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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 18th May 2019, 07:14 AM   #491
edbarx is offline edbarx  Malta
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Modifying further the DRC filter to use two 100uF capacitors one directly after the diode and another one after the resistor, resulted in a ripple reduction of 17/0.2 = 85 times. The amplifier's power supply was simulated with a 2 Ohm resistor connected from rail to ground on both rails to simulate extreme loading. I am posting a screenshot of LTSpice with the output and Fourier Transform.


Post Scriptum:
Adding a 100p capacitor immediately before the rectifier and two 1uH inductors at the secondary windings' output resulted in heavy 2000Hz charging current oscillations with an amplitude of 60A. The output still seems immune to this according to LTSpice.

I will further increase the frequency to see what happens to the output.
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Last edited by edbarx; 18th May 2019 at 07:37 AM.
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Old 18th May 2019, 01:45 PM   #492
edbarx is offline edbarx  Malta
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I ordered those four chunky, 100V, 1000uF electrolytics, another four 0.1uF Mylar capacitors and a packet of 10 22.1 Ohm resistors. In the meantime, I will make the PCBs and plan where they will be securely installed.


I will build the filters as suggested by Mjona.
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Old 18th May 2019, 06:35 PM   #493
mjona is offline mjona  New Zealand
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It is hard to read some of the images for your power supply however if you are up for redesigning your boards you might consider a separately rectified supply for the small signal stages.

These are not going to demand much current from the transformer so the ripple will be less from this reason plus the isolation from the main rectification for the output stage.

The charging currents for capacitors occur on half cycles towards the peak of a 50 cycle mains frequency so the larger the peak current drawn is the larger the ripple spike will be since the charging has to happen in the same length of time.

You will have to test this by simulation however the best outcome might be achieved by reducing the capacitors supplying the output stage to 2000uF and making those for the small signal stages double at 4700uF.

There is an example of a Rotel high power amplifier in Linsley-Hood's book about Valve and Transistor amplifiers. I am away from home at present and will be until Monday week and I don't have the exact details but if you are interested I can give these later.

My thinking is if space is a problem this scheme may give the results you want within less space.

Also you could put the RC network on a board with the separate rectifiers ( 1A types will do) for the small signal supply in which case you could use your existing power amplifier boards by using a spot face cutter tool to separate the feed to the small signal stage - neatly and drill holes for a pcb pin or header to take the connection from the separate supply.

Last edited by mjona; 18th May 2019 at 06:37 PM.
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Old 18th May 2019, 10:13 PM   #494
edbarx is offline edbarx  Malta
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To avoid having the filters' 1000uF capacitors from feeding current to the output stage when the voltage of the main reservoir capacitors drops, I inserted a rectifier diode on both rails. The result is a reduction of 50% of the ripple at the output. Therefore, I will add a diode to the each filter. Simulations are showing that this is equivalent to using a separate power supply. In fact, the inrush charging currents drop to zero every half cycle like what happens with a separate transformer winding and rectifier.

The next 'problem' to solve is to sufficiently block incoming RF interference like that encountered with unclean mains. I am referring to interference that is emitted by large currents being cut off, interference from triacs and thyristors, unfiltered drills, insufficiently filtered switch-mode-power-supplies, malfunctioning power inverters, etc. I am not planning to use my amplifier near a radio repeater churning RF power at several megawatts.

P.S. (an anecdote)
I remember once I was told a large crane was being used near a 1 MW, 1.6MHz radio repeater. The size was such that resonance was set up and the crane started to get hot. To continue working, the repeater had to be temporarily switched off.
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Old 19th May 2019, 07:29 AM   #495
Mooly is offline Mooly  United Kingdom
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My attempts at a design of a 3 stage amplifier
Quote:
Originally Posted by edbarx View Post
P.S. (an anecdote)
I remember once I was told a large crane was being used near a 1 MW, 1.6MHz radio repeater. The size was such that resonance was set up and the crane started to get hot. To continue working, the repeater had to be temporarily switched off.
Wow, that sounds scary and yet you can absolutely imagine something like that happening under the right conditions.
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Old 19th May 2019, 08:21 AM   #496
edbarx is offline edbarx  Malta
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With the filters in the circuit attached to this post, I am getting these results:
One such filter is shown. It is made up of C7, C15, C13, R47 and D5

Rail ripple (intentionally made huge): 41.8V - 25.2V = 16.6V
RF voltage superposed on -ve rail: 50mV @ 200kHz

Results (after filters):
Minor rail voltage ripple (for sensitive parts): 1.02mV - 0.91mV = 0.21mV
RF voltage: 1.09mV - 0.81mV = 0.28mV

RF attenuation factor: 50mV/0.28mV = 178.6
Ripple attenuation factor: 16.6V/0.21mV = 79.0

RF attenuation in dB: 20*log(178.6) = 45dB
Ripple attenuation in dB: 20*log(79.0) = 38dB

Post Scriptum:
The space issue and my intention of using the existing PCBs favour using this filter as it uses far less bulky components. It is also more effective to reduce ripple but less effective on reducing RF.
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Last edited by edbarx; 19th May 2019 at 08:26 AM.
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Old 20th May 2019, 08:22 PM   #497
edbarx is offline edbarx  Malta
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I have a blank PCB sheet, electrolytic capacitors, polyester capacitors, resistors and diodes. This should enable me to build the filters PCBs. Hopefully, the amplifier output will be quieter with no audible hum and other noises.
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Old 21st May 2019, 06:06 AM   #498
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by edbarx View Post
With the filters in the circuit attached to this post, I am getting these results:
One such filter is shown. It is made up of C7, C15, C13, R47 and D5

Rail ripple (intentionally made huge): 41.8V - 25.2V = 16.6V
RF voltage superposed on -ve rail: 50mV @ 200kHz

Results (after filters):
Minor rail voltage ripple (for sensitive parts): 1.02mV - 0.91mV = 0.21mV
RF voltage: 1.09mV - 0.81mV = 0.28mV

RF attenuation factor: 50mV/0.28mV = 178.6
Ripple attenuation factor: 16.6V/0.21mV = 79.0

RF attenuation in dB: 20*log(178.6) = 45dB
Ripple attenuation in dB: 20*log(79.0) = 38dB

Post Scriptum:
The space issue and my intention of using the existing PCBs favour using this filter as it uses far less bulky components. It is also more effective to reduce ripple but less effective on reducing RF.
The point I am about to make has probably been made before - this is to add capacitors in parallel with the diodes in the bridge rectifier. The separate rectifier scheme for the small signal section is better since the large half wave signals induced by the output stage at high powers will provide the best isolation. These will still get through the extra diode you have added in the line to the small signal section - albeit mitigated by the RC filter.

If you run with this dual supply system add the suggested capacitors in parallel with the diodes in the power section bridge rectifier.
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Old 21st May 2019, 04:44 PM   #499
edbarx is offline edbarx  Malta
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Thank you for posting. The existent rectifier bridges are accessible with a rating of 400V, 25A. If I were to solder a filtering capacitor across the four diodes, which capacitors would you suggest? The rectifier bridges already have a filtering capacitor across the AC terminals.
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Old 21st May 2019, 06:13 PM   #500
mjona is offline mjona  New Zealand
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With the power level of your amplifier it would be useful to replace the potted bridge with one having a higher current rating.

The conventional filter arrangement is to fit a capacitor between each secondary and the transformer center tap which connects to earth.

The idea of a parallel capacitor in parallel with a rectifier diode is not too far removed from that of the capacitor across the points in the long outmoded Kettering ignition system.

The spark plug leads in these systems also serve to suppress e.m.i. which interferes with television reception. A neighbour in the vicinity of the property where I am holidaying at has a lawn mower which does that.
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