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Designing new remote preamp

Discussion in 'Preamps / Channel Strips' started by Zilla, Apr 21, 2006.

  1. Zilla

    Zilla Active Member

    ....um, returning to the topic of this thread (but maybe not 'to die for')...

    I am developing a mic pre solution for myself specific to location ribbon mic pre-amplification. Since there seems to be plenty of Royer enthusiasts in this forum, maybe I could layout my ideas and get some feedback. Anybody interested?
     
  2. DavidSpearritt

    DavidSpearritt Well-Known Member

    Absolutely. I am a ribbon fan, but only have an SF24 now. Hoping to get some Coles 4040's when funds allow. I am keen to hear the AEA ribbon preamp, but would love to discuss this with you Scott. Doesn't the transformer in the SF24 somewhat isolate the preamp influence? In other words, I suppose you are mainly concerned with amplifying passive ribbons?
     
  3. Zilla

    Zilla Active Member

    The added gain of the active royer version means that the pre-amp does not need to do as much heavy lifting (in this case ~14dB less), thereby reducing pre-amp influences. But this is at a cost of additional electronics in the signal path. I compared the active against the passive and preferred the later.

    So... YES, amplification of passive ribbons (and dynamic mics, too). My goals at the moment are...

    1. Very portable: 1.75"x4"x6" case.
    2. Independence from venue power: batteries.
    3. Ability to place pre-amp at mic stand: batteries, low profile case.
    4. Eliminate distortion generating components: DC coupled.

    Restricted to the above, I have sketched 3 potential designs:

    1. One channel, unbal out, ~12hr battery life.
    2. One channel, bal out, 6dB less noise, ~6hr battery life.
    3. Two channels, unbal out, ~6hr battery life.

    All designs to use a 6 position gain switch: +40,45,50,55,60,65dB.
    All designs to incorporate a battery level indicator.

    So, any comments?
     
  4. DavidSpearritt

    DavidSpearritt Well-Known Member

    Sounds ideal. I would add that for my application, stereo inputs is preferred but the specs, portability and battery design are all spot on. :cool:

    I know this is an expensive option, but I have also wished for remote gain control, ala crookwood, so that I can hang the stereo ribbon from the concert hall ceiling, place the preamps in the ceiling, as close as possible to the mic and control gain from down at side stage.

    Should we start another thread for this?
     
  5. ptr

    ptr Active Member

    Very interesting. The basic design decription sounds a little like the Sound Devices MP1 that I'm using right now. (Not on Ribbons, but in general use. Ribbons are a field I'm just starting to explore)..

    I use the MP1 when I need to drive very long lines (100 feet or longer) in enviroments without house power, so for me an unbalanced output is out of the question.

    /ptr
     
  6. Zilla

    Zilla Active Member

    The form factor on that MP1 is really nice! Very handsome.

    But I question whether some of its design principles fit with my needs...

    1. If its power source is two AA's (+3V), then how are they getting +15V, -15V, and +48V? The only way I know is via a dc-dc switching power supply. I prefer not to have switching supplies in my audio devices.

    2. Since it can provide phantom power, there must be a reactive ac coupling network on its input (xfrmr, capacitors, or both). Passive microphones do not need blocking caps, therefore I can eliminate their distortion, as multiplied by the preamp's gain, all together.

    3. To support all its features, they must stuff much circuitry into that tiny box. I must assume that they cannot be using components that I personally find desirable. The components I prefer would just be too physically large to fit.

    Of course, this is all supposition. I have not looked inside one of these units, nor have I listened to one. As far as I know these things could sound excellent, but I suspect my preamp will be more to my liking.
     
  7. ptr

    ptr Active Member

    Indeed it is. I'm not an electrical engineer enough to answer your questions, so I'll leave it for someone more lerned in that area to comment. Having had a look inside, I can confirm that its "crammed".. As with any production unit its not as tailored as one perhaps might want..

    I have not done any shoot out; the MP1 is quite silent but does not match the musicality of my DAV Broadhurst Gardens Pre’s. Pity I'm on the opposite side of the globe, had Hollywood been in my neighborhood I’d be over with one instantly for You to scan!

    /ptr
     
  8. jahtao

    jahtao Guest

    do you guys know about the lab forum?!!!!!!!!!!!!!!!!!!!

    http://www.prodigy-pro.com/forum/index.php

    they rule
     
  9. dizziness

    dizziness Guest

    Yes, I did a shootout of the newly added inputs on my Millennia HV-3C (minus blocking caps) and yes, they do sound better. I was tempted to just use the SF-12 and HV-3c combo without the touch of ambient mics as the tone and clarity were SOOO good.

    Chris
     
  10. Zilla

    Zilla Active Member

    @ jahtao,

    Yes, I am aware of the lab forum. Good resources there. However, I don't really need technical input. At the moment I am more interested in feedback from fellow practicing acoustic music engineers

    @ dizziniess,

    Exactly; clarity and resolution are much greater without the caps. I am beginning to believe that phantom supplies generally reduce achievable resolution, even for solid-state condensers. For example, compare the 4003 vs 4006, or 221 vs CMC6, or SF12 vs SF24. Of course there are other factors involved, but having made many such comparisons I seem to be sensing a consistent degradation. Not totally sure yet, but am very suspicious...
     
  11. DavidSpearritt

    DavidSpearritt Well-Known Member

    Scott, as you said, these examples contain many more variables in addition to just phantom caps, ...

    4003/4006: big polarisation voltage difference, 130 to 48, big preamp differnces as well
    221/CMC6: valve presence, no valve
    SF12/SF24: no transformer vs presence of transformer

    I think isolating the sound difference between these pairs as being influenced by phantom caps, maybe drawing a long bow. Very difficult study, and these other factors are big sound changers.

    I must say that my impression of SF12 vs SF24, is that the SF12 sounds slightly more open, albeit with a higher noise floor. This is a tiny, unconfirmed impression having used both these mics in the same room with similar material.
     
  12. Zilla

    Zilla Active Member

    A very long bow, indeed. The three sets of mics I listed are the best examples available because they can all have identical capsules. But the method of pre-pre-amplification does vary, creating an x-factor.

    However, my hunch is also based on other clues. For example, working for durations of time with only tube and ribbon mics, then returning to phantom powered sessions, then back again. There is a significant resolution difference. It has taken many years of exposure to finally notice this general 'vibe' shift.

    Unfortunately, there is almost no way to test this theory. But using basic engineering knowledge I can say the following...

    1. Going through decoupling networks reduce resolution, not increase it.

    2. Connecting the output of a power supply to the input of a high gain amplifier can only influence the sound quality on its output.

    3. The way power is transfered and received at the mic and put to useful work is not as clean as a dedicated mic power supply. Also, many phantom powered mics have a little switching power supply in them to get +60V to polarize the capsule.

    So I put this knowledge together with my aural experience and feel justified in suspecting phantom power may not allow condenser mics and mic-pre's to perform to their fullest.

    But, yeah, basically this is just a hunch.
     
  13. Zilla

    Zilla Active Member

    just to clarify some things...

    When I read "polarization voltage", I think capsule polarization. The DPA's have pre-polarized electret capsules. The power supply does not enter into their capsule polarization.

    My interpretation of Royer's product information page is that actually both models have transformers.
     
  14. Simmosonic

    Simmosonic Active Member

    I did this with the help of an excellent audio designing friend by the name of Terry Demol, some time ago. I have discussed it elsewhere on this forum once or twice before.

    My system was specifically designed to allow a Royer SF12 to drive long cable runs in concert halls and similar. The system went through three designs and re-builds until I thought it was as good as it could possibly be. The final version had a 'head amp' that was placed with the microphone and had fixed gain (IIRC, it could be switched from +33dB to +39dB) and there was an accompanying preamp that sat with me and offered an additional gain from +2db to +22dB or so.

    This turned out to be a better option than giving the head amp remote-controlled gain. Instead, we gave it a fixed gain, and had variable gain at the other end.

    The whole thing was powered from +/-24DC, using a number of 12V gell cells in the preamp. We devised our own form of phantom power for the head amp by sending +24DC down one of the balanced lines, and -24DC down the other. Of course, this power ended in the head amp, it didn't get near the microphone. This was the best solution we could find without making the head amp outrageously heavy.

    (An earlier version, which was just a head amp only, used four 9V rechargable nicads to provide +/-18V DC. It worked quite well, certainly solved the problem, but it didn't sound anywhere near as nice as the later version, where we split it into two parts. And it was damn heavy, some winch systems couldn't cope with it.)

    The input of the head amp was tailored to provide the best match possible for the SF12's output transformer. Also, it was actually four high quality unbalanced amplifiers, the signal remained differential thoughout the whole system, it was never unbalanced. The concept was for it to be a balanced line with gain.

    Terry also borrowed some ideas from medical electronics and radio telescopy (where they have to send very small signals over long lengths of cable) that effectively took the cable out of the equation. Clever stuff, but I'm not at liberty to talk about it...

    As for SF12 vs. SF24, yes, they both have transformers. The transformer in the SF24 is different to the one in the SF12.

    I used the SF12 and custom preamp system for a few years, until the SF24 came out and I bought one immediately. It's a lovely mic, and doesn't need any of this help. But I tend to agree with David Spearritt that, in some cases, the SF12 sounds better - especially with my custom head amp/preamp system. I made some lovely and very natural-sounding recordings with that rig (Royer SF12, custom head amp/preamp system, Prism AD124 AD converter).
     
  15. Simmosonic

    Simmosonic Active Member

    I have often dreamed about this, too, but to be really useful and unobtrusive for your/our needs (hanging in a ceiling of a concert hall), I reckon it has to be either wireless or infra-red RC. Sending controlling signals down the same cable that the mic signal is travelling up on seems like a very flaky idea to actually implement properly - especially if we're concerned about capacitors and so on in the signal path.

    There are plenty of wireless remote control systems available in the hobby shops (designed for RC model cars, boats and airplanes) that could easily control a motorised pot. The electronics shops often have kits for infra-red volume controls for hi-fi systems, same idea. All you have to do is motorise the gain pot and you're done...

    A motorised pot would be easy to make with some of the other stuff the hobby shops supply - motors, gears, etc. Couldn't be too hard, but it adds weight.

    Then there is the problem of interference - I'd hate to be in the middle of a gig and something starts spewing forth RF (taxi going by? wireless mics or IEM on stage? you never know your luck in these situations) and suddenly your gain is going up and down! Or, alternatively, some geek in the audience has his Palm Pilot or similar and is pumping out IR at just the right angle. To avoid this, I'd have a function whereby, once I was fully happy with the gain setting, I'd press a button on the remote controller that disabled the remote control permanently by disconnecting the receiving element (antenna or IR receiver) from the RC circuit using a relay. This would, of course, be a 'do it once' operation, requiring you to physically press a button on the preamp to activate it again. BUT... it would totally remove any chance of such interference.

    RF or IR RC could be done with no compromise to the signal integrity at all.
     
  16. DavidSpearritt

    DavidSpearritt Well-Known Member

    Yes, my terminology is incorrect. I meant supply voltage, as you correctly point out, the 190V or 200V prepolarised capsules are common. IIRC the older 4003's that we have, had externally polarised caps though.

    With the Royers my simple assertion was also misleading. Simmo, correctly points out that these transformers are quite different. Sorry for the crappy typing of top of the head thoughts. :)
     
  17. Plush

    Plush Guest

    Just as a clarification--

    Crookwood implements its remote control by using a separate xlr mic cable run from the remote unit to the distant mic amp. Then, one daisy chains short xlr cables to the additional other remote controlled mic amps if they are
    being used.

    The remote control is not done on the mic cable.

    Please keep us posted on work and your new mic amp ideas.
     
  18. Simmosonic

    Simmosonic Active Member

    Yes... This is the only alternative to IR or RF that doesn't involve sending controlling signals down the existing mic lines. Good for any application where you can run an extra cable or two (for daisy-chaining). Not very useful for winch systems, unless you're able to hang your own cables.

    (Mr Spearritt, this would be possible with your rig where you drop your own cables, you could replace your stereo multicore with a four channel cable and have wired remote that way.)
     
  19. BRH

    BRH Active Member

    New user here. Good Forum, especially for Classical Music Recording.

    What's the reason for having the preamps at the mic stands? Are you saying that you would then send a line level signal from the pre to recording device, possibly a couple of hundred feet?
    How about a couple of alternatives.

    1. Make a passive box with high quality tranformer to boost the signal from ribbon mics by about 10-20 db., more importantly changing the ohms to a good 200 so you could run long mic cables without loss.

    2. Since ribbon mics can handle a high SPL, increase the sound going into the mics. Could be done using an electric train transformer..... one pole split off to each performer using something like an antistatic arm braclet, and the other pole connected to a conductive floor covering underneath the musicians. The transformer could be operated remotely by the recordist when he/she felt the need for more gain from the musicians. This could really add some zing to lack-luster recitals, possibly giving a condenser-like quality to those ribbons.
     
  20. Zilla

    Zilla Active Member

    Of course I had already contemplated this solution, but abandoned it due to the extra gear schlepping factor.

    Yes. Better to have a short cable run to the x1000 amplifier and then line drive at a lower impeadance (<50ohm) to the recorder. Better S/N and distortion; sounds mo' better.
     

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