things is - it is actually not possible to do perfect reproduction - despite all of our efforts !!!
so if a hundred amps come CLOSE to faithful reproduction, they will still differ in thousands of different ways compared with each other.
... and our ears / brains are very sensitive and it would seem that many people can hear at least some of these small differences ... and because we are all different, naturally each of us will have a preference.
even with live music some prefer to sit / stand at the front and get right on in there with the dynamics, the details and the 'buzz' while others prefer to sit a few rows back and savour the sound in a more relaxed 'air cushioned' kind of way - which, I believe, is why many prefer amps that are not so faithfull but never-the-less very pleasant to listen to.
...and because the whole idea of all this is that we get enjoyment from listening, anything other than subjective, it seems to me, would be daft. ... 🙂
so if a hundred amps come CLOSE to faithful reproduction, they will still differ in thousands of different ways compared with each other.
... and our ears / brains are very sensitive and it would seem that many people can hear at least some of these small differences ... and because we are all different, naturally each of us will have a preference.
even with live music some prefer to sit / stand at the front and get right on in there with the dynamics, the details and the 'buzz' while others prefer to sit a few rows back and savour the sound in a more relaxed 'air cushioned' kind of way - which, I believe, is why many prefer amps that are not so faithfull but never-the-less very pleasant to listen to.
...and because the whole idea of all this is that we get enjoyment from listening, anything other than subjective, it seems to me, would be daft. ... 🙂
Let's face it guys, we'd all be so much better off being satisfied with a mini hi-fi. Our problem is that we are never satisfied. The fun is in the chase, right?
It is a bit like reading a good book, wonderful, yet when it is finished, there is a feeling of wanting more. So we go in search of another.
It is a bit like reading a good book, wonderful, yet when it is finished, there is a feeling of wanting more. So we go in search of another.
Hi mikelm, I'm a lover not a fighter (so I'm not going to add anything further to this particular issue after this post) but I like truth. I also make my living by speaking (and listening) and I usually use words carefully. It is easy to put up a "straw man" and knock him down. What I said was "more nearly related to" and a little earlier "more faithfully". I didn't say "perfect". I know about imperfection....that's the only reason I have a job!
Nice fire works 20 miniutes ago! What happened here ? Both 0R1 ohmite resistors (either the R13 and R15 or R14 and R16 pair)blew wide open and split up in the middle...and the smell..
Do I assume both transistors are toasted as well?
Where do I begin fixing the amp? Any advise are welcome and appreciated..
Do I assume both transistors are toasted as well?
Where do I begin fixing the amp? Any advise are welcome and appreciated..
Attachments
I was listening to music over 3 hours today.. the amps are in daily use since 2005 but I have not done any Iq current checking or adjustment over 20 months or so since...
well perhaps it get bit too hot and then a transient come along and take it out.
perhaps ur bias drift.
about two years ago I changed the resistors like u use here and replace them with a lenght of "constantan" resistance wire. it sounded much better to me.
if you like I can send u some
email me if you want
this episode may have a happy ending !
mike
perhaps ur bias drift.
about two years ago I changed the resistors like u use here and replace them with a lenght of "constantan" resistance wire. it sounded much better to me.
if you like I can send u some
email me if you want
this episode may have a happy ending !
mike
Hi crhis ma
Was this a thermal runaway?
How long have you had amp on before without trouble?
How hot were your sinks before resistors were "lighting up"?
cheers
John
Was this a thermal runaway?
How long have you had amp on before without trouble?
How hot were your sinks before resistors were "lighting up"?
cheers
John
Mike, John
The amps were completed and in daily use since around Nov, 2003 and the last time the Iq bias were re-adjusted in 2005.
The amps were on well over 3 hours before it fried. But the night before that was on continuesly over 5 hours before I went to bed.
The heatsinks were normally at 42C after 1 hour with bias set at 3A. I meant to re-set the bias two months ago that I noticed one Mj21194 felt hotter than the other. May be one was dying slowly over time.
At the time of the resistors were lighting up the temperature of the heat sink felt normal. It was in the middle of a song that suddently the left channel's mid-high muted then I realized something odd happened. As I walked over to the amp and investigate I was in time to see the fire work sparked up, pop , flash and smoke. My first reaction was reaching over to the power switch and turned it off. This pair of my JLHs is used to power the mid-high section of my 3 ways active OB Jordans. I have not check to see if the speakers were damaged yet.
Shall I replace the two Mj21194 on this rail first before replacing the 0R1 power resistors?
Mike, I do not mind to try those wire you mentioned but do I have to all 8 pieces of the resistors ? What do you do to implement the constanan wire? A specific length for 0R1? and is it a direct replacement?
The amps were completed and in daily use since around Nov, 2003 and the last time the Iq bias were re-adjusted in 2005.
The amps were on well over 3 hours before it fried. But the night before that was on continuesly over 5 hours before I went to bed.
The heatsinks were normally at 42C after 1 hour with bias set at 3A. I meant to re-set the bias two months ago that I noticed one Mj21194 felt hotter than the other. May be one was dying slowly over time.
At the time of the resistors were lighting up the temperature of the heat sink felt normal. It was in the middle of a song that suddently the left channel's mid-high muted then I realized something odd happened. As I walked over to the amp and investigate I was in time to see the fire work sparked up, pop , flash and smoke. My first reaction was reaching over to the power switch and turned it off. This pair of my JLHs is used to power the mid-high section of my 3 ways active OB Jordans. I have not check to see if the speakers were damaged yet.
Shall I replace the two Mj21194 on this rail first before replacing the 0R1 power resistors?
Mike, I do not mind to try those wire you mentioned but do I have to all 8 pieces of the resistors ? What do you do to implement the constanan wire? A specific length for 0R1? and is it a direct replacement?
Hi Chris ma
difficult to tell what the problem was, but with Class A amps like this thermal runaway is a potential issue. Prolonged heating will cause the amp to warm up and the CCS to the input stage is not balanced against temperature change. Unfortunately. On the other hand, the CCS in the VAS (second ) stage will reduce the base current to the output and this could be a saving feature.
Your mid point will drift off with temperature. Most amps show some drift, but in a single ended non differential, it is likely to be more of a problem.
Don't know what speaker loading yours was, but a low impedance at DC could have caused a large offset current even for small offset voltage. Could the DC impoedance of your speakers contributed to the problem?
It might have been a dodgy joint in the power transitors- they were tightly screwed down I hope?
I think you will have to replace all output transistors and resistors.
cheers
John
difficult to tell what the problem was, but with Class A amps like this thermal runaway is a potential issue. Prolonged heating will cause the amp to warm up and the CCS to the input stage is not balanced against temperature change. Unfortunately. On the other hand, the CCS in the VAS (second ) stage will reduce the base current to the output and this could be a saving feature.
Your mid point will drift off with temperature. Most amps show some drift, but in a single ended non differential, it is likely to be more of a problem.
Don't know what speaker loading yours was, but a low impedance at DC could have caused a large offset current even for small offset voltage. Could the DC impoedance of your speakers contributed to the problem?
It might have been a dodgy joint in the power transitors- they were tightly screwed down I hope?
I think you will have to replace all output transistors and resistors.
cheers
John
how much current through each transistor / resistor will you you rebuild it to ?
I'm just thinking about the dissipation of the resistance wire
I'm just thinking about the dissipation of the resistance wire
I will try to keep it same as before like the right channel now that my multimeter will display 150MV when measure across R13 with 17MV DC offset at the speaker terminals after an hour power on with 38C heatsink in room temperature of 18C.
Do I really have to replace the power transistors and resistors on the other rail for R14 and R16, Q1 and Q1A? They seem to be fine.
My plan is to replace Q2, Q2A, R13, R15 only and unit test with variac once I can find the parts... is that a bad idea?
Do I really have to replace the power transistors and resistors on the other rail for R14 and R16, Q1 and Q1A? They seem to be fine.
My plan is to replace Q2, Q2A, R13, R15 only and unit test with variac once I can find the parts... is that a bad idea?
Of course you could try just changing the faulty ones ...
I think I would try that first but perhaps switch on with bias turned down and speaker disconnected.
but if you are interested to try the wire you will need to change them all.
here is a picture of how I do it using plain vero board.
the wire is 4.4ohms / meter so you will need about 23mm for 0.1
cheers
mike
I think I would try that first but perhaps switch on with bias turned down and speaker disconnected.
but if you are interested to try the wire you will need to change them all.
here is a picture of how I do it using plain vero board.
the wire is 4.4ohms / meter so you will need about 23mm for 0.1
cheers
mike
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hello to all, i am also building the JLH amplifier, my question is can i use +/- 35V rail supply on this amplifier?
Falmig hot?
Don't these things get flaming hot
?
Doesn't that increase the risc of thermal runaway (the resitance of these wires do change with temprature don't they)?
MArco
Don't these things get flaming hot
?Doesn't that increase the risc of thermal runaway (the resitance of these wires do change with temprature don't they)?
MArco
tiktak said:
Unless you are using speakers with a very high minimum impedance (>15R) I'm afraid the answer is no. To keep the dissipation in the output transistors to an acceptable level (<45W) the quiescent current would need to be reduced to a maximum of 1.25A which would limit power output and reduce still further the low efficiency of this design.
I would suggest a rail voltage of between +/-18V and +/-22V for the JLH (a lower rail voltage and higher Iq being more suited speakers that have a low minimum impedance), with a possible increase to +/-25V if paralleled output transistors are used and the speaker impedance does not drop below 8R.
thank you geoff, i think i'll be using the+/-22 v rail voltage for now, i will just revise my transformer
Re: Flaming hot?
You do have to consider power and heat, this is why I checked what current chris was planning to use
In this case if you do the maths - 0.15 Volts x 1.5 amps the power comes out at 0.225Watts.
From my experience, I know this will be ok. The resistors will get warm but nowhere near flamimg - perhaps about 50 deg C
This wire has been developed to have a low temperature coefficient - I guess that why it was called "constantan" wire.
for some reason it really improved the sound when I tried it - my guess would be because of it's low noise.
it is a copper / nickel alloy and can be found here
http://www.wires.co.uk/acatalog/constantan.html
hope this helps
mike
deduikertjes said:
Don't these things get flaming hot
Doesn't that increase the risc of thermal runaway (the resitance of these wires do change with temprature don't they)?
MArco
You do have to consider power and heat, this is why I checked what current chris was planning to use
In this case if you do the maths - 0.15 Volts x 1.5 amps the power comes out at 0.225Watts.
From my experience, I know this will be ok. The resistors will get warm but nowhere near flamimg - perhaps about 50 deg C
This wire has been developed to have a low temperature coefficient - I guess that why it was called "constantan" wire.
for some reason it really improved the sound when I tried it - my guess would be because of it's low noise.
it is a copper / nickel alloy and can be found here
http://www.wires.co.uk/acatalog/constantan.html
hope this helps
mike
Hi Chris ma
I think if you provide a way of limiting the volts and amps you can try just replacing transistors and resistors that look dead.
I would suggest you want to take a look at the VAS too - if your upper output device went skywards then maybe it took the VAS/driver with it....
maybe some protection is needed for the VAS?
cheers
John
I think if you provide a way of limiting the volts and amps you can try just replacing transistors and resistors that look dead.
I would suggest you want to take a look at the VAS too - if your upper output device went skywards then maybe it took the VAS/driver with it....
maybe some protection is needed for the VAS?
cheers
John