Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Equipment & Tools From test equipment to hand tools

Question on QA400
Question on QA400
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Closed Thread
 
Thread Tools Search this Thread
Old 9th July 2015, 06:43 PM   #1
Alan0354 is offline Alan0354  United States
Banned
 
Join Date: Apr 2014
Location: US
Default Question on QA400

I went through a few threads on this here but I still have questions. I just started looking into method to test THD, my experience in the pass was using spectrum and network analyzer at work. Obviously those are a little too expensive to buy for home use. So I am absolutely a newbie in all these sound card, and low budget equipments.

I read complains that the QA400 has only single ended input. For power amp, why is it important to have differential input? Signal is large. AND even if the ground noise show up, it's going to be a peak at 60, 120Hz. People should know those are ground issue. It should not affect reading at 1KHz and up.

I don't want to build a differential probe with opamp if I can help it, also I try not to spend $79 to buy the QA190. Attached is my interface circuit I think will work.

The top one is from the output of the power amp to input of the QA400. I use a 4ohm 100W load, then I use lower resistance value to do the dividing. I put the 470ohm in series to current limit and use the zener to limit to 4.7V or so. I have a rotater switch to select divided by 2, divider by 10 and 100. Don't read too much in the resistor values, it's approx.

For input to the amp, I take the frequency output from the QA400, just drive through a volume pot and drive the amp.

I do this is all passive so I don't add any distortion. I don't isolate ground. My laptop is powered by two prong plug, there is no ground. I don't need to worry about ground loop. worst come to worst, I unplug the laptop and run on battery!!

What do you guys think?

Thanks
Attached Images
File Type: jpg QA400 interface.jpg (99.7 KB, 128 views)
 
Old 9th July 2015, 06:52 PM   #2
Alan0354 is offline Alan0354  United States
Banned
 
Join Date: Apr 2014
Location: US
I also have a THD power attenuator actually designed for guitar amps to attenuate the power driving to the speaker. It is basically a power volume pot between the amp and the speaker. It has compensation inside to make the load reactive like a speaker. It even has line out. I have to look deeper into whether this can drive the QA400 directly.

This is for guitar amp, I don't know how well it will work. Also sadly, it's a 8ohm load, not 4ohm that I want.

Any guitar player have experience with this?

Mind you, this is for guitar amp with speaker that has NO CROSSOVER, usually the speakers are 12" with no mid and tweeter. The impedance might be a whole lot different from a hifi speaker. It's just so nice to all the load resistor, switches all packaged up in a box.
Attached Images
File Type: jpg DSC_1473.JPG (373.5 KB, 128 views)
File Type: jpg DSC_1474.JPG (341.3 KB, 127 views)

Last edited by Alan0354; 9th July 2015 at 06:56 PM.
 
Old 9th July 2015, 07:28 PM   #3
skidave is offline skidave  United States
diyAudio Member
 
skidave's Avatar
 
Join Date: Jul 2006
Location: Pennsylvania
I know this thread is long, but towards the beginning, there is voltage divider information and a schematic by 1Audio. Good reading, but a bit long.
 
Old 9th July 2015, 07:42 PM   #4
Alan0354 is offline Alan0354  United States
Banned
 
Join Date: Apr 2014
Location: US
Quote:
Originally Posted by skidave View Post
I know this thread is long, but towards the beginning, there is voltage divider information and a schematic by 1Audio. Good reading, but a bit long.
I read it and I downloaded the schematic. That's the reason I posted this. I don't think I need all that as described in detail in the OP.

On top, the schematic shows it drive through a 1K resistor. The AC input impedance is 10K, that itself creates error.
 
Old 9th July 2015, 10:46 PM   #5
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Question on QA400
Your attenuator will work fine. I would skip the divide by 2 step. There is still potential to damage the QA400 (2VP max really) and its noise and dynamic range is good enough to work with a divide by 10, and then the display is scaled correctly if you follow the instructions for setting it.

Add a 100 Ohm resistor in series with the ground so a balanced amp or other mishap won't fry anything.
__________________
Demian Martin
Product Design Services
 
Old 10th July 2015, 12:33 AM   #6
Alan0354 is offline Alan0354  United States
Banned
 
Join Date: Apr 2014
Location: US
Quote:
Originally Posted by 1audio View Post
Your attenuator will work fine. I would skip the divide by 2 step. There is still potential to damage the QA400 (2VP max really) and its noise and dynamic range is good enough to work with a divide by 10, and then the display is scaled correctly if you follow the instructions for setting it.

Add a 100 Ohm resistor in series with the ground so a balanced amp or other mishap won't fry anything.
Thanks for your answer, you are the one that really tested the QA400. I am waiting for answer from QuantAsylum about the 10K input impedance. In my circuit, I actually has 470ohm to try to protect the input, I hope it's a typo and I can raise to 1K like yours or even higher. They are just very slow in reply from reading the other posts.

I am still reading your long thread a little at a time.
 
Old 10th July 2015, 07:14 PM   #7
skidave is offline skidave  United States
diyAudio Member
 
skidave's Avatar
 
Join Date: Jul 2006
Location: Pennsylvania
I use this divider with my soundcard (I do not own a QA400). This works very well and I have no problem testing high wattage power amplifiers.

Click the image to open in full size.
 
Old 10th July 2015, 07:34 PM   #8
skidave is offline skidave  United States
diyAudio Member
 
skidave's Avatar
 
Join Date: Jul 2006
Location: Pennsylvania
For what it's worth...it took me a while to get to where I am at with device testing. Many members have helped and I thank them all the time.

Here is one of my amps after a repair. I wanted to see when I would reach 1% distortion; this was around 250 Watts. Top image is ARTA scaled to match my HP3850a spectrum analyzer (bottom).

Click the image to open in full size.

Click the image to open in full size.
 

Closed Thread


Question on QA400Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
QuantAsylum QA400 and QA401 1audio Equipment & Tools 2667 18th February 2020 10:58 AM
QA190 and QA400 Gains sgtnoah Equipment & Tools 4 17th July 2015 04:46 AM
BigE's F5T build (Split from QA400) BigE Pass Labs 141 4th February 2015 07:00 PM
FS QuantAsylum QA400 audio analyser jan.didden Swap Meet 3 29th September 2014 06:10 PM
newbie with input tran. question and wiring question imo Solid State 0 18th January 2006 11:10 PM


New To Site? Need Help?

All times are GMT. The time now is 03:50 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki