Krell kst 100 power amplifier review năm 2024
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Manual Library / Krell Show Stereo Power Amplifier (1990) SpecificationsPower output: 100 watts per channel into 8Ω (stereo) Frequency response: 20Hz to 20kHz Total harmonic distortion: 0.1% Damping factor: 150 Input sensitivity: 1.5V Signal to noise ratio: 96dB Speaker load impedance: 1Ω (minimum) Dimensions: 6.5 x 18 x 14.8 inches Year: 1990 Downloadsbrochure schematics (low res) schematics (rev c) stereo switch replacement (rev g) Krell Power AmplifiersKSA-50S MDA-500 I bought this from US Audio Mart a couple months ago. The seller played dumb and said it was in great shape and probably had never been opened... hahaha... When I got it (for a fair but not great price), I connected it up and it sound good (replacing a fully restored Audio Research D100B) but every once in a great while the right channel would drop down and distort. I figured that might be the output protection relays that the output goes through. I opened the amp up and saw that while all the output transistor date codes indicated they were original, there was a lot of half-assed soldering and repairs done on this unit. A bunch of small transistors had pieces of bus wire connecting tracks and the differential input connectors had wires from connector to circuit board instead of direct mount. The first thing I did was replace all the electrolytic caps except the big PS caps which measured good... besides, I couldn't find no similar sized replacements. I also replaced the diff input connectors and speaker binding posts along with the speaker relays. I also cleaned up all the previous 'repair' work. I put it in the system and adjusted the bias to 100mV and it worked great for a couple months. Recently the right channel started dropping/distorting again and I happened to notice one time when it did this that the bias for the right channel dropped way down. A little later when powering on the amp it hummed loud with bias staying at 0, and shut itself down. I took the amp apart and found that the right channel + rail was down to around 12 VDC. Seems like the bridge rectifier died for this rail (there are 4 bridge rectifiers, + and - for each channel. I noticed that there was some sparking and a small amount of smoke from around this part when testing just the PS so I'm hoping that the bridge rectifier is the source of all the issues since a drop of the + rail would certainly cause that channel to distort and the bias to drop... One thing I learned working on this amp is the circuit board is very thick with huge traces and takes a load of power to solder/desolder anything. I think part of the source of this amp's problem is cold solder joints in the power supply. So the questions... Anyone have experience working on Krell amps? Anyone seen power supply problems or issues with factory solder joints... Any comments or advice welcome. Since the bridge rectifier seemed to die during power-up surge, is that a common occurrence? These are 800V 35A bridge rectifiers.
Hello, I am working on some Krell equipment too. Two KMA-100-MKII monoblocks. They are my first experience with Krell. I am just a hobbyist but know how to work on equipment and solder. These Krell's though are different animals. Yes they do have thick boards and huge traces and components soldered on both sides of the board. So I am going to reserve judgement on anyones repair work on these amps. The old solder is very difficult to remove and the traces (at least on mine are very easily burned. I ended up purchasing a Hakko FR-301 desoldering tool and while it did help I still had to resort to the wick. I am sorry that I cannot offer any help but I saw your post and want to wish you luck. My post is in Solid State and I have gotten very little help. Footfungus has been the most help but apparently he must be very busy right now. He advised me to try Diyaudio and I am going to do just that. You may want to look into that also since your post has been up for a few days without a response. There just doesn't seem to be that many people who work on Krell's. Just saying.
The I bought this from US Audio Mart a couple months ago. The seller played dumb and said it was in great shape and probably had never been opened... hahaha... When I got it (for a fair but not great price), I connected it up and it sound good (replacing a fully restored Audio Research D100B) but every once in a great while the right channel would drop down and distort. I figured that might be the output protection relays that the output goes through. I opened the amp up and saw that while all the output transistor date codes indicated they were original, there was a lot of half-assed soldering and repairs done on this unit. A bunch of small transistors had pieces of bus wire connecting tracks and the differential input connectors had wires from connector to circuit board instead of direct mount. The first thing I did was replace all the electrolytic caps except the big PS caps which measured good... besides, I couldn't find no similar sized replacements. I also replaced the diff input connectors and speaker binding posts along with the speaker relays. I also cleaned up all the previous 'repair' work. I put it in the system and adjusted the bias to 100mV and it worked great for a couple months. Recently the right channel started dropping/distorting again and I happened to notice one time when it did this that the bias for the right channel dropped way down. A little later when powering on the amp it hummed loud with bias staying at 0, and shut itself down. I took the amp apart and found that the right channel + rail was down to around 12 VDC. Seems like the bridge rectifier died for this rail (there are 4 bridge rectifiers, + and - for each channel. I noticed that there was some sparking and a small amount of smoke from around this part when testing just the PS so I'm hoping that the bridge rectifier is the source of all the issues since a drop of the + rail would certainly cause that channel to distort and the bias to drop... One thing I learned working on this amp is the circuit board is very thick with huge traces and takes a load of power to solder/desolder anything. I think part of the source of this amp's problem is cold solder joints in the power supply. So the questions... Anyone have experience working on Krell amps? Anyone seen power supply problems or issues with factory solder joints... Any comments or advice welcome. Since the bridge rectifier seemed to die during power-up surge, is that a common occurrence? These are 800V 35A bridge rectifiers. No experience working on Krells, but the description of your problem doesn't sound like a bad bridge rectifier. I've seen quite a few bad rectifiers and they always fail to a dead short. This immediately leads to a blown AC mains fuse or rail fuses depending on how/where it's fused. You should be able to measure the bridge in question and see if all 4 diodes are operational. It's possible that one of the main PS capacitors is failing and loading the supply down. Here again any time I've seen this the capacitor failed to a dead short and blew the main AC fuse, but there might be a chance it could fail to a partial short.
Thanks for the responses. Yes, I have an old Ungar soldering iron that doesn't have the power to melt the solder on those big PS board tracks - got a higher power iron on order. I checked the caps when I originally did the amp and they all still measure fine (all still within 10% and esr < .10 and put 60 VDC on each). What boggled my mind was that I turned all the lights off when the turn-on stuff first started occurring and I could see light from arcing on the PS board around two of the rectifiers. As you mentioned the power switch breaker started tripping shortly after like something was shorting. My guess is that there was a cold solder joint under one of the rectifiers and there was some arcing and maybe the pulsing through the diodes into 33K uF caused some big current spikes and maybe one or two internal diodes shorted. Only a guess but I'll check it out once I get them off the board. Already have 5 new rectifiers from DigikKey ready to go and I tried putting an external voltage source on the bad channel with no excess loading so I don't think any of the power output transistors shorted.
Removed a couple of the rectifiers and see the telltale mark of some arcing around the + leg of the device for the right channel + rail. It looks like they cut the legs short to use wave soldering. I'm going to leave extra length and bend the leads over onto the pads to get a lot more solder area.
all 5 rectifiers replaced and PS worked fine so I buttoned up the amp and brought it up on a variac. Everything is good - rails are stable where they belong for both channels. Very odd failure mode but I'm glad it wasn't worse.
Nice find and that's great news! There's not much that's better than finding a cheap and readily available part that's the cause of your troubles. Much better than some obscure part failing and finding out that it's been out of production for >20yrs. FYI, I use one of these iron's for really high heat needs. It'll flow solder on a heavy Cardas binding post with a 6ga wire in about 4 secs flat. The only downside is that its a Japanese version so it needs to be hooked up to a variac or something to limit the power to 100VAC. Enjoy the Krell, I used to lust after the KST-100 back in the 90's when it hit the market.
OK, so now that the PS is back in operation I'm still left with the nagging problem I saw occasionally from the start, mentioned above. The right channel when playing quietly will drop in volume and distort intermittently. I'm trying trace the cause. When the output drops, the bias drops from 100mV to 0 volts. The output level drops as mentioned and only shows positive going waveform like a half wave rectifier when the amp is fed a sinewave. If I increase the input to the amp it starts working again. I originally thought it might be something in the bias circuit itself but that didn't seem to be the case - the DC component around the circuit didn't seem to change when the signal output dropped. In the below schematic, based on how this amp seems to have solder issues and maybe component issues I thought it might be the pair of 100 ohm resisters tying the bases of the output transistors together. If the bottom resister opens then that loses bias and requires a higher signal level to overcome the transistor junctions... I think. Those resisters are tucked under the circuit board and aren't accessible when the amp is in operation. It also might be the drive circuits prior to the bias circuit. You probably guessed I can trace signals and voltages but don't know a lot about amplifier design. If anyone can help out with how the sections of this amp impact bias or what I'm seeing, I'd sure appreciate it.
Not a Krell expert, but based on the schematic and my own experience The 100 ohm resistors opening will cause the bias to increase, not decrease. I'd be looking for cold or poor solders,but my first check would be connections to the collectors of drivers and outputs, especially the negative driver. The connections are through the mounting screws, and could give you intermittent issues. You also could have issues with the socket connections. Because of all the parallel oututs, not likely to be an issue with them, but you never know. Also, (minor nit pick) but your drawing has the voltage gain transistor bases connected to the emitter of Q61- should be collector.
Thanks Sregor for the comments. The copper buss bars connect the power supply board and outputs and bolt to the main board. All of these were tight as well as all hardware on the TO-3 devices. There are no sockets - everything is soldered in place. I followed your advice and took the amp back apart and carefully inspected most everything. Similar to what I found around the bridge rectifier, I found some indication of an arcing connection around the feed from the +39 VDC. The feed comes through a via and resistor leg to the main traces on the bottom of the board. This feeds both the servo circuit and most of the amp except the output. Have a look at these schematics and in the Power Amp 1 schematic just past the resisters, zener diode, and cap up top is where the line was flakey. I'm thinking this would cause the drops in the DC levels I saw around the bias circuit and would definitely mess things up. I have the amp back on the bench to monitor if the issue recurs. Note that I didn't draw these schematics but they best match the late revision amp I have. Let me know if you have any comments.
This is frustrating - After touching up the solder joints to keep connection for the +39V, I let the amp sit on the bench powered up all day yesterday without incident. This morning I powered it up and about an hour later, the issue is occurring. Bias dropped to 0, DC voltages are good. I probed around and don't see where the problem is. An item of note is I monitor the signals at emitter and collector of Q60 from the first schematic above. When running normally, the collector signal is riding on 1.25V DC level and the emitter is running on -1.1 DC level. This is the same as the 'good' left channel. When the issue occurs and bias drops to 0, these signals change to collector .65V and emitter -1.8V so it seems like something flakey with the + voltage supply which made the 39V flakiness seem like a likely culprit. As I was probing around the lower level stuff and the servo circuit, the issue disappeared. Since the signals never go away at output or in-circuit, it seems like this is something in circuit balance that's causing it. A better view of the schematic is below and note it's Q45 that I would be monitoring for DC level in this one. Any ideas on what could cause a drop of the signal DC of .6 volts at Q45 emitter and collector? Last edited: Jan 2, 2023
Well you're getting there, slowly but surely.... These intermittent problems are the worst because once they go away you have to wait for the problem to occur again. The fact you're just losing one Vbe drop and not dropping to zero volts at the base of Q45 when the failure occurs says you should be close to where the problem is. The schematic you last posted isn't a high enough res to make out the part numbers. Whats your voltage drop across the resistor in the prior stage (arrow pointing to it in attached pic) the one attached to the +V rail, also check the same resistor on the opposite polarity tied to the -V rail. These resistors should be dropping around 1.9v when working properly because you need enough voltage here to bias 3 Vbe drops + the extra mV for output bias in the later part of the circuit. If the voltage drop here is unchanged when the failure occurs then this would say the fault is after this point in the circuit. I'm guessing it is, but it would be best to verify. I'm guessing its going to be a cold solder joint or a cracked trace either of which are due to heat cycling over the years.
Thanks Mondialfan... Below is a more detailed version of the schematic along with a bunch of points measured both when in a bad state and not. I typically monitor with 10x scope probes at points 8 and 9 to detect when the issue is occurring but noticed if I remove the probes the issue occurs more easily... I'm thinking the noise pickup from the probes helps prevent the issue? From the measurements it seems like when the issue is occurring the rails are unloaded causing a bunch of the levels to increase slightly which I would guess is caused when the bias drops to 0 and the circuit is more or less just idling with no loads. You asked about the drops across the 2.6K resistors at the emitter of Q15 and 16 - the drops are 3.23 + and 3.2 - (good) and 3.35 + and 3.39 - (bad). The rail voltages wander around a bit whether working or not so some of the measurements may seem slightly off what they should be. Any help/guidance you or anyone can provide is greatly appreciated. Happy New Year! Note attached schematic still low res, see below Last edited: Jan 2, 2023
Just a shot - when malfunctioning, base of Q32 goes more negative, but no bias and output stays at zero volts - this suggests (to me) that either Q32 is opening up, or perhaps losing the negative voltage on it. or maybe connection between emitter and outputs... My 2 cents.
You're correct that amount of voltage drop across the 2.6 k resistors is to be expected with the voltage difference on The rail between bad and good. It's because they're using fixed resistors and not an active current source to control that branch of the circuit. Your voltage is at 4, 5, 6 and 7 seem balanced between good and bad so I don't expect there is a problem before there. I'm still thinking the problem lies somewhere after Q15/16 but before q27/33. I would test the voltage drops across r45 through r52. These should be fairly uniform. It could be that one of these mpsw devices is breaking down when hot. These are a one-watt rated transistor and based on your readings they should only be dissipating approx 175mw but there's likely a lot of heat in that environment so one of them may be failing. Last edited: Jan 2, 2023
Thanks Steve - The connections to Q32 and later in the circuit are all under the circuit board - I'm working on a way to take some measurements around Q31/32 and later. I'm wondering if there is something going on this area and the servo control compensates the imbalance in feedback but I would think the numbers at reference points 3/11 would shift. One item of note is that Q38 and Q39 are touch sensitive when the amp is in the bad state. Touching Q39 causes causes the level at schematic ref 8 and 9 to temporarily drift more negative. Touching Q38 causes ref 8 and 9 to go more positive. Touching these transistors when the amp is in the good state will causes changes but not as much as when in the bad state... Red herring? Thanks Mondialfan - I checked the drops across all resistors R45 - 52 and they stay fairly consistent: On the + rail, R45/47/48/49 are all 2.6 volts drop in the 'bad' state and 2.58 in the 'good'. On the - rail, R46/50/51/52 are all 2.7 volts drop in the 'bad' state and 2.64 in the 'good'.
Just wondering, have you tried using something nonconductive and putting some pressure on various areas of the board. I've found poor connections and cold solder joints in this manner before. It doesn't isolate the problem but if it suddenly goes away or it gets noticably worse when you press on something then it clues you into what the issue is. If it's a lot of work to tear into it then I'd probably shotgun a few parts just to eliminate them from the equation. The bias transistor, it's hard to tell but looks like Q45, and the 100ohm adj pot. I'd also replace Q31/32 all of these transistors are in stock at Mouser. I'd also consider replacing the Q31/32 sockets if they are socketed parts. If you don't replace them then make sure the pins fit tightly. I don't see where your test point 11, which is the voltage from the DC servo, is changing that much from good to bad. I don't think those parts q38 and q39 that you're seeing a shift when you touch them are a problem. Those two transistors are a current mirror and have to be matched pretty well so just touching them and the heat from your fingers throws that match off.
I've tried flexing the board (It's thick/heavy and well supported) and parts and I've also tried a heat gun and freeze spray to try to initiate something to no avail. My next steps will be to try to do some measurements around Q31 and 32, then I think I'm going to try to reflow some of the joints in the Q18-Q26 circuitry. None of the parts are socketed in this amp so everything is hard soldered in place. I have a bunch of the Q31/32 types already from when I was transistor matching my ARC D100B power amp so those would be a relatively quick/cheap replacement. I added the drawn layout and the one I've been using for the schematic in the google share. If you look closely you can see the left channel circuitry which is at the bottom of the photo has lots of little bus wires and blobs of solder, maybe for the same issue occurring in that channel in the past. The left channel works flawlessly at this point. At this point, I have too much time and money in this amp to just let it go so I'll keep at it - when it works it's a very nice amp. Is Krell a good amp?Krell amplifiers has long been synonymous with high-end audio excellence, standing as a trailblazer in the world of audiophile electronics. How much power does a Krell KSA 100 use?Power consumption: 100W, idle; 1400W, full power. Dimensions: 19" W by 8.5" H by 19.25" D. Weight: 80 lbs (net). Serial number of unit tested: 32-20216. Who makes Krell audio?Krell Industries. Which amplifier is best in the world?7 Best Power Amplifiers of 2024. Crown Power Amplifier.. Rockville Power Amplifier.. Gemini Sound Power Amplifier.. Fosi Audio Power Amplifier.. AIYIMA Power Amplifier.. Behringer Power Amplifier.. MUSYSIC Power Amplifier.. |
