Class A low power (5-10w) amp suggestion needed

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s3tup

s3tup

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Joined 2004
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#1
Hi all,

I'm building 3-way active xover speaker systems and stuck on selecting simple and quality amplifiers for duty of amplifying tweeters and midranges.
Not much power is neede in such duties, but quality is required.

Currently i'm using lm3886 chimp amps for all ways, but i think 50w is way too much, and although the chipamps are nice quality, i'd like to try going real solid state class A, without feedback if possible.

The power supply i have is giving 34v per rail (25v unrectified), 400va.

I'd like to stick to already made boards, because i can't make the boards on my own.


Thanks!
 
#2
The two that come to mind are the John Linsley Hood 10 watt Class and the Nelson Pass ZEN series. The JLH is just about to celebrate its 41st birthday but is still going strong. These two are originally single supply amps but with multi amping a small protective cap feeding the speaker can be made part of the cross over and the output cap dispensed with etc. (The later 1996 JLH came in split rail form.) The nice thing about both these is that the bugs have been ironed out of them. An exhaustively thorough website for the JLH can be found by entering "Class A amp" in Google and you'll find that it comes up pretty soon. Geoff Moss runs that. There is a thread here dedictaed to it with something over 10,000 posts. The Pass section of this forum is similarly detailed about the ZEN series.

I am not personally familar with the Pass design but both are highly regarded.

Edit: I think boards for both are available but both are v.simple and the JLH can certainly be made without a board by using point to point or "rat's nesting" to use the more formal expression....!

Good luck, Jonathan
 
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#3
I would also go for the old JLH design. I have build this design again and again... Very simple and always seems to surprice me. It really does something special. Only problem is the lack of control in the low frequency area. But if you are going to use it for tweeters only, this is no problem. Then you can also replace the large electrolytic capacitor in the output, by a small film capacitor.
This amp will give you "bang for the buck" like nothing else!
 
#4
...now that you mentioned JLH

Good day,

my JLH have been running (and heating my room in this tropical country:hot::hot::hot:) for the past six years. it's the 1969 design with dual rails, no output capacitor, I will not forget Rudy Van Stratum for this:worship:.

It has given me big pleasure in music 'till now. I did'nt know if anyone has tried but i recently discovered that by using tube buffers these amp sounded marvelously warm & "tubey"!!!

Great for those with inefficient drivers, as the JLH has 10W, compared to a SET that has 3-5W output, plus you'll be needing costly horn loaded, highly efficient drivers, unless you have a very healthy bank account😀😀😀

Best Regards🙂

efren
 
#5
Hi s3tup,

I'm doing exactly the same ;-)

For tweeters I'm going to build in the next days a low power ClassA Krell-clone with about 15W ClassA and low feedback.

If everything works well, I will offer boards.

Have fun, Hannes

PS: your toroid is not well suited for low ClassA-power amps as the rails are too high. Maybe you want to continue to use a chipamp for midbass?
 
#9
Might as well add Glen's K10A Hybrid amp :
www.diyaudio.com/forums/solid-state/119409-kleinschmidt-10a.html

Rotten shame that Glen Kleinschmidt recently decided to skippy these premises, but there are plenty of people left who have Glen's email adress in case you'd like to contact him for the K10A files.
Afaik, Glen sold all K10A boards of his 50 PCB run, but the single-sided board is easy to etch/drill yourself.
Other alternative is asking/posting for K10A boards, usually only a small fraction of distributed boards are put to use, most end up in a drawer to wander off in oblivion.
Your 25Vac transformer would fit right in.
 
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#10
If you are going for the JLH for the tweeters, use the '69 version, not the "updated" ones, since they ruin the excelleng HF performance of the original circuit.
The NOS output mosfets are possible to find with a little research.

It is not zero NFB, but the sound comes close to that.
 
#14
JLH and others

Telstar said:
They also change the output transistors and other little modifications to improve the LF... which worsen the HF performance, IMHO.
Click to expand...

I've got a JLH (original) as my #1 system. Can't complain. A good friend (PhD in electronics and an audio buff) has spent a decade or so trying to improve it (he runs ESLs) and thinks he's finally done so. Moral: improve with caution.

You could also look at Jean Hiraga's Le Monstre. Or, (shock, horror), the Patek chip amp which appears to go Class A at low levels. Or Musical Fidelity A1. None of which I've heard (I've heard Nelson Pass's Aleph, but not long enough to say more than: It is good)

Also check out this thread
 
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#15
Wow, many thanks for responses, i'll check them all!
Don't the minimalistic class-A designs have poor power supply rejection? The lm3886s swinging on the power rails could make a problem in this case for rock-solid constant current draw of class-a amps... Especially when speaker is connected directly to power rails (through current limiting resistor)

In addition i doubt my 400va toroid will handle well these little power eaters (as they tend to eat 10x more power than they able to give) 4x100 for 10w= all the toroid power is spoken for, and nothing left to feed the chipamps...
 
#16
S3TUP said:
Don't the minimalistic class-A designs have poor power supply rejection? The lm3886s swinging on the power rails could make a problem in this case for rock-solid constant current draw of class-a amps... .
Click to expand...

Definitely look at separate power supplies for A and AB sections.

With big class A designs swinging chokes or batteries are worth a look (less humm etc). O'wise you're looking at some pretty big hum numbers or another "amplifier" to regulate the power rails.
 
#17
Good blank PCB to start with

2pcs of 100W Sanken Mono Power Amplifier PCB DIY - eBay (item 160399965992 end time Mar-01-10 19:47:36 PST)

Found this board because of another thread. A little re-biasing of the front end (with gain set for no feedback) and increase of the number of diode (should be shottkey) in the output bias circuit and this would make a lovely small class A amp. This board has such a sound basic design I may buy a pair myself and build a small super wide band amp!

This will likely need a servo to control DC offset or at least a trim pot. Attached is a schematic showing how to the two pots to trim the input and output offsets.

Would probably regulate the front end (cut traces) to like 20-25 volts and run the output stage off your unregulated 34 volts
 

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#20
thoglette said:
Wahab,
I've been contemplating (a euphomism for "daydreaming") such an amp for a while. What examples do you have in mind? (The closest thing I can think of is the Musical Fidelity A1 )
Click to expand...

Threre s scores of schematics that can be found on the web...
Most, if not all , are designed for relatively high power utput.

Here an exemple that i designed, but it s also too powerfull,
as to met your requirements, 5 to 10 W , the power supply
should be no more than +-15V, and the schematic should be
slightly modded, 4 resistors and two zener must be modded...

Anyway, these kinds of schematics work very well...

http://www.diyaudio.com/forums/solid-state/155993-awb50-simple-power-amp.html#post1998387
 
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