Back in 2012, Dana Brock, Nelson Brock, and Mark Cronander [aka Variac] organized an "Amp Camp" in Northern California at the Brock's ranch. Nelson Pass, well known among audiophile circles, contributed with one of his designs and I'm sure they all must have supervised the participants as they went about soldering and building those first kits. The result was a simple, solid-state, Class A amplifier to take home. This amp was of course the Amp Camp Amp [or ACA] which we'll be talking about and measuring here.
You can see Pass' article introducing the amplifier with schematics and measurements. What a great idea! There is no substitute for the value of experience and the memories of building something, especially while sharing with a loved one; regardless of the subjective or objective performance of the final product.
The ACA has developed a healthy following over the years - check out the long-lived discussion thread on diyAudio. Nothing official, but I heard over the grapevine that there have been >2000 kits sold, suggesting conservatively >2500 actual stereo amps out there with no accurate way to estimate non-kit PCBs, unofficial "clones", or just completely DIY builds with the published schematics.
The device I tested, pictured above, is one of the early kit designs assembled as 2 monoblocks by Mitch Barnett [aka Mitchco, mitchba] a number of years back []. As you can see, it's in a nice Ferrari-red enclosure. The switching power supply he's using are high quality 24V/5A units [Mean Well GST120A24-P1M] with low noise, rated 180mVp-p, 90% efficient. These are an upgrade as the original ACA design used typical 19V/3A laptop computer power supplies. Remember that because this amp is a DIY project, it only makes sense especially for novices to avoid working on high voltage circuits connected directly to the wall - hence the external power supply.
Since I'm borrowing these units for testing, I did not open them up to poke around or take pictures of the innards :-]. Mitch tells me that the , the "1.1" version that upped the power a little and supposedly lowered distortion. These particular boxes were built in 2013. Here's a "borrowed" picture of Mitch's four "stuffed boards" from his post on the diyAudio forum:
As you can see, this is a low power, single-ended MOSFET amplifier. Depending on how you want to rate this, using a 24V/5A power supply, it's said to be "8W" on the DIYAudio Store into 8Ω "with clipping at 3% distortion" using the recent 1.6 version. Since we're still at version 1, the core amplification circuitry hasn't changed significantly over the years. The other spec we're given is that distortion is "0.7% at 1 Watt" presumably at 8Ω as well. We'll see if this version 1.1 amp can keep up with a 4Ω load when we put it on the test bench. As you can see in the introduction link above by Pass, there are measurements in there already, and Audio Science Review has measurements / impressions up as well based on the 1.6 version.
Despite all that has been written over the years, since Mitch had the amps on offer for a loan, I figure it would still be fun to have a listen myself and see what my test bench shows. Here are a few more pictures of one of the boxes with heat sink and Mean Well power supply:
BTW, for those looking to build this amp, it has evolved over the years and these days the new versions incorporate both channels in a single box and also have a balanced monoblock configuration advertised as "15W" into 8Ω available [among a few other monoblock options]. The current kit costs US$327 with stereo RCA inputs [buy 2 if you want stereo monoblock option of course]. Version 1.8 has just been released with easier switch access to stereo and the mono modes.
Before I begin testing and making an even bigger mess with cables everywhere, I want to show you my little basement set-up :-].
I'm starting now to use my Intel NUC 6i5SYH for data acquisition and analysis which is faster than the Asus laptop and Microsoft Surface 3 Pro I had been using all these years. Noise level through the measurement system remains very low. You can see the Rigol DS1104Z oscilloscope on the left, the Raspberry Pi 3 B+ "Touch" running Volumio with signals on a USB stick - convenient if I want to use the Topping DX3 Pro as signal generator. There's a 4Ω/300W power resistor in the middle. Behind the 2 amps are of course my RME ADI-2 Pro FS DAC/ADC which is the heart of the ADC measurement system sitting on top of the Linear Audio Autoranger MK II which I updated to a 2V firmware a few months back [originally 1V nominal out] allowing even better resolution [I know there's now an EPROM selectable up to 3V - can't keep up :-].
Nope, it's not an Audio Precision set-up but with carefully selected components, tested to verify functioning, there's a lot of flexibility one can enjoy. I can use different software and find new ways to measure different characteristics, without spending a ton of money as a hobbyist. It does take some time getting the pieces working, creating my own test signals and standardizing the procedure. But that's part of the fun! Furthermore, the bits and pieces like the DAC and ADC can be used elsewhere as playback equipment and for vinyl rips when not testing. Refer to my post on the "Measurement Of Amplifiers Rig" [MOAR] for details on how I'm doing this.
As you can see in the test bench picture above, I've put stickers on the front of the ACA boxes to remind me which is "right" and which "left" for the measurements. Obviously it's up to each user how they want to orient the mirror-image amp/heat sink. I'm putting the heat sinks facing inward as if I were using these in a sound room since I prefer the amps to be located close to my speakers on each side and because they get warm, I'd rather have the heat sinks medially oriented away from speakers placed further out.
I. Basic Amplifier Characteristics
As a start, let's just get a few noticeable nuances out of the way. When I turn these amps on, they send some odd "gurgling" sounds to the speakers for about 2-3 seconds. Nothing too loud or scary. Likewise when turned off there is what sounds like a brief DC "thud". I was told by Mitch that this is normal for the amp and he uses blocking capacitors with his sensitive compression drivers for protection.
These amplifiers also need to warm-up. In fact, I do not recommend listening to them until at least 15 minutes of warm-up has been achieved. During the warm-up period, the sound is unstable, demonstrating a noticeable meandering soundstage - it's like the amplitude level and frequency response appear to be fluctuating over those first few minutes and the instability can be seen while playing a steady tone and watching the oscilloscope levels. Things generally look/sound OK from 15-30 minutes and I would consider the amps ready for listening [and measurements] after 30 minutes to really get an idea of their performance.
As expected, the heat sinks do get warm as an inefficient Class A device that's always "on". Uncomfortable to the touch but not going to burn flesh - the large heat sinks worked well. After 2 hours in ~20°C ambient room temperature on my test bench, the contact thermometer probe tells me the heat sinks measure 45-46°C [~114°F] for both units.
With the units adequately warmed up, let's start with a look at the Amplifier Voltage Gain: - Right: +16.0dB - Left: +15.9dB
That's essentially an identical reading between the 2 monoblocks. Note that this gain level is quite low compared to most amplifiers. For context, the unbalanced input standard for THX is +29dB. My Emotiva XPA-1L last year measured +35dB, the cheap Yeeco TI TPA3116 class D was +25dB. My Hypex NC252MP DIY box measured +26dB. This means the ACA will need to be driven with higher input voltage compared to the other amps. This is obvious when hooked up and listening on my system - I need to push the preamp volume up by at least +10dB to get this amp near the output level of any of the others.
Pass' ACA intro article lists a gain of 14dB and I wonder if the mod Mitch implemented might have bumped the gain a little on these he lent me. I see that the Audio Science Review measurements only found a gain value of +9.4dB on the version 1.6. No wonder Amir asked "Is this thing on?" if either inadequately driven or paired with lower-sensitive speakers. Since version 1.5 [2018], the gain has indeed dropped down to 10dB. This means that a typical 2V source [like say the RCA output from the Topping DX3 Pro] would not be adequate to drive the amp to full output power. Make sure to check that your preamp is up to the task depending on which ACA version you plan to use.
Amplifier Damping Factor:
The ACA has low damping factor consistently around 2.0 - 2.3 across the audible frequencies into a 4Ω load. This means a rather high output impedance which also means it's not going to have tight control over a reactive speaker load. We can see this in the frequency response [~1.5V signal into load]:
I separated the frequency responses of the 4Ω resistive load from the Sony SS-H1600 bookshelves for clarity. Mitch's two amps appear to be well matched in volume and consistency across the frequencies.
Notice that the amps are able to achieve excellent flat frequency response to 20kHz with less than 1dB variation by 48kHz into a resistive load through my measurement system. When it comes to managing the Sony 8Ω speakers however, it's having some difficulty with around 2dB variation across the audio band [compare this to the Hypex's "iron grip" on those same Sony speakers!].
A side note to this is that by ACA version 1.5, as the voltage gain dropped down to +10dB, the damping factor increased to 10 from an initial "about 3" of version 1.0. Current ACAs therefore should be able to "control" the Sony speakers significantly better.
To mix it up a bit, here's the frequency response into an 8Ω load with phase data - nice and flat:
II. Single-Tone Harmonic Distortion and Noise
Given that this is by no means a "powerful" amplifier, let's have a peek at the Harmonic Distortion vs. Frequency at 2V/1W into a 4Ω load using Room EQ Wizard's stepped sine function:
The noise floor [brown] is quite good down around -90 to -110dB from the 2V fundamental. While I cannot fully capture what is heard over the first 15 minutes of warm up time with the fluctuating soundstage, we can see a slight difference in these graphs between "cold" compared to after warming up for 1 hour. As the unit warms up, the most prominent harmonics do increase by about 1dB across the audible spectrum; not a massive change. The most important difference I've found is that the amplitude stabilized after 15 minutes as noted above.
Even at just 1W, we're seeing quite a bit of harmonic distortion present already! By design, this amp uses relatively low 9dB feedback. The 2nd harmonic is prominent up around -36dB at 1kHz after warm-up and basically floats there without much variation. In fact, due to the amount of 2nd order harmonic, THD and THD+N would be basically pinned at just a tiny bit over the red 2nd harmonic line.
Here are some THD[+N] graphs at a few output levels to examine [lots of detail in there - click on image to zoom]:
As you can see, the graphs continue to demonstrate the high amounts of 2nd harmonic; not just in the frequency sweep above but also across power levels from 16mW all the way to 6.25W. I've highlighted the THD+N values but since noise is low, and harmonic distortions are high, there's essentially no difference between THD and THD+N.
Notice also that I'm using a 950Hz fundamental. Why you may ask? The answer is with those purple asterisks I've put over the 8kHz peaks. Initially, I thought these might be due to the USB packet noise we've talked about before. Nope. The 8kHz spikes and 16kHz harmonic are actually due to the Mean Well switching power supply. Evidence that at least with this amp, we can make out some noise contribution from the power supply selected. Note that while the 8kHz noise might affect measurement of a 1kHz fundamental thinking that there's a higher level of the 8th harmonic, that 8kHz tone isn't a big problem; it's only -90dB at 16mW so in reality nothing to worry about when listening. Notice by the time we hit 4W and 6.25W, the 8kHz noise is easily swamped by the higher order harmonics.
For completeness, here's a similar "matrix" for the same amp but with an 8Ω load attached:
In watt-to-watt comparisons, we can see that the amplifier prefers the 8Ω load over 4Ω with better THD+N. For example, at 4W into 4Ω, THD+N is a high -25.6dB/5.2%[!] compared to 8Ω THD+N of -34dB/2% [still high]. This amp appears to be current limited with lower impedance loads. Definitely will be a challenge with low impedance speakers with difficult phase angles!
If we graph THD+N vs. voltage output to summarize, here's how it looks:
We can easily see the difference the higher 8Ω load makes to harmonic distortion given the same output voltage. The question of course is just how sensitive your speakers are and if the sound is loud enough in your room. Usually, when I measure amplifiers, I like to see 0.1% distortion or less as a rough threshold for being "clean" or "linear". Well, this ain't going to happen here! If we say that the "first watt" is most important, what we see is that at 1W into 4Ω, THD+N is already -35.5dB/1.7% distortion. And 1W into 8Ω has a THD+N of -42.1dB/0.78% [which is about the 0.7% advertised for current versions of the product]. At 5W into 4Ω, we're looking at THD+N of -23.6dB/6.6%, and 5W into 8Ω is -31.4dB/2.7%; "impressive" numbers, but not in the usual objectively "good" way!
III. Multi-Tone Testing: Intermodulation Distortion and Triple-Tone TD+N
Already we can determine that this is a rather nonlinear amplifier with significant distortions even at low power. Let's try some intermodulation tones at my standard 2V/1W level into 4Ω:
As expected, we're looking at some rather significant amounts of IMD even at 1W into 4Ω. Across the different tests, we're seeing -23 to -33dB levels. For context again, a good modern, yet affordable, Class D like the Hypex NC252MP ranges from -78 to -97dB on these tests. My Class A/B Emotiva XPA-1L monoblocks sit around -80dB. Admittedly, these are more expensive amplifiers [the Hypex is actually only 2x the price of the ACA kit], but the difference is still rather stark if we just look at cost to objective performance.
Next, let's have a look at the synthetic TIM ["Transient InterModulation"] signal consisting of a 1kHz square wave with 12kHz sine created at 192kHz samplerate [96kHz bandwidth], measured again at 2V into 4Ω:
We're seeing some anomaly here and I've put the cursor at the level of those distortions around -70dBFS. Remember, as I've said previously, this is a challenging synthetic test signal and while from an engineering perspective we ideally would not want to see those -70dBFS peaks, I'm not sure just how audible this is. If you want to see essentially perfect TIM signal results at 2V into 4Ω, check out the Emotiva XPA-1L or Hypex NC252MP measurements again; both also achieve good results at 10V.
[Of interest, check out this recent Audioholics interview with Bruno Putzeys [May 2020] where there is discussion of TIM in relation to amplifier gain-before-feedback and the interplay with feedback allowing for lower TIM around 35 minutes. Remember that feedback is not in general a "bad" thing despite the audiophile mythology and love the discussion around 43 minutes that DSD able to sound good is an example of massive feedback.]
Finally to round off our multi-tone testing, here is the Triple-Tone Distortion and Noise measurement at 2V into 4Ω which I use as one of the summary metrics in my "AMOAR" rating system below:
As expected, very consistent results from the pair of monoblocks. We're looking at a TD+N of -30dB at 2V into 4Ω. Notice the rather significant amount of harmonic and intermodulation products throughout the audio spectrum [and into the ultrasonic range]. Clearly, this is not one of the better TD+N results I've found.
IV. Square Wave and Wideband Noise
Consistent with the extended frequency response, the square waves are looking very good. That tiny overshoot at the leading edge is from the DAC I used as signal generator [Topping DX3 Pro V2 at 24/384], but otherwise the tops and bottoms are flat, nothing like the superimposed ultrasonic noise we see with Class D amps. Mitch and his daughter did a great job with the build along with good matching components to achieve excellent channel balance between the two monoblocks.
And for completeness, here's a peek at the wideband FFT up to 600kHz to make sure there are no nasty ultrasonic signals up there headed for your speakers:
Again, you can compare this with Class D and note the absence of things like 400kHz noise as found in the Hypex NC252MP. There's some stuff around 250kHz for example; nothing major and could be switching power supply related.
V. Impressions, Discussions and Summary
Here's the AMOAR Composite Score graphic for this amp:
Permit me to be blunt. This is the worst measuring amplifier I've done to date and these measurements will likely not be "outdone" by another device soon in this blog!
The damping factor is very low at just over 2 with a 4Ω load [I'd like to see 20+]. Notice how the 8Ω Sony SS-H1600 speaker [impedance measurement here] frequency response fluctuates across a 2dB range with this amp. Distortion is high at -30dB/3.2% on the triple-tone signal at 2V output level into 4Ω. And when it comes to "clean" power, it appears to be incapable of 2V to drive low voltage gain amps] into the dual Pass ACAs to my Paradigm Reference Signature S8 v.3 speakers with an in-room sensitivity of 92dB/W/m. I used basic 12AWG OFC wire between the ACA and speakers. Listening position is about 10' away. Because of the amps' low power, I cannot run a proper DSP room correction [which typically will attenuate the signal another 3-6dB], and I have the subwoofers turned off [I would need to readjust the subs for integration].
What I heard from this amplifier was clearly a device with much less power than I'm used to either with the Emotiva XPA-1Ls or the Hypex NC252MP. With the low gain, I have to push the preamp up to about "-10dB" just to approach normal listening levels when typically -20 to -30dB is all I need. These ACA amps will not "rock" like the others [remember, I "need" something like 70W into 8Ω in this room and with these speakers]. Through my speakers, the ACAs sounded quite good with classical quartets, jazz ensembles, and intimate solo vocalists. Pushing the volume up higher with well-mastered, higher dynamic range rock [like say the DR11 version of Aerosmith], or pump up Eric Johnson's soaring guitar work on Ah Via Musicom's rock tracks [DR12, tracks like "Desert Rose" or "Cliffs of Dover"] can sound strained on my system and I start getting worried about damaging the borrowed gear! Generally I listened with average of