We think that 2 watt resistors should do the work.
As to the capacitor... do you mean to substitute our 0.1 uF with a 47-100 uF capacitor? ... or to add a 47-100 uF to our 0.1 uF?
Have you considered that the resistive divider must provide the +62.5 volt to 10 channels (each absorbing 365 mA) ? In this view...have we to change any parameter?
If 1 volt is enough to drive the transistor a 3 ohm series resistor could be fine as sense resistor. We just need to increase the heater supply of about another 1 volt (26.3 instead of 25.2). We will test the circuit.
Many thanks for your always helpful suggestions and precious lessons.
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Ian,
we tried to create a circuit for the relay. It utilizes the 26.4 Volt of the heater supply and a 3.3 sense resistor; this resistor gives 1.2 volt drop out to drive the base of PNP transistor (configured in negative logic, e.g. it open the anodic circuit when no voltage is present on its base).
We are not able to calculate the value of R4 and, honestly, we believe that we have made many errors (horrors).
Moreover, in the orange box we have also include a R-C net that could play as delay circuit (for the anodic supply) when switching on the mixer.
Please, do not blame us.
we tried to create a circuit for the relay. It utilizes the 26.4 Volt of the heater supply and a 3.3 sense resistor; this resistor gives 1.2 volt drop out to drive the base of PNP transistor (configured in negative logic, e.g. it open the anodic circuit when no voltage is present on its base).
We are not able to calculate the value of R4 and, honestly, we believe that we have made many errors (horrors).
Moreover, in the orange box we have also include a R-C net that could play as delay circuit (for the anodic supply) when switching on the mixer.
Please, do not blame us.
Hi Antonio,
1. 2W resistors are fine for the heater pot divider.
2. I meant to ADD the 47uF in parallel with the 0.1uF.
3. Looking at your relay drive circuit I think you have your logic inverted. If you replace the transistor with an NPN type, put the relay between its collector and the 88.9V and connect is emitter to the 62.5V it should work.
To calculate R4 you need to think about the relay current. When a transistor is turned hard on (in saturation) its current gain is a lot lower than when biased for class A operation. As a rule you should assume the saturation current gain is no more than 20. Your relay needs about 50mA of current so you need to inject at least 50/20 = 2.5mA into the base of the transistor.
The base emitter junction drops about 0.7V and the voltage across the sense resistor is 1.2V so R4 needs to drop 0.5V at 2.5mA i.e. R4 = 0.5/2.5 K = 200 ohms.
Cheers
Ian
1. 2W resistors are fine for the heater pot divider.
2. I meant to ADD the 47uF in parallel with the 0.1uF.
3. Looking at your relay drive circuit I think you have your logic inverted. If you replace the transistor with an NPN type, put the relay between its collector and the 88.9V and connect is emitter to the 62.5V it should work.
To calculate R4 you need to think about the relay current. When a transistor is turned hard on (in saturation) its current gain is a lot lower than when biased for class A operation. As a rule you should assume the saturation current gain is no more than 20. Your relay needs about 50mA of current so you need to inject at least 50/20 = 2.5mA into the base of the transistor.
The base emitter junction drops about 0.7V and the voltage across the sense resistor is 1.2V so R4 needs to drop 0.5V at 2.5mA i.e. R4 = 0.5/2.5 K = 200 ohms.
Cheers
Ian
Ian,
we woul like to leave, basically, unactivated the relay and activate it only when no current flows in the filament line (and so, no voltage accross R sense and no voltage on the base of transistor). The relay is a n.c. one!
Ian,
remove what reported above. We reported the first proof of the power supply schematic.
SPECIFIC REQUIREMENTS
For heater supply:
A) For each of the 10 channel boards: anodic supply in the channel must be interrupted when one of the 4 filaments breaks.
B) DC regulated supply.
For anodic supply:
A) Anodic supply with a low ripple.
B) If possible, a choke input type.
SPECIFIC QUESTIONS ABOUT THE SCHEMATIC
For heater supply:
A) Is the current sense circuit (interrupting anodic supply in the case a filament breaks) correct now?
B) Are the value of R3, R4, and R5 correct?
C) Which is the power required for R4 and R5?
D) The relays interrupting anodic supply (that we already have) are of 12V - 264 ohm. Are necessary to insert a resistor between Q2 and relay coil (or between the collector of Q2 and the negative line of filaments) … in order to produce a 14.4 Voltage drop and obtaining the 12 volt? If so, can a 320 ohm (2W) resistor do the work?
For anodic supply:
E) Do you suggest (your opinion) the implementation of a choke input supply? If so, is the proposed one (Hammond 132 -10H-500mA) a good choice? Consider that each channel absorb about 35-40 mA … X 10 channels = 400 mA.
F) The designed anodic supply includes a total of 3 pi-greek filters, one of which (LC) is shared among all 10 channels and are displaced on the power supply board; the last (RC filter, one for each E88CC) is displaced on the channel boards, near the tubes. Is that enough (for a low ripple) and correct?
G) Is 1K value of R3, R4, R5, etc (the resistor of the second pi-greek filter) correct? Is 3W enough?
H) Which value for C1?
I) Is 100uF of C4, C5, C6 correct?
Ciao
remove what reported above. We reported the first proof of the power supply schematic.
SPECIFIC REQUIREMENTS
For heater supply:
A) For each of the 10 channel boards: anodic supply in the channel must be interrupted when one of the 4 filaments breaks.
B) DC regulated supply.
For anodic supply:
A) Anodic supply with a low ripple.
B) If possible, a choke input type.
SPECIFIC QUESTIONS ABOUT THE SCHEMATIC
For heater supply:
A) Is the current sense circuit (interrupting anodic supply in the case a filament breaks) correct now?
B) Are the value of R3, R4, and R5 correct?
C) Which is the power required for R4 and R5?
D) The relays interrupting anodic supply (that we already have) are of 12V - 264 ohm. Are necessary to insert a resistor between Q2 and relay coil (or between the collector of Q2 and the negative line of filaments) … in order to produce a 14.4 Voltage drop and obtaining the 12 volt? If so, can a 320 ohm (2W) resistor do the work?
For anodic supply:
E) Do you suggest (your opinion) the implementation of a choke input supply? If so, is the proposed one (Hammond 132 -10H-500mA) a good choice? Consider that each channel absorb about 35-40 mA … X 10 channels = 400 mA.
F) The designed anodic supply includes a total of 3 pi-greek filters, one of which (LC) is shared among all 10 channels and are displaced on the power supply board; the last (RC filter, one for each E88CC) is displaced on the channel boards, near the tubes. Is that enough (for a low ripple) and correct?
G) Is 1K value of R3, R4, R5, etc (the resistor of the second pi-greek filter) correct? Is 3W enough?
H) Which value for C1?
I) Is 100uF of C4, C5, C6 correct?
Ciao
If you connect R4 to the other side of R3 it will be correct!!!
R3 and R5 are correct. However, because Q1 only has to sink about 6mA when it is hard on, you only need to supply about 0.3mA base current to ensure it turns on so R4 can be increased. As before:
R4 = (1.2-0.7)/base current = 0.5/0.3 K = 1.6K ohm. A 1K5 would be fine.
R5 consumes a maximum of nearly 6mA at about 26V or around 150mW. A half watt type for R5 would be OK. R4 consumes very little power; a normal 0.25W type will be OK.
This looks OK in theory. The 320 ohm at 3W should provide the correct voltage drop and be able to dissipate the power. However, I have never run a relay before with such a large voltage drop across an external resistor so I cannot be certain it will work.
I have no experience of choke input filters for pre-amplifier power supplies so I cannot comment if your design is good or bad. My own designs all use CRCRCRC filters. You design is essentially a CLCRCRC type which should be at least as good. However, I notice that the dc voltage after the choke filter is 250V but after the first RC filter it has somehow managed to increase to 255V.
With appropriate values of R L and C that supply topology should give low enough ripple.
I try to make the voltage drop in each RC section about the same. Since R3,R4, R5 take three channels of current I would reduce the 1K resistor to 330 ohms to keep the voltage drop similar. I would use 470uF for C4,C5 and C6.
Sorry, I cannot help with that one.
Yes, they are fine.
Cheers
Ian
Ian,
we have a big problem.
There are numerous cables from each preamplifier board to the panel. If we want to have an individual shileding for each section (see schematic) ... the number of cables is very high. Specifically, this approach produces 4 big groups of cables (see panel):
1st group (from the board to the panel commands) = 10 cables.
2nd group (from the board to the input-output jacks) = 4 cables
3rd group (from the board to the ouput stages) = 9 cables
4th group (power supply - anodic - heaters - 12-0-12 for ICs) = 7 wires.
Managing all these cables is not simple at all and we (so far) have not been able to find a solution. A possible solution could be that of using 3 big multipolar cables (each with a single common shield): 1 cable (with 10 wires) for the fisrts group, 1 cable (with 4 wires) for the second group, and 1 cable (7 wires) for the third group...leaving the power supplies to individual wires.
At this poin, however, the question becames: can the wires included in each big cable interact with the other wires?
Are there other solutions?
Antonio
we have a big problem.
There are numerous cables from each preamplifier board to the panel. If we want to have an individual shileding for each section (see schematic) ... the number of cables is very high. Specifically, this approach produces 4 big groups of cables (see panel):
1st group (from the board to the panel commands) = 10 cables.
2nd group (from the board to the input-output jacks) = 4 cables
3rd group (from the board to the ouput stages) = 9 cables
4th group (power supply - anodic - heaters - 12-0-12 for ICs) = 7 wires.
Managing all these cables is not simple at all and we (so far) have not been able to find a solution. A possible solution could be that of using 3 big multipolar cables (each with a single common shield): 1 cable (with 10 wires) for the fisrts group, 1 cable (with 4 wires) for the second group, and 1 cable (7 wires) for the third group...leaving the power supplies to individual wires.
At this poin, however, the question becames: can the wires included in each big cable interact with the other wires?
Are there other solutions?
Antonio
If we directely connect the board with the panel (see direct conncection), the distance is about 10-25 cm.
Conversely, if we want a better distribution (by means of a single multiwires cable or by means of mutli-wires connected together) (see long connection), the distance becomes 50-60 cm.
In both cases, long connections with output buffer stages must be protected by means of shielded cables.
Power supply must be positioned at 30-40 cm from the nearest channel (or it could also be in a separate cabinet and connected with the mixer console by means of two power cables).
We have not decided if realize a single top pannel including the commands of all 10 channels (and in this case we are forced to adopt long connections...in order to allow the hinge effect) or realize 10 separate small indipendent cabinets, similar to those reported in the images (and in this case we could also utilize short connections).
P.S.: the image reported include only few cables (just for a representative description); indeed, the number of cables we need is far greater !
Conversely, if we want a better distribution (by means of a single multiwires cable or by means of mutli-wires connected together) (see long connection), the distance becomes 50-60 cm.
In both cases, long connections with output buffer stages must be protected by means of shielded cables.
Power supply must be positioned at 30-40 cm from the nearest channel (or it could also be in a separate cabinet and connected with the mixer console by means of two power cables).
We have not decided if realize a single top pannel including the commands of all 10 channels (and in this case we are forced to adopt long connections...in order to allow the hinge effect) or realize 10 separate small indipendent cabinets, similar to those reported in the images (and in this case we could also utilize short connections).
P.S.: the image reported include only few cables (just for a representative description); indeed, the number of cables we need is far greater !
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OK Antonio, now I have a much better idea of the mechanical construction of your mixer.
The first thing to say is that with (relatively) high impedance circuits it is important to keep screened cable lengths short to avoid deterioration of high frequencies due to cable capacitance. I do not have the circuits in front of me but for example a 100K volume control has a worst case source impedance of 100/4 = 25K ohms. 100pF of capacitance in a cable connected to the control will make the HF response 3dB down at about 64KHz. Not much to worry about you might think but small diameter audio coax cable has a capacitance of typically 300pF per metre, so 66cm of this cable will be 200pF which will give a response 3dB down at 32KHz and 1dB down at 16KHz. So use cables as short as possible and potentiometer values as low as possible. With a 10K volume control there is no problem. Cables connected directly to low impedance output buffers are also not a problem. You can get low capacitance coax cable with about 100pF/metre but it tends to be about 6mm in diameter!
I can understand your difficulty in deciding whether to use a single control panel or separate modules. The single hinged panel was popular with early mixers from the 1940s onwards but they were nearly all 600 ohm sources and controls throughout so cable capacitance was not an issue. I know of only one tube mixer made by RCA that used 100K volume controls and a single hinged control panel. The hinge is attractive because it means that access to the wiring and other parts of the console is easy. However, it does mean longer cable lengths and if a single control fails the whole mixer is out of action.
I think this is why later mixers used a modular approach. Individual modules are easier to build and test and if one goes wrong it can be quickly replaced by a spare - probably something that is very important for a PA mixer.
Many modular mixers have plug in modules. As well as simplifying changing a module this also means that common signals like power and mix buses can be run easily across the connectors into which the modules plug. The big disadvantage of plug in modules is that you have to get the plug in mechanics just right and that's a lot of work for a single DIY project.
So, on balance I would recommend you take a modular approach but not with plug in modules. Cable capacitance and volume control impedances dictates how long the cables can be.
Lastly, you suggested using multi-core cables with a single screen (to save space) and asked if the cables inside this multi-core can interact with each other. The answer is similar to the cable capacitance question but this time we are interested in the core to core capacitance. I regularly use twin core cable to connect to and from a volume control and I have not been able to measure any detrimental effect of doing this, but that is for a single signal. The problem with several different signals in one cable is crosstalk between them which is again governed by the core to core capacitance. I have had a look at some typical multi-core screened cables on the net and it looks like the core to core capacitance is typically about 60% of the screen to core capacitance. At 1KHz, the impedance of 100pF is about 1.6 Megohms. Let's assume we are using a 10K volume control then the cross talk is effectively a potential divider with 1.6 Meg at the top and 10K at the bottom. This pot divider has an attenuation of approximately 1.6Meg/10K = 160 or 44dB. That is not a good figure for crosstalk and if the frequency is 10KHz the crosstalk gets even worse at 24dB. Therefore I would say do not use a multi-core cable with a single overall screen.
One last point. I notice your microphone transformer is not on the PCB. You need to keep the transformer secondary leads to the tube as short as possible to avoid picking up interference so the transformer should be very close to the PCB.
You also say the power supply should be at least 30cm from the modules or in a separate enclosure. With sensitive microphone transformers in the mixer I would recommend using an external power supply in a separate screened enclosure.
Cheers
Ian
The first thing to say is that with (relatively) high impedance circuits it is important to keep screened cable lengths short to avoid deterioration of high frequencies due to cable capacitance. I do not have the circuits in front of me but for example a 100K volume control has a worst case source impedance of 100/4 = 25K ohms. 100pF of capacitance in a cable connected to the control will make the HF response 3dB down at about 64KHz. Not much to worry about you might think but small diameter audio coax cable has a capacitance of typically 300pF per metre, so 66cm of this cable will be 200pF which will give a response 3dB down at 32KHz and 1dB down at 16KHz. So use cables as short as possible and potentiometer values as low as possible. With a 10K volume control there is no problem. Cables connected directly to low impedance output buffers are also not a problem. You can get low capacitance coax cable with about 100pF/metre but it tends to be about 6mm in diameter!
I can understand your difficulty in deciding whether to use a single control panel or separate modules. The single hinged panel was popular with early mixers from the 1940s onwards but they were nearly all 600 ohm sources and controls throughout so cable capacitance was not an issue. I know of only one tube mixer made by RCA that used 100K volume controls and a single hinged control panel. The hinge is attractive because it means that access to the wiring and other parts of the console is easy. However, it does mean longer cable lengths and if a single control fails the whole mixer is out of action.
I think this is why later mixers used a modular approach. Individual modules are easier to build and test and if one goes wrong it can be quickly replaced by a spare - probably something that is very important for a PA mixer.
Many modular mixers have plug in modules. As well as simplifying changing a module this also means that common signals like power and mix buses can be run easily across the connectors into which the modules plug. The big disadvantage of plug in modules is that you have to get the plug in mechanics just right and that's a lot of work for a single DIY project.
So, on balance I would recommend you take a modular approach but not with plug in modules. Cable capacitance and volume control impedances dictates how long the cables can be.
Lastly, you suggested using multi-core cables with a single screen (to save space) and asked if the cables inside this multi-core can interact with each other. The answer is similar to the cable capacitance question but this time we are interested in the core to core capacitance. I regularly use twin core cable to connect to and from a volume control and I have not been able to measure any detrimental effect of doing this, but that is for a single signal. The problem with several different signals in one cable is crosstalk between them which is again governed by the core to core capacitance. I have had a look at some typical multi-core screened cables on the net and it looks like the core to core capacitance is typically about 60% of the screen to core capacitance. At 1KHz, the impedance of 100pF is about 1.6 Megohms. Let's assume we are using a 10K volume control then the cross talk is effectively a potential divider with 1.6 Meg at the top and 10K at the bottom. This pot divider has an attenuation of approximately 1.6Meg/10K = 160 or 44dB. That is not a good figure for crosstalk and if the frequency is 10KHz the crosstalk gets even worse at 24dB. Therefore I would say do not use a multi-core cable with a single overall screen.
One last point. I notice your microphone transformer is not on the PCB. You need to keep the transformer secondary leads to the tube as short as possible to avoid picking up interference so the transformer should be very close to the PCB.
You also say the power supply should be at least 30cm from the modules or in a separate enclosure. With sensitive microphone transformers in the mixer I would recommend using an external power supply in a separate screened enclosure.
Cheers
Ian
Ian, you will not believe … we have come to the same conclusions. But now, with your support and always precious explanations, we are much more safe of our choices.
We have already arranged the power supply section (see power supply schematic) and also ended and tested the heater sensor section. In this arrangement, we should have (see channel 1 power supply):
1) One “Main power supply board” (in the power supply cabinet).
2) Ten little power supply boards hosting the sensor resistor and the relay + the second anodic R-C group (in the mixer console).
3) Ten preamplifier boards hosting the third R-C group of the anodic power supply (in the mixer console).
The external power supply, however, leads to ground connections problem. We know that all the negative ground returning wires should be connected on the negative of the first capacitor of the power supply (main ground star), but this component is into the power supply cabinet. So, if we consider a separate power supply, we should have at least 10 negative returning wires (+ one positive anodic wire) and so a complex multiwire cable and a complex panel connector to adopt.
We’d like to ask if the proposed organization of the ground wires ( see ground connections) could be appropriate. Specific questions for that schematic are
1) Are the ground connections correct?
2) Where we have to connect G1-ch1, G1-ch2, G1-ch3 … ? to the corresponding ground star of the own preamplifier board (G2-ch1, G2-ch2, G2-ch3 …) ? … or, separately, to Main G?
Ciao
We have already arranged the power supply section (see power supply schematic) and also ended and tested the heater sensor section. In this arrangement, we should have (see channel 1 power supply):
1) One “Main power supply board” (in the power supply cabinet).
2) Ten little power supply boards hosting the sensor resistor and the relay + the second anodic R-C group (in the mixer console).
3) Ten preamplifier boards hosting the third R-C group of the anodic power supply (in the mixer console).
The external power supply, however, leads to ground connections problem. We know that all the negative ground returning wires should be connected on the negative of the first capacitor of the power supply (main ground star), but this component is into the power supply cabinet. So, if we consider a separate power supply, we should have at least 10 negative returning wires (+ one positive anodic wire) and so a complex multiwire cable and a complex panel connector to adopt.
We’d like to ask if the proposed organization of the ground wires ( see ground connections) could be appropriate. Specific questions for that schematic are
1) Are the ground connections correct?
2) Where we have to connect G1-ch1, G1-ch2, G1-ch3 … ? to the corresponding ground star of the own preamplifier board (G2-ch1, G2-ch2, G2-ch3 …) ? … or, separately, to Main G?
Ciao
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Ian,
we forgot a little but important detail. G1-ch1, G1-ch2, G1-ch3, and G1-ch10 represent the star for all the shields of the cables connecting the preamplifier board with potentiometers and input-output jacks.
They are connected only on the pramplifier board side... and are free on the other side.
we forgot a little but important detail. G1-ch1, G1-ch2, G1-ch3, and G1-ch10 represent the star for all the shields of the cables connecting the preamplifier board with potentiometers and input-output jacks.
They are connected only on the pramplifier board side... and are free on the other side.
Hi Antonio,
It is good that we have both come to the same conclusion!
Your proposed earthing scheme is a 'star of stars' and is quite a common technique. Ideally all 0V returns should be starred at the PSU but this is impractical and in practice is not necessary except in power amplifiers where there are significant signal currents flowing in some 0V lines. In a class A mixer the signal currents are small and the main concern is to avoid unnecessary crosstalk because of common impedance paths. Taking an individual 0V from each channel star to a common star will work but it may not be necessary. The important thing is to keep the 0V impedance as low as possible and often this is achieved using a single thick bus bar that runs the length of the mixer and to which all channel grounds are connected - it is effectively a long thick star. Most mixers today do something like this and the old tube RCA ones certainly used this technique. Here is a picture from an old RCA console. You can see the bus bar held between two tag strips.
I do this in my own mixer designs. Here is a picture of the (incomplete) back plane of a 6 into 2 mixer I am working on right now.
The blue and brown wires are the 12V heater supplies, the red wire is the HT+ and the bare copper wire is the 0V bus.
The other thing you need to do is to ensure the metal chassis (screen) and 0V are kept separate and only joined together at one point (also in the PSU) and you need to maintain this screen between the PSU and the mixer.
This is what I do.
1. A screened multi-way connector takes power from the PSU to the mixer.
2. 0V from modules connects to a bus bar in the mixer and the bus bar is connected to a thick wire in the PSU cable.
3. The metalwork of the mixer is connected to the screen of the cable.
4. In the power supply, mains safety earth is connected by a short single wire to the PSU chassis star point.
5. The screen of the power cable is connect to the PSU chassis star point.
6. The smoothing cap -ve is connected directly to the PSU chassis star point
7. The 0V cable is connected directly to the smoothing cap -ve.
This way the (screening) chassis, safety earth and 0V are connected together at one point only. This is my first rule of grounding.
Cheers
Ian
It is good that we have both come to the same conclusion!
Your proposed earthing scheme is a 'star of stars' and is quite a common technique. Ideally all 0V returns should be starred at the PSU but this is impractical and in practice is not necessary except in power amplifiers where there are significant signal currents flowing in some 0V lines. In a class A mixer the signal currents are small and the main concern is to avoid unnecessary crosstalk because of common impedance paths. Taking an individual 0V from each channel star to a common star will work but it may not be necessary. The important thing is to keep the 0V impedance as low as possible and often this is achieved using a single thick bus bar that runs the length of the mixer and to which all channel grounds are connected - it is effectively a long thick star. Most mixers today do something like this and the old tube RCA ones certainly used this technique. Here is a picture from an old RCA console. You can see the bus bar held between two tag strips.
An externally hosted image should be here but it was not working when we last tested it.
I do this in my own mixer designs. Here is a picture of the (incomplete) back plane of a 6 into 2 mixer I am working on right now.
An externally hosted image should be here but it was not working when we last tested it.
The blue and brown wires are the 12V heater supplies, the red wire is the HT+ and the bare copper wire is the 0V bus.
The other thing you need to do is to ensure the metal chassis (screen) and 0V are kept separate and only joined together at one point (also in the PSU) and you need to maintain this screen between the PSU and the mixer.
This is what I do.
1. A screened multi-way connector takes power from the PSU to the mixer.
2. 0V from modules connects to a bus bar in the mixer and the bus bar is connected to a thick wire in the PSU cable.
3. The metalwork of the mixer is connected to the screen of the cable.
4. In the power supply, mains safety earth is connected by a short single wire to the PSU chassis star point.
5. The screen of the power cable is connect to the PSU chassis star point.
6. The smoothing cap -ve is connected directly to the PSU chassis star point
7. The 0V cable is connected directly to the smoothing cap -ve.
This way the (screening) chassis, safety earth and 0V are connected together at one point only. This is my first rule of grounding.
Cheers
Ian
Hi Antonio,
I do not know how far you have got with the PCb for the LED signal level display but I thought you should know about an integrated circuit I have just come across that combines the audio amplifier and LED drive into a single SIP device. It is the LB1443N made by Sanyo. It is a five LED VU meter.
I have also found a 12 LED VU meter made by Rohm, type number BA682A.
There may well others so it is probably worth Googling.
Cheers
Ian
I do not know how far you have got with the PCb for the LED signal level display but I thought you should know about an integrated circuit I have just come across that combines the audio amplifier and LED drive into a single SIP device. It is the LB1443N made by Sanyo. It is a five LED VU meter.
I have also found a 12 LED VU meter made by Rohm, type number BA682A.
There may well others so it is probably worth Googling.
Cheers
Ian
Ian,
thank you for the suggestions about the grounding and ground scheme. I have always utilized the star point in my previous realizations. However, I understand that, in the case of a mixer, this approach is difficult to realize. Your suggestions are clear but some points needs to be clarified (for us).
1) Is it correct to connect all the transformer shields to the PSU chassis star point?
2) Is it correct to connect all the cable screens (G1-ch1, G1-ch2, G1-ch3…) to a bus connected to the screen of
3) the main cable (… in its turn connected to the metalwork of the mixer)?
4) Do you think that the isolation of the 0V from the hearth through a RC net (10R, 0.1uF) is necessary?
5) Is the 0V of the dual power supply for the IC sections (op-amp, various ICs, etc) to be connected to the
PSU 0V star point ?
6) The bus bar for 0V should be a 1.5 mm silver copper wire. Is it appropriate?
7) Where to connect the shells of potentiometers ? directly on the mixer chassis? …or isolated from it and connected to the PSU chassis star point?
8) Similarly … where to connect the shells of jack inputs ?
As to the IC for leds and op-amp ...we'll have alook to the datasheet.
Thank you
thank you for the suggestions about the grounding and ground scheme. I have always utilized the star point in my previous realizations. However, I understand that, in the case of a mixer, this approach is difficult to realize. Your suggestions are clear but some points needs to be clarified (for us).
1) Is it correct to connect all the transformer shields to the PSU chassis star point?
2) Is it correct to connect all the cable screens (G1-ch1, G1-ch2, G1-ch3…) to a bus connected to the screen of
3) the main cable (… in its turn connected to the metalwork of the mixer)?
4) Do you think that the isolation of the 0V from the hearth through a RC net (10R, 0.1uF) is necessary?
5) Is the 0V of the dual power supply for the IC sections (op-amp, various ICs, etc) to be connected to the
PSU 0V star point ?
6) The bus bar for 0V should be a 1.5 mm silver copper wire. Is it appropriate?
7) Where to connect the shells of potentiometers ? directly on the mixer chassis? …or isolated from it and connected to the PSU chassis star point?
8) Similarly … where to connect the shells of jack inputs ?
As to the IC for leds and op-amp ...we'll have alook to the datasheet.
Thank you
Ian,
we have had a look to the IC you suggested.
Francesco want a 10-led series vu-meter (7 green, 1yellow, and 2 red).
We have already realized the pcb with the LM3915 (see schematic adopted). We have also tested the circuit and it seems to work properly. It is important (with the LM3915) the possibility of adjust the led brightness and the dot/bar selection.
Ciao
Antonio
we have had a look to the IC you suggested.
Francesco want a 10-led series vu-meter (7 green, 1yellow, and 2 red).
We have already realized the pcb with the LM3915 (see schematic adopted). We have also tested the circuit and it seems to work properly. It is important (with the LM3915) the possibility of adjust the led brightness and the dot/bar selection.
Ciao
Antonio
For mains transformers yes.
No. The screens of these unbalanced cable carry signal current so they should not be connected to the chassis. The screen of the cable should be retuned to the local 0V at the sending end only. See the image below for how to connect pots and insert jacks.
An externally hosted image should be here but it was not working when we last tested it.
I prefer to use insulated (plastic) jack sockets so I can be sure there are no additional local connections to the chassis.
Many people are of the opinion it is necessary and as many are of the opinion that it is not. It is a subject that is still debated today:
XLR pin #1 Connections
I connect directly.
Yes.
Yes, that is OK.
Potentiometer shells are screens (they do not carry signal current) so they should be connected to the local chassis by as short a connection as possible (this assumes the shell is NOT connected to any of the three pot terminals).
Input jack shells carry signal current so they should not be connected to the chassis. As I said above I prefer insulated jacks. For inputs connect the screen to the shell at the jack end and connect the screen to the 0V nearest the tube the signal goes to.
Cheers
Ian
Ian,
there is a little misinterpretation.
In our arrangment we have both +ve signal and negative (e.g. jack screen) signal wires running into cables with a proper screen (see screen cable). Negative part of the signals (i.e., 0V references) often run (black wires) together with other wires but they are always protected by a screen cable.
Moreover, all these negative (0 reference) wires are connected to the star point of the preamplifier board (and this is already done on the pcb)... BUT they are connected only on the star poin side (they are free on the panel board side !).
We enclose a schematic reporting the connections of both screen cables (pale green lines) and negative wires (blacK lines) (see screen cable connections).
In this contest (assuming it is correct),
1) where to connect the screen cables ?
2) ...and were to connect the potentiometer shields?
there is a little misinterpretation.
In our arrangment we have both +ve signal and negative (e.g. jack screen) signal wires running into cables with a proper screen (see screen cable). Negative part of the signals (i.e., 0V references) often run (black wires) together with other wires but they are always protected by a screen cable.
Moreover, all these negative (0 reference) wires are connected to the star point of the preamplifier board (and this is already done on the pcb)... BUT they are connected only on the star poin side (they are free on the panel board side !).
We enclose a schematic reporting the connections of both screen cables (pale green lines) and negative wires (blacK lines) (see screen cable connections).
In this contest (assuming it is correct),
1) where to connect the screen cables ?
2) ...and were to connect the potentiometer shields?
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Hi Antonio,
Thanks for the diagrams. I understand what you are doing now. The simple answer is to connect the screens to 0V at the PCB end. The reason is that although these are screens they are surrounding signals. The potential of the chassis is unknown - it will be similar to the potential of 0V but not identical. If the screen is connected to chassis then the chassis voltage variations are transmitted directly to the signal conductor and noise is induced rather than being kept out. SO the best thing to do is connect the screen to the local 0V.
For the same reason you may prefer to connect potentiometer shields to the screen of the cable connected to it.
Cheers
Ian
Ian,
we always thank for your precious suggestions and I personally add that it is a pleasure to read your answers; they are true lessons of Electronic.
We have fully understood your answer to our previous questions. The rationale of your suggestion is clear but one thing escape us.
You suggest to connect the shields of potentiometers to the screen of the cables connected to them (and not to the chassis). Indeed the shield of potentiometer is electrically connected with its thread. So, de facto, the shield is already connected to the chassis. What benefit might result from a connection from the shiled with the local 0V ?
... or do you mean that we should isolate the thread (and any othe metallic portion) of the potentiometer from the chassis?
Ciao
we always thank for your precious suggestions and I personally add that it is a pleasure to read your answers; they are true lessons of Electronic.
We have fully understood your answer to our previous questions. The rationale of your suggestion is clear but one thing escape us.
You suggest to connect the shields of potentiometers to the screen of the cables connected to them (and not to the chassis). Indeed the shield of potentiometer is electrically connected with its thread. So, de facto, the shield is already connected to the chassis. What benefit might result from a connection from the shiled with the local 0V ?
... or do you mean that we should isolate the thread (and any othe metallic portion) of the potentiometer from the chassis?
Ciao
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