Hi RL,
These are wonderful pictures, both of the amp build and the oscillographs. The bode graph is outstanding, and the transient performance terrific. It's rounding pretty much at 10KHz, but no issues at 1KHz and no ringing. It is clear it should sound very good. Thank you for posting these piccies!
On any LTP you have to have same impedances on the two bases. If you move T1 base to ground to 47k5, you should also change the fb series to 47k5 too, and of course double the shunt resistor to 4k4, and adjust for zero offset at R105 pot. You should then increase the fb nesting resistor of 221k R112 to 470k as well. If the LTP is not balanced, the H2 level is higher than H3, if balanced correctly, H2 should be less than H3. But the remaining signal path is set up for a monotonic decrease of harmonics, so there is some advantage in achieving balance.
For some reason XRK has not suffered failures with the IXYS trench mos on his two ALPHAs, but yes, it seems there MAY be an issue. I would suggest using the OnSemi/Vishay IRFP264 which is a 250V, 280W 38A TO264 package with a slightly different device topology and low transconductance. I have used the FQA40N25 with no issues, so this trench mosfet may be quite OK too. But thank you for the words on this, and for your review.
I am thrilled you like the sound. This is a very, very good amp, with a truly musical delivery with SS muscle and tube grace. I comment the ALPHA to any audiophile.
Cheers,
Hugh
These are wonderful pictures, both of the amp build and the oscillographs. The bode graph is outstanding, and the transient performance terrific. It's rounding pretty much at 10KHz, but no issues at 1KHz and no ringing. It is clear it should sound very good. Thank you for posting these piccies!
On any LTP you have to have same impedances on the two bases. If you move T1 base to ground to 47k5, you should also change the fb series to 47k5 too, and of course double the shunt resistor to 4k4, and adjust for zero offset at R105 pot. You should then increase the fb nesting resistor of 221k R112 to 470k as well. If the LTP is not balanced, the H2 level is higher than H3, if balanced correctly, H2 should be less than H3. But the remaining signal path is set up for a monotonic decrease of harmonics, so there is some advantage in achieving balance.
For some reason XRK has not suffered failures with the IXYS trench mos on his two ALPHAs, but yes, it seems there MAY be an issue. I would suggest using the OnSemi/Vishay IRFP264 which is a 250V, 280W 38A TO264 package with a slightly different device topology and low transconductance. I have used the FQA40N25 with no issues, so this trench mosfet may be quite OK too. But thank you for the words on this, and for your review.
I am thrilled you like the sound. This is a very, very good amp, with a truly musical delivery with SS muscle and tube grace. I comment the ALPHA to any audiophile.
Cheers,
Hugh
Congratulations to the latest builds from aive, bigaudioscotto and raptorlightning !
The Alpha is getting momentum
I've also started with soldering an Alpha4R.
@raptorlightning : nice report,
the alumina insulators are doing great.
Too bad that the IXTH76N25T didn't work, on paper it has all the correct parameters,
did you also have the snubbers on the Ixys ?
Indeed I did have snubbers, as well as reducing the 100K resistor to 68K, reducing the gate stopper resistor, and putting 1nF in parallel with the new 68K. None of these tricks helped to save that specific device in this position.
I apologize. The rounded square wave was the 100kHz results, not 10kHz. I mathed the horizontal scale incorrectly.
I originally had made the same resistor changes you suggest and I decided not to balance the LTP in that way for some reason a while back. I repeated my LTSpice sims with the resistor changes for balancing and, remembering the results I saw before, THD increases (slightly - 0.011% to 0.014% for normalized input voltages, likely due to reduced NFB) and the FFT noise floor is higher. I have images of the before and after, but I intentionally didn't make these changes as they proved (slightly) detrimental. HD2 remains above HD3 regardless of component values with a monotonic decrease. The HD1-4/HD1-15 ratio is >99.5%, actually closer to 99.99%, in either case.
Overall, it doesn't really seem to matter in simulation at least, whether or not these component values are changed. Is there perhaps something I'm missing in simulation?
Note that the FQA40N25 isn't a trench MOS - it's planar+DMOS technology. And the often noted IXFH74N20P is something called "HiPerFET", which I can't seem to find the structure of. Nor can I find the architecture of the "Polar" series IXTH88N30P and IXTH90P10P, but these don't seem to suffer the same deficiencies that the "TrenchFET" devices do.
Thank you so much for the help in debugging. It surely was a surprise that the 76N25 device didn't behave at all like simulations. Linear operation seems indeed disastrous for some of these devices.
I'm glad I could catch the issue before others stumbled across it!
It sounds great. Very clear and transparent. Authoritative across the volume knob as well without losing any steam as it gets too loud for me. Very flat and clean across the frequency spectrum with nice, smooth high frequencies. I'm sensitive to amps with glare in the high end, and this has none of that.
As far as hum and ground loops... what hum and ground loops?
It is dead silent with no input. The SLB and Alpha makes a good pair! I do recommend hooking every ground (except signal in) to the SLB board to avoid ground loop potential.
I'm glad I could catch the issue before others stumbled across it!
It sounds great. Very clear and transparent. Authoritative across the volume knob as well without losing any steam as it gets too loud for me. Very flat and clean across the frequency spectrum with nice, smooth high frequencies. I'm sensitive to amps with glare in the high end, and this has none of that.
As far as hum and ground loops... what hum and ground loops?
Hi RL do you mind sketching your grounding and earthing scheme? I get a little buzz/hum when I put my ear to the cone, but not sure if it's worth resolving tbh.
Yes, there is a faston tab on the SLB connected to an NTC and 22nF cap as a ground loop breaker, to isolate the clean 0v GND from the dirty chassis/IEC/earth GND. That should be the only place the amp or PSU touches the chassis/IEC/earth GND. If you still have hum, try connecting the audio input RCA input jack GND to the SLB 'Star Hub' rather than the amp boards.
Here are some very good articles for zero noise and hum : "Advanced Grounding Guruship" and "The library of grounding problems"
Regards,
Danny
Regards,
Danny
Honestly the pictures in my post do a better job than any sketch. If you look in the top right to the back of the chassis by the small transformer, you can see a small nylock nut. That's the single earth ground chassis connection for the IEC jack, transformer shield, and SLB earth wire (single black wire heading back across the bottom). Speaker negative and Alpha grounds all come back to the SLB board. The speaker protection board just passes ground straight across so it's a direct connect back to the SLB from the binding posts. Since the input is shielded wire, it's connected at the Alpha input terminal.