Does anyone care to ask an amplifier designer a technical question? My door is open.

I closed the cable and fuse thread because the trolls were making a mess of things. I hope they dont find me here.

I design Tube and Solid State power amps and preamps for Music Reference. I have a degree in Electrical Engineering, have trained my ears keenly to hear frequency response differences, distortion and pretty good at guessing SPL. Ive spent 40 years doing that as a tech, store owner, and designer.
Perhaps someone would like to ask a question about how one designs a successfull amplifier? What determines damping factor and what damping factor does besides damping the woofer. There is an entirely different, I feel better way to look at damping and call it Regulation , which is 1/damping.

I like to tell true stories of my experience with others in this industry.

I have started a school which you can visit at There you can see some of my presentations.

On YouTube go to the Music Reference channel to see how to design and build your own tube linestage. The series has over 200,000 views. You have to hit the video tab to see all.

I am not here to advertise for MR. Soon I will be making and posting more videos on YouTube. I don’t make any money off the videos, I just want to share knowledge and I hope others will share knowledge. Asking a good question is actually a display of your knowledge because you know enough to formulate a decent question.

Starting in January I plan to make these videos and post them on the HiFi school site and hosted on a new YouTube channel belonging to the school.

@ffzz, you’re welcome!

My former VAC Renaissance 70/70 MkIII amp is a class A amp employing four 300B power tubes per channel, in a push-pull parallel configuration, and is rated at 70 watts per channel. It is a 100+ pound beast, which I believe consumes something like 700 watts of AC at all times, converting most of that power into heat that is injected into the room.

The Pass XA25, as you realize, is rated at 25 watts into 8 ohms and 50 watts into 4 ohms, operating in class A, and it is specified as consuming 240 watts of AC. John Atkinson’s measurements that were provided in conjunction with Stereophile’s review, though, indicated that it is capable of providing 80 and 130 watts into those impedances, respectively. He stated that some of that disparity is due to the fact that Pass bases its power ratings for the amp on much lower distortion percentages than JA uses, and presumably a lot of that disparity reflects the XA25 transitioning to class AB when outputting more than a certain amount of power.

As I mentioned in my earlier posts that you saw my speakers are nominally 6 ohms, and have an unusually flat impedance curve as well as relatively high sensitivity, which makes them very versatile with respect to amplifier selection. Since like most solid state amps the XA25 is designed to provide an essentially constant output voltage into varying load impedances, for a given input voltage, (as long as the amp is operated within the limits of its maximum voltage, current, power, and thermal capabilities), even if we assume the very conservative 25 and 50 watt numbers it can be calculated that it is capable of providing at least 33.3 watts into 6 ohms.

I wouldn’t say that the XA25 has "an edge over the VAC amp." They are both wonderful amps, in their own ways, and some non-sonic reasons factored into my decision to change. See the comment I left two days ago near the end of the following thread, as well as a subsequent comment by member "1markr":

Best regards,
-- Al
A question on another topic. My Marantz SA-11S2 has an Spdif through rca. I would like to add a BNC connection to go to my dac. Can I just install the BNC and jumper over with 75 ohm coax inside the player?

Thanks again for your thorough explanation. As a big fan of SET amps, I am delighted to hear your experience of XA25. Should I ever endeavor into the ss world, it seems that XA25 would be the natural candidate to try out.

@flashbazbo, the caveats that occur to me regarding adding a BNC output to your player are:

1) Be sure to select a 75 ohm BNC connector, rather than a 50 ohm BNC connector.

2) Looking at rear panel photos of the player, as might be expected it appears that the ground shell of the RCA jack is isolated from the metal panel. So you would want to select a BNC connector that is similarly isolated. Such connectors are readily available, but many BNCs are designed such that their ground shell is connected to chassis.

3) Waveform quality might benefit, at least slightly, if you were to disconnect the internal connection to the RCA jack and connect it directly to the BNC, rather than jumpering between the two connectors. You would not want to use both connectors simultaneously anyway, as significant impedance mismatches would result if 75 ohm loads were applied to both outputs.  And if a need ever arose to connect an RCA plug to that output, BNC-to-RCA adapters are readily available.

-- Al
What Al said- The issue here is something called 'characteristic impedance'. The BNCs are designed for cables with a characteristic impedance of either 50 or 75 ohms. Without getting into the designer's head, it uncertain what CI the RCA connection is actually working with, since RCAs have no termination standard and are used with all sorts of impedances. Hence Al's comment about waveform quality; reflections of the signal (noise) may be the result. 

could you explain what shorting plugs are?
They are connectors that have all their connections shorted together. Installed at the input of an amp or preamp, in this way one can be sure that no noise is present at the input of that piece.