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?

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?

Sounds ideal. I would add that for my application, stereo inputs is preferred but the specs, portability and battery design are all spot on. 8-)

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?

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

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.

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

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..

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

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

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

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

they rule

Zilla wrote: 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.

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

@ 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...

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.

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.

just to clarify some things...

DavidSpearritt wrote: 4003/4006: big polarisation voltage difference, 130 to 48,

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.

DavidSpearritt wrote: l
SF12/SF24: no transformer vs presence of transformer

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

Zilla wrote: 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?

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).

DavidSpearritt wrote: 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.

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.

Zilla wrote: just to clarify some things...

[quote=DavidSpearritt]4003/4006: big polarisation voltage difference, 130 to 48,

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.

DavidSpearritt wrote: l
SF12/SF24: no transformer vs presence of transformer

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

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. :)

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.

Plush wrote: Crookwood implements its remote control by using a separate xlr mic cable run from the remote unit to the distant mic amp.

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.)

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.

BRH wrote: Since ribbon mics can handle a high SPL, increase the sound going into the mics. Could be done using an electric train transformer.....

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

BRH wrote: 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?

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.

Scott,
I know you have a train transformer somewhere. Time to break it out.

Brooks: aka "Homie"

I thought that was you!!!

Why can't you make a pre-pre......Just a slight passive transformer to boost up to a solid mic level with appropriate ohms and then send this mic level to a transformerless pre? I'm more comfortable with sending longer mic lines than line level..... you know us old dudes!

Scares me to leave a 'box' near a mic stand. Someone's kid will most likely start playing with it before showtime.

Read some of your other stuff about phantom interfering with the mic.... not letting the mic's full range come through. I think I read something similar on Neve's website. Could it be that the phantom is not properly balanced from differences in cables within the cables? or connections?

BRH wrote: Why can't you make a pre-pre......Just a slight passive transformer to boost up to a solid mic level with appropriate ohms and then send this mic level to a transformerless pre?

The most voltage gain you could get from a mic transformer is about 20dB. But the better sounding xfrmrs have only 6dB. To achieve best performance from a xfrmr it will need to be terminated properly. Hanging variable lengths of cable (capacitance) off the xfrmr would probably result in less-then-optimum performance.

BRH wrote: I'm more comfortable with sending longer mic lines than line level..... I'm more comfortable with sending longer mic lines than line level..... you know us old dudes!

If both lines are balanced and have low z source outputs (ie not 600ohm) then the advantage goes to transmitting at Line Level for better s/n. Look at it this way: Do you want to hang a 100' antenna on the input of a +66dB (x2000) gain amp, or only a 15' antenna. Which do you think would run into less problems with noise, emi, rf, blue tooth, and other noises and intermodulations?

BRH wrote: Scares me to leave a 'box' near a mic stand. Someone's kid will most likely start playing with it before showtime.

In practice I have not had any problems. Anyway, my prototype has recessed controls to discourage unauthorized tinkering.

As far as reasons to avoid phantom input circuitry:

1. Potentially input circuit z balance reduced, therefore reduced CMRR.
2. Due to standard 6k8 phantom resisters, max possible input z=13k6 (but is typically 2k). More z can be beneficial for ribbons.
3. Phantom blocking caps introduce coloration/distortion. These are unnecessary for ribbon and tube mics.

BRH wrote: Why can't you make a pre-pre......Just a slight passive transformer to boost up to a solid mic level with appropriate ohms and then send this mic level to a transformerless pre? I'm more comfortable with sending longer mic lines than line level..... you know us old dudes!

Scares me to leave a 'box' near a mic stand. Someone's kid will most likely start playing with it before showtime.

My two cents on this.

The mic already has these circuits. A modern phantom powered mic is made to run at least several hundreed feet of mic cable. I would not expect it to be a good idea to add only a transformer or other "unpowered" circuits to this kind of mic -- it is already optimized for driving the cable so it will most probably not get any better.

For older mics / dynamic mics it might be different. Roede does a pre-pre that supposedly improves dynamic mics. Takes phantom power.
http://www.rodemic.com/?pagename=Products&product=D-PowerPlug¨

And of course, an unbuffered ribbon is of course not made to drive long cables. (The Royer SF24 is an example of a ribbon with preamp inside, drives long cables).

Having a box at the bottom of the mic stand has not been any problem for me so far as far as audience goes. I tend to have a DPA 4003 pair with the HMA5000 at the bottom. Never had any interference. It can be a bit of hassle though to get both line cable and mains power out to the mic stand.

All-in-all in my experience running long cables to the mic generally works. But getting that last ounce out of the signal may involve having the mic preamp close to the mic. When that extra little is needed than it would be very nice to be able to remotely control the pre. And then the extra hassle would be OK.

Gunnar

I assuming that this is for a minimal setup. Like 2-3 mics. Doesn't seem like it would be convenient. Every recital or 'church gig' I did had more problem with backround noise and room sound. Seems like you would want to squeeze the last bit of dynamic range out of your rig when in a studio with a 20-25db noise floor and not a recital hall where it's at least twice that.

I haven't done any location recording of music for a few years, but when I did I liked it and had a chance to try different things. Now, I'm "reduced" to engineering voice overs, ADR and foley.

As I get older, I'm wanting a drier, closer mic'd sound, and not take the chances of the room sound. (I gotta use that Waves IR for somthing!)

One time I was recording a somewhat famous harpsichordist, and had my 2 U89s on a wide cardiod pattern on seperate stands not spaced too far, right in front of the instrument. A "Groupee" from the harpsichordist
came to me and pointed his finger at the stage 10 minutes before starting with 400 people there and said. "That's NOT how you record Classical Music! You record with microphones back a lot further in the hall." I said OK and ran up there and took down the U89s and stands and hooked up the SM57 I had in the ceiling, used that for the recording, and handed it to them on the way out.

P.S. Sold the U89s for what I paid.

BRH wrote: I assuming that this is for a minimal setup. Like 2-3 mics. Doesn't seem like it would be convenient.

Yes, minimal setup. Actually it is extremely convenient. My entire rig, except the stand, fits in a back-pack. I don't have to do a big load-in, instead I do a walk-in. I don't have to search for a/c power or run extension cords. My control location is not predetermined by where an outlet is. I can set-up where ever I like. I can literarily have my rig up in 12 minutes, record ready! And once its up I don't have to contend with any suprises like ac hum or dimmer buzzes, etc. I am then free to concentrate and maximize my time on mic position, the most significant part of minimal acoustic recording.

BRH wrote: Every recital or 'church gig' I did had more problem with backround noise and room sound. Seems like you would want to squeeze the last bit of dynamic range out of your rig when in a studio with a 20-25db noise floor and not a recital hall where it's at least twice that.

But why not have wide dynamic range always? My philosophy: better is better under any circumstances. But its not just about dynamic range for me. Its about the excessive amounts of electronic hissy noise one usually has to put up with when using ribbons under high gain. This noise is usually apparent even over a venue's noise floor. I am finding my pre's noise contribution is low enough that I am now happy to use a ribbon where I might not have in the past.

BRH wrote: Sold the U89s for what I paid.

I should have bought those from you when I had the chance.

Ya, and they were consecutive serial numbers. Now I'm looking at the prices....JEZ..!

But....... I did go over to Audio Rents and rented the AGK426 for $50 for the weekend. Pickup Friday-returned Monday. Can't bet that, and it was in really good shape, not a beater. They threw in the mic stand with extension tubes for free.

Zilla, I've got a couple of Schoeps caps to liquidate.

1 figure 8
1 cardiod or hyper
1 swivel cap extender
2 low cut adapters-70hz and 600hz (Both)
Older ones-grey. you wouldn't want the body, it's T powered.

Had a pair over here and someone rip one off. Now I've got the left overs and using the remaining one in the sound room. PITA to run the T power because everbody leaves the battery on and I'm always looking for fresh 9Vs.

You must be using your laptop and some lightweight aluminum stands.
You put you expensive mics on lightweight aluminum stands!????.
That's some big ones,man.
You actually timed youself?...... this sounds like a Navy drill.

BRH wrote: Zilla, I've got a couple of Schoeps caps to liquidate.

I sold all my phantom powered schoeps, so I can't really use those caps. I will ask around and see if others are interested.

BRH wrote: You actually timed youself?...... this sounds like a Navy drill.

Yeah, that's how much a nerd I am. At least now if I don't set up in time there is not a reactor core meltdown!

BRH wrote: You must be using your laptop and some lightweight aluminum stands. You put you expensive mics on lightweight aluminum stands!????

Royer->RABBIT mic pre->Custom USB interface->Laptop. All battery powered. The stand is light weight so I also carry a small, black sand bag to prevent any tragedies.

OK, Back to the preamp you are building/designing. I've got a question Mr. Zilla. I'm looking at the AEA The Ribbon Pre and the input impedence is 18,000 ohms! What up wit dat? A long time ago (1984) I bought a Valley People QLZ transformerless pre and it's input was 10,000 ohms. I thought that was a little high, but I guess it let the signal flow with ease, and had an open sound. Now Neve has come out with his new Portico
5012 and it's input is 10,000ohms. He's says the higher ohms affects the mic less. BUT, other pres have a 500ohm switch on them, saying that this is 'more correct' for the lower impedence of ribbons. What-up? What are your thoughts on these impedences and how they affect the mic, gain, frequency response and the design of the new GodZilla ribbon pre?

Also, have you used the AMEK 9098 DMA? Could be a real sleeper if found on the used market. Probably cheap.

i have read that the minimum recommended mic to preamp impedance ratio is 1:5, and the optimal is 1:10. Clarifications...?

A good friend, Richard Hess, gave an AES paper on Voltage Transmission Systems for Audio that covers this topic. Check it out on his website:

http://www.richardhess.com/be/aes-80.htm

Peter.

MM, that was interesting. Looks like 50ohm line out can go a long way and arrive at 10-20k input with no loss. I guess that's why you guy/gals are saying to put the pre on stage and run line level.

But mic level goes good into 10 times input. OK. 300 goes into 3k...?
Why the 18k? Then there is something (AEA Website) about Royer ribbions 1800 ohm bass resonance peaks. Is this normal for all mics or just ribbons or just Royer?

Adjustable impedence inputs might be nice. Or has it gotten so esoteric that ever mic should have it's own matched boutique pre?

BRH wrote: Adjustable impedence inputs might be nice. Or has it gotten so esoteric that ever mic should have it's own matched boutique pre?

I have used preamps with adjustable input impedances, very interesting on mics that have a relatively high output impedance (such as ribbons) when driving long cable lengths. The impedance switch acts like a tone control.

I've also had a hand in designing a custom preamp for a specific microphone, with highly satisfying results. But it is not something you can just sit down and do and be done with. There is a lot of testing and refining required because, ultimately, you end up with a circuit that measures correctly, but changing one or two little things here and there changes the tone considerably *when in use with the particular microphone*. The measurements on the bench are still essentially the same or within the same window of deviation, so you have to choose your preference. (You end up trying all the different 'tones' until you find the one you like the most. This is an expensive process, by the way. The stereo preamp I had custom-made for the Royer SF12 costs me nearly $10k US over two years before I was totally satisfied with it. Then Royer brought out the SF24 and I stopped using my custom system...)

I think it's fair to say that for any given mic and cable, there'll be an optimum load impedance. The question for every preamp designer is: how far can you deviate from that optimum-for-one-microphone-and-cable and still get great results across the board? It's always a compromise.

rfreez wrote: i have read that the minimum recommended mic to preamp impedance ratio is 1:5, and the optimal is 1:10. Clarifications...?

These are good rules of thumb, but they're not going to tell you much about how a particular mic *sounds* when faced with a particular load (cable and preamp input impedance).

I'm a bit rusty on this, so correct me if I'm wrong or being too simplistic (I'm a big boy, I can take it), but...

If we consider the microphone to be a voltage source, then its output impedance and the preamp's input impedance form a voltage divider. With a 1:10 impedance ratio, 1/11th of the source voltage will be lost across the microphone's output impedance, with the remaining 10/11ths of the voltage source output appearing across the preamp's input impedance. So a 1:10 ratio might offer the minimum loss you want, with higher ratios being theoretically better. (A 1:5 ratio means 5/6ths of the source voltage actually make it into the preamp, maybe that is too much loss?)

And we have to remember that these are impedances, not necessarily purely resistive, and therefore their behaviour may change with frequency resulting in different tones from different combinations of microphones and preamps. Especially with microphones with passive outputs, such as ribbons and dynamics.

This stuff is easy at line level, where signals levels are relatively high and we try to keep input impedances at least 10x higher than output impedances. It is much harder at mic level because we are facing a few problems. Firstly, the microphone is not a very powerful source and its output is likely to be reactive (rather than purely resistive, especially if it is a passive device like a ribbon or dynamic) and therefore sensitive to what it is connected to; secondly, we'd like a highly sensitive (i.e. high impedance) input to 'reflect' the output of the microphone into the preamp as faithfully as possible with as little loss as possible; thirdly, because the output of the microphone is very small, we need a *lot* of gain to get it up to nominal level. A high input impedance coupled with a lot of gain can be a recipe for disaster in terms of noise and so on. Keeping the input impedance down helps the preamp be a preamp instead of being a radio or a noise generator. It's not ideal for the microphone, but it's ideal for the system.

Compromises, compromises, compromises... All good reasons for having a preamp custom-made for a particular microphone (as if we could afford to do that for every mic in our collection).

I'm assuming that these custom made/design pres are transformerless. Has there been any thought to using a high quailty tranformer on the input to help stablize this fluxuating impedence and help reject unwanted noise? Seems like we'all started going transformerless right around the end of analog tape, I'm guessing to get a getter sound picture to print to tape. But now we have digital and I'm seeing a possible return to transformers.
Kind of like those condenser mics were good with analog, making up for some lost HFs, but now with digital purists, I'm noticing a return to ribbons.
Transformers can sometimes be a dirty word.

BRH wrote: I'm assuming that these custom made/design pres are transformerless. Has there been any thought to using a high quailty tranformer on the input to help stablize this fluxuating impedence and help reject unwanted noise?

My custom-made preamp for the Royer SF12 had two big and lovely Lundahls on the inputs (one for each channel), but they could be switched in or out of the circuit, depending on my needs and tastes. I liked the sound they offered on some jobs, but not on others. And, on some jobs, they were absolutely essential to get a useful signal.

I might also mention that the solid state components of that system remained balanced throughout. It was actually four high quality unbalanced preamplifiers arranged as two matched pairs, FWIW.

Here are two aspects to consider when contemplating preamp input z...

1. The output z of the mic feeding into the input z of the preamp forms a voltage divider. Therefore every interfacing inherently has some amount of passive attenuation. For example: A given mic outputs a 10mV signal through its 50ohm output z. If this is connected to a 2K preamp, this interface essentially forms a pad with an attenuation of 2000/2050 or about -0.22dB. This is good and typically of, say, a scheops looking into a modern pre. But here is another scenario: a Royer's nominal 300ohms looking into a 1k2 input. Now the attenuation is 1200/1500, or -1.9dB. That's a lot of signal loss before we even get any pre-amplification. So you can see, the higher the input z, the lower interface signal loss will be.

2. The other side of the coin is noise. The higher the z on the preamp input, the more noise there will be due to self thermal noise. The good news is that the output z of the mic will be in parallel with the input z of the preamp. So if you have 300ohms//1k2, you have a total z of 240 ohms. With 300//18000 you have 295. The 55ohm difference will contribute a higher self noise.

So it is a trade off. A designer balances signal loss against self noise. Two sides of the signal-to-noise coin.

Zilla wrote: Here are two aspects to consider when contemplating preamp input z...

1. The output z of the mic feeding into the input z of the preamp forms a voltage divider.

SNIP!

2. The other side of the coin is noise. The higher the z on the preamp input, the more noise there will be due to self thermal noise.

SNIP!!

So it is a trade off. A designer balances signal loss against self noise. Two sides of the signal-to-noise coin.

Is there a faint echo in here? :wink:

Ummm... Oh yeah, just like what you already said!!

OK, Zilla, u da man.
So, ya don't like phantom. What about 12V T-powered mics. Reading some old info on Schoeps and Neumann talking about Phantom being an inferior powering source. I've got 2 Schoeps CMT 441 HyperCardiod T-powered, good for location film sound ( I LOVE film, especially those 35s). Do these have blocking caps, or is it the preamp that's got the caps? Modded a Chinese tube mic and took out the caps, they were in the mic.

Too bad the Schoeps aren't matched, at least I don't think they are.
One in the 700nr range, the other 1500nr. Got a chamber to test these?