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Old 29th January 2013, 10:28 PM   #11
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A class A amp driven by an smps? Sounds almost obscene

A linear regulator for a 100W class A amp at least to me does sound like a serious project. And in order not to limit transients it should probably be an order of magnitude more powerful than the amp.

Once i owned a Naim with a regulated supply. In complexity the regulator was similar to the amp. It did some things quite well but neither bass, nor dynamics were anywhere close to acceptable.
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Old 30th January 2013, 01:32 AM   #12
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for class AB: I am a firm believer in BL**DY big caps. A 100% over-rated [regulated] power supply [with a cap multiplier] is a given

the reasoning: class AB draws per the signal, so the ps needs to be ready to 'dump' heavy current


for class A: a floating power supply, with cap multiplier, although I still put in bl**dy big caps.

the reasoning: class A draws steady current, but needs to have as little ripple as possible

cap multiplier is so easy, I have no idea as to the question.............especially if you are already using a series pass regulator.
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Old 30th January 2013, 02:17 AM   #13
ilimzn is offline ilimzn  Croatia
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If properly designed, a series regulator will be far less stressed than either a class A or class AB output stage - simply because you want to keep the voltage drop on it minimal, usually well below pass transistor SOA starts limiting available current below the simple P=VxI law. Not so easy with the output stage, especially for reactive loads!
In theory, with a high PSRR design, a regulator is superfluous. Adding one for the input stages is a far less involved affair aind simply put, it keeps a lot of unwanted stuff off the power rails before PSRR even comes in as a consideration. However - there are relatively simple topologies that never the less work well but have relatively low PSRR and a regulated power supply, or, keeping it simpler - a capacitance multiplier - is of great help.
Keep in mind that you can only use the available power supply voltage up to the bottom of the ripple waveform, so if your cap multiplier or regulator regulates to a voltage just a bit below, the losses are minimal but the amp is not subjected to ripple + harmonics from the power supply, and it clips cleanly. Also, it's easier to add a protection mechanism in the power supply, especially for a simple amp, where it will be further from any interaction with the actual audio signal.
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Old 30th January 2013, 03:17 AM   #14
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Quote:
Originally Posted by analog_sa View Post
A class A amp driven by an smps? Sounds almost obscene

I smiled at that too but resisted the urge to comment...............your mind is possibly as perverted as mind?
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Old 30th January 2013, 03:46 AM   #15
Bonsai is offline Bonsai  Taiwan
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Attached is a picture of the ripple eater (AKA cap multiplier) I used on my e-Amp. This only powers the front end, but you can see the very dramatic improvement you get - about 30-40dB reduction in ripple. Especially important is that the HF ripple on the supply is removed.

It is quite easy to apply this concept to power the whole amp. Because the series pass device is close to saturation, the power dissipation is very manageable.

I will be trying this on my next class A amp for sure (although I can say my current sx-Amp is very quiet even without a ripple eater).

The top trace is the PSU ripple before the ripple eater
The solid trace below it is the output side of the ripple eater.

Amplifer output was 150W into 8 Ohms (scope probe switched to 10x) and the driving signal into the load was 4 kHz.

I think a full linear reg is another story completey and in my view not a smart approach on a power amplifier - better to focus on improving PSRR, good layout and ripple eater (especially for the front end circuit). Just connecting your comp cap to the right node in a MC VFA can gain you 15-20dB of PSRR improvement (see Harry Dymonds paper and Doug Self for some examples and ideas).

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File Type: jpg Ripple eater.jpg (532.8 KB, 183 views)
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Last edited by Bonsai; 30th January 2013 at 03:52 AM.
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Old 30th January 2013, 03:52 AM   #16
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Y'know, class-A amplifiers can be quite happy with choke-input filters. The inductor won't be the smallest thing in the circuit, but then it is class-A after all.
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Old 30th January 2013, 04:18 AM   #17
qusp is offline qusp  Australia
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you know, if you actually looked at the topology its not really an SMPS. more a class A amp with differential feedback, powered by an SMPS, the noise is obscenely low and the transient performance quite remarkable. get with the program its a way to get extremely good regulated performance at 2 x 600W without building an amp larger than the amp you want to power to power your amp (in the case of a linear regulated supply)

I knew it would cause a stir, I took some convincing myself before deciding to get a couple. the amps i'm powering arent pure class A, rather high bias AB (600ma @ +/-65vdc for the lateral outputs) so a somewhat different use-case to here, but it will be in Class A for most of my usage at normal listening levels.

a linear regulated front end with unregulated outputs and a fully linear regulated linear bench supply were compared to it, powering the amp in question. the difference in performance was significant across all measures with high end agilent and AP2. square wave, multitone and IMD were like nothing I had seen before, so I decided it was worth a try. the savings in space and expense were icing on the cake.

I will also be trying them on the outputs (the FE supply is too high voltage) of a babelfish JX, jfet circlotron (R125) and aleph JX, maybe the F6 (all partially finished projects) just for kicks, I expect it to do well with them too

Last edited by qusp; 30th January 2013 at 04:43 AM.
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Old 30th January 2013, 05:36 AM   #18
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I will look into the capacitor multiplier circuit.

Is there any issues of using a linear regular for the input stage and predriver and a capacitor multiple circuit for only the output stage? I suppose as long as the output stage collector voltage is higher than the predriver there won't be any issues of over driving the output stage.
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Old 30th January 2013, 06:30 AM   #19
kf_tam is offline kf_tam  Hong Kong
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Quote:
Originally Posted by KMossman View Post
for class A: a floating power supply, with cap multiplier, although I still put in bl**dy big caps.

the reasoning: class A draws steady current, but needs to have as little ripple as possible
Only Balanced Class A draws steady current! Ordinary Class A current changes according to the output waveform. Just that there is a high idle current and no cross-over so the current change is a lot less drastic than Class AB/B.
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Old 30th January 2013, 06:36 AM   #20
cotdt is offline cotdt  United States
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Quote:
Originally Posted by kf_tam View Post
Only Balanced Class A draws steady current! Ordinary Class A current changes according to the output waveform. Just that there is a high idle current and no cross-over so the current change is a lot less drastic than Class AB/B.
This is just trying to be overly technical. In practical terms, the average current draw in Class A is constant and you will not see voltage sag.
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