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Old 11th July 2011, 09:58 PM   #651
Baztien is offline Baztien  France
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Default ground center tap

Hi all
On my instructions it is specified to connect the center tap of the transformer to ground. I understand I can connect it to the safety earth wire of my supply. is this correct?
thanks in advance
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Old 11th July 2011, 11:12 PM   #652
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Baztien View Post
On my instructions it is specified to connect the center tap of the transformer to ground. I understand I can connect it to the safety earth wire of my supply. is this correct?
NO !
the Safety Earth wire must be connected permanently to the chassis. Nothing else is allowed to interfere with this permanent mechanical fixing.

The Centre Tap is connected to the PSU Zero Volts.
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Old 12th July 2011, 08:57 AM   #653
Baztien is offline Baztien  France
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Default zero volts

Quote:
Originally Posted by AndrewT View Post
NO !
the Safety Earth wire must be connected permanently to the chassis. Nothing else is allowed to interfere with this permanent mechanical fixing.

The Centre Tap is connected to the PSU Zero Volts.
Ok understood for the safety earth wire.

I've just read Elliot's guide on hard wiring the PSU.
It seems to me that the zero volts is also connected to the chassis on a separate point than the earth point.

thanks for your advice. I'm using a PSU on a PCB (from a kit) and a EI transformer. So all the arrangement described are often different to mine and I have to transpose as best as I can.
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Old 14th July 2011, 10:34 AM   #654
Baztien is offline Baztien  France
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Hi
As I've got no answer, I'm thinking I didn't ask the question:
Should the zero volt or ground be connected to the chassis?

cheers
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Old 14th July 2011, 11:48 AM   #655
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Quote:
Originally Posted by Baztien View Post
Hi
As I've got no answer, I'm thinking I didn't ask the question:
Should the zero volt or ground be connected to the chassis?

cheers
The forum has a good article on grounding. This section applies to you question:

Audio Component Grounding and Interconnection
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Old 1st October 2011, 07:12 AM   #656
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Question "Basic" PSU -vs- switched

A lot of information on this thread - so I may be mixed up on what I am about to ask.

I am in the process of building a PSU for a stereo LM3875 kit (Peter Daniel's). I originally planned on having only one supply power both amps. But I bought a 160VA transformer, as that was the highest VA I could get a hold of with 22-0-22 secondaries, which appears to be slightly under rated for both amps at max power. So I will buy a second 160VA transformer and more MUR860 for dual mono. I plan on powering an Alpair 12.2 full-range driver on each channel. This load is rated at 7 ohms.

The question I have is this - and it has been asked before - and not answered: Is there any sonic benefit to building a regulated switched PSU for this application? I gather from some posts there is no benefit, just more work. But surely there is some kind of "diference". Or is this another one of those highly contriversal topics like the whole snubberized -vs- not bit!?! Feel free to direct me to other threads.

Thanks in advance

Allen
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Old 1st October 2011, 09:50 AM   #657
AndrewT is offline AndrewT  Scotland
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I hope the kit designer/supplier comes in with an opinion.
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Old 3rd October 2011, 01:24 AM   #658
nige838 is offline nige838  United States
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Default What does the term "single supply" mean?

Excuse my lack of knowledge, but what does the term "single supply" mean. I'm working on a gainclone and I intend to power it using only one power supply supplying B+ to both channels. Is this known as a "single supply"? If so, what do I need to do to ensure success?

Thanks in advance,
Nicholas
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Old 3rd October 2011, 06:37 AM   #659
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Quote:
Originally Posted by ArtsyAllen View Post
Is there any sonic benefit to building a regulated switched PSU for this application?
Quote:
from Doglas Self: Audio Power Amplifier Design Handbook 5th Edition
Linear Regulated Power Supplies

Advantages
  • A regulated supply-rail voltage means that the amplifier can be made to approximate more closely to a perfect voltage source, which would give twice the power into 4 Ω than it gives into 8 Ω. This is considered to have marketing advantages in some circles, though it is not clear why you would want to operate an amplifier on the verge of clipping. There are, however, still load-dependent losses in the output stage to consider. More on this later.
  • A regulated supply-rail voltage to a power amplifier gives absolutely consistent audio power output in the face of mains voltage variation.
  • Clipping behavior will be cleaner, as the clipped peaks of the output waveform are not modulated by the ripple on the supply rails. Having said that, if your amplifier is clipping regularly you might consider turning it down a bit.
  • Can be designed so that virtually no ripple is present on the DC output (in other words the ripple is below the white noise the regulator generates) allowing relaxation of amplifier supply-rail rejection requirements. However, you can only afford to be careless with the PSRR of the power amp if the regulators can maintain completely clean supply rails in the face of sudden current demands. If not, there will be interchannel crosstalk unless there is a separate regulator for each channel. This means four for a stereo amplifier, making the overall system very expensive.
  • The possibility exists of electronic shutdown in the event of an amplifier DC fault, so that an output relay can be dispensed with. However, this adds significant circuitry, and there is no guarantee that a failed output device will not cause a collateral failure in the regulators that leaves the speakers still in jeopardy.

Disadvantages
  • Complex and therefore potentially less reliable. The overall amplifier system is at least twice as complicated. The much higher component count must reduce overall reliability, and getting it working in the first place will take longer and be more difficult. For example, consider the circuit put forward by John Linsley-Hood [1]. To regulate the positive and negative rails for the output stage, this PSU uses 16 transistors and a good number of further parts; a further six transistors are used to regulate the supplies to the small-signal stages. It is without question more complex and more expensive than most power amplifiers.
  • If the power amplifier fails, due to an output device failure, then the regulator devices will probably also be destroyed, as protecting semiconductors with fuses is a very doubtful business; in fact it is virtually impossible. The old joke about the transistors protecting the fuse is not at all funny to power-amplifier designers, because this is very often precisely what happens. Electronic overload protection for the regulator sections is therefore essential to avert the possibility of a domino-effect failure, and this adds further complications as it will probably need to be some sort of foldback protection characteristic if the regulator transistors are to have a realistic prospect of survival.
  • Comparatively expensive, requiring at least two more power semiconductors, with associated control circuitry and over-current protection. These power devices in turn need heat-sinks and mounting hardware, checking for shorts in production, etc.
  • Transformer tappings must still be changed for different mains voltages.
  • IC voltage regulators are usually ruled out by the voltage and current requirements, so it must be a discrete design, and these are not simple to make bulletproof. Cannot usually be bought in as an OEM item, except at uneconomically high cost.
  • May show serious HF instability problems, either alone or in combination with the amplifiers powered. The regulator output impedance is likely to rise with frequency, and this can give rise to some really unpleasant sorts of HF instability. Some of my worst amplifier experiences have involved (very) conditional stability in such amplifiers.
  • The amplifier can no longer deliver higher power on transient peaks.
  • The overall power dissipation for a given output is considerably increased, due to the minimum voltage drop through the regulator system.
  • The response to transient current demands is likely to be slow, affecting slewing behavior.
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Old 3rd October 2011, 06:56 AM   #660
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Quote:
Originally Posted by nige838 View Post
I'm working on a gainclone and I intend to power it using only one power supply supplying B+ to both channels. Is this known as a "single supply"?
Yes, a single supply needs two rails, one positive and one negative, where the negative is usually at the same time the ground. A split supply has three rails, positive, negative and ground, where the ground potential is in between positive and negative.

Quote:
Originally Posted by nige838 View Post
If so, what do I need to do to ensure success?
The basic differences are usually that you
  • need a virtual ground between the positive rail and the real ground, to which the input signal is referenced. That means usually three more resistors and two more capacitors. An additional transistor or even op amp is sometimes used to improve the stability of that virtual ground.
  • need a (big) DC blocking capacitor between the amp output and the speaker. The virtual ground is present at the amp output and the resulting DC signal at half the rail voltage could destroy the connected speakers without that capacitor.
Depending on the chip amp IC you use, some of those components may already be built-in. You need to read the corresponding datasheet. Usually there is a schematic of how the IC you use must be implemented with a single supply.
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