ACA amp with premium parts

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dfoulk

Member
Joined 2013
close but I give my nod to the IRFP044's

To my hearing, the midrange is a bit richer with the IRFP044's in both the Q1 and Q2 positions. The IRFP 140 for Q2 and the IRFP440 in Q1 position has more of a leanest in character, especially listening to piano and male voice. It is obviously a subjective call, but aren't they all to some extent. I tried a 2SK2221 as Q1, but it would not bias correctly with the IRFP 140 as Q2? Not sure what was going on with that, but guess I will put it back together and listen for a while. Happy listening, testing or not:)







dfoulk said:
think I will try a 140 for Q2 and the 440 for Q1 to see how they pair. I have a spare board to try it on so I don't have to disturb the dual 044 setup. If the results floor me, I will simply have to do some unsoldering:D
Click to expand...
 
Puma Cat

Puma Cat

Member
Joined 2020
TungstenAudio said:
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.
Click to expand...

Hi Tungsten,
Quick question for you: What is the (original) resistor number for the 90.9 kOhm resistor that you substituted for to provied a bit less negative feedback?

Stuffing my boards now, and don't want to solder this in, only to have to remove it later.

Thank you,
Stephen aka PC.
 
M

motospark

Member
Joined 2005
Applied to the current Amp-Camp 1.8?

TungstenAudio said:
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.
Click to expand...

Hi, I also have the original Vandersteens 2Ci, my questions is can I apply these drop in upgrade replacement parts to the new Amp-Camp kit 1.8? I will also be re-wiring my original 1.6 Amp-Camp Amp to 1.8 and run as mono-blocks.

Thank You and hope all is well and safe.

Mark
 
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TungstenAudio

Member
Joined 2014
Yes, the ACA 1.8 is functionally identical to 1.6.
For driving a pair of large speakers like the Vandersteens, it would be better to use two ACAs in parallel bridged mode. Better still, use a bigger amp ;)

All these options will sound very good. A bigger amp such as the M2x or Aleph J will be a revelation. Make sure you're taking advantage of the bi-wire feature on these speakers.
 
Puma Cat

Puma Cat

Member
Joined 2020
So...quick question for the gang of gurus that have built up ACA 18s.

I did a "mockup" of mounting the PCB on the heatsinks last night and noted that once the boards are soldered to the power transistors, accessing the backside of the board to connect the chassis and connector (binding posts and RCAs) wires would not really be practicable. Does it make sense to solder the (chassis) wires to the boards before mounting the boards on the heat sinks and soldering the power transistors to the boards?

Board-and-Heat-Sink.jpg
 
Puma Cat

Puma Cat

Member
Joined 2020
TungstenAudio said:
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.
Click to expand...

Sorry if this is an obvious question, but I'm a newbie to this amp and the distinctions between bridging mono configurations and paralle mono configurations.

My question is, do you leave this 68.1K ohm resistor installed as you've described if you are using the parallel mono configuration?
 
T

TungstenAudio

Member
Joined 2014
This is a good drawing that shows how the different bridged modes of operation get connected. The 39k resistor is replaced by 68.1k to get a better match between the two boards with the different input and feedback resistor values. You will notice that in the parallel mono configuration, this resistor is not in the signal path.
 

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Puma Cat

Puma Cat

Member
Joined 2020
Thanks, Tungsten.

Can someone please confirm for me the power rating for the resistors that come in the ACA kit? The kit BOM lists resistor values, but no other specifications than that (which technically means its an incompete BOM). I just ordered some Vishay resistors for the custom changes at R11 and R12 that are 0.4W, and they are a notably smaller than the resistors that come in the kit. I am reluctant about using a resistor of this size in these positions. If need be, I can put up a photo to show the difference in size. Thanks in advance to the gang.
 
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motospark

Member
Joined 2005
Hi Tungsten A, about these:

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

I can find the Silver Mica, but I'm having trouble sourcing the 2 Ohm, 2W 2% Metal Oxide part, I can find 5% but not 2%. Can you tell me where you found these for purchase? Also, the changes by adding these extra components, does this in any way change the function of the 3 position switch in the back of the Amp?

Thanks

Mark.
 
T

TungstenAudio

Member
Joined 2014
As usual, there is more than one way to get the desired resistance for R3 and R4. The 2.0Ω add-on resistor is useful if you have already installed the standard 0.68Ω resistors in their places. It may be 1%, 2% or even 5%, depending on what you can find, If you haven't stuffed these parts yet, then a combination of 0.68Ω and 0.5Ω for R3 and R4 works well.
 
Puma Cat

Puma Cat

Member
Joined 2020
TungstenAudio said:
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.
Click to expand...

Hi James,
Paralleling these two resistors gives a resistance of 0.51 Ohms. Could you please provide the rationale for lowering the resistance of R3/R4 to this value and is it possible to do this mod with custom parts without making this resistance change at R3/R4? In other words, what will be the effect of making the resistance change or not?

While I really appreciate all the info on what the modifications are to this circuit topology with premium parts, it helps my learning process to also have the context of why these changes are being made (in other words, how does the resistance change impact the functionality of the circuit topology?)

Thanks in advance,
Stephen aka PC
 
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M

motospark

Member
Joined 2005
Finally Fount them.

TungstenAudio said:
As usual, there is more than one way to get the desired resistance for R3 and R4. The 2.0Ω add-on resistor is useful if you have already installed the standard 0.68Ω resistors in their places. It may be 1%, 2% or even 5%, depending on what you can find, If you haven't stuffed these parts yet, then a combination of 0.68Ω and 0.5Ω for R3 and R4 works well.
Click to expand...

I found the two last Pieces to the upgrade. The Silver Mica Capacitor 10pF 500Vdc SMICA-54388 from Parts Connexion, and the 2W 2.0 OHM 5% Metal Oxide from DigiKey. Five day wait to ship on the MO. Blocked

Just a clarification, The Metal Oxide 2w 2 ohm 5% is used as an addition to the or in Parallel (same hole) with the exiting kit part for location R3? I know Stupid Question.


Mark
 
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TungstenAudio

Member
Joined 2014
Here is a helpful formula for finding the total resistance of a number of resistors connected in parallel:
1/Rtotal = 1/R1 + 1/R2 + ... + 1/Rn

The original values for R3 and R4 have a total resistance of 0.340 Ohms. We have found (not just me) that a lower total resistance of about 0.288 Ohms (plus or minus a little) helps improve the sound of the ACA. There are a couple things working here. First is the quiescent current through Q1 and Q2, which is increased by the new values for R3 and R4. Second is the balance of dynamic current contribution between Q1 and the interactive current source made with Q2, while music is playing. It is up to us, as the builders, to decide what sounds best in our system.
 
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Puma Cat

Puma Cat

Member
Joined 2020
This explanation was exactly what I was looking for with respect to the rationale for the slight change in resistance value for R3/R4.

Very, very helpful. I'm all up for "what" to do to improve functionality, but as a scientist, I really value understanding "why".

Thanks very much for the explanation. :)
 
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