Output Impedence:
100 O balanced.

the shinybox 46 :
Impedance: <= 200 ohms
Recommended Load Impedence: 1000 ohms

but we're talking diy. sorry.

chris.

 

[Rossi]

I saw those at the Frankurt fair. The question is wheter the equivalent input noise is low enough. The noise spec reads "-116 dB A-Wt RTI." If my English is correct, RTI means referred to input. -116 dB input noise may suffice for normal dynamics, but doesn't sound usable on a ribbon. But who knows, maybe the noise performance is better than spec'd or maybe the thing can be modded to better EIN with low rbb transistors.

 

[mfdu]

point taken on the noise issue. and also those of you who piped up about the mismatch of impedence - mic looking for 1KO and finding 100KO.

as kev advised me, i reckon i may try the internal rode unit (d-power) and pop it in a little box of its own, and add "termination resistors" on the input from mic

now. termination resistors methodology to achieve 1KO visable to the mic. comments?

so now it becomes a valid diy discussion again. hurrah!!

chris.

 

[Kev]

my suggestion of the resistor was more about keeping the mic happy
... rather than about controlling noise floor

If the noise floor is too high then you will need another option I suspect.

$50 was cheap enough to have an experiment

These things are never clear until you actually try them ... in your situation and in context.
It can only not work
and if it doesn't then you have an interesting toy for a solution to another problem, one day in the future.

 

[mfdu]

gotcha.

best i can do is scratch and sniff.

hell, i'm happy with that.

will report results early next week.

thank you very much for your outlook on this.

chris.

 

[clintrubber]

After having toyed around with the Thomann-ribbon & loud amps I found most mic-pre's do fine.
But understandably for M/S the output-level of the ribbon used as fig-8 drops a lot, so some additional gain required. Nice thread, I'll be building (finally).

 

[Rossi]

Yeah, no problem with loud sources, but on acoustic guitar, vocals or distant micing, some extra gain is always welcome.

Which one did you get the RM700 or the RB500?

 

[clintrubber]

[quote author="Rossi"]Yeah, no problem with loud sources, but on acoustic guitar, vocals or distant micing, some extra gain is always welcome.

Which one did you get the RM700 or the RB500?[/quote]
The RB500, I think I have it for about nine months now.

I only just read/heard/saw about that silver one (and it's tempting :wink: - I also saw your mag-review), but the RB500 does fine for what I'm doing and I'm still in the phase of getting to know it better.
I like its kind of universal use: nice for guitars and being that fig-8, it's nice for M/S too.
(We from Holland like deals like that :twisted: :wink: )

Bye,

Peter

 

[Rossi]

Yeah the RB500 is a bit more universal. I like it a whole lot. I have two, and I'm actually considering a third one. A guy from Thomann told me that the newer ones have higher output than the early ones. Probably a 600 ohms secondary transformer vs a 200 or 300 ohms transformer on the early ones. I also wanna get a second RM700 so I can do stereo recordings with them, too. Both M/S and L/R work great with the RB500. The RM700 might even be a litte better for stereo, because the ribbon is a bit shorter, and you can get the two elements much closer together than those of the large RB500s. The difference between front and back is bigger on the RM700s, though. Reportedly that's also true for the Royer originals.

I'm gone for this year. [fake Arnie accent] I'll be back!

Guten Rutsch, as we say in Germany!

 

[clintrubber]

[quote author="Rossi"]Yeah the RB500 is a bit more universal. I like it a whole lot. I have two, and I'm actually considering a third one. A guy from Thomann told me that the newer ones have higher output than the early ones. Probably a 600 ohms secondary transformer vs a 200 or 300 ohms transformer on the early ones. I also wanna get a second RM700 so I can do stereo recordings with them, too. Both M/S and L/R work great with the RB500. The RM700 might even be a litte better for stereo, because the ribbon is a bit shorter, and you can get the two elements much closer together than those of the large RB500s. The difference between front and back is bigger on the RM700s, though. Reportedly that's also true for the Royer originals.[/quote]

W.r.t. M/S:

Last summer I've been playing with this combination of re-*mping & M/S:
if you are re*mping anyway, why not do M/S with the same mic ? (double amount of tape-runs of course)
IMNSHO it's a fine trick !

Now you can place the same mic in the very same spot, only rotated 90 degrees to be the 'M' on the first run and being 'S' on the second.

It turned out fine for me, but what do you all think about this ?
Anyone took the trouble of the two runs & has also done this before ?

It's about micing an amp+FX here, never exactly the same, but I figure it's the-same-enough not to spoil the M/S-combination too much.
I feel it has more advantages that disadvantages, but haven't toyed around it more than a few times so far.


Those ribbons are too cheap ! As in: it's too tempting to get more & more of those goodies :wink:

I'm gone for this year. [fake Arnie accent] I'll be back!

Guten Rutsch, as we say in Germany!

Enjoy !

Peter

 

[Rossi]

M/S with the same mic is an interesting idea, acutally. :thumb:

I personally wouldn't do it on electric guitar, though. Mono is fine with me, I rarely use any fancy effects on guitar.

Yeah, those ribbons are insanely cheap for how good they sound. I'm thinking about getting one of the newer RB-500s with higher output and build a booster preamp directly into the mic & maybe add a low cut switch. A hi shelf switch would be great as well, but I don't think it's possible without degrading the S/N ratio.

 

[clintrubber]

[quote author="Rossi"]M/S with the same mic is an interesting idea, acutally. :thumb:

I personally wouldn't do it on electric guitar, though. Mono is fine with me, I rarely use any fancy effects on guitar. [/quote]
The idea is to get a somewhat broader sound without having to use the more artificial ways (FX-boxes, dly,reverb...). I may skip using it, but the thought came up so I tried it & was not underwhelmed :wink:

Yeah, those ribbons are insanely cheap for how good they sound. I'm thinking about getting one of the newer RB-500s with higher output and build a booster preamp directly into the mic & maybe add a low cut switch. A hi shelf switch would be great as well, but I don't think it's possible without degrading the S/N ratio.

Hi shelf as for instance in the MD441 ? Don't know how they do it in that mic. Or perhaps better simply after the mic-pre.

 

[clintrubber]

[quote author="PRR"]I hacked-up a device model that approximately matches the 2SK170BL Vg/Id curve. (I did NOT fool with capacitance and other parameters.)

[/quote]

Just observing here:
Looks like Erwin from Alkmaar has seen the Neumann-logo... would he have seen the circuit above as well ? :roll: Dunno if he's a member here.

http://tritonaudio.com/tritonaudio/...ent&task=category&sectionid=4&id=17&Itemid=33

Interestingly he mentions application for a '441, as done by Rossi in his mag-article. Again, don't want to be a clone-cop, just observing.


And before someone jumps, let's keep this product from the same maker for The Brewery, OK ?

 

[Rossi]

The MD441 is notorious for good sound and low output, so it's a usual suspect for a mic booster. I miss essential data for this product. What's the input impedance and most of all, what it is its input noise figure? Without that kind of information I'd be very hesitant to buy that plug. Rode build similar plugs, as well, and from what I've heard, they're not quiet enough for use with ribbons.

FET circuits are nice for "real" active ribbon circuits (using a very high step-up transformer), but I'm not sure a FET circuit will do very well in combination with a normal low impedance ribbon mic.

 

[clintrubber]

[quote author="Rossi"]The MD441 is notorious for good sound and low output, so it's a usual suspect for a mic booster.[/quote]
OK, we give him that, my conspiracy-hat may have fitted me too well there :wink:

I miss essential data for this product. What's the input impedance and most of all, what it is its input noise figure? Without that kind of information I'd be very hesitant to buy that plug. Rode build similar plugs, as well, and from what I've heard, they're not quiet enough for use with ribbons.

FET circuits are nice for "real" active ribbon circuits (using a very high step-up transformer), but I'm not sure a FET circuit will do very well in combination with a normal low impedance ribbon mic.

Right, easier to make a BJT work for that application, as we saw for instance in this thread & others. The PR-talk of that site (not the PPR-talk on this site) might sound nice to an non-tech person but has very little hard info.

No idea what the Rode plug costs, but he's not asking insane prices though, I'll be checking what he asks for empty non~Neumann~branded shells. Must add that I like his logo & the colour of it though; we have already AEA & MA dreaming of RCA, so why not a brand/garage-company doing the same of Neumann ?

Regards,

Peter

 

[Kingston]

What an interesting thread!

I private messaged Rossi about his experiences with the circuit since this discussion is a little old. He suggested these discussions work better in public so here's the short exchange which may help others too:

I have several RB500 mics upgraded with Lundahl LL2913. They are my favourite mic of all time, but as you know, getting enough gain with low noise can get troublesome. In the thread you talk about this same mic, but it was never quite clear to me if you had actually used the booster circuit on the RB500. Looks to me like it could be easily built and installed inside the mic, and maybe you did?

What was your experience with the circuit, and was it ever agreed on what revision worked best in this application, the FET version(s) or the plain original by PRR?

yes I built this circuit (into an external enclosure), and it works very well with the right low Rbb transistors (as I said in the thread). The 2SC3329 is very hard to get these days. The 2SC2546 is easier to obtain.

I never built the FET version, as I'm pretty sure the BJT version is lower noise with normal Low-Z ribbon mics. The FET version might work better in connection with super high ratio transformers (1:100).

The Lundahl ribbon tranny sounds fantastic, but it isn't particularly low noise as the primary resitance is a bit too high for the RB-500. The stock transformer is actually a bit lower noise - but it doesn't sound as nice.

Slightly off topic, but thanks for the information about the Lundahl as well. I never knew about the primary resistance, but indeed it sounds nicer, more accurate somehow.

 

[zebra50]

I think this thread needs a link to this one...
http://www.groupdiy.com/index.php?topic=37546.0