Aussies - Silicon Chip 20W Class A Amplifier opinions?

Hi there, it catched my eye as well, did somewhone here already build it? Before I spend 500 bugs on a kit I rather would like to know how how das it really perform.
Looking at the picture I have my doubts about the pream and the capacitors it seems pretty cheap stuff for a 500bugs kit.
  • Like
Reactions: 1 user
Well I dont need much power for a single fullrange speaker therefore 20W are OK.

Never the less do you know what type of transistors do they use?

Im not only looking at the THD, but at the amp as it performs, therefore I would like to see an oppinion of somewhan who accualy did it and copared it with other A class amps.
Joined 2010
Paid Member
Hi Imagios
The trannys are MJL21193/4. The parts are certainly not boutique grade at that price but middle grade, all 1% MF resistors, MKPs and many 6.3 mm spade connections. Get a decent ratchet crimper unless you have a swage to suit or just mighty strong. The caps are as would be supplied in midrange commercial quality amps up to even $2,500 AU.

If you believe class A will be adversely affected by using such as Panasonic FC,M grade PSU caps, then replacing 6 x 10,000uF 50V will not be an obstacle for you.

I will say that distortion as low as claimed and without crossover artifacts is certainly distinct, dead quiet and genuinely the closest I have heard to studio production quality sound in the 20W design. If there is a problem, it is the specially wound toroid which is prone on some samples to a buzz at the constant 2.5 amps load which can be audible at close proximity. Resilient mounting has solved the problem for me in two builds.

The remote controlled preamp, incidentally, is buffered and amped with LM4562s which are amongst the very few opamps even in the running for the job.

I would not try to compare sonic qualities with any other amplifier as this is not a "musical" amp. However, if you need accuracy, IMHO, this is it for DIY. Frankly, I do like it very much because the imaging and soundstage is genuine, pin sharp, and absolutely stable. The rest is in the clarity to resolve massed voice sounds and and details that class AB generally struggles with. Try it with headphones, near field monitors or any decent, efficient speakers of 6 ohms and above and you will hear a difference to the rest....absolutely!

There are tweaks to the input pair balance, as mentioned here, which can make a significant improvement but its also impressive as it comes too. You will have to add your own little five button Philips RC5 code remocon and 230V power, however.
hi wixy,

The standard kit sounds really good. If you mod it to SandyK's recommendations you will have an amp that will be hard to beat. Do search for other threads about the older 15W version.

If you have a Altronics account you will save $60.


Agreed! I have the 15w version with Alex's mods, this amplifier has lots of potential . I've tried a lot of amps and this is one of my favourites
To be honest most likely I would order the kit just PCB´s with a partlist and the case, and exchange some of the parts as you wrote. I mean look at a amp with that low distortion and on the in put site is a cich, just how much gets lost from the pream to the amp if insted of solderig wire from part to part, you solder the cinch socket and than make the cable and conect them...
Still I really concider the kit, but definately with changes in the area of capacitors and even I would like to change the resistors to for example Vishay resitors and so on. If the distortion is by design so low than it should be supported by matching quality of components and I mean they are not so much more expensive than the standart ones.
IMHO, Silicon Chip should not have included the preamp as part of this kit.
It is similar to what is used in the Studio Series Preamplifier, and it is very mediocre indeed, and does not do justice to the power amplifier section.
For starters,there are several unnecessary coupling capacitors, and there should be a series resistor (e.g. 47 to 100R) for stability reasons at the output of the first section, to the cable going to the volume control.
The designer, conceded that in hindsight several of the coupling capacitors were not needed, due to the low offset of the I.C.s used. However, he never did put anything to this effect in subsequent issues. They could have done a lot better than using those MJL output devices too. SC has even mentioned that in a recent issue.

Last edited:
Joined 2010
Paid Member
Hi SandyK
I believe that the main issue, stability concerns or nay, is to avoid DC of any flavour on the volume control. The ICs run very stably in my examples, and as far as I am concerned, DC is the much greater worry.

Oooh! Those oh, so nasty NP caps are solely for that purpose! I removed them initially and listened to the even nastier rasping sound as I turned VR1, exactly as predicted in the SC text and accompanying the kit.

I even placed a pro. JVC pot there which is known to outperform ALPS blue vevets and their successors. Yes, it still made an annoying groaning sound! If you have a motorised volume control like this and are lazy when really enjoying your music like me, you will not like it all.

Caps it is, unless you prefer the hair shirt routine; jumping up at every need to hit the quiet knob(s).

As far as what SC design and publish, I think it's obvious that commercial demand drives all serious kit design. Retailers like Jaycar and Altronics thrive on strong full kit sales and profit flows back through advertising, right? The DIY segment is strong but not the whole market nor the full kit sales market either.

These retailer guys push the wheels with like 40 years of experience at predicting what will sell and what will die on the dusty shelves. I suspect that whatever our spartan monobloc or mega-featured integrated amp dreams may envision, there will be another guy who wants plain vanilla class D and half price wares. If he is flashing the cash and we don't, there'll be no nice surprise, sorry.
Hi Ian
I wasn't suggesting that all of them be removed.The I.Cs used should have had a typical DC out of 2mV or less. I had no problems with mine.
Another person found that series resistor necessary when he swapped in an LM4562.I replaced the 22uF 35V with a 100R 1% , and left in the 2.2uF 35V, but removed the 47uF 35V NP as it was not needed due to the low DC out of <2mV.
Nevertheless, the SQ of that preamp is far below that needed to get the most out of the Class A.
Long relay PCB traces also resulted in quite a bit of channel separation decrease at >10KHZ with the original preamp. Although sounding much better than it did originally, mine now languishes in the back of a cupboard somewhere.
IMHO, Silicon Chip should not have included the preamp as part of this kit.

Well right now the preamp is my biggest concern,... I mean OK I will definately exchange the capacitors in the power section, and on the amps, to some Mundorf´s and Wima´s. The transistor MJL21193/4 would be by quality as well depending on the manufacturer, but here I dont know any good one. I might as well concider to buy them by a sell who sells them "paired" .

Newer the less I would like to have a remote pre amp, I would really like to know the figures of this one, distortion ect. I saw on the photos of it they use a chip, do you know by any chance what type it is?
Looking at the photos I really beliwe the engeneer made a nice job, but the amp seems really to deserve bether parts and preamp. May be Sandyk is right. I would like to know how the amp would perform with a tube preamp :D
newer the less I for my self dont have a work banch at home I do all my work on my computers desk and therefore I need a as much as a complete kits as it gets. Thats why I like this one...
Joined 2010
Paid Member
Errr, before jumping into the cap issue, Imagios, they are 22uF! Wanna swap those for for MKP or even MKT?.....I didn't think so!

Realistically, whatever you feel may be a potential preamp improvement, both OPA2134 and the supplied LM4562 opamps come in below typical measurement THD + noise floor. Comparisons of improvements will remain a moot point in the application. Maybe someone will volunteer a new AP 2720 system to resolve such questions but probably not today, I think.

The limiting factor is actually PSU radiation, not amp or preamp shortcomings, according to designer/presenters John Clarke and Greg Swain. Sort the radiation first and maybe then you could justify digging down to O.00001% THD for some reason that escapes me for the moment. Perhaps a Halcro amplifier is hiding inside the SC 20 Watt!
I wonder do anyone can tell me that the replacement of the MJL21193/4 cause my country cannot found this transistor! I think i only got the following list to replace~

NJL3281DG /MJL3281AG NJL1302DG


MG9413 MG9412

2STC5200 2STA1943


Which will have the best audio result, i am making a DIY circuit by using this amplifier design, and the current mirror design can have the best current flow and balanced the input stage!
I’ve just started construction of my SC 20W class A but it contains some variations because it will be used to drive a pair of RAAL ribbon tweeters above 3kHz via a passive external crossover network.
The crossover slope is optimum with a 5.6 ohm resistor at the crossover input, so this resistor is now moved into the amp itself which negates the need for the output inductor.
The load impedance is hence really 5.6 + 8 = 13.6 ohms.

Main modifications are as follows:

1. Rail volts raised from 22 to 42.
2. LTP current reduced from 6mA to 3mA and 220R emitter degeneration.
3. Use of 3 transistor current mirror. (all mpsa-18)
4. 22 ohms of VAS emitter degeneration added
5. VAS and current source transistors changed from BC546 / 556 to Toshiba drivers (2SC3423 2SA1360 Transistors - Buy Online @ AmpsLab)
6. Driver transistors as above (Toshiba)
7. Outputs stage emitter resistors – Mills 3W 0.1R
8. Output Transistors changed to Sanken LAPT (
9. Compensation changed from CMC to TMC.
10. Output inductor removed and amplifier output impedance set to 5.6R.
11. Quiescent output stage current reduced from 1.2A to 600mA
12. LTP / CM devices and resistors are matched to <0.2%. High HFE devices used here too.
13. Beta enhancer transistor changed to high HFE MPSA-18 @ Iq = 1mA.

This configuration will provide 10W of class A into 13.6 ohms and up to 50W class AB.
I’ll provided some measurements when completed.