Tube pre with SS HT

Discussion in 'AV Receivers' started by Yogi, Mar 24, 2003.

  1. Yogi

    Yogi Screenwriter

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    Well finally I was able to integrate a cheap tube pre (2ch) with my SS HT gear. The tube pre was an ASL2004DT integrated with a Denon 3802 hooked to a Proceed AMP 2 for the mains and a B&K Ref7250 for the rest. The speakers are all Vienna acoustics. Out of the box the tube pre sounded fantastic with clean highs and taut bass and a lush midrange. The real kicker came in when last weekend we were watching a DD movie in 2 channel via the tube pre and on a restaurant scene my wife tells me that it looks like the neighbours are having a party since she can hear their music play. I too thought the same until I paused the movie and the neighbours music went away. And we both were wowed by the immense soundstage this preamp was projecting considering that our nearest neighbours home is about 100 feet away. The soundstage was completely three dimensional and extending beyond the physical speakers. I am a tube believer now. No questions.

    There is however a problem. Just for kicks I cranked up the preamp all the way and although I couldnt hear any distortion the sound broke up during heavy transients. I listened to some classical scores and when ever there was a drum beat I could hear a crackling noise. I always believed that tubes never go into hard clipping and always distort gracefully so that is something that didnt make sense to me. Whats more baffling is that I hooked up the tube pre directly to my Tape and MD player and I couldnt hear any crackling at any volume level on my preamp. So it baffled me as to why this problem was source dependent. I also tried some higher quality interconnects and while the crackling is reduced to a minimum it still is there on transients. I only have two explanations for that.

    1) Either the tubes are defective or highly microphonic.
    2) Or there is some kind of impedance mismatch between the source DVD player and my tube preamp. I am thinking its the later as the DVD player is supposed to output 2V RMS and my preamps input impedance is 100K. since I dont know the output impedance of my DVD players analog outs I am only assuming that its in the 10K range, which means my tube pre is seeing almost 20V at its input and on heavy transients some of the internal components are getting saturated leading the the crackling sound. Does that make sense? Any expert opinions on this? I have a set of new tubes ordered so I'll be able to check for tube defect and microphonics tomorrow.

    Any input would be greatly appreciated.
     
  2. Chu Gai

    Chu Gai Lead Actor

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    How exactly do you have your system hooked up Yogi?
    I'll just offer the following general comments. I think you'll find the distortion characteristics of tube preamps to be different from tube amps. As you get into saturation and hence distortion with the preamp the sound tends to get buzzy, or gritty sounding. It seems to lack dynamics if you will. Tube amp distortion is entirely different. We might be talking about the same thing. So just back off the gain on the preamp. Another thought Yogi, and you'd really need the vendor to give you the info on this (assuming they can since I believe they're primarily importers), is that you just might have a preamp whose distortion characteristics are not only non linear with gain, but result in odd-order (3rd I'd guess) harmonics.
    On the other hand, the output of your preamp is on the order of 10 volts, fully cranked. You just might be giving headaches to your amp which has an input sensitivity of 1.6 volts for full output. I've no idea what the output impedance of your tube preamp is but i'd suspect it to be on the high side. Ever thought of giving that Behringer a try that Brett's been messing with? Less than 1/2 the price of those ASL's!
     
  3. Yogi

    Yogi Screenwriter

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    I have the DVD players analog outs going to the 2004DT's inputs and its output going directly to the external inputs of my 3802. The 3802's preouts are going to the Proceed amp. Whenever I listen to music I put my 3802 in ext-in mode. That way it just acts as a straight gain without any DSP. I dont know the output impedance of my 2004DT
     
  4. Chu Gai

    Chu Gai Lead Actor

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    well then if the 3802 is being bypassed entirely so that it's passing the signal of the tube through unscathed, then it sure sounds like you're overdriving the amp. maybe you'll bounce your findings off of ASL and see what their position is.
     
  5. Yogi

    Yogi Screenwriter

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    I dont have an email for ASL but I did shoot off an email to Divertech (their NA distributor) and still havent heard a thing from them. You might be right I might be overdriving the amp.
     
  6. Chu Gai

    Chu Gai Lead Actor

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    well it's potentially over 5x the voltage needed for the amp...something's got to give!
     
  7. Darrel McBane

    Darrel McBane Second Unit

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    I've been thinking of adding a 2 channel tube preamp into my system for about a year or more. I've had my eyes open on Audiogon for a VTL 5.5 preamp. I just love the sound of this piece. I'm not sure how it will match up with my Revel F-30 and Rotel 1090 SS amp. I've seen several come and go through Audiogon and their has alway been some other HT piece of equipment that has steered me away. But, I added two of the last piece into my system recently ( Aragon Stage One and Sensory Science OTA HD receiver) and this may be the year I pick one up. Then a decent turntable so I can play the vinyl LPs I've had in my closet for the last 15 years.

    Good to hear your impressions of adding a tube preamp to a HT setup.
     
  8. Scott_N

    Scott_N Second Unit

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    You could also look for a Rogue Audio Magnum 99 a Cary SLP-2002 or a Audio Research LS16. All solid tube preamps in the same price range. Myself I want a Cary SLP-98L and Cary V12i combo.
     
  9. Ricky T

    Ricky T Supporting Actor

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    Darrell,

    I use a Rogue 66 magnum tube preamp with HT bypass unity gain with my NHT 3.3s and Parasound 2200II amp...love it! You can even buy a used Rogue 66 (magnum or non-magnum) on audiogon and send it to Rogue for the $150 HT bypass option.
     
  10. Yogi

    Yogi Screenwriter

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    I actually had been thinking about the Rogue 66 or 99 (or CJ PV14) as my next upgrade. Actually it would be a completely separate 2 ch cd/sacd music setup with Vienna Acoustic Beethovens (I luv VA speakers and dont want to change to another brand). I would then move my Proceed amp to this setup. I dont want to go to tube amplification as I feel there are somethings better left for transistors to do[​IMG] I will then get a separate B&K 2 ch amp for my mains. I would still keep the 2004DT for my two front channels in the HT setup to soften some of the harshness in digital movie tracks. But all this not for another few years (hopefully). Untill then its 2004DT for my music and HT needs. I just got some new tubes for it and I cant wait to get home to do some tube rolling[​IMG].
     
  11. Martin Rendall

    Martin Rendall Screenwriter

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    When you say "external inputs", are you referring to the 5.1 (or perhaps 7.1) direct inputs?

    So your chain is:

    DVD Player -(RCA)-> ASL2004DT -(2 of 7.1 direct inputs)-> 3802 -(preouts)-> Proceed (yum, by the way).

    If you replace the DVD player with another device, you don't get the distortion? Are the speaker volumes comparable? Do you have an SPL meter?

    OK, regarding impedences. Impedences should be at least an order of magnitude greater as you go from source to target. More is even better. So, the DVD player will have a low output impedence (around 600ohms?), while the ASL2004DT has a 100K input impedence. This is actually very optimal. I have a DIY tube pre I use with a CD source (and tested with a DVD source), and it worked great for me.

    I didn't follow the argument that the impedence differences increases voltages. I presume you used ohms law and held the current constant? Not sure about that... I *think* the voltage is held constant. You've got me puzzling now.

    So, if the problem is an impedence matching one, I would strongly suspect this part of the chain: ASL2004DT -(2 of 7.1 direct inputs)-> 3802. Can you take the 3802 out of the chain entirely, and see if that fixes the distortion? It's possible that the ASL2004DT has a fairly high output impedence, while the Denon 3802's external inputs have a fairly low impedence. An issue like this would manifest more like a loss of the higher frequencies due to cable capacitance, as opposed to distortion, IMHO. Not really what you are describing.

    I will say that I find it highly unlikely that a DVD player is driving the pre-amp tubes to distortion, unless the player's output is very high (>> 2V), and/or you have the volume on the pre cranked all the way to 10. If you've done that, you should back off the volume on the pre, and turn up the volume on the 3802. I stress, the pre should not be distorting.

    Another possibility is that the DVD player is broken and is outputting some DC in the signal. Do you have a volt meter?

    Regarding microphonics: you would notice issues at the same volumes, regardless of the source device. Microphonics is a physical vibration issue.

    Hmmm. Puzzler. Keep us informed.

    Martin.
     
  12. Scott Oliver

    Scott Oliver Screenwriter

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    I think you should do as Martin suggested, take the deno out of the chainand try the same stuff.

    I think you are the first person I have ever read or seen that has put a 2 channel pre in front of the HT receiver/processor. Most always have it placed in the chain after the rec/pro using that device's preouts, hence the popularity of unity gain and HT passthough on preamps.
     
  13. Brett DiMichele

    Brett DiMichele Producer

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    I think the culprit is the source (DVD) I know with my DVD
    the output is extremely high and I can overdrive my tubes
    and get the same effect as you (it's much softer than SS
    Clipping still...)

    I believe it's the DVD-P I bet the OUT-V is 2V

    Try turning the Pre gain down and just using the Denon's
    Gain and see what happens.
     
  14. Yogi

    Yogi Screenwriter

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    OK guys problem solved. The ASL schematics that came in the manual are wrong. They suggest placing the 12AX7 tube where there should be a 12AU7. The tube positions were swapped. I looked at the circuit diagram and also opened up the chassis and underneath the circuit board was clearly marked where the 12AX7 and the 12AU7s should go. Once the tubes were swapped the problem was completely gone. No more crackling at any output levels on the preamp. I am surprised ASL didn't fix this problem in their manual. Also at normal volume levels on the preamp (between the 10-2 o clock) position there is no difference in sound when you have the two tubes swapped. Its only when the tubes are fully opened that you would notice any problems and that too only on dynamic tracks. Sorry guys to bother you all.

    Also on a related note, the reason I have the preamp before the processor is that I only want to use if for my 2 ch listening untill I get a dedicated 2 ch system for my 2 ch needs. If I have the tube pre after the processor it will have to be on for both 2 and multichannel HT use.

    Thank you guys for the helpful suggestions.

    Cheers.
     
  15. Yogi

    Yogi Screenwriter

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    Scott, I did like you suggested and put the preamp between the processor and the amp. Guess what? The sound improved even more. The soundstage got more dimensional and real. The instruments developed more 'air' and 'life' around them. So the Denon even thought in a straight pass thru mode (ext-in) does modify the signal enough to mask those tubey characteristics. Hmmm I wonder why? Also I got more tube magic into the sound by having hum in my speakers[​IMG] thats audible from 3 ft away and is independent of the volume level of the pre or the processor. Now I have to make a choice, either live with a slight hum but with better sonics or live with a quieter system with inferior sonics. I chose to live with a slight buzz and have better sonics. Am I crazy? (thats apart from the fact that I am an audioholic and that makes me crazy in the first order) or is that something most of us would have choosen to go with?

    Also what are some ways of reducing tube hum? is there a cure?

    Regards,
     
  16. Chu Gai

    Chu Gai Lead Actor

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    possible grounding issue Yogi?
     
  17. Yogi

    Yogi Screenwriter

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    Could be. I dont know if that would cause the hum to increase as I get near the 12AX7 tube (sorry I left out that little detail). When my finger is 1 cm away from the 12AX7 tube the hum increases and if I touch the tube it makes a slight 'ching' sound. Although I have listened to many tube products at stores etc this is my first experience owning one of them and I am on the learning curve and any input from fellow tubeophiles would be greatly appreciated.

    Wondering where other tubophiles like Larry and Saurav went. I haven't heard from them in a long time.
     
  18. Frank_S

    Frank_S Supporting Actor

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    Hi Yogi, I've been using a tube preamp(EAR864)for well over a year with and love it for 2 channel playback, especiallly vinyl. Until then I had been using my Classe SSP-25 pre/pro which is no slouch. The difference in my setup is that I completely bypass the pre/pro when listening to 2 channel, I swap the interconnects between the preamp and pre/pro.
    Unless you are willing to do that, I would do what Ricky T recommends and get a preamp with unity gain. Good luck! [​IMG]
     
  19. Yogi

    Yogi Screenwriter

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    Well, I plan to continue using this preamp but will completely bypass the processor for 2 ch music by having the analog outs from my DVD player to go directly into the tube preamp (CD in) and have the digital output going to the receiver. Then I'll have the receiver preouts going into the aux input on the tube pre. The tube pre outputs will go to the amp. For 2 channel music I'll use the volume control on the tubepre. For HT I'll calibrate the volume on the tube pre to achieve the same SPL levels in all channels. Then I'll mark that volume level on the tube pre's volume pot. That will be my HT bypass volume. When watching movies I'll keep the tube pre at that bypass volume level and control the overall volume from my receiver. I think that would work quite well. Any comments on the proposed system setup are more than welcome.

    Regards,
     
  20. Martin Rendall

    Martin Rendall Screenwriter

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    Yogi,

    I subscribe to the viewpoint that hum isn't a function of tubes, but rather poor design. However, it does come down to tolerances. Sand amps tend (generalization here - beware) to be sensitive more than tube amps. That is, they achieve full output with a lower input voltage. They also tend to have more power, which increases their sensitivity even more. So a tube pre with hum may not be audible when the signal is sent to an amp with lower sensitivity. Which means that it's not worth the tradeoffs required to lower the noise floor.

    That's not just theoretical, but also leads me to a viable solution to your problem. Since you obviously have extra pre-amp gain you can afford to sacrifice, you can buy a couple of inline attenuators. As long as you put them right next to the power amp inputs, you should not see much if any noticeable audio degredation. But you will lower the hum by 12dB, which should reduce your hum audibility to under 1 foot.

    I have the same problem with my tube pre and power amp, and this solution works great.

    Unrelated however: you better check to see if the pre causes a phase shift. Some pre amps will shift the output signal by 180 degress, which when used for the mains in a multichannel environment, causes cancellation between the mains and the center channel. I presume this could happen to you since you've now got the tube pre between the pre/pro and amplifier.

    Martin.
     

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