Hello all!
I recently finished to modify my home made Quad405 following Bernd Ludwig's tips and the input stage as specified at dc-daylight.ltd.uk and everything worked perfectly up to a point.
I used to test my amplifier on the floor because the wall plug was easier to access from there 🙂 , when I moved it on my table a strange low frequency hum appeared. I moved back to the floor and it went dead quiet again.
I use a 400VA toroidal transformer, made here in Romania. The input of the amplifier is in the air, so no signal at all, the DC offset on the output is around 0.2mV, so it is ok. The input OP is an OPA627 with stabilized source (7815, 7915 and 24V zenners before them).
I tried everything from changing the power cables to even ticker ones, to double the caps in the rectifier from 10.000uF to 20.000uF, but the problem remained when testing it on the table.
I'm out of ideeas and I don't want to place my amplifier on the floor 🙂 so I'm looking forward to any ideeas.
Many thanks,
Zsolt
I recently finished to modify my home made Quad405 following Bernd Ludwig's tips and the input stage as specified at dc-daylight.ltd.uk and everything worked perfectly up to a point.
I used to test my amplifier on the floor because the wall plug was easier to access from there 🙂 , when I moved it on my table a strange low frequency hum appeared. I moved back to the floor and it went dead quiet again.
I use a 400VA toroidal transformer, made here in Romania. The input of the amplifier is in the air, so no signal at all, the DC offset on the output is around 0.2mV, so it is ok. The input OP is an OPA627 with stabilized source (7815, 7915 and 24V zenners before them).
I tried everything from changing the power cables to even ticker ones, to double the caps in the rectifier from 10.000uF to 20.000uF, but the problem remained when testing it on the table.
I'm out of ideeas and I don't want to place my amplifier on the floor 🙂 so I'm looking forward to any ideeas.
Many thanks,
Zsolt
Hello edl,
I tought that I might have a ground loop somewhere but the thing that I could not understand and drive me crazy is that it's working fine if the whole stuff (toroidal + PSU + 1 side Quad) is on the floor.
One thing I forgot to mention is that in both cases I have the earthing connectec to the PSU ground via a 10R.
However, when I tested it on the floor I tried to remove the earthing 10R and the hum was there too, like testing it on the table;
and by putting my hands near the input (not touching it) a modulated sound appeard in the speakers.
For this reason I suspect that is somehow related to the electromagnetic field of the toroidal. Also, if the toroidal is on the table it has a small 50Hz hum/noise comming from it, however this noise disappears on the floor (everything beeing connected in the same manner as on the table). I live in a flat wich has a metal structure, maybe that's the reason my toroid feels better there? Should I try to isolate the toroidal in some metal case?
Zsolt
I tought that I might have a ground loop somewhere but the thing that I could not understand and drive me crazy is that it's working fine if the whole stuff (toroidal + PSU + 1 side Quad) is on the floor.
One thing I forgot to mention is that in both cases I have the earthing connectec to the PSU ground via a 10R.
However, when I tested it on the floor I tried to remove the earthing 10R and the hum was there too, like testing it on the table;
and by putting my hands near the input (not touching it) a modulated sound appeard in the speakers.
For this reason I suspect that is somehow related to the electromagnetic field of the toroidal. Also, if the toroidal is on the table it has a small 50Hz hum/noise comming from it, however this noise disappears on the floor (everything beeing connected in the same manner as on the table). I live in a flat wich has a metal structure, maybe that's the reason my toroid feels better there? Should I try to isolate the toroidal in some metal case?
Zsolt
Zsolt
I see from your comments that you applied my non inverting front end modification to your 405 - I hope you found it made a great improvement - did you also move C8 as I suggest ??
with regard to the hum problem - You may have noted that I wire every ground to the buss between the supply caps including the connection to the heatsink after removing the bolt - Soldered a wire on to the PCB where the bolt connects the PCB to heatsink - take this to the centre of the PSU caps or the 0V output terminal.
Hum can also be a problem if the input sockets are not connected to the chassis directly at the point they are mounted - also try to keep the connections from pre-amp to 405 together if they are separate cables.
I hope this helps
Best regards
Keith (dc-daylight ltd.)
I see from your comments that you applied my non inverting front end modification to your 405 - I hope you found it made a great improvement - did you also move C8 as I suggest ??
with regard to the hum problem - You may have noted that I wire every ground to the buss between the supply caps including the connection to the heatsink after removing the bolt - Soldered a wire on to the PCB where the bolt connects the PCB to heatsink - take this to the centre of the PSU caps or the 0V output terminal.
Hum can also be a problem if the input sockets are not connected to the chassis directly at the point they are mounted - also try to keep the connections from pre-amp to 405 together if they are separate cables.
I hope this helps
Best regards
Keith (dc-daylight ltd.)
Keith
Thank you, indeed the input sockets were not connected to the ground. I connected them and I also chnaged the PSU as you suggested.
After I connected the sockets to the ground the hum disappeared, however, a small ssssss noise remained wich was not there before. It's not a constant hum, but it's a faint noise audible in the bass and in the tweeters too.
I don't have an oscilloscope to see the output but I observed that the DC offset on my voltmeter is fluctuating now between +- 1mV.
Regarding to your question: the answer is yes 🙂 I found your non inverting front end modification a great improvement. With my TL071 the noise went so down that I was realy undecided if I have to change them to the new OPA627, but of course I was curious and I did the modification. The noise I think remained the same, however the sound of the amplifier is much more pleasant. Also the DC offset went down from 39mV to around 0.5mV.
As a note: the operational is decoupled by 100nF right on the IC's pins and by 100uF after the stabilizers. I had to use stabilizers with the OPA627 because the original zenner + resitor started to warm up.
From the sound/oscillation point of view, is it important where do I solder the ground of the stabilizers regarding to the other components ground (like: R1, R3, C2 etc.) ?
Do you have any suggestion how to get rid of that faint noise? Could it be that my amplifier is sort of oscillating now?
Best regards,
Zsolt
Thank you, indeed the input sockets were not connected to the ground. I connected them and I also chnaged the PSU as you suggested.
After I connected the sockets to the ground the hum disappeared, however, a small ssssss noise remained wich was not there before. It's not a constant hum, but it's a faint noise audible in the bass and in the tweeters too.
I don't have an oscilloscope to see the output but I observed that the DC offset on my voltmeter is fluctuating now between +- 1mV.
Regarding to your question: the answer is yes 🙂 I found your non inverting front end modification a great improvement. With my TL071 the noise went so down that I was realy undecided if I have to change them to the new OPA627, but of course I was curious and I did the modification. The noise I think remained the same, however the sound of the amplifier is much more pleasant. Also the DC offset went down from 39mV to around 0.5mV.
As a note: the operational is decoupled by 100nF right on the IC's pins and by 100uF after the stabilizers. I had to use stabilizers with the OPA627 because the original zenner + resitor started to warm up.
From the sound/oscillation point of view, is it important where do I solder the ground of the stabilizers regarding to the other components ground (like: R1, R3, C2 etc.) ?
Do you have any suggestion how to get rid of that faint noise? Could it be that my amplifier is sort of oscillating now?
Best regards,
Zsolt
Hi,
Now that you have the supply working you may want to consider running a resistor Zener//cap AFTER the regulator. An experiment that may sound better.
That hum. Was it electrical? or mechanical coming from the transformer vibrating the table?
running the zener warm is achieveable. Much more likely is that the Zener will run hot. This seems to do no harm to the following circuit.
Now that you have the supply working you may want to consider running a resistor Zener//cap AFTER the regulator. An experiment that may sound better.
That hum. Was it electrical? or mechanical coming from the transformer vibrating the table?
AndrewT
I had to run 24V Zenners before the 7815/7915 regulators because their max voltage is +35V/-35V.
Do you suggest me to put back the original 3.3k (or some smaller values) R7 and R8 after the regulators (red square in the attachement picture)?
Regarding to the hum, it was an electrical hum. A small mechanical hum could be heard because the transformer is not fixed but the one I'm talking about was clearly comming from the speakers.
Regards,
Zsolt
I had to run 24V Zenners before the 7815/7915 regulators because their max voltage is +35V/-35V.
Do you suggest me to put back the original 3.3k (or some smaller values) R7 and R8 after the regulators (red square in the attachement picture)?
Regarding to the hum, it was an electrical hum. A small mechanical hum could be heard because the transformer is not fixed but the one I'm talking about was clearly comming from the speakers.
Regards,
Zsolt
Attachments
Hi,
do you really have a resistor in the Vrail feed to the opamp after the regs? This will kill effective regulator action.
I am suggesting that the the opamp MAY operate more cleanly if a zener stabilised voltage was fed to it rather than the dirty PSU voltage that comes out of these one chip regulators.
The easy way to achieve this is to add an extra regulating stage after the 7824 & 7924. The extra stage would be a dropping resistor (270r) and Zener (15V 500mW). This would provide about 30mA of peak current from the Zener and any additional current demand from the opamp would be met by the decoupling caps.
do you really have a resistor in the Vrail feed to the opamp after the regs? This will kill effective regulator action.
I am suggesting that the the opamp MAY operate more cleanly if a zener stabilised voltage was fed to it rather than the dirty PSU voltage that comes out of these one chip regulators.
The easy way to achieve this is to add an extra regulating stage after the 7824 & 7924. The extra stage would be a dropping resistor (270r) and Zener (15V 500mW). This would provide about 30mA of peak current from the Zener and any additional current demand from the opamp would be met by the decoupling caps.
Hi,
the ground symbol on the input side looks the same as the ground symbol on each of the regulator and decoupling returns.
Have you returned each of these grounds SEPARATELY to your central ground? Otherwise you may get contamination of your input signal from ground rail modulation. I wonder if the hiss you heard comes from here? or is it simply low level noise from the gain of your circuits?
the ground symbol on the input side looks the same as the ground symbol on each of the regulator and decoupling returns.
Have you returned each of these grounds SEPARATELY to your central ground? Otherwise you may get contamination of your input signal from ground rail modulation. I wonder if the hiss you heard comes from here? or is it simply low level noise from the gain of your circuits?
AndrewT
No, I haven't returned them separately to my central ground and I'm susspecting that this is the cause of the faint hiss.
Maybe the regulators are ok as they are and I don't need any extra regulating stage but a separate ground return for the decoupling caps and the regulators.
This faint hiss is not the low level noise comming from the amplifier's gain. I changed the regulation stage because of the higher current need of the OPA627. Before of that the noise could be heard only if you sticked your ears to the speakers, especially after appling the non inverting design for the input.
Using the original regulation design for the OPA627 the sound was suppressed (maybe is not the right word), especially listening to piano (Keith Jarrett) the tail of the notes were clearly distorted and the resistance's R7 and R8 and zenners were getting hot after a couple of minutes.
That's why I tried the regulators for the input as in tvicol's design:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=51036&perpage=10&pagenumber=2
I have to try to separate the regulators and decoupling caps return from the rest of the input. Thanks.
No, I haven't returned them separately to my central ground and I'm susspecting that this is the cause of the faint hiss.
Maybe the regulators are ok as they are and I don't need any extra regulating stage but a separate ground return for the decoupling caps and the regulators.
This faint hiss is not the low level noise comming from the amplifier's gain. I changed the regulation stage because of the higher current need of the OPA627. Before of that the noise could be heard only if you sticked your ears to the speakers, especially after appling the non inverting design for the input.
Using the original regulation design for the OPA627 the sound was suppressed (maybe is not the right word), especially listening to piano (Keith Jarrett) the tail of the notes were clearly distorted and the resistance's R7 and R8 and zenners were getting hot after a couple of minutes.
That's why I tried the regulators for the input as in tvicol's design:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=51036&perpage=10&pagenumber=2
I have to try to separate the regulators and decoupling caps return from the rest of the input. Thanks.
Hi,
try hard wiring the dirty returns together using the shortest insulated wire links possible.
Then run the shortest route to the central ground.
The short wires are to minimise inductance in the connections.
try hard wiring the dirty returns together using the shortest insulated wire links possible.
Then run the shortest route to the central ground.
The short wires are to minimise inductance in the connections.
Hi,
I think that resistors must be removed after regulators , as Andrewt suggested.
Also , zeners will open at something like 0,5mA or so (i am not sure).
You may try putting resistors to ground after zeners , so that they draw constantly a few mA of current and ensure that zeners are always open.
If the zeners would start opening/closing there mayl be some noise present...
Not sure about this , just some ideas ...
Regards,
Lukas.
I think that resistors must be removed after regulators , as Andrewt suggested.
Also , zeners will open at something like 0,5mA or so (i am not sure).
You may try putting resistors to ground after zeners , so that they draw constantly a few mA of current and ensure that zeners are always open.
If the zeners would start opening/closing there mayl be some noise present...
Not sure about this , just some ideas ...
Regards,
Lukas.
I missunderstood what Andrew wrote in
In the quote he suggested to add an extra stabilizing stage using a dropping resistor (270r) and a Zenner (15V 500mW), but first if all I will try to hard wire the dirty returns toghether and run them to the central ground.
Regards,
Zsolt
I drawed the resistor in the red sqare just to ask if that is what he suggested, but no, I do not have any resistor there.
In the quote he suggested to add an extra stabilizing stage using a dropping resistor (270r) and a Zenner (15V 500mW), but first if all I will try to hard wire the dirty returns toghether and run them to the central ground.
Regards,
Zsolt
Thank you guys,
I was able to resolve the hum/noise problem 🙂 .
The first noise problem was that I haven't connected the input sockets to the chassis directly at the point they were mounted.
And, indeed, the faint ssss noise in the speakers was due to bad grounding of the regulators and decoupling caps used by the input operational. Hard wiring them to the central ground removed the faint hiss too.
Regards,
Zsolt
I was able to resolve the hum/noise problem 🙂 .
The first noise problem was that I haven't connected the input sockets to the chassis directly at the point they were mounted.
And, indeed, the faint ssss noise in the speakers was due to bad grounding of the regulators and decoupling caps used by the input operational. Hard wiring them to the central ground removed the faint hiss too.
Regards,
Zsolt
A friend of mine was trying to chase down a mechanical hum from an early 405. He tried mains cleaners, DC blockers on the AC-line, the lot. He figured out eventually it was the laminations in the mains transformer itself.
Listening closely, with the lid removed, he managed to strap his EAR across the 240vAC terminals on the top of the transformer!
I learnt new words that day...
Listening closely, with the lid removed, he managed to strap his EAR across the 240vAC terminals on the top of the transformer!
I learnt new words that day...
Hello all again!
I'm still upset with something 😕 and I couldn't find a solution for it:
Despite that there is no noise (hearable only if you stick your ears to the speakers) when the input is plugged to the CD player or grounded (volume to min), there is still a hearable noise (both 50-100Hz rail hum and noise) when the input is in the air. Also when I plug the audio cable to the input RCA (and the cable is not connected to the cd player) this hum+noise increases.
Although I know this is not a big problem it bugs me, so if you have some suggestions...
Regards,
Zsolt
I'm still upset with something 😕 and I couldn't find a solution for it:
Despite that there is no noise (hearable only if you stick your ears to the speakers) when the input is plugged to the CD player or grounded (volume to min), there is still a hearable noise (both 50-100Hz rail hum and noise) when the input is in the air. Also when I plug the audio cable to the input RCA (and the cable is not connected to the cd player) this hum+noise increases.
Although I know this is not a big problem it bugs me, so if you have some suggestions...
Regards,
Zsolt
Hi,
it's good if there is no noise when the cables are connected and no signal is present.
Do you ever listen music on your system with the cables disconnected?
Have a look at the schematic. Is there a DC blocking cap on the input? Is there an input impedance setting resistor to ground from the amp side of that same cap to signal ground? Is there a DC referencing resistor on the other side of the cap (RCA end) to ground? If not then adding a high value 1M0 to 2M2 resistor from RCA input pin to RCA ground might help. But equally it might not, worth trying.
it's good if there is no noise when the cables are connected and no signal is present.
Do you ever listen music on your system with the cables disconnected?
Have a look at the schematic. Is there a DC blocking cap on the input? Is there an input impedance setting resistor to ground from the amp side of that same cap to signal ground? Is there a DC referencing resistor on the other side of the cap (RCA end) to ground? If not then adding a high value 1M0 to 2M2 resistor from RCA input pin to RCA ground might help. But equally it might not, worth trying.
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