DIY Simple Class A Amp - How to get started?

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47688

Disabled Account
Joined 2018
Hey guys,

I am pretty new to the DIY hifi world and would like to build a simple class A amplifier as a first project. I have done some soldering before of which I would not be afraid, but I am far away from being an electronics buff. So please bear with me... :blush: The amplifier would be used mainly for Rock music in an office room of 4 x 4 x 3m.

A model that seems to have been recommended a lot for A model projects on a budget is John Linsley Hood's 1969 design.

Would you consider that model a good starting point? :rolleyes:

Considering that I am a beginner, I would also prefer a complete kit of components, rather than having to search and purchase individual electronic parts.


So far, I found the following Chinese kit options:

- DIY Option 1 - Aliexpress

- DIY Option 2 - Aliexpress

- Soldered Aiyima Kit - Aliexpress


What do you think of those? Are there other options from other suppliers?

Which other components would I need and could you recommend, in particular in terms of power supply?

Would appreciate your feedback and advice. :)


Thanks a lot,

J.
 
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xrk971

xrk971

Founder of XSA-Labs
Joined 2012
etsy.com/shop/XRKAudio
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JLH 1969 is a fine amplifier, but ask yourself if you will be happy with only 10w on your speakers. You need really sensitive speakers to make that work.

A variant of JLH on Ali is a 25w version that uses modern easy-to-mount TO-264 BJTs:

25Wx2 Hood 1969 Amplifier Kits Class A Power Amplifier Audio Board 15A 2SC5200 Tube HD1969 With Electronic Filtering AC12V AC28V-in Amplifier from Consumer Electronics on Aliexpress.com | Alibaba Group

But I think JLH is now about 50 years old. There are a lot of newer and better topologies since then - and they are available on DIYA if you look. For example, here is a Pass F5 parts kit:
F5 parts Kit – diyAudio Store

And here are the boards:
F5 – diyAudio Store

Here is the PSU board:
Universal PSU – diyAudio Store

Or you can do the dual-SMPS trick like I suggest below - it's actually quieter than a linear supply. Look here for proof:
Strange Forest of Noise with Linear PSU

With any amp though, a good high current PSU and the case with heatsinks will be the backbone of the system and the major costs.

If starting out, you can save a lot of money and complexity by using a pair of 24v 5amp SMPS connected int series for +/-24v PSU that doesn't require connecting a large mains transformer, rectifier bridge, cap bank etc...

You can also use a CPU heat-pipe cooler with PWM fans and spend maybe $50 on a cooling solution vs hundreds of $ for a passively cooled finned case. Here is how I did it with the Alpha amp (another excellent newer Class A topology that happens to be 100% adjustment-free and zero-DC offset self adjusting):
Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier
 
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huggygood

huggygood

Member
Joined 2011
"With any amp though, a good high current PSU and the case with heatsinks will be the backbone of the system and the major costs."
"That J L Hood amplifier and the article about it is my recommendation.
Go for it and choose an kits. :)
DSCN3336.JPG

the most simplest of all :)
 
J

john_ellis

Member
Joined 2006
The original JLH used Veroboard IIRC. There are not many components in the original design, so you may find it suitable to build. I second the recommendation to go for the original circuit as it is simplest; you could use (modern) 2N3055's for the OP transistors, but see my notes on optimising this - they need higher supply voltage (and bigger heatsinks) than originally spec'd to keep distortion low at 10W output (see the JLH thread, recent posts).
My concern too is that you might find the output power a little low. You could however build several and use parallel speakers.
 
avtech

avtech

Member
Joined 2017
The JLH is a mostly easy kit to assemble if you can solder. The only tricky part I found was mounting the transistors - care must be taken to insulate them from the heatsink. I like the idea of the 2SC5200 kit now I finished the 2N3055 kit!

I got the single supply version of the JLH kit which I powered with 1 SMPS brick. It is dead quiet with no input but required properly shielded input cables.

This kit runs hot so you need to use big heatsinks - I used undersized heatsinks supplemented with fans that keep things nice and warm.

As others mentioned, you need sensitive speakers to get the best out of those JLH kits - research your intended speaker set up as well and then find an amp kit + speaker combo that suits you, then have fun!!
 
J

johnsurnamerobinson

Member
Joined 2018
I actualy built the hood amp back in the 1980s sounded very very good
circuit is nice and stable and not too fussy about transistors used one thing I did find
was the amp was sensitive to output impedance and the constant current supply (bias current) had
to be individualy adjusted to speaker load ( to avoid premature clipping ) also do make sure your
heat sinks are big and i mean BIG as amp is single ended class A and will dissapate a LOT of heat!!!
Overall an exellent choice for a starter project
 
Ian Finch

Ian Finch

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Joined 2010
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There are a number of complete and tested Chinese JLH'69 and other small class A amps on the market which give an indication of an acceptable minimum heatsink size for home use. You can run amplifiers at temperatures up to the max. 85°C capacitor temp. rating but you wouldn't want to hold onto anything above 55°C unless you're OK with pain and blisters.

I have this one and in warm weather like 30°C, it becomes too hot to hold anywhere for long. Raising it on ~20mm blocks or feet allows much better airflow and lowers the temp. by 5-10C. In cooler climates, it may be acceptably warm, as it is here in Oz during winter.
 
xrk971

xrk971

Founder of XSA-Labs
Joined 2012
etsy.com/shop/XRKAudio
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