ACA amp with premium parts

A reader of these forums writes:

Capacitors C2 and C4 play a couple important supporting roles in the ACA design. Selection of C2, as noted by the Mighty ZM, may include good quality industrial parts. I chose an Aluminum Organic Polymer device for its low ESR and relatively high ripple capacity. There are a number of other parts that would be quite suitable for this position, including 25V parts in the Nichicon KA or KW series. Panasonie and Kemet also offer Organic Polymer devices that will fit. The footprint on the PCB has a 5mm lead spacing, so that may help narrow the selection. (My suggested part actually has a LS of 3.5mm, which I consider acceptable, if non-ideal.)

My 'upgrade' of C4 was intended to address any possible switching artifacts from from the SMPS brick. I wanted to increase the capacitance while maintaining low ESR, high ripple and a 3.5mm lead spacing. It also should have a voltage rating of 35V. This area of the board would not be a good place to install oversize components with flying leads. Again, Panasonic and Kemet may have suitable substitutes in their Organic Polymer series.

Why Organic Polymer? In low voltage applications, they provide good characteristics in compact form factors, and have made their way into a number of modern industrial designs. Also, for what it is worth, I believe the Organic electrolyte may be the next closest thing to the 'silk' substrate used by the famed Elna Silmic II series.



anything higher than the stated volt should be fine is it? if i cant source the particular parts which you have recommended.
 
What a delightful wonder this ACA 1.6 with 'TungstenAudio' mods is (Many thanks for sharing) The pre is a B1Korg. The ACA at present running as a stereo jobbie. Its driving a pair of JR149's (87db). What a delight, as its known that these speakers are a bugger to drive but this amp is so good it find it a breeze. Sweet as you like and musical too.

On the bench PSU (3.0amp) during setup and test, the amp was drawing 1.47amps per channel at 24vdc and with the two channels my bench PSU was struggling. (getting very hot indeed.

Know doubt If I build another and bridge them for two mono's it'll be worth it.

Sorry cant post/upload Photos but will...

:)

Thanks Papa
 
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Hi James, find attached a couple of images showing my current ACA (including your recommendations / changes).

When I started the layout-release shown below, using the Vishay LVB2560 bridge rectifier, I had severe problems concerning the thermal dissipation of the rectifier.
I had to finally replace it with a DIOTEC KBU8B, which has very long legs (see image 3) and does a very good job (0.8V drop per diode) as well.

My current ACA is driven from a 230VAC / 19VAC INDEL toroid with 2 secondaries, each secondary rated at 4.21A and resulting in about 25VDC "quiescent voltage".
It is "powered" from a "JKuetemann CAP-multiplier".

And it sounds: "absolutely great".

I will not try to improve the current scheme ("Never change a winning Team"), but will work on the case from now on.

If somebody is interested in this ACA:
I have 2 bare PCBs along with a couple of components for this extraordinarily well sounding ACA: 2SK170BL, DIOTEC rectifiers, 2.2µF WIMA, MPC resistors, …)

Contact me (via PM), if you are interested to have - Rudi
 

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^ As with many other things, the choice of capacitors may depend on more than one consideration. In this case, the original 10 uF Elna Silmic capacitors which were included in the ACA kit were known for their high quality and transparency. The same value was easy to use in both the C3 and C4 positions.
With the modified ACA, the value of C4 was increased to assist in power supply filtration. It is reasonable to make a different choice for C3 as well. I haven't tried other values in my ACA builds yet, but have a few 2.2 uF Nichicon ES series bipolar caps that I may try sometime if I do another build from bare boards.
 
I have a pair of ACA's in parallel mode driving my 15 ohm Rogers LS3/5a's very nicely (medium sized room, medium volumes, etc). Do you have a rough idea of the power output from parallel monos?
The short answer is no.

The long(er) answer is it really depends on how your ears respond to what your speakers are doing when driven by the amp at higher power & distortion levels. As I mentioned a couple posts back, I voiced my ACA mods to make the amps sound better when driven close to their power limits. I recently made some THD measurements with one of my original modified ACAs in simple stereo mode driving an 8 Ohm load. At 1% measured distortion, the value often given for the onset of clipping, the output was 6.5 Vrms, or a little over 5 Watts. The residual waveform indicates primarily negative H2, as is expected from the ACA. That is one way to judge output power. With that amp, my ears tell me that the amp starts to sound a little compressed (loss of dynamic detail) before it sounds like it's clipping.
If your ears like the way your parallel bridged amp sounds with your 16 Ohm speakers, then that's mostly what you need to know. If you really need to hear what a more powerful amp sounds like, build one of the larger FW clone amps. I alternate between my M2x and Aleph J, depending on whether I feel like tinkering (M2x), or just what to hear what a glorious tube amp sounds like without the limitations of running through lots of iron after the output stage.
 

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I have a pair of ACA's in parallel mode driving my 15 ohm Rogers LS3/5a's very nicely (medium sized room, medium volumes, etc). Do you have a rough idea of the power output from parallel monos?

You would better off with bridge drive for a 15 Ohm load. Parallel mode drives low impedance better.

Here are the numbers for version 1.6 I published in the ACA thread.

Single channel, 1KHz
8 Ohm 7W at 1% THD
4 Ohm 3W at 1% THD
4 Ohm 10W at 3% THD

Bridge mode SE drive 1KHz (the way the kit does bridge mode)
8 Ohm 6W at 1% THD
8 Ohm 20W at 4% THD

Bridge mode balanced drive 1KHz (you need a balanced preamp for this)
8 Ohm 12W at 1% THD
8 Ohm 20W at 3% THD

With balanced drive you get the THD lowering effect of 2nd harmonic cancellation. With SE drive, the slave side adds to the THD of the driven side. I posted oscilloscope photos of this in the ACA thread.

For Parallel mode, I would expect 14W at 1% THD with a 4 Ohm load, but I have not tested this. If your speaker dips below 4 Ohms, this is what you want.

For a 16 Ohm load I would expect half the 8 Ohm bridge power numbers with perhaps half the THD. Again, I have not tested this.
 
To all who have been following this thread, know that I'm grateful to be part of this community. I am especially humbled by the praise of the one and only N. Pass who has so generously given of his designs and his experience.


For Rafa,
I'll try to find a good way of editing or drawing a schematic showing the changes that I have made to the ACA. In the meantime, here is a list of what I have done.

On board, drop-in replacements

C1: 4700 uF, 35V; Nichicon KG series "Gold Tune"
C2: 1000 uF, 16V; Nichicon RNL series Aluminum Organic Polymer
C4: 100 uF, 35V; Nichicon RL8 series Aluminum Organic Polymer
R11: 20 kOhm, 1/4W, 1% metal film
R12: 90.9 kOhm, 1/4W, 1% metal film


On board add-on components
R4b: 2.0 Ohm, 2W, 2% metal oxide; added in parallel w/ R4
C101: 10 pF, 500V, 5% Silver Mica; added in parallel w/ R12


Off board substitution
Bridging Resistor: 68.1 kOhm, 1/4W, 1% metal film


Note that I left C2 as the 10 uF, 25V Elna Silmic II, and have been quite pleased with the overall results. Recently I happened to acquire a Naim NAP 150, and discovered that it also uses the exact same component for its input coupling capacitors. Given Naim's extremely selective practice for their components, it seems that we are all in good company.


For Anand,
The output impedance of the modified ACA is a function of the added 2.0 Ohm resistor R4b, as well as the substitute value for the feedback resistor R12. I don't have a formula at hand for calculating this.

My loudspeakers are an older pair of Vandersteen 2C, purchased as store demos with the original Sound Anchor stands around 1988. They have a nominal impedance of 7.5 Ohms, with a minimum of 6 Ohms. Efficiency is listed as 87 dB. They were known for being fairly easy to drive with reasonably uniform phase and impedance curves. They are one of the few multi-driver speaker designs that is phase coherent and time-aligned. I'll see if I can dig up the original spec sheet & owner's manual for more information.
I'm sure an upgraded pair of speakers, such as a new set of 2Ce Signatures, could be even more revealing / sound even better, but my old Vandys have been a constant part of my system over these many years as I have been adding & upgrading other components.





for the bridge resistor, is it possible to ignore if i install the amp without the switch . ?
 
Looks like the discussion of the bridge resistor and switch is being handled in the main sticky ACA thread.


Of note: I am still using essentially the same component substitutions that are mentioned above. I have simply swapped a 0.50Ω resistor for R4, while keeping R3 at 0.68Ω. One may also use 0.56Ω resistors for both R3 and R4 to the same effect, without needing the addition of R4b.
The rest of my experimentation with the ACA has been trying IRFP140 Mosfets and higher voltage linear power supplies. The IRFP140s were a good substitution, and I may also try IRFP150 or even FQH44N10 devices in the future. In order to support higher supply voltage, it was necessary to change the simple C4 substitution into an RC dropping network, with 499 Ohms for the resistor. This protects the Q4 Jfet against excess voltage.
 
Been in discussion with the MFR about these:

ULN-PS7 Ultra Low Noise, 6.0A High Current Power Supply for High Quality Audio | eBay

He can supply them without the heat sink and with the transistors & rectifiers unsoldered so that you can mount them then solder the board over them like you do the ACA boards.

So - can the ACA take 25volt without modification or would i need a couple of resisters to work as a voltage divider to drop the output back to 24 volt? I've got wall warts and bricks all over my system now - time to tidy up replace and upgrade. I'm considering same cases as ACA with a hole in the top with a Chrome Toroidy showing through :)
 
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If you intend to get the PSU without the heat sink, and mount it directly on one side of a same enclosure as the ACA, keep in mind that the PSU board width is 9cm. It is more than the ACA side height, therefore I don't think that the board could fit that way.
 
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If you intend to get the PSU without the heat sink, and mount it directly on one side of a same enclosure as the ACA, keep in mind that the PSU board width is 9cm. It is more than the ACA side height, therefore I don't think that the board could fit that way.

Will bolt it to the bottom and if necessary get a thicker bottom plate made - but the bottom plate should conduct the heat to the sides quick enough?
 
Question to TungstenAudio Post - #171

Your scope shows the same result as my simulation in LT Spice (aca_ltspice.jpg),
indicating a negative H2. (And so do other FW amps I have simulated so far.)
But, the ACA is an inverting amplifier and therefore normally it is recommended
to flip the speaker wires to reestablish the proper absolute phase.
This then results in positive phase H2 (aca_ltspice_flipped.jpg) at the speaker.
Was GND of your measurement setup connected to GND of your ACA ?
 

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