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#1
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Variable Z Mic Preamps
How is variable impedance being implemented on some of the mic pres
(Vipre, Digital MPA etc) I would guess it would be a dual resistance to ground, but it would have to be on the inboard side of the transformer (or capacitors in a non-transformer design). It seems that all the pres have some set impedance just inside the dc blocking like 10k or so. What range of impedance is appropriate? |
#2
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Variable Z Mic Preamps
R. Foote wrote:
How is variable impedance being implemented on some of the mic pres (Vipre, Digital MPA etc) Some are using multitap transformers. Some are using shunt resistors. I would guess it would be a dual resistance to ground, but it would have to be on the inboard side of the transformer (or capacitors in a non-transformer design). Shunt resistors are not a good thing from a S/N standpoint, so if you are going transformerless, a transformer with a multitap primary is the way to go. It seems that all the pres have some set impedance just inside the dc blocking like 10k or so. Often much lower. Often the input stage is fairly low Z to begin with in a transformerless type. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#3
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Variable Z Mic Preamps
Thanks Scott.
I guess if I can find a multi-tapped primary Jensen transformer I will try refitting my diff amp input with that. (It has coupling capacitors now) Right now, the amps inputs are at 10k, but the transformer will probably want it's own specific secondary impedance? I wonder if Jensen supplies that info when you but a transformer. Obviously, the multi tap transformer is the way to go. Roger Foote |
#4
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Variable Z Mic Preamps
R. Foote wrote:
I guess if I can find a multi-tapped primary Jensen transformer I will try refitting my diff amp input with that. (It has coupling capacitors now) Right now, the amps inputs are at 10k, but the transformer will probably want it's own specific secondary impedance? I wonder if Jensen supplies that info when you but a transformer. Yes. Look at the Jensen web site. Each of the transformers basically want to see a particular load impedance, and in the case of the input transformers, if the load impedance isn't right, they will ring. Output transformers are usually designed to work into a wider range of load impedances, at the expense of efficiency and noise performance, because that's a lot more important in that application. Obviously, the multi tap transformer is the way to go. If you like the sound of transformers. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#6
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Variable Z Mic Preamps
Well I found a transformer (JT-MB-CPCA) that seems suited to work with
my preamp which has a differential input structure. Right now the diff amp is at 20k impedance and this transformer wants 3k, so it would be an easy thing to replace the 2- 10k resistors with 1.5k resistors... I wonder if there is any benefit going with a single ended transformer tied to an AD743 for instance versus retro-fitting this existing differential amp? Of course this is not going to be a variable Z pre, but that seems a little tough to get parts for, and I can't just mess with the secondary loading of the transformer without risk of ringing. Anyway I just want to say thanks for taking the time to help me out on this Scott! Roger Foote |
#7
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Variable Z Mic Preamps
R. Foote wrote:
Well I found a transformer (JT-MB-CPCA) that seems suited to work with my preamp which has a differential input structure. Right now the diff amp is at 20k impedance and this transformer wants 3k, so it would be an easy thing to replace the 2- 10k resistors with 1.5k resistors... Sounds reasonable. Run a square wave through and see what it looks like on the other end. Set the load capacitance and resistance so you get a nice-looking 1 KC square wave on the scope. I wonder if there is any benefit going with a single ended transformer tied to an AD743 for instance versus retro-fitting this existing differential amp? Depends on the stage itself. A differential amp stage, if it's well designed, can give you some linearity benefits over a simple one-transistor input stage. All op-amps basically have a differential front-end going into an output stage (usually push-pull but sometimes single-ended). Of course this is not going to be a variable Z pre, but that seems a little tough to get parts for, and I can't just mess with the secondary loading of the transformer without risk of ringing. Why not? You can see ringing on the scope. You can "select on test" until it looks good on the scope. Anyway I just want to say thanks for taking the time to help me out on this Scott! The Jensen applications notes have a couple nice designs for transformer input preamps with various technologies. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#8
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Variable Z Mic Preamps
I am studying the Jensen schematics as we speak. Thanks Scott and Mike
for the invaluable input! |
#9
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Variable Z Mic Preamps
Well I ordered a Sowter #3678 multi-tap mic transformer. Tapped at 50,
200 and 600 ohms. Wish it had some higher taps (1k or so) but at least it has some variation. All of the Jensens seem to have between 1.2 and 1.4k primaries, so if a person was to put a resistance3 across pins 2 and 3, it would lower the impedance even more.. So maybe 600 ohms won't be so bad. I think I will try a transformerless design with variable z. I guess Panasonic bipolar caps would be best? |
#10
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Variable Z Mic Preamps
R. Foote wrote:
Well I ordered a Sowter #3678 multi-tap mic transformer. Tapped at 50, 200 and 600 ohms. Wish it had some higher taps (1k or so) but at least it has some variation. the people who do it "right" (Great River, myself, certainly others) simply use a relay to strap a dual-primary transformer for series or parallel connection. This gives you nominally 150 or 600 ohm impedance. I make a circuitboard that handles this function along with polarity switching, -20dB pad, phantom power, and XLR input jacks all on a 3x3" 2-channel board. http://www.rollmusic.com/projects/3125.shtml All of the Jensens seem to have between 1.2 and 1.4k primaries, so if a person was to put a resistance3 across pins 2 and 3, it would lower the impedance even more.. So maybe 600 ohms won't be so bad. Ultimately these numbers are pretty meaningless most of the time. A transformer doesn't have an actual impedance of its own, only an impedance ratio. So the actual source and load impedances are dependent on the microphone, the transformer ratio, and the amplifier. Along with any terminating resistance and/or capacitance. I think I will try a transformerless design with variable z. I guess Panasonic bipolar caps would be best? Transformerless preamps with a variable input impedance are simply a bad idea if you ask me. Unles you come up with some very sophisticated circuit to vary the actual input impedance of the amplifier, then you're forced to use loading resistors which throw away precious mic-level signal voltage and really defeat the purpose of the whole concept. The reason impedance-variance works so well with transformers is that varying the ratio lets you trade impedance for gain without a noise penalty. You still have to face the limitations of high-ratio transformers, but if you're buying new transformers from the likes of Jensen, Sowter, or Lundahl then you've got a big technology advantage over the iron of yore which suffered phase, frequency, and distortion penalties in high-ratio arrangements. ulysses |
#11
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Variable Z Mic Preamps
R. Foote wrote:
I think I will try a transformerless design with variable z. I guess Panasonic bipolar caps would be best? What are you going to do with the bipolar caps? ulysses |
#12
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Variable Z Mic Preamps
Justin Ulysses Morse wrote in message ...
R. Foote wrote: I think I will try a transformerless design with variable z. I guess Panasonic bipolar caps would be best? What are you going to do with the bipolar caps? ulysses The de-coupling caps in a transformerless preamp's input structure. Thanks |
#13
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Variable Z Mic Preamps
R. Foote wrote:
Justin Ulysses Morse wrote in message ... R. Foote wrote: I think I will try a transformerless design with variable z. I guess Panasonic bipolar caps would be best? What are you going to do with the bipolar caps? The de-coupling caps in a transformerless preamp's input structure. Decoupling or coupling? Decoupling caps should be the last thing to worry about. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#14
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Variable Z Mic Preamps
Scott Dorsey wrote:
R. Foote wrote: The de-coupling caps in a transformerless preamp's input structure. Decoupling or coupling? Decoupling caps should be the last thing to worry about. Perhaps he's referring to DC blocking caps, to keep the phantom supply out of the inputs. Non-polar would be good for when the phantom power is off. Maybe an input amp that can run with +48 on its inputs could even eliminate those. After all, it *is* common-mode. |
#15
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Variable Z Mic Preamps
Steve O'Neill wrote:
Scott Dorsey wrote: R. Foote wrote: The de-coupling caps in a transformerless preamp's input structure. Decoupling or coupling? Decoupling caps should be the last thing to worry about. Perhaps he's referring to DC blocking caps, to keep the phantom supply out of the inputs. Non-polar would be good for when the phantom power is off. Maybe an input amp that can run with +48 on its inputs could even eliminate those. After all, it *is* common-mode. For input blocking caps, I see no reason not to just go with huge mylars. Depends on your input Z of course, but 22 uF is plenty. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#16
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Variable Z Mic Preamps
Howdy Steve...
I meant DC blocking. That's the trouble with looking at a schematic for one thing and talking about something else. Scott, I might try the mylar idea. I got to thinking about non-polars from a recommendation from Deane Jensen's application notes. thanks all! |
#17
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Variable Z Mic Preamps
In article ,
R. Foote wrote: I meant DC blocking. That's the trouble with looking at a schematic for one thing and talking about something else. Scott, I might try the mylar idea. I got to thinking about non-polars from a recommendation from Deane Jensen's application notes. The upside is that they'll last forever, unlike an electrolytic. The downside is that they're physically large and somewhat costly. The physical size can cause some problems with coupling between different parts of a circuit, especially if you're retrofitting film caps into an existing circuit, floating them above the circuit board. If you're making a new layout, this isn't a problem at all, but make sure not to run other stuff near the input caps, especially the amp's output traces. Finally, IMHO, mylar won't sound as good as a fresh electrolytic, but a better dielectric like polypropylene will. 10uF is probably the bare minimum size and 22uF will be enough. Best of luck, Monte McGuire |
#18
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Variable Z Mic Preamps
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#19
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Variable Z Mic Preamps
In article ,
R. Foote wrote: What are your thoughts on solid tantalum? They're pretty fragile and they _must_ be run forward biased. If phantom is _always_ on, then one might be able to get away with using one as an input blocking cap, but they are very very picky about being reverse biased. The only place I'll use them nowadays is in single supply, class A amplifiers as cathode or emitter bypasses. They have an interesting sound there... But, even if you think it can almost always be forward biased, consider what happens when it fails in a mike pre. Your preamp front end or the mike or both might get seriously annoyed by a randomly shorted out component that's biased up by P48. That could be pretty expensive. For this application, phantom power DC blocking, I'd never consider a tantalum electrolytic. It's just too risky, and 40uF total of 60V high quality (Kemet brand) tantalum is pretty darn expensive anyway. These days, a nice Panasonic FC (or the obsolete HFQ) series aluminum electrolytic seems to make the most sense. You'll probably have to swap them out every 5-10 years or so, but they do sound good and you can get a lot of capacitance in a small place. In some cramped layouts, the small size of an aluminum coupling cap makes the circuit work better than a film, if only because the thing isn't marginally unstable from random coupling to other parts of the circuit. For a new design where you have the space to avoid coupling problems, a quality metallized polypropylene (or two or three) would work quite well. Use 10-20uF per input leg at the smallest voltage rating you can get - probably 100V but maybe you could find 63V rated parts (which would be smaller). The best caps are noninductively wound and use pure copper leads (not copper clad steel). You can go for MultiCaps and other exotica, but I see no reason as long as the film is clean, the foil is clean and the cap is put together competently. A simple dielectric absorption test seems to weed out the bad caps and it's easy to do. Take a 9V battery, charge up the cap, remove the battery, short the cap for exactly one second and then measure the voltage that ends up across the cap. It should be in the millivolt range with a good cap, the lower the better. BTW, this is an easy way to identify the type of dielectric used in a cap when you're at a hamfest or surplus store. A 9V battery and a cheapo multimeter work just fine. You'll find that polystyrene, polypropylene and teflon have the lowest recovery voltage, polycarbonate are higher and mylar is higher still. I've seen some defective polypropylenes that read high with this test, so it seems to be a good thing to do. Best of luck, Monte McGuire |
#20
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Variable Z Mic Preamps
Monte P McGuire wrote:
These days, a nice Panasonic FC (or the obsolete HFQ) series aluminum electrolytic seems to make the most sense. For a new design where you have the space to avoid coupling problems, a quality metallized polypropylene (or two or three) would work quite well. Use 10-20uF per input leg at the smallest voltage rating you can get - probably 100V but maybe you could find 63V rated parts (which would be smaller). The best caps are noninductively wound and use pure copper leads (not copper clad steel). I was wondering about this lately while flipping through the Digikey catalog. Other than the fact that it's usually the better caps that they bother using OFC leads for, how much does it really matter if the leads are copper-clad steel? I mean obviously copper is a better conductor than steel, is the difference measurable? In any event, taking a small, strong magnet to the suprlus shop sounds like a good idea as well. I went through my capacitor bin with a magnet recently and found about half the stuff I had on hand was copper, half steel. Not counting electrolytics, which are all steel. Is the steel cheaper, or is it a matter of current handling? A simple dielectric absorption test seems to weed out the bad caps and it's easy to do. Take a 9V battery, charge up the cap, remove the battery, short the cap for exactly one second and then measure the voltage that ends up across the cap. It should be in the millivolt range with a good cap, the lower the better. BTW, this is an easy way to identify the type of dielectric used in a cap when you're at a hamfest or surplus store. A 9V battery and a cheapo multimeter work just fine. You'll find that polystyrene, polypropylene and teflon have the lowest recovery voltage, polycarbonate are higher and mylar is higher still. I've seen some defective polypropylenes that read high with this test, so it seems to be a good thing to do. Good idea. But I bet it's very hard to short each cap the exact same amount of time. Differences you measure could be due to timing errors. It's interesting to note how in general, all the relevant properties of capacitors tend to follow one another among different dielectrics. Mylar, polypropylene, polystyrene, teflon seems to be an ascending order of preference almost no matter the qualitative parameter. Size and cost go the other direction. Monte, what do you know about the PPS film caps, such as the Panasonic ECHS series? I gather there's been some supply disruptions due to materials availability. But are they worth it? ulysses |
#21
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Variable Z Mic Preamps
Justin Ulysses Morse wrote:
Monte P McGuire wrote: These days, a nice Panasonic FC (or the obsolete HFQ) series aluminum electrolytic seems to make the most sense. For a new design where you have the space to avoid coupling problems, a quality metallized polypropylene (or two or three) would work quite well. Use 10-20uF per input leg at the smallest voltage rating you can get - probably 100V but maybe you could find 63V rated parts (which would be smaller). The best caps are noninductively wound and use pure copper leads (not copper clad steel). I was wondering about this lately while flipping through the Digikey catalog. Other than the fact that it's usually the better caps that they bother using OFC leads for, how much does it really matter if the leads are copper-clad steel? I mean obviously copper is a better conductor than steel, is the difference measurable? The argument is that the steel/copper junction makes a little semiconductor and causes a little bit of nonlinearity at low levels. I don't know about cap leads, but I know I can tell the difference in sound between a couple hundred feet of RG-174 which has a steel-clad center conductor, and a run of a similar copper-core cable. There's a difference between a couple hundred feet and a 1/8 long cap lead, though. In any event, taking a small, strong magnet to the suprlus shop sounds like a good idea as well. I went through my capacitor bin with a magnet recently and found about half the stuff I had on hand was copper, half steel. Not counting electrolytics, which are all steel. Is the steel cheaper, or is it a matter of current handling? The steel is cheaper and easier to manufacture. Also since it is stiffer, it's less apt to get deformed in transit and jam-up on pick and place machines. All of the parts you buy today are basically designed for machine insertion. A simple dielectric absorption test seems to weed out the bad caps and it's easy to do. Take a 9V battery, charge up the cap, remove the battery, short the cap for exactly one second and then measure the voltage that ends up across the cap. It should be in the millivolt range with a good cap, the lower the better. BTW, this is an easy way to identify the type of dielectric used in a cap when you're at a hamfest or surplus store. A 9V battery and a cheapo multimeter work just fine. You'll find that polystyrene, polypropylene and teflon have the lowest recovery voltage, polycarbonate are higher and mylar is higher still. I've seen some defective polypropylenes that read high with this test, so it seems to be a good thing to do. Good idea. But I bet it's very hard to short each cap the exact same amount of time. Differences you measure could be due to timing errors. I strongly recommend the ESR meter from Dick Smith electronics. Doesn't do real DA tests, but it does a great job for selecting capacitors and also for in-circuit capacitor testing. I have been amazed how much easier troubleshooting is with that thing. It's interesting to note how in general, all the relevant properties of capacitors tend to follow one another among different dielectrics. Mylar, polypropylene, polystyrene, teflon seems to be an ascending order of preference almost no matter the qualitative parameter. Size and cost go the other direction. Monte, what do you know about the PPS film caps, such as the Panasonic ECHS series? I gather there's been some supply disruptions due to materials availability. But are they worth it? The PPS films aren't so bad. The polycarbonate stuff was wonderful... density almost as good as mylar, sound quality and DA specs almost as good as polystrene, and low price. Then the one manufacturer of the film raised his prices and most folks discontinued them and the ones that are left are not cheap any more. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#22
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Variable Z Mic Preamps
Steve O'Neill wrote:
Perhaps he's referring to DC blocking caps, to keep the phantom supply out of the inputs. Non-polar would be good for when the phantom power is off. Maybe an input amp that can run with +48 on its inputs could even eliminate those. After all, it *is* common-mode. FWIW, I played around with the idea of letting the 48Vdc just ride on through to eliminate the capacitors, but I could never achieve the necessary CMRR to avoid contaminating the audio. My latest attempt involves dropping the 48Vdc down to about 5Vdc, since a smaller voltage ought to be easier to make disappear, or at least not cause problems at the first stage output. Bill |
#23
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Variable Z Mic Preamps
In article ,
Justin Ulysses Morse wrote: I was wondering about this lately while flipping through the Digikey catalog. Other than the fact that it's usually the better caps that they bother using OFC leads for, how much does it really matter if the leads are copper-clad steel? I mean obviously copper is a better conductor than steel, is the difference measurable? As Scott mentioned, the possibility of a nonlinear junction exists, and also at the very least, you have a little bit extra inductance from the ferrous lead material. Probably not a big deal, but what the heck. I have used some of the Panasonic caps with copper clad steel leads and they seem to work OK, but I'm not crazy about the concept. Good idea. But I bet it's very hard to short each cap the exact same amount of time. Differences you measure could be due to timing errors. Try it out and see. When you short out the main charge on a cap, the secondary charge doesn't get shorted out right away and it slowly recovers in a few seconds. So, the amount of recovery voltage is not all that dependent on how long you short out the cap. In practice, I found it good enough to identify different dielectrics. And heck, you're an engineer, right? You've probably got a good enough sense of timing from punching in tape machines, so you can probably short some leads pretty consistently. It's interesting to note how in general, all the relevant properties of capacitors tend to follow one another among different dielectrics. Mylar, polypropylene, polystyrene, teflon seems to be an ascending order of preference almost no matter the qualitative parameter. Size and cost go the other direction. Monte, what do you know about the PPS film caps, such as the Panasonic ECHS series? I gather there's been some supply disruptions due to materials availability. But are they worth it? PPS is useful because it can withstand the heat of surface mount reflow. Most all of the other dielectrics can't, although I'm hoping for a Teflon comeback for this reason alone... ;-) I don't know how good it sounds, but it may be OK... never tried it yet. The numbers aren't fantastic, but it may still sound OK. Best of luck, Monte McGuire |
#24
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Variable Z Mic Preamps
In article ,
Justin Ulysses Morse wrote: I think it makes sense to build separate preamps for use with and without phantom power. Build a transformerless preamp with phantom power (and electrolytic blocking caps) for use with condensers, and you never have to turn off the phantom power, so the caps are always biased. Build a separate preamp for dynamics and eliminate the coupling caps entirely in that one. A great idea... there's no cap better than no cap at all. The design challenge in a mic preamp is trying to accommodate such a huge range of input signals without sacrificing performance. The trade-off between input overload and maximum gain is a tough one when some mikes put out 5mV and others put out 5V. I think if you're building your own mike preamps, you'll get much better results if you don't make compromises to accommodate every imaginable microphone with one preamp design. A BK-5 has different needs than a TLM103. If you're building preamps for your own personal use, it makes sense to think about what you're going to use them with and build them accordingly. For example, you could build one preamp with a hefty step-up transformer and no phantom power on the input and a gain range of 50-70dB just for use with low-output ribbons on vocals. You could build another preamp that would have no gain at all, just phantom power (and maybe a JT-16B input transformer), for use with ultra-sensitive mikes like the TLM103. The whole point of DIY is you get exactly what you want and need. You just have to decide what that is. Great ideas here - worth quoting again... For 'colorless' preamps, perhaps this isn't so important, because it's not that hard to get a similar tone across a wide input / gain range if you're striving for complete accuracy. Of course, reality sets in and nothing's all that colorless, but you get my drift: many of the transformerless designs are pretty much uniform sounding across their gain range, as long as you aren't asking for gobs of gain. But... for deliberately colored preamps, it is definitely worth optimizing a preamp to a mike to an input signal. Many of the preamps I use for lead vocals have a sweet spot that isn't all that wide, and if you're lucky enough to get a singer and mike and preamp that hits the right spot, things work really well. If you fall out of that level/gain pocket though, the preamp won't sound very good at all... There are some topologies that help you widen that sweet spot, and this can sometimes help out a lot. The A-Designs preamp does this quite well. It's a fixed gain amp preceded by a volume control and a transformer, so you can hit the sweet spot pretty easily by adjusting the attenuator after the input tranny. Turn the pot up for more drive, turn it down for less. Pretty simple. I made some low gain mike preamps designed for drum use that use a similar design - a fixed gain amp driven by a transformer feeding a pot. It's a great idea for a limited range amp - this thing is designed for drum use, so maximum gain is pretty low, around 30dB or so. But, minimum gain is also quite low (negative infinity, because of the input pot), so it's easy to handle high outputs from close mikes on loud drums. The only thing left 'flapping in the breeze' is the input transformer, which has to handle the full mike input signal. Fortunately, there are quite a few low to medium ratio transformers that can handle high input levels without problems. I used the UTC A20 as the input tranny for this preamp, wired as a 1:2 step up, and it can handle +15dBm according to the manufacturer. In practice, it's worked just fine. Another concept related to all of this is "what are you tracking to" and what levels it can deal with. When I ran analog tape, the signals I asked from my mike preamps were a good bit lower than what I ask of them when tracking to a modern ADC. This means that if you have a signal at a preamp's sweet spot that causes an output of +10dBV, then it might be tough to deal with this preamp in an analog tape situation, but it might work just fine with digital where converter sensitivities are a good bit lower. Sometimes it's worth recalibrating a converter to a different sensitivity to deal with a specific preamp's sweet spot. As you said initially, all this stuff is under your control if you make your own preamps, but also keep in mind that the same gain staging concepts apply to storebought electronics too. Evaluating a preamp has to include deciding what sort of input range the pre is happy with and what sort of output levels it's best at providing, and somehow scaling that to your recording devices with pads, trims or clever console use. All the best, Monte McGuire |
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