Power amplifier 2x15W for 800 ohm speakers


Designed by my old friend Jon Idestam-Almquist

From an article in the Swedish magazine "Radio & Television" April, 1964.
With kind permission by Jon Idestam-Almquist 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