The wattage dissipated by the resistor is microscopic. Even if you placed it across the supply rail it would only dissipate around 15 milliwatts. In its intended use it will dissipate under 2 milliwatts.
Forumula is V squared/R or alternatively, I squared*R
Quick approximation... a 24 volt rail theoretically could support an 8.4 vrms output, in practice the amp will not reach that, but assuming it did you get (8.4*8.4)/39000 which is 0.001809 watts.
I'm not 100% sure of the reasoning behind the resistance value because the high output impedance of the amp means that the 'B' channel (which gets its input from the 'A' channels output) will experience pretty large variations in final output voltage.
For example if you match the gains off load by selecting an appropriate resistor value, and then load the bridged amp by 8 ohms, the output voltage of 'A' will fall by say 1.8db while the 'B' channel output falls by around 2.6db
I guess the chosen value allows both to clip at about the same time under maximum drive conditions while accepting that at differing impedances there is a mismatch... which means don't stress over the exact value
Forumula is V squared/R or alternatively, I squared*R
Quick approximation... a 24 volt rail theoretically could support an 8.4 vrms output, in practice the amp will not reach that, but assuming it did you get (8.4*8.4)/39000 which is 0.001809 watts.
I'm not 100% sure of the reasoning behind the resistance value because the high output impedance of the amp means that the 'B' channel (which gets its input from the 'A' channels output) will experience pretty large variations in final output voltage.
For example if you match the gains off load by selecting an appropriate resistor value, and then load the bridged amp by 8 ohms, the output voltage of 'A' will fall by say 1.8db while the 'B' channel output falls by around 2.6db
I guess the chosen value allows both to clip at about the same time under maximum drive conditions while accepting that at differing impedances there is a mismatch... which means don't stress over the exact value
I am considering the ACA as my first diy and I have 2 questions:
1- Is using a higher powered SMTP - let's say 200W for both channels beneficial at all?
2- Regarding to bias adjustment and working temperature, would it be damaging to the sound quality if I run the mosfets too cool? I am planning in using a computer fan assisted cooler instead of a passive radiator and I am afraid that the mosfets temperature will have a hard time going above 40C. I can always fine tune by lowering the fan or turning if off though. The idea of the CPU cooler is a matter of form more than function. I know very little about electronics, but love the mechanics and aesthetics of putting anything together.
1- Is using a higher powered SMTP - let's say 200W for both channels beneficial at all?
2- Regarding to bias adjustment and working temperature, would it be damaging to the sound quality if I run the mosfets too cool? I am planning in using a computer fan assisted cooler instead of a passive radiator and I am afraid that the mosfets temperature will have a hard time going above 40C. I can always fine tune by lowering the fan or turning if off though. The idea of the CPU cooler is a matter of form more than function. I know very little about electronics, but love the mechanics and aesthetics of putting anything together.
I would suggest running smps close to full rating, read match va rating to current draw to avoid idle/low power whine
Running mosfets "hot" is not a feature in itself (as i understand it), it's just an result of high bias (in our pass examples).and the resulting dissipation.
If you can bias hard and keep the temperature down you are on the right path in my world
Running mosfets "hot" is not a feature in itself (as i understand it), it's just an result of high bias (in our pass examples).and the resulting dissipation.
If you can bias hard and keep the temperature down you are on the right path in my world
200W is not that "over powered" for two channels. You would never run them at their max. power. The most important is that it is "noise free" and free of any switching noise. As long as you can get the temperature stabilized so you can make a final bias adjust after warm-up 40 degree is probably ok but I would not like to have an electrical motor running close to the amp with all that electrical and magnetic noise it would generate. It does nothing good for the sound. After all the sound must be the most important.
So, better to stick with the 120W specified for 2 channels? I had this problem of low power whine before (on a cheap SMPS though) and definitely do not want to go there again.
You're saying I can ideally bias as hard as possible and even if the mosfets don't get hot (just assuming so), it would work just fine?
The CPU cooler I'm planning to use (and bought already) should be good for 150W dissipation with a fan. I might get away without a fan if my positioning idea works (I'll position the fins vertically to help natural air movement). The problem is that dissipation efficiency lowers as input heat lowers and the 4 mosfets are only dissipating what? I assume 60W total?
I can understand and relate to your concern with noise by using a fan. If I end up needing one I'll get a thermo controlled one to keep speeds as low as possible. The SMPS I'm looking at has a connection for a fan, I am expecting it to be isolated from the rest of the circuit so electrical noise won't travel to the amp.
But my understanding is that if the bias is right (or high) as long as the mosfets are not hot, I'm good to go? Let's hypothetically say that I adjusted the bias hard but the mosfets won't get past 40C, in this case that's fine, they don't NEED to be hot, being hot is just a consequence of the cooling capacity of the heatsinks not being that great, and that's fine.
I'm just careful about this because although I've seen CPU heatsinks on the hybrid Alephs, they are little furnaces. The ACA doesn't generate nearly as much heat. And I haven't seen a build with CPU coolers.
For the first Diy project is better to try build with the same parameters ( psu voltage, bias ) heat sinks dissipation like on the original ACA schematic and article. After that is easy to know sweet spot sound quality.
Experiments can be interesting probably in the next step
For the first Diy project is better to try build with the same parameters ( psu voltage, bias ) heat sinks dissipation like on the original ACA schematic and article. After that is easy to know sweet spot sound quality.
Experiments can be interesting probably in the next step
Agreed! It is just because as the kit is not available and I am putting together myself. A few things like the power supply are not much more money for a lot more capacity so I thought that could be a no-brainer. I'll stick with the 120w meanwell as designed. I'll just go with a open chassis one because I want it integrated and not as a separate brick (WAF points here)
On the heatsinks I'll probably be stubborn since the huge standard heatsinks don't quite fit my visual taste.
In class A with right high bias mosfet always run hot but we protect this transistor with heat sink's. To much aluminium is not one problem ( temperature goes down but bias is at right level )
but too little heat sink can damage mosfet ( specially in the hot Summer days )
with too much heat to dissipate....or You need low bias but have more distortion and compromise sound quality
Expect the .65 cent 1 watt models will work just fine in this application. The link was more to get you a vendor that had that value and made in the USA you were in search of. I can tell you for absolute sure you can twist the two together that you already bought and they will also do a very nice job. So if you want to save a 1.30 cents and shipping - might try those.
While adjusting the offset of my ACA's, observing THD in the process, I found the distortion under almost full load (at around 1%THD) to be lowest while the amps were cold. With hot amps, THD was a lttle worse.
Ok.....my observations was that bias was 1.45 A when cold and settled to approx. 1.4 A when warmed up. Both channels behaved exactly the same. So if high bias = lower THD then there is an explanation.
@ Preamp
Useful informations about ACA better sweet spot , distortion level , damping factor ,
frequency wideband response , low noise 15 uV new version start at 45:30
Burning Amp 2017 - Nelson Pass
Have a nice day
I have on order one- 1.6 kit. but intend to do two monoblocks at some point. Apparently not very good searching for information. Would like the dimensions of a board so as I can plan out for the future. I have a chassis that may work on hand but don't know the board dimensions. Thank you in advance.
Did you try looking at the DIY Audio Store?
amp-camp-amp-pcb
DuckDuckGo is my friend. Works like a charm for me.
Yes it shows 88 by 57mm but doesn't new board VS1.6 have ouput transistors on both ends? Hey thanks for the "am I blind tip for the day". geez sometimes wonder how I get home some days. That will be close enough I think. I have about 235 mm for end to end. Can't wait to get to building it.
@ Preamp
Useful informations about ACA better sweet spot , distortion level , damping factor ,
frequency wideband response , low noise 15 uV new version start at 45:30
Burning Amp 2017 - Nelson Pass
Have a nice day
Most interesting was that Nelson lowered the gain even further to decrease distortion. I'll adapt that for my amps, too, once I can afford do disconnect them from my current setup
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