Upgrading a simple Class A amplifier with better components

[triamping]

I just bought a class A amplifier, that comes with a separate case with power transformer.
This component shows I think the important audio signal path.

1. Which components here might be worth desoldering and replacing with higher audio grade components?
I've heard good things about capacitors from: Nichicon KZ Muse, Nichicon KG gold Tune and Elna Silmic II.
Is it possible that I would hear better audio if I would replace these caps with "better, audio grade" caps?
I love soldering so if theres any chance I will take it!

2. In persuit of higher quality audio, am I allowed to simply ENLARGE the voltage or the capacitance (or both) without any riscs?
I want warm mids, deep base, and enough headroom for momentary energy going to the voicecoils of the speaker (active system, so no passive filters inside the speaker).

3. What are the two blue potentiometers for? I asked the distributor about what these are/do, and he simply replied:
"Please don't touch them: the designers have tested every single model by hand before shipment!"
So upon receiving the amplifier, what should I measure where, with my voltmeter sothat I can check (over time) that the potentiometers are indeed set correctly?

Thanks!

 

[DF96]

Is it possible that I would hear better audio if I would replace these caps with other caps?

Unlikely, unless the original caps are faulty.

In persuit of higher quality audio, am I allowed to simply ENLARGE the voltage or the capacitance (or both) without any riscs?

No. In order to successfully modify the circuit you first have to understand at least as well, if not better, than the original designer. Even if not harmful, there is no reason to suppose that increasing voltage or capacitance will necessarily improve audio.

What are the two blue potentiometers for?

They are probably for setting output stage quiescent bias. The correct setting depends on circuit details. They could be for setting the quiescent output voltage. The circuit diagram will tell you.

 

[triamping]

They could be for setting the quiescent output voltage. The circuit diagram will tell you.

Could I measure the voltage (set voltmeter to millivolts) on both speaker terminals and see if I get a sensible reading that is the same on both speaker terminal posts?

What range (in millivolts) would be "normal" or "expected" for a 10 watts class A amplifier? What range would be off / show a sign of a problem?

Thank you very much

 

[SyncTronX]

Hi,

Without a schematic etc, that's going to be difficult, at least for me.
Before making any changes hook it up and listen to it for a while.

From where did you purchase this amp?
Link please.

What is on the underside of board?
Pics please. I think this must be a kit?
Instructions that came with it would be helpful. Take pics and post.

The input section is in the center then moves outward to either side,
which would be left channel and right channel. (Assuming this is
the whole amp, stereo pair.

The output devices the the TIP41Cs from what I can tell,
from Fairchild/OnSemi. They are 6 amp Complementary
Silicon Power Transistors, General Purpose Amplifier type.
Probably 100v 65 Watts Max.

NOTE: With this small amp you probably aren't going to have big
booming deep bass unless you have high efficiency speakers. It should
sound nice I presume.

Solid OSCON type caps 220uf 25V, by the small signal transistors in the center. Those are fine. What transistors are these two?

These are the initial gain stage of the amp.
Small signal transistors and gain.

The green box poly or mylar caps are fine 1.0uf or .1uf
Signal coupling/blocking cap.

The Nichicon Vx (220uf 25V) are good caps.

Resistors all look good, either metal films and in some places carbon
films so those should be fine.

The big electrolytics 2200uF 16V 105C I don't know I don't see brand.
Maybe the KO 021 Type. If those are BC Vishays, those are good caps
too. 5000 Hour 2 (85C) to 200,000 40C hour (40C) life. DATASheet

The two pots, leave them alone, they are either bias, voltage adjust,
or DC offset adjust.

I'd say to hook it up and listen before you do anything. Have fun.
What speakers are you connecting them too?

Cheers,

 

[globalplayer]

You could upgrade the heatsink.

 

[analog_sa]

You could upgrade the heatsink.

This is hinted to be a JLH type amp, which fortunately is explicitly covered on this board. The Chinese are, as usual, highly optimistic about the performance: the advertised parameters are just silly.

If it were mine i would first of all draw an accurate schematic, measure the factory bias and supply voltage and decide what is a realistic speaker load if class A operation is preferred. The type and capacitance of the PS caps is also very important.

Aliexpress.com : Buy Pure Class A 1969 Amplifier Computer Desktop Mini Merge Split Type HIFI Audio Power Amp from Reliable audio power suppliers on Zhang Bao electronic commerce co., LTD Store

 

[analog_sa]

As already mentioned there are huge resources devoted to the JLH 1969 amp and here is a reprint from the original article. Most probably the Chinese interpretation is very close and differs only in transistor types. Judging by the modest heatsink size 10W per channel in class A into 8ohms are probably impossible. 20W into 4ohms are completely out of the question.

The caps possibly worth replacing are the input and output coupling caps. The input seems to be a small footprint polyester film and the output is unfortunately an axial which severely limits the possible replacements.

 

[cumbb]

Simple circuit and simple setup and good psu and 5 watt sound much better, cleaner, clearer and powerful than 100 watt complex circuit and complex setup and ordinary/usual psu;-)

This one will sound very good;-)

I would replace the little green caps. May be the output-caps too. Both 2 x 220 µF(?) should be good enough. Do all caps not bridge with little ones;-!
Resolder and set new joints I would.

 

[SyncTronX]

@Everyman,

Y'all are pretty good with answers to some of my questions along with some from EFTs as well.

@ analog_sa from the link and article with schematics, what recommendations for power supply caps would you recommend as EFT has yes to ask about the power supply module. Let us assume that he or EVERYMAN will be using the similar power supply.

And the answer is....

Note: Everyman shall be construed to include both sexes of GS:Homo Sapiens. Note2: never mind.

 

[analog_sa]

It seems the PS box has a small collection of Epcos caps inside. Plenty good imho.

 

[triamping]

Thanks sooo much for your elaborate written reviews all!

@SyncTronX:
"What is on the underside of board?"
I will listen indeed first and then (carefully) open everything up, make detailes pictures of top and underside and report back here.
As another poster statet its a pre-built cheap amp. I have no expectations at all. None! I could be surprised by terrible audio or good audio I am unbiased. My "reference" is the 26 kg sony ta-n80es. I will swap that amp with this 3.5 kg new amp and see what I descover. I will relisten al my music see hoe it goes.

@cumbb:
"I would replace the little green caps."
What other caps are better? Please provide a link of something better

"May be the output-caps too."
Which are the output caps? What would you advise replacing them with? Please provide link of better component you advise mate.
It is okay to replace with higher capacitance OR higher voltage OR both?

 

[traderbam]

Note what DF96 says.
Before you solder the power transistors, why not take the board off and draw the schematic diagram including all part values? It's a simple circuit so it shouldn't take long. Then post it. Without this it is sort of wreckless to suggest changes.

 

[e_fortier]

The only JLH I’m aware of that uses TIP41 is from e-bay and is called mini JLH which is rated at 3W output. That could explain the smaller heat sink

See here, the schematic is the second pic. 2 X MINI JLH HOOD 1969 CLASS A 3W AUDIO POWER AMPLIFIER KIT | eBay

Ps I don’t know this seller.
Eric

 

[cumbb]

@Eftegari

The green little ones: If MKS - sound dark and gritty - replace with elcos. The most sound better. Or polystyrene...-)))
Never big ones like MKP, FKP, Oil ...
My suggestion;-)

I would, moreover, connect booth "+" of the two 100 µF smd caps;-)

 

[SyncTronX]

That fleaBay ad, those pics make it look real bad.
and the transistors look like fake. Whatever Eft has
and others linked to, looks a whole lot better and with
quality parts.

Yes, you can upgrade them, but why not start with something better.

@cumbb, what are elcos capacitors? I googled and came up with
some kind of eye make up.

Good luck and keep us informed.

Cheers,

 

[ashok]

..... what are elcos capacitors?....

Electrolytic capacitors !

 

[analog_sa]

I googled and came up with
some kind of eye make up.

Cause you googled with the wrong spelling. Try "elko".

 

[Alan Frobisher]

I would think the pots are to trim the output offset to zero.Have a look at TCAAS site ,hosted by Rod Elliott, a slightly modified J.L-H. design incorporates an 7815 regulator to provide an offset voltage. The heatsink, as shown, will run VERY VERY hot,especially with both channels, the life of the transistors will be severely compromised,not to mention the electrolytics,which do not like elevated temperatures (they dry out and lose their capacitance).The most expensive items when building an amp. are the heatsinks,the transformer and the case. For best results,do NOT skimp on the first two items!Heatsinks should run at a temp.which is comfortable to the hand.To set up the offset,warm up the amp.then trim for an offset as close to zero as possible,if using a dual voltage PSU.,or half the rail voltage a single supply.

 

[AndrewT]

Cause you googled with the wrong spelling. Try "elko".

electro might be less ambiguous if one feels the need to shorten from electrolytic.

 

[Joao Costa]

Unlikely, unless the original caps are faulty.

I'm sorry to disagree, better components = better sound
In this order, why would you buy a high end amplifier, if the better quality components wouldn't made a difference? I have upgraded my speaker crossover and amp capacitor with better ones, and the sound difference was HUGE!!!!