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My attempts at a design of a 3 stage amplifier
My attempts at a design of a 3 stage amplifier
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Old 26th November 2018, 08:06 PM   #301
Mooly is offline Mooly  United Kingdom
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My attempts at a design of a 3 stage amplifier
Using voltage/current limited supplies is a good plan and is something I do myself.

Solid state relays for speakers are becoming popular. They consist two suitable power FET's connected back to back. FET's with an Rds as low as a few milliohms are common these days. The gates are driven from a floating voltage source, typically a photovoltaic coupler. Very simple and very reliable.

The LT image shows how to set up a simulation. V1 is the floating voltage source, >7 volts is on and 0V is off.
Attached Images
File Type: jpg SS Speaker Relay.JPG (67.3 KB, 159 views)
File Type: png Annotation 2018-11-26 200436.png (26.7 KB, 161 views)
Attached Files
File Type: pdf AVAGO Photovoltaic MOSFET Driver..pdf (244.6 KB, 16 views)
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Old 27th November 2018, 02:04 AM   #302
mjona is offline mjona  New Zealand
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If you are looking for inspiration with electronic relays the attached link may be of interest.
TGM10 - based on NAIM by Julian Vereker
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Old 10th December 2018, 09:43 AM   #303
edbarx is offline edbarx  Malta
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The physical modifications of one channel are ready with the next phase being testing for current leaks. Low voltage testing at +/-15V, total 30V DC, is OK. The circuit is the modified output with large smoothing capacitors, 6600uF per supply rail. Since PCB cutting may short previously existing tracks, I am convinced making sure there are no short circuited tracks is extremely important at this stage. Therefore, I will use two mains 25W filament bulbs connected in series with the amplifier's power supply, with a bulb per rail, and see what happens. The modified circuit must pass these tests before the current limiting bulbs will be removed.

When this test is passed, the next phase will be of building a small PCB to host the VBE multiplier and variable resitors. The VBE transistor will be mounted physically on top of one of the 2SC5200/2SA1943 power transistors to sense the temperature of the power stage. This will ensure the bias voltage drops as power stage's temperature rises.

The succeeding testing will involve setting the proper VBE multiplier bias voltage. I will use two resistors instead of the VAS's driver and current source. The resistor values will be such that the VAS's standing current flows in the resistor, VBE multiplier, resistor chain. Again, the first step in the test will involve using protective mains filament bulbs. When this test is satisfied, I will use the full voltage without protection and test for stability over a period of time.

Readers may think this is unnecessary, but high voltage has no mercy on parts, which are costly to replace and take time to arrive by post.
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Old 10th December 2018, 02:40 PM   #304
Mooly is offline Mooly  United Kingdom
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My attempts at a design of a 3 stage amplifier
Hope all the testing goes to plan, and yes, very true that high voltage (with the current behind it) shows no mercy.
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Old 11th December 2018, 11:43 AM   #305
edbarx is offline edbarx  Malta
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There is not enough space for a separate PCB hosting the VBE multiplier. This means, I will make the final PCB that hosts the input stage, voltage amplifier stage, VBE multiplier and output current overload limiter. The latter two circuits will be assembled first to allow me to set the proper bias voltage with a collector current of 10mA per output transistor. As posted before I will test for stability with this quiescent current, then it will be time to assemble the input stage and VAS. The latter two stages will be tested before connected them to the output stage. I will take the negetive feedback from the collector of the VAS constant current source. This will allow me to first ensure the output voltage is kept at 0V with no signal. After this test, a signal will be connected and the AC signal strength measured at VAS's current source's collector.


The next step is connecting the VAS to the VBE multiplier, and moving the negative feedback point from the VAS's current source's collector to the output point. The next test will again limit the current with light bulbs to prevent damage to the output stage in the event high frequency oscillations occur. A frequency counter will be connected to the output to make sure there are no high frequency oscillations.


Next, it will be time for a proper power test with high power resistors across the output.


This is becoming interesting... it is a pity I am not doing it for a course of study.
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Old 11th December 2018, 06:39 PM   #306
Mooly is offline Mooly  United Kingdom
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My attempts at a design of a 3 stage amplifier
I find your test procedures quite intriguing... very thorough... but intriguing non the less.

Hoping it all works and behaves as it should.
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Old 12th December 2018, 12:49 PM   #307
edbarx is offline edbarx  Malta
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The preparation and etching of the PCB to host the input stage, VAS, VBE multiplier and output current limiter is etched and ready. I used an old 'fusty, musty, dusty' method and drilled all the holes before I completed the tracks. Thanks to the poster who suggested this simple yet extremely helpful tip.

My etch resist was not some difficult to get chemical, but correcting fluid from a local stationer. The correcting fluid remained solid through the entire etching process.

The next thing to do is to check for shorts. I can also reuse some components from the original circuit. There is a 50 Ohm ten turn preset and a 470 Ohm variable resistor that I can use. I can also use resistors, diodes and capacitors.
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Old Yesterday, 03:30 PM   #308
edbarx is offline edbarx  Malta
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Testing Results:
a) High voltage test for ten minutes: Passed.
b) Leakage test: supply rails to heatsink: Passed.
Estimated insulation resistance is 300 megaohms. This is based on the measuring instrument's resistance.

Here are some photos of the real thing. There is also a photographs of the high voltage test setup. The photo was taken during the test.


Attached Images
File Type: jpg amplifier-side1.jpg (501.3 KB, 27 views)
File Type: jpg amplifier-side2.jpg (495.1 KB, 26 views)
File Type: jpg etched-pcb.jpg (411.1 KB, 27 views)
File Type: jpg high-volt-test.jpg (561.9 KB, 17 views)
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Old Yesterday, 06:50 PM   #309
Mooly is offline Mooly  United Kingdom
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My attempts at a design of a 3 stage amplifier
That's looking pretty good to me, well done Its good when a plan comes together.
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