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Old 10th October 2018, 03:50 AM   #11
Extreme_Boky is online now Extreme_Boky
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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 ?

...you 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)?

Thanks,
Nick
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Old 10th October 2018, 05:37 AM   #12
TungstenAudio is offline TungstenAudio  United States
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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.
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Old 10th October 2018, 11:32 AM   #13
RafaPolit is offline RafaPolit  Ecuador
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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.

Thanks!
Rafa.
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Old 10th October 2018, 11:48 AM   #14
poseidonsvoice is online now poseidonsvoice  United States
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ACA amp with premium parts
Quote:
Originally Posted by RafaPolit View Post
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.

Thanks!
Rafa.
+1.

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).

Best,
Anand.
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Old 10th October 2018, 06:20 PM   #15
TungstenAudio is offline TungstenAudio  United States
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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.
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Old 10th October 2018, 08:48 PM   #16
TungstenAudio is offline TungstenAudio  United States
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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.
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Old 11th October 2018, 03:20 AM   #17
Mazeppa is offline Mazeppa  United States
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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,
Franco
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Old 12th October 2018, 02:49 PM   #18
TungstenAudio is offline TungstenAudio  United States
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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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File Type: jpg ACA_final (1).jpg (242.7 KB, 275 views)
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Old 12th October 2018, 05:50 PM   #19
Rudi_Ratlos is offline Rudi_Ratlos  Germany
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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
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File Type: jpg ACA_SCH.JPG (128.0 KB, 265 views)
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Old 12th October 2018, 06:37 PM   #20
TungstenAudio is offline TungstenAudio  United States
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Quote:
Originally Posted by Rudi_Ratlos View Post
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.


Thanks,
James
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