Power amplifier 2x15W for 800 ohm speakers


Designed by my old friend Jon Idestam-Almquist
who just turned 87 and is still going strong.


From an article in "Radio & Television" April, 1964.
If you visit any public library in Sweden and want to read this number of the magazine, you will most likely find that these pages are missing. This amplifier and the associated loudspeakers were very popular during that period. The pictures and test data below are from one unit found on the Swedish version of Ebay, called Tradera.




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

Parts list

trådl. = wire wound

When testing I used an attenuator to reduce the output signal from 100 V to 1 V in order to protect the instruments.
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 %

We tested three amplifiers and all had different test values.
Here is a summary of our 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 . We 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 we did not have.