My 12AX7 zero negative feedback preamp, with measurement

[MQracing]

I have to jump on this bandwagon.

AlexW88--- what a beautiful job. those terminals just knock me out completely.

If it sounds half as good as your construction quality--- you'd have a real ear-pleaser!!!

kudos to you,

MSL

[alexw88]

Originally posted by jackinnj
Had to change the last pic since I had the BW set at 80kHz -- this is a plot of THD% from 20Hz to 100kHz -- but this isn't the entire story:

It is expected that the 2nd harmonic (and THD) will go down without the bypass capacitor, but did you noticed that 3rd harmonic went up? (as suggested by http://www.tubecad.com/2005/March/blog0039.htm )

Originally posted by MQracing

If it sounds half as good as your construction quality--- you'd have a real ear-pleaser!!!

MSL

Did I mentioned it sounds good? I really like how it sounds

[SY]

Alex, in your design criteria, you mentioned second harmonic distortion. I didn't understand what you wrote; did you mean that you deliberately wanted to have some second harmonic or did you mean that you wanted it as low as possible, but with whatever is there dominated by second harmonic?

[alexw88]

SY, I actually want some 2nd harmonic distortion because I want some "sweet" sounds.

I have done a Opamp balanced preamp which gives 0.0007% THD, with 2nd harmonics at 132dB below the fundamental. It sounds very accurate but at the same time it sounds too plain. That's why I want to try tube for its sweetness.

May be I am totally wrong about the relation between sweetness and harmonics distortion.... please share your view

[abzug]

Well, there was a much cheaper way to find out than build a tube amp--take your low distortion preamp and add some 2nd harmonic by software to a music file you rip to your computer...

If an amplifier is completely transparent, any euphonic distortion can be added by signal processing, which costs nothing.
If the amplifier is not transparent, then the type of distortion one can produce from it is limited by its inherent distortion, since it's very hard to fully characterize that distortion so that it can be canceled by predistortion in the signal processing.

[SY]

No "view," really. One can design a preamp to be an effects box or to reproduce the signal as exactly as possible. It's the designer's choice. If the former is your choice (which it was), you made exactly the right design decisions to get 2nd harmonic distortion.

[jackinnj]

Here's Alex's preamp with equal 1 volt output -- the lower series is with the 680R bypassed with 470uF -- the response on the analyzer is set to its initial condition of 10Hz to 500kHz

[rdf]

Distortion predominantly second and a substantial 240 (?) Hz component on the power supply?

[alexw88]

Originally posted by jackinnj
Here's Alex's preamp with equal 1 volt output -- the lower series is with the 680R bypassed with 470uF -- the response on the analyzer is set to its initial condition of 10Hz to 500kHz

How come you are getting 0.4% THD? That's 10x higher than mine. Can you post a FFT frequency spectrum showing the distortion content?

[jackinnj]

Originally posted by alexw88



How come you are getting 0.4% THD? That's 10x higher than mine. Can you post a FFT frequency spectrum showing the distortion content?

Because I am running the program flat out -- the THD% in an absolute sense is less important than the comparison between the bypassed and unbypassed RK -- done under identical conditions. If I engage the 400 Hz filter to get rid of the bumpies which arise due to the non-shielded nature of the filament leads, and the halogen which illuminates my workspace, etc.., etc. I will get a lower number. All I wanted to do was illustrate the THD% at identical output levels.