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Old 30th August 2018, 07:03 AM   #5661
Skylar88 is offline Skylar88  South Africa
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Quote:
Originally Posted by Extreme_Boky View Post
Fellas, it is also very important that your DC bus is connected to the output transistors (located on the PCB) via a bus that has the lowest possible impedance, i.e. via a large, short, thick DC bus bars (if possible)...

...Using thin wires, multiple connections, crimps, and capacitance multipliers' PCB on various boards with more thin wires... ain't gonna cut it...

Good point Extreme_Boky. If not done right, a capacitance multiplier or any other in-between component is gonna make things worse, not better.

Thanks for reminding us.
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Old 30th August 2018, 07:32 AM   #5662
astromo is offline astromo  Australia
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Quote:
Originally Posted by Extreme_Boky View Post
Fellas, it is also very important that your DC bus is connected to the output transistors (located on the PCB) via a bus that has the lowest possible impedance, i.e. via a large, short, thick DC bus bars (if possible)... that DC bus has to deliver a current swing from nothing, to a full required current (at all frequencies, i.e. within a full frequency range of at least 20Hz - 20kHz) in the shortest amount of time possible, while preserving the DC voltage constant. That carefully catered for (inside the amp) current, has to be passed now onto a speaker uninterruptedly (think of the same DC bus requirements, and distances, with regard to your speaker cables!!!)

...

Using thin wires, multiple connections, crimps, and capacitance multipliers' PCB on various boards with more thin wires... ain't gonna cut it...
... duly noted ... I'm intent on gaining advantage from the experience of others, while sticking with a plain vanilla, standard build - especially for my first go. When I've got sufficient confidence, then I'll think about kicking the trainer wheels off.

Thanks for helpful advice.


Quote:
Originally Posted by Extreme_Boky View Post
I know you asked specific people here to answer your question... but my small contribution will go down the line of thinking what is paralleling going to do to input impedance which is already quite low for a single ACA, and requires a dedicated pre-amp with fairly low output impedance (while also trying to minimise that interconnects capacitance...) to get a decent sound out of single ACA

I'll exit now quietly...
From the ACA store page, an excerpt from the spec section reads:
Input Impedance 10 k Ohm

I've seen various rules of thumb in terms of ratios around pre-amp impedance matching but your advice doesn't include any of that quantitative detail. I'm missing the subtlety of the message I suspect...
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Old 30th August 2018, 08:07 AM   #5663
Extreme_Boky is offline Extreme_Boky
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I like to see 50K input impedance amplifier, driven by a maximum of 500ohms output impedance pre-amp. This gives a linear drive at full audio band frequency spectrum and also removes the constraints that could be imposed by highish interconnects' capacitance. So, the ratio is 100:1.

With ACA, and the same logic, you'd expect a pre-amp to have 10ohm output impedance...

With two ACA's connected in parallel, the pre-amp would need to have a 5ohm output impedance.

Let's not forget that the ACA is designed to show the virtues of the 2-gain stage, deep class A design while providing a FULL kit and a very reasonable cost. It also comes from the audio legend who pioneered class A designs. So, to expect an amp with 50K input impedance, with only 2 transistors; a second one being driven off the 1K resistor, is really unfair.

All I wanted to do was to warn you guys that ACA has its expected limitations, and as long as you are aware of them... it's all good. But, paralleling ACA's is not a good idea simply from the input impedance point of view.

The logical next step would be Alep J. That is the whole idea, to move forward and build an amp with a good input stage, correct drive for up to 2 MOSFET's, very good current source while staying simple and providing single ended pure class A goodness.
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Old 30th August 2018, 09:33 AM   #5664
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Old 30th August 2018, 09:36 AM   #5665
Skylar88 is offline Skylar88  South Africa
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Great post Extreme_Boky. Thanks for putting the ACA scenario into perspective.

I totally agree with you. And to use a bad analogy: If I have a 1300cc budget car, there is only so much I can do to improve performance. It would be best to get a car with a bigger engine.

This is not meant as a put-down for my beloved ACA, but I have already purchased M2 boards.
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Old 30th August 2018, 01:56 PM   #5666
RafaPolit is offline RafaPolit  Ecuador
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Quote:
Originally Posted by Extreme_Boky View Post
...So, the ratio is 100:1.

With ACA, and the same logic, you'd expect a pre-amp to have 10ohm output impedance...

With two ACA's connected in parallel, the pre-amp would need to have a 5ohm output impedance...
Ok, wait... I know very little (or nothing) about audio design, but simple math I do very well! 10Kohm at 100:1 ratio is 100 Ohms, which is, conveniently, the output impedance of a lot of PREs out there.

The parallel would require 50 Ohms as per your ratios, not 10 and 5!



Best regards,
Rafa.
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Old 30th August 2018, 03:16 PM   #5667
Skylar88 is offline Skylar88  South Africa
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Just had a revelation regarding the need for a preamp with gain (for use with the ACA).

As source for the ACA, I've been using a PC + DAC + B1 buffer and 97dB speakers. The volume is quite satisfactory for 95% of my music, but then my 53% Karlsonators with dual FE25-16's are on the sensitive side of the scale.

Just now, I connected my new phone with FLAC playing capability. With all controls turned to the max, the volume is only good for near field listening. So, I understand now why some guys complain about a lack of volume. The ACA needs a bit of gain with efficient speakers, and a lot of gain with inefficient speakers.

In contrast, I recently connected an AKSA Lender Preamp with 15dB gain to the ACA and the amount of volume blew me away. I had to turn the pots on the Lender preamp right down. It goes to show that the ACA really needs a preamp with gain; exactly how much will depend on the sensitivity of your speakers.

Hint: If you have a problem with low volume on the ACA, there is a neat trick mentioned in post #5151 to increase volume past 100% using certain media players, i.e. Foobar and VLC Player.

I'm posting this for the benefit of others who, like me a year ago, isn't sure what preamp to use with the ACA. When I asked which preamp would be best for the ACA, the B1 was suggested by many - and I now realize that it wasn't the best advice for me. YMMV as they say, but maybe this post helps someone.


PS. I don't regret building the B1. I can always use it with other amps.

Last edited by Skylar88; 30th August 2018 at 03:19 PM.
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Old 30th August 2018, 03:35 PM   #5668
RafaPolit is offline RafaPolit  Ecuador
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Yes, I reported the same findings a few pages ago. With a DAC / PRE with little gain and the 'standard' line output of 2V, I get average volume and decent sound. With a PRE that can output 4V into the line (albeit I don't have access to its gain value per-se) the volume is enough even for 4 Ohm speakers in a living room, and the sound is full of body, detail and bass.

I said the same: make sure you pair the ACA with a nice PRE (with some gain) and you will be surprised!

Rafa.

Last edited by RafaPolit; 30th August 2018 at 03:54 PM. Reason: added attribute of GAIN to the pre.
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Old 30th August 2018, 03:51 PM   #5669
Skylar88 is offline Skylar88  South Africa
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Quote:
Originally Posted by RafaPolit View Post
I said the same: make sure you pair the ACA with a nice PRE and you will be surprised!


I have to say that the B1 is a great preamp/buffer. The sound is exceptional, and as others have mentioned, transparent. With the right DAC (with gain) and efficient speakers, the B1 can be a good match for the ACA.
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Old 30th August 2018, 03:55 PM   #5670
RafaPolit is offline RafaPolit  Ecuador
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I have corrected my post! I did not mean to imply AT ALL that the B1 is not a (VERY) nice PRE. I meant to say that the ACA benefits from a bit of additional gain, having itself only about 10dB of gain, when a PRE/AMP combo usually have around 30dB or so of combined gain.
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