Amp Camp Amp - ACA

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This is exactly why member 6L6 strongly recommends you use a "Mega328 component tester" to doublecheck every single part, 1 second before stuffing it. He's putting this recommendation into all of the documentation he writes for diyAudio, and has been doing so for several months. It's good advice.

Besides measuring resistors, capacitors, inductors, and bipolar transistors, it will ALSO tell you which end of an LED is positive, and if the LED is water-clear, when it lights up during Mega328 testing, you can learn whether you've got a red or a green or a yellow LED in the socket.

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Soldering on the resistors near C3 needs a makeover from the top side - there’s one hole with no solder showing. A few blobby looking joints. It’s almost always the soldering that causes this kit to fail. The bizarre thing is all four of your boards have failed. Did you put them together one at a time to completion or all four simultaneously? Same mistake repeated on all four boards at the same time. Worth sticking up a photo of the underside too.
 
Ironically i did measure all the resistors except the 4 with the value printed on them! and orientate them so you can see the value when in postion,as regards soldering i was under the impression you soldered most of the components on the back of the PCB so no need to do the top side as well?
 

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Hi Brian yes solder almost all of it from one side only. To know you’ve made a nice joint you generally want a nice conical shape that’s quite shiny after. Lumpy looking solder can indicate it hasen’t flowed properly and might not be a good.conection. Can just be the solder though. I had some high silver content stuff sold for Audio use that was very hard to use compared to regular flux cored lead tin mix. I’m sure you’ll be very happy with the amp once those resistors are swapped out.
 
Two quick questions.

1/ Is the bias preset around its midpoint (set in the middle) in order to get the 12 volts midpoint or is it 'one ended'?

2/ Could there be any doubts over issues with the semiconductors? Were they from a reputable source or supplied with the kit?

It is odd for all four of your boards to be the same.


Hello Karl,

1...i never really noticed nor consciously measured, as long as i can set the output node the 1/2 of B+, then that is it...

2....i have over 70+ boards building experience, from build outs and teach ins... never had any real issues, except maybe one or two occasions where parts were installed wrong....

this ACA will work the first time if you made no errors, and the fact that you can adjust/calibrate the 1/2 b+ point is the confirmation....

what we did with our boards is the part values are printed on the silkscreens so that it will be so hard to commit mistakes...
 
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The ACA is remarkably tolerant of even huge variations in many of the parts, certainly as a simulation anyway. Crazy things like R7 (10k) which still works if you fit 1k or 100k.

Nothing obviously changes DC wise.

That it still biased up to 12 volts midpoint with 680 ohm fitted for a 0.68 was totally unexpected.
 
A friend is interested in using this amp for his inefficient HiFi man Susvaras which are 55 ohms and very inefficient. 83dB/1mW

Many folks use speaker amps for these...so I am wondering how well the ACA might drive these difficult headphones? The ACA is rates at 8 watts at 8 ohms so I assume the power will be less as the impedance load goes up?

If this is do-able will replacing the 3 pin XLR on the rear with a 4 pin work?

Any other modifications needed etc..

Thanks In Advance!
Alex
 
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There should be no issue with power. 55 ohms is an easy load and the ACA would develop just over 1 watt rms into that value.

I'm not familiar with the headphones and their terminations but just remember that the ACA is phase inverting so you need to reverse the leads to preserve absolute phase.
 
Hi there,

first, English is not my mother tongue, second, despite having quite a bit more than a doctorate in medical and some others in soft sciences. My electronic background is nearing zero.

About 40 years ago I built loudspeaker kits, notably a 3 way passive system, an Hiraga 20watts class A amplifier, a "pseudo" Kaneda DC1 preamp (including a formidably ugly homemade chassis from a huge aluminum plate). Then a Nemesis amplifier and an Hiraga prepre amp for my MC Dynavector.

This all said, you must probably infer that I'm living in France, and that I was assiduously reading "L'Audiophile" in the heroic days, 35 years ago.

Time passed, yearly income increased, but my ability to understand more or less "intelligent" silicon device remained to the same sub-zero level.

I will not even speak about vacuum tubes with too many exotic connectors and life-threatening currents.

I must confess that my only interest in these "prosthetic devices" were due to the lack of live classical music in my area.

All this to say that I'm at a crossing road : I can afford a number of somewhat expensive amplified-DSP high-end monitors. But ... my "epistemolophilic pulsion" is still there.

I'm currently bulding a 4-5 way multi amplified horn system.
For the high mid (2kHz to 8 kHz) and the upper range, I'm looking for 2 pairs of stereo amps.
My first thought was to build a second pair of Nemesis monotransistor amps with those enormous output transformers, the huge passive power supply. I do have the SK135, the output transformers and all the heavy iron needed, so I just need to buy the bunch of capacitors the size of a family vegetable can, a few passive components AND also make a chassis able to survive all this weight.

Here enters two other contenders :

-A French made simili Kaneda 209, a relatively light device, not a power hog like your regular class A amp, with outstanding sonic performances, very similar to the original kaneda 209 amp.

-Then the Amp Camp : a simple enough device for my capabilities, but with 3 questions :

*is a SMPS power supply really good enough concerning the sound ?

* would this problematic chemical 3K output capacitor could be replaced by something more "audiophile compatible" non polarized equivalent or a lower capacity, since I absolutely don't need to go under 800 Hz. Is there a risk of oscillation and HF noise pollution ?

* Last an ecological concern about class A amps : is it worth it ?considering the poor efficiency in these times of CO2 awareness ?

I may add that most of my everyday travel is on a non-electrical bicycle....

Thanks for your advices !
 
frugal-phile™
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The ACA is a very good value. I do not think the SMPS is a big issue. People have built them at all levels of tweakishness. My 3 are stock.

Do note that people here are still building the Hiraga Class A amp and althou i have not heard it, i expect it would outclass the ACA. And i am sure people here have tweaked the design.

Any smallish Class A amplifier (say 20w) even being the power hogs they are, use less power than an big incandescent light bulb.

And there are quite a few other Class A designs here to select from. With horns and multiamping you won’t need much power, even a couple watts would likely do.

dave
 
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Hi Yves. My thoughts for what they are worth :)

*is a SMPS power supply really good enough concerning the sound ?

Tbe SMPS specified for the project is absolutely ideal and is both cost effective and efficient.

A linear supply built to equal the performance of this would be bulky and heavy and also much less efficient... something you are concerned over.

* would this problematic chemical 3K output capacitor could be replaced by something more "audiophile compatible" non polarized equivalent or a lower capacity, since I absolutely don't need to go under 800 Hz. Is there a risk of oscillation and HF noise pollution ?

There are no stability concerns at all with the ACA. If you don't need go under 800Hz then a smaller cap could be used however there is something to be aware of... something that stops it working as you expect.

The feedback for the ACA is taken from after the coupling cap and so this means that the amplifier will try and maintain the output level regardless of a smaller cap. It is an important point.

So in practice what this means is that as the frequency falls, the amplifier increases the level before the cap such that the level at the output (where the feedback is taken from) remains constant.

Even a 100uF coupling cap will still see the response extend down to 20Hz but the amplifier output before the cap will be very large and probably running into clipping.

So the answer here is that it is much better to compromise and perhaps look at reducing the input value coupling cap as well as the output coupling cap.

* Last an ecological concern about class A amps : is it worth it ?considering the poor efficiency in these times of CO2 awareness ?

I may add that most of my everyday travel is on a non-electrical bicycle....

Thanks for your advices !

Only you can answer that... and remember the SMPS goes some way to offsetting the inefficiency.

The heat of the ACA will add to room heating in colder weather and so you gain there as well.