Yeah... I tried 10K resistor and that was waaay too bright, but only with a blue LED - the red LED was kinder to my retinas, but the colour was too girly... a macho amp deserves a macho LED colour!
Almost all DAC's use (unfortunately) OP IC's in their analogue stage. The gain can be easily adjusted - increased.
If you get a DAC with discrete analogue stage, the gain can be adjusted as well - depending on how that analogue stage was executed, the approach may vary, but only slightly (global / local feedback.. some don't use feedback at all, in which case the individual transistor gain stage has to be adjusted)
If you get a really good stuff, just a resistor after a DAC chip, then you can't do much... increasing the resistor value (to get more volts out of it), will not be to DAC current-out IC expectations.. at all...... In this case, just make do with what you have.
I find the ACA to have enough gain, and enough power, to drive 86dB efficient speakers to a more than reasonable level - loud! The balance between damping factor/gain/power capabilities of the output stage... is just perfect!
I'd avoid a separate pre-amp box at all costs... (or savings..?)
Once you hear what ACA can do, maybe Aleph J could be the next logical step up
It has + and - 24V DC supply -> perfect for a simple, DC coupled, pre-amp stage that can be easily installed inside the Aleph J enclosure - point-to-point wired on the Aleph J PCB, right at the +in and -in eyelets. You could even use a really nice, cheap, easy to implement, LT3045 voltage regulator boards -> to get a very clean and noise-free power supply rails for that point-to-point wired pre-amp. And, there you have it - an ultimate amp!!!! and... you don't have to use those darn JFET's, BC***C will do the job as good as those JFET's with a bit of an effort around base pin biasing.
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A question.
I notice Mr Boky has bypassed C1 and C2. Obviously in the signal path.
Wouldn't it make sense to bypass C3 as well being the main input cap ?
C4 is across the main power supply, would increasing its value be of any benefit ?
I'm guessing a bypass on C4 wouldn't be of much use.
Thanks.
I notice Mr Boky has bypassed C1 and C2. Obviously in the signal path.
Wouldn't it make sense to bypass C3 as well being the main input cap ?
C4 is across the main power supply, would increasing its value be of any benefit ?
I'm guessing a bypass on C4 wouldn't be of much use.
Thanks.
The caps come from what was close at hand... they are also the most common values that can easily be sourced. Simic II caps are "nice sounding"...
The choice purely depends on how much money one wants to spend, bearing in mind that the ACA is an introduction to single-end pure class A design.
C3 can be (previous stage output impedance-permitting), a 4.7uF cap like AuriCap, or BlackGate C type - if you can find them, or even Mundorf EVO silver/gold/oil - with a structural support, of course. I do not believe in bypassing with a lower value film cap here - i.e. I prefer a single coupling cap for the low input signal coupling duties. So, probably AuriCAP would give a very nice sound for the money spent.
This cap cannot be bypassed with a link (removed) because of that R10 that sets a half voltage at output via input JFET gate...
C4: Stay with 10uF and bypass with 0.068uF or so... polypropylene, polyester/ solid... WIMA... Vishay....all these dielectrics have their own signature and they can influence the sound in a positive or negative way... depending on other elements in your audio signal chain. Try and see what's good for you/your system.
C1: I used 10,000uF and 2.2uF film cap. You could try whatever suits your taste. You could even use a 10,000uF / 4.7uF / 0.47uF combo...
C2: 1000uF silmic II would be great here... but it is too big - I tried. Changing the value may influence startup/shutdown behaviour and/or stability. So, a bit of experimenting is required here. However, there's nothing stopping you in providing a very low impedance path to high-frequency noise, by installing a 0.068 cap there...in parallel.
Again... -> just some basic guidelines, but your own ears and the rest of your system, shall decide.
The choice purely depends on how much money one wants to spend, bearing in mind that the ACA is an introduction to single-end pure class A design.
C3 can be (previous stage output impedance-permitting), a 4.7uF cap like AuriCap, or BlackGate C type - if you can find them, or even Mundorf EVO silver/gold/oil - with a structural support, of course. I do not believe in bypassing with a lower value film cap here - i.e. I prefer a single coupling cap for the low input signal coupling duties. So, probably AuriCAP would give a very nice sound for the money spent.
This cap cannot be bypassed with a link (removed) because of that R10 that sets a half voltage at output via input JFET gate...
C4: Stay with 10uF and bypass with 0.068uF or so... polypropylene, polyester/ solid... WIMA... Vishay....all these dielectrics have their own signature and they can influence the sound in a positive or negative way... depending on other elements in your audio signal chain. Try and see what's good for you/your system.
C1: I used 10,000uF and 2.2uF film cap. You could try whatever suits your taste. You could even use a 10,000uF / 4.7uF / 0.47uF combo...
C2: 1000uF silmic II would be great here... but it is too big - I tried. Changing the value may influence startup/shutdown behaviour and/or stability. So, a bit of experimenting is required here. However, there's nothing stopping you in providing a very low impedance path to high-frequency noise, by installing a 0.068 cap there...in parallel.
Again... -> just some basic guidelines, but your own ears and the rest of your system, shall decide.
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Just learning to understand the bypass purpose of capacitors
Mr. Boky I have not experimented with changing capacitance and bypassing with different types like polyester poly styrene poly propylene. I think I remember reading ceramic capacitors already no no in audio because they could possibly cause ringing ? Or some sort of oscillation ?. I ask this because at our college electronic‘s class we were donated by Lawrence Livermore labs and Hewlett Packard Percision instruments a tons of resistors and capacitors and other electronic‘s. I’ve been rummaging through and I see some extremely high and ludicrously expensive resistors and capacitors that were hand-picked and matched down to third digit decimal point . Should I totally skipped not even attempting to grab the ceramic capacitors because they’re not suitable for audio?. Learning through experimentation and a lot of reading just too much to remember .
Mr. Boky I have not experimented with changing capacitance and bypassing with different types like polyester poly styrene poly propylene. I think I remember reading ceramic capacitors already no no in audio because they could possibly cause ringing ? Or some sort of oscillation ?. I ask this because at our college electronic‘s class we were donated by Lawrence Livermore labs and Hewlett Packard Percision instruments a tons of resistors and capacitors and other electronic‘s. I’ve been rummaging through and I see some extremely high and ludicrously expensive resistors and capacitors that were hand-picked and matched down to third digit decimal point . Should I totally skipped not even attempting to grab the ceramic capacitors because they’re not suitable for audio?. Learning through experimentation and a lot of reading just too much to remember .
Ok, I had no spare parts at hand, called Dad and had a remote testing session of LEDs.
I have a few pointers:
- Up to 20K Ohms is ok with both transparent case LEDs of blue light and also with Blue opaque encasing
- Intensity is dimmed much 'nicer' with opaque blue crystal. The transparent one is almost as bright as it was with the 10K.
The LEDs we have at hand have about 1.5V ~ 2V drop on the LED and start transmitting at around 1.7V. Even at 2V with the 10K resistor, there is light on the LED, so one could go even higher than 20K with the 24V PS.
Hope this helps,
Rafa.
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I did not install the R13 resistor and have not implemented the LEDs in my version. This mens simpler circuit, less wiring and less "disturbtions" during listening. I try to dim every light on my equipment if possible or to try to blind it in other ways.
There are alternative ways to check if ACA is on or off
There are alternative ways to check if ACA is on or off
Do not use ceramics.
Try polypropylene for sound coupling.
Also, there are articles that talk about using two electrolytes, back-to-back; this reduces distortion... I experimented with this topology, and then applied a common point (between two capacitors) bias voltage, as suggested by the article... which sounded even better. So, if you want to experiment... go crazy.
On the topic of LED brightness...
In my experience you will never calculate or arrive mathematically at a current that gives a brightness to suit all people.
The only way to do it is trial and error, and a 9 volt battery and resistor + preset is a good way to find out what current suits you. Always have a resistor in series with the preset as a safety net, something like 220 ohm for a 9v battery would prevent killing or impairing most LED's. Having found the current needed you can then calculate a suitable value for the actual supply voltage in use.
Modern LED's can be super bright and a few hundred microamps may be plenty for a softly glowing power indicator.
I have some tiny orange ultra high brightness LED's that give off a detectable glow (in a darkened room) from 9 volts and many meg ohm of series resistance. Amazing.
In my experience you will never calculate or arrive mathematically at a current that gives a brightness to suit all people.
The only way to do it is trial and error, and a 9 volt battery and resistor + preset is a good way to find out what current suits you. Always have a resistor in series with the preset as a safety net, something like 220 ohm for a 9v battery would prevent killing or impairing most LED's. Having found the current needed you can then calculate a suitable value for the actual supply voltage in use.
Modern LED's can be super bright and a few hundred microamps may be plenty for a softly glowing power indicator.
I have some tiny orange ultra high brightness LED's that give off a detectable glow (in a darkened room) from 9 volts and many meg ohm of series resistance. Amazing.
Dedicate one of the buttons on the infrared remote control, to LED Brightness. It'll get twiddled and finagled and micro-optimized, an astonishing number of times. Even when the amp doesn't move and the ambient light doesn't change. Co-owner #1 likes the brightness at one certain setting, co-owner #2 likes another. You know, spouses.
There are alternative ways to check if ACA is on or off
Indeed!
I've found a fantastic way to determine if the ACA is on and connected properly to it's associated up- and down- stream components is to apply a small AC signal - random, patterned, or some combination of the previous mentioned, to the input, and that signal will be processed and conditioned by the ACA circuit and then sent via a set of copper conductors to your pair of mechanical/electro transducers, which will in turn vary the air pressure in the room. These air pressure variations will then superimpose on your tympanic membranes, thereby registering in your body and finally this signal of the amp's proper operation will be transmitted via your nervous system to your brain.
