Saturday, 11 July 2020

OWNER'S COMMENTS: Pass Amp Camp Amp (ACA) 1.1 by Mitch Barnett

[What follows are comments from Mitch Barnett, the owner of the Pass Amp Camp Amp 1.1s that I listened to and measured. I appreciate Mitch's input on the matter, it's a great example of how owners of the ACA might put them to use in one's system, recognizing one's needs.]

Thanks Arch for the great time with your family for a fun night of good food, wine and of course music and movies. Great job on the objective measurements and subjective listening impressions, “It actually sounds good…” with the caveat of not asking too much from the amp. In my mind, this is the dichotomy between objective measurements versus what sounds good to our ears.

I purchased these kits seven years ago to drive high efficiency compression drivers, which are typically rated at over 113 dB/W/m. Having tried amps with higher voltage gains, I found I could hear the hiss from those amps at the listening position. I tried a number of L-pads and resistor networks, but was really never happy with the sound quality of padding down high power amps. Enter the ACA’s. With their low voltage gain meant I did not have to pad down the output to the compression drivers or the analog signal going to the amps to level match the other drivers in my tri-amped system. Also, they are wonderfully silent. I can only hear a slight hiss if I put my ear right next to the compression driver in the waveguide. Also, with 113 dB/W/m efficiency the ACA’s never get anywhere near 1 watt of output as I mostly listen to compressed rock music at reference level of 83 dB SPL some 9 feet away.

Mitch's current system. Notice that Mitch is using the little ACA's powering the ultra-sensitive horns (JBL 2453H-SL compression drivers + 2384 waveguides) in his JBL Pro 4722 speakers. Notice the use of Crown XLS 1502 (525Wpc into 4Ω) for the dual 15" drivers on each side. Plus dual Rythmik F18 18" subs with 900W each for <50Hz frequencies!

What is interesting is that I have had several lower distortion amps like the Purifi 1ET400A and active speakers like the Kii THREE and Dutch &Dutch 8c here. With the ACA’s running my HF compression drivers, I have not had any thoughts about distortion from the ACA’s as compared to those much lower distortion gear. But that is largely due to the efficiency of the compression drivers maybe requiring up to a couple hundred milliwatts of power. As Arch has alluded to, with that amount of 2nd harmonic distortion, this reminds me of tube sound. It has that “glassy” sound to the top end, which I am a sucker for. I know the objective measurements are not good, but listening to music, I don’t find anything really objectionable about the sound quality. So what if I like some sugar on top of my accurate soundJ

I also run a pair of the same ACA’s Arch measured on KEF LS50’s which are notoriously inefficient [85dB/W/m]. Sure, with these amps, they don’t go very loud and increasing the volume sounds like increasing the preamp gain on my Traynor Class A guitar tube amp. But backed off, they actually sound good, and oh, that top end... Yes, the bottom end is “euphonic”, but it doesn’t stop me from enjoying the sound. The ACA’s “sound like” they are louder than they really are. So when I listen to rock music at moderate volumes on the KEF’s, there is a spot where it sounds “ripping” or perceived louder than it is. For sure that is the 2nd harmonic distortion, but sounds “alive” to my ears and a pleasant effect, at least to me. Aside from being a cool amp project for one to learn about and build electronics, from my experience, it offers a surprisingly pleasant sound at the finish line.

The KEF’s really benefit from a much higher power amplifier with tighter control. But it is not to say that the ACA’s sounded bad, they actually sound good. And that seems to be the general consensus from the mass majority of folks who have soldered these kits up. So what does that say about the objective measurements? As mentioned by Arch, one needs to drive that input level up and hooking the amp up to much more efficient loudspeaker is also beneficial. Or in my case, used in a digital XO system to drive high efficiency compression drivers with low noise and a bit of character on top.

[I asked Mitch to discuss more about the protection capacitors he uses - I suspect this will be beneficial for folks to be aware of.]

With respect to protection capacitors, whenever a midrange or high frequency compression driver is connected directly to an amplifier, it is recommended that a capacitor be wired in series with the driver in order to attenuate any low frequency and switching transient signals, and to block DC components that can damage (expensive) compression drivers. Note that this is not specific to the ACA, but all amps connected directly to compression drivers. Safety first J.

I used a chart like this: to determine the value of the protection cap. Best practice is to select a capacitor that is active at approximately one octave below the crossover frequency, assuring minimal acoustic interference with crossover region performance while maintaining a good degree of protection.

In my case, the crossover frequency is 630 Hz so the cap would “kick in” at around 300 Hz and below. My JBL 2453H-SL large format compression drivers are 8Ω. So looking at the chart, we see a value of 66μF to 80μF for 300 Hz to 250 Hz range. Since I could not find a 66μF cap at the time, I went with these high speed, 82μF Solen caps:

[Great discussion and links! Thanks again Mitch. For those who might be unaware, Mitch also runs his Accurate Sound service where you can reach him for consultation around sound calibration and digital room correction needs.]


  1. So, Mitch, did you try the LS50s with the Purifi and if you did how did they sound?

  2. Hi Unknown, yes I tried the Purifi amp and a Crown XLS 1502 with the LS50s. As mentioned in the article, the ACA's don't drive the LS50s very loud, but louder than one would think. So if I kept the ACA's from too much 2nd harmonic running the LS50s and compare to the Purifi or Crown, at the same (low) volume level, there is not much audible difference. Both the Purifi and Crown sound a bit tighter in the bass and cleaner overall, with the ACA having a nicer sounding top end, but that is all likely sighted bias. I also did not spend a lot of time comparing as the LS50s were not my target loudspeaker for this amp. I wish I had this ABX Switch Comparator to set up a proper test.