I thought 1% tolerance was too much for an XLR input stage. If your experience is the opposite, I will be more than happy.Mooly said:
Do you have a circuit for an XLR input with balanced impedances? Since, I will be using resistor dropper for this stage, I am also tempted to use the servo circuit you posted earlier. If it further reduces noise, whatever its source, I will be more than happy.
If I can find a solution to have no mains hum injected, there will be no need to further complicate the circuit by adding an XLR input stage. Searching this forum, I found that even XLR inputs can suffer from huge hum intensities that completly swamp an audio signal. The reason given is Pin 1, the ground. If the latter is connected to chasses on both signal source and amplifier one would be asking for trouble, if I understood correctly.
So, my question is: Since the amplifier circuit is grounded to chassis by its own large heatsink, should I use signal ground for signal input sockets? If that is the case, I should make sure the input PCB does not get ground from its mounting pillars.
I would have thought 1% was fine for a set up like this tbh. I mentioned it because of what you said in post #422 about values reaching 10%
I've never built any balanced input stages I'm afraid and so would be starting from scratch in designing one.
Yuo could always do what the late JLH did and inject a little hum in antiphase (cancellation)
Grounding the chassis is normally OK but of course you don't want multiple ground routes caused by connecting other parts of the circuit to chassis. So input grounds should be returned to a designated 'clean' ground that is not influenced by other circulating currents.
I've never built any balanced input stages I'm afraid and so would be starting from scratch in designing one.
Yuo could always do what the late JLH did and inject a little hum in antiphase (cancellation)
Grounding the chassis is normally OK but of course you don't want multiple ground routes caused by connecting other parts of the circuit to chassis. So input grounds should be returned to a designated 'clean' ground that is not influenced by other circulating currents.
This means I have a small ground loop inside the amplifier box. The input is grounded at the front panel, and the amplifier is grounded at the 0V point between the two large smoothing capacitances. In the same box there is a huge toroidal transformer, which usually leaks a weak magnetic field. This in turn, generates a voltage in any conductive loop it encounters. As a precaution, I fixed the input cabling with tape to make sure the cabling is in contact with the metal chassis. However, I cannot predict from where electrons flow in the metal box, and in this way, a loop can form.
I rerouted the input signal cabling. It seems the hum was injected through parasitic capacitance between the heavy supply tracks and the input cabling.
For now, I am satisfied with the results although there is always room to improve things. I am starting to make the second channel PCBs using the same old fashioned method.
I rerouted the input signal cabling. It seems the hum was injected through parasitic capacitance between the heavy supply tracks and the input cabling.
For now, I am satisfied with the results although there is always room to improve things. I am starting to make the second channel PCBs using the same old fashioned method.
The second channel PCBs are etched. I made a mistake in one, but it can be easily overriden. Mistakes are part of human nature, what is important is not being discouraged by them, but having the motivation of exploiting them.
One positive aspect of this second input-VAS-output-protection-circuit PCB, is that the tracks came out smoother than the first one.
Tomorrow, I will purchase the remaining components, hopefully, and start installing them. This second channel should not offer the same uphill learning curve like the first. This should be far easier because of the experience gained.
This thread should be seen as a walkthrough exposing the ups and downs of trying to build something usually built in highly equipped environments like factories. It is human to lack courage and motivation when things go not as planned.
One positive aspect of this second input-VAS-output-protection-circuit PCB, is that the tracks came out smoother than the first one.
Tomorrow, I will purchase the remaining components, hopefully, and start installing them. This second channel should not offer the same uphill learning curve like the first. This should be far easier because of the experience gained.
This thread should be seen as a walkthrough exposing the ups and downs of trying to build something usually built in highly equipped environments like factories. It is human to lack courage and motivation when things go not as planned.
this one works first time.
When this channel is tested and installed, the next step is to modify the existing cooling system to control the ventilation fan according to temperature. But there is a problem, the cooling fan is powered from a coil tap on the primary side of the toroidal transformer. This means, the fan's control circuitry is not isolated from the mains. There are two NTC resistors from the original circuit that respond very well to temperature changes. The problem is fixing these to the main heatsinks and playing safe according to electrical safety regulations.
Winding another coil on the toroidal transformer requires the removal of hard resin, something that requires hours of great patience.
Winding another coil on the toroidal transformer requires the removal of hard resin, something that requires hours of great patience.
The fan is a 24V DC type. As soon as I can, I will inspect the power supply PCB and measure the voltage that is supposed to power the cooling fan. That should allow me to modify the already existing circuit to feed more power as heatsink temperature rises.
P.S.
The second channel amplifier passed the first test: I used it to listen to music at a low volume for an hour or slighly more.
P.S.
The second channel amplifier passed the first test: I used it to listen to music at a low volume for an hour or slighly more.
The cooling fan power supply provides a voltage of 12V with a mains voltage of 120V ac. The design operating voltage is 24V DC with a european mains of 230V ac. At 12V DC the fan works with sufficient speed which is good news to me. The fan is supplied through a medium power transistor which is turned on with an opto coupler. It seems there is an issue with the opto coupler or the power transistor. The fan should not run at full speed without a signal when the primary is powered with 230V ac mains.
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The fan's control circuit contains only a few components. There is only one medium power transistor mounted on a small heatsink, an optocoupler and a few resistors. The use of a heatsink indicates this circuit is not a PWM chopper.
Simulating on LTSpice for this circuit shows it will not be as effective as intended. R1 is a negative coefficient resistor (thermistor) in thermal contact with the heatsinks.
Simulating on LTSpice for this circuit shows it will not be as effective as intended. R1 is a negative coefficient resistor (thermistor) in thermal contact with the heatsinks.
That's a very basic kind of a circuit where the voltage for the fan creeps up, also 0.22 ohm as a load seems a little low tbh.
I would use something like a simple opamp based comparator that gives you say three (or as many as you want really) discrete speeds such as off, low and high or off, very low, low, med, high and very high.
I would use something like a simple opamp based comparator that gives you say three (or as many as you want really) discrete speeds such as off, low and high or off, very low, low, med, high and very high.
Taking experimental data of the thermistor confirmed it is a positive coefficient thermistor. At 17.2C, its resistance is 89.6 Ohms which increases to over 40kOhm when the temperature approaches 100C. I used a hot air gun with precise air temperature control.
This kind of exceptional sensitivity should allow me to build a simple voltage regulator whose voltage output is a function of temperature. Taking data from the fan, this starts rotating steadily at 5.54V DC, the speed stays constant until the voltage reaches 7.11V DC, then the speed increases steadily until it reaches full speed at around 16.69V DC.
This data will allow me to design a simple series pass regulator that effectively controls the fan.
I can greatly simplify my circuit by using a 7805 regulator chip. Its voltage output can be increased by using a two resistor chain with one between the output and common, and the other between common and ground. The latter resistance should contain the thermistors in parallel with a 12V Zener diode. The output voltage can be shifted up by about 0.6V to 5.6V by putting a signal diode between ground and the thermistor Zener diode parallel network.
The only problem with the latter circuit is the whether the 7805 IC can withstand a 30V DC power supply. The voltage should reach a maximum of 25V DC, but I want to widen the safe margin.
This kind of exceptional sensitivity should allow me to build a simple voltage regulator whose voltage output is a function of temperature. Taking data from the fan, this starts rotating steadily at 5.54V DC, the speed stays constant until the voltage reaches 7.11V DC, then the speed increases steadily until it reaches full speed at around 16.69V DC.
This data will allow me to design a simple series pass regulator that effectively controls the fan.
I can greatly simplify my circuit by using a 7805 regulator chip. Its voltage output can be increased by using a two resistor chain with one between the output and common, and the other between common and ground. The latter resistance should contain the thermistors in parallel with a 12V Zener diode. The output voltage can be shifted up by about 0.6V to 5.6V by putting a signal diode between ground and the thermistor Zener diode parallel network.
The only problem with the latter circuit is the whether the 7805 IC can withstand a 30V DC power supply. The voltage should reach a maximum of 25V DC, but I want to widen the safe margin.
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The second channel is complete and installed together with the other channel in the amplifier box. I am testing the amplifiers at 82V DC rail to rail. This is half the nominal voltage. The AC mains supply is from a large auto transformer with an output at 120V AC. Till now, the amplifiers are behaving as they should with the heatsinks barely getting warm, although the ambient tempreture here is 17C. I am still testing using very inexpensive speakers. To use better speakers I need more testing to properly evaulate the amplifier's reliability.
The next project seems to be an analogue signal processor and mixer. I tried to purchase a ready made mixer with 3 channels or 2, but I was told small mixers do not come with an equilizer built in. The only signal filtering is done through a bass, midrange and treble filter. This forum is a motivator giving me assurance that where my knowledge is lacking, there are helpful people who are ready to share their knowledge.
The next project seems to be an analogue signal processor and mixer. I tried to purchase a ready made mixer with 3 channels or 2, but I was told small mixers do not come with an equilizer built in. The only signal filtering is done through a bass, midrange and treble filter. This forum is a motivator giving me assurance that where my knowledge is lacking, there are helpful people who are ready to share their knowledge.
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