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ACA-Nelson Pass's Amp Camp Amp


The Dude

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1 hour ago, captainbeefheart said:

The ACA amps are very similar to my favorite SS topology, single ended Class A, current source loaded Mosfet output taken from drain. It was based off the Zen amplifiers which are also very cool if people have not tried them. Some use light bulbs as drain loads instead of a CCS which is neat. But they are not great at driving tough loads and have fairly high distortion. Back to the ACA amps, the third version is the best in my opinion which has several additions including feedback to improve upon the design and making it more speaker friendly for people with speakers that present tough loads. Many of Klipsch speakers are easy to drive in regard to power but as for impedance and reactive nature of the loads they can be tough on amplifiers. The mini ACA is two stage push pull complimentary amplifier. It does have feedback which is good but the fact it's a push pull design puts me on the I will most likely never build the amp list. Still looks very promising and I am certain it will be a great amp for people to build and get great results. If I did go SS push pull I'd do something like the mini for sure.

And the price is supposed to be cheaper than the ACA being that it does not require a case. I believe the ACA is $299 so less then that. 

 

My comments are not the actual kit build. I just ordered the board and sourced the parts myself. Also I have a special case with a linear PS section I use to test amplifiers with. The neat thing is most all of the NP Firstwatt offerings use the same voltage rails enabling swapping out just the amplifier boards. 

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1 hour ago, henry4841 said:

The neat thing is most all of the NP Firstwatt offerings use the same voltage rails enabling swapping out just the amplifier boards. 

 

Yes that's true. One of NP's greatest quotes I have heard from him is when he is asked why he chose this or that for parts or whatever in his builds and his answer is because he has a drawer full of them. He build with what he has, very resourceful and proves you can build great amps from just about anything. I am very similar, if I see a very good deal on surplus parts or whatever I get them, those parts end up in all sorts of things I build. Why? Same as NP, because I have a drawer full of them already.

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On 1/14/2022 at 1:25 PM, CWelsh said:

I will be looking forward to your impressions after a couple of weeks of living with it.

Well its been about a month and half and I am still very please with this amp.  Clean, clear, for the price of the kit, I'd be surprised to find a better deal on an amplifier.  Sorry, but I couldn't provide much more.  Im a fan of Crown, McIntosh, and Parasound and this amp runs with them all. 

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5 hours ago, The Dude said:

Well its been about a month and half and I am still very please with this amp.  Clean, clear, for the price of the kit, I'd be surprised to find a better deal on an amplifier.  Sorry, but I couldn't provide much more.  Im a fan of Crown, McIntosh, and Parasound and this amp runs with them all. 

Perfect little kit for those wanting to build with the reward of sound that should cost much more. The bigger Nelson Pass projects do sound better but one cannot fault the sound of the little ACA. Standing it's own with McIntosh and Parasound says a lot. 

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1 hour ago, CWelsh said:

I ordered an ACA today. Got the notice they were back in stock and available, so I pulled the trigger.

 

$336 easily worth it, neat little amps. Probably have a free power supply laying around or pick one up for the price of a bag of peanuts. Strange design from what the majority of amps use, it is push pull but the output is taken from the drain not the source so it's a common source push pull output stage. It's also a current feedback design and not voltage feedback, the former typically has better bandwidth and slew rate characteristics, it's a tradeoff and current feedback isn't necessarily better, just different. One off the wall idea is the P channel output Mosfet has different gain than the N channel opposite to it giving the stage non-linearity, purists and people that hate single ended tube amps need not apply here as that is the effect you get, high 2H distortion, about 1%THD at 5 watts similar to many 300b SET amps. No worries Nelson has you covered if you don't want that sound, add the jumper which AC bypasses the source degeneration around the lower gain P channel Mosfet effectively increasing gain closer to the N channel Mosfet making the stage more linear and voila less distortion. I also like the idea of the standby mode switch that cuts the bias current to the transistors but keeping the power supply capacitors charged up.

 

Audio Science review rates this amp at the very bottom because they use the SINAD scale for rating quality which is a ratio between signal and distortion+noise. As long as you keep in mind it's not sound quality, it's purely an objective quality where you're compared to a theoretically perfect amplifier which is the output signal is the exact same as the input signal only larger to power loudspeakers. I am in the camp where a little second harmonic at the threshold of hearing seems to sound the best, not sterile but still accurate. This amp does that well with efficient speaker because you will be literally using mW's of power, yes less than 1 watt. I say this like I know you lol but seriously I find many of us efficient horn speaker users are listening to 1 watt or less the vast majority of time with average listening. This puts you with around maybe .2%THD at listening levels and it's going to be second harmonic dominant without the jumper, with the jumper it's down to .06%THD. I would try the amp without the jumper first to see if you like it, then add it back in again. I am willing to bet if you are one of us that listens to 1 watt or less most of the time that you will prefer the amp without the jumper. I am curious to hear your impressions after trying both settings. As I tell lots of people almost daily, you don't need tubes to get tube like sound as it's all in the circuit design and in this case using mismatched push pull devices. A normal engineer would purposely try and match devices as close as possible for lowest distortion but here we have a situation where you can make an effects box but then cleverly "adjust" the gain of one output device to force them to match closer and lower distortion. Neat way to have two sounds with one amp.

 

Keep us updated with your impressions.

 

 

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7 hours ago, captainbeefheart said:

Audio Science review rates this amp at the very bottom because they use the SINAD scale for rating quality which is a ratio between signal and distortion+noise. As long as you keep in mind it's not sound quality, it's purely an objective quality where you're compared to a theoretically perfect amplifier which is the output signal is the exact same as the input signal only larger to power loudspeakers. I am in the camp where a little second harmonic at the threshold of hearing seems to sound the best, not sterile but still accurate. This amp does that well with efficient speaker because you will be literally using mW's of power, yes less than 1 watt. I say this like I know you lol but seriously I find many of us efficient horn speaker users are listening to 1 watt or less the vast majority of time with average listening. This puts you with around maybe .2%THD at listening levels and it's going to be second harmonic dominant without the jumper, with the jumper it's down to .06%THD. I would try the amp without the jumper first to see if you like it, then add it back in again. I am willing to bet if you are one of us that listens to 1 watt or less most of the time that you will prefer the amp without the jumper. I am curious to hear your impressions after trying both settings. As I tell lots of people almost daily, you don't need tubes to get tube like sound as it's all in the circuit design and in this case using mismatched push pull devices. A normal engineer would purposely try and match devices as close as possible for lowest distortion but here we have a situation where you can make an effects box but then cleverly "adjust" the gain of one output device to force them to match closer and lower distortion. Neat way to have two sounds with one amp.

 

Keep us updated with your impressions.

 

 

I don't know how quickly I will get around to actually building the amp. We moved back into our house about five weeks ago and I still have a shop/storage room full of boxes and my electronics tools are in there somewhere. I do look forward to hearing this thing, though.

 

My typical listening level is under 70db so, yes, most of the time the amp will be in the fraction of a watt range. I like your suggestion about trying it with and without the jumper. One of the main reasons I bought this amp is to learn something new through the process of building it, and because it seems to be a great platform for experimenting with switchable options for stereo/mono/balanced, etc. I also want to try bi-amping with the ACA driving the upper end and the Aiyama Class D amp driving the bottom. Should be a lot of fun.

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6 hours ago, The Dude said:

I don't know about any jumper, mine also came with a power supply

 

The jumper should be right on the PCB board, without it the two push pull devices have different gain structures and create nonlinearity, if you didn't know about the jumper than I doubt you installed it so I'd be confident to say you have been listening to the amp with the higher distortion setting. When you install the jumper it places a Capacitor in parallel with the degeneration resistor at the source of the PNP output device which increases the gain closer to the gain structure of the NPN. The result is better linearity and less distortion.

 

I wonder how many people that built these amps haven't played around with the two settings, personally I'd just install a switch to the jumper traces so you can change settings on the fly and do some fast A/B listening tests. Maybe even hide the switch out of sight on the chassis and have a helper around the house randomly change the switch setting without letting you know and see if you can blindly tell the difference between the two settings. It's a great amp to be able to tell if you like the addition of some second harmonic distortion.

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On 1/12/2022 at 2:39 PM, grendel23 said:

If you like the ACA, you would love some of the other Pass DIY amps. I have built an F5 and an M2x. IMHO, the M2x is an end game amp as long as 25 WPC is enough. I am using mine on the mids with BMS 4592ND drivers on K-402 horns and the sound is very good.

Did you use one of the diyAudio chassis for your M2x build?

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Mine is the latest 1.8 version, but the differences are minimal. They've added a switch and done a little rewiring to make it easier to move from stereo to mono to balanced mono.

 

It arrived today and I'm just a little anxious to get to work on it.

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