[ndm]

Okay, what method would I use to measure the frequency response of an amp with no speakers connected? I am one of those nitty gritty type of guys and would like to test the actual frequency response of the amps, dsp, etc...

This is more of a curiosity type of thing than anything else. I mean, I know that I can simply RTA my system with a mic and make adjustments after that but again, I want to know everything about the equipment that I am putting in.

There are some amps that folks used to say did not reproduce good midbass. I want to know why. My old alpine setups seem to have had a signature sound. I want to know why.

Hopefully you guys can understand the reason why but even if you dont and still have an answer then please do share. I am hoping for maybe a software solution (complete with instructions on hook up and evertyhing) but I guess hardware solutions will work too. I am not a rich man so hopefully I wont have to sell my kids to do this.(Never happen!!)

 

[Gadget01]

An oscilloscope is needed for that kind of signal analysis.

 

[Jepalan]

A lot of good information here...
Basic Amplifier Measurement Techniques | Audioholics

 

[TrickyRicky]

A lot of good information here...
Basic Amplifier Measurement Techniques | Audioholics

That resistor looks like my 1ohm 1kwatt wire wound resistors..those things are HUGE.

 

[Justin Zazzi]

Did you see the voltage divider method I posted in the other amplifier testing thread?
http://www.diymobileaudio.com/forum/2217283-post157.html #157

You can use Room EQ Wizard and the tutorial by Hanatsu to measure frequency response. Just add the voltage divider to protect your sound card, and make damn sure your output voltage stays below 0.3v

 

[ChrisB]

When I was into testing amplifiers, I used a M-Audio USB Mobile Pre, Rightmark Audio Analyzer, a Countryman Type 85 direct box, and dummy loads.

That combo allowed me to get pretty graphs like these:

 

[sqnut]

The only response that matters is the one at your ears.

 

[ndm]

An oscilloscope is needed for that kind of signal analysis.

I was not able to figure out how to test the frequency response with the O-scope that I own. I actually have 3 scopes but the other two are very old and both finally quit on me not to long ago. I loved the small portable one and the other was good but just huge. Both the old ones were made by tektronics

A lot of good information here...
Basic Amplifier Measurement Techniques | Audioholics

Yes that was good reading.

Did you see the voltage divider method I posted in the other amplifier testing thread?
http://www.diymobileaudio.com/forum/2217283-post157.html #157

You can use Room EQ Wizard and the tutorial by Hanatsu to measure frequency response. Just add the voltage divider to protect your sound card, and make damn sure your output voltage stays below 0.3v

This is the route I am going to go since I already have REW and just need to build the divider

When I was into testing amplifiers, I used a M-Audio USB Mobile Pre, Rightmark Audio Analyzer, a Countryman Type 85 direct box, and dummy loads.

That combo allowed me to get pretty graphs like these:

That looks like a good route if the voltage divider does not pan out.

The only response that matters is the one at your ears.

Yes but I want to know everything about my signal before it gets to my ears.

 

[ndm]

Did you see the voltage divider method I posted in the other amplifier testing thread?
http://www.diymobileaudio.com/forum/2217283-post157.html #157

You can use Room EQ Wizard and the tutorial by Hanatsu to measure frequency response. Just add the voltage divider to protect your sound card, and make damn sure your output voltage stays below 0.3v

How does this look? I am hoping that it will allow me to test all my amps and all my RCA signals for each channel.
front 1- jl 300/2 max 24.5v to tweeters
front 2- jl 300/2 max 24.5v to mids
front 3- jl 300/2 max 24.5v to midbass
Sub - jl 1000/1 max 63v to sub

Also, this should allow me to test at the rca's too right? And should I be able to use my laptop mic in port?

I made the divider at 70 volts in just in case. I also made it so I never exceed .279v to my sound card.

 

[TrickyRicky]

pm sent on the tektronic scopes, lol...I need spare parts so hopefully you still have it.

 

[ndm]

pm sent on the tektronic scopes, lol...I need spare parts so hopefully you still have it.

Waiting on the photos I just took to load...then you will have a PM back.

 

[ndm]

pm sent on the tektronic scopes, lol...I need spare parts so hopefully you still have it.

I am not sure I really want to get rid of the smaller one I would like to fix it some day.....gotta think about that.

I figured I would post here instead of the pm.

 

[Justin Zazzi]

How does this look? I am hoping that it will allow me to test all my amps and all my RCA signals for each channel.
front 1- jl 300/2 max 24.5v to tweeters
front 2- jl 300/2 max 24.5v to mids
front 3- jl 300/2 max 24.5v to midbass
Sub - jl 1000/1 max 63v to sub

Also, this should allow me to test at the rca's too right? And should I be able to use my laptop mic in port?

I made the divider at 70 volts in just in case. I also made it so I never exceed .279v to my sound card.

This looks good for protecting your sound card, but there is something else you need to keep in mind. 70 volts across a 2kohm resistor will mean you need to dissipate about 2.5 watts of power in that resistor network. Most resistors you buy are either 1/4 watt or maybe 1/2 watt. So keep the ratio of the resistors the same, but bump them up by an order of magnitude. Use about 20kohm and 80ohm for the same effect and make sure you get 1/2 watt (or higher) resistors, then you'll be fine.

I do not think this will work with a microphone input on a sound card. 0.3v is the max for a line input, and microphone inputs work off much less voltage so you'll likely distort or clip the signal if you use a mic input. Use a line input instead.

And yes you can use this setup to test RCA cables too, but you would be best to use a different set of resistors because low-level signals are somewhere between a volt and a few volts.

Be sure to calibrate your laptop sound card line in/out using REW, and also make sure you check everything with a volt meter before plugging it into your laptop!

 

[Jepalan]

Awesome thread. Can't wait to see OP's plots. This is exactly the kind of discussion that keep me coming back to DIYMA.

 

[ndm]

This looks good for protecting your sound card, but there is something else you need to keep in mind. 70 volts across a 2kohm resistor will mean you need to dissipate about 2.5 watts of power in that resistor network. Most resistors you buy are either 1/4 watt or maybe 1/2 watt. So keep the ratio of the resistors the same, but bump them up by an order of magnitude. Use about 20kohm and 80ohm for the same effect and make sure you get 1/2 watt (or higher) resistors, then you'll be fine.

I do not think this will work with a microphone input on a sound card. 0.3v is the max for a line input, and microphone inputs work off much less voltage so you'll likely distort or clip the signal if you use a mic input. Use a line input instead.

And yes you can use this setup to test RCA cables too, but you would be best to use a different set of resistors because low-level signals are somewhere between a volt and a few volts.

Be sure to calibrate your laptop sound card line in/out using REW, and also make sure you check everything with a volt meter before plugging it into your laptop!

Now I am confused. What is the difference between a line in and a mic port?

Pardon the stpidness on my part.

 

[ndm]

Awesome thread. Can't wait to see OP's plots. This is exactly the kind of discussion that keep me coming back to DIYMA.

This I admit is actually a second attempt to get an answer. I originally posted another thread asking this same question in a different way but was never able to quite grasp the answers I was given. So I changed the question slightly and low and behold....I'm finally catching on. I also cannot wait to actually see the goofy frequency response of my stoopid factory stereo. It is driving me bonkers.

Once I know exactly what the signal is doing at each point(high level, rca after dsp, amplified) I will really be able to (hopefully) get somewhere with this factory integration before I simply give up and ditch the factory radio for good.

I will as always follow up on this thread though as I get really annoyed when people don't post a follow up after they have fixed an issue.

 

[TrickyRicky]

I am not sure I really want to get rid of the smaller one I would like to fix it some day.....gotta think about that.

Does the larger unit have the four feet on the back? I need the trigger level knobs as well.

 

[t3sn4f2]

This might be worth reading also.

Real-Life Measurements | Stereophile.com

Here's a typical FR for non highend fullrange class D amp when presented with a loaded versus simulated load.

Fig.1 Sonic Impact Super T, frequency response at 2.83V into (from top to bottom at 2kHz): simulated loudspeaker load, 8, 4, 2 ohms (0.5dB/vertical div., right channel dashed).

 

[ndm]

Does the larger unit have the four feet on the back? I need the trigger level knobs as well.

Yes and yes

 

[TrickyRicky]

How much for the four feet and the trigger level knobs, shipped. Also does the smaller one have the same style feet as the larger one? If so then I'll take those as well....I have two large tektronix missing all their plastic feet.