A U D I O... P A P E R


DESIGN NOTES on the original SE84CSP

Oct 2004 by Steve Deckert

 

It was a dark and rainy October night when on an impulse I decided to make an unannounced visit to a friends house who frequently gets the opportunity to test anything new that we design in the way of electronics.  We call it the "Dennis Test" and it is a valuable part of a network of field testers who run our new designs through the paces before products become available for sale.

This is a story about a type "B" preamp.  If you don't know what a type B preamp is, please read the following short article:

Preamps - Do they help or hurt the sound

As you've learned from the above link, we have basically type A and type B to work with.  Type A being a circuit that you simply can't hear requires a near perfect recording to achieve musical bliss.  Since most recordings being digital now days sound rather anal or analytical type A preamps often get a bad rap and are falsely labeled with these two adjectives.  Nevertheless, hard core audiophiles will often accept nothing less.  Unfortunately these "hard-core" audiophiles didn't get that way without spending as much on stereo gear over the years as the average American spends on their first house.  They have the cost no object sources and have spent years collecting nearly flawless recordings so for them a type A is the only possibility worth considering when choosing a preamp.

Having said that, for the rest of us who still choke a little bit when we tell people our CD players cost almost a thousand dollars because it took some sacrifice to come up with that cash, welcome to the real world.  In the real world of majorities and averages the bottom line involves listening to CD players and CDs where the recordings are not all produced by Maple Shades, Chesky or Shefield Labs and so on.  So it goes without saying a large part of the music we all listen to was not recorded live two-track using tube microphones by skilled engineers who's sole objective is to impress fellow audiophiles.

What does this mean?  Basically you can have the best tube amps and speakers in the world and it's not going to change what's on that recording.  If the recording is dry, lacks dynamics, sounds compressed or two-dimensional then that is exactly what you are going to hear.

ENTER the TYPE B preamp - a magic device not unlike a good phono cartridge that makes what comes out sound better than what went in.  Is this possible?  Yup.  In fact there has been many manufactures who have addressed this goal with their products.  Unfortunately most simply came up with products that were so poor at resolving detail and timbre that they veiled or homogenized the recording to a point where all of the annoying things in the recording were no longer emphasized and often missed entirely by the listener.  The result was - well, less annoying listening experiences with sub par recordings.  

This encompasses (at least in my observation) the majority of audio gear today.  You may think it sounds pretty good but it all stems from a point of reference.  When you hear REALLY good stuff for a period of time and go back to the mainstream audiophile crap that's been shoved at you all these years you quickly realize that it actually sucked. Whoops, I'm probably getting a little too honest there.

So anyway, back to the story.  I have been in the infamous (yes that's what I mean) listening room at the Dennis test facility many times.  It's 1/2 glass with wood floors, no drapes, low ceiling, no rugs, no furniture.  Certainly it possess a huge potential for poor acoustics.  The only time the sound in this room impressed me was one night when I inhaled a gas station sandwich and chugged a couple beers.  I got instantly sick, a cold sweat came over me and down I went.  While laying there on the floor roughing it out for the next 30 minutes or so I was really rather impressed with a 24 bit recording we listened to - but that was the only time I really got off on the sound in this room.  Of course in the interest of fairness I have heard things sound respectable in there at least a good portion of the time.  This is what makes this test facility so valuable to me.

Well, last night I arrived with a couple new toys to listen to, specifically the new type B preamp we're selling called the SE84CSP.  Prior to hooking it up, I suggested we get everything that was already there playing for a period of time - at least until it started sounding good.  Once it starting sounding good we would replace the preamp in the system with the CSP and observe the changes.  Of course as always, the first 10 minutes of sound made me wonder why I even wasted my time coming over.  It was simply really bad.  But after the usual rotation of people checking how the speakers were hooked up & playing with all the knobs and switches it turned out to just be a dirty tube socket in the main amplifier. Everything was going to be okay.

During the next hour of listening I sat there as I usually do with fantasy's about room treatments and how much better it would make things sound.  Now, this is important. The reason I often fantasize about room treatments here is because just when you start to enjoy the sound of something, a note or passage in the music comes up and spanks you on the forehead.  It's so annoying.  And the bass often never seems right, making you wonder if the speakers are in the right location or perhaps just designed to always get it wrong.  Has this ever happened to you?  Well, I know it has, it does, and probably will many more times before its over.  On this particular night I had formulated in my mind a specific plan to address all these room issues and fully intended on making a future unannounced visit with a truck load of treatments - if for no other reason than to prove to myself that I was right.

Since the main amplifier at the Dennis test facility is an original SE34-I single-ended triode of around 7 watts per channel it is sometimes easy after a few beers to run into volume problems.  You know the deal, it starts sounding really good so you push it a little harder, it gets better, you go a little more, it starts to get worse.  Specifically certain notes or passages in the music become hard or brittle, so you reduce the volume to a more sane level and everything gets fine again.  Well last night was no exception and even though I've heard it get substantially louder in the past, I can't say that it ever did with the recordings we were listening to at the time.  I probably said, or at least thought, "put in a decent recording so we can crank it up".  I usually do.

Time to swap in the new preamp.  Do not change the selected recordings, lets just see what it can do with what we're listening to.  The preamp was installed, someone pushed play, the volume adjusted to the same level we had been struggling with all evening and suddenly it got really really good.  I knew it probably would be, but I didn't know that my fantasy's about installing room treatments would simply and abruptly end.  

Frankly I was really shocked because each and every note, and "dangerous" passages in these non-audiophile recordings was suddenly balanced and well behaved.  Nothing would fall apart.  It wouldn't shout at you.  It sounded good.  (For the first time I realized that this room didn't require room treatments) After a short time of amazement we realized we could turn up the volume without negative consequences and did.  Then after a bit, we turned it up some more.  Amazing - it just wouldn't sound bad and was now playing way louder than should even be possible at 10 times the power level we had.  This was another epiphany for me as I realized just how valuable this preamp actually was.  It had a way of making music out of pure crap.  It let the notes hang there in the air with long articulate decays that were to put it mildly, seductive.  Of course I knew it would do that and was anxious for Dennis and Paul to witness it.  But I had no idea it would regulate and govern recordings with such grace as it did.  And I think everyone would agree it did it with no loss of detail, in fact quite the opposite.  We were simply hearing WAY more music come out of these recordings and with the best overall balance I've heard in this type of system.  The only thing that I can say I have heard with this type of finesse is my reference vinyl rig in my personal listening room.  And I have to say that the rich harmonic content that oozed from every CD we played was much closer to analogue than it was to digital.

Now, you might be thinking, okay, the preamp that was swapped out for this new CSP must have been a real piece of crap to evoke such a contrast and I would accept that assumption in most cases if this story was told to me.  But the shocker is that the preamp we replaced was good.  It was one of my own SE84CSP type B preamps!

Huhhh?  Yes, folks, it's always a point of reference equation.  We all thought the original CSP was good.  Me, Dennis, Paul, and quite a large number of customers who own one are raving about it.   Damn, here's more honesty - fasten your seatbelts.  The original CSP was and is in fact very good, this new one is just better.  Better sonically and more user friendly.  The problem with the original CSP was that because of it's rather esoteric SRPP output stage, you have to run really GOOD spec tubes in it.  Once a customer starts tube rolling with it, it can develop noise and or hum problems.  You might say, well,, so, just run the factory tubes in it.  Many still are I'm sure.  But once a person who is into tube rolling finds a particular 6DJ8 or whatever that he or she likes the sound of better, the added noise or hum is overlooked.  My feelings on this are A) DECWARE has never produced a product that hums, and B) Someday someone will visit these original CSP owners who did roll tubes and bring them a preamp that sounds just as good, but doesn't hum.  Wow!  The customer is impressed and buys the other preamp.  He then sells his CSP on Audiogon or ebay.  The new buyer always wanting to try a Decware product purchases it.  He finds it hums and somewhat annoyed gets on the Internet and blows off some steam about how he or she tried the supposedly great CSP preamp and it hummed.  You can see my concern.  So since this is a brand new product and it's still early in the game I decided to completely redesign it so that you can stick any damn tube in it and never have a noise problem - ever.  The result was a rock steady design with an average hum of 0.7 millivolts which is as good as it gets, and superior to most.  I didn't know until I started voicing the new design that it would sound better than the original, but it simply did.  I could tell that within 30 seconds even out on my test bench.

Last week I personally gutted 6 brand new CSP preamps before they went out the door and rebuilt them on my dime.  And of course if anyone with an original CSP and is not happy with it I will update them too.  I should warn anyone in that position that the two units do not sound the same.  The original was more dynamic, perhaps slightly more detailed. On a really good source the original is wonderful sounding.  Even though it was a type B designed to make things sound better, I now have to update the definition of type B to the revised model.  By comparison, the original falls between a type A and type B, while the revised is solidly and completely a type B.

Even though a type A is usually all I will use in my reference system with the vinyl, I have spent many hours in my listening room listening to this new type B and nothing is lost.  The music, like I said, hangs in the air longer. The decays are clearly heard and sustain is noticeably longer.  It's incredibly musical with my 10,000.00 source, and was incredibly musical with a 500.00 CD player.  In my 30 years of this hobby I have never had a preamp that would do both so well.

Significant Design changes between the original CSP (serial numbers below 020 or so) and the updated model are as follows:

A more classic circuit design with a straight gain stage feeding a cathode follower. The original was a cathode gain stage driving an SRPP output stage.

The two output tubes like before are dual triodes, but now are wired using only one side of each tube.  And the left tube uses the left side, while the right tube uses the right side.  This means that after your tubes wear out you can simply swap the left for right channel and you have a fresh set of tubes again!  Twice the tube life.

The voltages driving each stage in the new design have been reduced from around 300 volts to 80 and 90 volts, and the current increased by the same ratio.

And finally because of the 62 % drop in supply voltage changing rectifier tubes will make about 32% more difference on the signature of preamp.  Specifically, the 5Y3GT has the largest voltage drop (around 30 volts more than a 5U4 in this circuit) on the B+ supplies for each stage. A 5AR4 has less of drop and consequently when compared to a 5Y3GT increases the B+ supply voltage.  A 5U4 has the least voltage drop so it raises the B+ supply voltage even more.  A 30 volt increase of a supply voltage that was only 90 volts to begin with is large change.  As the voltage changes on this circuit the signature changes from very euphoric and smooth to having more force and weight.  Think of the 5Y3GT giving you the most type B signature possible and a 5U4 tilting the scale to somewhere between a type B and type A, similar to the original circuits signature.   This should make tube rolling with different rectifiers EXTREMELY rewarding.  Of course, as before, the CSP is not hard on tubes, does not create much heat and can still be left on 24 hours a day 365 days a year with no worries.

 


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