Updated: 25-Nov-01
Doede's Tube Stories...

Project: the DDPRE1, a complete new PRE AMP with Phono Stage

For the first part of this story see: item1

Final Construction:

After the mechanical construction and the basic wiring all components are being mounted. Actually, I was surprised how many components had to go in !! Specially the Capacitors are taking quit some space. As you see at the picture (better Click it for more detail) I have been struggling with space again. What actually is a pity, because now servicing or tweaking becomes a pain in the .... But what is the alternative? A chassis twice as big? I thought this 19" rack was already huge for a pre amp... Or you do it as Triodedick with his CLEO4 a separated Power supply and Amp module. Any way every one can make his own decision. In the meantime I have mounted the frontpanel and the amp is playing !

The Volume Switch:

In the middle of the amp you can see the input selector and the stepped volume switch. Just have a look how I did it: Cheaper is not possible: a standard piece of metal, A standard rotary switch with 12 positions and metal film resistors. Total cost below 10DM. Why so cheap? Doesn't it sound bad than? And what about these extremely expensive rotary switches with 24/36/48 steps which cost 50 times more ?? ( !!! ). Well I rather spend that money on the right tubes and capacitors. I hear no limits with this simple construction. It is a passive linear device. May be the resistors could be replaced for Rikens or so. Will only cost you aprox 100DM more. And believe me, you do not need more than 12 positions. I made the last 3 positions 6dB each, and than go lower gradually. This is good enough for my use..... Oh yea, the little ax and shaftconnector are also coming from Conrad (6DM). Could you believe I saw a similar thing in a HIGH END COMPONENTS CATALOGUE for 25 US$ ???? Grrrrrrrrr, I am getting frustrated sometimes of the ripping off tactics from these High End Audio companies !!

First sound Impression's !!!

Line stage:
You could say, that the line part is identical with the tube stage I already had built for/in the DDDAC. This is true, except for the fact, that we now have a double mono version, better capacitors and resistors. Well, this is exactly what you hear. I have got used in the meantime to recognize the typical differences between the used components and between power supplies. And this is precisely what came back. No smashing smacking difference. Every thing is smoother, bit more pin point, more stable stereo image and softer highs. Again: a small step forward !! and you know, a lot of small steps makes a giant leap (for mankind?)...... Pffffffffffffff nice wording, but just realize the amount of effort, quality components and money you need to put into this result. It is clear, that the last small steps are costing exponential efforts !  Is it therefore it is called "High" "End" ? who knows...... At least it is my hobby and it keeps me of the street.........

RIAA PHONO Stage
Uhhhhhh , what did I just said about small little steps? Wow ! I am surprised and shocked !! And this does not happen oft to this sober Dutch guy.... Nothing small step; no concentrated A-B comparison/listening: this new RIAA design is playing in another league, I did not foresee, that my LINN / Jubilee combination was capable of this sound reproduction. This amp is taking away all limitations I must have had with my old simple SRPP design with the ECC83 (including simple solid state power supply). Yeah, yeah, that is what they always say, so what is the reference here? Good question, back to earth now (although the sound is heaven): If you have been reading all of my site, you know why I started to built the DDDAC. The final result was, an almost even score for both systems (even when compared the same CD and LP). They were very close. Well, the original distance I had between my LINN and CD10 CD-player is now back and the gap is wider. So now I have 2 winners, although I start thinking already, what to do with the DAC? Is it possible to tweak / develop the DAC further and close the gap?? or did I found the limits of the digital medium already ??? we don't know till I try and that is for sure what will happen in the future !!!! Let me give you an impression on what is sounding so good in ranking of impact

1st
The total sound stage has increased with meters in all directions. No full attention listening required. Switch from any  CD to any LP and you hear the soundstage behind your back grow immediately with many cubic meters.

2nd
In the highs, I experienced a micro detail level I never heard before; clear pin point, very natural. It is not only soft and smooth, you clearly can hear the timbre. Transients are precise and not just an attack. Instead of "TSSJJKK" from the HIATS, you hear the Bronze so real!! And I always thought it sounded good already (though it did...)

3rd
Voices and String instruments are sounding very coherent. What I mean, is that thanks too the good coherence between ground and upper tones, these sounds are  very very natural.

Wow, mission completed ! or............  no, not that fast, Ofcourse I will do some nice tweaking and besides that, I did had some nasty problems, which are solved now for 90%, but I am not satisfied yet. Keep on reading below what happened, specially if you want to build this design !!

First Problems:

The first time, I switched to Phono, I got HUMMMMMMMMMMMMM. Nice deep 50Hz Humm from my subwoofers. Unacceptable high level of Humm. I was very disappointed. My old SRPP stage was very quiet you must know. The best thing you can do here is play Sherlock Holms. You know him? this wiseguy solved crimes, only by eliminating all other possibilities, deduction he called that. So that is what I started. I used the digital oscilloscope and the FFT function to get a pattern of the HUMMMM:

As you can see, the 50Hz component is there, but also a 100, 150, 250 etc Hz. This doesn't look good at all! This stinks and looks like AC heater HUMM. Was I out of my mind to feed the phono tubes with AC? May be I was, I just wanted to try, but I failed clearly..... even the symmetrical approach did not help. So I measured the 3.15 Volt tabs on the transformer: brrrrrrrrrr looks very bad, no wonder, you see all this crap.

With those flat tops of the sinewaves, you will get great uneven harmonic garbish. The only way open was to do DC- heating. I made a choice for the current source again. Nice slow start of the tube and very simple to create. I did not wanted to built in immediately special transformers (the problem is, you need aprox 8-9V AC to feed the current source, I have 6.3 V available...) so did the quick and dirty trick below: (only one channel shown)

The problem I have here is the unbalanced situation with the AC heating for the Penthode D3A in the MU line stage. Also I needed to ground the circuit. If not more HUMMM is the result, because it is not really floating, also the Uk-heater from the top tubes is now too high. To be honest, my SRPP ECC83 stage had the same setup with an isolated transformer and is still alive.....

OK, the result is now, that the HUMM has disappeared for 90%. In the phono position, you can still hear some Humm, but as soon as you play the LP, it disappears. In-between the songs the vinyl noise is equal. Acceptable for listening music BUT NOT GOOD ENOUGH !! I will continue tweaking. Next steps will be (temporarily) battery filament supply, Sherlock Holmes, remember?? and a separate distant Power supply. I need to know where the rest is coming from, before I can take corrective action! I will succeed ! Oh yes, totally forgot to say, that changing tubes around in the first section and also the triodes 30, does makes audible differences in the level of HUMM, it is clear that these kind tubes are sensitive for Humm and you need to experiment for the best result !! Or: if you buy just one set of tubes, you might have bad luck !!! make sure to have the opportunity to exchange tubes. May be with those from a befriended DIY-er??? Could be your former friend when you give him back the tubes with the most Humm, hehehe......

I will continue to work on the HUMM issue and report back. When that is done, I will start tweaking the actual RIAA filter. After that I will publish the FINAL CIRCUIT (#3) with all measured values. So keep checking!

Update November 25th

Solving the problems.....

As described, the only problem left over was the HUMMM. At a certain point I found out, that when I was tilting the chassis a bit, the HUMM was less. Very strange??? I did several tests, with battery supply, changing wires etc, NO RESULT !! I reminded the chassis thing.... The left transformer had some mechanical HUMM. I reported this already to Wil Blauw and he promised already before to send me a new one. I started to suspect the 6SL7 of microfony and picking up the (Quit load, I must say...) vibrations through the chassis !! When plugging in less sensitive Tubes, like the 5691 RED (more about this one in a minute !!!) the Humm was also less. OK, I waited and a few weeks ago the new (silent) transformer arrived. Few hours soldering and screwing later, voila! the HUMM was much much more less now. I took the final step now and took separate transformers for the PHONO stage filaments.....  OK, I admit, there is still some noticeable HUMM if you turn the volume to the right, but when playing music, I can hear the noise of the LP again. If I listen very carefully, I can than also hear some HUMM, but it totally disappears at the first notes of the music. May be in the future, if I would build something new. I will construct a separate power supply to generate total silence. But for now it is just fine

And a little tweak....

As told, I was lucky enough to get some 5691's from audio and tube friend Dick. He explained me, that these military tubes are designed to last longer and be less sensitive for the environment. Or less microfonic as you will. Electrically, they are identical to the 6SL7, so I real drop in replacement, uhhhhhhhh almost, they take 600mA in stead of 300mA heating current. So the supply must be sufficient !!

Indeed, this tubes were definitively less sensitive, but also had a nice surprise for me! Probably thanks too the more precise construction, the sound is more transparent and the highs are extremely clear and defined !!  An extra reason NOT to remove this tubes anymore. Therefor you will find them in the latest schematics of the DD PRE 2001 ...........

to be continued ................ ??? oh yes, it will be with other line stage tubes like the VT67 and 226 !! and some measurements...

IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma