Hi,
I am not sure I'm helping you at all, because I don't make the measurements myself. Working from a schematic is tricky.
The only obvious thing is that the voltage across the emitter resistors is too high. See, you have about 0.92V drop across 2x0.22 ohm resistors, still you don't have any current flowing! If there was continuity, the current would be huge. So, one of those resistors should be open (or some connections on the board are missing). Otherwise it doesn't make any sense - Ohm's low!
Did you check your driver and output transistors out of the circuit? Are you sure that they are correctly oriented on the board, collectors and emitters not swapped, or NPN-PNP transistors swapped? ... Anyway, first confirm what's the situation with the emitter resistors. What's the output voltage by the way? Close to 0V or close to -0.9V. This may give you idea which one of the resistors is possibly open.
I am not sure I'm helping you at all, because I don't make the measurements myself. Working from a schematic is tricky.
The only obvious thing is that the voltage across the emitter resistors is too high. See, you have about 0.92V drop across 2x0.22 ohm resistors, still you don't have any current flowing! If there was continuity, the current would be huge. So, one of those resistors should be open (or some connections on the board are missing). Otherwise it doesn't make any sense - Ohm's low!
Did you check your driver and output transistors out of the circuit? Are you sure that they are correctly oriented on the board, collectors and emitters not swapped, or NPN-PNP transistors swapped? ... Anyway, first confirm what's the situation with the emitter resistors. What's the output voltage by the way? Close to 0V or close to -0.9V. This may give you idea which one of the resistors is possibly open.
Hi, thank you. Of course you are right, I had double checked, reading is right, so there has to be discontinuity possibly a faulty power resistor (one of the 0.22ohm ones, which were recycled from a another built). the output voltage (upper power resistor was close to 0 by the way). I could not work on it the past few days, will report back soon
Power Rail Fuse protection
Apart from the fuse before the transformer primary, R.Slone (Audio Amplifier Construction Manual) also appears to recommend fuse protection on each power rail between the transformer secondaries and the bridge rectifier (in case the rectifier develops a short), and on each power rail from the power supply to the amp power input.
The problem I envisage is that if only one of the two rail fuses blows then almost immediately the amp output will produce around 40v dc and with an 8ohm speaker load there will be about 5amps flowing through the speakers. Also other components such as the feedback cap will see about 40v dc very quickly thereafter.
I'm wondering therefore whether, given these possible problems associated with power rail fuse protection relative to the small risk of problems such as a rectifier failure is it worth doing?
Apart from the fuse before the transformer primary, R.Slone (Audio Amplifier Construction Manual) also appears to recommend fuse protection on each power rail between the transformer secondaries and the bridge rectifier (in case the rectifier develops a short), and on each power rail from the power supply to the amp power input.
The problem I envisage is that if only one of the two rail fuses blows then almost immediately the amp output will produce around 40v dc and with an 8ohm speaker load there will be about 5amps flowing through the speakers. Also other components such as the feedback cap will see about 40v dc very quickly thereafter.
I'm wondering therefore whether, given these possible problems associated with power rail fuse protection relative to the small risk of problems such as a rectifier failure is it worth doing?
AC secondary fuses
I think Slone's focus is on large PA amplifiers where every protection is worth the effort to limit damage at the high power in rough use by often untrained or inexperienced people. This measure is to protect the bridge and transformer, I think and seldom an issue with the massive overkill typical DIYs fit in those positions.
I've never seen this in domestic amplifiers and frankly, I can't see it being effective amplifier protection since it must use higher rated fuses to allow for capacitor surge currents. A better approach is a big, nasty relay in the output to protect speakers at least and I can't recall any DIY amp failures in many years that were not related to output/load faults or abuse.
I think too, if you go to the trouble of building an amplifier like the NAP 140 for your enjoyment, it should be at least as good as what you can buy so the less fusing, switching and connections at the sharp end, the better IMHO.
I think Slone's focus is on large PA amplifiers where every protection is worth the effort to limit damage at the high power in rough use by often untrained or inexperienced people. This measure is to protect the bridge and transformer, I think and seldom an issue with the massive overkill typical DIYs fit in those positions.
I've never seen this in domestic amplifiers and frankly, I can't see it being effective amplifier protection since it must use higher rated fuses to allow for capacitor surge currents. A better approach is a big, nasty relay in the output to protect speakers at least and I can't recall any DIY amp failures in many years that were not related to output/load faults or abuse.
I think too, if you go to the trouble of building an amplifier like the NAP 140 for your enjoyment, it should be at least as good as what you can buy so the less fusing, switching and connections at the sharp end, the better IMHO.
Ian, the reason I looked at this option was because I recently had a bad experience with amplifier oscillation. This caused part of both channel pcbs to be burnt, melted some components, and destroyed others through excessive voltages and currents. The speakers were intact because I was using speaker protection circuitry.
I was looking to rail fuses as a way of preventing a recurrence of the consequences of oscillation - obviously after doing everything else possible to prevent oscillation from ocurring in the first place.
The problem I have found when looking at fuse protection options is that with normal higher levels of amp output, power rail current is about 3.5 amps. In this situation, using simulation, it appears that if one rail is abruptly disconnected then the current in the other rail firstly drops to about 1 amp in 200 milliseconds, then rises almost instantaneously to about 4.6 amps which is when the damage starts to happen. So to prevent collateral damage the fuse has to act quickly and at the "right level".
Within this range of 3.5 to 4.6 amps, choose a fuse too low in value or too fast and this may blow prematurely and cause its own problems. Choose one too high in value or too slow and it won't protect. With a margin of 1 amp its very difficult to get it right. Only if the differential was much higher (say 3.5/10 amps) rather than (3.5/4.6) would I would find the fuse protection attractive.
Therefore, in this case I concluded that the cure was as bad as or even worse than the disease!
I was looking to rail fuses as a way of preventing a recurrence of the consequences of oscillation - obviously after doing everything else possible to prevent oscillation from ocurring in the first place.
The problem I have found when looking at fuse protection options is that with normal higher levels of amp output, power rail current is about 3.5 amps. In this situation, using simulation, it appears that if one rail is abruptly disconnected then the current in the other rail firstly drops to about 1 amp in 200 milliseconds, then rises almost instantaneously to about 4.6 amps which is when the damage starts to happen. So to prevent collateral damage the fuse has to act quickly and at the "right level".
Within this range of 3.5 to 4.6 amps, choose a fuse too low in value or too fast and this may blow prematurely and cause its own problems. Choose one too high in value or too slow and it won't protect. With a margin of 1 amp its very difficult to get it right. Only if the differential was much higher (say 3.5/10 amps) rather than (3.5/4.6) would I would find the fuse protection attractive.
Therefore, in this case I concluded that the cure was as bad as or even worse than the disease!
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Thankfully, I've never had such a catastrophe with amolifiers. I try never to risk it but it seems you built something more like an incendiary device than an amplifier. Sorry this happened to you, pattox. It must have been a really bad experience.
Anyway, the point is that this won't happen in a correctly designed and built amplifier like some recent NAP clones posted here, so you shouldn't have to be concerned about the finished product, just whilst it is under test. Usually, we employ lightbulb testers or replace the fuses with say, 100 ohm resistors during testing when the faults or their onset can be seen without causing damage. If delicate precision fusing was so important in a finished amplifier, it would never survive normal use anyway. It would be impractical and unstable.
Wire fuses just aren't accurate enough for your requirement as they have at least 25% tolerance from some suppliers. Bear in mind too, that as fuse rating current is approached, thermal distortion occurs and the cycling will soon cause failure. Some years ago, I even built Slone's regulated supplies for 40V and 50V rails with variable current limiting They worked OK but eventually I just went back to the simple expedient of resistors which work better in some cases and limit all significant faults whilst allowing full rail potential at no load.
Check the posts here. That's how most of us go about it with all the precautions on test and then just simple relays and nominal high rated fuses to isolate the load when mistakes are made in use. That's all many commercial products have too,
Sure, I have had oscillation but it doesn't cause much harm with such limiting and a resistive or no load attached. A 'scope finds any suspicious instability before it becomes a problem and if it only shows up at higher power, the design really needs more investigation than simulation. Unfortunately, sims tell you nothing about stray reactance or unsuitable part types on the board and connecting leads. Don't be misled by this tool - it only tells you about the models it uses, not the construction.
Anyway, the point is that this won't happen in a correctly designed and built amplifier like some recent NAP clones posted here, so you shouldn't have to be concerned about the finished product, just whilst it is under test. Usually, we employ lightbulb testers or replace the fuses with say, 100 ohm resistors during testing when the faults or their onset can be seen without causing damage. If delicate precision fusing was so important in a finished amplifier, it would never survive normal use anyway. It would be impractical and unstable.
Wire fuses just aren't accurate enough for your requirement as they have at least 25% tolerance from some suppliers. Bear in mind too, that as fuse rating current is approached, thermal distortion occurs and the cycling will soon cause failure. Some years ago, I even built Slone's regulated supplies for 40V and 50V rails with variable current limiting They worked OK but eventually I just went back to the simple expedient of resistors which work better in some cases and limit all significant faults whilst allowing full rail potential at no load.
Check the posts here. That's how most of us go about it with all the precautions on test and then just simple relays and nominal high rated fuses to isolate the load when mistakes are made in use. That's all many commercial products have too,
Sure, I have had oscillation but it doesn't cause much harm with such limiting and a resistive or no load attached. A 'scope finds any suspicious instability before it becomes a problem and if it only shows up at higher power, the design really needs more investigation than simulation. Unfortunately, sims tell you nothing about stray reactance or unsuitable part types on the board and connecting leads. Don't be misled by this tool - it only tells you about the models it uses, not the construction.
Hi, I went back to the other boards (140 C) where I had 200mv DC offset on one board and 25mv on the other one. Checked all you mentioned and I am running out of ideas. Dc offset is only 12mv with input shorted to ground, does this help understand where the offset is coming from? I have no input cap (just a link), could the VBE cap at Q5 cause the offset too if feedback cap is okay? Finally if al checks out, could just a mismatch of LTP transistors (bc550 b) alone cause a 200mv offset?. Thank you
Vbe should not be a problem.
LTP pair can be a problem if the mismatch is really big and I don't believe that. More likely it is related to the LTP components.
Another possibility is that you create the offset with the source that you connect (not having input cap). Good indication for that is that the circuit is fine with the input shorted. Why in just one of the channels though? ... I'm out of ideas as well. Try another source and see if the offset is still there. Does this channel, the one with the offset, plays music? If it does, then probably the source is causing it. Insert a cap and you'll be good to go.
Hi Ruwe, you were right, one of the o/p resistor was "open". I replaced it and the board is fine, I can bias quiescent current and DC offset is ~22mv. Thanks again for the precious input!
The strange thing is that the 200mv offset appears also with no source is connected at the input, it goes away when signal input is shorted to ground and the same happens with or without input cap. I tried the channel with an old speaker and it plays music fine. As the offset appears with no source attached to the input should I assume it could only come from the feedback loop to the base of LTP 2?
Ha, 1 down and 1 to go!
... I'd say the problem is on the input, because when you refer it to ground (0V), the output also follows and the voltage is close to 0V. First check resistors' combination on the input. Is their sum close to 27k? Also make sure that the feedback ground and the input ground are connected together. If all checks, then look in the feedback loop - the capacitor and the two resistors. If all is OK, then there is only one thing left - replace the LTP transistors.
Nothing else than the above should affect the DC at the output.
Hi Ruwe,
offset dropped to 40mv after replacing tr1 as you suggested The other board is 22mv, I will try to replace and match tr2 for balance. Thanks for the help. I am driving these boards with Pass B1, which has 10uf output cap. This means that I should be safe removing input cap from the boards, correct? By removing the cap the input resistors are becoming in parallel so input resistor to ground goes from 100k to 18k (100k+22k in parallel). Should something be done to compensate for that or it is not relevant?
offset dropped to 40mv after replacing tr1 as you suggested
The other board is 22mv, I will try to replace and match tr2 for balance. Thanks for the help. I am driving these boards with Pass B1, which has 10uf output cap. This means that I should be safe removing input cap from the boards, correct? By removing the cap the input resistors are becoming in parallel so input resistor to ground goes from 100k to 18k (100k+22k in parallel). Should something be done to compensate for that or it is not relevant?The changed total input resistance is not much of a problem, it will just slightly affect the DC balance.
You can safely remove the 100k, this resistor matters only with the capacitor in place as a discharge path in case the input has been disconnected. Without capacitor you don't need it at all.
Hi,
I recently purchased a coupleof these NAP 140 kits from eBay...i have read through muchof this thread and had a couple questions....it appears back in 2008 the boards were not printed correctly and that someof the transitors were wired backwards...does anyone knowif that is still an issue today, habe they worked these problems out in the past four years?
Also, how does one actually set the bias? Were do I take my measurements from and what is the optimal current?
Thanks,
Jeff
I recently purchased a coupleof these NAP 140 kits from eBay...i have read through muchof this thread and had a couple questions....it appears back in 2008 the boards were not printed correctly and that someof the transitors were wired backwards...does anyone knowif that is still an issue today, habe they worked these problems out in the past four years?
Also, how does one actually set the bias? Were do I take my measurements from and what is the optimal current?
Thanks,
Jeff
There are a few boards and kits available on Epay but I don't think any are now more than a couple of years old. Recent issues have not been with the board and overlay but the substitution of semis with transposed pinouts, or just plain assembly errors.
Make sure you have the specified parts or check the specific manufacturer's drawing for different types supplied, because different manufacturers work to different pinout standards internationally and again, for specific markets. Much of this thread is taken up with that problem for newbs and your bias question. The rest is "Lego" work.
Bias is the regulated DC current flowing through the output transistors to minimise distortion. It's conveniently measured in mV potential across each Re resistor. These are the large, low value resistors in the emitter leads of most output transistors but since this design is Quasi-complementary, the lower transistor in the schematic has this 0.22 ohm resistor in its collector leg. Whatever, you should read the Naim recommended equivalent of 30-38 ma which is around 7 mV across either Re. I suggest reading twice this across both resistors in series, for a little better accuracy with typical DMMs.
Errors lead to fireworks, so it's not a good idea to fire up an amp. on full power the first time. Solder 100R resistors in place of each fuse or the old favourite - light bulb in series with the AC power (A) in. It is safer but you'll need to readjust the bias after removing the bulb.
Might I suggest a good book for newbs that is relatively inexpensive and will get you up to speed in a consistent fashion, rather than trawling up opinions which may be great pieces of advice but hard to connect with other views.
Check Randy Slone's "High power audio amplifier construction manual" at Amazon and everywhere. It's not new or perfect but has a lot of info. and is suited to start and DIY with.
Make sure you have the specified parts or check the specific manufacturer's drawing for different types supplied, because different manufacturers work to different pinout standards internationally and again, for specific markets. Much of this thread is taken up with that problem for newbs and your bias question. The rest is "Lego" work.
Bias is the regulated DC current flowing through the output transistors to minimise distortion. It's conveniently measured in mV potential across each Re resistor. These are the large, low value resistors in the emitter leads of most output transistors but since this design is Quasi-complementary, the lower transistor in the schematic has this 0.22 ohm resistor in its collector leg. Whatever, you should read the Naim recommended equivalent of 30-38 ma which is around 7 mV across either Re. I suggest reading twice this across both resistors in series, for a little better accuracy with typical DMMs.
Errors lead to fireworks, so it's not a good idea to fire up an amp. on full power the first time. Solder 100R resistors in place of each fuse or the old favourite - light bulb in series with the AC power (A) in. It is safer but you'll need to readjust the bias after removing the bulb.
Might I suggest a good book for newbs that is relatively inexpensive and will get you up to speed in a consistent fashion, rather than trawling up opinions which may be great pieces of advice but hard to connect with other views.
Check Randy Slone's "High power audio amplifier construction manual" at Amazon and everywhere. It's not new or perfect but has a lot of info. and is suited to start and DIY with.
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Ian, best advice i have received so far, thanks. I have been contemplating actually learning what all the jargon means here in the SS side of the forum....it seems a daunting task, tubes are much easier! As said elsewhere, i have built several chip amps and as you poetically put it, they are just legos...easy easy....not much to learn to build them, you dont need to know whats actually going on to have success. I think after this project I will return to my first love, tubes...they seem to be a bit more forgiving unless you are talking aout a slip of the finger into a 500v B+.....
i was reading somewhere early on in this thread about disconnecting the V+ and putting an ammeter in series to check bias...what is that all about? Did I misunderstand something?
i was reading somewhere early on in this thread about disconnecting the V+ and putting an ammeter in series to check bias...what is that all about? Did I misunderstand something?
In a word, no. This checks total current drain on the positive supply rail. Just about every amplifier discussed here is a 3-stage affair and has bias currents to the input stage (~2 mA), the Voltage Amplifier Stage (VAS) (~10 mA) in addition to the typical 25 mA output stage bias for a Emitter Follower (EF) designs. Note that many here don't like lowest distortion bias setting and use much higher levels to more closely approach class A operation, but then we are not talking Naim clones.
If the comment referred to just the collector lead to the upper transistor, fine for p-p wiring but that would involve damage to basic PCBs like this one or some fancy rewiring around the fuse holder. Nahh - just measure the emitter resistors and calculate in the head. Once you know your target value, you deal in relative terms - forget current and think mV.
'Funny this; in principle we are trying to offset the base-emitter diode drops of the driver + output transistors anyway, so correct voltage (the bias voltage fed into the driver transistor bases) is the real aim, but I digress because regulating the current is the way we actually go about it!
Mods
You'll find a lot of chat about mods here but some guys have become gurus with their extreme mods and even some sensible upgrades. Bear in mind that most mods are evaluated well after our memory of the previous amplifier condition has faded so opinions just remain that - opinions and some are likely just about different sound quality rather than better. Consider that many modders have no clue about electronics and the mayhem they cause with some efforts, so I suggest starting with the basic kit first and leave soldering simple with leads straight so parts fall out if you go to change them.
I believe that the supplied MF resistors I have seen are fine - they have very little influence on audio unless you are comparing faulty parts or different types but folks have a delicate sense of audio quality that can be offended by anything less than best affordable quality components - a sort of blind faith that there will be audible improvement if we throw more money at our amps. You could then ask why we start with such cheap kits in the first place but that's probably not a polite question.
If long reliable life is an issue, perhaps yes. They may well help there but let's be realistic about DIY. We build, listen, bin or store this stuff and go on to our next inspiration.
Capacitors are a different matter and there is much discussion and poo-poohing about the TAG tantalum capacitors used by Naim since the '70s. The point is that their reputation was established using them, warts and all. Many posting here just want a pincushion for boutique parts rather than cheap access to the original design but size is a "big" problem because the best caps are mostly film types which are relatively huge for the same value and thus problematic for mounting and noise sensitivity. Some fit modern SMD parts but these can also be a bad idea unless you really know what you are doing.
Specialised sites for modders are Pinkfishmedia forum in the UK and this one which is interesting and has useful links. I warn about BS which is thick on these sites and expressing technical views may be met with hostility.
Modifying Naim Audio power amplifiers
You'll find a lot of chat about mods here but some guys have become gurus with their extreme mods and even some sensible upgrades. Bear in mind that most mods are evaluated well after our memory of the previous amplifier condition has faded so opinions just remain that - opinions and some are likely just about different sound quality rather than better. Consider that many modders have no clue about electronics and the mayhem they cause with some efforts, so I suggest starting with the basic kit first and leave soldering simple with leads straight so parts fall out if you go to change them.
I believe that the supplied MF resistors I have seen are fine - they have very little influence on audio unless you are comparing faulty parts or different types but folks have a delicate sense of audio quality that can be offended by anything less than best affordable quality components - a sort of blind faith that there will be audible improvement if we throw more money at our amps. You could then ask why we start with such cheap kits in the first place but that's probably not a polite question.
If long reliable life is an issue, perhaps yes. They may well help there but let's be realistic about DIY. We build, listen, bin or store this stuff and go on to our next inspiration.
Capacitors are a different matter and there is much discussion and poo-poohing about the TAG tantalum capacitors used by Naim since the '70s. The point is that their reputation was established using them, warts and all. Many posting here just want a pincushion for boutique parts rather than cheap access to the original design but size is a "big" problem because the best caps are mostly film types which are relatively huge for the same value and thus problematic for mounting and noise sensitivity. Some fit modern SMD parts but these can also be a bad idea unless you really know what you are doing.
Specialised sites for modders are Pinkfishmedia forum in the UK and this one which is interesting and has useful links. I warn about BS which is thick on these sites and expressing technical views may be met with hostility.
Modifying Naim Audio power amplifiers