Audiomat Owner’s Resource Over the last couple of months I have scoured the internet to find everything I could about Audiomat amplifiers, their design specs, and how to run them to achieve top performance. As many of you know, there is not a lot out there, a few crumbs here and there and a few forum comments from folks who have owned them. This thread will hopefully act as a place to deposit knowledge and experience on all Audiomat amplifiers.
For years I have lusted over and looked forward to acquiring one of these amps, last week I took the plunge and purchased a Prelude Reference. The amp arrived in great shape from a great CAM member, thanks Jim! Upon first listen I was very impressed by the power and authority from this amp. It never seems to lose composure and it is very quiet, not a bit of noise while idle. I do notice though that there is something uneasy about parts of the frequency spectrum, some very subtle distortion is making it hard for me to fully engage in the music. So what’s up with that?
These amps are special?The forums and some reviews elude to these amps being mysterious in that somehow, only an Audiomat tech is capable of working on them. I really hate not being empowered, so I need to know why it is so crucial. I have come to find out some of the things one must consider. Please feel free to correct any of my assumptions…..
1. Preamp is passive, the three tubes in the amp are drivers and a phase splitter.
2. Power tubes have a fixed bias, which means that tubes must be selected to perform in a specific range of operation that will require closer matching of power tubes.
3. I remember reading somewhere that this amp has no global negative feedback…this will become important later.
4. Prelude Amp was originally voiced and came stock with 1960’s Sylvania 12ax7’s and Winged C Svetlana EL34.
So, back to my little distortion bug…….
The first thing I do is suspect the tubes, so I roll most of my good vintage 12ax7’s in and out of the amp over the course of the next day. I notice differences in the presentation but the distortion stays, incidentally I observe less difference in the sound during tube rolling than you would normally expect to observe in a tube preamp.
Next, I roll in an identical set of winged C’s I have lying around, nope…same.
At this point I am convinced of a couple of things. I start to wonder if the nice Valvo 12ax7 tubes that came in the amp were stock? All indications are…no. So, based on that assumption I take the leap and assume that somewhere along the way somebody popped in these nice tubes and then neglected to adjust the two pots that control AC balance in the amp. This, I believe, is one of the reasons why folks are adamant about the need to have these amps “tuned” by Mutine.
AC Balance? Lots of amps have pots for AC balance. I have a Dynaco ST-70 with a new board that has AC balance pots on it, some Scott amps have them, and Citation, Audio Research……many amps have this adjustment. In most amps this adjustment is not that critical, but after reading some forums I am convinced that in some amps, this is very critical! I eluded to a post I saw about there being no negative feedback in the Audiomat amps, is this true? I also saw a post stating that amps with no negative FB will be very susceptible to distortion if AC balance is not properly adjusted…..as in just put the pot in the middle. Hmmmm…
I am now convinced that my amp had some tubes rolled in but the pots never adjusted, here is one way of explaining how to do it. The methods vary slightly but the idea is to have balanced signal at the power tubes.
http://dynacotubeaudio.forumotion.com/t270-ac-balance-pots-on-boardNow here is where I lay my disclaimer, I am not an electrical engineer so delving into the various designs and circuits to any degree is beyond my level. What I do have is pretty well trained ears, so based on my experiments leaning towards the amp being out of adjustment, I figure I should be able to hear improvement if I happen to make it happen. So, I start spinning pots…..I trim by ear listening for the distortion artifacts to guide my adjustment. The conditions for this were not ideal but I did manage to get the amp sounding much better, to the point where the nagging obstacle preventing me from fully enjoying the music, was now almost gone. Now, the amp sounds very good! I know I have some adjustment to do that will require more than my ears but I was pleasantly surprised to see that it was possible to achieve gains by ear and that my theory was starting to look correct. I’ve seen other threads relating to this ac balance topic with huge observable gains made after adjusting, but I do think it is very dependent on the circuit.
Today I have 4 nice 1969 Sylvania 12ax7’s coming in the mail, I will drop them in and attempt another “AC balance by ear” tuning on it, wish me luck! I do want to get this done properly and will either find a tech in Ottawa that can do it, preferably one who has a distortion analyzer…..but it can be done with a scope as well, or I will take it to Montreal to the Mutine suggested tech.
Please feel free to share your experience with these wonderful amps, especially if you have had one re-tubed or are able to share any knowledge in the effort to demystify these amps and educate other folks on how to keep them running the way they were intended.
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