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Hi all,I hope this is the right place for this post. I am new to this forum and the preamp was my first (and yet only) DIY tube project too. I am using it with a balanced step-up trans for my Denon Dl103. The problem is that I get toooo much gain in my setup (DL103, MC step-up, Siren Song, Sun Audio line preamp, Sun Audio SV-300B SET and Lowther Bicor 200). Since my line preamp has trans coupled balanced 600Ohm inputs and the Siren Song is mainly (besides the last stage) balanced I was thinking of the following mod:
get rid of the last stage (6N7) in the "Siren" and (optionally) insert a balanced output trans for coupling with the 600ohm line-pre input.
Has anybody suggestions to this?
Where and how do I have to "cut" the last stage away?
Where can I find suitable trans?
Is this idea feasible at all?I will appreciate any comments.
Best regards
Antonio
Follow Ups:
Why not keep the existing layout, but insert a Magnequest B7 linestage transformer at the end. This allows a 12:1 voltage reduction, and balanced output. It does seem silly to go from balanced to single ended and back to balanced, but in terms of ease of set-up and design this is really an easy way to go.I have both the Siren Song, which I thoroughly enjoy, and the B7's used in a para-feed line pre-amp with a 76 driver. Nice system.
Go to Bottlehead.com to look for specs and info on the B7. Use the C4S devices for plate loading. Takes a while for the devices to break in (transformer, cap and C4S) but it is worth the time.
Jon Ver Halen
I'm not surprised you have too much gain. It depends to some degree on which version of jc's you have. If I recall correctly jc moved the eq networks arounds in serial versions. To eliminate the last stage, you must take into consideration the change which might be introduced into the RIAA eq. I mean 600 ohms is drastically different then the grid of a tube impedance wise. To me, it might better to get ride of some line stage gain.
Thanks for your comment,I was not aware of different versions of the "Siren" (besides of two published in Sound Practice)! The one I build is from Sound Practices issue Summer 1993 (page 16). I only modified the PS, the last stage is a paralled 6N7. What about replacing this stage with a 1:1 output transformer (balanced). Is it then posible to change the second riaa stage into a balanced design like the first stage.
I am aware of the interaction between input impedance of the last stage and riaa filter but I am not very sure where the second part of the riaa eq ends and where the coupling of the last (6N7)stage starts (as I said I am a greenhorn).
Unfortunately I am no electronic or tube expert to design these changes by myself. Where can I find more/help information on the Siren Song?regards
Antonio
I'm looking at the schematic now. The 100k off the plate of the second half of the 6sn7 forms one of the poles of the time constants with the .033uF and 10K resistor. In fact, it is (100k +10k)*.033uF or the 3,630uS which when used in conjunction with the rest of what's there, namely the grid of the 6n7s in parallel and the coupling network comprised of .1uF coupling capacitor and the 249k grid resistor of the 6n7, yields the 3,180uS pole in the RIAA eq. In the same fashion, the 10k and the .033uF or the 330uS, used with the surrounding componets in like manner, yield the 318uS pole in the RIAA eq. So, as you can see the component values are a bit off in order to compensate for the actual in circuit result when used in conjunction with the other components.Having said that....in order to place anything in the circuit in place of the parallel 6n7, it would have to approximate the grids of these two tubes in parallel, such that the two previously discussed poles are the same and the eq corresponding to such are equal. This is the only way the RIAA eq will be correct.
Now, the grids of a couple of 6n7s tied in parallel are I'd guess approximately the 500k to more likely mega ohm range. There is also some parasetic miller capacitance to be dealt with.
Now, you wanna put a 600 ohm 1:1 tramsformer there??? Not even close! The component values will have to all be changed and not by a small amount. In short, for-get it. I'm not trying to hard here...just trying to help.
You need the 6n7 there to act as a buffer to isolate the RIAA eq from the rest of the world, dude!
If the gain of the last stage is something like 75% of mu which is 35 or 26....How does one reduce this? Not too easily since you really don't have a lower voltage to work with and the max plate voltage of the 6n7 is 300. This rules out using a step down transformer in place of the plate resistor which is 25K. You could go to a lower mu tube in this application....but, you still effect the RIAA eq a bit.
What is you line stage like???
Hi,thanks a lot, I start realizing that I was completely on the wrong path. Looks as if I have to read some more books (just had one very old german book about tube power supplies). Ok, cannot say anything about the line stage since this one is not homebrewed but an Sun Audio SVC-500. The only info I have is that it has balanced and unbalanced line inputs nd two ecc82. Ok, so another shot into the fog: can the 6N7 be wired as a buffer with no amplification, still decoupling the riaa from the rest? The other thing I was aiming at was a complete balanced path through the amp, s itpossible? Don't worry laughing about my comments, I guess I deserve it trying this kind of project without a clue.
Ok, I am just making you work for "my" project. I don't want to bother you. Just leave it if you feel it is too much. I am glad that you help me so far.PS: What literature on tube preamps do you recommend?
Best regards
Antonio
Larry, can't you just take the output of the preamp off the cathode of the final tube instead of the plate?-j
Hey, it has been a long day. I apologize for being short. I should have been more direct in answering your question. That's: The whole circuit is in the circuit and there really isn't a starting and stopping point with respect to the eq and the coupling network.Now, let's talk about this balanced thing. The input to the first stage in the Siren Song as shown in second SP is not balanced. You must have changed this following the first SP article. Easy enough by just putting the input across the grids of the two sections of the 6SL7 configured as a long-tailed pair. Between the 6SL7 and the 6SN7 you're balanced as well, the 6SN7 again being a long-tailed pair. From the output of the 6SN7 you are unbalanced into the 6N7.
One thing you could do is rewire the 6N7 like the first stage, another longtailed pair, applying the signal to the first grid and grounding the second grid, taking the output off the two plates into your transformer primary. Trouble is that the transformer would have to work with the plate resistance of the 6N7 which is I'm guessing 10k. Thus, if you have a 10k to 600 ohm transformer that might just get you there and you have lost gain in the step down ratio of this transformer. Doing the math in my head this is something like 16 to 1??? Check it it is late. This also matches you up for the balance input of your line stage. Not bad after a long day at law school huh?
Oh yea, a source for the tx...maybe jensen??
Now, I'm not necessarily suggesting this approach. You still will be off on the eq a bit since it is not parallel sections...but at least it is the same tube. I guess I really think you should leave the phono pre-amp alone.
Now, let's talk about the line stage. ECC82. 12AU7. I got some personal reservations about this tube; but, I'll keep those to myself since we are in public. Aside from that, a mu of 20. So no substitutions here....
Now I get to ask a question: Why the line stage? Why do you seem to insist one using it?
OK, I'll take two questions: Why the balanced? Audiophile dogma?
This is really the best thing going on the web right now...:> ))
Hi,I think Morgan Jones' "Valve Amplifiers" (Newnes 1996) should be of great help to you. Also, have a look at
http://www.tubecad.com
for more than a year's worth of free journals about tube theory. Could it get any better?! May be a bit too advanced to begin with, though.
Good luck!
Morgan
Hi,thanks a lot, I start realizing that I was completely on the wrong path. Looks as if I have to read some more books (just had one very old german book about tube power supplies). Ok, cannot say anything about the line stage since this one is not homebrewed but an Sun Audio SVC-500. The only info I have is that it has balanced and unbalanced line inputs nd two ecc82. Ok, so another shot into the fog: can the 6N7 be wired as a buffer with no amplification, still decoupling the riaa from the rest? The other thing I was aiming at was a complete balanced path through the amp, s itpossible? Don't worry laughing about my comments, I guess I deserve it trying this kind of project without a clue.
PS: Wha literature on tube preamps do you recommend?
Best regards
Antonio
Almost forgot reading source on preamps. Allen Wright, the tube preamp cook book. Should be perfect without you having to wade through the tube theory and not really addressing the topic at hand.
Hi L.D (Larry?) and Jon,sorry for beeing away so much time, I was very busy and then my car broke down (I am still repairing it).
Well, I am impressed about the help I am receiving on this matter, really! Some of the suggested literature I think I'll have a study during x-mas hollidays.
I think I got this "the circuit is in the circuit" thing. You are right when you suggest not changing the circuit a lot, Morrison has had his thoughts on it.
Ok but still: why balanced? A question of circuit elegance, as Jon Ver Halen stated going from balanced to SE and then to balanced again seems silly, so that's why I wanted to change the last SE 6N7 stage to balanced as well. The signal is balanced at the pickup and keeping it like this all the way into my line pre is elegant and may also improve noise level (my "Siren Song" IS noisy AND oscilating, the last especially when I open the volume pot - a different story but shurely the next one).
So the suggestion of keeping the last last stage and including a step down line trans looks perfect for my purpose. Why a line stage? Basically to switch between different sources, i got mine cheap and used.
Questions:
Can I change the last SE 6N7 stage to a balanced ("long-tailed" - what's that?)?
How do I have to modify the RIAA in front of it then?
What is the output impedance I'll get then so that I can look for an adecuate (linestage) trans?As usual: Any suggestions and comments are highly appreciated.
The very best regards
Antonio
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