Build options and tech questions

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Sesquipedalio

Sesquipedalio

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Joined 2018
#1
Hello Everyone,
I would like some tech build advice.
Background: I am relative beginner on amp builds having only started with ACA 1.6 a few months ago. I loved the build and adore the sound and am looking for a follow up - especially to put in front of a pair of Thiel 2.3 (4Ω, 87dB). I liked the clear direction and all-in-one parts kit. Due to the availability of parts and clear directions, my target for next build was either an F5 or Aleph J. I have searched the web for comparisons and it sounds like most people prefer Aleph-type amp sound - or maybe that from descriptions I believe I might enjoy it more. I am looking for something to be in the same vein as the ACA, but more powerful.

Some questions: When I looked at the F5 it is listed as - 25w into 8Ω and 40w into 4Ω (as I have commonly learned is normal to have inversely more W output into lower Ohm speakers). However, the Aleph J is listed on FIRST WATT PRODUCTS as 25w into 8Ω and 13w into 4Ω (to me strangely proportionally and lower Watt output into lower Ohm speakers).
Questions:
Why is the output power on the Aleph J LOWER into lower Ohm speakers?

Should I be concerned, given my (4Ω, 87dB) speakers, trying to put the Aleph J in front to drive them?
Understanding my background, and that I do not want to build a 3rd anytime soon, are there better suggestions/choices for my second build than either the Aleph J or F5 or if one of those is recommended, which one?
 
#6
BA3 would be my suggestion for speaker with that load. Perhaps BBA3 mono blocks, depending on listening habits.

M2 has some of the warm attributes you mention liking. If AlephJ is your thing, you might Query Zenmod about availability of his Bablefish version, which can ride up considerably on the power scale. Same as his kit for M25, or "Unconstipated M2."

IMHO, F5 isnt what you seem to be looking for. An excellent amp, can drive difficult loads, but not much gain. I would not classify its "flavor" as warm either.

If you are driving those speakers with the ACA amps now, I suppose Aleph J would do it a little better, but most here combine First Watt Amps with fairly efficient speakers....most.

The BA3 can be a warm sounding amp, P3 is adjustable and seems more warm to me than F5. If built like 6L6's build guide, and as I did, use a 24 volt transformer for (in my case) 31 volt rails which will give around 45-50 class A. I did it in Deluxe 5 chassis...otherwise mono blocks in Deluxe 4...but if going to expense of mono blocks, go Deluxe 5 cases and do BBA3 mono blocks. That should give around 100 watts class A and drive most anything.

One more thing, the ACA kits are complete kits designed to be put together successfully by most anyone so inclined. Like you said, clear instructions, everything there, most decisions made for you. These other projects are not that. They are boards and maybe partial packages like F6 which include the transformers. Chassis are available in the store, or can be made if you are up to it.

Having a complete kit, or refurbing an existing amp is one thing, but the jump from bare boards to a 3 dimensional amp is another leap. Be prepared to read the threads, including changes to BOMs etc. 6L6's build guides are much help here. I would read them all. most use the same chassis and very similar power supplies. Most wiring queries can be figured out by going over them. Take notes, go over your plans in your head until you can see a finished project. Ask questions for clarity. Get your boards and formulate or gather BOM lists for your part orders. Ask questions when you see something isnt the same everywhere, mistakes are made, part substitutions happen. Ask about your choices before you order. Have boards in hand so you can ascertain parts will fit.

Test power supply first, have at least a dim bulb tester, and preferably a variac as well.

Finally, build as intended. Unless you really know what your doing, forget about "better ideas" and build it as designed. Once working, you can ask about and make changes. Nice to have a working amp first, otherwise trouble shooting is much more difficult.

Source your jfets...non counterfeit are harder and harder to find. Mark's M2x uses alternatives to the hard to find toshiba jfets, as does Zenmods Bablefish and M25.

Good luck, and have fun.


Russellc
 
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#7
Understood. Correct on the Thiel recommended. I was hoping that some Firstwatt at 30-40 Watts -> 4 Ohms would work based on my experiment. I, of course, already attached my newly built ACA 1.6 (8W -> 8 Ohms) to the Thiels and it sounded just as sweet as as into my more efficient work desktop [Triangle Comete] speakers. The amp was clearly very under powered, but I was overall pleased and surprised that the sound character remained.
I am also entertaining moving away from the Thiel and to much more efficient speakers...
 
#10
Amps that lower Watts into lower Ohm speakers

Attila I said:
the output power/ohm discrepancy you noticed
is a result of different output stage concepts
(single-ended vs. push-pull) look at the data
Click to expand...


Thanks for the post Attila. I would like to have this explained to me like a kind teacher would explain it to the complete [electronics] noob that I am. I understand [in a basic way] the water flow analogy assigned in many online resources describing how amplifiers increase watts output to lower ohm speakers. I already spent WAY too much time trying to research this using "the data" and the web and got nowhere - hence the question in the post 😉
 
#11
If you drove those speaks with ACA, you understand the compromise clearly. Aleph J would maybe drive them a bit better, and would be a great amp when more efficient speakers come to you.

As qwertyl said, M2 would be a little more capable, it is push pull. Most Push pull amps, like F5 can put out more power, but slide into class AB doing so. With single ended, it is what it is, and more difficult lower loads result in the power observations you are making. They dont slide into AB like some pushpulls can.

BA3 with 30 volt rails will make the 40 plus watts you are looking for, but You are at the limit without cascoding to protect the Jfets. Mine has run fine for several years now. As this DIY bug sinks its teeth in, you will gravitate towards higher efficiency speaks. Your Theils are fine speakers they just prefer a little juice.

Russellc
 
#12
Sesquipedalio said:
...Questions: Why is the output power on the Aleph J LOWER into lower Ohm speakers?....
Click to expand...

Has nothing to do with how many ends the amp has.

This is a class A amp. When designing for A you have to pick an idle current similar to your Peak load current in your intended load impedance.

The Aleph J's manual says:
Maximum output voltage +/- 21 volts
Maximum output current +/- 2.5 amps

To get the most out of *both* these numbers you want a load of 21V/2.5A or 8.4 Ohms.

If you present a 4 Ohm load, the 2.5A times the 4 Ohms makes only 10V peak; you are not getting the full available voltage, the load Z is too low.

In principle you "could" redesign for 0.7X the voltage and 1.4X the current, arrive at the same 25 Watts and the same heat. However many details will change and must be checked. Do you need more big transistors for the higher current? Does the input stage have enough headroom at the lower supply voltage? And possibly higher gate capacitance? And how do you modify the current in that very-clever bias scheme? There's 3 resistors (7 parts) enforcing idle bias and push-pull action.
 
#13
Russellc said:
As qwertyl said, M2 would be a little more capable, it is push pull. Most Push pull amps, like F5 can put out more power, but slide into class AB doing so. With single ended, it is what it is, and more difficult lower loads result in the power observations you are making. They dont slide into AB like some pushpulls can.


Russellc
Click to expand...

At watt point 😉 does the M2 begin to slip into class AB, if at all.
 
#15
I have no idea as to M2. It is rated to deliver 25 watts into 8 ohms, and 40 into 4 ohms. What happens after this into stated loads I do not know. It isn't in the owners manual on the First Watt site, but is likely in the M2 thread. Obviously, it isn't the same amp as F5, which I understand does deliver more, just no longer in pure class A.

Russellc
 
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#16
if FW amp is declared as 25W/8R , it usually delivers all of these 25W in A class

then go to 4R load , A class envelope is restricted with Iq , so it goes to 50% of previous - around 12W5 ...... then you hear KLUNK! and amp goes to 40-50W in AB class (Aleph and u-follower OS stages being somewhat wimpier in final numbers, due to limited Ipeak)

only way to increase A class envelope (without obvious first chooice - rising Iq) is going XA25 style - removing source resistors , and mosfets are allowed to Berserk

Babelfish M25 is example od that , having 40W of A class on 4R load , then Klunk!-ing rest to 50W/4R
 
#17
Zen Mod said:
if FW amp is declared as 25W/8R , it usually delivers all of these 25W in A class

then go to 4R load , A class envelope is restricted with Iq , so it goes to 50% of previous - around 12W5 ...... then you hear KLUNK! and amp goes to 40-50W in AB class (Aleph and u-follower OS stages being somewhat wimpier in final numbers, due to limited Ipeak)

only way to increase A class envelope (without obvious first chooice - rising Iq) is going XA25 style - removing source resistors , and mosfets are allowed to Berserk

Babelfish M25 is example od that , having 40W of A class on 4R load , then Klunk!-ing rest to 50W/4R
Click to expand...

Thanks for the explanation Zen Mod! 👍🏻
 
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