Which of these designs would you choose?

[nobody special]

I'm trying to decide what to build, but have narrowed it down to a couple different choices...

System specs:

speakers: 4 ohm 92db/w
amp chassis (1 per channel): 120W of heat sinking.
26V+26V rails.

I have 2 designs I'm considering:
The first is an F4 power buffer biased up a little higher than normal. If I go this route, I will build an Aikido type tube front end for it that will also function as a preamp. This should do 35W into 8 ohms and be very close to class A into 4 ohms.
The second is similar to an X150- fully complementary folded cascode mosfet front end. I would build a matching X preamp. It should do about 35W class A into 8 ohms, about half that into 4 ohms and about 150W AB into 8 ohms.

I put a scope on my speakers while they were at a volume that was slightly louder than I ever listen, and I saw a voltage equivalent to about 60W into 4 ohms.
Right now, I'm leaning toward the first design, but I thought I'd get some opinions. I have enough material to eventually build both, but it takes me a LONG time to get around to finishing anything, so I wanted something good for a while. What do you think?

 

[Zen Mod]

BA3 Bal

plenty of bugzes in output , biased to whatever you need

or - if you don't need balanced - plain F4 , 5-6 deep , 22-24Vac xformers , biased as you prefer

 

[nobody special]

Thanks, Zen Mod. I'm not sure what a BA3 is, but I'll look it up.
I've got 700VA of xfmr, 66,000uf per rail, 26volts per rail, and lots of IRFP240/9240.
I really should look into different speakers, but I built these about 10 years ago and have enjoyed them. They are a somewhat difficult load, so it limits my amplifier options.
I really do appreciate your reply. I know I'm not really a regular around here, but I stop in from time to time. You used to be Choky, didn't you?

 

[nobody special]

BA3- very interesting. I had a similar design I was considering at one point that used feedback, but of course wasn't nearly as elegant as Mr. Pass's design (to be expected). I guess that's 3 options now. Thanks for complicating things for me, lol.

 

[Zen Mod]

..... You used to be Choky, didn't you?

luckily , I still am

 

[giuliodido]

Hi,

I'm using this thread since I have more or less the same doubt, and since I find redundant opening another one (I recall that in these days there is at least another active suggesting thread). I hope that nobody special is ok with this.

I have an 800VA 2x24V toroid which I would like to use in one of the Mr. Pass' designs. I already made the F5 and the Aleph J: I find both of them excellent, but I prefer by far the Aleph. I originally planned to build a F5T, but after the Aleph J, and reading about the various BA, I have some doubts.
I plan to use this amp with a pair of Maggies.

Any advice is really well accepted. Many thanks, also to nobody special for hosting me 🙂

 

[Zen Mod]

which Maggies ?

if I remember correctly - all of them are not so efficient , but ruler flat impedance module?

 

[6L6]

I already made the F5 and the Aleph J: I find both of them excellent, but I prefer by far the Aleph. I originally planned to build a F5T, but after the Aleph J, and reading about the various BA, I have some doubts.

Skip the F5T and build a BA-3 Complimentary. 😀

 

[nobody special]

Any advice is really well accepted. Many thanks, also to nobody special for hosting me 🙂

No problem at all with you posting your question! But, I do think you would get better results posting a new one, as I seem to be anathema here, lol (not sure why).
It sounds like you have a similar problem with speaker load which leads to a similar question. I really wanted to build an Aleph 30, but I didn't have enough heat sink to bias it up hot enough for the low impedance, and they weren't really known to drive a difficult load well. I like the idea of simplicity in a circuit, which is one of the reasons I'm looking into the X design, which can be very simple with a stout follower output stage. I'm currently thinking about doing a hybrid of the two concepts above: F4 type followers with a single 12ax7 in an X arrangement, with AC feedback around the outputs.

 

[Zen Mod]

12AT7 , if you must choose from that family

or 12BH7

or 6DJ8

 

[nobody special]

Why?^^
I would like to (and might) use 6SN7, but probably not enough gain.

 

[Zen Mod]

12AX7 is too wimpy

12AT7 still plenty of gain , but more muscle

12BH7 - as AT on steroids

6DJ8 ......... we are starting to be serious ....

 

[nobody special]

great, thanks!
Here's what I'm thinking for the front end, only substitute 6DJ8 and adjust values accordingly:
I think I will double up my intended output stage to 6 pairs each side followers, biased as around 120W dissipation, 26V rails.

 

[Zen Mod]

small trimpot from both lower cathodes , wiper to CCS - to maintain DC symmetry ;

besides that , bleeder resistors on outer side of output caps

 

[jacco vermeulen]

I'm currently thinking about doing a hybrid of the two concepts above: F4 type followers with a single 12ax7 in an X arrangement, with AC feedback around the outputs.

Yeah !

(afair, his name was ChokeMe)

 

[nobody special]

good ideas, and thanks again. It's not really a complete schematic- just a presentation of concept, so keep the ideas coming.

 

[nobody special]

Yeah !

(afair, his name was ChokeMe)

lol

 

[giuliodido]

which Maggies ?

if I remember correctly - all of them are not so efficient , but ruler flat impedance module?

Sorry for the late reply but I was completely burdened at work.
I have 0.5QR, they are 85dB efficient and should have an almost flat 4 Ohm impedance plot. But for the future I plan to upgrade to used 1.4-1.5-1.6, or possibly 1.7.

6L6, thank you for the suggestion. I see that BA-3 front end is quite similar to F5, and that your suggestion of complimentary pairs goes along with the F5 similarity. In this case, what would be the difference between the two amplifiers? And then, how many complimentary output pairs will be good enough?

Thanks again, also to nobody special 😉

 

[6L6]

Yes, the front end is very similar to the F5, except that it has no feedback and has an output capacitor. Where it differs is in the output - the complimentary BA is a 'source follower' configuration, meaning it's very neutral and musical, and has a lower output impedance, making it easier to drive lower-impedance speakers.

If you have the room, try 4-6 complimentary pair/channel. The limit is heatsink.