I can say at the moment that using around 18R as drain resistors for the J-fets in the input stage (around 45dB OLG) sounds very different and for my ears much better than the version with 70dB OLG.
When I used like Lynn some time ago a 1k load, I had not such a good impression.
But sound comparing in memory is fairly nonsense.
Of course the first nights with a new build are mostly the most exciting..... :--))
The sound is much more sweet, and airy. Maybe a bit too soft, I must try a bit higher values maybe up to 100R.
And Ian, I see no danger for the J-Fets or unusual hard work when I compare the situation to their place in F4 or F6. Here we have source resistors with about 10R and rails with +/-23V, don't they have a harder life here?
In my build they see only +/-12V because of cascoding.
When I used like Lynn some time ago a 1k load, I had not such a good impression.
But sound comparing in memory is fairly nonsense.
Of course the first nights with a new build are mostly the most exciting..... :--))
The sound is much more sweet, and airy. Maybe a bit too soft, I must try a bit higher values maybe up to 100R.
And Ian, I see no danger for the J-Fets or unusual hard work when I compare the situation to their place in F4 or F6. Here we have source resistors with about 10R and rails with +/-23V, don't they have a harder life here?
In my build they see only +/-12V because of cascoding.
Interesting. Perhaps some of the sound difference may be down to the different damping factors.
When I said "work hard", I meant that driving a lower load means more current variation through the JFETs for a given signal level which typically results in more distortion. I didn't mean to imply that you were exceeding their rating or anything like that.
When I said "work hard", I meant that driving a lower load means more current variation through the JFETs for a given signal level which typically results in more distortion. I didn't mean to imply that you were exceeding their rating or anything like that.
It might be easier to understand the signal levels by working backwards from the amplifier output. For the XA25, the two voltage gain stages are the JFETs (K170/J74) and the VAS (K2013/J313), ignoring the relatively small voltage attenuation from the output stage. For a given output level Vout, the input to the VAS must be Vout/A(VAS), where A(VAS) is the voltage gain of the VAS. Thus, the necessary voltage input to the VAS is unaffected by changing the JFET load resistors, but, as Ian said, the JFET currents are increased. Similarly, if the gain of the VAS is changed by lowering the load resistance on the VAS output, then the required input to the VAS must be increased, thus increasing the JFET currents.
What magnitudes are involved here? With VAS stage gain of 80dB (assuming a 33K load on the VAS output) and an output voltage of 4V Peak (1Watt into 8R), the input to the VAS must be 400uV peak. With 100R JFET drain load resistors the AC current must be 4uA peak. With 10R JFET drain load resistors the AC current must be 40uA peak. Are either of those AC currents large enough to cause significant distortion from the JFETs?
You wouldn't think that such low values of current variation would have much impact but then these are only at 1W output. However, I made my comments based on the distortion figures shown by simulations with varying resistances and the changes appear very real though I should perhaps add the caveat that we are talking fairly low levels of distortion anyway. All counts though if we are aiming at figures like .00x%
Here is another avenue to explore. Put feedback around the FE only, thus loading the FE with the bias neterok and the feedback network. The output damping factor will become low, determined only by the transconductances of the output FETs. And the harmonics will be mostly defined by the OS, since the FE will have very low distortion.
XA25 Home Brew Line Cord
I am hesitant to perpetuate snake oil. I have been building power cables for decades; not so much for high fidelity, but to accommodate needs of placement. I can't say that I could identify any one power cable over another. I have tried; both home brew and commercial. Recently I came to appreciate that my Equitech conditioner is said to have lower output impedance than the wall socket. This caused me to consider that maybe a line cord could contribute to high fidelity because it would not be shackled by a 50 ft run of Romex. Thus, perhaps transients would be available to the line cord that are not available at the wall receptacle.
Because the XA25 has incredible resolution; I thought I would go all out on a longer power cable from the XA25 to the Equitech 20A at the rack. If there was ever an amplifier that might reveal a nuance born of a power cord, it would be the XA25. I have been using the line cord that shipped with the XA25. The XA25 is on a stand between the speakers I am removing the remaining electronics from the sound stage area.
Thus, I built a 15 foot line cord. There is a separate run for each of three conductors. Each run consists of silver multi-conductor instrumentation cable. Each conductor is silver shielded with 4 X 20 ga (14 gauge effective) Teflon insulated conductors and Teflon jacketed over the shield. The conductors are terminated with Furutech FI-50 NCF(R). The shields are single ended at the male plug (Equitech end). This line cord was crazy expensive to build.
I may need to readjust my thinking about power cords. The XA25, to my ears, just took another step into a darkness that was already pitch black. The dynamic range kicked up a notch. Vocal definition is up a notch. Newly appreciated sounds. If you happen to enjoy haunting music; the XA25 may scare you to death.
I am hesitant to perpetuate snake oil. I have been building power cables for decades; not so much for high fidelity, but to accommodate needs of placement. I can't say that I could identify any one power cable over another. I have tried; both home brew and commercial. Recently I came to appreciate that my Equitech conditioner is said to have lower output impedance than the wall socket. This caused me to consider that maybe a line cord could contribute to high fidelity because it would not be shackled by a 50 ft run of Romex. Thus, perhaps transients would be available to the line cord that are not available at the wall receptacle.
Because the XA25 has incredible resolution; I thought I would go all out on a longer power cable from the XA25 to the Equitech 20A at the rack. If there was ever an amplifier that might reveal a nuance born of a power cord, it would be the XA25. I have been using the line cord that shipped with the XA25. The XA25 is on a stand between the speakers I am removing the remaining electronics from the sound stage area.
Thus, I built a 15 foot line cord. There is a separate run for each of three conductors. Each run consists of silver multi-conductor instrumentation cable. Each conductor is silver shielded with 4 X 20 ga (14 gauge effective) Teflon insulated conductors and Teflon jacketed over the shield. The conductors are terminated with Furutech FI-50 NCF(R). The shields are single ended at the male plug (Equitech end). This line cord was crazy expensive to build.
I may need to readjust my thinking about power cords. The XA25, to my ears, just took another step into a darkness that was already pitch black. The dynamic range kicked up a notch. Vocal definition is up a notch. Newly appreciated sounds. If you happen to enjoy haunting music; the XA25 may scare you to death.
XA25 Warm-up Plus Signal Processing Required?
A reviewer wrote that a month or more or burning in was required for HPA-1/XA25 to settle in. But then, the reviewer wrote that they were not sure of what, or who, was breaking in - the XA25 or the reviewer.
I have noticed that after many hours of quiet warming, the XA25 appears to require an additional time of playing music before coming entirely to life. I am wondering whether this is due to the XA25 or, instead, due to an adjustment toward my open loop gain as relaxation reduces negative feedback to the ossicular chain? By way of negative feedback,the stapedius and tenser tympani muscles have been said to adjust sensitivity and filtering at the middle ear. It might be helpful to hear the saber toothed tiger step on a twig. Perhaps the reviewer was onto something by questioning what or who was breaking in?
A reviewer wrote that a month or more or burning in was required for HPA-1/XA25 to settle in. But then, the reviewer wrote that they were not sure of what, or who, was breaking in - the XA25 or the reviewer.
I have noticed that after many hours of quiet warming, the XA25 appears to require an additional time of playing music before coming entirely to life. I am wondering whether this is due to the XA25 or, instead, due to an adjustment toward my open loop gain as relaxation reduces negative feedback to the ossicular chain? By way of negative feedback,the stapedius and tenser tympani muscles have been said to adjust sensitivity and filtering at the middle ear. It might be helpful to hear the saber toothed tiger step on a twig. Perhaps the reviewer was onto something by questioning what or who was breaking in?
Last edited:
