Adjusting P3 - a video

[radule]

radule,
What is your setup for testing. I'm assuming output from F5 is going into a dummy load of 8Ω or 4Ω and then into sound card? What sound card and do you need to attenuate signal before going into sound card?

Yes and for attenuation I using 1kohm helipot in paralel with dummy load to prevent overdrive of the sound card line in. If you want to use same, take care about disipation on the potentiometer, I dont care because I have really big one.

Sound card is Asus D1.

 

[AndrewT]

Use a protected input to your sound card.

Use diodes and/or zeners preceded by voltage dropping resistors.

I suggest you always connect via a sound card protection circuit.

 

[nashbap]

Great work

Looks like you are on the out of phase side of H2 minimum. So you have an out of phase 2nd harmonic.

Hope this all makes sense now.

2 picodumbs your explanation is good and makes it easy to understand since we are now referring to H2 with the reference of increasing or decreasing R3/R4.

Could you explain how this works with a differential signal where the two phases are combined. I have made a BA3bal pre and am feeding both output phases of a channel to a input transformer that then feeds my SE mono amps. If we were to look at Kevin's P3 table and assume that his left and right is now plus and minus how do we go about increasing H2, for example when we combine phases? Do I decrease R4 in the plus and negative phases of each channel by equal amounts?

Thanks. nash

 

[KevinHeem]

I'm back playing with P3 again on my BA3/FE. This time with Arta spectrum analyzer running on a MacBook Pro which is running boot camp.

What would be the difference in distortion at a 1V output vs. 10v output?

The reason I ask is I don't want to smoke the sound card in my computer, so I only dare to put 1V into the computer.

 

[gior]

Isn't it possible to measure output voltage via a voltage divider in parallel to the load?

 

[asanden]

Does somebody have a tip how you can show residual harmonic distortion waveforms like in the video with an usb soundcard (I have an E-mu 0202) and software (for instance Arta).
I manage to measure amp distortion harmonics on FFT, but it would be nice to monitor a residual waveform also.

 

[gior]

My 2 cents;
The link here (http://www.linkwitzlab.com/frontiers_7.htm#G2) shows a way to output at same level than input then maybe you could try to subtract input from output (opamp?) and use second channel of sound card to plot it

 

[vdi_nenna]

I'm back playing with P3 again on my BA3/FE. This time with Arta spectrum analyzer running on a MacBook Pro which is running boot camp.

What would be the difference in distortion at a 1V output vs. 10v output?

The reason I ask is I don't want to smoke the sound card in my computer, so I only dare to put 1V into the computer.

You have to put something in between the amp and sound card to not burnout the sound card. Search FFT spectrum analyzer: http://www.diyaudio.com/forums/pass-labs/258613-diyaudio-firstwatt-f6-56.html#post4352055

 

[asanden]

Thanks,
I could not find a software solution so far, so I'll indeed try a simple opamp solution (attenuator for amp signal, two source followers with an instrumentation amp stage w/ gain 1).
Also a signal trafo could work maby (2 windings in, 10k, 1/2 out) but I dont have much experience with them.

My 2 cents;
The link here (Issues-7) shows a way to output at same level than input then maybe you could try to subtract input from output (opamp?) and use second channel of sound card to plot it

 

[andyr]

Well, you can turn the pot, listen, take notes, and try again...

Do you have a signal generator and a soundcard? You can FFT the signal and see the harmonic spikes.

Don't wanna turn the pot and listen! I want to see it on the screen.

I do have a sig-gen and a CRO. As I understand the video, the bottom trace is the output of something ... can you tell me what this is?

Thanks,
Andy

 

[McLanz]

CD Player 1khz Source sufficient?

Hi 6L6,

I just bought a used HP334A distortion analyzer.

Do you think a CD Player as source (playing a test cd) is good enough? I see many videos with extreme low distortion signal generators.

 

[coolnose]

I "think" you are looking for THD spectrum, that is relative measurements, 2nd vs 3rd harmonics, not chasing absolute "low" numbers...

 

[coolnose]

I "think" you are looking for THD spectrum, that is relative measurements, 2nd vs 3rd harmonics, not chasing absolute "low" numbers...

 

[Pheonix NJ]

I watched this video several times. Please correct me if I'm wrong. You can make the adjustments shown in the video with just a scope. You don't actually need the distortion analyzer?
I understand you won't know the distortion value but you can try the match the waveforms as seen in the video. Do I have that right?

 

[Ryssen]

I did it on my computer using audioTester

 

[KevinHeem]

Listen/watch again at about the 1:12 point…he explains that the source of the second trace has been run through a spectrum analyzer “steep notch filter”.

 

[6L6]

You absolutely need the distortion analyzer to adjust it as shown.

 

[Pheonix NJ]

Yeah, I'm a dumbass. I was tired last night. It struck me this morning after coffee, the lower trace line was from the distortion analyzer. Thanks for the answer. I feel stupid asking.

 

[6L6]

There are no stupid questions. This is a hobby, its supposed to be fun, illuminating, and satisfy our curiosities.

 

[Andersonix]

Asking for an easier way to adjust p3 is not stupid, so keep 'em coming PheonixNJ!