Power amplifier 2x15W for 800 ohm speakers

Designed by my late friend Jon Idestam-Almquist (May 6, 1928 - Jan 23, 2022) From an article in the Swedish magazine "Radio & Television" April, 1964. With kind permission by Jon I have the priviliege to present not only the complete description for building the amplifier but also an earlier article that more deeply presents the construction. You will find these links at the bottom of this page. IMPORTANT NOTE: All material on this site is Copyrigt Jon Idestam-Almquist. You are only allowed to use the information for your own private purpose. You may not copy any pictures for use on your own site without permission.

Close up of the power supply, keep hands off, +460 Volts DC is present.

Fuses for the speakers in case you connect low-ohmic ones instead of the 800 ohms.

One of the pcbs.

Circuit diagram for one of the identical channels.

Power supply

When testing I used an attenuator to reduce the output signal from 100 V to 1 V in order to protect the instrument. THD at 0,5 W before replacing the ECC83s. L = -58 dB = 0,126 % R = -80 dB = 0,01 %

THD at 0,5 W after replacing the ECC83s with newer ones. -72 dB = 0,025 %

THD at 2 W output L = -71 dB = 0,028 % R = -73 dB = 0,022 %

THD at 8 W output -57 dB = 0,14 %

IMD at 2 W output. -54 dB = 0,20 %

I tested three amplifiers and all had different test values. Here is a summary of my test. The reason for different output levels on the channels is that the gain was not the same on both channels. An asterisk * means that the signal was clipped . I did not test the tubes for conductance or anode current. THD measured with 1 kHz IMD measured with 60 Hz and 8 kHz 4:1 DFD measured with 10 and 11 kHz

Amp 1 before changing ECC83s Amp 1 after changing ECC83s Amp 2 Amp 3 Tested above THD   L R L R L R L R 0,5 W 20 V -58 dB -80 dB -73 dB - 74 dB -73 dB - 72 dB -78 dB -79 dB 2 W 40 V -52 dB -73 dB -65 dB -67 dB -66 dB -66 dB -70 dB -73 dB 8 W 80 V -43 dB   -58 dB -60 dB -58 dB -58 dB -61 dB -65 dB 9 W 85 V   -64 dB         clipped clipped 13,8 W 105 V -35 dB*               15,1 W 110 V     -45 dB* -50 dB* -48 dB* -48 dB*     18 W 120V   -56 dB*             IMD                   0,5 W 20 V -47 dB -65 dB -60 dB -60 dB -60 dB -60 dB -62 dB -61 dB 1,5 W 35 V -41 dB               2 W 40 V   -58 dB -52 dB -54 dB -53 dB -53 dB -54 dB -54 dB 4,5 W 60 V     -47 dB -49 dB         5,3 W 65 V         -48 dB -48 dB     7 W 75 V             -41 dB -45 dB 8 W 80 V         -57 dB -57 dB     10 W 90 V -30 dB -47 dB -42 dB -45 dB -41 dB* -41 dB*     DFD                   0,5 W 20 V -60 dB -77 dB         - 75 dB -75 dB 2 W 40 V             -70 dB -71 dB 3,8 W 55 V -54 dB -70 dB             5,3 W 65 V             -64 dB -64 dB 8 W 80 V -45 dB               10 W 90 V   -65 dB             Tubes         ECC83 Philips other Philips One Philips One Telefunken Siemens EL86 Telefunken Telefunken Telefunken EI

The Telefunken tubes are probably quite used since the data are out of original specs. Test values will probably be much better with new Telefunken tubes, which I did not have.

Article from September 1962: Click on each picture to get a full size jpg approx 1240 * 1754 pixles Be aware of that the text is in Swedish Page 48 Page 49 Page 50 Page 51 Page 52 Page 53 Article from April 1964: Page 58 Page 59 Page 60 Page 61 Page 62 Page 63 Page 64 Page 65 Page 66 Page 67 Page 68