tube pre amp and Krell FPB 600
I've used lots of tube preamps with my 700c and never used the internal jumper and never have had a problem. The jumper defeats one of the better aspects of the amp by putting a capacitor in line with the signal. If a preamp leaks DC, I wouldn't want it. You can easily check the outputs of the pre by using a meter. |
Rwwear Ill defer to your expertise as youve actually used a tube pre with your 700cx and I decided to look elsewhere. But this was my exact dilemma just last month. I was looking at a tube preamp, either the AR Ref 3 or VTL 6.5/7.5. I contacted the preamp manufacturers as well as Krell and was told by both that in order to properly protect my FBP700CX I should absolutely engage the coupling capacitors when using a tube preamp. Unfortunately placing these caps into the signal path degrades the sound and largely defeats the design purpose of the FPB amps. It wasnt that the preamp was going to leak DC constantly and at a constant rate that would necessarily allow an easy meter read at the outputs. It was that tubes are not the most stable device in terms of consistency; they change constantly in small amounts over their operation period and more radically over longer periods of time. This can cause spikes to occur during lets say turn on until the circuit settles out, this can be enough to fry an amp or a pair of speakers in the right circumstance. I was told that the later designs have much better turn on, biasing, and dc offset circuits then they used too and to that end the risk has certainly been minimized, but not eliminated. If you do want to engage the coupling caps I have the procedure from Krell for all the FPB series amps. I can send it to you offline or you can contact Krell directly. They were very helpful and had lightning fast responses. Let me know what you decide and how it works out. I may revisit the tube preamp dilemma in the future but for now I opted for a KCT. Dave |
When I talked to Krell about using the VTL 7.5 with my FPB650M monoblocks they said no problem at all. I did for awhile and the sound was great, and no problems. I used two other tubed preamps with these amps. Again no problems. Right now I am waiting for a custom made tubed preamp(Singlepower Audio) to go with the Krells. I have never changed over the Krells to "adapt" them for tube preamps. If your thinking about the 7.5 with your Krell, IMO they go very good together. |
I compared two preamps over a local audiophile home who owned a FBT 300 amp. One was the tubed Modwright SWL 9.0 (w/ upgrades)and the other was a Belles 28a reference preamp solid state. There were no issues running the Modwright with his amp, but I did have to have caps installed in my Pass labs X-150 amp which is also servo I believe. All Pass amps have an input cap in them now to be safely used with tubed preamps. I don't think you'd have a problem. For what it is worth the Belles sounded better in his system. More depth, dynamics, soundstage and control over the Modwright, and you would not have to switch in any caps (if that is the case) in your amp to use it. Which would result in a cleaner sound. |
Well! My experience is different. What happened is that when I connected my BAT Vk-3iX preamplifier to Krell FPB 300 without knowing about the coupling capacitors engagement, the combo performed wonderfully. However, one day there was a loud popping sound from both speakers and right channel went mute. I reset everything but now Krell FPB 300s right channel had intermittent operation, it will work sometime but not the other times. I sent Vk-3iX to BAT and FPB 300 to Krell. BAT gave the ok signal that Vk-3iX was performing as per specifications but Krell told me that right channel of FPB 300 had intermittent short in the driver board and reason seems to be from high DC at input of FPB 300. Krell changed the board and a controlling processor but engaged the coupling capacitors in FPB 300. BAT Vk-3iX is from new line of BAT but I measured max of 11.3 mV DC at it output. BAT says up to 14 mV DC is normal for it. However, Krell says that it is not average level of DC that hurts direct coupled amplifiers as DC servo in amplifiers can and do counter it but it is the extremely fast spurts of DC as high as 100-200 mV DC from tube preamplifiers that does the damage. I measured max spurt of 56.67 mV DC at VK-3iX output. However, BAT says if max function in digital multi-meter is used then it can measure low frequency AC magnitude which can depend on line supply quality I can continue writing about this for one more page. I sold FPB 300 to experiment with Parasound JC 1s but I hated JC 1s sound. Whatever the Stereophile writes in favor of JC 1s, they are not even close to Krells FPB series. Now I have been using BAT with FPB 400cx with coupling capacitor engaged in Krell for six month and so far everything is fine. But this is not an isolated problem with Krell only. All amplifiers which does not have coupling capacitors as protection against DC and only rely on servo to counter DC face the similar risk. I once read somewhere about a Parasound JC 1 that got fried when the owner turned off the preamplifier before turning off the amplifier. Some preamplifier (even solid state) spurts DC when they are cycled on/off and this alone can fry an amplifier which doesnt have coupling capacitors as protection when its servo failed to counter high level of DC. JC 1 doesnt have coupling capacitors thus in above case it got fried. |
I realized that this is an older thread but I am concerned. I've purchased a vk-3ix (have not received it yet) used. However reading Haroon's comment on how it fried his JC 1 because it doesn't have coupling caps concerns me because I have a A21, which is basically the baby brother of the JC. What I don't understand is this from the Parasounds Owner manual: "Parasound power amplifiers incorporate ingenious and fast-acting DC servo circuits, which completely eliminate the need for coupling and blocking capacitors. The JC 1 is direct-coupled from its input jack to its speaker terminals.This advanced circuitry never needs adjustment or maintenance. It operates outside the audio signal path to keep the DC offset at the output of the JC 1 at a constant 0.00 Vdc.The results are startling clarity, freedom from listening fatigue, and formidable bass response." In relation to the comment should I not use the BAT? |
With due respect to Ralph, I think Krell's concern about tube preamps being used with their direct-coupled amps is a valid one. Many tube preamps have a substantial low-frequency pulse on turn-on and turn-off which can cause major problems for a high-powered direct-coupled solid state amp. Note that these pulses occur notwithstanding a coupling capacitor on the output of the preamp. Most modern tube preamps have a muting relay which allows the preamp voltages to stabilize before the amp is connected. However, not all tube preamps have muting relays and Krell is understandably going to err on the side of caution. No manufacturer wants an unhappy customer. Of course, Krell could have just warned that the power amp should always be turned on after the preamp has warmed up and turned off before the preamp. This simple procedure would also solve the thump problem, but then who wants to count on their customers actually following this advice. |
Of course, Krell could have just warned that the power amp should always be turned on after the preamp has warmed up and turned off before the preamp. This simple procedure would also solve the thump problem, but then who wants to count on their customers actually following this advice. This is a bit of info I can certainly use for my Krell 700cx, thankyou Salectric. |
Of course, Krell could have just warned that the power amp should always be turned on after the preamp has warmed up and turned off before the preamp. This simple procedure would also solve the thump problem, but then who wants to count on their customers actually following this advice. Salectric / Audiolabyrinth Krell makes a really big deal about this in my Krell manual. In fact it takes up a whole page. Its hard to miss. This process / procedure of amp/s turned on last and first off is an Audiophile 101 rule. For the last 30 -35 years with me anyway. Audiolabyrinth - see page 15 Amplifier Operation section of your manual. Cheers |
Salectric, Historically tube preamps have been in use with solid state amps for **decades**! Solid state amps by their very nature have to have a protection circuit, and there aren't any warmup thumps that a tube preamp can make that can damage the input of a transistor amp. The protection relay is there to protect the speaker, not the amp. Ever tried turning on a solid state preamp after the amp is already on? They thump too! Singling out a tube preamp when solid state preamps are just as guilty is ludicrous. Ct0517 got it right- **no matter what kind of preamp you have**, it gets turned on and stabilized first before you power up the amps. I think you will find that all preamp manuals state something to this effect unless the preamp has a warmup mute feature. That statement in their manuals is misinformation in a very pure and refined form. |
I agree that it is not likely a preamp turn-on or turn-off thump will damage the power amp. The real point is the damage to the speakers. Krell has good reason to be concerned about this in my opinion. Since I have been all-tubes for many years, including phono, linestage, power amps and even DAC, I really don't have a dog in this hunt. Whatever Krell does or does not do is of no concern to me. I would never underestimate, however, the ability of some well-meaning audiophiles to mess things up. I clearly recall a lengthy exchange I had on some forum, probably Audiogon but I am not certain, where the other party was insistent that a preamp should always be turned on after the power amp, never before. (That is not a typo.) He dismissed my concerns about possible damage by saying he had never had a problem or experienced any thump for 20 or more years. Of course, his good experience hardly proves the rule; it just shows the muting relay in his preamp works. |
CT0517, I do have the owner's manual, I have read exactly what you have specified many times, it is a whole page, I do not believe it would hurt nothing to do the power up and power down scheme that Salectric has told me, I never thought of it, makes sence to me,I'm going to do it, The tube/solid state digital source I use straight to my amp in no way could spike anything following Salectric's advice, I have always used all solid state, likly that may be the reason I never thought of doing a procedure for on power and off power of Tubes mixed with sold state audio equipment. |
I leave my SS amps on continuously per the manufacturer's instructions and get a thump through the speakers when I turn my tube pre-amp off. This is normal according to the manufacturer. There is some woofer excursion but not as great as it is when playing at very loud volumes, so I don't worry about it. |
12-04-14: Audiolabyrinth Hi Audiolabyrinth As explained by Krell to me its a small amount of DC that escapes through the outputs as the relays shut down. Not hazardous to speakers. My Krell amp makes a very subtle thump at shutdown only too. My previous OTL monoblocks would do it at startup and shutdown. The Classe amps were both startup and shutdown but only occasionally. The others I owned like multiple Bryston amps, and others I demo'ed like Pass Labs and others I can't remember. My Roger Modjeski Rm9 and Rm10 tube amps do not do it. Cheers |
Hi Knghifi, yes, I am going to do the power up and power down that salectric has specified in the above post, interesting, you said that by doing this, I may not get that turn off thump with My krell 700cx that will be back from krell today with up-graded power supplies and along with complete new and improved caps that they installed earlier this year that was not the stock caps that was in the amp when manufactured,I am going to do what you and all have said about power up and down scheme, Thankyou Knghifi. |
12-04-14: Audiolabyrinth Hi Audiolabyrinth Here is Rogers' US website. branded as Music Reference I discuss the rm9 and rm10 in my virtual page. Feel free to stop by and say Hi. Knghifi, A.L., et al Re; the amp shutdown noise we have been discussing. here is some more info on it from Krell that sheds more light. ---------------------------------------------------------------------------------------------------- The noise you are experiencing is not a problem with the unit nor is it a byproduct of your AC mains (*). This is normal for large powerful amplifiers that are not capacitor coupled (AKA: no capacitors in the signal path or at their outputs). *Were your line AC to consistently run low (at or below 100v) this could possibly (not definitively) cause the power supply to fall slightly out of regulation, which could possibly (again - not definitively) exacerbate this noise upward slightly. Also, if the amplifiers are turned completely off (including the rear panel breaker) between listening sessions, this will cause the noise to be louder. Leaving the units in standby allows part of the power supply to stay warmed up, thus reducing this noise. That said, should one wish to not leave them in standby due to their power consumption, this noise should never be loud enough to damage the speakers upon turn on or off. Regards, Patrick Patrick Bresnahan Sales, Technical Support, Service Krell Industries LLC 45 Connair Road, Orange, CT 06477 - USA |
HI Knghifi, Got my Krell 700cx amp back yesterday from Krell, Talking about holy fricken cow!, this amp sounds incredible with the modds we paid dearly for that appears to be worth every cent!, BTW, I did the power up and power down as specified, no matter what, I still get the not loud at all power thump over the speakers when I turn amp off at the rear panel breaker, I see in the post above this is normal,BTW, Patrick Bresnahan of Krell know's his product very well and then some, A very talented musician too!, we have become good friends over the course of the last 3 years, I talked to him yesterday, I recommend, that if you need any thing done at Krell, Patrick is the Man!, KNGhifi, I will try muting as you have said, that is informative, never tried that,I just turn the volume completly down, is that the same thing?, Oh, up packing the amp and doing re-set up was an incredible feat to say the least, you know, my amp does weigh 180 pounds, took hours, and a neighbor, plus the wife, whom took a vacation day from work to do all this, good thing she did, she was a huge help, I appreciate her, good woman to put up with this insanity. |
Are you having a problem? I would definitely turn the amp off before the preamp and wait for the preamp to warm up before turning the amp on. The 3 LEDs on the front of the amp will signify if there is a problem. Since your preamp is not balanced, I would put jumpers between pin 1 and 3 on the XLRs. I would contact Krell if you think you need to engage the internal jumpers. But they should be close to the rear inputs under the top cover.
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