Amp Camp Amp - ACA

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#5,923
If you measure AC voltage using the DMM at the speaker output when you are playing "loud" (with speakers connected).....what does it measure?
You should be able to see some RMS volts at the output.....let us say 3-4 Volts when playing loud. If you only see 0.2 VAC or so.....something must be wrong. But since both amps are bad it would be strange if you made the same error on both.
 
#5,926
Hi damkel - The boards and wiring look good at first glance. You can adjust the bias, so another positive.
What are you using as a source? They do not play loud with inputs like phones etc.

Now check the resistance between the red and black output terminals. It should be near 1k ohms on the 20k range (it takes a few seconds to get there). If it is very low you have a short somewhere. Do the same for the input socket, that should be about 50k on the 2 meg range, again it takes a few seconds. A low reading again indicates a short. Then do the test Meper tells you to, measure the ac volts on the output terminal with a loud source (or 1kHz test tone if possible).
 
#5,927
Numerically the voltage gain is low, only around 3.7 with no load attached.

Disconnect the speakers and apply a sine wave input signal of around 100 to 400Hz. If you measure the input voltage on your DVM (low AC volt range) then the output voltage should be approx. this value multiplied by 3.7

Is the bias current correct ? Check this by confirming the volt drop across the 0.47 ohm resistors which should be around 0.37 volts DC (no signal applied for this).
 
#5,932
oc11 said:
I have been thinking about the ACA, its case and the very hot heatsinks that are part of it. I have not been building amps, that is for sure, but I have built computers, desktop units primarily (with an occasional redoing of a laptop which is a near nightmare to do), since the 90's and fail to see why anything electrical that gets pretty hot cannot be cooled down to some degree.
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Uhm, yes. But why would you do that? It doesn't get too hot. If you're placing it in a tight cupboard you want to ventilate the cupboard, not the amplifier.

A silent fan isn't cheap. There are cheap ones available, but they turn into a noisy one after a while.
 
#5,933
oc11 said:
I have been thinking about the ACA, its case and the very hot heatsinks that are part of it. I have not been building amps, that is for sure, but I have built computers, desktop units primarily (with an occasional redoing of a laptop which is a near nightmare to do), since the 90's and fail to see why anything electrical that gets pretty hot cannot be cooled down to some degree.

So, let me talk about my ideas to make the rig a bit cooler. To increase cooling passively, the case (which is a Mini Dissipante 2U unit 200X300 mm) can be fitted with a pre-drilled bottom and top plates. They are sold at the diyaudio store under "We also recommend, chassis baseplate unit." Of course, I have to confirm that the sizes of the baseplates and top plates I received are the same and the screw holes are in the same place. Once this is done, the bottom and top plates that come with your case can be replaced with the pre-drilled ones. These are, I believe, pre-tapped to accept screws in the holes. All you really have to do is to decide which holes you want to keep as pre-tapped holes, and basically drill out the rest of the screw threads in the other holes to enlarge them and allow free air circulation. The legs of the unit can be replaced with taller legs to allow for freer passive air flow by convection from the bottom. Assuming you can also replace the top plate with a pre-drilled one, you can significantly increase the level of air circulation within the unit in a purely passive manner.

But why stay there, you can buy computer case fans which are silent and install them on the top of the units to extract hot air from them. A number of these fans come with USB connectors for external use. So all you need is to choose your fans for airflow, size and silent operation, install them and hook them up to an external USB hub that connects to AC. Make sure the top plates' pre-drilled holes left unused are also enlarged to facilitate the air flow. I honestly have no idea how much heat can be dissipated that way but it may be significant.

If you go passive the air convection through the units happens without additional noise. If you use fans you can expect background noise in the 28 dB level which may or may not be acceptable to you personally. Again this is nothing that I have already done with an amplifier but I have done it many times with computers. I think this is entirely feasible to do with the pre-drilled plates. The plates are 3 mm aluminum so drilling them is not impossible to do at home.

Hope you like this small contribution to possible amp cooling solutions from the PC world.
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As an initial response, I'd offer the old saying, "If it ain't broke don't fix it."

The amp has been designed to work just fine if you build it to spec. I don't think using a PC and their heat management is a valid comparison. I've got 24V power feeding mine and the heat sinks measure around 50°C with ambient air at around 25°C. Seems reasonable. Yes, 50°C is warm but it won't immediately burn the skin. You'd have to leave a body part parked against the heat sink for a while to cook it. So, as long as you've got a well aired place to house the amp, on its own shelf, you should not have a problem.

If you're planning to put the amp in a closed cabinet, then I'd think about ventilating the cabinet but leave the amp stock. But for a thing of beauty, why hide your own handiwork?
 
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#5,934
OK, valid points, however, I still think increasing convection by making holes in the baseplate will not ruin the appearance of the amp, nor will it increase noise in any way by adding a fan, and certainly it will not affect the way it works but it will provide better ventilation. If all the ventilation holes are in the top pane, the hot air will certainly come out, but where is the new cold air going to come in from?
 
#5,936
oc11 said:
If all the ventilation holes are in the top pane, the hot air will certainly come out, but where is the new cold air going to come in from?
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? the bottom ? (... was that a trick question? ...)

Don't forget that the bulk of the heat is being transfer to the cooling fins. Not that I'd bother (as explained) but if you want forced convection that's useful, that's where I'd be directing air flow. Those fins are really good at rejecting heat, so you'd be making better use of what you've got.
 
#5,937
Skylar88 said:
It's normal for the sound to be fuzzy the first minute or so - until the amp and heatsinks warm up.
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I have a 19V 1.5 ACA biassed to 10V and... not even in the first seconds would I describe the sound as fuzzy.

I have untrained perfect pitch, my ears can hear higher than 16Khz still (I'm 41), I can discern and score different melodic lines from a full orchestra, I can play by ear most any song I have heard... but every time I read here things like this, or the fact that after a few days the sound 'improves considerably', I really start doubting my ears.

Perhaps you guys have much better hearing comprehension, perhaps your systems are more resolving. I don't know, but I would say my amp is at 90% or better of its music-rendering capabilities during the first minutes, and up to 100% after that.

But fuzzy is not an adjective I would use, not even during second 1 of turning it on.

Again, I doubt my ears... but my 'credentials' would usually put me amongst a group with very good hearing.

For what is worth, others reporting low volume sound had indeed, as 6L6's question suggests, a short between the heatsink (ground) and one component's pin that was a bit long and ended up touching the heatsink.

Best regards,
Rafa.
 
#5,938
RafaPolit said:
... not even in the first seconds would I describe the sound as fuzzy.
Click to expand...
Rafa

Maybe I should have added that it is what I experience with my ACA... And now that I think about it, my ACA is not standard. I've added cap multipliers which ramp up the current very slowly. So that may be the reason for the fuzziness right after switching on. I find it kinda cute, though.

I thus retract my statement that it is normal. There is probably nothing wrong with your credentials or hearing. 🙂
 
#5,939
I have tried to do the resistance checks from the build guide for v1.6 step 55.
My numbers are off and are more or less the same on both modules.
Can I due the measurements with the boards completely stuffed and all wire connections made?

This is what I have measured:
R9.S should be 1K but is 0.5K
R10.G should be 335K but is 332K
Q4.D should be 1K but is 0.5K
Q1.S should be 0 and is 0
Q1.D should be 5.2K but is 5.53K
R15.B should be 5.8K but is 5.21K
R15.E should be 5.2K but is 4.52K
R5.C should be 10.9K but is 10.4K
Q2.G should be 11K but is 10.55K
Q2.D should be 1K but is 0.5K
Q2.S should be 5.2K but is 4.52

It is like there is missing 0.5K somewhere, or...???
Is this enough to point at what is wrong or should I continue with the voltage checks?

(Measuring is not nearly as fun ad soldering….)
 
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