I don't recall any TL431's in the XA25, so those would be opto isolators.
Additional comments about setup for the BCD-3/HPA-1/XA25. These components were phased into my system over a few weeks.
First the HPA-1 replaced my Classe/Perreaux preamplifiers. Very much to my surprise, I noticed a huge jump in bass definition and extension. I was not expecting a preamplifier to effect the bass. I was using a Perreaux power amp at the time. There was an overall jump in musicality and darkness. I was extremely happy.
A funny thing about the HPA-1. I have a partially built B1 on the shelf. I have separate resistance ladders for each channel. I read everything that I could find about the HPA-1. There appeared to be no provision for balance control. I wrote Pass Laboratories and asked if the volume shaft was ganged. I received a single sentence reply from Pass. "There is no balance control on the HPA-1." I got the sense that asked an offensive question. Nonetheless, I bought the HPA-1 and the sound stage was perfectly balanced. I was relieved. But then......
After a few weeks the XA25 arrived. The XA25 may have shipped with the power button activated. I plugged it into my powered down 20A Equitech. I flipped the power switch on the Equitech and blew a 20 amp circuit breaker. One of the reasons I stayed with lower power was to avoid needing a mega-dollar power plant in the basement. I unplugged the XA25, reset the breaker, and powered the Equitech. Thinking that the XA25 might have shipped on. Or that I might have flipped the switch while unpacking. I flipped the XA25 switch, powered down the Equitech, plugged in the XA25, and then flipped the power switch on the Equitech. This time the breaker did not trip. The XA25 was off. I turned on the XA25 and all has been fine since. But.....
The XA25 blew me away from the start. And it just got better. There was nothing to miss about my previous hardware. But then I noticed that I was having difficulty locking in a sweet spot. I was listening to the CM Savant Windows CD and lamenting that the piano was so biased to the left. Blaming the sound engineers. But then I noticed that other CD's that I could have sworn were center stage were leaning to the left. I noticed vocals were leaning to the left depending pitch. I was wearing out the carpet shifting my chair. I went to work on the speakers with levels and protractors and tape measures. Silver lining; I got the geometry absolutely perfect. But still there was smearing to the left. It is perhaps a good thing that there is no balance control on the HPA-1. I might not have noticed that I had a suboptimal speaker connection. The insane resolution of the HPA-1/XA25 alerted me to the problem in the first place.
A funny thing about the XA25. The fiercely transient attack on a high piano note is unbelievable. A pin point image floats into the listening room. A call to Ghost Busters. Then, I listened to a CD that is entirely unlikely I would ever listen to. The recently released self titled Slowdive by Slowdive. I think they call it toe gazing music. The sound stage floods the entire room in an overwhelming way. As the music moves past you, you move forward. You are captain Kirk. Total sensory overload. All of this at a comfortable listening level - 10 o'clock on the HPA-1
My point is that I never had any reason to consider that the XA25 may have another gear beyond burn in. My Equitech has additional filtering and isolation options. After about a month of listening to the XA25 I thought to ask myself, "Did I plug the XA25 into the high current receptacle?" I had not. The XA25 was plugged into a filtered outlet specifically not recommended for power amplifiers. I changed the line cord to a high current receptacle. The XA-25 had another gear.
First the HPA-1 replaced my Classe/Perreaux preamplifiers. Very much to my surprise, I noticed a huge jump in bass definition and extension. I was not expecting a preamplifier to effect the bass. I was using a Perreaux power amp at the time. There was an overall jump in musicality and darkness. I was extremely happy.
A funny thing about the HPA-1. I have a partially built B1 on the shelf. I have separate resistance ladders for each channel. I read everything that I could find about the HPA-1. There appeared to be no provision for balance control. I wrote Pass Laboratories and asked if the volume shaft was ganged. I received a single sentence reply from Pass. "There is no balance control on the HPA-1." I got the sense that asked an offensive question. Nonetheless, I bought the HPA-1 and the sound stage was perfectly balanced. I was relieved. But then......
After a few weeks the XA25 arrived. The XA25 may have shipped with the power button activated. I plugged it into my powered down 20A Equitech. I flipped the power switch on the Equitech and blew a 20 amp circuit breaker. One of the reasons I stayed with lower power was to avoid needing a mega-dollar power plant in the basement. I unplugged the XA25, reset the breaker, and powered the Equitech. Thinking that the XA25 might have shipped on. Or that I might have flipped the switch while unpacking. I flipped the XA25 switch, powered down the Equitech, plugged in the XA25, and then flipped the power switch on the Equitech. This time the breaker did not trip. The XA25 was off. I turned on the XA25 and all has been fine since. But.....
The XA25 blew me away from the start. And it just got better. There was nothing to miss about my previous hardware. But then I noticed that I was having difficulty locking in a sweet spot. I was listening to the CM Savant Windows CD and lamenting that the piano was so biased to the left. Blaming the sound engineers. But then I noticed that other CD's that I could have sworn were center stage were leaning to the left. I noticed vocals were leaning to the left depending pitch. I was wearing out the carpet shifting my chair. I went to work on the speakers with levels and protractors and tape measures. Silver lining; I got the geometry absolutely perfect. But still there was smearing to the left. It is perhaps a good thing that there is no balance control on the HPA-1. I might not have noticed that I had a suboptimal speaker connection. The insane resolution of the HPA-1/XA25 alerted me to the problem in the first place.
A funny thing about the XA25. The fiercely transient attack on a high piano note is unbelievable. A pin point image floats into the listening room. A call to Ghost Busters. Then, I listened to a CD that is entirely unlikely I would ever listen to. The recently released self titled Slowdive by Slowdive. I think they call it toe gazing music. The sound stage floods the entire room in an overwhelming way. As the music moves past you, you move forward. You are captain Kirk. Total sensory overload. All of this at a comfortable listening level - 10 o'clock on the HPA-1
My point is that I never had any reason to consider that the XA25 may have another gear beyond burn in. My Equitech has additional filtering and isolation options. After about a month of listening to the XA25 I thought to ask myself, "Did I plug the XA25 into the high current receptacle?" I had not. The XA25 was plugged into a filtered outlet specifically not recommended for power amplifiers. I changed the line cord to a high current receptacle. The XA-25 had another gear.
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he's mostly one sentence guy
when you think of it .... in 95% of situations , that's practically ideal approach
In one sense, you set them loose by eliminating the degenerative; In another sense, you make them behave with the negative?
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I don't recall that the amplifier has a 60 dB open loop.
The point of eliminating the degeneration is not to increase the gain, but
to achieve better complementary push-pull cancellation of distortion. Two
complementary purely square law parts operating push-pull end up with
zero distortion. Adding resistance in series with the Sources muddies
that up.
Getting around this for the XA25 vas stage actually involves more loading
of the input devices, which is compensated by the greater gain of the
undegenerated vas transistors. Thumbnail calculation says the olg
should be roughly the same as previous XA circuits.
In any case, we are only interested in the sound, and our tests clearly
reveal a desirable effect of eliminating degeneration in pp circuits.
What can I say? We can't make enough of them.
Thank you for the explanation about push/pull cancellation of distortion. And thank you for making the XA25. It inspired me to purchase the listening chair I have wanted.What can I say? We can't make enough of them.
I was nearly ready to throw in the towel on digital and go with an analog front end. Not so fast. The BCD-3/HPA-1/XA25 has astounding musicality. It is incredibly easy to listen do. Music simply happens. It puts the life into you rather than sucking the life out of you. I still have tweaking to do and then my analog friends will be auditioning the XA25.
My salesman doubted that the XA25 would handle Aerial 7Bs (86db/50W Minimum). I don't get out much, but I don't appreciate any lack of power. In fact, the Aerials, to my ears, have more weight on the XA25 than on my old CA200.
I can't imagine what a speaker "upgrade" would sound like. Nonetheless, I am auditioning Devore Super 9s and O96s in the coming weeks. It is not clear to me that I should even be concerned about efficiency in the case of XA25. 99% of my listening is at 10 o'clock on the HPA-1. The sound is not thin. But, I have an open mind.
Any thoughts about what to expect from the XA25 driving Devore Super9s (91db/8 ohms) compared to the O96s (96db/10 ohms) would be greatly appreciated. Thanks again to folks for commenting.
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Thank you for commenting. The 7Bs sound great to me. As I said, I don't get out much. This is my first major upgrade binge in nearly 20 years. I would like to get through it and then simply listen to music. There is a Devore dealer less than 90 min from me. One the reasons I am auditioning Devore is simply because I can. I am certainly not offering the 7Bs for trade in. The dealer will be using Luxman. I don't know whether I should take the XA25 with me.According to steteophile measurements your speakers are much better than ones your sales person suggest you to get. Never liked Devore Super 9s or O96... xa25 has plenty of juice to deal with any speakers especially if you have a gain stage in front.
Further, I may be retiring soon. I have a ton of electronic components on the shelf and a day/night job that keeps me from them. I hope to build/design low wattage audio amplifiers in retirement. The O96 interests me for that reason. Then a friend's son may soon build a Spawn Haruna cabinet for me. And so forth and so on. I have a large finished basement to play in. Thanks again.
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I don't recall that the amplifier has a 60 dB open loop.
...
What can I say? We can't make enough of them.
It is good to hear that the XA25s are doing so well.
I have built something similar to the XA25 which has the circuit topology shown in the image below. Simulations of the amplifier show the open-loop gain (OLG) as follows:
[FONT=Helvetica, Arial, sans-serif]Rldfe OLG[/FONT]
[FONT=Helvetica, Arial, sans-serif]none 70dB[/FONT]
[FONT=Helvetica, Arial, sans-serif]2.2k 52dB[/FONT]
[FONT=Helvetica, Arial, sans-serif]1k 45dB[/FONT]
[FONT=Helvetica, Arial, sans-serif]The OLG of the built amplifier is slightly lower than the simulations.[/FONT][FONT=Helvetica, Arial, sans-serif]none 70dB[/FONT]
[FONT=Helvetica, Arial, sans-serif]2.2k 52dB[/FONT]
[FONT=Helvetica, Arial, sans-serif]1k 45dB[/FONT]
Attachments
more loading - most probably means lesser (than usual) ohmic value of drain resistors on input JFets , thus smaller gain (than usual) of input JFets
though , by simulations of Real McCay ....... OLG is damn high ......... even if we count on fact that in vivo it is smaller , it seems Pa started making MC preamp , ending with wakoo power amp
.....
What can I say? We can't make enough of them.
Baaaad Pa !
Goood Pa!
