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George S.

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#1
The QA403's are back in stock as of this morning. Just ordered one.
Was considering waiting on the QA404 that are projected to be available in June-July, but given the world situation and the incremental improvements of the QA404, decided it was best to pull the trigger for the QA403.
Should meet my very modest needs. More of a learning tool for me than anything else.
 
Hexis22

Hexis22

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I've been looking for a more cost effective method of performing THD+N analysis, this looks very interesting.

It would be interesting to use this for guidance in different internal amp wiring approaches, specifically potential degradation due to AC coupling.

Looking forward to George's evaluation of this tool... I'll be waiting for June-July as well.
 
WOPL Sniffer

WOPL Sniffer

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I plan on replacing my QA401 with the new QA404. Pair it with the programmable load (QA451) and yer all set. Even though it aint got enough beans to hold up to the WOPL, combine the 451 and an external load and the sky's the limit. Then I can put up the QA401 for sale.
 
G

George S.

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The QA403 came today. Using a old Lenovo T410 laptop with a fresh install of Windows 10, and a powered USB hub to ensure the QA403 is adequately powered when the laptop is on battery.
Was concerned about the processing power of the old laptop, but the Windows Task Manager shows the QA uses very few Windows resources. Hardly a blip on the CPU or memory usage from the QA.
The old Acer monitor was a curbside find that needed new caps on the power supply board, recapped it years ago. It's a little scuffed up, but was worth fixing and using.
This thing is great and was worth waiting for, almost 2 years.
Now I can learn about those mysterious dB's, FFT's, and THD's.
For you guys interested in tapes, there is a wow and flutter utility built into the software.
 

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George S.

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Been working through the "Getting Started" documentation. Lots to learn and it makes logical sense. There are instructions for noise and THD measurement setup in loop back. Once completed, the base line is established and the DUT is inserted for measurement, Very helpful for learning the proper settings. Here's a example of the units THD on loop back. As the FFT values are increased, the noise floor drops, revealing the harmonics. Not bad THD for such a device.
Notice my old friend, the 60 Hz spike.
I have cables everywhere, and am going to address this best I can. And, I've seen similar spikes on other spectrum analyzer screen shots, and have read that some employ a 60 Hz notch filter.
This link will be my guide in reducing noise. Pretty interesting as it shows setups for testing amplifiers.
https://hifisonix.com/wp-content/uploads/2021/09/Measurement-with-the-QA401.pdf
 

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Gepetto

Gepetto

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#11
George S. said:
Been working through the "Getting Started" documentation. Lots to learn and it makes logical sense. There are instructions for noise and THD measurement setup in loop back. Once completed, the base line is established and the DUT is inserted for measurement, Very helpful for learning the proper settings. Here's a example of the units THD on loop back. As the FFT values are increased, the noise floor drops, revealing the harmonics. Not bad THD for such a device.
Notice my old friend, the 60 Hz spike.
I have cables everywhere, and am going to address this best I can. And, I've seen similar spikes on other spectrum analyzer screen shots, and have read that some employ a 60 Hz notch filter.
This link will be my guide in reducing noise. Pretty interesting as it shows setups for testing amplifiers.
https://hifisonix.com/wp-content/uploads/2021/09/Measurement-with-the-QA401.pdf
Click to expand...
Yup, 60Hz is everywhere...
 
Gepetto

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George S. said:
The QA403 has differential inputs and outputs while the QA400 is single ended.
Did some reading about differential inputs and outputs today.
Click to expand...
It is like the HP inputs (and possibly other gear as well) in that it has +/- inputs and outputs for each channel.

This means to get the benefit of the differential loopback you are working on, you will have to connect two coax cables from + to + and also from - to -
 
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George S.

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For preamp testing, need to build four differential cables. Am short on those awesome Amphenol 69475 BNC and Switchcraft 3502A jacks I've been recycling for reuse. Ordered what I need to build them using the Beldon 83284 50ohm RG-316 mil-spec cable. I have plenty of that cable on hand.
Found a nice way to the bush the Switchcraft RCA's with a rubber grommet from the bent nail collection.
 

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George S.

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Gepetto said:
It is like the HP inputs (and possibly other gear as well) in that it has +/- inputs and outputs for each channel.

This means to get the benefit of the differential loopback you are working on, you will have to connect two coax cables from + to + and also from - to -
Click to expand...
Joe, I see your point on the loopback. Was following the "how to" at that point and they hadn't explained the differentials yet. Did some more reading today on differentials themselves to understand how they work and discovered I need to make some preamp test cables. I think this will do it?
Two cables to a single RCA. On one, center cable conductor wired to center RCA terminal, no connection to RCA ground or cable shield. Shield floats.
On the other, center cable conductor wired to RCA ground only. No connection RCA center conductor or cable shield. Shield floats.
On both cables to the RCA, the cable shield should have no continuity with the RCA plug.
Then install both BNC's normally, and mark them for polarity + and -.
 
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George S.

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Differential loopback on the left channel. Thanks Joe! 50 ohm shorting caps on the right input.
Ditched the powered USB hub. Laptop provides sufficient voltage and current on battery. Using a Monoprice Monolith "audio" heavy USB cable.
The software monitors the USB voltage and current and the box changes color if it drops out of spec. Nice!
 

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George S.

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So on loopback, the gain is appx 6 dB higher using a differential configuration versus single ended configuration.
Being that these are marketed primarily for audio engineers and manufacturers, I've got a lot to learn.
That's good though. I can concentrate on this and quit spending money on other stuff.:p
 
Gepetto

Gepetto

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George S. said:
So on loopback, the gain is appx 6 dB higher using a differential configuration versus single ended configuration.
Being that these are marketed primarily for audio engineers and manufacturers, I've got a lot to learn.
That's good though. I can concentrate on this and quit spending money on other stuff.:p
Click to expand...
Correct George, you should have picked up that extra 6dB since the signal is differential. Was there any reduction in the background noise floor and the 60Hz spike as a result of going balanced?
 
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George S.

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Gepetto said:
Correct George, you should have picked up that extra 6dB since the signal is differential. Was there any reduction in the background noise floor and the 60Hz spike as a result of going balanced?
Click to expand...
Joe, yes, noise appeared to drop substantially. Wish I could put a number on it, but I was rather focused on the increased gain. Looking back on the material and pictorials I read about differential inputs and outputs, it makes sense.
Going to do a lot more reading about it this work week. Lots of digging to do on the QA Forum. I have yet to post a question there yet. Want to try to understand as much of this as possible first. I have noticed that many of the guys there do operate single ended. May have to do with lack of documentation.
I'll get this figured out. The built in attenuators are also somewhat of a mystery at this point in regards to the big picture. Interesting how the internal relays click as internal attenuation is changed.
Probably be a couple weeks until I can get back to it, but I can read in the meantime. Thanks Joe.
 
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