According to the current, revised BOM the only place you should use a 6.8K is at R0.
Cool plan? Dont forget, the ACA mini is not a cool amp, it is getting quite warm or hot. Not the ideal amplifier to build into a turntable system!
Will, following up on this point raised by rusomon, if your kit is based on the new BOM, what happened to the new R4 value 22.1K resistors? Since 22.1K is not in the old BOM, could you have accidentally used them in the R2/R3 positions?
Last edited:
I've been remiss in not reporting my results. I completed it sometime right before or after XMAS (I'm retired).
The build went pretty smoothly except for a few novice missteps. I've only ever built one other amp, the ACA Amp. One misstep was that I soldered the LED pilot lamp in the wrong position and upon de-soldering it to place it properly, I realized that I had already trimmed the leads and had no way of knowing which lead was positive. I figured that a 9V battery would help and it appears that the voltage requirement for the LED is much much less than that so a new LED is on order from DigiKey (but back ordered until early March or April).
The second misstep was dropping the unit while I was attempting to bias it. This drop was only 2' or so but was enough to bend the power socket which eventually began to short periodically so it's been replaced already.
Lastly, as a novice and absent a good VMM that autoranges, I found the biasing a bit intimidating. This was compounded when I had no readings at all on one channel and attributed it to the drop. After a cooling off period (me, not the amp), I went back to it and started with the other channel and proceeded to get it somewhat biased. Mysteriously, to me anyway, the first channel was now showing readings at each test point and the process began to make sense to me. So much so I rebiased it several times and it was easier with my new Klein autoranging meter and feel confident doing so.
I posted all the above as encouragement to other novices that problems in the build are not insurmountable.
So how does it sound? Fantastic to my ears. Based on about 50 years of playing with stereos (user interfaces only), IMHO amazingly good.
I had the Mini on top of the ACA Amp on my garage desk and will go out on a limb and say that it is noticeably better to my old ears anyway. I've had in this garage desktop system using an iPad, Schiit Mani, Schiit Modi DAC, Schiit Mani Phono Section, Schiit Saga pre, old Dual 510 belt drive TT (ADC XLM MKIII cart with Jico stylus), old Kenwood KT-8007 tuner, and all driving Elac Debut 6 (original model) speakers. This system is my "daily driver" and is on 4-5 hours minimum each day.
How good is it? I was invited to hear my best friend's new PS Audio FR-30 Aspen speakers (100W minimum recommended). On a lark, I packed the Mini for my visit. He laughed when I held it in my hand to show him when I first arrived. We listened to his new speakers and they are phenomenal via his Mark Levinson ML1 pre and 23.5 power amp. Vinyl via a Linn LP12/Troika and digital via a PS Audio streamer/DAC. Huge sound stage and prodigious bass whether he had his new REL subs in or out.
After a while, we substituted the Mini for his ML 23.5 power amp. Both of our jaws dropped. There really was no immediate loss of sound quality although the Mini obviously couldn't play as loudly or with such great authority. But talk about punching above your weight. Great sound stage and a wonderful purity to the sound but in a natural and not clinical way.
I mention all this not to "name drop" about some great equipment. Instead, I mention it to encourage others to at least hear one of these gems. Strongly recommended.
The build went pretty smoothly except for a few novice missteps. I've only ever built one other amp, the ACA Amp. One misstep was that I soldered the LED pilot lamp in the wrong position and upon de-soldering it to place it properly, I realized that I had already trimmed the leads and had no way of knowing which lead was positive. I figured that a 9V battery would help and it appears that the voltage requirement for the LED is much much less than that so a new LED is on order from DigiKey (but back ordered until early March or April).
The second misstep was dropping the unit while I was attempting to bias it. This drop was only 2' or so but was enough to bend the power socket which eventually began to short periodically so it's been replaced already.
Lastly, as a novice and absent a good VMM that autoranges, I found the biasing a bit intimidating. This was compounded when I had no readings at all on one channel and attributed it to the drop. After a cooling off period (me, not the amp), I went back to it and started with the other channel and proceeded to get it somewhat biased. Mysteriously, to me anyway, the first channel was now showing readings at each test point and the process began to make sense to me. So much so I rebiased it several times and it was easier with my new Klein autoranging meter and feel confident doing so.
I posted all the above as encouragement to other novices that problems in the build are not insurmountable.
So how does it sound? Fantastic to my ears. Based on about 50 years of playing with stereos (user interfaces only), IMHO amazingly good.
I had the Mini on top of the ACA Amp on my garage desk and will go out on a limb and say that it is noticeably better to my old ears anyway. I've had in this garage desktop system using an iPad, Schiit Mani, Schiit Modi DAC, Schiit Mani Phono Section, Schiit Saga pre, old Dual 510 belt drive TT (ADC XLM MKIII cart with Jico stylus), old Kenwood KT-8007 tuner, and all driving Elac Debut 6 (original model) speakers. This system is my "daily driver" and is on 4-5 hours minimum each day.
How good is it? I was invited to hear my best friend's new PS Audio FR-30 Aspen speakers (100W minimum recommended). On a lark, I packed the Mini for my visit. He laughed when I held it in my hand to show him when I first arrived. We listened to his new speakers and they are phenomenal via his Mark Levinson ML1 pre and 23.5 power amp. Vinyl via a Linn LP12/Troika and digital via a PS Audio streamer/DAC. Huge sound stage and prodigious bass whether he had his new REL subs in or out.
After a while, we substituted the Mini for his ML 23.5 power amp. Both of our jaws dropped. There really was no immediate loss of sound quality although the Mini obviously couldn't play as loudly or with such great authority. But talk about punching above your weight. Great sound stage and a wonderful purity to the sound but in a natural and not clinical way.
I mention all this not to "name drop" about some great equipment. Instead, I mention it to encourage others to at least hear one of these gems. Strongly recommended.
Last edited:
I have been enjoying my ACA Mini since I built it a few months ago. The transistors came with a sticker that said they were matched by Nelson Pass. How awesome is that? It sounds great, but now I need some trouble shooting help. A few weeks ago, I noticed the left channel was silent. Both channels bias just fine, but the voltage doesn't change on the left side when music is playing.
I had time today to reflow each solder point on the left channel but that channel remains silent. The cables all work since both channels play fine when I swap a different amp. The RCA plug has connectivity on both legs. Everything looks okay to me.
I am at a loss as to what could be wrong. Photos are attached (I didn't clean the flux after reflowing and adding a bit more solder). Thanks for any suggestions to get this back up and running.
I had time today to reflow each solder point on the left channel but that channel remains silent. The cables all work since both channels play fine when I swap a different amp. The RCA plug has connectivity on both legs. Everything looks okay to me.
I am at a loss as to what could be wrong. Photos are attached (I didn't clean the flux after reflowing and adding a bit more solder). Thanks for any suggestions to get this back up and running.
Attachments
Thank you so much, Nelson. I am a troubleshooting newb who went to college to study EE and eventually design stereo equipment but went to the wrong school and ended up with a chemical engineering degree. Haha, long story. Thank you for openly sharing your designs, thoughts, and expertise. Because of you, I am able to fulfill my dream of studying concepts that make incredible-sounding music. You rock!
I will try following the signal path to test for continuity with my DMM and report back. Can I assume the transistors are okay since the channel biases? 🙏
I will try following the signal path to test for continuity with my DMM and report back. Can I assume the transistors are okay since the channel biases? 🙏
I'd suggest carefully examining your speaker termination, if you haven't already. Possibly a stray strand or two at the screw terminal (I think we've all had something like that before). Also try connecting your speaker directly to the screw terminal. What does the amp sit on when you're using it? Anything conductive touching the board?
If you're still stumped, start comparing voltages at different resistor points on the board from the good channel to the bad, if you have good mini-grabber leads for your meter(s). Write them down on the schematic. That would likely turn up something off.
This is how I discovered (rather, someone else noticed) an input short on my VFet Lottery amp build. The voltages were off at some point and someone wise recognized it.
Good luck! As you know, the ACA Mini is worth the effort!
Edit: what's going on here?
If you're still stumped, start comparing voltages at different resistor points on the board from the good channel to the bad, if you have good mini-grabber leads for your meter(s). Write them down on the schematic. That would likely turn up something off.
This is how I discovered (rather, someone else noticed) an input short on my VFet Lottery amp build. The voltages were off at some point and someone wise recognized it.
Good luck! As you know, the ACA Mini is worth the effort!
Edit: what's going on here?
Attachments
Last edited:
It is working again! Thanks for taking a look through my photos, von Ah. That goober is thermal paste that I need to clean up. I added the paste last night, concerned that I had somehow overheated the Harris transistors.
The issue was that I didn't install star washers on the back side of the heatsinks. I only installed them on the front side like a goofball. Now that I have a good ground connection, both channels are making beautiful music again. Yeah! Now I can reclean it and put my Elekit TU-8600 up for sale to fund my next project.
Thanks again!
The issue was that I didn't install star washers on the back side of the heatsinks. I only installed them on the front side like a goofball. Now that I have a good ground connection, both channels are making beautiful music again. Yeah! Now I can reclean it and put my Elekit TU-8600 up for sale to fund my next project.
Thanks again!
Attachments
Last edited:
It is odd, huh, @Mikerodrig27? It looks like the PCB track directly connects the drains, with or without the heatsink.
I reflowed the entire left/problem channel last night to no avail, but today noticed a connectivity problem from the center pins of the 520's to the heatsink post. Once I added a star washer to the backside of the heatsinks, connectivity happened. Every other connection on that channel also beeped on my DMM. I'll get it cleaned up and touch up the bias since I added goop to the 520's. Let's hope this was it. I love this little amp.
I reflowed the entire left/problem channel last night to no avail, but today noticed a connectivity problem from the center pins of the 520's to the heatsink post. Once I added a star washer to the backside of the heatsinks, connectivity happened. Every other connection on that channel also beeped on my DMM. I'll get it cleaned up and touch up the bias since I added goop to the 520's. Let's hope this was it. I love this little amp.
Hey all, Got my Mini kit yesterday, and am about to finish it up, except for deciding on its final look. I have been through a bunch of pages here, and see the word electrocution used at least once. What are the electrical risks with this amp in it's stock form? I do have a couple cats, and the do get into absolutely everything and epsecially new and odd looking things. This amp happens to check all of those boxes. Long term, I think I am going into a cigar box with this one. Any thoughts would be appreciated. I will understand the heat situation very soon. TIA. Looking forward to hearing this today and how it differs from my ACA.
Plastic Switcher Brick? You mean the switch? Mine is standard and is metal. Thanks again.
This paragraph had me wondering:
Lastly, you will have noticed that there is no chassis provided with this kit. This is where you
are on your own. Beyond the aesthetic considerations, a chassis provides additional hazard
safety. 24 volts is not a lot, and the heat sinks on top are at 12V, but you still would not want
these exposed to toddlers or small animals.
This paragraph had me wondering:
Lastly, you will have noticed that there is no chassis provided with this kit. This is where you
are on your own. Beyond the aesthetic considerations, a chassis provides additional hazard
safety. 24 volts is not a lot, and the heat sinks on top are at 12V, but you still would not want
these exposed to toddlers or small animals.
