Powered up the one channel of the SECFA amplifier without drama or black smoke, Haha! Very smooth 
Borko’s Recomendation to center VR6 before installing it was perfect, the 'O' node was only 1.5v off from half the psu voltage.
I’m set up to use just VR20 for bias adjustment, no additional switches or resistors. For initial bias current VR20 trimpot was adjusted to 50k, after the amp settled down and heated up, it’s currently adjusted to 63K and thats good for 1.3A with a 39.5vdc psu. The psu consists of an Antek AN6432 trafo and a single rail SLB psu with the PNP attached to the heatsink via flying leads.
The DC offset is virtually 0, hovering between -1mV to 2mV.
So far so good!! Next phase will be MUSIC!!!!
Borko’s Recomendation to center VR6 before installing it was perfect, the 'O' node was only 1.5v off from half the psu voltage.
I’m set up to use just VR20 for bias adjustment, no additional switches or resistors. For initial bias current VR20 trimpot was adjusted to 50k, after the amp settled down and heated up, it’s currently adjusted to 63K and thats good for 1.3A with a 39.5vdc psu. The psu consists of an Antek AN6432 trafo and a single rail SLB psu with the PNP attached to the heatsink via flying leads.
The DC offset is virtually 0, hovering between -1mV to 2mV.
So far so good!! Next phase will be MUSIC!!!!
Attachments
Powered up the one channel of the SECFA amplifier without drama or black smoke, Haha! Very smooth
Borko’s Recomendation to center VR6 before installing it was perfect, the 'O' node was only 1.5v off from half the psu voltage.
I’m set up to use just VR20 for bias adjustment, no additional switches or resistors. For initial bias current VR20 trimpot was adjusted to 50k, after the amp settled down and heated up, it’s currently adjusted to 63K and thats good for 1.3A with a 39.5vdc psu. The psu consists of an Antek AN6432 trafo and a single rail SLB psu with the PNP attached to the heatsink via flying leads.
The DC offset is virtually 0, hovering between -1mV to 2mV.
So far so good!! Next phase will be MUSIC!!!!
Vunce, could you please measure few DC voltages, Vgs of J112 (Q2) and voltage over R7? for comparing...
Thanks Gary
Got first sound tonight! Music sounds good, very quiet background, I couldn’t hear any noise, hum or hiss at idle. As usual the real listening can’t happen until the second channel is born and moved to the big system. But all signs are good and it passed the ‘quiet’ test. Heatsink temperature is 47°C, directly on the mosfet/BJT devices is 50°C @1.4A bias
Hi Borko,
I attached a marked up schematic with the vital health statistics of the SECFA. How does she look Doc?
Note: R9 is 22R
Got first sound tonight! Music sounds good, very quiet background, I couldn’t hear any noise, hum or hiss at idle. As usual the real listening can’t happen until the second channel is born and moved to the big system. But all signs are good and it passed the ‘quiet’ test. Heatsink temperature is 47°C, directly on the mosfet/BJT devices is 50°C @1.4A bias
Hi Borko,
I attached a marked up schematic with the vital health statistics of the SECFA. How does she look Doc?
Note: R9 is 22R
Attachments
Last edited:
Thanks Gary
Got first sound tonight! Music sounds good, very quiet background, I couldn’t hear any noise, hum or hiss at idle. As usual the real listening can’t happen until the second channel is born and moved to the big system. But all signs are good and it passed the ‘quiet’ test. Heatsink temperature is 47°C, directly on the mosfet/BJT devices is 50°C @1.4A bias
Hi Borko,
I attached a marked up schematic with the vital health statistics of the SECFA. How does she look Doc?
Note: R9 is 22R
everything is ok, the only difference is that I get little higher (2.7-2.8V) Vgs but that`s due to J112 jfets... T1 has 0.3W dissipation at idle and 0.6W at max output swing, maybe it`s better, just in case, to attach smallest heatsink you have, little metal plate or something like that... But I didn`t had a problem during the measurements at full swing..
Any data about max dissipation for TO126 without heatsink?
Maybe, for the numerous noobs reading this thread and interested in trying this amplifier, it would be good to add some more info about the power-supply.
For exemple, basic info like power/voltage of the transformer (and fuse value) to reach the (recommended?) +45V at the outputs of the power-supply at both R and L outputs
For exemple, basic info like power/voltage of the transformer (and fuse value) to reach the (recommended?) +45V at the outputs of the power-supply at both R and L outputs
Maybe, for the numerous noobs reading this thread and interested in trying this amplifier, it would be good to add some more info about the power-supply.
For exemple, basic info like power/voltage of the transformer (and fuse value) to reach the (recommended?) +45V at the outputs of the power-supply at both R and L outputs
Ok, no problem, I`ll include that in the article update...
