Questions about setup and ACA one year after the build.
Hi Amp camp amp ..err, camp
---->Please skip directly to questions below if you dont like stories, philosophy or impromptu poetry
About a year ago I've bought and built the ACA kit and have been using it as a main amp regularly. Motivation behind it was that I started to get annoyed by the mainstream. Shiny boxes on shiny stands. Powered by more shiny boxes on their shiny stands. Shiny cables in between them, sometimes even they on shiny stands.
DIY audio, I thought, was beyond reach for someone with limited knowledge in electrical circuits and acoustic design. But reading through popular magazine reviews, product descriptions and certain audio science reviews and juxtaposing this with what I heard in hifi stores or at peoples homes insulted the logic. Also it didn't look or sound any fun.
So the reasoning was that to have fun, one must dig a bit deeper and start experimenting - firstly with room acoustics and (electrically) simple concepts. I sold my shiny boxes, found some 2nd hand Fostex FE206En in factory BLH and started playing with that. While flawed in some 'objective' ways, they were (and still are) quite spectacular and engaging. Following this a search for a more appropriate amp ensued. The simple 20w TI-based chipamp I liked on a few british monitors wasn't pairing as good on the Fostexes. So here I landed and started discovering the various ideas for amplification.
Amp Camp Amp not only seemed like a good project for a beginner, but also spoke to my minimalist (purist?) sensibilities. A circuit schematic that fits on a post-it is making people excited, but in a a school-girl kind of way, that sounded fun! Also, like the Fostexes, I expected that it won't be the finest, high-endest or most refined. But also like the Fostexes, I hoped it would show a glimpse of that murky back room of audio world that i find exciting. It did!
-----
Been listening with ACA and Fostex horns exclusively for more than a year now. Imperfect, sure, but fun, exciting, intimate, epic.. as well. Recently, I'm finding some motivation to dig a bit deeper and have some questions regarding my "system" (pic bellow). I'm also tweaking the speakers, discussed in another thread.
1.Need for a preamp. I've read a few comments that because of the low gain of aca a "real" preamp is needed. Dac with volume control isn't optimal, they postulated, or even a buffer like Nelson recommend. I have a rpi4+pi2aes streamer feeding a chord mojo2 dac which is connected directly to aca with a headphone jack to 2x rca splitter and some belden microphone wire. Mojo is also a class A amp that can output something like 700mW into 8ohms. It can even drive the Fostex horns by itself to reasonable levels. With ACA, according to the colour chart included below, it's feeding the ACA 0.5V at most, before it gets unbearably loud. Do you see any problems with this setup? Would I benefit in some ways with the buffer pre like the B1, or even a "real" preamp? I want to add a turntable down the line, so im wondering what would be the smartest way of handling that.
2. Linear PSU and other ACA tweaks. I thought a linear PSU would be the bare minimum upgrade down the line, but reading through this thread I found mixed experiences. Many even describe a loss of dynamics and energy, even with (allegedly) well designed outboard linear psu. Are there any obvious (certain) improvements to be made on ACA itself?
3.When you've adjusted the bias the first time round, have you found that it drifted and needed to be tweaked later?
4. Longevity. As mentioned, I like the thing and am using it almost daily. I'm wondering how long can it go before it might need som tlc? And what would be the first things to look for?
Appreciate your wisdom,
Happy camping!
Hi Amp camp amp ..err, camp
---->Please skip directly to questions below if you dont like stories, philosophy or impromptu poetry
About a year ago I've bought and built the ACA kit and have been using it as a main amp regularly. Motivation behind it was that I started to get annoyed by the mainstream. Shiny boxes on shiny stands. Powered by more shiny boxes on their shiny stands. Shiny cables in between them, sometimes even they on shiny stands.
DIY audio, I thought, was beyond reach for someone with limited knowledge in electrical circuits and acoustic design. But reading through popular magazine reviews, product descriptions and certain audio science reviews and juxtaposing this with what I heard in hifi stores or at peoples homes insulted the logic. Also it didn't look or sound any fun.
So the reasoning was that to have fun, one must dig a bit deeper and start experimenting - firstly with room acoustics and (electrically) simple concepts. I sold my shiny boxes, found some 2nd hand Fostex FE206En in factory BLH and started playing with that. While flawed in some 'objective' ways, they were (and still are) quite spectacular and engaging. Following this a search for a more appropriate amp ensued. The simple 20w TI-based chipamp I liked on a few british monitors wasn't pairing as good on the Fostexes. So here I landed and started discovering the various ideas for amplification.
Amp Camp Amp not only seemed like a good project for a beginner, but also spoke to my minimalist (purist?) sensibilities. A circuit schematic that fits on a post-it is making people excited, but in a a school-girl kind of way, that sounded fun! Also, like the Fostexes, I expected that it won't be the finest, high-endest or most refined. But also like the Fostexes, I hoped it would show a glimpse of that murky back room of audio world that i find exciting. It did!
-----
Been listening with ACA and Fostex horns exclusively for more than a year now. Imperfect, sure, but fun, exciting, intimate, epic.. as well. Recently, I'm finding some motivation to dig a bit deeper and have some questions regarding my "system" (pic bellow). I'm also tweaking the speakers, discussed in another thread.
1.Need for a preamp. I've read a few comments that because of the low gain of aca a "real" preamp is needed. Dac with volume control isn't optimal, they postulated, or even a buffer like Nelson recommend. I have a rpi4+pi2aes streamer feeding a chord mojo2 dac which is connected directly to aca with a headphone jack to 2x rca splitter and some belden microphone wire. Mojo is also a class A amp that can output something like 700mW into 8ohms. It can even drive the Fostex horns by itself to reasonable levels. With ACA, according to the colour chart included below, it's feeding the ACA 0.5V at most, before it gets unbearably loud. Do you see any problems with this setup? Would I benefit in some ways with the buffer pre like the B1, or even a "real" preamp? I want to add a turntable down the line, so im wondering what would be the smartest way of handling that.
2. Linear PSU and other ACA tweaks. I thought a linear PSU would be the bare minimum upgrade down the line, but reading through this thread I found mixed experiences. Many even describe a loss of dynamics and energy, even with (allegedly) well designed outboard linear psu. Are there any obvious (certain) improvements to be made on ACA itself?
3.When you've adjusted the bias the first time round, have you found that it drifted and needed to be tweaked later?
4. Longevity. As mentioned, I like the thing and am using it almost daily. I'm wondering how long can it go before it might need som tlc? And what would be the first things to look for?
Appreciate your wisdom,
Happy camping!
Attachments
Im wondering if H2 with ACA wouldnt be "too much of a good thing"? Also missing is basic functionality like multiple inputs, volume control etc.
As mentioned, mojo dac/amp can drive tha ACA plenty loud at ca. 0.5v output and has invisibly low Rout.What would change if a buffer pre (like B1) would be added? Or a "real" line stage? Can there be any objective improvements?
I'm confused by some of the comments, saying ACA needs a proper line preamp to really perform.
As mentioned, mojo dac/amp can drive tha ACA plenty loud at ca. 0.5v output and has invisibly low Rout.What would change if a buffer pre (like B1) would be added? Or a "real" line stage? Can there be any objective improvements?
I'm confused by some of the comments, saying ACA needs a proper line preamp to really perform.
Hm. As your Mojo (or my Furutech)easily drives the ACA, a nice line-stage would just add „something“ to the signal, and makes the SQ, like, „nicer“ (less fatiguing, silky, whatever subjective adjective may fit). It is a matter of psychacoustic, thd, second-order, this sort of things…
In the end, if adding a pre improves the entertainment factor, I‘m in.
In the end, if adding a pre improves the entertainment factor, I‘m in.
IF you don't need gain, then you don't need gain. Some people might. Putting something before the ACA that increases the likelihood of driving the ACA to clipping and beyond (and/or using more attenuation) is counter-intuitive to me, but to each their own. Do this for fun. Go find the input sensitivity for the ACA. Decide if you need more gain after that. Better yet, go with what you already know... at less then the full output of your DAC... it's already too loud.
Not getting into that discussion.
Why do you need a buffer? Don't get me wrong, something like the B1 could be fun. I love several of the pre-amp projects for their own reasons.
No. It sounds wonderful.
Maybe. You seem to enjoy the things you have. Adding a pre-amp may or may not work well for you. When you need something for your turntable / phono pre to feed into and want some input switching, a buffer could be wonderful... or depending on the output of the phono pre... you may then decide you need more gain.
Can't help you there.
No. Nothing is certain. Those that claim improvements do so subjectively. I trust and believe them, and I appreciate their viewpoints and impressions of the performance in their systems with their ears for their enjoyment. Can you expect the same with any level of certainty? No, not in my opinion. With that said, we all love to tweak... it's fun.
Yes, but only once. Then, I just leave it alone.
I wouldn't worry about it. Write back in 20 years.
It may or may not be. You may like to set it up so that the distortions cancel each other... or add a bit. The H2 is a super-neat project and an absolute bargain to use just as a learning tool. You can add inputs and a volume control if you want, but I doubt that the H2 + ACA would be your choice ... however, it may be. H2 + F4 is super fun. You can't go wrong for the price... Give it a whirl. Twirl the pots... see what you like. If you've got a way to measure the distortion, that's even better.
It depends on the objective. Seriously. If your objective is to add "something" whatever that "something" may be, then the objective has been met.
I'm not sure if it's a fun thing to do where you live to read fortune cookies and add the words "in bed" after the fortune.
I do a similar thing with all subjective comments posted in threads. I add the words "in my opinion" to everything. People are posting their honest opinions for the most part. Everyone has different systems, brains, ears, rooms, tastes, and goals than I. Their opinions are valuable and fun to read. However, in a number of situations, my opinions differ from the majority. It could be that I have a tin ear...
No matter what you choose, you've made the right choice.
... in my opinion.
Have fun!
Thank you both for replies! I too appreciate subjective reports and consider them just as valid as measurements. As someone still a lot to learn, I do want to make sure I'm not committing a faux pas over something that might be quite obvious to a more experienced person. So thanks for putting my mind at ease!
The starting point was indeed to have as few components in the chain so that it would be easier to understand basic principles, easier to troubleshoot, hear differences in things I'm fooling around with etc.. Now getting up every time to adjust the volume is getting tedious and vinyl playback has been on the horizon for a few years now. Hence, trying to figure out what I actually need/want in front of the ACA.
The answer in my case is probably as straightforward a preamp as possible, adding a few inputs and a volume control with remote. B1 in short i guess. But I'm not sure I understood Nelsons B1 article on why a passive preamp wouldn't work quite as well as a buffer. Something about output impedance not jiving with ACA. Can someone elaborate on this in layman terms?
Also, whats a neat way of adding remote functionality to B1?
The starting point was indeed to have as few components in the chain so that it would be easier to understand basic principles, easier to troubleshoot, hear differences in things I'm fooling around with etc.. Now getting up every time to adjust the volume is getting tedious and vinyl playback has been on the horizon for a few years now. Hence, trying to figure out what I actually need/want in front of the ACA.
The answer in my case is probably as straightforward a preamp as possible, adding a few inputs and a volume control with remote. B1 in short i guess. But I'm not sure I understood Nelsons B1 article on why a passive preamp wouldn't work quite as well as a buffer. Something about output impedance not jiving with ACA. Can someone elaborate on this in layman terms?
Also, whats a neat way of adding remote functionality to B1?
My other ACA (running on 26V, using irfp150's) is driven by a B1, and seems a bit underpowered. It's being used by my brother, and it could just be that his speakers are inefficient.
As to the H2, just add a couple extra rca jacks, source selector switch, and volume pot and you have a pre amp. It's right there in Nelson's H2 article.
As to the H2, just add a couple extra rca jacks, source selector switch, and volume pot and you have a pre amp. It's right there in Nelson's H2 article.
Passive pre-amps can be fine. I think he's just explaining situations where a buffer can be beneficial.
Rather than me try to explain it... I think Nelson does an exceptional job. Maybe take this section I've copied below from the B1 article and see what doesn't make sense. Emphasis added is mine.
Text copied from - https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp
"Is impedance matching an issue? Passive volume controls do have to make a trade-off between input impedance and output impedance. If the input impedance is high, making the input to the volume control easy for the source to drive, then the output impedance is also high, possibly creating difficulty with the input impedance of the power amplifier. And vice versa: If your amplifier prefers low source impedance, then your signal source might have to look at low impedance in the volume control.
This suggests the possibility of using a high quality buffer in conjunction with a volume control. A buffer is still an active circuit using tubes or transistors, but it has no voltage gain – it only interposes itself to make a low impedance into a high impedance, or vice versa."
This is where I go off book - The ACA has an input impedance of ~10k. Is that relatively high or low? Do you think it might prefer a relatively low source impedance or a relatively high source impedance (all things remaining equal)?
IF someone were to go and buy a 20k Alps "Blue Velvet", stick it in a box with some input and output wiring... would that be ideal for an ACA? Would the Mojo care?
What if the input impedance of the ACA was 100k (like a First Watt F5) and the pot was 10k?
Last, but not least... these folks are trying to sell you something, so take everything in that context. However, it might help.
https://tortugaaudio.com/what-is-a-passive-preamplifier/#:~:text=Using a passive preamp between,lacking in dynamics and punch.
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If you plan to use a turntable, you should check for the output-difference between the phono-pre and the mojo, as phonos tend to have a lowish output (this also may depend a bit on the cartridge-phonostage-settings and stuff). Me, I couldn’t be satisfied without a line-stage (preamp). But as I said, it all depends on what your phono-stage is delivering…
A b1, as cool as they are and look, don‘t help with a problematic gain-structure <— but they are great for that impedance-thing. (Which is just as important)
A b1, as cool as they are and look, don‘t help with a problematic gain-structure <— but they are great for that impedance-thing. (Which is just as important)
This component interaction via different in/out impedances is exactly the thing I'm struggling with. Maybe someone can link a good resource to learn up on it?
And these folks did try to sell the ACA and I'm glad they did! I've had so much fun learning to solder (nicely), learning about how amps work, building something relatable from what to my family looked like "funny pins and needles and rainbow pasta" and finally enjoying it.
But there is no need to defend this amp by this aspect of it. For the right speaker it sounds great. I got curious and hooked Vista Audio Spark 2 to Fostex blh:s. Its a 20w class a/b amp based around LM3886, premium, well measuring, extremely clean sounding TI chip. Looks like a decent PS in there too and cost about the same as ACA.
Kicker, I unhooked the thing after 3 tracks and went "camping" again. On Fostexes, the detail and cleanliness was too much. Images were so pinpoint that the piano keys juxtaposed perfectly over books I was staring at and I could track them book by book. But they were also tiny and flat, didnt smack, ring or linger. A few of my favourite singers also suddenly now had tiny heads. Very clean and smooth vocals. But heads (and chests) were two sizes smaller. I wonder how one would measure this effect? 🤔
So folks, if you have 96db efficient speakers and like tiny heads, DON'T get an ACA.
^ Did you read the last link? IMO it gives a nice overview and what it means in layman's terms. Those are the people I meant re: trying to sell you something (their 'passive' pre-amp).
Or just search for "impedance mismatch" and read happily along. Show us what you've searched for and found, and tell us what you don't understand. What was unclear from the last link I posted?
Perhaps cut exactly from each article what is not making sense. That might help. Just saying, I don't understand... is hard to work with.
Basically...
"Low" output impedance of a source relative to "high" input impedance of what it's driving => good.
"High" output impedance of a source relative to "low" input impedance of what it's driving => bad.
That's over-simplified, but it may be what you're looking for.
Or just search for "impedance mismatch" and read happily along. Show us what you've searched for and found, and tell us what you don't understand. What was unclear from the last link I posted?
Perhaps cut exactly from each article what is not making sense. That might help. Just saying, I don't understand... is hard to work with.
Basically...
"Low" output impedance of a source relative to "high" input impedance of what it's driving => good.
"High" output impedance of a source relative to "low" input impedance of what it's driving => bad.
That's over-simplified, but it may be what you're looking for.
Sorry, misunderstood that! Yes that's the article I was thinking. I realize low Rout into high Rin is what most designers aim for (good, as you describe it), but simply lack knowledge to understand it. In simple terms is it "easy" to drive? I.e. because of the high Rin, the load will be more stable. But even then, current source amps and line stages seem to be still in discussion and that works differently it seems. A "famous" swede wrote a whole book about it, Nelson made the F2J and many "highend" products use it, Albeit the current driving might still be converted to a Voltage source at the output. These concepts are very fuzzy so perhaps better I revisit the topic after some more reading!
^ OK... let me try one more time. I appreciate you trying to learn. I may just not be a very good teacher. As mentioned, do a little reading. Look for the answer. These topics are widely covered, and many people can do a much better job of explaining than I could.
Here is a nice way to get started.
https://letmegooglethat.com/?q=what+is+wrong+with+low+input+impedance
This is the first article that appeared in my search. It uses production equipment and instruments as the examples, but the section below is really nice, IMO. That's about the best I can do. I hope it makes sense. Mostly, I hope you really enjoy whatever you choose to go along with your ACA. It's a wonderful hobby!
https://www.whirlwindusa.com/tech-articles/high-and-low-impedance-signals#:~:text=If the input impedance of,and frequency response will suffer.
Emphasis added is mine.
"Low Feeds High
In order to preserve signal level and frequency response, it's important to drive equipment with a source signal that is lower in impedance than the destination equipment's input impedance. If the input impedance of a device is not significantly higher than the source impedance, the signal will be reduced or "loaded down" and its signal to noise ratio and frequency response will suffer.
Think of this as having a nozzle at the end of a garden hose. The garden hose is a low impedance source (there is little resistance to the flow of water) and the nozzle is the higher impedance of the input being fed by the hose.
When the nozzle valve is closed (open circuit):
Now open the nozzle just a little:
As you continue to open up the nozzle:
With the nozzle open all the way:
In the case of a high impedance guitar output (7,000 to 15,000 Ohms or more) driving a relatively low impedance input of a mixer (2,000 to 10,000 Ohms), it's like connecting a garden hose to a fire nozzle. The hose just can't produce enough flow (current) for the size of the opening (impedance) to maintain the pressure (voltage)."
---------
So, back to the questions I had asked previously...
Input impedance of the ACA is?
What happens if you put a 20k pot between your source and the ACA?
I hope that helps. Others may have a better way to explain.
Here is a nice way to get started.
https://letmegooglethat.com/?q=what+is+wrong+with+low+input+impedance
This is the first article that appeared in my search. It uses production equipment and instruments as the examples, but the section below is really nice, IMO. That's about the best I can do. I hope it makes sense. Mostly, I hope you really enjoy whatever you choose to go along with your ACA. It's a wonderful hobby!
https://www.whirlwindusa.com/tech-articles/high-and-low-impedance-signals#:~:text=If the input impedance of,and frequency response will suffer.
Emphasis added is mine.
"Low Feeds High
In order to preserve signal level and frequency response, it's important to drive equipment with a source signal that is lower in impedance than the destination equipment's input impedance. If the input impedance of a device is not significantly higher than the source impedance, the signal will be reduced or "loaded down" and its signal to noise ratio and frequency response will suffer.
Think of this as having a nozzle at the end of a garden hose. The garden hose is a low impedance source (there is little resistance to the flow of water) and the nozzle is the higher impedance of the input being fed by the hose.
When the nozzle valve is closed (open circuit):
- Input impedance is VERY high
- Pressure (voltage) is at maximum
- Flow (current) is zero
Now open the nozzle just a little:
- Input impedance reduces but remains high
- Pressure reduces but remains high
- Flow is small
As you continue to open up the nozzle:
- Input impedance reduces further
- Pressure reduces
- Flow increases
With the nozzle open all the way:
- Input impedance is very low
- Pressure falls dramatically
- Flow is greatest
In the case of a high impedance guitar output (7,000 to 15,000 Ohms or more) driving a relatively low impedance input of a mixer (2,000 to 10,000 Ohms), it's like connecting a garden hose to a fire nozzle. The hose just can't produce enough flow (current) for the size of the opening (impedance) to maintain the pressure (voltage)."
---------
So, back to the questions I had asked previously...
Input impedance of the ACA is?
What happens if you put a 20k pot between your source and the ACA?
I hope that helps. Others may have a better way to explain.
thanks, ben mah. i saw that spec on the amp camp page:
"Heat dissipation: 20 Watts per Channel"
but, it sounded low for a class a amp, and i wasn't sure it was the same thing as idle current. my pass f2 clone, which doesn't make quite the power of this amp, idles at 200w, and it gets really hot...
doug s.
"Heat dissipation: 20 Watts per Channel"
but, it sounded low for a class a amp, and i wasn't sure it was the same thing as idle current. my pass f2 clone, which doesn't make quite the power of this amp, idles at 200w, and it gets really hot...
doug s.