Hello!
apologies if this was asked before.
is there a protocol available to measure riaa properly? Does anyone have any experience with that? I did some freq response measurements that worked OK but the gain disparity is causing all kinds of issues with thd +N measurements. I was starting from -80dB to -60dB, I also tried in line attenuator but of course it kills the noise. I read somewhere that a protocol was available.
thanks!
I presume you are looking to quantify RIAA deviation. You want to load the high resolution RIAA weighting file by pressing the ‘user’ button on weighting, select and navigate to the file, load it and you will see it on the screen.
Then set your output to something suitable (5mV or -46.1dBV) for MM input stages. Then activate the frequency response button and view the plot. Scale for dBR +/- however many dB you want.
This example below is a truly excellent phono stage and I had to zoom in to +/- 1dB or less to see deviation of less than 0.1dB.
Here’s a THD for the same preamp (1.41kHz)
No weighting is needed when you test for THD unless you want to switch in A-WTD
You don’t need attenuators as the QA-403 has its own internal attenuators work to keep the D/A in its sweet spot to minimise noise. Bear in mind, if you use the IDLE generator to set things up, there is a check box to prevent range change glitches when changing output levels.
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I will try this out tomorrow. My main problem has been the excessive amplification at LF and high attenuation at HF in a thd v frq measurement I am getting weird looking curves because of that and I have to scan using very low output levels to avoid overloading the input stage of the qa403 at LF. I am sure there is a right way to do this.
Hi! I hope I get your issues right. “excessive amplification at LF and high attenuation at HF”: this is the RIAA curve of your phono preamp. Use the RIAA weighting file as John explained, this will fix the levels. Build a resistive voltage divider to attenuate the generator signal from the QA403 down if you test an MC stage. I built specific cables for that and use smd resistors that I solder to the plug that goes into the input of the phono stage.
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Do you want to share your pad down values and build, it could be helpful for members. Cheers.
This might look a bit wild. Connector is not an RCA but a Redel as I like these better on my personal gear. Voltage divider is differential 5k 5R 5k. The center 5R is chosen to emulate a 5R coil which is a typical value for the MC cartridges I use. This is then connected to 2 BNC cables for balanced connection to the QA403.
Also check this thread: Phono preamp measurement setup
No sure I get what you’re doing here with a 5k5R5k. you mean your loading the QA output with 10005ohm and taking the 5Rin into the riaa? A 10000:5 att ratio? What dB output do you pick to run the measurement to start with? Of course that would be scaled for the riaa curve.
This kind of goes into how the signal is attenuated within the instrument.
I am trying to understand is what is the quality of the waveform coming out the instrument as a function of dB setting? Meaning for example is -80dB wavefom any good or is it super noisy to start with? There must be a balance between using a high output and then attenuating it with a divider vs taking the low output that maybe more noisy to begin with.
thanks
Not really a balance, having a resistive attenuator on the connector means the noise will be that of a 5 ohm resistor, in other words super low (300pV/√Hz) - you definitely want a reasonably high output from the QA to get its best SNR and distortion performance from its DAC.
So say 6dB out, 66dB attenuation → -60dB signal (1mV). Sounds about right for an MC preamp. For MM preamp I’d say 46dB attenuation would be reasonable.
You can evaluate the QA in loop back with a plot of THD+N against output level.
So the 5k +5k has no contribution on the noise?
What MarkT said. In other words: I start with a 1Vrms output and scale that down to 0.5mV right at the input of the DUT. That way noise that might get picked up between generator and DUT is attenuated as well. Together with the QA RIAA weighting I get useful measurements.
For fixed frequency RIAA checks I have used a Rohde & Schwarz SPN signal generator that lets me pick a 5R output Z and 0.5mV level. The QA403 can’t do that without help so I built the attenuator cable to emulate my own standard that I’ve used for a long time to build MC phono stages.
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For MM front ends, you can run it straight from the QA403 and get excellent results. The D/A is operating in its sweet spot and a 2.5mV-5.0mV level will give excellent N and THD due to the internal gain/attenuators.
This is the MM input stage driven directly from the QA403 on a 42 year old Denon preamplifier.
unwtd:
awtd:
If you want to get really serious, you can place a typical MM cartridge in a can and series connect it to the MM front end in order to assess “real world” S/N. I’ve got a nice little load box and PCB designed by Michael Fidler (UK) of phono preamp ‘fame’ with a classic AT-9x cart inside, along with a dedicated MC front end load for testing in the same box.