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Low-mu preamp (from AudioXpress 2/04)

This is a preamp I designed using a low-mu power triode (7236, 5998, 6AS7, etc.) with a pentode (EL34) CCS load.

Thanks to AudioXpress and Ed Dell, the full magazine article is now available for download!

"A Low-Mu triode Preamp" (LARGE 1.3MB PDF file)

Here's the schematic (33 kB PDF file)

Here's the mechanical layout as a AutoCAD DWG file (290 kB) or an AutoCAD R12 DXF file (501 kB).  You can download DXF viewers and conversion programs from the net, just do a search.

 

Some more photos (click on the picture for a full-sized image):

 

 

Parts sources:

 

McMaster-Carr Stainless steel and Aluminum sheets, hardware, 3M high-performance transfer tape adhesive

www.mcmaster.com
Telephone (630) 833-0300

 

Cambion Single-point terminals (like part number 572-4827-01-05-16)

www.cambion.co.uk

Telephone (800) 947-1256

 

USECO Fasteners Single-point terminals (like part number 1417)

Telephone (401) 461-5554

Specs:

7236 tube:

 

THD + noise (800Hz 80kHz bandpass):    

                1kHz, 1V RMS out, 100k load                          0.05%

                20kHz, 1V RMS out, 100k load                       0.06%

                1kHz, 1V RMS out, 10k load                             0.07%

                20kHz, 1V RMS out, 10k load                          0.07%

                1kHz, 2V RMS out, 100k load                          0.10%

                20kHz, 2V RMS out, 100k load                       0.10%

                1kHz, 2V RMS out, 10k load                             0.14%

                20kHz, 2V RMS out, 10k load                          0.13%

 

Frequency response (using 250k pot see text):

                2V RMS in, 1V RMS out, 100k load

20Hz: -0.09dB                     20kHz: -0.45dB                   -3dB @ 65kHz

                2V RMS in, 1V RMS out, 10k load  

20Hz: -0.13dB                     20kHz: -0.41dB                   -3dB @ 70kHz

                2V RMS in, 2V RMS out, 100k load

20Hz: -0.06dB                     20kHz: -0.95dB                   -3dB @ 40kHz

                2V RMS in, 2V RMS out, 10k load  

20Hz: -0.11dB                     20kHz: -0.96dB                   -3dB @ 40kHz

 

Output impedance:             450 ohms

 

Noise:                     -55 dBV (mostly 60 Hz)

 

Voltage gain:        3.96

 

6AS7G tube:

 

THD + noise (800Hz 80kHz bandpass):    

                1kHz, 1V RMS out, 100k load                          0.15%

                20kHz, 1V RMS out, 100k load                       0.12%

                1kHz, 1V RMS out, 10k load                             0.16%

                20kHz, 1V RMS out, 10k load                          0.13%

                1kHz, 2V RMS out, 100k load                          0.18%

                20kHz, 2V RMS out, 100k load                       0.24%

                1kHz, 2V RMS out, 10k load                             0.33%

                20kHz, 2V RMS out, 10k load                          0.27%

 

Frequency response (using 250k pot see text):

                2V RMS in, 1V RMS out, 100k load

20Hz: -0.05dB                     20kHz: -0.40dB                   -3dB @ 65kHz

                2V RMS in, 1V RMS out, 10k load  

20Hz: -0.11dB                     20kHz: -0.43dB                   -3dB @ 65kHz

                2V RMS in, 2V RMS out, 100k load

20Hz: -0.08dB                     20kHz: -0.60dB                   -3dB @ 55kHz

                2V RMS in, 2V RMS out, 10k load  

20Hz: -0.12dB                     20kHz: -0.56dB                   -3dB @ 55kHz

 

Output impedance:             324 ohms

 

Noise:                     -55 Dbv (mostly 60 Hz)

 

Voltage gain:        1.92