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New Preamp - Versatility Dilemma? #3017355 11/25/19 07:16 PM
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KuruPrionz Offline OP
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This weekend I worked a win/win craigslist trade (with cash on my end) for a Focusrite ISA One preamp. I've done basic testing, the unit is very clean and well kept. It works fine.

I now have options that were previously unavailable. In my experience, lots of options always sounds good on paper but can become a bit of a PITA in the real world if you are both the Artist and the Engineer.
One of the reasons I love my Cloudlifter CL-1 is that there are only two options, plug it in or don't plug it in. The process of testing is very simple and easy to remember. So far I've preferrred it on every mic I've tried.

The ISA One offers 4 switchable impedence settings. The manual describes how these settings change the tone and output. I plan on learning what these settings actually do for some of my mics soon. I've narrowed down my first test subjects to three mics for now - Shure KSM8, CAD D-82 and CAD Equitek E-100 (2). Those three are more than enough to start with!!!! Eventually I'll test a few other mics I've picked up along the way.

Next "Big Deal" option was an unexpected bonus. My Presonus Quantum interface sees all XLR plugs as mic inputs and re-configures internally to use it's own mic pres. These are clean, quiet and have good headroom, I like them.
The ISA One has an XLR out, meant to be a Line Out. I can either use the ISA One as a sort of "uber-versatile Cloudlifter" into the mic pres or use my XLR to TRS pigtail and bypass the internal preamps. Both choices have potential.

Now we have 4 x 2=8 options per mic x3 mics to 24 options.

There is an insert in the ISA One with a bypass switch. I've gotten good results using an FMR RNC in Super Nice mode with a 1:4 ratio pre conversion. It makes it easier to get good vocal take keepers with inexperienced vocalists while being almost impossible to notice on playback. It can increase the noise floor a bit, sometimes that is a worth compromise. I haven't tried it with the new pre. That's 2 options, on or off.

2 x 8 = 16 options per mic x 3 = 48 options.

That's just with the ISA One. It is situational but there is also the Hi-Pass filter (x2) and the possibiltiy of using the Cloudllifter in front of the ISA One when running Line In (x2).

32 or 64 options per mic depending and 96 or 192 options if both Hi-Pass and Cloudlifter are included.

I am just going to start simple, with the Shure KSM-8 and the ISA One running Line In. The 4 impedence settings and compressor on/off is plenty for now!!!!!
And yes, I get that some of these may be "splitting hairs" but I won't know unless I try them. I think once I understand the basics I'll be good to go without testing every single option.

I plan on writing down just my favorite settingd for each of the 3 mics and posting that on my desk.
I suspect my brain hurts more from thinking about it than it will from doing the testing.

Thoughts, feedback? Cheers, Kuru

PS, I have another experiment pending regarding transformer saturation, just got sidetracked. I'll get to it!!!!



There is never enough time to be in a hurry...
Re: New Preamp - Versatility Dilemma? [Re: KuruPrionz] #3017478 11/26/19 03:56 PM
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Mike Rivers Offline
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Originally Posted by KuruPrionz
This weekend I worked a win/win craigslist trade (with cash on my end) for a Focusrite ISA One preamp.

I now have options that were previously unavailable. In my experience, lots of options always sounds good on paper but can become a bit of a PITA in the real world if you are both the Artist and the Engineer.


Yup - the more choices you have available, the longer it takes you to complete the project. . . at least until you learn enough about what those various options do, and can have confidence that you're making your first choice be the best choice. That takes time and experimenting, so it's good that you're getting started on the right track.

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The ISA One offers 4 switchable impedence settings. The manual describes how these settings change the tone and output. I plan on learning what these settings actually do for some of my mics soon. I've narrowed down my first test subjects to three mics for now - Shure KSM8, CAD D-82 and CAD Equitek E-100 (2).


The first (and only) microphone review that I wrote was for the CAD Trion series of mics, a conventional dynamic, a condenser, and a ribbon. I had just gotten a Mackie Onyx 800R preamp with an input impedance switch on its first two channels, so I was eager to experiment with that switch as well, particularly since at the time the rumor going around (which turns out to be false) is that ribbon mics like to work into a low impedance because they're inherently very low impedance, and I didn't have any other ribbon mics at the time other than Beyer M260s, which are so much not like conventional ribbons that they don't count. Anyway, what I found was that the CAD ribbon in this lot actually did sound best when loaded with about 300 ohms, the Mackie's lowest impedance choice. This mic sounded rather flabby on the low end when used with a more conventional 2 kΩ or so preamp, but the lower impedance tended to dampen the motion of the ribbon and the low end cleaned up nicely. Paul Stamler did some experimenting with an SM57 in a class he was teaching, the outcome of which was that when running into 500 Ω. the SM57 sounded like a completely different and much better mic.

The preamp's input impedance works on a mic in several different ways. Most noticeable is a drop in level. First, though, let me point out that most variable input impedance preamps do the trick by just shunting a resistor across the input terminals. Resistance is only part of "impedance" but, when varied, usually does change the sound coming out of the preamp. This is pretty obvious when you think about Ohm's Law - lowering the load impedance causes more voltage to drop across the mic's output transformer or whatever is between the microphone element and the output connector. It's less apparent with transformerless condenser mics than any other type of mic because the output stage of the mic's internal amplifier is a buffer that provides enough current to work just fine into 100 Ω or less. Put a transformer in there though (and remember that nearly all dynamic mics have a transformer and they all have a coil of wire which has inductance) and you get the inductance combining with the load resistance to create a filter that changes the frequency response of the electrical circuitry with a changing load. This may or may not be significant depending on the design of the transformer. But, like with the CAD ribbon mic that I was playing with, transformers can ring, and finding the proper load impedance can be the best compromise between reducing the signal level and damping the ringing of the transformer.

The alternative to a resistor across the preamp's input terminals is a transformer with multiple taps. I don't know what method the Focusrite ISA uses, but it's expensive enough so that it could be a tapped transformer rather than resistors. And, by the way, while your Cloudlifter doesn't have any switches on it, the Cloudlifter-Z has a knob that adjusts the load from the natural 3.5 kΩ or so down to a couple of hundred ohms. So much for simplicity!

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Next "Big Deal" option was an unexpected bonus. My Presonus Quantum interface sees all XLR plugs as mic inputs and re-configures internally to use it's own mic pres.


Well, sort of, unless they're doing something really different with this interface. Typically, interfaces of this type that use XLR-TRS "combo" input connectors just put an attenuator network (a couple of resistors) between the 1/4" jack that's in the center of the XLR jack and the preamp's input. That's actually not a bad way to do it, particularly if the preamp circuit is well designed, but so many people were complaining that they didn't want another stage of electronics in line with their $1000 preamp. So what PreSonus did with the Quantum was to right it so that when you plug into the 1/4" input, the output of the preamp stage gets disconnected from the input of the A/D converter and the signal from 1/4" jack replaces it. But the XLR inputs are mic level (and mic impedance).

Never mind . . . I see from your next sentence
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The ISA One has an XLR out, meant to be a Line Out. I can either use the ISA One as a sort of "uber-versatile Cloudlifter" into the mic pres or use my XLR to TRS pigtail and bypass the internal preamps. Both choices have potential.

that you understand this.


Quote
I am just going to start simple, with the Shure KSM-8 and the ISA One running Line In. The 4 impedence settings and compressor on/off is plenty for now!!!!!


When conducting your experiments, don't be tempted with other possibilities. If you want to learn what your mics sound like when switching impedances, make sure you don't change anything else. Assuming you'll be recording something and then listening to the playback, probably the best signal path would be from the ISA line out to a line in on the PreSonus. And while you're at it, pull the ISA out of the circuit and connect the mic you're working with directly to the PreSonus preamp. That will give you one more data point, which isn't going to be too much of a burden. You might find that for certain things, with certain mics, the PreSonus by itself gives you a more useful (as in you don't have to fool with it to get it to mix right) recording.

When you're evaluating the recordings, remember that the level is going to change when you change impedance, so now you have another choice - do you change the gain on the preamp to give you the same recording level at whatever impedance you're testing, or do you leave the preamp gain knob untouched and adjust the playback level when you're listening? You might consider setting up a small speaker that you can feed with pink noise, and play the noise into the mic under test from a measured distance between mic and speaker so you have a standard for overall sensitivity. If you record a short blast of noise before recording whatever you want to use as a test source for the mic, you can apply a volume adjustment to the DAW tracks so that the playback volume of the noise matches up. A digital volume change is one of the things you can do in a DAW that only affects one things.

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PS, I have another experiment pending regarding transformer saturation, just got sidetracked. I'll get to it!!!!



One thing at a time, please. wink

Re: New Preamp - Versatility Dilemma? [Re: Mike Rivers] #3017491 11/26/19 05:48 PM
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KuruPrionz Offline OP
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Originally Posted by Mike Rivers
[I don't know what method the Focusrite ISA uses, but it's expensive enough so that it could be a tapped transformer rather than resistors. And, by the way, while your Cloudlifter doesn't have any switches on it, the Cloudlifter-Z has a knob that adjusts the load from the natural 3.5 kΩ or so down to a couple of hundred ohms. So much for simplicity!
One thing at a time, please. wink


Thank you Sir Mike for the informative post!!! I really appreciate it, this is why I love to "waste time" in forums.

Yes, the Focusrite does have a multi-tap Carnhill transformer in the front end. It is supposed to be a "to spec" copy of the transformer in a Neve circuit, this is one channel of a mixer from a noted British recording studio. At least, that's the bling. All the interactive "push-pull" of coil based inductors and coil/magnet based transducers does seem to add a certain pleasantry to our tones. Tube driven guitar amps owe more to their tone from the battle between the output transformer and the speaker (a dynamic mic in reverse) than they do from the tubes themselves. My first Mesa Boogie had a line out that bypassed the OT/speaker connection and it sounded like a cheap solid state amp distortion box thingie. Ugh, nasty!

I'm familiar with the Cloudlifter line and would buy more of their goodies. I'm good for now, already enough distractions!! :- D

A circuit that is switched by the jack is still the Quantum reconfiguring internally, no? The gain controls disappear for line inputs, one less variable.
The tone generator is a good idea for consistency. I had a 1khz generator used to test phone lines for a job installing phone systems. Pretty sure it's long gone.
Seems like a cheap Casio or Yamaha thrift store keyboard could provide a reasonably consistent signal in one of the organ voices. Maybe I'll pick one up, doesn't need to be fancy. I could just run it into a DI box to tweak mic gain, as long as not settings or gear is changed it should be "good enough" to keep levels consistent when the preamp impedence is switched.

Yes, I've already created a session in my DAW for recording mic tests. Will use the same procedure with each mic so I have less labeling to do. Name of mic on first run, done. The 3 additional impedence tests below the labelled track will be for the same mic, running from lowest impedence to highest. I'll add a Presonus preamp track to each mic as well, I do like their pres. Not "character" pres but they sound really nice and have lots of headroom with a very low noise floor.

Simple, 16 total tracks should tell me a great deal. Refinements beyond that can be applied to favored impedence settings. Process of elimination, I'll miss a few things that way but I'll get to a "let's record!!!!" place sooner. In the end, that's where I want to be. Cheers, Kuru

PS, sometimes when I go to Baskin Robbins, I just want vanilla!!!! :- D


There is never enough time to be in a hurry...
Re: New Preamp - Versatility Dilemma? [Re: KuruPrionz] #3017592 11/27/19 01:35 PM
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Mike Rivers Offline
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Originally Posted by KuruPrionz

Yes, the Focusrite does have a multi-tap Carnhill transformer in the front end. It is supposed to be a "to spec" copy of the transformer in a Neve circuit, this is one channel of a mixer from a noted British recording studio. At least, that's the bling. All the interactive "push-pull" of coil based inductors and coil/magnet based transducers does seem to add a certain pleasantry to our tones.

Tube driven guitar amps owe more to their tone from the battle between the output transformer and the speaker (a dynamic mic in reverse) than they do from the tubes themselves. My first Mesa Boogie had a line out that bypassed the OT/speaker connection and it sounded like a cheap solid state amp distortion box thingie.


I'm not aware of a studio console with multi-tapped transformers on the mic inputs, but there were a lot of custom built consoles.

The output stage of a tube guitar amplifier is indeed a significant contributor of the "tube sound" but the input stages also contribute to the sound. Overdriving the input tends to produce odd harmonics, while a push-pull tube output stage, when pushed hard, tends to produce even order harmonics. That's why there's a "master gain" setting - so you can run the input stage clean and push the output stage, run the input stage dirty and amplify that fairly cleanly, or get some of each flavor of distortion.

When it comes to small-signal tube-based devices like compressors, equalizers, and mic preamps, transformers indeed contribute more than the tubes to the "tube sound" that they're famous for. More often than makes sense people use there devices with all the controls set for minimum processing just to get the "tube sound" without messing with dynamics or frequency response.

Have you run across the Whitestone Audio P331? It has two 6SN7s and a couple of transformers and costs $5000. Famous mastering engineers (one of whom is the designer's wife) love it because it's so subtle and hardly changes the sound at all. At least it has some knobs and switches that change where the signal lies along the tube's operating curves.

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A circuit that is switched by the jack is still the Quantum reconfiguring internally, no? The gain controls disappear for line inputs, one less variable.


Semantics, not worth arguing about. Basically, it just disconnects the preamp circuit, which is where the gain control is located, so it won't contribute any noise and won't load the line level input. "Reconfiguring" to me means something like switching a pentode so that it runs as a triode to provide two different "tube sounds." Whatever.

Quote
The tone generator is a good idea for consistency. I had a 1khz generator used to test phone lines for a job installing phone systems. Pretty sure it's long gone.
Seems like a cheap Casio or Yamaha thrift store keyboard could provide a reasonably consistent signal in one of the organ voices.


Got a smart phone? There are signal generator apps. Or got a spare computer for an off-line tone generator? Here are a couple of places where you can download WAV or MP3 files:

https://www.wavtones.com/functiongenerator.php
https://www.audiocheck.net/testtones_index.php

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PS, sometimes when I go to Baskin Robbins, I just want vanilla!!!! :- D


Do they still have jamoca almond fudge?

Re: New Preamp - Versatility Dilemma? [Re: KuruPrionz] #3017651 11/27/19 06:57 PM
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KuruPrionz Offline OP
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I need to read more carefully before posting, my apologies.

Your smartphone suggestion is excellent, thanks!

I've done some reading and re-reading. The transformer is actually a Lundahl, not a Carnhill. It may be a "clone" but I didn't find that information. I suspect some of what I posted above is a lack of careful reading and some is "internet information". :- D

A search for info on the transformer does not lead to any information about multiple taps. The Focusrite users manual and spec sheets do not indicate it.
So it probably is as you have said above, using a shunted resistor. I may pop the lid someday but not any time soon.

Focusrite is saying that the ISA preamps were used in a commissioned console for AIR studios (George Martin's place), that layout included EQ, which the One does not have.
http://www.airstudios.com/about-us/history/
There are 2 AIR studios, one built in 1969 and one in 1991, I didn't see which one had the commisioned console but guessing the later iteration.
The ISA One is supposed to be based on part of that circuit.

I'm done doing research, I just want to learn to use what I have now to it's best potential. Simple experiments will get me there.

This all started because the lack of a HPF on my FMR RNP seemed highly unsatisfactory for my purposes. I emailed them, got an answer from Mark McQuilken, the founder/owner. Nice guy, very open to the observations of his user base.
I want to insert a compressor - FMR RNC - pre-conversion and I don't want to compress all the useless low frequencies that are generated by proximity effect on microphone when tracking vocals.
The Focusrite allows me to do that, was the primary reason I picked it up.

The versatility is more or less another "beautiful distraction" at this point. Once I have it sussed out I am sure there will be benefits. If the Quantum had pre-conversion channel inserts and an HPF I would just use that and be totally fine with it.

Here is Hartley Peavey's white paper on his work with developing Transtube guitar amps. I found it interesting, you may as well:
https://peavey.com/support/whitepapers/Chapter3.pdf

FWIW, I have thousands of gigs behind me as a guitarist in a variety of popular dance music bands. I've owned at least 7 Mesa amps, a variety of "clones" "boutique" and straight up vintage Fender, Supro, etc. in the 40 years I've been gigging. I've played many gigs with Peavey Transtube amps and been very happy with the tones compared to actual tube amps. Currently using a Boss Katana Combo 100 as it has made my life much simpler without sacrificing tone.

Tube amps may be reliable but experience tells me that tubes are not. When I am booked to play, I just want to play - not trouble shoot gear.

This weekend is already gone and I haven't gotten there yet!!!! I'll return when I know more about my test results. Cheers, Kuru


There is never enough time to be in a hurry...
Re: New Preamp - Versatility Dilemma? [Re: KuruPrionz] #3018238 12/03/19 05:39 AM
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Tonight I hooked up the mic signal chain I wanted to test. I decided to do an overview tonight and go deeper when I am recording.

Mics tested were Shure KSM8 - dynamic, CAD D82 - ribbon and CAD Equitek E100 (2) medium diameter condenser.

I used a Focusrite ISA One preamp with an FMR RNC inserted. The ISA has an Insert switch.

As discussed, I used an XLR out to TRS iline in, Presonus Quantum. Obviously, recording my tracks and listening back is the only way to get a more subjective test result. I chose to zip through these with headphones on and get an idea what I am working with.

I tried each mic with all 4 impedence settings, HPF on and off, RNC inserted or not. Quite a few combinations!

I also tried out Mudder side address pop filters, see my thread on those. Very useful.

I didn't hear anything that I hated.

The D82 without the HPF would be fantastic for drums but too much bass for vocals. Easy fix, sounded smooth, full and lively. I am pleased. The 4 impedence settings all made a noticeable difference, the output certainly increased when the resistence increased. The Med setting seemed to boost both output and low frequency response, it sounded huge compared to what I am used to sounding like.

The KSM8 provided a great "intelligebilty", pronounciation was clear without being overly sensitive to sibilence or plosives. I tried it with and without a Blue pop filter about an inch off the grille. It does clean it up a bit, not sure it's needed.
I'll test a foam windscreen, that was nice for my now-broken Shure Beta 87a. The KSM8 is a great addition, one of the best dynamic mics ever.

The E100 (2) with the Mudder pop filter provided a clear, full, present tone and was not at all harsh or over-accentuated in the upper frequencies. It responded well to all impedence settings and the HPF.

For where I am and what I am doing, I am delighted to have these tools. I haven't had a sound this good before, I am excited to jump in and make some music.

I've also been treating my room, there is unavoidable exterior noise. I hit thrift stores every week. I've been snagging cheap mic stands and lighting stands, curtain rods and large puffy quilted blankets. Easy to set up and take down, by the windows they really reduce the noise geting in. I need to put at least one more up and a sign to remind me to turn the refrigerator back on. Little things but they will bug you if you leave them!!! Cheers, Kuru



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Re: New Preamp - Versatility Dilemma? [Re: KuruPrionz] #3018565 12/05/19 05:50 PM
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Last night I plugged a bass into the DI. Another XLR out that is Line Level, with the mic pre in the Quantum on 0 and the gain on the Focusrite DI circuit on 0 I got a healthy signal.
The impedence switch makes a notable difference. I use active EMG Pa pickups on my basses. The roundwound strings sounded huge and the nylon covered Tru Bass strings had that "thump" in abundance.

I'll swap the XLR to TRS cable in and see what happens when the ISA DI gain is turned up a bit. Gotta dig around, I have another cable.

I want to test the ISA mic pre as a "cloudlifter" too so for now I'll just swap cables.
I've still got line in and instrument in options on the preamp channel to test.

So far it is a big improvement.


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