This amp has almost zero dc offset when stabilized after powering, atleast I haven't seen it so low in any of my previous class-A builds! And this also sounds amazing, now there's option for 4R version, nice tips and tricks from all of you guys so I can't see any problems here, quite the opposite! I would also love to build4R version even I dont have 4R speakers
Don't forget the easily adjustable gain.
All those options in a class-A amp, nicely done Hugh !
Don't forget the easily adjustable gain.
All those options in a class-A amp, nicely done Hugh !
Oh, forgot that
I have 2x pots per channel in my build, both are in recommended position and no need to adjust. Could be resistors instead even I could not get very tight matching with devices.
X and I hope that this amp, based on the AKSA/Lender front end and the amended Aleph J output stage, will become a classic of the genre.
Being somewhat OCD I have designed my own pcb to include two independent CRCRCRC power supplies and hope to build and test it in the next few weeks. I am very impressed with JPS 'euro card', a magnificent piece of layout, but I love to do my own pcbs, it soothes me as I spend hours polishing it to suit my whims........
I will put up a picture when done. It will be standard 20W 8R version, using a 350x100 heatsink for each channel. I will try it out on my 87dB/watt/metre TL speakers, it should go loud enough.......
Keeping building guys!! This gets me out of bed every day to see the posts!
Ciao,
HD
Being somewhat OCD I have designed my own pcb to include two independent CRCRCRC power supplies and hope to build and test it in the next few weeks. I am very impressed with JPS 'euro card', a magnificent piece of layout, but I love to do my own pcbs, it soothes me as I spend hours polishing it to suit my whims........
I will put up a picture when done. It will be standard 20W 8R version, using a 350x100 heatsink for each channel. I will try it out on my 87dB/watt/metre TL speakers, it should go loud enough.......
Keeping building guys!! This gets me out of bed every day to see the posts!
Ciao,
HD
My apologies in advance , but I am slightly confused regarding the values of R131,132,128. For 8ohm use I used the following values: R131 0.33ohm R132 0.17ohm ( two 0.33 in parallel ) and for R128 CCS base drive 680ohm. Are these still the default values to use? All other components are as original circuit shown at the start of the thread, with the exception of the Mosfets that are FDA38N30 and FQA36P15.
Regards
Alan
Hi Aksa, thanks for your effort too! You guys are brilliant minds
I hope this one will become classic, atleast it has all the possibilities to do that!
You could fabricate extra board for me too with integrated supply
? B.B. and maybe one more Alpha later will probs be my last builds for a while.
Minimum impedance or graph? If it is high -and real 87 dB- it must enough SPL I think.
Pro-Audio and lighting Calculators. Calculator page
In phase (you are in the vertice of the triangle) you must add 3 dB.
3m - 96.5 dB
With a good coaxial speaker or tweeter with waveguide... random phase must be almost in phase. Well, better between.
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You are fine for 8ohms - what Hugh has been suggesting are changes for a 4ohm version of Alpha 20.
For your 8ohm speakers you might want to try reducing the bias current to 1.35amps using R131 of 0.47R and R132 of 0.22R. It works quite well and reduces heat output quite a bit and causes less rail voltage sag for smaller transformers. It’s optional though.
The VU meter will be something for JPS64 to answer - it would require an ADC, which I believe an Arduino has several 10-bit ADCs built in, so seems like it should not be a problem. The trick may be some gain/range settings to get it to correspond to the correct max output depending on rail voltage.
Can we use opto-isolator (analog LED to photodiode ouput)?
Luna Optoelectronics - NSL-32 - 40 mA (Max.) 2.0 V@ 16mA Photocell Axial Analog Optocoupler - Allied Electronics & Automation
Here is a nice dual channel from Vishay:
http://www.vishay.com/docs/83647/ild205t.pdf
With 8pin TH DIP breakout board:
BOB-09118 SparkFun Electronics | Development Boards, Kits, Programmers | DigiKey
Luna Optoelectronics - NSL-32 - 40 mA (Max.) 2.0 V@ 16mA Photocell Axial Analog Optocoupler - Allied Electronics & Automation
Here is a nice dual channel from Vishay:
http://www.vishay.com/docs/83647/ild205t.pdf
With 8pin TH DIP breakout board:
BOB-09118 SparkFun Electronics | Development Boards, Kits, Programmers | DigiKey
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A hypothetical question, if each mosfet had its own CPU cooler, could the mosfet be mounted directly without an insulator, as long as the coolers were separated
. Obviously they would be mounted inside the enclosure so that they could not be touched.
Are there downsides, I am just trying to reduce the mosfet temperatures as much as possible.
Regards
Alan
. Obviously they would be mounted inside the enclosure so that they could not be touched.
Are there downsides, I am just trying to reduce the mosfet temperatures as much as possible.
Regards
Alan
Hi Hugh,
I tried to simulate the damping factor for the Alpha20 8R.
I did an ac simulation with the fb resistor connected to Vout and another with the fb resistor connected before the sensor resistor,
like in standard Alpha20 and BB.
The difference between the two is huge.
But is my simulation correct ?
Because I should see a DF of 200 with the standard Alpha20.
I've also included the ltspice file of my ac analysis
Regards,
Danny
I tried to simulate the damping factor for the Alpha20 8R.
I did an ac simulation with the fb resistor connected to Vout and another with the fb resistor connected before the sensor resistor,
like in standard Alpha20 and BB.
The difference between the two is huge.
But is my simulation correct ?
Because I should see a DF of 200 with the standard Alpha20.
I've also included the ltspice file of my ac analysis
Regards,
Danny
Attachments
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Hi Danny (and others interested in Zout/damping factor on audio amps),
There are now three versions; ALPHA 20W for 8R, ALPHA 20W for 4R, and ALPHA BB 52W for 8R/4R. I think we might finish at this point, it's getting complicated!!
I cannot answer whether your technique is correct because I use a different measure and don't fully understand V(vin2)/V(vout). I have not seen this approach but I'm sure it works fine. Using fb from the LEFT side of the 0.12R output series resistor, I simply set the load at 400R (a load impedance so high the amp is hardly seeing it at all), measure the output at around 10Vpeak, then change the load to 4R and measure off Spice at 9.675Vpk. Then calculate 1 - (9.675/10) = 0.0325, then multiply by 4R to get the Vout which is 0.13R. DF is then 4/0.13, which is 30.8. You notice that this is almost the same as the 0.12R resistor, not surprising; the 0.12R resistor is outside the fb loop so lies between the amp and the speaker, so it's a simple addition of impedance to the intrinsic Zout of the amplifier.
Now, if you take fb from the output node, RIGHT side of 0.12R output series resistor, you get very different figures. For 10Vpk into a 400R load, it drops to 9.953Vpk, and the sums are [1 - (9.953/10)] x 4R = 0.0188R, that is near enough to 19 milliohms, giving a DF of 212.7!! VERY different....... but of course, this is sine wave, 10Vpk, 1KHz, so at all other amplitudes, waveform and frequency it will be different.
The fact is that you can manipulate these aspects pretty easily, BUT, what is the sonic effect?
The high DF will have very punchy bass with a slight 'dry' feeling because the note finishes very quickly, moving onto the next. With a DF of 30, there will be a slight loose sensation to the bass, 'overhang' as the cone is not controlled so well by the amp. You might actually like this effect, particularly with this type of amplifier which needs sensitive speakers which have a very tight suspension.
Cheers,
Hugh
There are now three versions; ALPHA 20W for 8R, ALPHA 20W for 4R, and ALPHA BB 52W for 8R/4R. I think we might finish at this point, it's getting complicated!!
I cannot answer whether your technique is correct because I use a different measure and don't fully understand V(vin2)/V(vout). I have not seen this approach but I'm sure it works fine. Using fb from the LEFT side of the 0.12R output series resistor, I simply set the load at 400R (a load impedance so high the amp is hardly seeing it at all), measure the output at around 10Vpeak, then change the load to 4R and measure off Spice at 9.675Vpk. Then calculate 1 - (9.675/10) = 0.0325, then multiply by 4R to get the Vout which is 0.13R. DF is then 4/0.13, which is 30.8. You notice that this is almost the same as the 0.12R resistor, not surprising; the 0.12R resistor is outside the fb loop so lies between the amp and the speaker, so it's a simple addition of impedance to the intrinsic Zout of the amplifier.
Now, if you take fb from the output node, RIGHT side of 0.12R output series resistor, you get very different figures. For 10Vpk into a 400R load, it drops to 9.953Vpk, and the sums are [1 - (9.953/10)] x 4R = 0.0188R, that is near enough to 19 milliohms, giving a DF of 212.7!! VERY different....... but of course, this is sine wave, 10Vpk, 1KHz, so at all other amplitudes, waveform and frequency it will be different.
The fact is that you can manipulate these aspects pretty easily, BUT, what is the sonic effect?
The high DF will have very punchy bass with a slight 'dry' feeling because the note finishes very quickly, moving onto the next. With a DF of 30, there will be a slight loose sensation to the bass, 'overhang' as the cone is not controlled so well by the amp. You might actually like this effect, particularly with this type of amplifier which needs sensitive speakers which have a very tight suspension.
Cheers,
Hugh
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Allan,
You may get away with using no mica/ceramic/Al203 washer on each cooler, HOWEVER, if you touch one or two of the coolers with a probe or set them onto a ground point on the enclosure, there may be smoke and flame........
I ALWAYS use an insulating washer on all mosfets, big and small. With electronics there are always disasters waiting to pounce, and they often do because you cannot see or hear or touch low voltage high current energies. And of course, if a probe can stray, and short something out, it will.
So don't avoid using insulating washers. Disaster waiting to happen........
Hugh
You may get away with using no mica/ceramic/Al203 washer on each cooler, HOWEVER, if you touch one or two of the coolers with a probe or set them onto a ground point on the enclosure, there may be smoke and flame........
I ALWAYS use an insulating washer on all mosfets, big and small. With electronics there are always disasters waiting to pounce, and they often do because you cannot see or hear or touch low voltage high current energies. And of course, if a probe can stray, and short something out, it will.
So don't avoid using insulating washers. Disaster waiting to happen........
Hugh
Typically the body tab on a vertical MOSFET is the same as the Drain or pin 2. So it is same as the live rails (+/-24 or +/-35v) full voltage and full current. Please use an insulator. I have the ceramics on order and can send those out to all who ordered them once I get them in. I think a silicone pad should work fine.
Hi Hugh,
You have explained this trick on how to estimate DF in LTspice for me before. I am curious to try it with a real amp. So I have some 270ohm 10W resistors and my usual 8ohm 25w resistors. I should set the amplitude to 10.0vpp with the the 270ohm load then swap to 8ohm load and remeasure and use the formula you provide. This will tell me experimentally what the DF is right? Is there any danger to doing this? I seems safe enough. It’s only 1.56wrms into 8ohms.