It may be useful to point out that windowing in FFT measurement can produce a frequency dependent level ripple into the measurement. The Hanning window for example can produce a 3dB level error depending on the frequency measured. The harmonics of the fundamental will also be effected by the same ripple. This is well documented in the QA400 manual and I suggest all users of the QA400 review this chapter. The flattop as the name implies does not introduce a level ripple and can be used as a reference for other windows. Flattop will produce an accurate level measure but the noise floor will be higher.
Hope this helps.
Cheers,
Hope this helps.
Cheers,
LOL. Well didn't hurt did it?
And you forced me tho look up the term RTFM.
Actually it's been there since beta testing the QA400.
They lowered the price too.
It looks great but I'd like to hear from others as well.
It would be nice if it had print features rather then copy to bit map.
There is a very active forum of QA100 users here https://www.quantasylum.com/Content/Default.aspx?tabid=64&g=topics&f=5
I don't think its as capable or refined as the Picoscope stuff Oscilloscope | Pico Technology but lots cheaper and probably more than enough for most applications.
I don't think its as capable or refined as the Picoscope stuff Oscilloscope | Pico Technology but lots cheaper and probably more than enough for most applications.
Ok 290 posts later... (fast reader, gotta be if ur on DiyAudio!)
...about ready to pull the trigger and buy one of these boxes. I have been procrastinating for a long time on getting a PC based measurement system.
But one thing is a constant with the QA400 or a soundcard, you gotta have a "box" to protect and condition the input or you get <poof> and the magic smoke comes out.
Spent some time looking at Pete Millet's solution, but fully stuffed it costs more than the measurement box! It seems one could skip the digital meter (maybe one with wires to make the bend, would do the same job for less? Did not get that far). Thinking out loud, a 90 deg header and a standard mount meter might be a way to go? But someone in another thread on the PMillet box said that (http://www.diyaudio.com/forums/tubes-valves/155405-test-measurement-interface-soundcard-6.html) if you leave out some stuff it's about half price... before you buy the box and then shield it.
That brings me to Damien's offering, which looks promising. But, I think I need attenuation. I want to measure amps and speakers (get to that in a minute). So I'd vote for the option of having the space on the board for switched attenuation. Of course using switches of the rotary type bought new adds expense. Switches from the junque box may not. That means generic space for wiring, or a layout for an obtainable switch OR generic wired switch(es).
Also, I need a mic pre. Well the QA will apparently not do things like waterfall plots and the other more exotic speaker oriented tests/displays. But it is perfectly useful for frequency response. Will it permit looking at impulse response directly? Dunno, didn't get that far with the QA400 info.
Power supply. Yeah it is nice to run off the USB port IF the USB port has enough juice to run the thing. So a provision for external power input would be useful. Might need to be switched or jumper'd to keep from having an "antenna" hanging out the back side of the box? Battery power is an option for purest source.
Chips. Other than simplicity (nothing wrong with that) and power draw, or expense, is there a benefit to using a THAT chip or that output amp chip that I did not recognize, as compared to making an instrumentation amp out of several LME4XXXX chips? Yes, yes, there is trimming and precision resistors possibly required... but does one gain back that 10dB or so of stuff at the bottom this way?
Gotta run out to the store, ha ha ha... more later...
...about ready to pull the trigger and buy one of these boxes. I have been procrastinating for a long time on getting a PC based measurement system.
But one thing is a constant with the QA400 or a soundcard, you gotta have a "box" to protect and condition the input or you get <poof> and the magic smoke comes out.
Spent some time looking at Pete Millet's solution, but fully stuffed it costs more than the measurement box! It seems one could skip the digital meter (maybe one with wires to make the bend, would do the same job for less? Did not get that far). Thinking out loud, a 90 deg header and a standard mount meter might be a way to go? But someone in another thread on the PMillet box said that (http://www.diyaudio.com/forums/tubes-valves/155405-test-measurement-interface-soundcard-6.html) if you leave out some stuff it's about half price... before you buy the box and then shield it.
That brings me to Damien's offering, which looks promising. But, I think I need attenuation. I want to measure amps and speakers (get to that in a minute). So I'd vote for the option of having the space on the board for switched attenuation. Of course using switches of the rotary type bought new adds expense. Switches from the junque box may not. That means generic space for wiring, or a layout for an obtainable switch OR generic wired switch(es).
Also, I need a mic pre. Well the QA will apparently not do things like waterfall plots and the other more exotic speaker oriented tests/displays. But it is perfectly useful for frequency response. Will it permit looking at impulse response directly? Dunno, didn't get that far with the QA400 info.
Power supply. Yeah it is nice to run off the USB port IF the USB port has enough juice to run the thing. So a provision for external power input would be useful. Might need to be switched or jumper'd to keep from having an "antenna" hanging out the back side of the box? Battery power is an option for purest source.
Chips. Other than simplicity (nothing wrong with that) and power draw, or expense, is there a benefit to using a THAT chip or that output amp chip that I did not recognize, as compared to making an instrumentation amp out of several LME4XXXX chips? Yes, yes, there is trimming and precision resistors possibly required... but does one gain back that 10dB or so of stuff at the bottom this way?
Gotta run out to the store, ha ha ha... more later...
There always the possibility of grabbing something out of the bone pile and modding it for use with the QA400. Lots of broken HP stuff out there that have precision attenuators built in. Maybe a broken 339A or something like that.
My other thought, such as it or they are or may be, is that there's no intrinsic benefit in making the board layout or size as compact as possible. I'd vote for a slightly more "relaxed" layout.
Maybe even a modular approach to the PCB stuff...
Pete Millet's layout is very compact and seemed designed to fit a nice small compact case. You're sort of locked into his component choices though. Which, I suppose is an ok way to design.
But there are two main situations where this device will be used. One is on the bench. The other is to carry around for portable situations.
For the former, there is not usually a need for it to be particularly compact, especially if it is going to have a more or less fixed location amongst other test gear. For the portable situation the unit needs to be relatively compact. But otoh, this is where all the interfacing cables to the DUT will go. A very small lightweight compact box is going to get pushed and pulled all over the place by the wires. I'd want something that at least gives the cables some resistance.
For me, not often traveling about to make tests the need for a very small unit is limited. I'm ok with something that is equal to or a large fraction of the laptop it will be working with. As long as it and the QA400, the necessary cables and the like fit into a typical laptop computer carry bag, that's more than good enough for me. Or maybe one of those "attache case" style that have the foam cutouts inside - with or without the laptop included in there.
And on connectors, I think I'd rather have larger connectors (XLR, BNC) with adapters back to the little stuff, as needed rather than the other way around (little plugs and jacks with adapters back up in size).
Knobs, can't forget the knobs. Gee I do love the way little aluminum knobs look. And those little pushbuttons too. Very exotic. But in everyday work I prefer larger stuff, and toggles over pushbuttons. The 'scope people have pretty much the right idea, the big knobs for the stuff that gets used a lot and the little knobs for stuff that isn't. The trend to make every control micro need not be followed by us in the DIY world. I'd rather go back a bit a few generations in test gear to slightly larger layouts and bigger controls and switches myself. Why? Just easier to actually use.
The other aspect of this is the box turning and moving whenever a switch is pushed, knob is turned, or connector inserted or removed. That makes all of it a two handed operation. Don't always have two hands available or want to have to use two hands...
Obviously there has to be a balance, and in a bit of portable gear you can't get too big or heavy...
After dinner random musings... feel free to disregard.
Maybe even a modular approach to the PCB stuff...
Pete Millet's layout is very compact and seemed designed to fit a nice small compact case. You're sort of locked into his component choices though. Which, I suppose is an ok way to design.
But there are two main situations where this device will be used. One is on the bench. The other is to carry around for portable situations.
For the former, there is not usually a need for it to be particularly compact, especially if it is going to have a more or less fixed location amongst other test gear. For the portable situation the unit needs to be relatively compact. But otoh, this is where all the interfacing cables to the DUT will go. A very small lightweight compact box is going to get pushed and pulled all over the place by the wires. I'd want something that at least gives the cables some resistance.
For me, not often traveling about to make tests the need for a very small unit is limited. I'm ok with something that is equal to or a large fraction of the laptop it will be working with. As long as it and the QA400, the necessary cables and the like fit into a typical laptop computer carry bag, that's more than good enough for me. Or maybe one of those "attache case" style that have the foam cutouts inside - with or without the laptop included in there.
And on connectors, I think I'd rather have larger connectors (XLR, BNC) with adapters back to the little stuff, as needed rather than the other way around (little plugs and jacks with adapters back up in size).
Knobs, can't forget the knobs. Gee I do love the way little aluminum knobs look. And those little pushbuttons too. Very exotic. But in everyday work I prefer larger stuff, and toggles over pushbuttons. The 'scope people have pretty much the right idea, the big knobs for the stuff that gets used a lot and the little knobs for stuff that isn't. The trend to make every control micro need not be followed by us in the DIY world. I'd rather go back a bit a few generations in test gear to slightly larger layouts and bigger controls and switches myself. Why? Just easier to actually use.
The other aspect of this is the box turning and moving whenever a switch is pushed, knob is turned, or connector inserted or removed. That makes all of it a two handed operation. Don't always have two hands available or want to have to use two hands...
Obviously there has to be a balance, and in a bit of portable gear you can't get too big or heavy...
After dinner random musings... feel free to disregard.
Bear-
I included drawings for attenuation cables. I thought I described the decision process earlier.
While the circuitry isn't difficult execution is more involved than it may first seem.
I posted the expresspcb files and you could modify them to suit pretty easily. Changing the connectors to XLR's is not difficult. But the box gets much larger, the connectors more expensive and you are not really closer to a usable product.
Adding a balanced attenuator will add much to the complication and cost. It would need to be Hi Z, frequency compensated, overload protected (the hard part) and use a multiwafer switch. Execution will be expensive. That is why I chose to make cables for different functions. You would not connect an RCA connector to speaker level signals. Nor would you want to use an unshielded connection (bananas) to a low level signal. The adapters will need to be fabricated anyway so I combined them. Minijack connectors don't seem right but they actually work well in this application. The connectors have gotten more reliable (500 million a year in cell phones has produced some demand for quality) and molded cables to make the interface cables are really cheap on eBay.
I'm using the standard miniboard because its cheap and quick. Everything fit the footprint. Using the existing layout in a larger box would be simple especially with panel mount connectors (unfortunately not available for the USB connectors).
When Audio Precision addressed this same product concept (APx515) they wound up with an almost rack width box that does a little more and has similar performance.
Perhaps the right move is a new box big enough for both boards.
I included drawings for attenuation cables. I thought I described the decision process earlier.
While the circuitry isn't difficult execution is more involved than it may first seem.
I posted the expresspcb files and you could modify them to suit pretty easily. Changing the connectors to XLR's is not difficult. But the box gets much larger, the connectors more expensive and you are not really closer to a usable product.
Adding a balanced attenuator will add much to the complication and cost. It would need to be Hi Z, frequency compensated, overload protected (the hard part) and use a multiwafer switch. Execution will be expensive. That is why I chose to make cables for different functions. You would not connect an RCA connector to speaker level signals. Nor would you want to use an unshielded connection (bananas) to a low level signal. The adapters will need to be fabricated anyway so I combined them. Minijack connectors don't seem right but they actually work well in this application. The connectors have gotten more reliable (500 million a year in cell phones has produced some demand for quality) and molded cables to make the interface cables are really cheap on eBay.
I'm using the standard miniboard because its cheap and quick. Everything fit the footprint. Using the existing layout in a larger box would be simple especially with panel mount connectors (unfortunately not available for the USB connectors).
When Audio Precision addressed this same product concept (APx515) they wound up with an almost rack width box that does a little more and has similar performance.
Perhaps the right move is a new box big enough for both boards.
Thats what i would do.... take the pcb from the qa400 and put it in a larger box... maybe with own PS. and add other pcb inside with it. Only one box with fewer cables/noise.
-RM
Demian,
I'm not raining on the parade here, just thinking out loud with the idea that others may have the same need, thoughts, or better ideas/solutions.
In general, and not just with this project, the general idea is to have a board with everything brought out to the edge, and then a thin front panel. I think this stems from pro use of rack panels, and trying to conserve vertical space in a rack. Bah.
Personally I would prefer to have a larger front panel, less depth, and have the pcb parallel to the front panel (which could be thought of as the top or bottom in an enclosure would be a standard type), flip out legs to stand it up, sort of like the inexpensive China digital 'scopes in concept.
What about the "chip choice"? Anything to gain in terms of noise floor/distortion reduction in that department, and what about the "roll ur own" instrumentation amp (going in or out)?
I have to admit that I would greatly prefer to buy something off the shelf, because I want to spend the little time I have to do this stuff working on the DUTs, not building up the interface box.
Oh yeah the other thing that concerned me a bit about PMillet's box is the use of input caps...
Richard, but the QA400 box is so cute and it has that nice red/clear two tone anodize!!
I'm playing about in my peanut sized brain with how to do the interface thing (now that I went and bought the unit).
I'm not raining on the parade here, just thinking out loud with the idea that others may have the same need, thoughts, or better ideas/solutions.
In general, and not just with this project, the general idea is to have a board with everything brought out to the edge, and then a thin front panel. I think this stems from pro use of rack panels, and trying to conserve vertical space in a rack. Bah.
Personally I would prefer to have a larger front panel, less depth, and have the pcb parallel to the front panel (which could be thought of as the top or bottom in an enclosure would be a standard type), flip out legs to stand it up, sort of like the inexpensive China digital 'scopes in concept.
What about the "chip choice"? Anything to gain in terms of noise floor/distortion reduction in that department, and what about the "roll ur own" instrumentation amp (going in or out)?
I have to admit that I would greatly prefer to buy something off the shelf, because I want to spend the little time I have to do this stuff working on the DUTs, not building up the interface box.
Oh yeah the other thing that concerned me a bit about PMillet's box is the use of input caps...
Richard, but the QA400 box is so cute and it has that nice red/clear two tone anodize!!
I'm playing about in my peanut sized brain with how to do the interface thing (now that I went and bought the unit).
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You are VERY talented.
The input caps are not going to compromise the performance but I did not include them. They do complicate the input interface a lot.
The THAT part is very good. Its distortion is definitely below the QA400 and includes a network of .1% resistors that would be a pain to deal with other ways. Same for the output driver, which also isolates the ground of the DUT.
To put this into perspective- a traditional analog distortion analyzer has a 20 dB indicator range and gets its dynamic range from the attenuator + internal amp. The QA400 has a 140 dB indicator range so you can see most of anything interesting with only a few input range options. Unfortunately there is no mechanism for external autorange or reporting external gain changes back to the QA400. This means that external stuff should be either even decibel or decade ranges, which further limits some options.
My choice was to set the input/output at a level that would be useful for a line level device- approx 7V RMS max. This ensures driving an amp to clipping and not overloading the input from a CD player or DAC. Then an external attenuation cable (X10 or -20 dB) with clips is well suited to power amps, balanced or single ended.
I'll look inside the box and see if its possible to graft the board into it. That would be the nicest solution. It may be possible with some adjustments.
The input caps are not going to compromise the performance but I did not include them. They do complicate the input interface a lot.
The THAT part is very good. Its distortion is definitely below the QA400 and includes a network of .1% resistors that would be a pain to deal with other ways. Same for the output driver, which also isolates the ground of the DUT.
To put this into perspective- a traditional analog distortion analyzer has a 20 dB indicator range and gets its dynamic range from the attenuator + internal amp. The QA400 has a 140 dB indicator range so you can see most of anything interesting with only a few input range options. Unfortunately there is no mechanism for external autorange or reporting external gain changes back to the QA400. This means that external stuff should be either even decibel or decade ranges, which further limits some options.
My choice was to set the input/output at a level that would be useful for a line level device- approx 7V RMS max. This ensures driving an amp to clipping and not overloading the input from a CD player or DAC. Then an external attenuation cable (X10 or -20 dB) with clips is well suited to power amps, balanced or single ended.
I'll look inside the box and see if its possible to graft the board into it. That would be the nicest solution. It may be possible with some adjustments.
A quick look inside suggests the following-
1) there is space
2) The BNC's may need to be removed to access the connections.
3) A new front panel would be the clean revision. The board would mount on the panel. maybe add an extra set of minijacks to directly access the QA400. Then- mini or 1/4" jacks for the I/O? The 1/4" jacks will make for larger and more expensive cables etc. but it may be possible to use the existing holes and panel.
Respinning the board for this should be too difficult. The 1/4" stereo PC mount connectors are large footprint which may force a shift to surface mount. The BNC's would be removed and wires connected to the pads. And two wires are needed for the USB power. Fortunately it would not go past the USB2 limit. I may be able to use a charge pump and eliminate the expensive DC-DC converter.
1) there is space
2) The BNC's may need to be removed to access the connections.
3) A new front panel would be the clean revision. The board would mount on the panel. maybe add an extra set of minijacks to directly access the QA400. Then- mini or 1/4" jacks for the I/O? The 1/4" jacks will make for larger and more expensive cables etc. but it may be possible to use the existing holes and panel.
Respinning the board for this should be too difficult. The 1/4" stereo PC mount connectors are large footprint which may force a shift to surface mount. The BNC's would be removed and wires connected to the pads. And two wires are needed for the USB power. Fortunately it would not go past the USB2 limit. I may be able to use a charge pump and eliminate the expensive DC-DC converter.