• Content count

  • Joined

  • Last visited

Community Reputation

0 Neutral

About montreal

Contact Methods

  • ICQ
  1. popcorn noise in legacy amp

    I don't think the noise is activated by a spike or dropout on the power line. The internal power supply of the CT-400 is very robust and regulated and I doubt if a dropout would get through. There are varistors added inside the electrical panel and the amp is fed by current passing through a computer power director which contains additional varistors. The amp has been in the basement since 1993. In 2002 it was placed in a rack inserted into a partition wall. The room temperature and humidity of the basement vary throughout the year, but the noise was more prevalent in summer. The amp is left on 24/7 and feeds speakers in the basement and two other ground floor rooms. including the master bedroom. I have been awoken from my sleep in the middle of the night by the bedroom's right side speaker starting to emit the popcorn noise. If I shut down the amp and restart it imediately, the noise returns. If the noise is precipitated by a neighbor's wireless device, I can't explain why such a device would be operated at 3 AM in the morning. It may be a coincidence that the noise stopped when my neighbor moved away. It may be that keeping the chassis as cool as possible by having the chassis fan stay on permenantly is the reason the noise has stopped occurring. Today I removed the patch wire that was keeping the fan on all the time. The basement room is now very quiet compared to before. It may be months from now before the same conditions repeat themselves which lead to the noise returning. Thanks for your comments.
  2. popcorn noise in legacy amp

    What I remember doing a few years ago was swapping the quad op-amp chip between channels and the problem did not move. Then I cross wired the output of the 2nd op-amp (right channel) of the quad to the input of the 3rd op-amp (left channel) of the other quad and the problem did not move. That told me that the problem was in one of the first two op-amps stages of one particular channel and was not related to a particular quad op-amp chip. Since the Crown CT-400 is basically a DC amplifier, there are no de-coupling electrolytic capacitors between stages. Whatever few bypass electroylitics there are on the main board, I replaced these without improvement. That left me with all the low wattages carbon resistors and ceramic capacitors and the volume control surrounding the first two input stages as being a potential source of the thermo-sensitive pop-corn noise. My conclusion is that perhaps the passive resistors and capacitors in the first two stages are causing the op-amp chip to oscillate at RF frequency and the popcorn noise is a by-product of the oscillation. I cannot prove that the cataylst for the noise is my former neighbour's wireless signal, but if my Crown amp was designed in the 1980's before the introduction of domestic 2.4 giga-hertz RF signals, then it is possible that the original circuit has no protection against this. Again, we will see if my restoring the fan's original control thresholds causes the popcorn to re-appear. Thanks for you comment.
  3. My 20-year-old CT-400 amp occasionally generated popcorn noise in one of the two channels. I noted this problem on this forum prior to 2008. I traced the noise back to the second op-amp stage of the input since the noise continued when I cross wired the first op-amp stage from left to right and the noise disappeared when I closed the volume control on the back of the amp. Swapping the Motorola 2-stage op-amp chip did not change the situation. Then my neighbour moved away 18 months ago and I have not had the problem since. My conclusion was that one of the neighbour's domestic wireless devices (wifi router or cordless phone) may have been broadcasting into my basement and then via the cabling into the Crown amp. Another corrective measure which I undertook 5 years ago to reduce (but not entirely eliminate) the frequency of incidents of popcorn noise was to hot wire the internal fan on 24/7 in order to maintain a lower chassis temperature when idling. Now that the noise appears to be permanently gone, I am thinking about removing the fan bypass jumper wire in order to reduce wear and tear on the fan and create a quieter background in the home theatre room. If the popcorn noise stays completely away without the fan operating continuously, then I will conclude that the neighborhood wireless transmitters had something to do with the popcorn noise.
  4. CT-400 with 2.7 ohm load

    Thanks for the warning. I was rather hoping that the fact that I'm keeping my peak output voltages so low (10-15 volts), the peak output current into 2.7 ohms (4-5 amps) would never trigger the current limiter protection. I'll look into getting a supplemental amp. Thanks again.
  5. Hello, Touch wood, my 16 year old CT-400 (rated 225 watts per channel into 4 ohms) is still going strong in my home theatre environment. I have recently added two additional pairs of stereo speakers in other parts of my house and I would like to parallel all speakers, meaning there will be three 8 ohm speakers in parallel per channel. In theory, I will have 2.7 ohms of inductive speaker load. In practice, I already know that my DC resistance is at 2.2 ohms. The three speaker pairs in question are all high efficiency vintage Tannoy drivers (15 and 12 inch) and they use relatively little power in order to generate between 80 and 90 decibels. In spite of the increased speaker load, I still anticipate never drawing more than a few dozen watts from the amp. I put an AC ammeter on the power input to the amp (120 VAC input) and at idle the amp draws 0.5 amps and with the speakers driven to high volume, the current bounces around between 0.6 and 0.9 amps. The OPED lights never dim and IOC lights never flash. Everything sounds great throughout the house. So my question: Can I operate like this indefinitely without creating any harm to either amp or speakers? Thanks
  6. Thermal noise in CT-400B

    One of the additional measures that I did was to re-seat the PIP cable in the main board socket. I reinstalled the amp in the rack, but left the bottom cover off. I placed a digital thermometer remote probe in the narrow space above the main board and noted that the temperature varies between 90 degrees F and 110 degrees F, and the fan never comes on as very little power is drawn by my high efficiency speakers. A few inches below this bottomless Crown amp is another power amp that throws off a few BTUs of heat. This explains what drives the main board temperature up to 110 degrees. The good news is that the popcorn noise has completely disappeared. Could that be uniquely due to removing the bottom cover or to re-seating the PIP cable? I put back the bottom cover and the main board temperature is now stable at 120 degrees F with very little variation. And still no noise. So I am wondering if 120 degrees is a reasonable operating temperature for a Crown main board, since most active and passive electronic components have a permitted operating temperature range up to 85 degrees C or 185 degrees F.
  7. Thermal noise in CT-400B

    Sorry Kip, I copied in your last name which you had mistyped in another post. It's Whitehead.
  8. Hello, In an adjacent thread called "Popcorn noise in CT-200", I previously discussed two issues I was having with my CT-400 (I thought it was a CT-200 when I started the post). The CT-400 series uses "Grounded Bridge" technology. One problem I noted was the fan failing to stop and the other was an intermittant thermal noise which only occured during warm summer weather when the ambiant basement temperature rose a few degrees above average. I was able to solve the fan problem. With respect to the thermal noise, Kip Whitehad suggested that I swap the front end op-amps between channels to see if the problem moved with the opamps. Only this summer did the noise problem return often enough that I was finally ready to follow through on Kip's advice. Yesterday I swapped the front end quad opamp (33079P) between channels and I also changed all the electrolytic caps (2 per channel) that were used in the front end. The problem did not move when the opamps were swapped, so all the active components held within the opamps are working fine. As well, the sporatic bursts of noise, which sound like you tuned an old style AM radio between stations during a thunderstorm, this noise completely disappears as soon as the volume control is lowered to position zero. The front end is made up of three stages, the Balanced Gain Stage (BGS), the Variable Gain Stage (VGS), and the Error Amp. This leads me to conclude that the Error Amp stage must be OK. The fact that the music is not distorted when the noise is present leads me to believe that the DC balance of the BGS and VGS opamps is also OK, because if these opamps were not balancing DC wise, they would be biased into saturation on either the positive or negative swing and the music would sound terrible. So how to explain the source of the thermal electrical noise? I suspect that even though the values of the resistors stay within range when heated in order to maintain a DC balance, any one of them might generate thermal noise at some slightly elevated temperature (my fan never comes on, but my chassis never draws more than 50-60 watts with my high efficiency Tannoy speakers). As well, in the BGS and VGS, there are a few small ceramic capacitors designed to keep RF from getting into the opamps and being amplified, and if any of these is acting up, then that could also be a source of the noise. Kip suggested checking the "header on the control board for cold solder joints" because headers can have "crack solder connections". Maybe he had my fan control problem in mind when he wrote that. Given what I have written here, could we agree that the problem is probably only on the "main module" where the BGS and VGS are located? And if you agree, apart from reheating the solder pads of all the resistors and capacitors that are located around the opamps of the BGS and VGS, as well as using circuit chiller spray, is there any other obvious way to pin point which passive component is the source of the noise? In an extreme case, I could isolate the BGS from the VGS by breaking an etch and cross wire each BGS to the opposite channel. Because the the noise disappears when the volume control in the VGS is turned down completely, I would like this to be a reason to eliminate the VGS as suspect. However, the VGS opamp uses a grounded non-inverted input and I am suspicious that the problem could still be within the VGS but masked because the volume control somehow alters the gain at radio frequencies even if the DC balance remains constant. If the ceramic cap associated with the VGS is acting up and generating RF noise, then lowering the volume control might naturally eliminate the resulting noise that we hear in the audio spectrum. Any comments would be appreciated, as I imagine thermal electrical noise in general is a bug that shows up in all stages of amplifiers, whether built using old or new technology. Thanks.
  9. popcorn noise in CT 200

    Thanks for your update and I hope you had a pleasant trip across the pond.
  10. popcorn noise in CT 200

    ........I asked Kip to address your question on the U904 since he has already been working with you on it........ Please tell Kip that his answer would be much appreciated. Thanks
  11. popcorn noise in CT 200

    Hi Mr. Glass, Thanks for your comment. It was never my intention to modify the cooling circuit. Initially I failed to appreciate how the opamp U904 operates basically because I forgot that it was using a mono-polar power supply and the output was already biased off when the chassis is at idle or near so. That sent me on a wild goose chase looking for a replacement U904. My substituted opamp was a bad choice because it is not optomized to be bias off with a mono-polar power supply. In the end, it appears that by my correcting the ODEP voltages using the trimmer resistors has resulted in the raising of the threshold heatsink temperature when the fan will first turn on. My fan no longer pulses. Pulsing can cause any amp owner to worry about the performance of the fan circuit. As KIP points out in his above post: "The fans can run at low speed even at idle." In my case, my fan rarely ran at idle when my amp was new. I suspect that my ODEP references voltages drifted over the last 13 years and this resulted in the fan's operate threshold moving close enough to my heatsink temperature so that my fan started to pulse due to the fan control circuit not having a different turn on and turn off temperature set point. I have several schematics of the CT400 and CT400B and the values of the half dozen or so resistors that surround U904 and U905 have evolved. This indicates to me that Crown has changed the way the ODEP voltage affects the voltage that is fed to the comparators U905. Knowing what the output voltage from U904 should be when the chassis is at idle holds the key to diagnosing any fan problems. Would it be possible for you to post what you think this output voltage should be at pin 1 of U904a? Thanks
  12. popcorn noise in CT 200

    UPDATE: August 24, 2005 The replacement dual opamp chip U904 (LM 1458) that I installed turned out not to be equilavent to the original LM358, even though it had the same pin layout. When I installed the correct opamp, I now have 0.66 volts feeding the comparator instead of the 4.8 volts produced by the LM1458 opamp. The LM1458 probably couldn't sink as much current as the LM358. This 0.66 volts which is fed simultaneously to pins 5,7, and 9 of comparators U905a,b,c is much closer to what I originally started out with before I changed anything, so my original U904 may not have been dead after all. Having 0.66 volts going to the comparators will result in the fan being turned completely off. And from what I can tell, it will take quite a bit of heat from the output transistors to get the fan to come on at its lowest speed. It is a bit of a mystery to me how U904 operates. It seems to be acting as a current sink when the chassis is idling. For the fan to come on, U904 will have to turn off. That will only happen when the ODEP voltage coming from the main board falls enough below its starting point of 7.7 volts and thus cause U904 to switch off. I used a hair dryer to heat up the output transistors and that wasn't enough to trigger the fan to come on. So it will take some serious power levels from to reach the threshold where the fan comes on. I must admit I am happier with the fan staying completely off when the amp is idling or putting out the dozen or so watts required to bring my home cinema up to 100 decibels. Having the fan stay on 24/7 can only cause it to wear out faster, although hearing the fan gave me reassurance that the chassis was being protected even if it wasn't hot enough to require the fan to be on. It remains to be seen if any pulsing of the fan will reoccur in the future. But if it does it means that this circuit can teeter-totter when the heatsink temperature is at some precise and stable value. If no pulsing occurs in the future, then that means that my original opamp U904 may have been defective all along.
  13. popcorn noise in CT 200

    Happy ending, After spending about 4 hours today slightly resetting the bias and OPED reference voltages, I still could not figure out why the fan wasn't coming on when I applied some heat to the heat sinks. It appeared to me that dual op-amp chip U904 was dead. Fortunately I had a spare (TOSHIBA) dual op-amp chip around and for the first time in what I believe is years, the fan comes on at idle and stays on. It purrs. Given that I have been running this amp without the benefit of a fan for quite some time now, I feel very gratetful that I have never needed to draw more than a dozen or so watts from it. Else, I would have surely cooked the components. Perhaps the popcorn noise that I noticed (which went away last week on its own), was an indirect result of the chassis running at a higher temperature than would have been the case had the fan been working. Remember if you have the complementary problem of the fan refusing to go off, it can be due to the opto-triac (U901/U902) on the control board, as was the case for me 6 years ago. Thanks again to the team at Crown for helping me through this problem with their advice.
  14. popcorn noise in CT 200

    Thanks Kip for your answer. No I'm not looking to delaying the fan coming on. In fact it's reassuring to hear the fan kick in and stay on for a minute or so while the amp is idling. This tells me that everything is operational and ready for some heat generating periods. What is spooky is when I never hear the fan come on at idle or at my usual low wattage. Also spooky is when the fan pulses on for a few seconds and sleeps for 10 minutes. In either of these cases, I am left with the incorrect impression that there is something wrong with the circuit and it might not perform when I might really need it. In order to avoid the situation where the fan is going to pulse due to my room being at a particular temperature, I would sooner change the comparitor's resister value so that the fan comes on at a lower chassis temperature rather than a higher chassis temperature. This gives me the most reassuring feedback. I just need to be careful not to set the resistor value such that the room air temperature is too warm for the fan to achieve its new goal and then shutdown. The fan could end up staying on all the time. It's a delicate balance that should take into consideration the warmest temperature that the room is likely to be at in summer. After all, the fan can never force the chassis to cool lower than the room temperature.
  15. popcorn noise in CT 200

    Thanks Kip for your detailed and very helpful post. In an adjacent thread called: CT800 HELP NEEDED the poster MiamiU mentions that his amp has a fan that cycles on and off every few seconds, indicating a problem with hysteresis in the circuit (design?). I have a similar problem with my fan only it is on for a few seconds and off for 10 to 20 minutes. Would you agree that when one looks at the way this fan control circuit has been designed, it could be possible that when this amp is at idle in a room with a particular ambiant temperature, the heat sink temperature could be such that the steady voltage delivered by the ODEP to the voltage comparitor (U905c) could be at the threshold where U905c begins to toggle continuously? In otherwords, the voltage required to make U905c toggle on could be the same as the voltage required to toggle U905 off. If so, then that could explain why our fans can pulse on for a few seconds at a time while our amps are at idle for long periods of time.