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Old 26th August 2012, 10:02 PM   #1
fotios is offline fotios  Greece
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Default SET-NO FEEDBACK amplifier performance

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.
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File Type: png 300B-PSE 0_THD+N.png (129.5 KB, 250 views)
File Type: png 300B-PSE 0_THD+N_FULL8.png (132.6 KB, 243 views)
File Type: png 300B-PSE 0_THD+N_BAND.png (110.0 KB, 241 views)
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Old 26th August 2012, 10:11 PM   #2
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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
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Old 27th August 2012, 04:37 AM   #3
fotios is offline fotios  Greece
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Quote:
Originally Posted by ruffrecords View Post
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
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Old 29th August 2012, 03:56 PM   #4
palmas is offline palmas  Portugal
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looks quite good, even too linear for a SET...
Congrats
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Old 29th August 2012, 04:42 PM   #5
fotios is offline fotios  Greece
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Quote:
Originally Posted by palmas View Post
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.
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