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Old 31st July 2011, 07:32 PM   #11
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Default From another forum we have a french guy that also produced 3D

His 3D was more real.

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 1st August 2011, 11:26 AM   #12
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Default Detailing the MKIII-Hx

Capacitor substitution and some tips and tricks:

‪Monologue detailing the MKIII-Hx power audio amp.‬‏ - YouTube

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 1st August 2011, 02:18 PM   #13
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Post numbered schematic & board layout

Greetings,

I put together this numbered schematic and board layout to help myself make a parts list. Note that these have not been carefully scrutinized by either Carlos or Alex. There may be mistakes. The point of posting them is to make it easier for others to put together their parts list. In addition, in the process, people will (hopefully) post my mistakes or make suggestions, so everyone benefits.
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Old 1st August 2011, 02:19 PM   #14
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and the board . . .
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Old 2nd August 2011, 05:20 AM   #15
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Default Comment on use of 3W Resistors

Hello Carlos,

Thanks for getting this builder's thread going.

I would just like to comment on your explanation regarding the use of the 3W resistors back on post #7. If you don't mind, I'll try to explain this as simple as possible, so please bear with me.

Below is the upper half (positive-going) output section. Let us do a DC analysis to see what the power is across those 0.47 ohm resistors. The speaker load is RL.

A1.jpg

To make the explanation simple, let's assume that for all transistors the Collector-Emitter voltage drop (Vce) is 5V when all the transistors are fully saturated (e.g. full output). With a 5V Vce drop on the transistors, we can simplify the circuit. This is an equivalent circuit:

A2.jpg

Notice that we now have a voltage divider circuit where the speaker load is the lower resistance; and the upper resistance is formed by (5x) 0.47ohm degeneration resistors. Note that five 0.47 ohm resistors in parallel is 0.094 ohms.

Therefore, the voltage across the (5x) 0.47 ohms is calculated as:

V = (60 V * 0.094 ohms) / (0.094 ohms + RL)

First correction:

Quote:
.... the only condition you may have high wattage there is if your power transistor blow out and produces a short from colector to emitter...then you will have 64 volts from the supply crossing it...136A or something alike that will flow...and now you will have the need of a 870 watts resistor...
Not TRUE --- just looking at the equation above, RL (speaker load) actually determines the maximum voltage across the five 0.47 ohm resistors. The lower the RL, the more voltage those resistors will have across them.

Correct answer is worst case condition for those resistors occur when the output load (RL) is shorted to GND. With this condition, those resistors will see the 60V (on this example). However, a shorted power transistor will remove the effect of the Collector-Emitter voltage drop.

Now going back to the main topic, regarding the use of 3W resistors:

Assuming the speaker load is 2 ohm resistance (we are still in DC analysis). Then the actual voltage and power across all five 0.47 ohm resistors is:

V = (60 V * 0.094 ohms)/(0.094 ohms + 2 ohms) = 2.6934 V <--- Very small, right?

However:

P(total) = (2.6934 V * 2.6934 V) / 0.094 ohms = 77.175 Watts

This total power of 77.175W is spread around 5 resistors. Therefore, each resistor will dissipate about 15.4W.

Don't dispair, since this is only the positive half of the signal, the resistors are powered for only 50% of the time. Also, the scenario above occurs only when the amp is clipping. When it clips, chances are (depending on the signal), it only clips for say about 10-20% out of the complete period.

The scenario above is an extreme case (imagine connecting 2ohm load) so at normal operations (typical home application), those 3W resistor might be fine.

However, for professional applications, those 3W resistors will burn at continous high output with critical speaker loads.

Also, I noticed that on the schematic, the circuit doesn't have an output protection where the voltage across the 0.47 ohm resistors are monitored. Some designs have this protection where it turns off the output when a certain voltage threshold is present across those resistors.

Other things to consider:

1. The above calculations are based off on nominal calculations. It might be wise to factor in the resistor's tolerance rating as well as other parameters (e.g. Vce, supply ripple and actual power supply rating)

2. As explained, the speaker's impedance is really critical on this issue. All speaker impedance rating is somewhat an "average resistance" on the frequencies it was designed for. The actual "resistance" is determined on a particular frequency that is being passed (e.g. some 8-ohm speakers will be 4-ohm at certain frequencies).

3. The number of output pairs will help in dissipating the power. Therefore, when you plan to reduce the output pairs, you need to increase those resistor's power rating (say from 5 pairs to 2 pairs) to handle difficult loads.

Last edited by soltari_knight; 2nd August 2011 at 05:35 AM.
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Old 2nd August 2011, 07:58 AM   #16
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Default Thank you soltaire

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 2nd August 2011, 08:21 AM   #17
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Default Thank you very much dear Byron

Your work will be very helpful for our builders.

I just cannot wait to see these boards....please, charge your camera's batteries to produce pictures for us.

regards,

Carlos
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Old 16th August 2011, 07:26 PM   #18
a.wayne is offline a.wayne  United States
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Can MJL1302a/13281a be used in driver stage ..?
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Old 16th August 2011, 07:34 PM   #19
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No ... simply because their grid is too large.

Best regards - Rudi_Ratlos
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Old 16th August 2011, 10:01 PM   #20
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Default They can work there.... of course dimensions will be wrong

And also you will have lower gain in these power units used in the driver position..they work...but not that good.

I have used in many amplifiers.... i really could not perceive difference in sonics.... maybe for low impedances we gonna need the original transistor gain.

You see the current gain is the multiplication of driver and power output gain.... voltage gain there is 1....means no gain as multiplication by 1 does not increase value.... gain of current is the one is very high....output just add current..voltage comes from the Voltage Amplifier Stage (VAS).

Imagine that we can have gain 40 in the output power transistor(s) while high current will be crossing these output units...and maybe we gonna have gain 90 to the power transistor while working as driver....multiplication result is 3600..

Now imagine we can have gain 40 in the output power transistor(s) and 200 in the original MJEXXXX driver.... result is 8000..... so...original will help the amplifier to drain much more current from the power supply...and transfer much more current to the speaker...and this means power..... using 8 ohms we will not perceive..but using 2 ohms, for sure you will have losses...got it?

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg

Last edited by destroyer X; 16th August 2011 at 10:08 PM.
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