Hi Everyone, Here is the answer I sent earlier in response to the PM (I deleted some extra information that is not germane to this thread)
The 2.2K resistors do two things. First they provide isolation between the two outputs. If you don't have the need for two you can eliminate one of the resistors. The second function is to help mitigate the effect of cable capacitance on the preamp. You can lower the resistance if the cable runs are short and / or low capacitance. With cables under 6 feet and of reasonable capacitance feeding power amplifier impedance over 10K you can go to a value in the 1K range.
The output impedance of the stage itself is in the 3K range. This is a fairly typical value and will work with nearly any power amplifiers. The best matches are when the ratio of impedance (preamp output to amp input) are more than 1 to 3 (approximately). That is why I said it was really best with tube amps. They are nearly always above 50K and more usually in the 100 to 500K range. However, most newer solid state amps now have impedances over 10K and for them it seems normal to be in the 50K to 100K range.
Yes the preamp uses DC heaters. I have not been able to determine which polarity through the heaters is better than any other one. Generally if I use a dual triode (like the ECC803S) with six volts on the heaters I will generally connect the center tap (pin 9 in this case) to the ground side of the heater supply. A consideration sometimes occurs. That is in the SRPP design when you are running in the 300B+ range or higher. The middle cathode will be at about half the B+ applied to the upper anode. To prevent heater to cathode failure you need to apply a dc reference voltage to the heater supply (like in the preamp). At the higher B+ levels it does make a slight difference. Then you need to figure out which half of the tube's heaters is heating which triode. Then you want to make sure (on 12 volt heater systems) which heater is going to be +12 and which will be 0. The six volt difference can make a difference in the dc reference needed. All this is to say that unless you use really high B+, the actual place you apply the dc reference is not important.
None of the components in the Forewatt are difficult to get. Most can come from Parts Express, but Mouser is probably a better source. Oddwatt Audio is not presently providing parts or partial kits.
The SS regulator is quite helpful in making the preamp quiet and a consistent performer.
Added: I want to thank all the folks that have interest in my projects. I try to design things that excel in performance and are fairly easy to duplicate. I appreciate the questions such as above as it is a trigger to either learn something new or pass on knowledge to others so that they may learn. That is a key aspect with my conception of diy.
Good listening
Bruce
Login
Register
FAQ
