frequency response setup question

[andi]

Hello: this is a really simple question i hope but i'm struggling to find an answer elsewhere so i'm hoping you can help so that i can move on.

I am looking at measuring the frequency response of the pickups that I make for the guitars that I have built.

I have read all the main threads and am looking to use a  Focusrite audio interface to feed an excite coil and measure the response in RMAA...pretty standard setup.

Before is start building the integrator etc i wanted to check that my interface and PC were up to it.

My question is this...if i plug the output(headphone) of the Focusrite Scarlett directly into the input of the Focusrite and run a sine sweep...what should i see?

I was hoping to see a flat(ish) line ...but i dont. Does this mean there is a problem?

Apologies for no pics as I am at work...will try and post later.

I did try and test a random unloaded pickup into this circuit (as i have made the exciter coil) and the response was not the beautiful response graphs that i see others getting. 

My though is that my setup is flawed hence not seeing a flat response to a direct out to in test and that this flaw will overlay its self on any further tests.

Sorry for the basic question but hope someone can help

Thanks 

[stratotarts]

What software are you using to make the sweep? Also, on the FocusRite, the "input monitor" selection must be turned off. If not, some of the pickup response is fed back into the test coil and disturbs the readings.

Also, I can't see your gain settings but you also need to verify that the sine waves at the input are not clipping (that the input is not saturated). Rightmark displays the input on a scope in real time for that.

Otherwise, you are correct, you should see a virtually flat line.

[andi]

Thank you so much..yes it was the monitor being mixed back in....got a pretty flat line for the cable...weird FR for the random pickup but that could be a number of thing...at least I know the rig works..thank you again

[stratotarts]

The plot you posted looks completely wrong for any normal pickup. Something is definitely very wrong with your setup. If that is a plot of an input-output loopback it is also completely bad. Please post more information in order to get help. Which measurement circuit are you using, and what component values?

Unless you are just showing us the plot with the monitor switch engaged...

[andi]

I agree ..it doesn't look as I would expect. I've update the setup and its still odd. The plot below is from a new exciter coil (500 winds) attached to a new pickup that I would this afternoon...it then goes through standard tele wiring and then back into the focus rite. Using RMAA Pro sine sweep.It's a very similar shape...the exciter coil doesn't have a resistor in it...which I have seen in some setups.
Strangely when I half the volume, the peak gets higher...thoughts??

[antigua]

Feb 27, 2022 13:33:30 GMT -5 andi said:

Strangely when I half the volume, the peak gets higher...thoughts??

The resonant peak is lowered due to cable capacitance. When you put it to half volume, there's some amount of resistance between the pickup and the guitar cable, which therefor impedes the capacitance between the pickup and the cable, causing the resonant peak to increase. If the volume pot were somehow at the amp end of the guitar cable, that wouldn't happen. It goes to show that the tonal effects of a passive circuit extent from the pickups all the way the guitar amp's first buffering stage.

[andi]

thanks for the explanation...still I strange shape graph tho?

[antigua]

Feb 27, 2022 16:27:35 GMT -5 andi said:

thanks for the explanation...still I strange shape graph tho?

It does look like the roll-off is too soft, even with the pickups connected to the tone and vol pot, but I've rarely used an audio interface and Rightmark software for testing pickups, so there might be some oopsie pitfalls I can't think of, there always seems to be a few. 

It might be better to test pickups disconnected from the wiring, in order to get an isolated pickup measurement.  I looked at the spec of you audio interface focusrite.com/en/audio-interface/scarlett/scarlett-8i6  and I see instrument input impedance is 1.5 meg ohm, so it is suitable hardware.

500 turns on an exciter coil is also ideal. Make sure the exciter has no metal pole pieces in it, that it's just air in the core. 

[andi]

Hi thanks again for your comments. The first image I posted which showed the flat response of a cable a the response of another pickup was done without the guitar electronics. I attached it through the test guitar as I thought that was the problem 😂..there’s no slugs in the exciter coil it’s just a humbucker bobin. I’m going to add a resistor to the coil to see what happens then.

Thanks again…. I don’t really have a deep understanding of this stuff, I’m just following other peoples instructions

[antigua]

Feb 28, 2022 1:14:49 GMT -5 andi said:

Hi thanks again for your comments. The first image I posted which showed the flat response of a cable a the response of another pickup was done without the guitar electronics. I attached it through the test guitar as I thought that was the problem 😂..there’s no slugs in the exciter coil it’s just a humbucker bobin. I’m going to add a resistor to the coil to see what happens then.

Thanks again…. I don’t really have a deep understanding of this stuff, I’m just following other peoples instructions

I looked again at your pic and I saw that you have the headphone volume maxed out, and the input gain maxed out, try turning those down. 

Make sure to always use the "instrument" inputs, all other inputs won't work for this. I see its used in the picture, but just to reiterate. 

[stratotarts]

I agree, I have seen this kind of result from signal chain overload. Try monitoring the waveforms visually during the peak readings. I think you will see some distortion there, which you can fix by adjusting the levels in your signal chain.