Influence of power supply in audio amplifiers. - Page 1
Forum section: electronics / projects

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: January 09, 2023, 07:39:00 am
content: In engineering point of view, as long as the supply is more than the drop out voltage, the amp should be able to draw all the current it needs to power the output. Although with generator and scope that exactly what happens but when it comes to music, the low frequencies get seriously effected. The more mystery is that at low frequencies the NFB is very high and the feedback has all the time and gain to bring the output into authority.The classic solution is to power with an armada of reservoir capacitors that can reach 100,000uF per rail as if just letting to sag a few percent and the bass dries out.I experience a systematic loss when powered with independent SMPS on each rail. I always thought that the single with dual SMPS will resolve this issue, not at all. I added a 10,000uF on each rail and got it a little better but when I wired in single floating supply without extra reservoir capacitors, just 2x1000uF and I got full bass.If someone can suggest a point of view I can investigate the cause of this effect.I want to note that single floating supply makes full wave rectified current draw from the supply.

author: jonpaul
author_rank: Super Contributor
timestamp: January 09, 2023, 12:38:28 pm
content: The use of dynamic tracking bus power to feed an audio pwr amp output stage is decades old.See papers and amps from BTL, HP, Tom Holman, THX, many others.Some use SMPS to drive a bus that tracks audio output V plus a margin eg 5V.Idea is patented.please Do a deeper R&D.j

author: Terry Bites
author_rank: Super Contributor
timestamp: January 09, 2023, 01:27:12 pm
content: Envelope tracking amplifers were first developed at RCA in the 1930s by Loy Barton.Not a new idea as you pointed out.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: January 10, 2023, 02:14:24 am
content: Bob Carver produced such high power amplifiers since late 70's.My question involves fixed voltage standard SMPS.The amplifier is behaving as if it is trying to keep the Vcc-Vee voltage constant while pulling current from either rail. With split supply things going wrong but with bridged or single supply, there is no problem.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2023, 05:32:44 pm
content: For bipolar transistors the current gain and cutoff frequency decrease at high collector current which is also where the supply voltage sags from high current demands. The drive current also increases more than linearly because of the drop in current gain which may affect the voltage amplifier stage if there is not enough current gain.The lower voltage across the transistor as it nears dropout also has an effect, but not as much as the increased collector current unless the voltage becomes very low.These problems with bipolar transistor Ft and hfe sagging can be made significantly better by using perforated or ring emitter transistors, which not only have better Ft and hfe, but also better performance at higher collector current, which is why they are preferred for high performance audio output stages. Alternatively, derate your standard output transistors for current.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: January 15, 2023, 06:21:11 am
content: I am reconsidering the definition of ground with rectified power supply.The ground in split rectified supply is considered as solid 0 impedance point, it isn't. When the rectifying diodes are not conducting, which is most of the time, the ground is simply floating held by the impedance of the reservoir capacitors. The amplifier which using the split supply has it input referenced to find itself in positive feedback mode as the load will drive the ground towards either rail which in turn the ground referenced input will get more positive feedback and drive the load more towards the rail until the rectifying diode switches on and pulls back the ground to the centre. Will it pull really to the centre? It has a non zero impedance made of inductance and resistance, seen at the secondary. Note that when bridged or single floating supply, the input has an independant ground hence no positive feedback.I will dig in further with simulator to see how the ground impedance is effecting.

author: magic
author_rank: Super Contributor
timestamp: January 15, 2023, 09:59:47 am
content: And what would be the point of analyzing circuits in such a backwards way?Ground is the point that you connect to ground, by definition. It is a fairly low impedance thing, although there are ways to screw it up by using high resistance traces carrying large currents, but I'm digressing...The supplies are held by capacitors while they are not being recharged by the transformer/rectifiers.Yes, the supplies sag and ripple under load, nothing new here.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: January 15, 2023, 12:27:18 pm
content: To show the presence of positive feedback, I made the following model. With the ground having 1mH and 1ohm transformer impedance, I impose a dc offset of 1 volt at the input, I provoke a current of 1/6 amp. The positive feedback pulls down the ground with respect to floating ground by 4 volts instead of 1/6 volts. If the transformer has 8 ohms the pull down is 13 volts instead of 4/3 volts.

author: dmills
author_rank: Super Contributor
timestamp: January 15, 2023, 12:39:00 pm
content: I fucking hate the concept of 'ground', it always seems to come with all sorts of magical thinking. Its ONLY real use (Outside of the completely unrelated safety circuit in a class I appliance) is to set the common mode voltage, nothing else.Look, currents flow in loops, and voltages are developed across impedances in response to the current, end of.In a classical audio amp the loops most of the time involve one of the bulk caps (Which one depending on the direction of load current), and this switches back and forward as the load current changes from source to sink. Note that this can be a reasonably potent source of EMI and correct cable dress is important here. The caps are then recharged by a fairly large current pulse from the transformer as soon as the diodes become forward biased, and some care needs to be taken to keep this out of the amp itself, think common impedance coupling.The loop goes bulk cap -> amp board -> load -> bulk cap, so if you want to minimise the potential for magnetic nonsense you want both the speaker leads to head to the amp board (Also lets you pick the PAIR of traces for the feedback up at the right point) then all three conductors head to the power supply board (Ideally twisted).It is not magic, and just takes a little thought.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: January 16, 2023, 03:20:22 am
content: To destroy the positive feedback and transforming into negative one I biased the power opamp with inverted mode but the non inverting input is linked to an artificial center point instead of the ground. Now, the ground feedback became negative. Again with 1V on the output with 8 ohm ground resistance, instead of getting 4/3 V, I get 50uV where with conventional way I had 13V.The story is not yet finished, I have sharing the same supply with other channel, what will be crosstalk?