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Regarding ABX testing of the amplifiers, I believe using a comparator that can switch at any desired rate is a better way to test but we can do it without the ABX comparator. David did testing of the comparator before he would use it and found it to be transparent. He was so impressed with the usefullness of the technique that he is researching building his own.

The same effect can be achieved by printing out a random sequence of trials which can be done with tools on the web. One person (the switcher) who will not participate in the test voting will switch the amps back and forth during the familiarization phase announcing to those at the test which amp is in play. When the identification part is desired, the listeners will inform the switcher who will connect the amp so indicated on the random trial sheet obviously without letting anyone know which amp it is. The users will then vote. It is slower than the comparator box but if there is any doubt it can be run that way. There can then be no doubt about a comparator causing differences because it will not be in the loop.

Tom

My test on the ABX box was one of looking for issues with distortion in the relay contacts which I measured on my curve tracer. I could not find any problems with the contacts, but after a lifetime of having issues with relay contacts, I still have skepticism. Also, I did not check to see about things like whether the chassis was steel, etc. But the ABX box is very useful for checking for issues of tonal balance between amplifiers, and helped me correct an issue in one of my proto amps that had a high-frequency anomaly when compared with Tom's Crown. I addressed something that was affecting the region of 50 kHz-100 kHz and this fixed it. It only happened with some music, and it would be harder to discover without the ABX box. That said, I expressed an interest in building a solid state ABX box in a non-magnetic box to see if this would be better, worse, or no difference. Still on my to-do list.

David

In addition to the measurements taken with the curve tracer, we drove a resistive load with music from an amp through the box. We then connected an input lead from another amp that was connected to a speaker across the relay contacts and listened to hear any distortions that may be the result of rectifying action by the relay contacts. We could not hear any distortion confirming that the contacts were basically a very low resistive load.

Tom

Thanks for starting a new thread.

As for me personally, I'd love to lay this thing to rest. Either I can hear differences between different audio components, or not. I'd be happy to be a "listener" in this test if Dave and Charlie are both on board. It would have to be done manually (without the ABX box) and without anything attached to the 845 amps besides interconnects and power cords. Add anything you like to the Crown. Voltage match the amps with a 1khz tone for level.

I do appreciate you inviting me to hear your system back in March. I know Dave's 300b amp sounded exactly the same as your Crown, however it also "performed" exactly the same way. That's a new term I'm throwing in here. By that I mean the "presentation" was exactly the same also. No difference in the image, width, or depth to the soundstage. I've never heard 2 amps "sound" and "perform" exactly the same way. At this moment, I have to believe it's the devices in circuit that are adding "their sound" to the amplifiers. Digital crossover, ABX box, etc.

I would love to be proved wrong. I don't think I will be, but I would love to be wrong. The savings account I opened last week for a pair of Dave's amps could go to better use. At least in my wife's opinion.

I wish there wasn't a significant difference in how amps sound/perform/present/soundstage etc. Jim, Greg Takesh, Ira, Charlie, and myself have observed big differences in amps. In one case in Charlie's system, a mono set of Hypex amps sounded just fine until the music became complex, loud, or dense, and it fell apart- for instance Olga Kern playing Islamey on piano, beautiful on Charlie's amps, each note individual even when a torrent of keys were played; the Class D amps @ 200 WPC were close on slow paced segments, but as soon as Olga turned up the wick, the piano became a homogenized mess with no individual notes and it sounded like she was banging on the keyboard? It was an unexpected surprise how much better David's 845 amps were, and a depressing feeling as $73,000 is not in the budget, but we all wanted a pair.

The past few comments are exactly why I would love to run this test. The amps may sound the same or they may sound different. We can do it without the ABX box. BTW, if David ever gets his box done it might be a hoot to test both his and mine against no box in a double blind test. That would confirm whether or not the boxes add/subtract sound, although in the grand scheme it is a moot point because other than testing you don't listen through an ABX box and you can do the double blind tests without them if desired. It is just harder and you lose some of the ability to control the time interval between the selections.

The main thing is to eliminate what I call the "wandering in the desert" syndrome where you have no base reference for your decisions. Remember, the purpose of a double blind test is not to decide what component is "better" or even what you like, but whether or not there are audible (forget about measuring in this context) differences in two components. I realize this is asking a lot of Charlie and David, but I will be willing to contribute whatever time and effort it will take to do it properly because it should be very revealing regardless of the outcome.

Tom

The "wandering in the desert" phenomena is why I keep my system pretty stable. Gives me a reference. My DAC is more than 20 years old, turntable and tonearm are more than 10 years old, speakers are 12 years old.... you get the idea. That way when I change something I have a pretty good idea where we started.

I'm willing to do the comparison although I won't have time until June sometime, the next month is crazy for me. (Anything to get those 845 amps back in to the system)

I will point out however that we listened to 3 different amps last week and it was pretty obvious that each of them sounded different. Two of them were Dave's designs so we know that they were competently engineered -- always a question with my stuff.

Over the past year I've listened to several amps in the system at various people's request. You saw what Kemper had to say. We once tried one of Dave's smaller production amps in the system and they didn't get along very well either. In each situation the differences were obvious. Maybe the speakers are just a hard load to drive.

Charlie

Charlie:

Thank you very much for offering to do the test down the road. I hear what you say about amps sounding different and I also know based on my testing over the years that all amps do NOT sound the same. There are lots of amps out there with significant shortcomings that prevent them from providing the best sound. I have heard terrible sounding transistor and tube amps. In fact I have owned some of both of them. In some cases, limitations in an amp if they are relatively benign can also offset other problems in a system. Not my favorite way to fix problems though.

The point I am trying to make is that if you have really good amps regardless of the design philosophy used they will not be the limiting factors in the sound. Although it is not a trivial matter, it is possible to make extremely good sounding amplifiers with a range of technologies. Each technology has a set of complications that must be solved for the results to be "great". Mess up any of them and the performance will be sub par.

I have a question. I don't remember what your preamp is. If it has high output impedance, ie more than 1K, I will have to build a buffer for the Crown so that it does not drag the preamp signal down. The input impedance if used in the unbalanced input mode is 5K.

Tom

Tom,

I use a passive preamp with 10K pot, DAC is SS so output shouldn't be a problem. Output on my phono stage is a 6SN7 cathode follower so it should be OK. Few hundred ohms at most although I haven't measured it.

Of course the different input impedences will change the characteristic of the sound but then again so would an input buffer. And so it goes.

Charlie

Isn't it fun! If the output impedances of the DAC and phono stage are low, it should be no problem. During the test we will drive both amps at the same time so that there will be no change to the drive because of different impedances as we switch between the amps. Only the output will change. I'll provide short "Y" cords to do that. The lead length to your amp will be very short in the range of inches with a longer cord from the "Y" at your amp to the Crown.

Tom