Recently re-built st-70 report

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Recently re-built st-70 report

Postby xlr8 » Wed May 05, 2010 11:53 am

Hello all,

I have recently re-built 2 st-70's with VTA boards (high gain) and original transformers (except for one replacement output tranny). All of the cap upgrades have been included. I opted for the russian PIO coupling caps over the auricaps they offer. I'm using EH EL34's for now, sovtek GZ34 rectifiers and JJ 12AT7's on the boards. As I come closer to finishing my speaker project, I have been listening to these 2 amps driving the midrange and tweeters in a vertical biamp config. Over a period of about 2 months, these amps have burned in and have morphed into lush and very liquid sounding pieces of equipment. I am very impressed to say the least. I have been working out of town for the better part of april and have had only cheap headphones and my computer to listen to. Coming home to my system has caused me to think crazy thoughts like shutting down my business and just sit here and listen.

The folks at Audio Alternative in Lilburn Georgia have always been very nice to me and although I have NEVER had the cash to drop on their EXTREMELY expensive gear, they have always been very helpful. In fact, if not for them, I may have never started this DIY journey in the first place. They were the ones who backed up my idea of tri-amping my old set of Maggie MG3a's. I drove those speakers into the ground with a 6 channel kenwood home theater amp that was rated at 110wpc with very good results. Not even close to what I have now though. The Maggie tweeters driven with solid state equipment border on the harsh side of sound. Having owned a couple of st-70's in the past, I wondered if tubes would help solve that problem, which they certainly have. As you all probably know, Maggies need quite a bit of power to produce bass. I have found the dirty little secret is that the tweeters do not need a terrible amount of power to produce truly excellent highs. In fact, they seem to be very sensitive and when the stock crossover is removed and replaced by an active filter, even better. Crystal clear piano, cymbal, tympany, triangle etc.

The ONLY reason I even tried to build a set of speakers in the first place was because my old maggie's coils just would not stay glued to the mylar anymore. Probably just needed to be taken down and cleaned really well, but the fact that they needed so much power was discouraging. I knew the amp I was using was a POS, and longed for tubes but could not afford a lot of the gear that is out there. I recalled my old set of st-70 driven Lascalas. I remembered how the music moved me in a way that now makes me sad that I ever sold them or the amps. Also, I had what I knew was an excellent set of tweeters on hand and ready to be utilized in some way. Magnepan offers a truly awesome trade-in deal on these, so I took advantage of that and now have a brand new set. Thanks to the modern age and the advent of the internet, I was able to find examples of speakers that others have built over the years and I began to put a set of design goals together. They are as follows:

Fairly high sensitivity
Simple to construct
Easy load to drive
Crystal clear sound
Loud enough to hear while in the backyard
Cheapest components
Flat response
True "High End" sound

I can proudly state that I have certainly accomplished these goals. Ok, so maybe "simple to construct" depends on one's ability and the availability of tools, but fairly straightforward nonetheless. As I was wondering how I could emulate maggie operation and integrate the tweeters into the design, I stumbled upon the line array theory. This made total sence to me, as a line of stacked drivers should operate in a similar manner as the maggie diaphram, yet without the load problem. Turns out I was half right. Dipole midrange is spot on, but bass is a different story. The line consists of 12 Vifa logic 5.25" truncated drivers per speaker. After a bit of experimentation, I decided to suplement the mid bass with a couple of cubes which contain 4 Vifa Logic 7" drivers in a compound push-pull arrangement. They are driven with a Crown XLS 402 that was sitting in a closet for several years. Finally happy with the bass, I decided that the icing on the cake would be a good sub. I buit a box for 2 JL Audio 12w7 drivers and power them with a new Behringer EP4000. All cables are DIY and consist of RG-6 innerconnects and cat 5e for speaker wire.

Sound:

Ok folks, so you may ask yourself "why in the world would someone build such a monstrosity and put it in their home?" The simple answer is that I love music. I have been playing musical instruments since I was a small child. I started with the piano, then moved on to brass (high school marching band), some percussion and have finally landed on the guitar. Asking me what my favorite type of music is is like asking a woman what her favorite type of chocolate is. A better question would be "Is there any music you don't like?" The answer would be most rap and a lot of the new country music out there. Other than that, I'm open to just about anything. I have surpassed my design goals. I no longer feel that the closest I'll ever get to the music is a live performance or hanging out at the shop I mentioned. These 2 70's I re-built have been given the opportunity to truly shine here and I consider them the heart and soul of my system. These amps allow me to hear John Coltrane breathing into the sax. I can hear the reed vibrating on the mouthpiece. I'm hearing things in recordings I've been listening to for years that I never heard before. Instruments have a sort of realistic texture that I'm not at all used to hearing in my own home unless I'm the one playing it. I have read so much garbage about "realistic representation" of performers and their instruments played back through a good Hi Fi, but I'm learning that you can only get as good as the recording. I now understand what people mean when they say that certain components are "transparent".

http://www.flickr.com/photos/48990852@N08/

The above link brings you to a shot of a set of home made speakers that are really too large for the room, as well as the cheap equipment I'm using. When I open the double doors in the back of the room, the stereo image extends out into the yard. I think they actually sound better when listening in the far feild like this. It sounds as if there is an actual band playing inside my house when I go outside to listen. I have literally hundreds of photos of neighbors who have come over and danced all night listening to my progress, and that is one of the things I have enjoyed most...ENJOYING the music. People have said that at high volume it is extremely loud, yet zero audible distortion. "As if i'm in the front row at a concert" when inside the house. More like the lawn seats at Chastain Ampitheater when outside. That's one design goal I am proud to have met.

Just had to share, as I am very close to completing the speakers. I have been on this journey since my teens and although I may continue to play with different Preamps, crossovers, eq, cables etc., the speakers and the amps that drive them will probably be here, even after I'm gone. I'd like to thank the ENTIRE DIY community for sharing your thoughts and your expertise with all of us laymen out here who cannot afford the comercially available stuff out there. When I listen to my system, I sort of feel sorry for the entire audio industry. It takes serious coin to get sound like this unless you're willing to take the time to design and build something affordable and scatch that itch yourself.

Again, happy listening and thanks to all of you!!!!

Oh, BTW, WAF becomes a non-issue once they hear what something like this can do.
xlr8
 
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Postby mesherm » Wed May 05, 2010 2:44 pm

Those speakers remind me of Infinity Reference speakers from the 80's.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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Postby xlr8 » Wed May 05, 2010 3:59 pm

Those speakers remind me of Infinity Reference speakers from the 80's.


Were they line arrays? I seem to remember a model they made in the early 90's that used the EMIT tweeter and several drivers in a different arrangement, but don't remember seeing a line array.

I know one thing for sure, these make my ex lascala/dynaco combo sound like the AM radio and single 6x9 in my '66 Barracuda I used to own. No kidding...
xlr8
 
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Location: Flowery Branch GA

Postby mesherm » Wed May 05, 2010 4:34 pm

Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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