Does your direct coupled preamp put out a DC surge when it turns on? If someone has their direct coupled solid state amp on and you turn on your preamp.....could it blow up the amp? Long ago, I had heard that your preamps could blow up things (is this true or not?) I made a pair of custom mono block amps that used the original Hypex class D modules (modified by me). That pair of amps was sold years later to someone else and they were working fine when sold. The new owner had one of your preamps and had the newly purchased amps on and then turned on the preamp. One of the amps immeditately blew up. He called me and we discussed it....that is when I learned that he had one of your preamps and I told him that I had heard your preamps put out quite a DC surge.
@ricevs Our preamps don't make 'quite a DC surge'... During warmup our balanced preamps do put out a low frequency signal but its low in amplitude (about 1/2V is typical); less than most tube preamps make. I'm not aware of any amplifiers ever damaged by it in the last 35 years.
I'm sure you are aware that a DC Offset at the input of a class D can result in the amp attempting to put out thousands of Volts, hence any class D should have an input servo to kill any offsets that might be present. This should be built into the input buffer circuit. Our class Ds seem to be fine if the preamp is turned on first rather than last and our tube amps of course don't care either.
there were many krell amps damaged from BAT tube preamps, they got a bad batch of Jensen capacitors. If it would have only been one amp damaged, krell wouldn't have went to the trouble of putting capacitors on the input of their amps with jumpers to bypass the caps if not needed.
@invalid Ah! Thank-you for that. We were sent samples of various oil-filled capacitors about 20-25 years ago but all of them we tried seemed prone to some electrical leakage (as @erik_squires alluded to; when subjected to vacuum tube operating Voltages at the input of the part there was a slight DC Voltage present at the output); enough that we felt it unwise to use them in an audio coupling application; such leakage could throw off the bias in an output section or cause a DC Offset to be present at the output of a preamp. Not all the caps did this but we found over time that some that seemed OK at first developed this problem later so we never used them in any of our products.
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I've used tube preamps with many a Krell amp over the years with no problem.If there is a problem the amp should detect it and shut down until the fault is removed. Years ago when I sold BAT and McCormack. The VK-3 would always shut down the McCormack amps as soon as they were turned on. Removing the pre from the circuit rectified the problem.
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I had a krell ksa-100 that I used with a don sachs pre for 3 years and no issues. I now bought a simaudio 760a and it goes into dc protection mode within 10 min of operation. The sachs does leak some dc at the outputs. Some amps just don’t jive with some tube preamps due to dc leakage. We just put the sachs internally and everything is fine. A transformer coupled tube pre is the safest bet. Krell usually respond very well to tube preamps. It might be that since the recap your amp is now too sensitive. It’s a journey man, it can be frustrating but it sure is fun.
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"The cap failure you experienced might have been caused by not speccing the cap right. If its connected to a plate circuit, it must be rated at a value 15% or so above the no-load DC Voltage of the power supply in the preamp! If not, it can short on turn-on. I have seen some oil-filled parts that have developed 'leakage' but its worth noting that they are not considered to be 'film caps'."
It's possible that I cut it a bit close, with a 300VDC rated cap in a circuit that, worse come to worse, might see 300VDC before settling to 150VDC--though I took plenty of precautions. I'm certainly more prone to err on the side of caution since then. ;-)
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FTR: When I mentioned that film caps get leaky, I meant the DC voltage leaked. This may have no visible effect.
Electrolytic caps OTOH can literally leak, or blow open from internal gasses.
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@atmasphere there were many krell amps damaged from BAT tube preamps, they got a bad batch of Jensen capacitors. If it would have only been one amp damaged, krell wouldn't have went to the trouble of putting capacitors on the input of their amps with jumpers to bypass the caps if not needed.
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Ralph,
Does your direct coupled preamp put out a DC surge when it turns on? If someone has their direct coupled solid state amp on and you turn on your preamp.....could it blow up the amp? Long ago, I had heard that your preamps could blow up things (is this true or not?) I made a pair of custom mono block amps that used the original Hypex class D modules (modified by me). That pair of amps was sold years later to someone else and they were working fine when sold. The new owner had one of your preamps and had the newly purchased amps on and then turned on the preamp. One of the amps immeditately blew up. He called me and we discussed it....that is when I learned that he had one of your preamps and I told him that I had heard your preamps put out quite a DC surge. He seemed to not know this about your preamp.
There is no way to make sure you always remember the WAY to turn on your stereo. We are living in another world/time now, we should not need to "KNOW HOW" to turn on the stereo. There should NEVER, EVER be a product sold to the general public that puts out a serious DC surge when it turns on. Every preamp and every CD player and DAC that I have EVER worked on has DC surge protection. They cannot hurt anything. The orginal Audio Research preamp had no protection....Instead you were suppose to plug in your amp to the back of the preamp and there were two on buttons on the front. You would turn on the preamp first and whistle dixie for a minute and then turn on the amp. And when you wanted to turn off the system you would turn off the amp first and then the preamp....Of course, this was the 1970s.....I owned one of those Audio Research SP-3A1 preamps things. Very soon after that Audio Research put a DC protection circuit on their preamps....so it was NEVER an issue after that......that was the early 80s.......This is 2024. By the way, i have never seen a solid state preamp of recent vintage (within the last 30 years) with a DC surge on turn on or turn off. Most of them are direct coupled and/or DC servoed.....and protected.
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@atmasphere Thank you so MUCH for taking the time to reply. I do feel a bit better. My reply this morning on this thread at 8:16 A.M. lays out my problem that prompted this thread in the first place. Something did happen to my KAV250a. But there was no smoke, no smell, just a soft malfunction-left channel first, right channel second. I'm taking the amp back to where it was re-capped and refreshed in 2022, so next week I'll have more idea about what caused it. Might not have been the pre-amp at all. But, thanks again for your input.
regards,
judsauce
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I have an older Krell KAV amp that has been recapped and refreshed. A technician that I respect very much said do NOT use a vacuum tube preamp with my KAV 250, or any other Krell amp for that matter. Can someone with more technical knowledge than myself tell me why I should not be using a vacuum tube preamp with my Krell? Are there some technical specs that I should be aware of when pairing?
@judsauce There's no problem using almost any tube preamp with a Krell. They have been promoting the myth that you can't for a couple of decades at this point.
Here's the truth of the matter: Regardless of the preamp or amps you own, ALWAYS turn the preamp on first and allow it to stabilize. With a tube preamp this means a 30 second warmup time. Then you can power up the amps. Solid state preamps can have a substantial turn-on thump too, which can easily damage loudspeakers or an amplifier, so this is simply good practice. Many modern preamps have a warmup mute function so people have gotten complacent and I'm sure this is how this myth got started; literally one person damaged their Krell amp in this manner and Krell decided to spread this myth.
People have been using tube preamps with solid state amps for decades (our preamps have a direct-couple output and we have lots of customers with solid state amps; we even make a solid state amp...); clearly this isn't a problem!
@erik_squires mentioned something about film caps (used at the output of many tube preamps; ours have a patented direct-coupled output so not us...) getting 'leaky' over time. To be clear, 'over time' means about 50 years or more. I've seen some film caps from the 1950s and 1960s that have gone bad but since then not so much, and FWIW, they usually open up rather than get 'leaky'. That's something that electrolytic caps, which are not as coupling caps in tube preamps, do.
@dogearedaudio The cap failure you experienced might have been caused by not speccing the cap right. If its connected to a plate circuit, it must be rated at a value 15% or so above the no-load DC Voltage of the power supply in the preamp! If not, it can short on turn-on. I have seen some oil-filled parts that have developed 'leakage' but its worth noting that they are not considered to be 'film caps'.
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There was a thread in another forum recently wherein a fellow was using a highly-regarded tube preamp with a very sensitive SS power amp. The amp kept going into shutdown, and after much discussion with the preamp's designer, it was determined that there was just enough DC leaking from the preamp to trigger the amps input sensors. So, yes, it's a thing, unfortunately.
A while back I was using some very pricey output capacitors in my homebrew tube preamp. One day the sound disappeared on one channel. After a lot of fussing around I finally figured out that one of those pricey caps had shorted, sending 150VDC to my homebrew tube amps. Fortunately, as Lynn says, tubes are pretty tough and there was no meltdown. But I learned a) to avoid pricey boutique caps and b) that tube preamps can definitely be tricky with sensitive amps.
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Regarding the power going out or flickering during storms: I have both preamp and amp going straight into fresh/new sockets in the wall. Whenever there is even the slightest prediction of a storm, I yank the power cables on both until the threat passes. And if I'm away for a long time, I pull them just as a precaution.
This is good practice, but about a third of the time I've seen gear go up it has been stormless. About half of the power events at this location in SC were in clear blue skies, due to some transformer issue, and I'm often not even aware of a short intense storm about to roll through. YMMV, but I've seen enough gear lost to something coming down the power or coaxial cable that I'm willing to "sacrifice" for surge protection.
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judsauce
You will get some great advice here, and many of us use tube preamps in front of our tube and solid state amplifiers......the combination can be a beautiful pairing.
I'd recommend reaching out to some of the best tube preamp manufacturers and have a conversation with them; most of them will give freely of their time.....as Lynn has done already. Ralph at Atma-Sphere and Aric Kimball of Aric Audio sometimes post here as Lynn has.
Some of those that I'd recommend are: Bob at Backert Labs (Rhumba Extreme & Rhythm preamps), Lynn Olson/Don Sachs (Raven preamp), Mick at Supratek, (several great preamps), Ralph at Atmosphere (MP-1 and MP-3 preamps), and Aric Kimball at Aric Audio (Motherlode II and XL and Super 6SN7 preamps).
I spoke with each of them before I purchased my Aric Audio Motherlode XL preamp, and they all were very giving of their time.....and you cannot go wrong with any of those preamps
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Hey @judsauce -
If a tube swap fixed your issues you have your answer. Any leakage at the outputs would not change based on the tubes, AFAIK. In either case, this problem is super easy to measure. Get a multimeter that measures at least down to 0.01 V and put it across the preamp outputs. You should measure very little, if any, DC.
I could imagine it is possible that swapping tubes caused oscillation though, which is a different issue than DC. That's when the pre or an amp starts outputing very high frequencies (the very opposite of DC) which are inaudible, but still heat up the amp and voice coils.
There are some good reasons for mixing tube pre/solid state amp, such as wanting to drive very difficult speakers. Electro-statics for instance can really shine this way.
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Interesting read. I used a tube preamp with a SS amp for a year without any issues, but reading all this, I will definitely stick to SS for both from now on. Tubes was not my thing. Too many updates temptations.
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@mattw73 Thanks for your reminder about the sequence of powering on and off.
I have to admit, I sometimes forget and then end up crossing my fingers. Not good.
Regarding the power going out or flickering during storms: I have both preamp and amp going straight into fresh/new sockets in the wall. Whenever there is even the slightest prediction of a storm, I yank the power cables on both until the threat passes. And if I'm away for a long time, I pull them just as a precaution.
Like you, I also prefer some tubes in the chain. The Krell paired with a SS preamp sounds a bit bright for my taste. And there is nothing like high volume creamy distortion when driving a class D tube guitar amp. Buttery smooth and so fun to play. But a vacuum preamp, for my taste, rounds out the brittle high end and adds a bit of warmth.
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@lynn_olson Your input is much appreciated.
HERE IS THE SKINNY:
I have 2 KAV250a's. In May, 2022, I had the oldest one recapped with some new resistors, etc, to the tune of $1,600.
Being a guitar player with several tube amps, I always loved the warmth and body vaccum tubes offered.
In March of 2023 I bought the Audio GD Vacuum HE1-XLR BALANCED LINE PREAMP. I used that along with my OPPO BD-95 for cd, and my Sonic Frontiers SFP1 phono pre-amp. The phono preamp was recapped and reconditioned by Glenn at Parts Connection. Those sources, along with the pre-amp and amp have logged over 220 hours in my diary with absolutely no issues. The sound, to me, was fabulous.
Fast forward to this past week.
THESE ARE THE EXACT CONDITIONS IN PLACE WHEN I HAD A MALFUNCTION: I had just purchased and installed a pair of NOS Mullards and a pair of Gold Lions in the Sonic Frontiers phono pre-amp. I fired everything up and put on Dire Straits lp, Leaving Elvis. Before the song was over, the left channel went silent. Then about a minute later, followed by a mild and dull low volume "thud", the right channel went out. No smoke, no smell, just no music. My trouble shooting revealed that the amp was not working. So, I swapped in my second KAV250a, immediately put back in place my original tubes in the phono pre-amp, and have only played the OPPO blue ray so far. And everything is fine.
The Krell has 26.4dB of gain, input sensitivity of 2.15 Vrms, and 100K ohms input impedance.
The Audio GD Vacuum HE1 XLR LINESTAGE PREAMP has an excellent review by STEREOPHILE, whose link I will attemp to paste below. I bought it from Walter and Underwood Hi-Fi (whom will be retiring this week, he sold the company.)
The amp is going back to the technician next week. So, is this a bad combo, in your opinion? By the way, the malfunction could be independent from the tube swap in the phono preamp, and it could be coincidental.
All opinions welcomed. Have a great Holiday Weekend, and pray for our soldiers.
Jud
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I think the thing to consider is the DC offset or surge from the preamp. Turning the tube preamp on first & waiting a couple minutes to stabilize usually does the trick. Turning the preamp off last during shutdown of the system is also common knowledge. I run tube preamps with solid state and on a couple of occasions have turned on the tubes not knowing the solid state power amp was already on. One of my amps has protection so it will go into protection mode if I screw up. My other amp blew a fuse & shutdown after being lackadaisical with my on/off process. I initially thought my subwoofer blew up due to the loud thump. When my amp with protection shut down it first sent a loud squealing noise through the speakers.
I've never had any significant or permanent damage but the risk of it happening is real. It's a scary feeling when you think you fried your gear. Another thing to consider is a quick power outage..that just as quickly power's back on. This can cause the DC effect as well off certain preamps. I don't know the correct terminology or actual effects from an engineering standpoint but I know it's not good.
With all that said I do enjoy my digital music thru some type of tube gear.. preferably a preamp. So that's what I choose to do. I don't really worry about it & enjoy the music...& give my tube gear time to stabilize before turning other components on. One should always make sure that their tube pre or power amp is up to spec. Buying used vintage tube gear should be looked over before hooking it up to expensive equipment or speakers.
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The big thing to confirm is the preamp under consideration has an auto-mute circuit, which triggers both for turn-on and turn-off, as well as brief power drop-outs. Not every preamp has a power management circuit that protects against all sources of transients. This is ESSENTIAL if you intend to mix a tube preamp with a transistor power amp.
Voltages over 20 to 50 volts will kill or degrade the input transistors, which are not designed for high voltages. A transistor power amp has no way of defending itself against transients of this magnitude ... I’ve yet to see a transistor power amp with an input relay. For that matter, coupling caps are quite rare in the transistor world. In any event, an input coupling cap will not protect the input section against a transient that large.
By contrast, a tube amp will tolerate a transient of hundreds of volts, even at the input. Not for long, of course, but for a few seconds, yes. To damage a tube amp the plate of the tube actually has to melt down, or more likely, burn up a cathode resistor. That takes time. By contrast, a transistor will fail from over-voltage or over-current in less than a millisecond, too fast to run across the room and turn it off. You’ll see a little puff of smoke emerge from the power amp and that will be it ... next step, a new circuit board with all-new parts. (I’ve seen this happen.)
There is no practical way to protect a transistor amp from what any manufacturer considers abuse (yes, the warranty will be voided, and yes, they can tell when the tech sees a burned circuit board with dead input transistors).
For better or worse, the burden of protection lies with the design of the preamp. Even transistor preamps can be the culprit if it has a split +/- 15 volt supply and one side folds down because it doesn’t feel like working any more (or if a regulator quits). That will yank the preamp output to either plus or minus 15 volts, it will stay there until repaired, and bye-bye power amp.
The real function of muting relay circuits is preventing these disasters. As mentioned earlier, output transformers can also prevent DC from getting through, and DC transients don’t make it through, either, because the transformer rejects all signals below 14 to 20 Hz.
Direct coupling has many advantages, but unfortunately, failure modes can propagate through the entire electronics chain unless protective measures are taken.
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Krell does not use coupling caps at the input which can result in problems if DC is leaked in from the preamp (both for the amp and speakers). The upside is better transient response as the signal is not impeded a constant charging and discharging capacitor.
Even though the amp will have some sort of DC protection, it may not be enough to overcome a tube preamp DC spike per lynn_olson’s post above. You should contact that tech again and ask if the amplifier has coupling caps that can be activated in order to use a tube amp.
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Ps, recapping updates power caps, rarely signal caps.
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@immatthewj you're too funny. Nothing to get off the pot on. Check my virtual system. It's the preamp that I have that's in question. It has the best music coming out of my system that I have ever heard. That's why I'm scratching my head.
@lynn_olson thanks so much for that information. It was very detailed and much appreciated. A big wealth of knowledge that I hope others studied as well.
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I know zero about the technical issues being discussed in this thread. I can speak only of my own experience. I ran a Krell KAV 250a with an Audible Illusions Modulus 3A preamp for over 20 years….without a single technical problem. And it sounded great. YMMV
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May be worth putting this into perspective. Your typical SS pre may have +- 15V rails. You could put your tongue on that and survive.
A tube preamp, like an old TV or amp, has potentially lethal voltages.
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Conventional vacuum tube preamps often use a "cathode follower" to assure low output impedance and compatibility with a wide range of power amps ... vacuum tube, transistor, Class D, GANFet, etc.
The plate circuit of the cathode follower is typically at 300 volts DC, and the cathode (the output) is typically at half that, or 150 volts DC. A blocking capacitor is used to remove the high voltage, leaving the output at zero volts DC, but when the preamp is turned on or off, significant transients can make it through the blocking capacitor. The most common strategy to deal with this is a shorting relay with a time-delay circuit ... that’s the faint "click" some preamps make when they are turned on or off.
Although it is good practice to turn on the preamp first, wait a minute for warm-up, then the power amp, and turn off the power amp first, followed by the preamp, this mostly protects the speakers from loud transients. A transistor power amp can be damaged, even if it is turned off, if the input transistors are exposed to more than 20 volts. That’s why a good vacuum tube preamp will have a muting relay as part of the circuit ... but not all tube preamps have a muting relay.
The preamp I designed for Don Sachs uses an output transformer, which are free of turn-on and turn-on transients. But coupling capacitors are much more common than output transformers, and muting relays are pretty much a requirement for that kind of circuit.
Old-school 1950’s preamps almost never had a muting relay, but all power amps used tubes back then, and they aren’t affected by transients (although speakers are). It was OK to simply turn on the whole system at once, since the tubes in the power amp took the longest to warm up, which protected the speakers.
Things got more tricky when DC-coupled transistor amps became the norm in the early Seventies. A mix-n-match of a traditional tube preamp could easily damage a first or second-generation transistor power amp, and maybe take out the speakers, as well. Modern tube preamps typically have a muting relay, which is there to prevent transients or failure-mode DC offsets.
You are not necessarily home free with a transistor preamp. Some use DC servo circuits to automatically zero out DC offsets, and if this servo circuit fails, and presents the power amp with several volts DC, that can take out a DC-coupled power amp, and damage the woofer, as well. Loudspeakers can only tolerate less than a watt of DC offset.
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@judsauce , well, we’ve got the long weekend coming up, so maybe that is having an effect on whose there to answer the phone. We’ll hope so anyway. Maybe if you give them a call on Tuesday you will get through. Have you got your eye on a pre that you are thinking you better get crap or get off the pot on? If so, I do know the feeling.
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To help explain, the maximum voltage in an amp is around 8V for pro audio gear, and around 2 I think for consumer. So even a few millivolts offset can cause a pop.
The issue isn't just the offset it's a step. So consider a preamp that's off, but normally has a few milliamps of offset. When you turn the preamp on, if it has no delay relays, you go from 0V to say 0.010 V, and that sudden step is the popping you hear. A cap in the way still passes the step, but over time blocks the 0.010V. So if you looked at the wave form on the other side of the cap you'd see a spike, then decline to zero, even though the preamp circuitry may still have 0.010V on the output. Of course, some of it may just be the random nature of the amp getting powered up even without a final offset.
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@immatthewj - It's usually a little DC. DC = direct current which can happen from leakage or a preamp which doesn't settle at exactly 0 volts.
Easy and save enough to measure. Any multimeter will let you know how much DC offset there might be in your preamp. A few millivolts is fine.
OP: I should mention that most leakage is in the low V range, so your average input coupling cap will handle it. You can always measure the DC yourself.
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@erik_squires , can you explain what happens that makes that "pop" when a preamp is inadvertantly turned off before the amp?
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I tried calling Krell close to where I am, they're in Connecticut. But when I went to leave a message with tech support I got a nice sounding man who said, "please leave me a message". Then his mailbox promptly told me, "voice mail box full". So, I left a message with sales. 50 bucks says I'll never hear back. Not like the old days, for sure. Wife runs the company now, Dan is off selling, well, you know what.
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If you haven't bought a tube preamp yet, you could buy a transformer coupled tube preamp, then you would never have to worry about it. Most capacitor coupled tube preamps are fine with solid state amps. Again just remember to turn your system on in the correct sequence because the tube preamp has to stabilize before the amp is powered up.
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OP:
Tube preamps use high quality film caps that usually have a life span longer than ours, but failures do happen and they may be small (few millivolts) to larger.
The input coupling caps on amps may vary depending on the quality of the amp. Lots of Yammies used bi-polar electrolitic. 40 years or so?
But worth getting into some details. The preamp coupling caps are rated for the voltage in the preamp, so 650V caps are not uncommon. Amp caps though may only be 50 V. Should the preamp cap fail and deliver 500V, it will absolutely break through the amp’s coupling cap and go boom. :)
If your preamp caps have just been replaced with high quality films they’ll last you a hundred years if they last you five. :)
The real guru on this subjecte though is going to be the manufacturer, @atmasphere who will correct me at length if I'm wrong.
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@immatthewj
@invalid
Thank you both!
@erik_squires
When you say "over time, the coupling capacitors can leak", what if they were replaced during the reconditioning? And, what would you consider a time frame for malfunction: 50 hours, 150, 250, or over depending on use?
Thanks again, everyone!.
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Typically you want the input impedance of your amp to be more than 10 times the output impedance of the preamp for optimal performance. The input impedance of your amp is 10 kOhms so as long as the preamp is below 1k Ohms (as many are) it should be fine unless your amp is different from almost any other solid state amp. I’d shoot Krell a call just to make sure, but I highly doubt it’d be a problem to use a tube preamp. As mentioned above, many people here use a tube pre with a solid state amp with no problem and wonderful results.
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Actually, OP, after reading @erik_squires reply, I do now remember someone posting about losing a pair of speakers due to what @americanradiance just described. That must be what your tech was talking about.
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The combination is often used and can be a magical. The tech is being overly cautious, but not entirely making things up.
Solid state preamps and tube preamps produce the same output voltages when working correctly. The issue with tube gear is that internal voltages are often 400V or greater. Most tube preamps use a "coupling capacitor" at the output to block that high DC voltage to nearly zero and let only the music go through.
The problem is on the rare occasion and over time the coupling caps can leak. If the amplifier lacks it’s own coupling cap on the input, or that coupling cap is not rated for the leaked voltage you can push the amp into overdrive, not to mention send your speaker cones ballistic.
Most solid state amps are "ac coupled." Meaning they use a cap on the input to prevent just such a problem. I believe I read somewhere that some Krell amps have an internal jumper in case you are not using a tube pre and want to avoid your music going through it. Worth checking on it.
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I was told that vacuum tube equipment was "old 1940's technology." And that tubes created conditions and extraneous signals that could/would fry certain components within the amp. Is that so?
I don't have the knowledge and understanding of electrical theory to comment on this from a technical perspective, but I would say that if you scroll through tyhe pages of this particular forum (amps and preamps) you will come across a ton of threads about, or icluding, people running tube preamps with ss amps. I suppose there may be some, but off hand I cannot remember anyone posting about the combination damaging the amp. But, as I typed, there may be some, but I think far and few between. I cannot comment from my own experience because I have been running tubed preamps in front of tubed amps.
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I was told that vacuum tube equipment was "old 1940's technology." And that tubes created conditions and extraneous signals that could/would fry certain components within the amp. Is that so?
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I’ve been using a tube preamp for several years with my krell ksa 300s, it’s just as safe as using a solid state preamp. The reason this subject always comes up is because there was a certain BAT tube preamp that leaked DC and damaged amplifiers. Just make sure you turn your system on in the proper sequence, amp should be last to turn on and first to turn off
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Didn't the tech that told you not to do that tell you why?
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