TL;DR: My ACA Mini amp sounds great now that the bias is set properly. @Nelson Pass says "it wants to be at 350 - 390 mV." Check your bias. You may want to bump it up.
The long version:
WOW! My ACA Mini amp has just become the best amp I've ever used. The lower mids and bass are there in full. Everything sounds so clear and clean. I am simply astounded by what this little kit amp can do.
What happened?
I reported to @Nelson Pass that my ACA Mini seemed to roll off the lower frequencies. I thought it ought to be able to drive my JBL Arena 170 speakers (8 Ohm, 7 inch woofer, soft dome tweeter in a fairly large, ported enclosure, 86 db efficiency), at least to the low levels I typically use (80db peaks, sometimes 85db). He agreed, and he said that someone with test gear should take a look. That someone was Nelson himself!
What Nelson discovered was that I had set the bias far too low: about 185 mV (I was shooting for 300 mV). He added that the amp "wants to be at 350 - 390 mV," i.e., higher than the 300 mV mentioned in his original write up. He set the bias and sent the amp back to me. I tried it yesterday and some more today, and I can now say, as above, that this is the best amp I have ever used.
Once again, Nelson to the rescue. I hope it's clear how grateful I am to this generous man who helped a 68-year-old novice builder end up with a truly spectacular amplifier.
I said earlier that my Aegir had a warm sound, with a slightly rolled off treble. The ACA Mini confirms this, as it has an extremely clear treble. What I also learned is that the Aegir has a somewhat blurry or muddy treble, again in contrast to the ACA Mini, which has a very clean treble. The first thing I noticed in this case was that the really high notes on a piano sounded so right with the ACA Mini: "tink" with no overhang or false resonance. How wonderful that was to hear!
The other thing that impressed me right away was that vocals were far more intelligible with the ACA Mini than with the Aegir.
And then there are trumpets, string plucks, ad infinitum.
Suffice it to say that now the ACA Mini is my main amp, and I don't think I'll put the Aegir back in my system unless the ACA Mini fails somehow...in which case I'll start building another one.
Prior to this, I had never heard an amp designed by Nelson Pass. I knew they were famous for their clarity. Now I have an almost free sample of this amazing technology in my house. What a thrill!
The long version:
WOW! My ACA Mini amp has just become the best amp I've ever used. The lower mids and bass are there in full. Everything sounds so clear and clean. I am simply astounded by what this little kit amp can do.
What happened?
I reported to @Nelson Pass that my ACA Mini seemed to roll off the lower frequencies. I thought it ought to be able to drive my JBL Arena 170 speakers (8 Ohm, 7 inch woofer, soft dome tweeter in a fairly large, ported enclosure, 86 db efficiency), at least to the low levels I typically use (80db peaks, sometimes 85db). He agreed, and he said that someone with test gear should take a look. That someone was Nelson himself!
What Nelson discovered was that I had set the bias far too low: about 185 mV (I was shooting for 300 mV). He added that the amp "wants to be at 350 - 390 mV," i.e., higher than the 300 mV mentioned in his original write up. He set the bias and sent the amp back to me. I tried it yesterday and some more today, and I can now say, as above, that this is the best amp I have ever used.
Once again, Nelson to the rescue. I hope it's clear how grateful I am to this generous man who helped a 68-year-old novice builder end up with a truly spectacular amplifier.
I said earlier that my Aegir had a warm sound, with a slightly rolled off treble. The ACA Mini confirms this, as it has an extremely clear treble. What I also learned is that the Aegir has a somewhat blurry or muddy treble, again in contrast to the ACA Mini, which has a very clean treble. The first thing I noticed in this case was that the really high notes on a piano sounded so right with the ACA Mini: "tink" with no overhang or false resonance. How wonderful that was to hear!
The other thing that impressed me right away was that vocals were far more intelligible with the ACA Mini than with the Aegir.
And then there are trumpets, string plucks, ad infinitum.
Suffice it to say that now the ACA Mini is my main amp, and I don't think I'll put the Aegir back in my system unless the ACA Mini fails somehow...in which case I'll start building another one.
Prior to this, I had never heard an amp designed by Nelson Pass. I knew they were famous for their clarity. Now I have an almost free sample of this amazing technology in my house. What a thrill!
@Mikerodrig27 : Please see my new post. You may be able to improve your ACA Mini's sound with a higher bias setting.
Very cool! Never clean that amp! 🙂
I have run mine as high as 550mV as I have the mosfets attached to large heatsinks (large by ACA mini standards). I agree with you that it likes bias. I need to pull that amp out and play with it again at some point.
Audiosteve drew up a board where each channel was mounted on it's own board. That is the board that I used. It is the same circuit.
Since you are enjoying these little amps, there is a new one out called the Zenductor. It should find it's way into the store at some point. Maybe you should check that one out too 🙂
I have run mine as high as 550mV as I have the mosfets attached to large heatsinks (large by ACA mini standards). I agree with you that it likes bias. I need to pull that amp out and play with it again at some point.
Audiosteve drew up a board where each channel was mounted on it's own board. That is the board that I used. It is the same circuit.
Since you are enjoying these little amps, there is a new one out called the Zenductor. It should find it's way into the store at some point. Maybe you should check that one out too 🙂
Attachments
Two things. And two bonus things at the end.
TL;DR: Bias voltage is very temperature dependent; I don't like the sound with the jumpers removed.
First, I find that the ACA Mini bias voltage is very dependent on ambient temperature. We had a couple of warm days when it got up to 78 degrees F in my house, and my bias was about 410 mV with Nelson's settings. I was concerned that this was too high, so I took it down a little bit.
This morning it is a more normal 68 degrees F in the house, and after an hour of warm-up (it really takes that long), the bias was at 280 mV. I am bringing it back up to 370 mV, which means it will run very hot if the house gets back up to 78 degrees F.
Nelson tells me that it's OK to run hot with a high bias, as long as no one gets burned by the heat sinks.
Incidentally, Nelson also said that the VO needn't be 11.5 V. You want it to be at least that high and under 12.0 V. Mine tested at 11.81 V with Nelson's settings. I think it's because my power supply puts out 12.35 V and maybe his puts out closer to 12.0 V. In any case, I am compromising on 11.66 V (10% over 11.5 V).
I'll reiterate that my ACA Mini amp takes a full hour to warm up and sound good, even when the house it hot. Once I have it on, I leave it on until I go to bed.
Second thing: Last night, the music I was playing with my ACA Mini was sounding absolutely great. I tried removing the jumpers. I didn't like the results.
In my system, when the jumpers are removed, the treble moves forward, collapsing the soundstage. The treble sounds thin, and there is noticeable sibilance.
Bonus thing #1: Here is a track that really makes you glad you have a clear and clean sounding amp: "Spain" by Chick Corea on the live "Trilogy" album. Give it a listen!
Bonus thing #2: Today I am starting to build a second ACA Mini amp. It's so fun to do! And I can use it in my bedroom for late night listening.
TL;DR: Bias voltage is very temperature dependent; I don't like the sound with the jumpers removed.
First, I find that the ACA Mini bias voltage is very dependent on ambient temperature. We had a couple of warm days when it got up to 78 degrees F in my house, and my bias was about 410 mV with Nelson's settings. I was concerned that this was too high, so I took it down a little bit.
This morning it is a more normal 68 degrees F in the house, and after an hour of warm-up (it really takes that long), the bias was at 280 mV. I am bringing it back up to 370 mV, which means it will run very hot if the house gets back up to 78 degrees F.
Nelson tells me that it's OK to run hot with a high bias, as long as no one gets burned by the heat sinks.
Incidentally, Nelson also said that the VO needn't be 11.5 V. You want it to be at least that high and under 12.0 V. Mine tested at 11.81 V with Nelson's settings. I think it's because my power supply puts out 12.35 V and maybe his puts out closer to 12.0 V. In any case, I am compromising on 11.66 V (10% over 11.5 V).
I'll reiterate that my ACA Mini amp takes a full hour to warm up and sound good, even when the house it hot. Once I have it on, I leave it on until I go to bed.
Second thing: Last night, the music I was playing with my ACA Mini was sounding absolutely great. I tried removing the jumpers. I didn't like the results.
In my system, when the jumpers are removed, the treble moves forward, collapsing the soundstage. The treble sounds thin, and there is noticeable sibilance.
Bonus thing #1: Here is a track that really makes you glad you have a clear and clean sounding amp: "Spain" by Chick Corea on the live "Trilogy" album. Give it a listen!
Bonus thing #2: Today I am starting to build a second ACA Mini amp. It's so fun to do! And I can use it in my bedroom for late night listening.
humm ,no jump ,sound good from start maybe better after 5min no more....use small cpu fan on hotter days no need to touch the bias ,
@Mikerodrig27 : Nelson said to me that the Zenductor runs even hotter, and no one has died yet. 🙂
Still (@nicoch58), I might get a little fan for the very few days when it gets over 78 degrees F here.
Today the high was about 68 degrees. I had the window open, since I was soldering up a second ACA Mini amp. It took me 7 hours. Last time it was 7 days!
Still (@nicoch58), I might get a little fan for the very few days when it gets over 78 degrees F here.
Today the high was about 68 degrees. I had the window open, since I was soldering up a second ACA Mini amp. It took me 7 hours. Last time it was 7 days!
Yep. I think the idea of keeping the temps low on these amplifiers is more of a safety thing versus it hurting the actual amplifer. The capacitors are the only obvious thing that may be compromised. But with the Zenductor and the ACA mini, the caps are all exposed to ambiant temps anyways.
I guess Nelson the scientist is using degrees Celsius in his comment. 🙂
I ordered a 6 inch clip-on fan with USB power ($14) to hang from the shelf above my ACA Mini amp. It will be easy to turn on, if needed. It's supposed to be quiet....
I ordered a 6 inch clip-on fan with USB power ($14) to hang from the shelf above my ACA Mini amp. It will be easy to turn on, if needed. It's supposed to be quiet....
Synergy!
That seems to be the key to great sound with the DIY ACA Mini amplifier. My JBL Arena 170 speakers, 2-way, 7 inch, large [almost 3 cubic feet inside, I'm guessing], hollow floor standing speakers [$150/pair, delivered, when I bought them on closeout] sound incredible when driven by the ACA Mini. I moved them about four inches further from the back wall since switching from the Schiit Aegir because the ACA Mini does a better job with bass, and these speakers are rear ported.
I've been listing to a lot of piano music [right now, Mozart piano sonatas] because this is the first stereo setup I've owned that reproduces the sound of a piano convincingly. I can hear the strings vibrate when they are struck. I can hear the piano resonate. Nothing is blurred. What fun!
Back to synergy: So far I haven't been able to get good sound with my second ACA Mini amp and the bookshelf speakers in my bedroom. I have two sets of small sealed speakers [JBL and NHT]. With the ACA Mini these have clear treble and almost nothing in the midrange and bass. I think the woofers in the sealed boxes must be hard to drive. My PSB Alpha P5 speakers are rear ported and sound better, but still they sound very small, smaller than with another amp like the Emotive BasX A-100.
In the living room, I have some PSB Alpha T20 floor standing speakers. These tend to sound small with any amp, I guess because of their "5 1/4 inch" drivers which seem more like 4 inch drivers. They sound OK with the ACA Mini, but nothing like the JBL Arena 170s.
In short, my experience is that relatively large, simple, easy-to-drive speakers are key to getting audiophile sound from the DIY ACA Mini amplifier. And when you have such speakers...WOW!!!
A piano sounding like a piano!!!
That seems to be the key to great sound with the DIY ACA Mini amplifier. My JBL Arena 170 speakers, 2-way, 7 inch, large [almost 3 cubic feet inside, I'm guessing], hollow floor standing speakers [$150/pair, delivered, when I bought them on closeout] sound incredible when driven by the ACA Mini. I moved them about four inches further from the back wall since switching from the Schiit Aegir because the ACA Mini does a better job with bass, and these speakers are rear ported.
I've been listing to a lot of piano music [right now, Mozart piano sonatas] because this is the first stereo setup I've owned that reproduces the sound of a piano convincingly. I can hear the strings vibrate when they are struck. I can hear the piano resonate. Nothing is blurred. What fun!
Back to synergy: So far I haven't been able to get good sound with my second ACA Mini amp and the bookshelf speakers in my bedroom. I have two sets of small sealed speakers [JBL and NHT]. With the ACA Mini these have clear treble and almost nothing in the midrange and bass. I think the woofers in the sealed boxes must be hard to drive. My PSB Alpha P5 speakers are rear ported and sound better, but still they sound very small, smaller than with another amp like the Emotive BasX A-100.
In the living room, I have some PSB Alpha T20 floor standing speakers. These tend to sound small with any amp, I guess because of their "5 1/4 inch" drivers which seem more like 4 inch drivers. They sound OK with the ACA Mini, but nothing like the JBL Arena 170s.
In short, my experience is that relatively large, simple, easy-to-drive speakers are key to getting audiophile sound from the DIY ACA Mini amplifier. And when you have such speakers...WOW!!!
A piano sounding like a piano!!!
I have heard JBL's from that series. They are pretty nice speakers for the price. So you need more power... It sounds like the DIY shark must move forward which brings the question, what is your next amp going to be! 🙂
It sounds like the F5 or F6 may be up your alley. They both will increase power into 4 ohms. Clean detailed and decent bass
If you want a big soundstage type of sound, Randy's UMS Aleph 30 boards make a great amp. Not a bass machine but pretty sounding. A little sweeter but less oomph, the Aleph J.
It sounds like the F5 or F6 may be up your alley. They both will increase power into 4 ohms. Clean detailed and decent bass
If you want a big soundstage type of sound, Randy's UMS Aleph 30 boards make a great amp. Not a bass machine but pretty sounding. A little sweeter but less oomph, the Aleph J.
@Mikerodrig27: Thanks for the feedback. Yes, some day when I have more time for projects, I may take on a bigger amp.
Re the JBL Arena 170s, I keep saying to myself, "These are cheap 'home theatre' speakers. How can they possibly sound so good?" One thing I particularly like about them is that they have wide dispersion, so I don't have to sit in the sweet spot to enjoy music. They even sound very good and realistic when I'm in another room.
For now I am quite content. 🙂
Re the JBL Arena 170s, I keep saying to myself, "These are cheap 'home theatre' speakers. How can they possibly sound so good?" One thing I particularly like about them is that they have wide dispersion, so I don't have to sit in the sweet spot to enjoy music. They even sound very good and realistic when I'm in another room.
For now I am quite content. 🙂
Update 2 months later:
I still think the DIY ACA Mini amplifier and the JBL Arena 170 floor standing speakers in my living room system are an incredibly good combination. Everything sounds so clear and true, especially pianos, as I mentioned before.
I earlier reported that the ACA Mini didn't do a great job with the bookshelf speakers in my bedroom. I now want to take that back.
I'd been using an Emotiva BAS-X A-100 amp with my bedroom speakers. It seemed like it provided more lower midrange and bass than I got from the ACA Mini. But every time I played music in the bedroom, I was sorry that it sounded so much worse than in the living room. The sound was so muddy when contrasted with the ACA Mini's clarity.
So I took out the second ACA Mini and redid the bias to make sure it was 370 mV or a bit higher and exactly 11.5V VO. [I did the same with the living room ACA Mini recently.] I then put it back in the bedroom and hooked it up to my small JBL Arena 120 speakers that hang on the wall. Well...it sounds great! The same clarity as in the living room, but, yes, less bass. Well, after all, these little speakers don't produce a lot of bass, so that's just how it is.
I have Mozart's piano sonatas playing in the bedroom right now, while I type this in the dining room. It sounds very much like when I listen to the living room system from another room. Wonderful! And of course even better in the bedroom.
So, what I learned is that a muddy sounding amp gives the impression of a fuller mid/bass, perhaps because its treble is so dull.
Very happy to have two ACA Mini amps in the house!
Thank you again, Nelson.
I still think the DIY ACA Mini amplifier and the JBL Arena 170 floor standing speakers in my living room system are an incredibly good combination. Everything sounds so clear and true, especially pianos, as I mentioned before.
I earlier reported that the ACA Mini didn't do a great job with the bookshelf speakers in my bedroom. I now want to take that back.
I'd been using an Emotiva BAS-X A-100 amp with my bedroom speakers. It seemed like it provided more lower midrange and bass than I got from the ACA Mini. But every time I played music in the bedroom, I was sorry that it sounded so much worse than in the living room. The sound was so muddy when contrasted with the ACA Mini's clarity.
So I took out the second ACA Mini and redid the bias to make sure it was 370 mV or a bit higher and exactly 11.5V VO. [I did the same with the living room ACA Mini recently.] I then put it back in the bedroom and hooked it up to my small JBL Arena 120 speakers that hang on the wall. Well...it sounds great! The same clarity as in the living room, but, yes, less bass. Well, after all, these little speakers don't produce a lot of bass, so that's just how it is.
I have Mozart's piano sonatas playing in the bedroom right now, while I type this in the dining room. It sounds very much like when I listen to the living room system from another room. Wonderful! And of course even better in the bedroom.
So, what I learned is that a muddy sounding amp gives the impression of a fuller mid/bass, perhaps because its treble is so dull.
Very happy to have two ACA Mini amps in the house!
Thank you again, Nelson.
After finishing the ACA build, i would like to use my new digital soldering station again along with my multi meter. So the mini is on my mind. Thinking of an artsy steam punk treatment with a Fosi tube pre and a monaural no baffle driver aimed at my right ear ( very near field ). Yes, there will be limited bass. Maybe a Tang band 4" full range driver. Perhaps a framework to hold it all made of welded half inch rusty rebar. Of course this concept might be a total failure but nothing ventured, nothing gained.
I was right! The ACA Mini is a fantastic headphone amp. I bought it from a neighbor, bolted both chassis into a discarded bureau drawer, and added the jack.
Holy cow! I am feeding it 96/24 from an Astell&Kern AK120 mk1. The phones are open back Sennheiser HD 600. But it's not about hardware anymore. There is a closer connection to the intent of the musicians. Thank you, Nelson Pass and everyone involved with kits, especially Zen Mod.
Holy cow! I am feeding it 96/24 from an Astell&Kern AK120 mk1. The phones are open back Sennheiser HD 600. But it's not about hardware anymore. There is a closer connection to the intent of the musicians. Thank you, Nelson Pass and everyone involved with kits, especially Zen Mod.
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ZM is having zerzilchnuttinnada contribution in ACA Mini endeavor
credit where credit's due
that, of course, having nothing with your amount of happinesssssssssssssssssss
credit where credit's due
that, of course, having nothing with your amount of happinesssssssssssssssssss
@Jlhommedieu10 : Can you hear any on/off pop or thump with headphones? I don't hear anything with speakers but I'm curious if there's something that might be audible with headphones.