Which version alpha are going to power with your delicious chokes?
I am very happy with my alpha running 1.9 amp of bias current and thankful I decided to go with a 4 ohm capable design. I picked up a pair of MartinLogan Motion 15 floor models from Best Buy this weekend for a deep discount, which are speced at 5 ohm. As of now I can't really hear much of difference from the SMPS with cap Mx and the linear with SLB. I will need to move to a better room for auditioning and more ears would help!
The biggest surprise was that Dayton bookshelf speakers I made are on par, sound wise (no comparison with looks!) with the Martin Logan. The highs from the ribbon tweeter are better than the silk domes and bass response is similar despite the 0.75 inch difference in size of the woofers.
I am very happy with my alpha running 1.9 amp of bias current and thankful I decided to go with a 4 ohm capable design. I picked up a pair of MartinLogan Motion 15 floor models from Best Buy this weekend for a deep discount, which are speced at 5 ohm. As of now I can't really hear much of difference from the SMPS with cap Mx and the linear with SLB. I will need to move to a better room for auditioning and more ears would help!
The biggest surprise was that Dayton bookshelf speakers I made are on par, sound wise (no comparison with looks!) with the Martin Logan. The highs from the ribbon tweeter are better than the silk domes and bass response is similar despite the 0.75 inch difference in size of the woofers.
Here's the version that will be choke powered.
I'll run it also at 1.9A with a 0.33 source resistor
Other changes:
- added a voltage/FB divider 66-33ohm over the 0.15 output/FB resistor,
this doubles the damping factor
- changed the 820r shunt FB resistor to 1.8k for less gain and 4R stability
- added a ground lift to the input signal,
for this I needed to cut 4 traces (left of c111 and above r101, both sides pcb),
soldered four 10r resistors over the traces cut.
- use of IXYS IXTH64N10L2 and IXTH90P10P
- added snubbers 220p-47r to both mosfets
That's it, the pcb and the components are already soldered,
now I need to put everything in place and add some wires
I'll run it also at 1.9A with a 0.33 source resistor
Other changes:
- added a voltage/FB divider 66-33ohm over the 0.15 output/FB resistor,
this doubles the damping factor
- changed the 820r shunt FB resistor to 1.8k for less gain and 4R stability
- added a ground lift to the input signal,
for this I needed to cut 4 traces (left of c111 and above r101, both sides pcb),
soldered four 10r resistors over the traces cut.
- use of IXYS IXTH64N10L2 and IXTH90P10P
- added snubbers 220p-47r to both mosfets
That's it, the pcb and the components are already soldered,
now I need to put everything in place and add some wires
Attachments
Very nice! Can you hear the difference with the snubbers and increased damping factor? I'd imagine damping factor is very important when driving larger woofers...
I have built mine with 2.2K for the shunt FB but I would like to change it for lower value to yield higher gain because my source (DragonFly Black V1.5) peaks at 1.2 RMS. What value would I need to achieve full power with 1.2Vrms input? I am using 100uF for C111 and would like to keep it unless it really needs to be changed.
I have built mine with 2.2K for the shunt FB but I would like to change it for lower value to yield higher gain because my source (DragonFly Black V1.5) peaks at 1.2 RMS. What value would I need to achieve full power with 1.2Vrms input? I am using 100uF for C111 and would like to keep it unless it really needs to be changed.
Assuming you want 35w into 5ohms, that’s 37Vpp. If your DAC puts out 1.2Vrms that’s 3.4vpp. You only need 11x or 21dB gain. The Alpha has more than 21dB gain so you should be able to hit 35w no problem. That’s the stock Alpha with 820R feedback shunt. As you set yours at 2k2 the gain is (22k+2k2)/2k2=11 or 21dB exactly. You are marginal so maybe drop 2k2 to 1k8 and you should be able to hit clip.
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Yes, I have also installed 1k8.
The snubbers are just preventive to avoid oscillations, they're soldered directly on the mosfet pins.
The increased damping will definitely be noticeably with my double 10" woofers.
Also with 4ohm speakers the damping factor is halved compared to 8ohm speakers.
So with this trick the DF for 4ohm speakers is the same as in the original Alpha20 with 8ohm speakers.
Thanks Hugh !
The snubbers are just preventive to avoid oscillations, they're soldered directly on the mosfet pins.
The increased damping will definitely be noticeably with my double 10" woofers.
Also with 4ohm speakers the damping factor is halved compared to 8ohm speakers.
So with this trick the DF for 4ohm speakers is the same as in the original Alpha20 with 8ohm speakers.
Thanks Hugh !
Ok, it’s been a while now that B.B. monos have been playing with my friends huge JBL monitors with Transcendent 300B Masterpiece pre. I was thinking that I’ll give them a real challenge and bring some new amps to play with.
Well, after listening session it was clear that B.B. won this battle, have to say that I was little surprised. I know, it’s much about synergy between everything, but man, B.B. really drive those speakers with authority! I have one more pair B.B. and ABBB pcb’s, I have to finally start planning how to build second pair for myself.
Well, after listening session it was clear that B.B. won this battle, have to say that I was little surprised. I know, it’s much about synergy between everything, but man, B.B. really drive those speakers with authority! I have one more pair B.B. and ABBB pcb’s, I have to finally start planning how to build second pair for myself.
According to the schematic, the impedance is 22k as set by R102.
Excellent, thank you X!
Hugh/X,
What is the smallest value input capacitor one can use with this design without affecting the LF extension of the amplifier (I'm not sure what -3dB point you were aiming for)?
And assuming a best case scenario with a source that has <10 ohms output impedance, 4V RMS of voltage swing, and of course short interconnects (1M or less)?
Thanks,
Anand.
What is the smallest value input capacitor one can use with this design without affecting the LF extension of the amplifier (I'm not sure what -3dB point you were aiming for)?
And assuming a best case scenario with a source that has <10 ohms output impedance, 4V RMS of voltage swing, and of course short interconnects (1M or less)?
Thanks,
Anand.
Hugh,
Thanks. I did think that the 10 uf cap recommendation was absurdly large but then again, if an electrolytic was the thought process (and it is easy for quick testing as X had graciously done) then there it is. Seems like a bare minimum of 2.2uf is recommended and most of us diy'ers have a small arsenal of film caps waiting to be used.
I realize this is the wrong thread but I bet the Alpha Nirvana would be similar (similar input stages), i.e. no need to go all the way to 10uf for the input cap, 2-10 uf will probably work fine, correct?
Thanks
Anand.
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It depends on what the impedance is and what you consider flat deep bass.
For an input impedance of 22k set by R102, the characteristic RC time constant is 0.7RC or 0.7x22000ohmx2.2uF=34mS or about 30Hz. That’s the -3dB point. I would aim for a value of 10Hz -3dB to make sure there’s not too much attenuation at 20Hz.
10Hz = 1/(0.7 x R x C) solve for C
C = 1/(0.7 x 22000ohm x 10Hz) = 6.5uF
So 6.8uF is what I think is needed if you want flat to 20Hz.
For an input impedance of 22k set by R102, the characteristic RC time constant is 0.7RC or 0.7x22000ohmx2.2uF=34mS or about 30Hz. That’s the -3dB point. I would aim for a value of 10Hz -3dB to make sure there’s not too much attenuation at 20Hz.
10Hz = 1/(0.7 x R x C) solve for C
C = 1/(0.7 x 22000ohm x 10Hz) = 6.5uF
So 6.8uF is what I think is needed if you want flat to 20Hz.
According to one of the reactance calculator in the net for 2.2uF and 10Hz, Xc is this:
XC = 1/(2piFC) = 1 / (2 x 3.141592653 x 10 Hz x 2.2 uF)
= 7.23E+03 ohms (to 3 significant digits)
= 7,234 ohms
This means that with a Zin of around 20k this gives us a ratio of 20/27.23 reaching the input base which is 0.73, a drop of 2.69dB from unity. (20log0.73).
In conclusion, if you use 2.2uF input cap in the Alpha Nirvana, at 10Hz the HF will be down 2.69dB on the 1KHz datum.
Hugh
XC = 1/(2piFC) = 1 / (2 x 3.141592653 x 10 Hz x 2.2 uF)
= 7.23E+03 ohms (to 3 significant digits)
= 7,234 ohms
This means that with a Zin of around 20k this gives us a ratio of 20/27.23 reaching the input base which is 0.73, a drop of 2.69dB from unity. (20log0.73).
In conclusion, if you use 2.2uF input cap in the Alpha Nirvana, at 10Hz the HF will be down 2.69dB on the 1KHz datum.
Hugh
It depends on where you draw the threshold. The circa 50% voltage threshold occurs at time = 0.7RC.
https://www.electronics-tutorials.ws/rc/rc_1.html