ACA amp with premium parts

I am sorry if this question had been asked. Had Anyone here built an aca amp using more expensive parts and can you hear any differences if any?

Info for Amp Camp Amp (updated by Moderators)

V1.8 builders - please note below the slight but important wiring changes between V1.6 and V1.8 and use the V1.8 wiring diagram.

Build Guides

ORIGINAL build guide: ACA illustrated build guide

Here’s a thread discussing the kits from the DiyAudio store for V1.6 and V1.8
Amp Camp Amp Kit 1.6/1.8

And here’s the link to the kits:
Amp Camp Amp – diyAudio Store

Old step by step build video by Patrick Norton on his TekThing website. He makes an error in cutting out the thermal insulator and leaving the backing on. The backing just peels off, no cutting required:
February 2016 Patreon Build Video: Amp Camp Amp Dual Monoblocks! - YouTube

See post #1453 for a tweak to the original V1.0 circuit boards. It’s not essential but should improve the sound. This tweak has been incorporated in subsequent versions (1b & 1.5, 1.6,1.8). if you have those circuit boards or kits it's already added to the board, and the part (R15) included in the kits
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Paid Member
2016-12-20 4:53 am
Huntington Beach, CA
What would make a premium parts version? Does not seem much to upgrade besides expensive capacitors and building a custom PSU. Now I am new to all this so I sincerely don't know what other parts would be considered upgradeable from the standard parts list.

At the other end of the spectrum for practice I bought some cheap ACA clone boards and plan to populate them with a combination of cheap parts from the local electronics store (nte labled) and recycled bits. After that I will assemble one using the parts I purchased from Digikey with the published BOM and the new 1.6 boards that I ordered yesterday.

Curious if my wholly untrained ears will hear a difference.


Paid Member
2008-12-27 7:27 am
Connecticut, USA
You can also build with Semisouth at Q1 position as I did. Most other components were what papa called out for on the original parts list. Mentality being that it is quite a bit lower distortion part than the IRF. The subject is discussed in his F2J notes on the First Watt site. I collected some extra Semisouth while stocking for my F2J project so I put them in the ACA with sockets for fun so I could “chip roll” so to speak. Honestly, I haven’t bothered as they sound nice just how they are.
Since the ACA is not my first amp build, simply the first NP class A design, I decided to have some fun upgrading a few of the parts. I was careful to make sure that the new parts were straightforward drop-in replacements for the originals, with one exception as noted below.

My first pick was the output capacitor. Since this has no provision for bypassing, I wanted to use a higher quality part that still adhered to the basic philosophy of the design. For C1 I chose a Nichicon KG series Gold Tune of 4700 uF, 35V. This part has the same footprint and is just slightly taller than the original.

The next cap I chose to upgrade was C4, the power rail bypass. With other builds that I've done, I've noticed that improved local bypassing can help with transparency and channel separation in the upper mids and higher frequencies. While the original 10 uF Elna Silmic II is certainly a high quality part, I wanted something with higher capacitance and low impedance. For this I chose a Nichicon Aluminum organic polymer RL8 series 100 uF, 35V. This has a larger diameter, but the same 3.5mm lead spacing as the PCB, and is an easy drop-in substitute.

Finally, I replaced C2 with a Nichicon Aluminum polymer RNL series 1000 uF, 16V. This has a 3.5mm lead spacing which is slightly less than the 5mm on the PCB, but is still a reasonably easy replacement.

This is how the initial board configuration looked:

Initial power-on and bias setting went without any trouble. The heatsinks were plenty warm, but not hot. I set the bias voltage at 12.2 V, since I'm using the 24V SMPS power brick. The initial listening impressions were wonderful. I was pleasantly surprised at the strength of the bass and overall easy presentation.

For the record, my reference system is a Naim NAC 82 preamp, HiCap, NAP 250 power amp (modified), playing through an older pair of Vandersteen 2C speakers.

Substituting the ACA for the NAP 250 required a higher setting on the volume knob, but the 82 had no troubles driving the ACA to it's full power capability. For most of my listening, I've been running the ACA at a level that is close to flat-out, or just below the level where it's sound becomes noticeably compressed. Fortunately this happens to be just enough to drive the Vandersteens so they pleasantly fill my large-ish living room.

There is a part 2 to the process, which I'll post next.
I too built an ACA from a board parts kit. Since I have a rather large stash of salvaged parts I also substituted some parts. I used "premium, upgraded parts" for inline signal circuit. Inline resistors are all vishay or mepco metal film or film foil types. A MKC film cap replaced the 10uf silmic input cap. The output cap is a 3300uf silmic 2, bypassed with a small auricap.

I can't tell you if it sounds better than a "stock kit" ACA. But it has replaced my Threshold amp in my main system. Since there is a small part count in the ACA, it affords an nice opportunity to change parts to experiment thus prove or disprove to yourself if different parts make for a change in sound.

have fun.....
ACA mods, part 2

The first set of modifications to the ACA consisted of simple component upgrades. The second set make minor modifications to the circuit, but are still in keeping with the original design philosophy.

In addition to the Naim system which I mentioned earlier, I have a second system which is all tube based. The preamp is an Audible Illusions Modulus 3A and the power amp is a modified Dynaco Stereo 70, using an EF86 front end and ECC99 phase inverter / driver. This system has been wonderful to listen to, but has a couple minor quirks that are frequently associated with some tube designs.

Substituting the ACA for the tube power amp was quite pleasantly revealing. The ACA shares much of the delightful liquidity of the all tube system, but brings better extension to the bass and more transparency to the upper frequencies. The standard ACA circuit, however, requires a much higher setting of the volume knobs on the Mod 3A. I felt it was difficult to get the most out of the little solid state amp, as it started to sound slightly compressed before reaching a volume level for more spirited listening.

This led me to alter the input and feedback resistor values. Since I had originally planned to build a pair of ACA amp as mono blocks, I had a second kit with which to experiment and compare results. Figuring that the little amp would benefit from both a higher input impedance and less negative feedback, I changed the input resistor R11 to 20k and the feedback resistor to 90.9k. The ratio was chosen to be the same as the Zen V4, a design which I've been studying for a while, and has a similar topology.

The results were better than I would have predicted! The dynamics of the ACA were greatly improved, so much so that I used even lower volume knob settings than would have been suggested by the modest increase in gain. Running a 1 kHz sine wave into the amp confirmed that the gain had increased from 10.5 dB to 11.4 dB.

Shortly after making this change, I also added a 2.0 Ohm resistor in parallel to R3 and R4, and reset the bias voltage to 12.4 V. I let this version of the ACA play for quite a while as I took my time changing the first amp to match the second.

The higher gain and quiescent current version of the amp was an absolute pleasure to listen to; the kind that made be want to sit with multiple albums. I found myself listening all the way through, rather than skipping songs or indulging in the usual bit of web surfing on my iPad while listening. The upgraded ACA also continued to sound better the more it was played. I would say that it probably needs around 20 hours of break in before it really begins to shine.

The final step was setting up the pair of ACAs to run as bridged mono blocks. I'll save the description of this for Part 3, which includes another minor tweak and final listening impressions.
ACA mods, part 3

Given how good the ACA sounded as a simple stereo amp, I really wanted to try a pair as bridged mono blocks. Since I had modified the input and feedback resistors, I knew that a different resistor value would be required between channels to make a bridged amp. After some experimentation, I settled on a value of 68.1k from the left (-) output to the right input. My experience plus some other findings on the Pass forums suggest that the standard recommendation of using the same value as the feedback resistor is not the best way to go. For those who wish to bridge the standard configuration amp (v 1.6), I would recommend a value no more than 33k, and preferably 30.1k in order to get more symmetrical response between the two halves of the bridged amp.

As bridged mono, these amps are amazing! As expected, both dynamics and stereo separation improved, but the degree of improvement is both subtle and profound. The enhanced dynamics open a clear window into the details of each recording, without throwing them in your face. The soundstage has improved to the point where, on good recordings, the 3D image is completely separated from the speakers.

Having the pair of bridged ACAmps has induced many extended listening sessions, and revisiting quite a few favorite albums. On many recordings, I'm hearing details that I hadn't heard before, and some of my favorites just beg to be played over and over again. This quality is a testament to the simple and elegant circuit design, which responds well to careful device selection.

Though the improved dynamics have been quite satisfying at my usual listening levels, the medium efficiency of the old Vandersteens still has me running the amps close to flat-out much of the time. To address what I perceived as a slight upper treble harshness, I eventually added a 10 pF Silver Mica cap in parallel with the 90.9k feedback resistor on each board. While the effect of this is subtle, the ease of presentation makes it worthwhile.

That's all I've done for now. While I originally was thinking of building a linear power supply to replace the SMPS bricks, I'm happy enough with the way things are to start working on other projects. Given the revealing nature of the amps, I'm sufficiently motivated to finally build an improved power supply for the NAC 82. I'm hoping that a DIY version of a dual HiCap DR to replace my single Olive series HiCap will be a worthwhile effort.

As for other NP amps, I already have an F4 clone in the works, and am planning an Aleph J, and even an M2X somewhere down the road. Too much fun!
So, just to clarify on feedback resistors, and on the value of the resistor that takes the signal from the output (negative) of one ACA, reduces the amplitude, and then feeds the signal to the other ACA...

The feedback resistor is 90K on each ACA PCB ? Or 30K ?
The resistor that (let's call it a bridging resistor) reduces the amplitude from the negative binding post is 90K ? mentioned 30K resistor values... where would these go ?

Do you still have a 20K resistor in series with the JFET's gates (instead of 10K)?

Yes, for my configuration of the bridged ACAmps, the feedback resistor (R12) on each board is 90.9k to go with the input resistor (R11) of 20k. The bridging resistor is 68.1k.

For the standard configuration which uses a feedback resistor of 39.2k, I recommend a bridging resistor of 30k.

Note that I also ended up adding a 10 pF silver mica capacitor in parallel with each of my 90.9k feedback resistors.
Tungsten, thanks for sharing all this evolution of your ACA! You received quite a deserved compliment from Papa Nelson!

Any chance of sharing a final schematic of your ACA with all the changes? It would help a lot to visualize the extent of you mods.



Moreover, I am curious to know what has happened to the output impedance of the ACA in both the stereo and monoblock configurations from your modifications. In addition, if you have an impedance/phase curve of the loudspeakers you auditioned your modified ACA’s with, that would be useful as well, for comparisons sake.

I am quite certain that the modified Dynaco ST70 you have has a fairly high output impedance (ie > 3 ohms).

To all who have been following this thread, know that I'm grateful to be part of this community. I am especially humbled by the praise of the one and only N. Pass who has so generously given of his designs and his experience.

For Rafa,
I'll try to find a good way of editing or drawing a schematic showing the changes that I have made to the ACA. In the meantime, here is a list of what I have done.

On board, drop-in replacements

C1: 4700 uF, 35V; Nichicon KG series "Gold Tune"
C2: 1000 uF, 16V; Nichicon RNL series Aluminum Organic Polymer
C4: 100 uF, 35V; Nichicon RL8 series Aluminum Organic Polymer
R11: 20 kOhm, 1/4W, 1% metal film
R12: 90.9 kOhm, 1/4W, 1% metal film

On board add-on components
R4b: 2.0 Ohm, 2W, 2% metal oxide; added in parallel w/ R4
C101: 10 pF, 500V, 5% Silver Mica; added in parallel w/ R12

Off board substitution
Bridging Resistor: 68.1 kOhm, 1/4W, 1% metal film

Note that I left C2 as the 10 uF, 25V Elna Silmic II, and have been quite pleased with the overall results. Recently I happened to acquire a Naim NAP 150, and discovered that it also uses the exact same component for its input coupling capacitors. Given Naim's extremely selective practice for their components, it seems that we are all in good company.

For Anand,
The output impedance of the modified ACA is a function of the added 2.0 Ohm resistor R4b, as well as the substitute value for the feedback resistor R12. I don't have a formula at hand for calculating this.

My loudspeakers are an older pair of Vandersteen 2C, purchased as store demos with the original Sound Anchor stands around 1988. They have a nominal impedance of 7.5 Ohms, with a minimum of 6 Ohms. Efficiency is listed as 87 dB. They were known for being fairly easy to drive with reasonably uniform phase and impedance curves. They are one of the few multi-driver speaker designs that is phase coherent and time-aligned. I'll see if I can dig up the original spec sheet & owner's manual for more information.
I'm sure an upgraded pair of speakers, such as a new set of 2Ce Signatures, could be even more revealing / sound even better, but my old Vandys have been a constant part of my system over these many years as I have been adding & upgrading other components.
Getting back with an update on my speakers...
There were no specified impedance or phase graphs for the Vandersteen model 2C. Also, to correct what I said earlier, nominal impedance was 7.8 Ohms, with a minimum of 6 Ohms. Sensitivity was listed as 88 dB at one meter with 1 Watt of pink noise input.
Various incarnations of the model 2C have been reviewed over the years. If I remember correctly, Stereophile published some pretty extensive measurements of the 2Ci or 2Ce.
I recently completed a pair of ACA's which for me, like many, are my initial foray into DIY amplification.

Could hardly be happier with the results thus far.

Have my name on the list to get a couple of ACA parts kits which will no doubt get some, or all, of the "TungstenAudio" treatment.

As an electronics challenged retired electrician that can solder, I muchly appreciate you sharing your "tweaks" in detail.

Interesting that your Vans are fully time aligned, my curiosity is piqued about them.

Cheers to all,
I haven't found a good way of editing the schematic for the ACA. When I opened the .png file in Photoshop, it was evident that this is a somewhat low-res scan of a paper printout. I wasn't happy with the results of trying to edit the component values. It would be great to know which program has been used to make some of the other schematics.

Since there seems to be some question about the method of bridging the two halves of this amplifier, hopefully a picture can help.
The "bridging" resistor is indicated by the red rectangle. It is connected from the left channel's negative (-) output to the right channel's input through a switch. The method is explained in the excellent pictorial build guide by 6L6. I had a couple locking toggle switches from my local electronic surplus store, and decided to use them here.

Note that the right input is left unconnected externally when running the amps in bridged mode. The negative speaker connection goes to the left (-) output and the positive speaker connection goes to the right (-) output.


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Hi, I have added your changes to the current schematic of the ACA.
Is it o.k. so far?

Best regards - Rudi_Ratlos

Much appreciated! This looks good - I would make just one minor correction. The output capacitor C1 is 4700 uF, as I have implemented this circuit.

The difference is small, but might be noticeable with full range speakers of a lower impedance. I've been listening to some recordings lately that feature bass guitar low tuned to BEAD, or 32 Hz on the B-string. A full-range response gives a wonderfully visceral experience.

I will make one other note while we're here. For new ACA builds, one couild use 0.68 Ohms for R3 and 0.50 Ohms for R4, thus avoiding the need to tack on an extra resistor. The resulting resistance is practically the same.