One possible explanation is that 9 KHz signal is relatively pure, and the 10 KHz tone has some modulation distortion, perhaps jitter.
The explanation is likely in large measure that the freqs need to be "in the bins", which are made inside the QA400. Once you get off the center freq of the bin, the base looks like it starts to spread.
They do not use a 9.00000kHz exact or 10.0000kHz exact freq.
When I used an external source (in this case a VP7725A) I had to tweak the freq (which can be adjusted down to quite a few decimal places) to get "in the bin".
_-_-
They do not use a 9.00000kHz exact or 10.0000kHz exact freq.
When I used an external source (in this case a VP7725A) I had to tweak the freq (which can be adjusted down to quite a few decimal places) to get "in the bin".
_-_-
Hi
I am seriously considering the Q400. My purpose is for testing my power amp and tweak the distortion. I read two or three pages here only. I just want to know whether this is a good one for me.
I have only laptop, I don't think I can add a sound card. I am looking at the Q400 and the old HP334A analyzer that is about $250. Of cause the HP is old and just use front panel adjustment and cannot plot graph.
I know I need to build a divider box bring the input down to a safe level for the Q400, what else do I need in order to start analyzing the THD of my amp?( I know I need cables etc.)
Is it true from reading the spec, Q400 has internal signal generator to drive the amp under test. Do I need another attenuator to adjust the input level?
Thanks
I am seriously considering the Q400. My purpose is for testing my power amp and tweak the distortion. I read two or three pages here only. I just want to know whether this is a good one for me.
I have only laptop, I don't think I can add a sound card. I am looking at the Q400 and the old HP334A analyzer that is about $250. Of cause the HP is old and just use front panel adjustment and cannot plot graph.
I know I need to build a divider box bring the input down to a safe level for the Q400, what else do I need in order to start analyzing the THD of my amp?( I know I need cables etc.)
Is it true from reading the spec, Q400 has internal signal generator to drive the amp under test. Do I need another attenuator to adjust the input level?
Thanks
$250 is way high for an HP334. The value is not there. (Although there are a bunch on eBay for $200+) Another $50 and you should be able to get an ST1700, more than good enough for most projects and way lower residual distortion plus more features.
Cables/adapters/level pads and load resistors and you should be on your way. Start simple and understand the tool before testing for max output. Mistakes can be costly.
The QA190 is a good tool for interfacing high output amps.
Cables/adapters/level pads and load resistors and you should be on your way. Start simple and understand the tool before testing for max output. Mistakes can be costly.
The QA190 is a good tool for interfacing high output amps.
Thanks for your reply. What is ST1700? I went on ebay and got no result. Can you give me the brand?
More importantly, is the Q400 good enough for high end audio amps? I have every intention designing a very good amp.....if not this one, I'll get there one day. I want to make sure Q400 is good enough.
From scanning through the spec sheets, it can read all the way down to -120dBm, that seems reasonable. Only problem is it can only read to 0dBm, so if I put a 30dB attenuation to test large signal full power, then the Q400 can only read to -90dBm only(because of the 30dBm attenuation). That's a little low.
If there is any suggestion of schematic of the attenuator box, can you kindly point me to the post? That will save me scanning through the over 100 pages here.
Many thanks
Alan
Last edited:
The ST 1700 is a legacy product of a company called Sound Technology. They still seem to be in business but long ago they switched over to software-based solutions.
My minimal benchmark for a test set that does a reasonably complete set of different and relevant tests is a software-based freeware analyzer called The Audio Rightmark.
Download. Audio Rightmark
I frankly can't tell if the Q400 does everything that The Audio Rightmark does. To its favor, the Q400 seems to address the big problem with PC sound card based solutions which is the matter of managing tests over a wide range of signal levels.
My benchmark for the does-it-all category is a fairly expensive software suite called Spectra:
Spectra Series Signal Analysis & Data Acquisition Software Solutions...
I am looking at the schematics, I have a few questions:
1) I am not familiar with the term "Loop back". Is this the case you use the signal generated by Q400 to feed to the input of the amplifier. Then the output of the amplifier driving back to the input of the Q400 through an attenuator for THD analyzing?
2) You have talked about the BNC input of Q400 is single ended. I read your first schematic. Are you using the signal of the input device to drive R5 and the ground to drive R6?
3) In the first schematic, you have a switchable attenuator to either using the full signal or divided by 10 signal to drive the input of the Q400. I assume the input of the schematic is from the output of the amplifier, the output drive back to the input of Q400. My question is if the input is from the amplifier, why do you need another amplifier on your interface board to buffer the signal. Seems like you can just take the output of the amplifier and drive the select switch to either route the full signal or divided by 10 signal to the Q400. Then you don't need any power for the board.
4) I see you have a 1K resistor in series with the output of your board before driving the input of the Q400. Do you know whether this will prevent the large signal from burning the input of the Q400? Also, I thought the AC input impedance of Q400 is 10Kohm only, will this cause error in the measurement?
Thanks
Last edited:
1) You may be reading the stages backwards. The output of the QA400 is a little over 1V. Not enough to drive an amp to full output. I added a balanced output circuit with low Z drive to address this issue.
2) i don't have the schematic in front of me however that sounds right.
3) The input amp is fully differential to deal with balanced amps and or ground loops. The 2 gain settings are for dynamic range optimization. The second gen design addresses this with adapter cables with internal attenuation
4) The input Z of the QA400 is supposed to be 100K. There is a potential error from that but its small. IK was to protect the QA400 from excess current if is not powered for some reason.
Thanks for your answer.
The spec said the AC input resistance is 10K. Actually I post this question to QuantAsylum forum asking the same question. There is absolutely no reason to make it that low. In audio circuit, you can easily make it 1M input impedance for any opamp input.
In this thread: https://www.quantasylum.com/Content/Default.aspx?tabid=64&g=posts&t=207
They use OPA1642 FET input opamp, there is absolutely no reason it has to be 10K input impedance. I am waiting for their answer.
Yes, I see you have a 1K resistor to limit the current.
I got the response from QuantAsylum, they said they did it that way, they might change to higher resistance in the future.
It totally does not make sense to use a 10K. This is audio frequency, no termination for transmission line to talk about. They use a JFET input opamp, they could have use 1Mohm if they want to and won't make a difference.
question to you guys that have the unit is whether it is easy and straight forward to change that 10K resistor to 1M.
This is my questions on their forum:Newbie's questions - QA400 - QuantAsylum Forum - Forum
It totally does not make sense to use a 10K. This is audio frequency, no termination for transmission line to talk about. They use a JFET input opamp, they could have use 1Mohm if they want to and won't make a difference.
question to you guys that have the unit is whether it is easy and straight forward to change that 10K resistor to 1M.
This is my questions on their forum:Newbie's questions - QA400 - QuantAsylum Forum - Forum
Actually no a 1 mega ohm could not be used. The resistor is connects to Vcom to bias the op amp input. The noise rises significantly with resistance. Mat tried it with higher value resistors but the noise is too high. They would have use 1k if they could get away with it.
Quite so. The input impedance is what ever the DUT output impedance is plus what ever
series Z at the QA400 input. But because of the Vcom bias arrangement the noise rises with the Vcom supply resistance and this is QA keep the value low. I worked with Mat on this a bit
just before the QA400 was officially released. The Vcom is extremely sensitive to the reactive and resistive load on it as well. It shapes the noise floor curve of the ADC. Any influence from input loading plays a part in the noise shape.
series Z at the QA400 input. But because of the Vcom bias arrangement the noise rises with the Vcom supply resistance and this is QA keep the value low. I worked with Mat on this a bit
just before the QA400 was officially released. The Vcom is extremely sensitive to the reactive and resistive load on it as well. It shapes the noise floor curve of the ADC. Any influence from input loading plays a part in the noise shape.