Big Transformers use to hum if the mains voltage is not symmetrical. If nothing else helps you should check the mains.
I have used PS Audio Humbuster to remove such hum. Perhaps it works a little bit different compared to your devices.
I have used PS Audio Humbuster to remove such hum. Perhaps it works a little bit different compared to your devices.
It's loader in real life. As I said, walk in the room and it's obvious. Even just audible from outside the room with door closed. Mostly from amp itself. Less from speakers thankfully.
The DCT-03 is exactly for that purpose. Asymmetry and DC offset are two ways of saying the same thing. I really appreciate everyone's input BTW!
The hum from the transformer and the speaker hum are two different things. If you remove the DC "dirt" on the AC mains, the transformer should be silent at idle. If loaded class A style the could make some noise again. So going from standby to On should start speaker hum. Please keep the different hum events cleanly separated! The external device will never fix speaker noise!
As far as I have understood, the "upgrade house keeping" powers the main transformer even when the amp is off. It only connects voltage to the amp when switching from standby to On. So the supply is hot all the time. (***)
The small transformer looks burned and may not be up for the task. Such upgrades often only help the seller and are a convenient way of generating secondary profit from products sold a long time ago. Such a cheap, too small transformer, doesn't build trust in this "improvement". Not publishing the schematic after 45 (!) years speaks the same language.
This amp has so many wires, if only one is connected wrong or running in a bad position, speaker hum can go from hardly audible to anoying. You may connect headphones to the speaker output and just carefully move different wires a little. If you can modulate the speaker hum, I'm right.
I build quite some amps with the Hitachi power MOS fet's, in some moving wires for only 2 centimeters cured or produced hum.
By the way, a choke between the main power supply was a common idea with such amps, transferred from valve amp supplys. Even if it may not have the promised benefits, it should not be a part of your problem. So better ignore them for now.
The repair guy knowing about hum and not having a solution is not very promising. Did he offer to fix it? How much $?
*** The whole "house keeping" may only be a complicated looking way to prevent the amp from activating the mains fuse by having the large torrodial On all the time. Without any influence on the amps sound.
It may be possible to get rid of it and only use a very common, reliable soft start. Any second transformer can induce hum in the whole construction. The problem is, you should be quite experienced in such amps to find out.
A way of isolating hum in an amp is to remove any component not neccesary for the pure amplification, then adding them again. At some point the hum will re-appear and you know the cause.
Then, there is the option to draw a ground connection scheme. This will clearly show you critical points or even mistakes, but only if you don't miss any wire!
As far as I have understood, the "upgrade house keeping" powers the main transformer even when the amp is off. It only connects voltage to the amp when switching from standby to On. So the supply is hot all the time. (***)
The small transformer looks burned and may not be up for the task. Such upgrades often only help the seller and are a convenient way of generating secondary profit from products sold a long time ago. Such a cheap, too small transformer, doesn't build trust in this "improvement". Not publishing the schematic after 45 (!) years speaks the same language.
This amp has so many wires, if only one is connected wrong or running in a bad position, speaker hum can go from hardly audible to anoying. You may connect headphones to the speaker output and just carefully move different wires a little. If you can modulate the speaker hum, I'm right.
I build quite some amps with the Hitachi power MOS fet's, in some moving wires for only 2 centimeters cured or produced hum.
By the way, a choke between the main power supply was a common idea with such amps, transferred from valve amp supplys. Even if it may not have the promised benefits, it should not be a part of your problem. So better ignore them for now.
The repair guy knowing about hum and not having a solution is not very promising. Did he offer to fix it? How much $?
*** The whole "house keeping" may only be a complicated looking way to prevent the amp from activating the mains fuse by having the large torrodial On all the time. Without any influence on the amps sound.
It may be possible to get rid of it and only use a very common, reliable soft start. Any second transformer can induce hum in the whole construction. The problem is, you should be quite experienced in such amps to find out.
A way of isolating hum in an amp is to remove any component not neccesary for the pure amplification, then adding them again. At some point the hum will re-appear and you know the cause.
Then, there is the option to draw a ground connection scheme. This will clearly show you critical points or even mistakes, but only if you don't miss any wire!
Thank you very much Turbowatch2!
With respect to the *** section of your comment I'll tell you what little I know. I know that the housekeeping board is primarily concerned with providing safety. It provides thermal protection when it senses that the heatsink is too hot. It looks at other parameters too, which I'm not knowledgable enough about to understand much. The upgrade, which a previous owner would have had done by Museatex, was strictly to have the skritch soft-start put in, so I'm told.
I'm tempted to try temporarily bypassing the housekeeping board and connecting the transformer's primaries to the mains and connecting the secondaries directly to the rectifier. Then, if the hum resolves, I can consider settling for the protection afforded by a simple fuse, maybe? If it doesn't resolve, at least then I know it's unlikely to be the housekeeping board, which narrows things down to sections that I understand better. If, after I do this I get no joy, I could try the headphone / wire manipulation investigation.
What do you think of this investigative approach?
The repair guy did have a proposed solution, which was to install anti-parallel diodes right after the mains enter the chassis. From my understanding of the circuitry of the DCT-03 filters I've used, plugging the amp into that would essentially be the same as what my repair guy proposed. I'll attach an image of the schematic from Sjostrom Audio so you might tell if I'm right or wrong in believing this. The repair guy is now reluctant to take on additional work due to concerns I'm not at liberty to discuss, but he is very experienced with these amps, extremely knowledgable and ethically upstanding. He just can't take this project on, so has nothing to gain from repair.
With respect to the *** section of your comment I'll tell you what little I know. I know that the housekeeping board is primarily concerned with providing safety. It provides thermal protection when it senses that the heatsink is too hot. It looks at other parameters too, which I'm not knowledgable enough about to understand much. The upgrade, which a previous owner would have had done by Museatex, was strictly to have the skritch soft-start put in, so I'm told.
I'm tempted to try temporarily bypassing the housekeeping board and connecting the transformer's primaries to the mains and connecting the secondaries directly to the rectifier. Then, if the hum resolves, I can consider settling for the protection afforded by a simple fuse, maybe? If it doesn't resolve, at least then I know it's unlikely to be the housekeeping board, which narrows things down to sections that I understand better. If, after I do this I get no joy, I could try the headphone / wire manipulation investigation.
What do you think of this investigative approach?
The repair guy did have a proposed solution, which was to install anti-parallel diodes right after the mains enter the chassis. From my understanding of the circuitry of the DCT-03 filters I've used, plugging the amp into that would essentially be the same as what my repair guy proposed. I'll attach an image of the schematic from Sjostrom Audio so you might tell if I'm right or wrong in believing this. The repair guy is now reluctant to take on additional work due to concerns I'm not at liberty to discuss, but he is very experienced with these amps, extremely knowledgable and ethically upstanding. He just can't take this project on, so has nothing to gain from repair.
See, I and many more in the field of repairing electronics, are often a little angry about others that keep schematics of old gear secret for egoistic causes. In the "old times", most the HIFI stuff you bought, came with a user manual and a complete schematic. The Chinese copy culture has made this nice touch disapear.
Anyway, so today there is an aged guy in Canada sitting on this papers and prohibiting the whole world from repairing these 1980's amps. Ethics? As a consumer you have the right to get the service manual for gear you own. No one is going to copy this old amp for commercial purposes. Anyone can re-engineer it, if he has enough time, but no one does, because it is not worth it. All this secrecy does, is to make it impossible for entusiasts to keep this vintage gear running at reasonable cost.
So don't expect any sympathy from me for him.
The diode/ capacitor thing is a very common "keep the DC out" construction. It may work to reduce transformer noise or not.
In my world, if you listen to a transformer in a silent room, you often can hear the hum get louder and then disapear again. It depends on how your AC mains line is used from others. Solar has quite some influence on this noise, so at night the current may stay cleaner.
I can try to explain to you a few things, but all the stuff you learned in decades of playing with amps is too much for a forum. You can't learn to swim from a book.
Your amp is nothing anyone would call a clean build. In fact, from the pictures, this looks like an unfinished DIYS product. Close to impossible that it is silent with all these randomly bundled wires. A nightmare.
A temperature control is stupid on these MOSfet amps, it is just something someone did to make it look more complicated. Or he did not understand the amp and it's components at all. You may find out that you can erase quite some stuff around the actual amp without making it worse.
Just to give you an example, with one amp we had an unsolvable hum problem. In the end we found out it was the loudspeaker DC protection that caused it over the relay coil... any non essential part can be your problem.
Sometimes in an amp a connection that should be avoided in school book amp gounding can be the final wire to make it 100% silent.
I would probably spent a few days on this amp reasembling it, but only if nothing is defective.
I maybe would reduce it into power supply and amp section first, to make it noise free, then see what else may be usefull or needed. Less is often more in HIFI!
Last question, is the amp worth it? Some of these amps with the HITACHI FET's did sound quite well, some not. If you are lucky it will sound as good as a 55 US$ Aiyima A07, but not better. It may have quite some power to drive current critical speakers, the number of FET's is overkill. 3 of them are fine for 300W/4 Ohm. It is possible the designer wanted to make it short circuit proof by using so many of them.
You may sell the FET's at eBay, as long as they are OK, keep the transformer and build another amp (from modules) in the vintage case. From what I see this would be a good idea. Mind that you don't have any schematic. Keep that as an option when you give up on the thing.
Anyway, so today there is an aged guy in Canada sitting on this papers and prohibiting the whole world from repairing these 1980's amps. Ethics? As a consumer you have the right to get the service manual for gear you own. No one is going to copy this old amp for commercial purposes. Anyone can re-engineer it, if he has enough time, but no one does, because it is not worth it. All this secrecy does, is to make it impossible for entusiasts to keep this vintage gear running at reasonable cost.
So don't expect any sympathy from me for him.
The diode/ capacitor thing is a very common "keep the DC out" construction. It may work to reduce transformer noise or not.
In my world, if you listen to a transformer in a silent room, you often can hear the hum get louder and then disapear again. It depends on how your AC mains line is used from others. Solar has quite some influence on this noise, so at night the current may stay cleaner.
I can try to explain to you a few things, but all the stuff you learned in decades of playing with amps is too much for a forum. You can't learn to swim from a book.
Your amp is nothing anyone would call a clean build. In fact, from the pictures, this looks like an unfinished DIYS product. Close to impossible that it is silent with all these randomly bundled wires. A nightmare.
A temperature control is stupid on these MOSfet amps, it is just something someone did to make it look more complicated. Or he did not understand the amp and it's components at all. You may find out that you can erase quite some stuff around the actual amp without making it worse.
Just to give you an example, with one amp we had an unsolvable hum problem. In the end we found out it was the loudspeaker DC protection that caused it over the relay coil... any non essential part can be your problem.
Sometimes in an amp a connection that should be avoided in school book amp gounding can be the final wire to make it 100% silent.
I would probably spent a few days on this amp reasembling it, but only if nothing is defective.
I maybe would reduce it into power supply and amp section first, to make it noise free, then see what else may be usefull or needed. Less is often more in HIFI!
Last question, is the amp worth it? Some of these amps with the HITACHI FET's did sound quite well, some not. If you are lucky it will sound as good as a 55 US$ Aiyima A07, but not better. It may have quite some power to drive current critical speakers, the number of FET's is overkill. 3 of them are fine for 300W/4 Ohm. It is possible the designer wanted to make it short circuit proof by using so many of them.
You may sell the FET's at eBay, as long as they are OK, keep the transformer and build another amp (from modules) in the vintage case. From what I see this would be a good idea. Mind that you don't have any schematic. Keep that as an option when you give up on the thing.
OK I have listened to your audio clips. Neither sound like mains hum, more like a relay chattering. To provide a soft start there must be a delayed switch on so a relay somewhere assuming it is the secondary (DC) supply that is switched (as mentioned above in the posts). If it is the mains supply to the toroid being switched it could be a solid state switch either way they don't seem to be latching on and vibrating at mains frequency. What is under the housekeeping board? Make sure the mains is disconnected before you touch anything here?
Hi Totally Analogue.
Thanks for that bit of insight! Could prove invaluable. The amp originally did not have a soft start. But at some point, some previous owner would have sent the amps in to Museatex to have a skritch soft-start added. As it seems you already suspected, the new circuitry is on a small board underneath the housekeeping board. I’ll do a bit of disassembly and post a picture of it.
Thanks for that bit of insight! Could prove invaluable. The amp originally did not have a soft start. But at some point, some previous owner would have sent the amps in to Museatex to have a skritch soft-start added. As it seems you already suspected, the new circuitry is on a small board underneath the housekeeping board. I’ll do a bit of disassembly and post a picture of it.
Hi totally analogue! Thanks for chasing me down. Life distracted me.
So, I temporarily (for the purpose of testing) removed the house-keeping board and the skritch. So, direct from (DC filtered) mains to the primary on the transformer, and direct from secondary to bridge. Results were a very slight decrease in physical chirp/buzz/hum at the amp. 90% unaffected. Also, my anti-parallel diode/capacitor DC filter does nothing to fix the noise (but does seem to inexplicably make other aspects of the sound better).
I did discover that maybe 50% of hum/chirp over speakers comes from preamp and follows along with increase and decrease of volume setting. I've got an older Pa6i preamp that I can substitute out for my preferred one. My substitute preamp contributes no noise. Unfortunately, it doesn't sound near as good otherwise.
So, concentrating strictly on the mono blocks,
I've replaced ALL electrolytic caps
I've replaced the bridge
I've removed the housekeeping board and skritch
I've put a DC filter between the mains and the amps
About 50% of the speaker buzz/hum/chirp can still be attributed the the amps
90% of the physical hum/buzz/chirp remains and can ALL be attributed to the amps
The physical noise (not speaker) accounts for probably 70% of the total noise contribution, so is the lion's share of the problem.
With the housekeeping board and skritch gone, it seems ALL of the noise comes from the chokes (chirp) and from the transformer (hum)
What else can I look at? I'm baffled. The write-up on the Museatex website says that the transformer was custom designed so I'd have to guess that it is not replaceable.
As an aside, to address the comments of other forum contributors, wire routing doesn't seem to make any difference in changing my amp's particular issues. Also, the rather rough appearance of the wiring is simply because I keep tearing apart and partially putting the amps back together just enough to try to figure out the source of the problems.
Cheers!
So, I temporarily (for the purpose of testing) removed the house-keeping board and the skritch. So, direct from (DC filtered) mains to the primary on the transformer, and direct from secondary to bridge. Results were a very slight decrease in physical chirp/buzz/hum at the amp. 90% unaffected. Also, my anti-parallel diode/capacitor DC filter does nothing to fix the noise (but does seem to inexplicably make other aspects of the sound better).
I did discover that maybe 50% of hum/chirp over speakers comes from preamp and follows along with increase and decrease of volume setting. I've got an older Pa6i preamp that I can substitute out for my preferred one. My substitute preamp contributes no noise. Unfortunately, it doesn't sound near as good otherwise.
So, concentrating strictly on the mono blocks,
I've replaced ALL electrolytic caps
I've replaced the bridge
I've removed the housekeeping board and skritch
I've put a DC filter between the mains and the amps
About 50% of the speaker buzz/hum/chirp can still be attributed the the amps
90% of the physical hum/buzz/chirp remains and can ALL be attributed to the amps
The physical noise (not speaker) accounts for probably 70% of the total noise contribution, so is the lion's share of the problem.
With the housekeeping board and skritch gone, it seems ALL of the noise comes from the chokes (chirp) and from the transformer (hum)
What else can I look at? I'm baffled. The write-up on the Museatex website says that the transformer was custom designed so I'd have to guess that it is not replaceable.
As an aside, to address the comments of other forum contributors, wire routing doesn't seem to make any difference in changing my amp's particular issues. Also, the rather rough appearance of the wiring is simply because I keep tearing apart and partially putting the amps back together just enough to try to figure out the source of the problems.
Cheers!
Interesting you still have the same hum with the soft start board removed. That rather does limit my thoughts as to where the problem might be. It is not unusual for Class A amplifiers to exhibit a bit of hum both mechanically and from the loud speakers but nowhere near the level that you are experiencing and certainly not a sharp buzzing sound. My next thought would be is the mains transformer being overloaded? Does the transformer have a label on it giving the VA rating, current and voltage? For 100 Watt class A amp I would expect that transformer to be at least 300 to 400 VA.
Your problem is going to be determining how much currently amp is actually taking without taking it to a tech. The advantage there is they could look at the DC supplies on an oscilloscope and see how much of that buzz is getting into the amp. Without the amp in front of me or a circuit diagram it's hard to go much further and I certainly wouldn't advise tweaking anything on the amp board until you have knowledge on how to accurately set the bias and DC offset. As knowledge on these amplifiers seems to be quite limited I wonder if it's worth trying on other forums not that I want to be disloyal to DIY audio but I would like to see you with the solution.
Your problem is going to be determining how much currently amp is actually taking without taking it to a tech. The advantage there is they could look at the DC supplies on an oscilloscope and see how much of that buzz is getting into the amp. Without the amp in front of me or a circuit diagram it's hard to go much further and I certainly wouldn't advise tweaking anything on the amp board until you have knowledge on how to accurately set the bias and DC offset. As knowledge on these amplifiers seems to be quite limited I wonder if it's worth trying on other forums not that I want to be disloyal to DIY audio but I would like to see you with the solution.
Yes, I am at a loss as to where to look next. The transformer is the one that came with the amp and so should be adequate, initially at least. There is a sticker on it, but it indicates the manufacturer and the model number only, with no other information. That manufacturer is now out of business. I’ll keep poking around to see what I can discover. I’ll post if I resolve this issue. Thanks so much for your attention and expertise!
If you didn’t already know …
An old tech once taught me a trick: Take an ALL PLASTIC screwdriver, put the handle to your ear and the tip to every component and you will immediately find out where the noise comes from. If you don’t have an all plastic screwdriver, any plastic insulated rod will do.
An old tech once taught me a trick: Take an ALL PLASTIC screwdriver, put the handle to your ear and the tip to every component and you will immediately find out where the noise comes from. If you don’t have an all plastic screwdriver, any plastic insulated rod will do.
I’ll take a listen to your files tomorrow night. I’m in the process of moving so have been tied up a bit with other things.