I have tested both. Linear is the conservative option. SMPS is cheaper, but has potential issues.
Hi TA
i am afraid of the SOA of that chip. Nelson Pass wrote in some very early comment that he tried a lot of MOSFET and if i remember right he give a kind of "warning" about SOA DC.
so the Figure 12--> SOA of that chip just show a DC area up to 25V.
just my 2cents
chris
I tested a pairing of FQA28N15 (still available) with IXTQ36P15P. It worked OK, but not nearly as good as with FQA28N15/FQA36P15.
It is important to consider that the "sound" of a particular FET combination is very dependent on the complete circuit topology. You should expect an undegenerated follower OS to sound different from a degenerated OS. This will be most apparent if the OS is operated without global feedback.
It is important to consider that the "sound" of a particular FET combination is very dependent on the complete circuit topology. You should expect an undegenerated follower OS to sound different from a degenerated OS. This will be most apparent if the OS is operated without global feedback.
Certainly it is best to exercise some judgement when using the IXTQ75N10P. I have been using them mainly in my F6 variants with +/– 26.5V rails, source degeneration and feedback. They also worked well in my souped-up ACA, which limits them to about 15V Vds.
The best thing about them, aside from sounding good, is that they are available. Unlike my previous faves the FQH44N10.
The FQA36P15 is rapidly going the way of the Dodo. Octopart listings are completely dodgy at this point, requiring an RFQ for 100 pieces or more to see if there are any parts actually in stock. I am tired of playing that game. I took a chance on Ebay with some NOS Fairchild FQH44N10 and got lucky, so I am using the last tube of those sparingly. I don't wish to rely on luck or committing to excessive quantities in sourcing parts for a new output stage, regardless of how well it simulates or measures in a sample of one amp.
If the "Holy Grail" output stage is to have some success, it will need to use parts that are really and truly available.
The best thing about them, aside from sounding good, is that they are available. Unlike my previous faves the FQH44N10.
The FQA36P15 is rapidly going the way of the Dodo. Octopart listings are completely dodgy at this point, requiring an RFQ for 100 pieces or more to see if there are any parts actually in stock. I am tired of playing that game. I took a chance on Ebay with some NOS Fairchild FQH44N10 and got lucky, so I am using the last tube of those sparingly. I don't wish to rely on luck or committing to excessive quantities in sourcing parts for a new output stage, regardless of how well it simulates or measures in a sample of one amp.
If the "Holy Grail" output stage is to have some success, it will need to use parts that are really and truly available.
I certainly welcome DIYers to try out any FET combos they choose. I am putting together a description of my FET gm measurements and pairing approach. But be ready to make resistor changes if needed for the bias circuit. So far the FET combinations I have tried only requires to 2 pot adjustments and changes to the R11 & R12 resistors in the optocoupler circuit.
