DIY ACA mini

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But will it actually work like this?
Errm... no!

I have Spiced a little, and this appears to be functional. It's inverting, yes, but the OS is regular follower, and the feedback is gone.
Am I missing something, or could this actually work?

Ultimately I would like it to be all follower, but I can't work out if that's even possible without adding lots of xtra parts.
 

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Well my flight home was canceled today, so I convinced my brother to build an ACA Mini with me (we each made one).

Both sound great and are reading around 310mv for Vb and close to 11.5V for Vo.

We didn't have jumper headers so currently playing Gershwin with them "out". Lovely sounds below 50% volume. Up around 75% volume distortion is pretty audible. I'm wondering if the cheaper 24V/5A cheap SMPS could be the culprit, or this just may be normal at high volume. Running from phone, to Korg B1, to ACA Mini.

This was my brother's first time soldering so I stuffed the boards and he soldered them. Got two built, biased up, and playing music in about 4 hours.

Thanks to all who made this amazing amp possible!

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Distortion is also dependent on the speakers you are driving with the ACA Mini. If they're low Z, low sensitivity the amp will distort more as it will be forced to run near its limit.
I've used my ACA Mini with 86dB KEF (which also happens to be 4 ohm) and although OK is not very pretty.
When connected to my 93dB, 6ohm OB's it is a completely different animal.

P.S. Just noticed your source and your pre are high distortion themselves, so that can also be the cause.
 
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Thank you! I'll do a few tests tomorrow including....
  • Using computer, to dac, to Mini
  • Change speakers up (my current listening is on Kef Q150's, 86db)
  • Use a 150W Meanwell SMPS
I'll report back on the effects of those changes.

Additional note, I was listening more this evening at lower volume levels sitting at a desk and the ACA Mini was sounding just superb. I think for normal listening levels the current setup is actually phenomenal for this little amp. I had a hard time turning it off to go to bed.
 
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As already posted... and some addition perhaps
1- you have already a high distorsion set-uo, too much "sugar" could also be the cause (I wouldn't run a Mini together with a standard B1K, but that's me and my set-up / LS etc.!)
2- the sound is indeed very much bias dependant, I run mine in the 340V region and with the right speakers the sound is just incredible!
3- IF your LS need a bit more power (or say the Mini is working near to its limit and produces too much distorsions as they rise with power, should it be the culprit), as you have 2 of them and possibly (?) a bi-wiring terminal, why not running your KEFs in bi-amp mode and see if the sound moves in the right direction?
4 - some speakers are just difficult to drive, advertised efficiency and simple impedance figure doesn't tell it all, (you want to look at least at the real efficiency and the entire (imaginary) impedance vs frequency variation
5- Make sure your source is set up at high volume level, possibly not the very highest (depending on product), but quite high so its presumably digital volume control plays less a role and volume is mainly controlled by your pré's pot

Just thinking out lound in hope to be of some assistance only

Have fun, as said fab litttle unit with the right LS, on my way to building the second one and trying various bi amp configuration for a friend...

Claude
 
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Preliminary testrun with the four channel, cool running, ACA mini, JeyDee Summer Edition amp. 43C at 25C ambient with stock bias. 🎸🙂🎺

The heatsink is rescued from my previous, outdated and now retired (it lacked Dolby Atmos) surround sound reciever.

This Summer Edition is not by any means a big power industrial arc welding unit, but: It Sings with a really! refreshing clarity and a happy pepp in the step!


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Hello,

Having used 605Bs in open baffles for a number of years, and built three ACAMinis, I can say that they should be an almost a perfect match in several combinations: bridged into passive crossovers, or with either line-level passive or active crossovers ahead of stereo or bridged ACAMinis. I'll send you a private message about your crossovers to avoid goin OT on this thread. I haven't tried any combination with mine yet but I fully intend to.

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I have Spiced a little, and this appears to be functional. It's inverting, yes, but the OS is regular follower, and the feedback is gone.
Am I missing something, or could this actually work?
Look at the dissipation of R8, R9, and R15 and the output MOSFETs. Should be very high. If you build it with a SMPS, it probably won't get out of hiccup mode. If you use a linear PS, you'll either blow a fuse or start a fire, but it may pass a signal for 30 seconds or so.
 
Distortion is also dependent on the speakers you are driving with the ACA Mini. If they're low Z, low sensitivity the amp will distort more as it will be forced to run near its limit.
I've used my ACA Mini with 86dB KEF (which also happens to be 4 ohm) and although OK is not very pretty.
When connected to my 93dB, 6ohm OB's it is a completely different animal.

P.S. Just noticed your source and your pre are high distortion themselves, so that can also be the cause.
I've been listening to the mini for a month or so on 99db 8 ohm Heresy IV and even at max volume I get no distortion that I can hear. I'm using a Schitt Saga+ preamp.
 
Sooo... I think I now have finalised my ACA Mini with its various tweaks. Was about time, I started assembling the second one :)

I tried daisy chaining 2 Murata coils for SMPS filtering, but I stick to one. 2 in a row sounds either identical, or on some music perhaps a tad bass heavier / groovier... but at the expense of some airiness and overall dynamics... perhaps a matter of taste, I prefer the orginal non muffled magic, but there isn't a lot of difference anyway.

I tried my smaller 1500uF/25V low ESR bypass caps on the PS caps and yes, I got the sameresults as with the bigger 3000uF caps they replaced. That's a keeper.

At the end the Mini is an astonishing performer, at first "glance" without jumpers its sonic signature reminds me of my beloved VFET... May try them on the same Klipsch to find out, time permitting.

While the original Mini is a real performer (what a great gift again from Papa!), to my ears and taste it can be taken to a another level with just 3 tweaks, each costing around 2$:
  • 1 x Murata 1433428C coil for additonal SMPS filtering
  • 2 x Nichicon UHW1E152MHD caps (1500uF / 25V) as PS bypasses
  • 2 x Panasonic ECW-FD2W335J PP caps (3.3uF / 450V) as DC decoupling output caps bypasses

Someone asked for pix, here we go. Underside picture - discard the yellow Blu-tack (!), that's me being paranoid (the Mini was built for friends to try it, so it will travel and be abused without me).

Top are the 2 brawn Panasonic PP bypasses, below the black PS bypasses. On the right hand side a Murata coil, something that can be mounted offboard between the SMPS and the Mini. That's all you need to try these 3 tweaks.

The other pix show the final product, ready to take off to bring you to music heaven!

The last tests will be around bi-amping variations with 2 identical Minis with these tweaks.

Enjoy music

Claude
 

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Reporting back on the distortion at high volume settings for two recently build ACA Mini's....

  • Using computer, to dac, to Mini
    • No change using a different DAC (CXA81) being fed by my computer. Anything above 55-60% volume creates very audible distortion in the 8ohm 86db speakers (KEF Q150's)
    • Anything below 50% is absolutely sublime music, no audible distortion.
  • Change speakers up.
    • I tried some Axiom Audio M3 Ti, and they had essentially the same exact distortion above 55-60% volume, whether through computer-DAC or if through phone-Korg B1. Of course, they are not much different from the KEF's at 8ohms and 86db.
  • Use a 150W Meanwell SMPS
    • I hooked up the Mean Well SMPS 150W PSU and it had no change in distortion, so the cheaper 5A SMPS PSU is not the culprit.
  • Adding the jumper and it's impact.
    • The jumper does have an impact on the sound. Not something I'm good at describing. However, didn't have much impact on the distortion at higher volumes (>60%).

Now on to what Papa suggested, "Very dependent on output stage bias, so you might want to crank it up a bit". I went with the "skyscraper" method that @lastnamenelson inspired. With a second stack of heat sinks, using thermal grease at the interface, I got much cooler temps at 300mV for Vb. So I dialed up the bias to a Vb reading of 420mV. This had a very minor improvement in when the distortion kicks in as audible, more like 60-65% volume level. Not a significant improvement, but overall, I love the approach and I plan to stick with using the skyscraper design as I have no concerns with the ACA Mini being "too hot" even at a much higher bias setting of 420mV for Vb. I can say that I highly recommend it for folks who want to crank bias up to a higher level to eek out a little more from the ACA Mini.

Of course, if you want to take it to another level, build the ACA Minmax that Nelson Brock has put together and is now in the store. I built one of the early pioneer versions and I absolutely love that amp! 36V and more heat sink gives that little extra to make the speakers sing just a little more. "Superb" is how I'd describe the sound of that amp. Very much like the ACA Mini, but with a little extra bump well worth the added cost to take it to the next level.

I'm assuming the distortion is simply because I'm driving the ACA Mini past it's designed power levels with the speakers I'm using. At normal listening levels, the ACA Mini is one, if not the best, sounding amps I have built, only to be rivaled by the ACA Minmax & Aleph Jzm. I'm still new to the whole hobby, so much more to learn and experience. Take my input with a grain of salt for that reason.

Thanks to all who helped provide input and certainly a big thanks to Papa for such a wonderful sounding amp to build and enjoy! What a gift! :cheers:

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Yeah, it seems you are just asking too much power from your Mini.

Had a quick look at the impedance curves of your KEFs and their efficiency and TBH you really want 10 times the Mini power to have some decent volume out of them. Low volume / on a desk probably acceptable, but TBH the Mini is not suited to push that kind of speakers to normal HIFI listening levels.

Your Axiom are very similar regarding what they ask from an amp in that regard, so no change to be expected.

I think you are very lucky the Mini produces soft clipping as the distorsion you are hearing is probably the Mini being overdriven and with most conventional amps you would have likely killed your tweeters...

So IMHO either low volume listening or another amp match, I am not sure any mod will help as to enable a bit more soundpressure you really need a lot more power from your amp...

Claude
 
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