so, we want the bias voltage at 12.5 instead 12.0v? I just finished mine like two weeks ago. Some parts were messed up, like the XLR connection couldn't be mounted in the direction it was supposed to, so I just didn't feel like point to point wiring it upside down or cocked off to the side in comparision to the build guide. I don't really use or have xlr output from any of my preamps except for a tape out on an NAD 1020B preamp...so I just didn't wire that up. I also didn't throw in the resistor/connections to the stereo/monoblock switch since I have plenty of speakers that are 8ohm and go from 91 to 98db/1m efficiency.....I have wondered though....in my situation or if anyone has noticed a difference in transparency or better quality sound going mono in terms of spending another 329 dollars? And lastly, man these things run some kind of hot...I have other class A single ended tube amps that get nowhere this temp...after about a couple days of playing with the ACA, I went and took the cover off and was going to let it warm up for 30 mins and just recheck the bias....I noticed on one pin that a solder joint looked like it exploded and another like someone reliquidfied it with a soldering iron!! And I'm telling ya...I might have a joint or two that could be suspect, but I doubt it...and these two that were messed up where previously solder joints of perfection....so I don't know what caused this....the general heat of the amp or some defect or problem....I just took the time to remove the old solder completely and reflow those two joints.
Yes its seems that way..I'll double check again to see if it happened again. I know these things are supposed to get hot...I have other class single ended....but man the temp just feels unhealthy...is was one of the middle mosfet joints...I had a perfect connection...it looked weird....like I dropped a dollop on it and it spread out like a start or snowflake pattern....I guess the best way to describe it. I may have to get some fans installed or something....the whole chassis is like a heat sink....in fact the top bottom and front plates are probably hotter than the heatsinks....I have never used thermasilk pads that camp with the ACA....or whatever they are called.....maybe they just don't work as well
I will within a couple days. I redid the joints so hopefully when I take the lid off this time they will look the same. I wanna get in there and twist the speaker signal wires tighter anyway as well....the build diagram I was looking at originally just had the power wires twisted for less interference, etc....I obviously just wasn't thinking about it and knew better, but forgot when I was just looking at the build guide and the wiring diagram..... which by the way is really really good, but I swear I think some of the steps need to be changed around a bit. But I'm telling ya, in terms of recapping and refurbing my vintage stereos, I've got my soldering capabilities down pat...and those 12 Q1 and Q2 joints were good and the easiest joints to make in the whole build. I wish I had a temperature reader or guage, etc. I have a window unit a/c in the room It's located in, maybe I might just have to put it close to the a/c vent 
Seemingly painful at first, but I can hold for 10 seconds....its just weird the front plate and top is probably hotter than the heatsinks.
btw, I have an NAD 1020b preamp I'm fixing up and it has a high output option...it goes up to a 15volt output instead of 2volt like the regular one....would that damage the ACA or just give me more volume. I figure the ACA is running 100% no matter what which is why its so hot, but would having a preamp with a 15volt output reduce the need for me to not bother with building monoblocks or would it stress the amp too much ?
btw, I have an NAD 1020b preamp I'm fixing up and it has a high output option...it goes up to a 15volt output instead of 2volt like the regular one....would that damage the ACA or just give me more volume. I figure the ACA is running 100% no matter what which is why its so hot, but would having a preamp with a 15volt output reduce the need for me to not bother with building monoblocks or would it stress the amp too much ?
10 seconds? It's not hot enough to melt solder. That temperature is normal.
More input voltage doesn't mean more output power. In practice it only means you'll have less usable range of your volume knob.
The ACA is a class-A single ended amp, and therefore is always sitting at it's max heat dissipation. (and will actually get a bit cooler if you run it near max output... )
You mounted the back panel on backwards if you couldn't get the XLR mounting holes to line up - an easy thing to have done, you are not the first.
The tape output of your NAD preamp is DIN, not XLR.
A second ACA will give you more than double the power into your speakers. It does sound better. (Quite a bit better, in my exprerience) Is that worth the expenditure? Only you can answer that.
More input voltage doesn't mean more output power. In practice it only means you'll have less usable range of your volume knob.
The ACA is a class-A single ended amp, and therefore is always sitting at it's max heat dissipation. (and will actually get a bit cooler if you run it near max output...
) You mounted the back panel on backwards if you couldn't get the XLR mounting holes to line up - an easy thing to have done, you are not the first.
The tape output of your NAD preamp is DIN, not XLR.
A second ACA will give you more than double the power into your speakers. It does sound better. (Quite a bit better, in my exprerience) Is that worth the expenditure? Only you can answer that.
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ha funny, your right...I don't know what I was thinking but since I'm trying to resolder some pads on the NAD today I gave myself the appropriate palm slap to the face for said statement Yeah, I'm really considering the second aca monoblock...because it doesn't quite get to the same volume level as my single ended class A tube amp thats also coincidentally rated at 8 watts....at least on my sansui speakers that are 98db efficient....but my investigation on these sansui speakers is that they are truly 6ohm speakers instead of 8ohm speakers to get the extra loudness numbers. And yeah most sansui speakers are kabuki stigmatized crap....but I've got 2 pair of the ones which have incredible build quality with high quality drivers....some of the most revealing speakers I've heard on the high end anyway.
Yeah, I'm really considering the second aca monoblock...because it doesn't quite get to the same volume level as my single ended class A tube amp thats also coincidentally rated at 8 watts....at least on my sansui speakers that are 98db efficient....but my investigation on these sansui speakers is that they are truly 6ohm speakers instead of 8ohm speakers to get the extra loudness numbers. And yeah most sansui speakers are kabuki stigmatized crap....but I've got 2 pair of the ones which have incredible build quality with high quality drivers....some of the most revealing speakers I've heard on the high end anyway.I'm having the common problem on the RCA outs on that NAD...at least I got the right side output going so I'm halfway there....but I did notice that side that did work resulted in in quite a bit more loudness, but it wasn't distorted. It was like the power doubled..same nice sound with more airy high details....typically with the aca, most of my preamps have to be turned mostly if not all the way up, even if I substitute two 12AT7s for 12AU7As in my tube Aikido preamp in order for it have more gain . This NAD 1020b high level output made it seem ironically more normal when turning the knob. Even though I have high efficiency speaker, it seems like the ACA really likes by Boston Acoustics A150s...it really does put out a solid crisp bottom end on those speakers....I'm not sure, but they are 8ohm and either 90 or 91db/m efficient.
I guess all in all, we know the addictive nature of this hobby and although I should spend money on other things, the pull and desire to pull the trigger on a second ACA kit is big. The force is strong with this little amp.....especially since last september I wanted to buy one so bad and they weren't in stock. I thought I was suppsoed to be notified when they came back in stock but I just ran across the site randomly and noticed...
and I yeah, with the rear panel it looked the same at first, I mounted it backwards because I had some scuffs on a few parts of my chassis...the rear and top and bottom panels had scuffs so I have those sides in the interior of the amp.
I guess all in all, we know the addictive nature of this hobby and although I should spend money on other things, the pull and desire to pull the trigger on a second ACA kit is big. The force is strong with this little amp.....especially since last september I wanted to buy one so bad and they weren't in stock. I thought I was suppsoed to be notified when they came back in stock but I just ran across the site randomly and noticed...
and I yeah, with the rear panel it looked the same at first, I mounted it backwards because I had some scuffs on a few parts of my chassis...the rear and top and bottom panels had scuffs so I have those sides in the interior of the amp.
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If you want more volume and better sound build one of the FirstWatt amps. F4, Aleph J, M2x, F6, etc... They are all more powerful than the bridged ACA, have better power into lower impedance loads, and really do the Class-A beautifully. There's not a bad amp amongst them, choose the one that seems the most interesting to you.
Ditto that from a builder of twin ACA's I ran in parallel mono.
They sounded better than any amplification I had tried previously, but the Firstwatt amps (recently put an Aleph J in service) are on a "different level" quoting 6L6 and believe me he is right.
I did the vintage audio for awhile, had a great time and put together some nice systems. Refoamed a pair of Boston A150's and wish I still had them.
I have recently thought of trying to find another pair to try with my Aleph J.
Keep yours and feed them some more Class A juice, I bet they'll sound terrific.
Believe I'd do an M2x for them if I had a pair.
Ok, there is probably something not right here:
- MOSFETs generate most of the heat
- Power resistors contribute another nice chunk of heat
If the Mosfets are correctly "coupled" to the heat sinks, most of the heat should go there. Then, by heat dissipation from he heatsink to the top and front, and by warming up the inside air, you should have lower temps on the top and front, up to identical temps.
If you have the air inside hotter than the Heatsinks, that probably means that the MOSFETs are not being "cooled" correctly by the Heatsinks and you are getting VERY hot air inside the case.
If that is true, you could have a problem in your hands.
Can you confirm this is indeed the case? Do you have a way to actually measure temps of heatsink and top lid?can you measure the air temperature inside?
Can you smell the characteristic "electronic burn" which is kind of sweet but burnt nonetheless?
in reference to the previous post...while rebuilding and recapping old vintage equipment and some tube amps is something I've done for years, this was my actual attempt at building a particular amp from scratch, but even then that's not totally true since the kit makes it a paint by number type level of difficulty......I imagine the other Pass amps like aleph J and F4/F5 up the difficulty and price level, but it still something I'd consider. This is probably the wrong thread to to get into the pros and cons of those particular amps, I'd imagine