Hi Freddy,
Older designs always ran the risk of oscillation with different output types. The MJ2119x series have an fT of around 4 MHz, which is close to the average output transistor used years ago. I have had very few problems unless the stability of the amp was marginal to start. I always check of oscillation even when using the same parts. That is one reason any audio service person requires a 'scope and THD meter (or better). These things are not optional, in addition to a good DVM. Every shop used to have at least these things and knew how to get the most from them. Today I hear people say they can test distortion with their hearing. Well people, that is not true, not even close. I get products in with several percent THD right from a "good" service person without equipment - but it passed the hearing test. Once you fix the unit, the owner hears the difference readily. Also, high distortion normally points to another problem, some serious.
-Chris
Older designs always ran the risk of oscillation with different output types. The MJ2119x series have an fT of around 4 MHz, which is close to the average output transistor used years ago. I have had very few problems unless the stability of the amp was marginal to start. I always check of oscillation even when using the same parts. That is one reason any audio service person requires a 'scope and THD meter (or better). These things are not optional, in addition to a good DVM. Every shop used to have at least these things and knew how to get the most from them. Today I hear people say they can test distortion with their hearing. Well people, that is not true, not even close. I get products in with several percent THD right from a "good" service person without equipment - but it passed the hearing test. Once you fix the unit, the owner hears the difference readily. Also, high distortion normally points to another problem, some serious.
-Chris
hi, Anatech
Of course , sorry translate french-english , first language is french ( belgium Wallon )
Goodnight all,
it's been a long time since I came to the forum, but I'm delighted to see the enthusiasm surrounding this amp which is often diverted from sound to stereo. its weight makes it a cannonball I agree, but its reliability and its musical rendering make it forget this weight.
I still use it a few times on my JBL E100, but in the meantime I have found my second QSC studio 1100 amp which I bridge per channel and which delivers 140W into 8 ohms. talking with other people on various forums, I hesitated to operate my 2 QSCs in bridge mode on the bi-amplification and I then passed the course. I switched to bi-amp mode with the BGW on the woofers and the 2 QSC studio 1100s, still in bridge on the mid-treble part, I was again amazed by the sound reproduction as well as the dynamics obtained, in terms of consumption it's not ideal to use 3 amps for listening, but I only allow myself to use 2 to 3 per month because I rarely have time to sit down to listen to my music at home .
Of course , sorry translate french-english , first language is french ( belgium Wallon )
Goodnight all,
it's been a long time since I came to the forum, but I'm delighted to see the enthusiasm surrounding this amp which is often diverted from sound to stereo. its weight makes it a cannonball I agree, but its reliability and its musical rendering make it forget this weight.
I still use it a few times on my JBL E100, but in the meantime I have found my second QSC studio 1100 amp which I bridge per channel and which delivers 140W into 8 ohms. talking with other people on various forums, I hesitated to operate my 2 QSCs in bridge mode on the bi-amplification and I then passed the course. I switched to bi-amp mode with the BGW on the woofers and the 2 QSC studio 1100s, still in bridge on the mid-treble part, I was again amazed by the sound reproduction as well as the dynamics obtained, in terms of consumption it's not ideal to use 3 amps for listening, but I only allow myself to use 2 to 3 per month because I rarely have time to sit down to listen to my music at home .
Hi titoon,
Welcome back firstly!
Using an electronic crossover is the best, specially if you can remove the passive crossover. One amplifier per driver, and you get amazing dynamics and much lower distortion. Oh yeah, it goes louder too!
Amplifiers operate better in normal mode. Bridging an amplifier brings problems and I think you may find it sounds cleaner in normal mode. For one thing, damping factor in bridged mode is 1/2 normal mode.
-Chris
Welcome back firstly!
Using an electronic crossover is the best, specially if you can remove the passive crossover. One amplifier per driver, and you get amazing dynamics and much lower distortion. Oh yeah, it goes louder too!
Amplifiers operate better in normal mode. Bridging an amplifier brings problems and I think you may find it sounds cleaner in normal mode. For one thing, damping factor in bridged mode is 1/2 normal mode.
-Chris
bonjour anatech
Je n'avais pas pensé à cette configuration, je l'utilise dans ma sono utilisation de mes T3V avec deux DCX2496 pour la séparation des 4 canaux (infra - grave - médium - aigu) mais je n'avais jamais utilisé cela en hifi stéréo .
ma configuration hifi stéréo, cela comprend un lecteur multimédia Auna ituner CD internet DAB+ et FM avec Lan et wifi. Double 1228 platine vinyle et mes 3 amplis . J'ai encore un DCX2496le libre mais passer par un système actif va compliquer le nombre de câbles à passer pour les 3 voies à séparer.
Phil
Je n'avais pas pensé à cette configuration, je l'utilise dans ma sono utilisation de mes T3V avec deux DCX2496 pour la séparation des 4 canaux (infra - grave - médium - aigu) mais je n'avais jamais utilisé cela en hifi stéréo .
ma configuration hifi stéréo, cela comprend un lecteur multimédia Auna ituner CD internet DAB+ et FM avec Lan et wifi. Double 1228 platine vinyle et mes 3 amplis . J'ai encore un DCX2496le libre mais passer par un système actif va compliquer le nombre de câbles à passer pour les 3 voies à séparer.
Phil
Hi Phil,
I'm using Google translate to understand you so I'll paste that here so others can follow.
My system uses a Revox B-261 tuner, Denon DCD-S10, Thorens TD-126 MKII + Ortofon 540 MKII, Node 2i streamer through a Marantz 300DC and currently Yamaha C-70 preamp (normally a Marantz SC-9 switch problems). Everything has been optimized and modified for increased performance. My current speakers are PSB Stratus Gold (original), looking at Klipsch RF-7 III. My current speakers are hard to beat, the system has been stable for over a decade. I also have a Nakamichi CR-5 cassette deck, sold my Tascam BR-20 (the cost of tape - yikes!). I have a few other good systems in other rooms, all have steamers. All my audio streamers and TV streamers are wired network connections. That is far better (good thing I wired the entire house when it was new!).
-Chris
I'm using Google translate to understand you so I'll paste that here so others can follow.
Just keep your filter slopes reasonable. Too steep is not good!
My system uses a Revox B-261 tuner, Denon DCD-S10, Thorens TD-126 MKII + Ortofon 540 MKII, Node 2i streamer through a Marantz 300DC and currently Yamaha C-70 preamp (normally a Marantz SC-9 switch problems). Everything has been optimized and modified for increased performance. My current speakers are PSB Stratus Gold (original), looking at Klipsch RF-7 III. My current speakers are hard to beat, the system has been stable for over a decade. I also have a Nakamichi CR-5 cassette deck, sold my Tascam BR-20 (the cost of tape - yikes!). I have a few other good systems in other rooms, all have steamers. All my audio streamers and TV streamers are wired network connections. That is far better (good thing I wired the entire house when it was new!).
-Chris
Thanks Duke!! going to get round to that soon.... can't believe where the time goes.
Not long after getting the 750 to work on both channels, it stopped, I ran out of free time, so it's been in the cupboard for a bit, just in the last week got the two channels going again, dry joints under the relay board! so now looking into getting the dim clip light banished ;-) and the other bits and bobs like all the old caps re-newed Still impressive sound repro as it is though :-D
Here is the photo which shows some improvement as described
1-complete disassembly and cleaning of the condos - transformer - relay board and the chassis.
2- Replaced the 3 330uf condos on the protection relay plate and re-soldered all the relay terminals because the welds were broken in 3 places.
3-Replaced the original fan with an adjustable 3-speed Antec PC fan.
(secondary 12vdc power supply to be provided) my initial idea was to take the voltage of the 15vdc available for the view-meters and use it for the fan but after advice from various people, it is preferable to isolate it by another power supply .
4- felt placements under the 2 transformer fixing points for its resonance on the sheet.
5- for the moment not yet received parts for the Amplifier part (transistors and 3 chemical condos)
6- change of the Red LEDs by a green and yellow part as on an American model. (waiting for leds)
photo after cleaning, replacements and modifications and reassembly.
Red sight-meter part which will be replaced by 5 green leds and 5 yellow leds as on this photo found.
to follow, I await my components , Phil
"re-soldered all the relay terminals because the welds were broken in 3 places"
Many Thanks for this! I could not locate a fault with mine, until I saw this and took the board out, I had two broken joints, now all is working lovely :-D Thank you 🙂
Bonjour,
Quelqu'un a déjà eu ce problème ?
court-circuit entre le refroidisseur et le connecteur.
le +78volts a touché le radiateur.
English please.
Translation:
Hello,
Has anyone had this problem?
short circuit between the cooler and the connector.
the +78volts touched the radiator.
Not on this particular amplifier, but I’ve had similar problems for the same reason. The solution is fairly obvious.
what are the possible problems for the amplifier circuits if one of the power rails is in short circuit?
If it’s just a power rail that shorted it usually won’t do anything to the amplifier portion. It will blow fuses, occasionally the rectifier. Pull it and check it first. Not good for the capacitors but one good spark wont destroy them either. With weird circuits (this isnt one) loss of a rail can upset the front end and damage components due to reverse biasing the B-E junctions, but I’ve never heard of it taking out an output stage. The op amp’s shunt regulators act as clamps that prevent that sort of thing from happening here. Rework the affected solder joint, Put in some insulation material so board flex won’t do it again, and fire it up on a dim bulb. If it comes right up you don’t even need to dig any further.
What to use? A piece of that reddish high temperature cardboard that’s usually used to prevent that sort of thing. I usually have some that’s been salvaged out of something. Any kind of plastic that will remain solid (and not get gooey) at 100C, several layers of kapton tape, even a thin piece of plexiglass. There is usually something around that can be used.
What to use? A piece of that reddish high temperature cardboard that’s usually used to prevent that sort of thing. I usually have some that’s been salvaged out of something. Any kind of plastic that will remain solid (and not get gooey) at 100C, several layers of kapton tape, even a thin piece of plexiglass. There is usually something around that can be used.
I'll bet somebody pushed too hard on that connector while it was powered up or shortly after turn off. The board is only spaced off of the heatsink by the thickness of those nylon shoulder bushings, not much. Maybe that lead wasn't trim short enough is another possibility. Once you get things sorted out and working if you get a bunch of strange problems that don't make any sense try cleaning and reseating those 11-pin connectors. Been there, done that. Nice amps when correct.
Craig
Craig
Good morning,
so short circuit is not a problem.
The power supply is correct (+78v -78v).
many sjxxxx transistors are short circuited on the left channel.
I'll have to check everything.
the right channel was OK but I damaged it while measuring it.
I would like to know if it is possible to operate a channel with a single pair of final transistors?
so as not to break several transistors during testing and adjustments.
so short circuit is not a problem.
The power supply is correct (+78v -78v).
many sjxxxx transistors are short circuited on the left channel.
I'll have to check everything.
the right channel was OK but I damaged it while measuring it.
I would like to know if it is possible to operate a channel with a single pair of final transistors?
so as not to break several transistors during testing and adjustments.
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If the amplifier was run very hot, or the outputs are over-tightened, it will compress the nylon shoulder washers. I have seen people do this. Replace the washers in that case. Also, if the solder joints on the connector have excess blobs, they could contact the heat sink. You could put some insulation behind the connector.
I think the plug was pushed on with the capacitors charged, perhaps even running (very poor practice!). The anodized heat sink generally doesn't short with light contact, but I would never bet on it.
Use a variac to ramp up the power for testing. You must replace all the outputs even when only one shorts. The drivers and other transistors are soldered from the top and you should be testing all for leakage and gain match before assembling it. Build it properly and bring it up on a variac.
I have had amplifiers come in where the idiot service person just removed the shorted outputs and called the amp repaired. Please just do it right. It is a nice amp.
I think the plug was pushed on with the capacitors charged, perhaps even running (very poor practice!). The anodized heat sink generally doesn't short with light contact, but I would never bet on it.
Use a variac to ramp up the power for testing. You must replace all the outputs even when only one shorts. The drivers and other transistors are soldered from the top and you should be testing all for leakage and gain match before assembling it. Build it properly and bring it up on a variac.
I have had amplifiers come in where the idiot service person just removed the shorted outputs and called the amp repaired. Please just do it right. It is a nice amp.
Yes, any dead output transistor means replace them all. In the affected channel. But it can be RUN on a single pair of expendable outputs (plus drivers), at highish impedance (say 20 ohm) to clear all other problems before committing a good matched set to it. That is one good use for the “survivors”. Good practice is to replace the drivers too, as they could have been overstressed when one of the outputs shorted. And it’s anybodys guess how much overstress they saw.
The channel that just saw the arc to heat sink is probably fine, if it was working before and otherwise undamaged. Just put in SOMETHING to insulate it - and if shoulder washers got squashed they are another risk item that must be addressed.
The channel that just saw the arc to heat sink is probably fine, if it was working before and otherwise undamaged. Just put in SOMETHING to insulate it - and if shoulder washers got squashed they are another risk item that must be addressed.
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