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Gear / Technical Help => Battery Boxes, Preamps, Mixers, ADCs, and Processors => Topic started by: poorlyconditioned on January 07, 2006, 09:40:49 PM
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Here are some pictures of my latest project, a DIY three-wire battery box + preamp:
Preamp/battery box connected to AKG CK91 capsules. Line in to minidisc:
(http://www.cs.uwaterloo.ca/~mannr/AKG/preamp.jpg)
Innards of battery box/preamp:
(http://www.cs.uwaterloo.ca/~mannr/AKG/preamp-innards.jpg)
The amplifier is based on an LT6234 chip. This is a low-power, high quality opamp (1.15mA per channel, tiny SOIC package). There are fixed gains of 12dB, 18dB, and 22dB (slider switch at bottom). The top switch is for power one way, and LED/battery test the other way. This is built into an old AT8352 battery module. I used this because it already has a nice 9V battery holder and one miniXLR jack. The other miniXLR was added (using a Dremmel tool!).
Details of the CK91 capsules (and larger pics of this project) are available at:
www.cs.uwaterloo.ca/~mannr/AKG
Schematics of the battery box and opamp are at:
www.cs.uwaterloo.ca/~mannr/AT853
PS: Please don't ask me to make you one of these. Soldering small components and fitting them in the case was very difficult! Anyone good with mechanical and electrical skills is welcome to use the design though.
Richard
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Nice.
I know a place where that would be useful. ;D
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Nice! Will you make me one? ;D (j/k) +T
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Nice Richard, I'd love to hear some recordings with that setup...
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Richard,
I hope you don't mind a couple of small suggestions about the design :)
First, I'd suggest increasing the input impedance. The current value (works out at 11K) is low enough to give some distortion as the gm of the mic FET varies with output level. One of the main points of the 3-wire connection is to decrease the dependance of output on gm, thereby reducing distortion.
Second, I'd suggest a change to the voltage divider at the input. Changing from:
(http://www.gcogger.dsl.pipex.com/stuff/cct1.gif)
to:
(http://www.gcogger.dsl.pipex.com/stuff/cct2.gif)
would drastically improve supply rejection (PSRR) which, in the current design, is in the order of 0dB at full gain. If this was a hi-fi pre-amp, I'd expect that change to give a noticeable improvement to transparency/detail (reduced 'smearing') but I don't know if that would be audible with these mics.
All the best,
Graeme
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Graeme, no professionals allowed ;D
How are you Sir?Havent seen you on the other place too much.
teddy
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Richard,
I hope you don't mind a couple of small suggestions about the design :)
First, I'd suggest increasing the input impedance. The current value (works out at 11K) is low enough to give some distortion as the gm of the mic FET varies with output level. One of the main points of the 3-wire connection is to decrease the dependance of output on gm, thereby reducing distortion.
Second, I'd suggest a change to the voltage divider at the input. Changing from:
(http://www.gcogger.dsl.pipex.com/stuff/cct1.gif)
to:
(http://www.gcogger.dsl.pipex.com/stuff/cct2.gif)
would drastically improve supply rejection (PSRR) which, in the current design, is in the order of 0dB at full gain. If this was a hi-fi pre-amp, I'd expect that change to give a noticeable improvement to transparency/detail (reduced 'smearing') but I don't know if that would be audible with these mics.
All the best,
Graeme
Thanks for the suggestions. I didn't expect such detail on this forum. You know about the "micbuilders" forum on Yahoo groups, right? I hang out there as well. I'll make the changes you suggest.
I've got a question about the 3-wire powering: Do you know the output impedance of a mic when I've got the FET wired as a "source follower". Consider the case of a FET with a source resistor (to ground) of 10K and a drain resistor (to +9V battery) of 3K. The output impedance is deterined by the drain resistor, right?
Anyway, thanks again for the help!
Richard
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Graeme, no professionals allowed ;D
How are you Sir?Havent seen you on the other place too much.
teddy
I'm not a pro any more - all this electronics stuff is fading into the haze of distant memories :)
I've not been posting much, as I haven't had a lot to say. I've acquired the SP-CMC-8 omni mics from Sound Professionals and have the choice of a Hi-MD recorder or iRiver iHP-120 with Rockbox firmware for recording. Unfortunately I've had little chance to use it yet. I need to make a pre-amp before the end of the month (my wife's next recital), and I'll probably be doing something very similar to Richard. Did you get someone to do your mods?
I've got a question about the 3-wire powering: Do you know the output impedance of a mic when I've got the FET wired as a "source follower". Consider the case of a FET with a source resistor (to ground) of 10K and a drain resistor (to +9V battery) of 3K. The output impedance is deterined by the drain resistor, right?
I'm not sure why you need a drain resistor in a source follower - your 'blackboard' circuit doesn't show one. It's not needed and can only degrade the overload and distortion performance. The output impedance is (to a good approximation) a resistance of 1/gm in parallel with the resistor from source to ground. Without having the details of the FET used, it's impossible to come up with a value. I tried some SPICE simulations, and the nearest JFET I could get a model for needed an 18K source resistor to drop 4.5V. It's actually about 30K on the mic capsules I've bought, but I couldn't find anything closer. This had a 1/gm value of about 3K, so I'd guess the source follower as in your circuit for the AT853 (with 27K resistors) would have an output impedance a little higher than that. A capsule that drops 4.5V with 10K would probably be a bit lower. It's pretty much guesswork, but should be in the ballpark. For best distortion performance, you need the source resistance and pre-amp input impedance (in parallel) to be as high as possible. The source resistor is limited by where you want the operating voltage at the FET source (a little under half the supply voltage for best overload levels) so you need to get the pre-amp input impedance a fair bit higher than the source resistor. You could easily go higher than the 47K I suggested in my circuit - should be fine for most op-amps.
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Did you get someone to do your mods?
Yessir, sure did!! Hope to have it back soon!Good to see you post. I think you have a lot to offer to the community here, from what I know of your EE knowledge. :)
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I think I said somewhere that I'd post the circuit when I settled on my pre-amp design, so here it is if anyone is interested. It's a combined 3-wire battery box (for the AT943 caps) and mic pre-amp.
(http://www.gcogger.dsl.pipex.com/stuff/micpre1.gif)
As you can see, it's very similar to Richard's. R1, R2 and C3 are shared between the channels to save space. Some of the values have been chosen because it's what I have lying around. I've kept it as small as possible, to fit in a small box. I'm sure it could be improved at the cost of space and power consumption (although I've not worried too much about the latter). Unfortunately, C2 needs to be an electrolytic since my iRiver iHP-120 has a very low 2K2 input impedance.
The input impedance is high (about 100K), so it wouldn't be too good with longer cables. The software I used to draw it doesn't do switches (!), but it should be clear what they do. SW1 is switchable bass cut with a DPDT, centre off switch (gives rolloffs at 3, 94 and 233Hz). SW2 is for power and SW3 is a -20dB setting - in one position the gain is 0-20dB and in the other 0-40dB.
All I have to do now is build it :)
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I think I said somewhere that I'd post the circuit when I settled on my pre-amp design, so here it is if anyone is interested. It's a combined 3-wire battery box (for the AT943 caps) and mic pre-amp.
(http://www.gcogger.dsl.pipex.com/stuff/micpre1.gif)
As you can see, it's very similar to Richard's. R1, R2 and C3 are shared between the channels to save space. Some of the values have been chosen because it's what I have lying around. I've kept it as small as possible, to fit in a small box. I'm sure it could be improved at the cost of space and power consumption (although I've not worried too much about the latter). Unfortunately, C2 needs to be an electrolytic since my iRiver iHP-120 has a very low 2K2 input impedance.
The input impedance is high (about 100K), so it wouldn't be too good with longer cables. The software I used to draw it doesn't do switches (!), but it should be clear what they do. SW1 is switchable bass cut with a DPDT, centre off switch (gives rolloffs at 3, 94 and 233Hz). SW2 is for power and SW3 is a -20dB setting - in one position the gain is 0-20dB and in the other 0-40dB.
All I have to do now is build it :)
+T, thanks for the drawing!
So, do you think the LT1469 is going to be better than the LT6233 I'm using? The LT1469 has better specs (and comes in a DIP package!) but takes 3.5mA vs. 1.2mA for each amp. I might build around the 1469 if it is better. Also, can you PM a source for LT1469? I checked Digikey but they don't have any in stock.
Another note is the gain. I like a gain switch instead of a pot, but that is personal preference.
Good luck, and please bring back photos when you're done!
Richard
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+T, thanks for the drawing!
So, do you think the LT1469 is going to be better than the LT6233 I'm using? The LT1469 has better specs (and comes in a DIP package!) but takes 3.5mA vs. 1.2mA for each amp. I might build around the 1469 if it is better. Also, can you PM a source for LT1469? I checked Digikey but they don't have any in stock.
Another note is the gain. I like a gain switch instead of a pot, but that is personal preference.
Good luck, and please bring back photos when you're done!
Richard
Hi again :)
It's difficult to say whether the 1469 will be better than the 6233 - it's a bit of a gamble until you listen to it, although either op-amp is probably better than the mics. I wanted something that was unity gain stable, and I wasn't all that worried about the current draw. I doubt I'll need more than about 25hrs between recharges! I preferred it to the 6233 because of lower input bias current and better supply rejection. The noise is a little higher, but I believe that the mic noise will the dominant factor. I also liked the circuit topology from the data sheet - looks like the kind of thing I might design :) One small issue is that it is not guaranteed to work within spec below 9V (it will typically work at 5V, but 9V is the worst case). Most PP3 rechargeables are actually 8.4V, but I managed to find one that is 9.6V (and still 220mAh).
I considered a gain switch but decided on a pot because this may sometimes be used into a minidisc (Hi-MD) recorder. On that, you can't adjust the record level without pausing the recording. Pretty daft, I think.
I'm afraid I'm in the UK, so I doubt my sources would be of use to you...
I'll post some pics when it's done. I've just worked out a layout that lets me use the box I wanted, but it only just squeezes in. It's a good job that I could only source the LT1469 in surface mount format, I guess ;)
Any thoughts on the values of the bass cut frequencies? I still don't know why it's needed - if the bass level is so high, surely the gig would sound awful? Still, I've put them on there just in case, at frequencies mostly decided by the capacitors I had available.
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+T, thanks for the drawing!
So, do you think the LT1469 is going to be better than the LT6233 I'm using? The LT1469 has better specs (and comes in a DIP package!) but takes 3.5mA vs. 1.2mA for each amp. I might build around the 1469 if it is better. Also, can you PM a source for LT1469? I checked Digikey but they don't have any in stock.
Another note is the gain. I like a gain switch instead of a pot, but that is personal preference.
Good luck, and please bring back photos when you're done!
Richard
Hi again :)
It's difficult to say whether the 1469 will be better than the 6233 - it's a bit of a gamble until you listen to it, although either op-amp is probably better than the mics. I wanted something that was unity gain stable, and I wasn't all that worried about the current draw. I doubt I'll need more than about 25hrs between recharges! I preferred it to the 6233 because of lower input bias current and better supply rejection. The noise is a little higher, but I believe that the mic noise will the dominant factor. I also liked the circuit topology from the data sheet - looks like the kind of thing I might design :) One small issue is that it is not guaranteed to work within spec below 9V (it will typically work at 5V, but 9V is the worst case). Most PP3 rechargeables are actually 8.4V, but I managed to find one that is 9.6V (and still 220mAh).
I considered a gain switch but decided on a pot because this may sometimes be used into a minidisc (Hi-MD) recorder. On that, you can't adjust the record level without pausing the recording. Pretty daft, I think.
With the HiMD you *can* adjust record levels while running. Here is what you do: hit pause-record, enter menu> REC SET >REC VOLUME > MANUAL. Then hit pause to start and stop recording. The "wheel" adjusts the record level, both in pause and record mode. You see meters on the main display. If you've got RM-MC40ELK remote, you see levels on the remote and *backlit* which is great!!! Now why can't other stealth gear do that so nicely???
I'm afraid I'm in the UK, so I doubt my sources would be of use to you...
I'll post some pics when it's done. I've just worked out a layout that lets me use the box I wanted, but it only just squeezes in. It's a good job that I could only source the LT1469 in surface mount format, I guess ;)
Any thoughts on the values of the bass cut frequencies? I still don't know why it's needed - if the bass level is so high, surely the gig would sound awful? Still, I've put them on there just in case, at frequencies mostly decided by the capacitors I had available.
I've never used the bass cut. I *think* if you use 3-wire mic powering your mics won't overload so you won't need it. You can fix it in post if you need I guess.
Again, please post pics when you're done. I love seeing gear innards.
Richard
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can the switches be combined for left and right? basfilter and the gainswitch switch? or is induvidual for each channel?
and can batterypower be shared over left and right? or does it need 2 battery's?
will you post your pcb drawing when it;s done?
looks prommising.
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Richard - I didn't realise you could adjust levels on the fly with the Hi-MD recorders. I'll have to play with that. Thanks for the info :)
Weazel - all the switches are double pole. In other words, they affect both channels equally. The battery is also shared. The op-amp bypass caps (C4 and C7) are shared because it's a dual op-amp with only one pair of supply pins. This is mostly being made on stripboard/veroboard, so no PCB. There seems little point for a one-off.
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I think I said somewhere that I'd post the circuit when I settled on my pre-amp design, so here it is if anyone is interested. It's a combined 3-wire battery box (for the AT943 caps) and mic pre-amp.
(http://www.gcogger.dsl.pipex.com/stuff/micpre1.gif)
As you can see, it's very similar to Richard's. R1, R2 and C3 are shared between the channels to save space. Some of the values have been chosen because it's what I have lying around. I've kept it as small as possible, to fit in a small box. I'm sure it could be improved at the cost of space and power consumption (although I've not worried too much about the latter). Unfortunately, C2 needs to be an electrolytic since my iRiver iHP-120 has a very low 2K2 input impedance.
The input impedance is high (about 100K), so it wouldn't be too good with longer cables. The software I used to draw it doesn't do switches (!), but it should be clear what they do. SW1 is switchable bass cut with a DPDT, centre off switch (gives rolloffs at 3, 94 and 233Hz). SW2 is for power and SW3 is a -20dB setting - in one position the gain is 0-20dB and in the other 0-40dB.
All I have to do now is build it :)
Hey Graeme, Teddy told me that people were over here talking circuits. I'm glad to see circuit discussions going on.
What's the purpose of C6? Is that just to make sure you don't get high frequency instability by cutting the gain at high frequencies?
Also, I notice that you are using a virtual ground on the positive input side. Why don't you connect your virtual ground on the negative input side by connecting the common terminal of R4 and R14 to the common terminal of R1 and R2, then deleting C1, C13, R3 and R15? If it's from worrying about signal loading on your virtual ground, you could put an emitter follower on your virtual ground and adjust R1 and R2 so that the output of the emitter follower is still halfway between the power supply rails.
Just curious...
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I think I said somewhere that I'd post the circuit when I settled on my pre-amp design, so here it is if anyone is interested. It's a combined 3-wire battery box (for the AT943 caps) and mic pre-amp.
(http://www.gcogger.dsl.pipex.com/stuff/micpre1.gif)
As you can see, it's very similar to Richard's. R1, R2 and C3 are shared between the channels to save space. Some of the values have been chosen because it's what I have lying around. I've kept it as small as possible, to fit in a small box. I'm sure it could be improved at the cost of space and power consumption (although I've not worried too much about the latter). Unfortunately, C2 needs to be an electrolytic since my iRiver iHP-120 has a very low 2K2 input impedance.
The input impedance is high (about 100K), so it wouldn't be too good with longer cables. The software I used to draw it doesn't do switches (!), but it should be clear what they do. SW1 is switchable bass cut with a DPDT, centre off switch (gives rolloffs at 3, 94 and 233Hz). SW2 is for power and SW3 is a -20dB setting - in one position the gain is 0-20dB and in the other 0-40dB.
All I have to do now is build it :)
Hey Graeme, Teddy told me that people were over here talking circuits. I'm glad to see circuit discussions going on.
What's the purpose of C6? Is that just to make sure you don't get high frequency instability by cutting the gain at high frequencies?
Also, I notice that you are using a virtual ground on the positive input side. Why don't you connect your virtual ground on the negative input side by connecting the common terminal of R4 and R14 to the common terminal of R1 and R2, then deleting C1, C13, R3 and R15? If it's from worrying about signal loading on your virtual ground, you could put an emitter follower on your virtual ground and adjust R1 and R2 so that the output of the emitter follower is still halfway between the power supply rails.
Just curious...
OK, switch off now if you're not interested in the tecchie stuff:)
C6 is there to help stability when driving capacitive loads. The op-amp data sheet gives some equations to calculate the value, but it's difficult to use when the gain is variable and you don't know what it's driving into! 22pF is as high as I wanted to go without much effect on the HF response at full gain.
As to the so-called virtual ground...
I don't like to think of it in those terms - trying to emulate a dual-rail supply. The mic output is relative to true ground, so if you make the op-amp work relative to the virtual ground then you are amplifying the difference between the two. This gives poor supply rejection (PSRR). The purpose of R1/R2 is simply to bias the op-amp mid-rail with as little supply noise at that point as possible. In other words, think in terms of a single rail supply rather than trying to fake a dual one.
Think of it this way - with your connection, any supply noise on the R1/R2 junction is seen at the op-amp negative input in full. The positive input, however, sees virtually none of it as the noise is reduced via R3 and the output impedance of the mic/FET. Therefore, the op-amp will amplify this difference by the full gain of the circuit (up to 40dB). I simulated the 2 circuits (assumed the op-amp has perfect PSRR for simplicity). I set R12 to be 5K to emulate the output impedance of the mic/FET - if the figure was lower, the difference between the circuits would be greater. This is done at max gain (i.e. worst case).
The following are the supply rejection figures (in dB) at 3 frequencies. In each case, the first figure is my circuit and the second is your suggested change.
20Hz: -65 -2
1kHz: -143 -36
20kHz: -195 -62
As you can see, the circuit as it stands will beat the specs of the op-amp at anything above about 100Hz - i.e. the op-amp is the limiting factor for PSRR. With the suggested changes, this is nowhere near true even at 20kHz.
Another thing is that your suggested change will amplify any input offset voltage (either from the op-amp or due to bias currents) by the full circuit gain. Not too much of an issue with the LT1469, but worth bearing in mind.
Finally, the suggested change would make the crosstalk between channels worse, unless R1/R2/C3 were duplicated for each channel.
HTH
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OK, I said I'd post some pics :) The pre-amp is made and briefly tested, but not used in anger yet. As you may be able to see, I'm better at designing things than putting them together.
Inside case, before wiring up to switches, sockets etc.
(http://www.gcogger.dsl.pipex.com/stuff/pre1.jpg)
The panel with the sockets will be right up against the circuit board once fitted, and the other end of the box is taken up with a battery. Maybe I'll use a bigger case next time! Unfortunately the surface mount chip is buried somewhere in the nest of wiring. If it goes wrong, it will probably be quicker to make another from scratch rather than try and take this thing apart :-\
Next, the finished unit connected to my iRiver iHP-120 and the SP-CMC-8 (AT943) mics.
(http://www.gcogger.dsl.pipex.com/stuff/pre2.jpg)
You can just about see the input and output sockets, power switch, -20dB and bass cut switches and level pot.
Finally, the obligatory size comparison (in case you don't know how big the iRiver is)...
(http://www.gcogger.dsl.pipex.com/stuff/pre3.jpg)
I'll be recording a classical music recital (mostly singing) tomorrow, so I'll post a clip when I can. So far it seems to have better clarity than using a 2-wire connectio straight into the iRiver.
One thing I've already found is that I need to fit a small resistor in series with the pot (say 100R). I get funny effects when the pot is reduced to minimum (zero resistance), probably due to the interaction with R7.
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OK, I said I'd post some pics :) The pre-amp is made and briefly tested, but not used in anger yet. As you may be able to see, I'm better at designing things than putting them together.
Inside case, before wiring up to switches, sockets etc.
The panel with the sockets will be right up against the circuit board once fitted, and the other end of the box is taken up with a battery. Maybe I'll use a bigger case next time! Unfortunately the surface mount chip is buried somewhere in the nest of wiring. If it goes wrong, it will probably be quicker to make another from scratch rather than try and take this thing apart :-\
Next, the finished unit connected to my iRiver iHP-120 and the SP-CMC-8 (AT943) mics.
You can just about see the input and output sockets, power switch, -20dB and bass cut switches and level pot.
Finally, the obligatory size comparison (in case you don't know how big the iRiver is)...
I'll be recording a classical music recital (mostly singing) tomorrow, so I'll post a clip when I can. So far it seems to have better clarity than using a 2-wire connectio straight into the iRiver.
One thing I've already found is that I need to fit a small resistor in series with the pot (say 100R). I get funny effects when the pot is reduced to minimum (zero resistance), probably due to the interaction with R7.
Excellent job! +T
Richard
PS: You might want to hard wire (solder) the mics in when you're done with the design. Also, hard wire a fixed connector ("pigtail") out with a male miniplug to go into the Iriver. I've had no end of problems with 1/8" plugs, especially for stealth gear.
Good luck!
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Very nice!
I found this nice article on building passive hifi preamps today. It has a lot of good tips about how the sound quality was improved and some interesting parts sources.
http://www.stereophile.com//amplificationreviews/54
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Excellent job! +T
Richard
PS: You might want to hard wire (solder) the mics in when you're done with the design. Also, hard wire a fixed connector ("pigtail") out with a male miniplug to go into the Iriver. I've had no end of problems with 1/8" plugs, especially for stealth gear.
Good luck!
Thanks :) That's an interesting idea about hard wiring the cables - might have saved me some space.
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Nice little box, looks cute
+T
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Nice little box, looks cute
+T
Thanks, but I wasn't really going for 'cute' :)
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OK, samples uploaded. Recorded with SP-CMC-8 omni mics (AT943), my pre-amp and Sony Hi-MD recorder. Very mild tweaking of tone response in CoolEdit.
http://www.gcogger.dsl.pipex.com/stuff/sample1.mp3 (http://www.gcogger.dsl.pipex.com/stuff/sample1.mp3)
http://www.gcogger.dsl.pipex.com/stuff/sample2.mp3 (http://www.gcogger.dsl.pipex.com/stuff/sample2.mp3)
http://www.gcogger.dsl.pipex.com/stuff/sample3.mp3 (http://www.gcogger.dsl.pipex.com/stuff/sample3.mp3)
http://www.gcogger.dsl.pipex.com/stuff/sample4.mp3 (http://www.gcogger.dsl.pipex.com/stuff/sample4.mp3)
The first and second are mezzo vocal + piano. The third is viola + piano. The fourth is soprano vocal + viola + piano.
The mezzo/soprano (and sometimes alto :) ) is my wife ;D
Overall, I'm very pleased with all the new gear.