NickC said:
Sorry to answer on Tim's behalf but a quick answer is yes. The feedback capacitor is there to maintain a low DC offset but it seems that removing that cap and leaving that 220 resistor only in the feedback loop can give you much more open sound and still maintain reasonable DC offset control. I did that mod in my JLH amp 6 months ago and still don’t see any need for that cap.
Argo
Nick,
As Argo seems to have answered on my behalf. I can only comfirm what he has already said, and elaborate a little. The 470uF cap was removed and replaced with a link. The 220 ohm resistor remained and was therefore connected to ground via the link. In my circuit, the dc offset variation increased from around 65mV with the feedback cap, to around 175mV without it, at an IQ of 1.5A/18.5V. Since then, Geoff has kindly supplied details of a slightly modified offset ccs which allows an adjustable degree of temperature compensation. Using this new circuit the hot/cold offset variation has been reduced to 50mV, though improvement on this figure might become progressively more difficult due to random dc fluctuations which can be >+-10mV. As I don't want to 'publish' Geoff's work without his prior consent, I will first seek his agreement, and hopefully post details on the new ccs in due course.
Tim.
As Argo seems to have answered on my behalf. I can only comfirm what he has already said, and elaborate a little. The 470uF cap was removed and replaced with a link. The 220 ohm resistor remained and was therefore connected to ground via the link. In my circuit, the dc offset variation increased from around 65mV with the feedback cap, to around 175mV without it, at an IQ of 1.5A/18.5V. Since then, Geoff has kindly supplied details of a slightly modified offset ccs which allows an adjustable degree of temperature compensation. Using this new circuit the hot/cold offset variation has been reduced to 50mV, though improvement on this figure might become progressively more difficult due to random dc fluctuations which can be >+-10mV. As I don't want to 'publish' Geoff's work without his prior consent, I will first seek his agreement, and hopefully post details on the new ccs in due course.
Tim.
New dc offset CCS
Nick,
As promised, here are the details of the slightly revised offset CCS, enabling variable temperature compensation and an improved hot/cold offset variation. Refering to the schematic posted by Les earlier in this thread, but with the feedback capacitor removed, replace R11 with a 20K preset in series with a 1K resistor (in my case, a 10K preset and a 10K resistor). By adjusting the new preset (in my circuit) a falling, or rising, or rising then falling, offset can be obtained. The existing preset is used to set the offset to zero, as before, once the amplifier is hot. Adjustment is very time consuming as the amplifier has to cool down completely before any adjustments can be evaluated. It is important to bear in mind that removal of the feedback capacitor in the standard 1996 JLH circuit will lead to a very much higher offset variation than would be the case in the circuit to which I have refered. I suggest a multi-turn preset for setting the offset to zero, and a single turn preset for the temperature compensation adjustment.
Tim.
Nick,
As promised, here are the details of the slightly revised offset CCS, enabling variable temperature compensation and an improved hot/cold offset variation. Refering to the schematic posted by Les earlier in this thread, but with the feedback capacitor removed, replace R11 with a 20K preset in series with a 1K resistor (in my case, a 10K preset and a 10K resistor). By adjusting the new preset (in my circuit) a falling, or rising, or rising then falling, offset can be obtained. The existing preset is used to set the offset to zero, as before, once the amplifier is hot. Adjustment is very time consuming as the amplifier has to cool down completely before any adjustments can be evaluated. It is important to bear in mind that removal of the feedback capacitor in the standard 1996 JLH circuit will lead to a very much higher offset variation than would be the case in the circuit to which I have refered. I suggest a multi-turn preset for setting the offset to zero, and a single turn preset for the temperature compensation adjustment.
Tim.
JLH modifications
For those interested, all the modifications that I mentioned in my previous posts, add some more, have now been included in full detail on Geoff's site.
http://www.gmweb.btinternet.co.uk/jlhupdate.htm
Tim.
For those interested, all the modifications that I mentioned in my previous posts, add some more, have now been included in full detail on Geoff's site.
http://www.gmweb.btinternet.co.uk/jlhupdate.htm
Tim.
Lucky
Thanks for the heads up TimA.
BTW folks, I do not think it was mentioned about the feedback take off point has been moved to the optimum location in the schematic. Just noticed it last night when I was fooling around with the amp.
After I re-wired the PSU filter caps, I notice one of the heat sink gets hot faster than the rest, but the amp sound normal. After 2-3 hours listening and constantly checking the Iq(voltage drop) on R9 and DC offset I notice one channel was a bit unsteady in terms of Iq and DC offset, it was hunting back and forth kind of. So off came the bottom cover mesh to take some measurements of each MJ15003 in that channel. It turned out that I have no voltage across R10 at all. The amp ran for 2-3 hours with only 3 MJ15003 with no problem and not noticed. So out came the transistor and checked, but it was ok, the problem was each of my MJ15003 has to have their legs extended to pass thru the thick heatsink, and this one was just made contact before, must be due to the banging and moving the amp when I did the rewiring of the PSU that it some how got lose and not conducting. Re-extended the legs put it back in, fire it up, everything is fine and back to normal now
Must try the feedback mod soon...
Regards,
Chris
Thanks for the heads up TimA.
BTW folks, I do not think it was mentioned about the feedback take off point has been moved to the optimum location in the schematic. Just noticed it last night when I was fooling around with the amp.
After I re-wired the PSU filter caps, I notice one of the heat sink gets hot faster than the rest, but the amp sound normal. After 2-3 hours listening and constantly checking the Iq(voltage drop) on R9 and DC offset I notice one channel was a bit unsteady in terms of Iq and DC offset, it was hunting back and forth kind of. So off came the bottom cover mesh to take some measurements of each MJ15003 in that channel. It turned out that I have no voltage across R10 at all. The amp ran for 2-3 hours with only 3 MJ15003 with no problem and not noticed. So out came the transistor and checked, but it was ok, the problem was each of my MJ15003 has to have their legs extended to pass thru the thick heatsink, and this one was just made contact before, must be due to the banging and moving the amp when I did the rewiring of the PSU that it some how got lose and not conducting. Re-extended the legs put it back in, fire it up, everything is fine and back to normal now
Must try the feedback mod soon...
Regards,
Chris
Very interesting reading - especially for Power Addicted Guys
Webpage:
Article written by John Linsley-Hood - september 1996
He describes his 10 Watt Class A project. (obs! I did not write wrong, it should not be 1000 Watt)
Some of my own selected points, in the article:
Webpage:
Article written by John Linsley-Hood - september 1996
He describes his 10 Watt Class A project. (obs! I did not write wrong, it should not be 1000 Watt)
Some of my own selected points, in the article:
JLH and MJ21194
If anyone has tried the MJ21194 in either the standard 1996 version or the modified circuit on http://www.gmweb.btinternet.co.uk/jlhupdate.htm
I would be very interested to know the results. When I tried them in my circuit (which at the time was the standard 1996 version) there was a faint hum, yet no hum was present with either the 2N3055 or the MJ15003. My reason for asking is that my impression of the sound of the amplifier with the MJ21194 was very favourable and possibly better than with the MJ15003.
Tim.
If anyone has tried the MJ21194 in either the standard 1996 version or the modified circuit on http://www.gmweb.btinternet.co.uk/jlhupdate.htm
I would be very interested to know the results. When I tried them in my circuit (which at the time was the standard 1996 version) there was a faint hum, yet no hum was present with either the 2N3055 or the MJ15003. My reason for asking is that my impression of the sound of the amplifier with the MJ21194 was very favourable and possibly better than with the MJ15003.
Tim.
chris ma said:
I would not mind to try it if they have the same/similar spec. Can you send me 10 of those matched ones I'll let you how well they work with my JLH
Regards,
Chris
I would be glad to send you some pieces if I were in US. These power transistors are very easy to find and cheap (less than 2USD) in my country. And according to my personal experience, Hfe of this transistor is very stable and will not need too much effort matching them.
Well, perhaps I'll try when I build my own JLH amp.
JLH and MJ21194
Did the hum you experienced seem to be at the
line voltage, or maybe double the line voltage?
I'm wondering if the MJ21194 wasn't picking up
mains harmonics carried along by diode HF ringing
that the other parts you mentioned weren't
responding to. Does the MJ21194 have a particularly
high Ft compared with the parts, for instance?
Scratching his head,
Erik Larson
Did the hum you experienced seem to be at the
line voltage, or maybe double the line voltage?
I'm wondering if the MJ21194 wasn't picking up
mains harmonics carried along by diode HF ringing
that the other parts you mentioned weren't
responding to. Does the MJ21194 have a particularly
high Ft compared with the parts, for instance?
Scratching his head,
Erik Larson
Ej,
I was just kidding you, anyway the heat sinks on mine were drilled for TO-3 case so they would not fit without a lot of mod anyway.
But the other Tim mentioned MJ21194 is very temping, I just have to take the MJ15003 mounting screws off and replace them, no soldering at all....will see...
Chris
I was just kidding you, anyway the heat sinks on mine were drilled for TO-3 case so they would not fit without a lot of mod anyway.
But the other Tim mentioned MJ21194 is very temping, I just have to take the MJ15003 mounting screws off and replace them, no soldering at all....will see...
Chris
EJ said:
They seem to be one of the best power BJTs around for Hifi and JHL really seems to gain a lot from good output transistors. Not long time ago I was forced to change my old original Motorola 2N3055 outputs (one leg of one transistor came lose from the transistor body and I only had 4 of these transistors) with newer ST Microelectronics 2N3055 devices and I noticed a big change in sound for worse. So I have decided to change the outputs with those well regarded MJ15003. 2SC3281, as a good candidate, also went through my mind but these devices used to be somewhat expensive (around 10 $US here) and hard to get. Now they should be obsolete but I found them in one place costing around 2 dollars and that made me suspicious, that those may be fakes. With 2SC5200 devices, which are newer replacements, the risk to buy fakes should be less. Only problem with them over here is, that you need to order them and it takes about for weeks to get them.
As for you, I wouldn’t hesitate to try them out, especially if you pay only two bucks per piece for them and you are sure they are not fakes.
Argo
JLH and MJ21194
Erik,
The MJ21194 datasheet quotes an fT of 4MHz, the MJ15003 datasheet quotes a figure of 2MHz. The hum was noticable only when close to the speakers (87dB/W). Though low level oscillation has been suggested as a possible cause, I had no access to a scope and was not able to determine the root of the problem.
Chris,
If you try the MJ21194s I shall be interested in your results.
Tim.
Erik,
The MJ21194 datasheet quotes an fT of 4MHz, the MJ15003 datasheet quotes a figure of 2MHz. The hum was noticable only when close to the speakers (87dB/W). Though low level oscillation has been suggested as a possible cause, I had no access to a scope and was not able to determine the root of the problem.
Chris,
If you try the MJ21194s I shall be interested in your results.
Tim.
Feedback pick up location
I have completed this modification to my JLH for ESL. It is very difficult to find words to describe the effect. Because it affects the entire spectrum, not just highs or lows. I thought my JLH is very good already, but after this mod I actually feel bad, because I realised now what was very good is not really. This feedback modification made me realise how inexperienced and untrained Hi Fi listener I am. Just been humbled further..
My thanks to Geoff Moss and NickGibbs.
Regards,
Chris
P.S. I think the tube guys would understand this, that the sound of darkness that the amp is producing.
I have completed this modification to my JLH for ESL. It is very difficult to find words to describe the effect. Because it affects the entire spectrum, not just highs or lows. I thought my JLH is very good already, but after this mod I actually feel bad, because I realised now what was very good is not really. This feedback modification made me realise how inexperienced and untrained Hi Fi listener I am. Just been humbled further..
My thanks to Geoff Moss and NickGibbs.
Regards,
Chris
P.S. I think the tube guys would understand this, that the sound of darkness that the amp is producing.
EJ,
It was just a simple mod, better if you are just building one, just look at the schematic posted by Les there you see the feedback loop from Vout to R8 was the change. Make sure you follow the schematic there you can't go wrong. I think D.Self discussed this particular area in his site that I read before. Funny that a couple of inches up or down a track make such difference.
Regards,
Chris
It was just a simple mod, better if you are just building one, just look at the schematic posted by Les there you see the feedback loop from Vout to R8 was the change. Make sure you follow the schematic there you can't go wrong. I think D.Self discussed this particular area in his site that I read before. Funny that a couple of inches up or down a track make such difference.
Regards,
Chris