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fotios 26th August 2012 10:02 PM

SET-NO FEEDBACK amplifier performance
 
3 Attachment(s)
Hello

I ask your forgiveness for the new thread but no one replies me in the most important thing for me in the other thread. So the only solution I had was to open a new thread for a clear technical discussion.
I built a SET amplifier without any feedback using the same circuit of “Glass House 300Bse” stereo amplifier designed by Andy Groves (Audio Note) on which the only modification I did was to convert it in monoblock. To this I simply added a second 300B in parallel with the single one of the original circuit and I converted the output transformer primary to 1.5ΚΩ/350mA.
What interests me is if the performance of amplifier as is presented in the three FFT plots is acceptable for its class of design. In the first are presented all main parameters at 1Wrms/8Ω “Un – Weighted”, in the second the same but in full output power at 8Ω “A – Weighted” and in the third the frequency response (or bandwidth -3dB). No filters have been used in FFT analysis (e.g. number of Harmonics), the vertical (Y) scale is in dBV so you can make a direct estimation of harmonics level in real Volts (0dBV=1Vrms) and only the frequency response plot is expressed in dBr for a reference max amplitude at 0dB. In all plots the FFT smoothing window used was “Kaiser 6”. For the frequency response as stimulus used White Noise.
Thank you for any comment positive or negative.

ruffrecords 26th August 2012 10:11 PM

Looks fairly typical to me. Over 1% THD distortion at 1W, mostly second and third harmonics with the harmonics falling off nice and rapidly. When the power is raised by 10dB the THD rises by about the same amount but the higher harmonics are much more relatively prominent which is an indication of the onset of clipping.

Cheers

Ian

fotios 27th August 2012 04:37 AM

Quote:

Originally Posted by ruffrecords (Post 3141058)
Looks fairly typical to me. Over 1% THD distortion at 1W, mostly second and third harmonics with the harmonics falling off nice and rapidly. When the power is raised by 10dB the THD rises by about the same amount but the higher harmonics are much more relatively prominent which is an indication of the onset of clipping.

Cheers

Ian

Thank you so much Ian for your detailed technical review. You should know that is precious to me, because this amplifier is the first tube project that i built in my life. This well focused review has enlighten me a lot.
Thank you again.

Fotis

palmas 29th August 2012 03:56 PM

looks quite good, even too linear for a SET...
Congrats

fotios 29th August 2012 04:42 PM

Quote:

Originally Posted by palmas (Post 3144391)
looks quite good, even too linear for a SET...
Congrats

Thank you so much palmas for your kindness to look on the FFT plots and above all for your positive comments + congrats :)
As a newbie in tubes, i feel a big relief after your confirmation - as well this of ruffrecords - that i have obtained a good result.
Fotis


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