RMAA - PM1000 Freq Sweep... Very Strange!

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khstudio

Well-known member
Joined
Dec 27, 2005
Messages
2,116
Location
New Jersey, USA
This pre sounds GREAT but what is going on in the low end? :shock:

The right channel is the raw, sound card looped back in & the white/left channel is the PM1000.

pm1%20-%20frq.png


The output cap is a Panasonic FC 100uf, stock value was 33uf & was Bi-Polar... could the fact that the new cap NOT being Bi-Polar cause this?
The EQ was set flat & it had the same curve with different gain setting. I also have 2 units & they both react the same. :roll:

Kevin
 
Cast your eyes to the db scale on the right of the graph...

Note where the -1db and -3db points are; 25hz and about 12hz...

So whats the problem? This is transformer coupled 'vintage' gear. Thats a fken good low end response if you ask me!



M@
 
That's what I wanted to hear :thumb:

I wasn't sure what the "V" was @ 10 hz :?

I've been doing a lot of testing of my equipment with RMAA (Thanks for the tip BTW) & NONE of my other gear looked like that in the low end... that's all.

I thought someone more experienced than me could tell me what I was looking at.

Remember Matt... I'm still pretty new to this DIY stuff but getting better.

Thanks to you guys.

Kevin
 
All of that subsonic weirdness is just the tester freaking out, you can ignore it. Just look at the -1 and -3 points and the general shape of the curve. There shouldnt be any drastic changes except below 10hz, where the tester seems to get confused sometimes. Remember youre not testing to make official freq THD specs to be published, just to make sure theres nothing weird going on that you missed in listening tests. Also its a quick way to see what the effect of changing transformer loads.

If your soundcard can go to higher sample rates, test at 96 or 192k. Then you can clearly see the effects of changing zobels/loads on the transformers without the anti aliasing filter getting in the way at 20k.


M@
 
I feel much better now, thank you :green:

If your soundcard can go to higher sample rates, test at 96 or 192k. Then you can clearly see the effects of changing zobels/loads on the transformers without the anti aliasing filter getting in the way at 20k.

I tried & it won't let me for some reason... my card CAN do 96k though.

BTW, The only thing I haven't tried (on this or my API/YAM pre) is loading the OT secondary with the 620 or 680 ohm resistor while testing with RMAA.


Kevin
 
This looks like FFT-based analysis, and the high value on the left-hand Y-axis is the d.c. from your soundcard. There are too few frequency bins in this part of the X-axis to show the bottom end response reliably. The program is simply joining what bin values it has with straight lines, hence the visually ugly result. The response looks fine.
 

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