Power for AMP 1

[locky]

Hi there. I have ordered an AMP 1 and looking forward to the project. Can anyone explain in beginners terms what is required in terms of a power supply for this amp. Is it possible to buy an external transformer that runs on 240v or do I need to construct something that will be sited in the casing I use? Anyone have any sources in the UK where I can get hold of what is needed?
Many thanks...

[tom]

Hi,
I'm also a "beginner" but cause there is no replie I will try to answer some of your questions.
I think an external transformer will also work but maybe that solution will be more expensive than a self made one. And the self made one can also be placed in an external case, of course.
I think, you have already found the PSU plan from jan on this site, is there still a question on this?
I don't know sources in the UK for getting the PSU parts but in germany, where I come from, finding a transformer with 2x22V and about 300VA was the most difficult thing. I've only found one shop here in germany (sells also to other countries).
But why do you ask for a 240V transformer, I thought there is 120V in the UK or is that wrong?

Tom

____________________
MPC Tutorial (http://www.angelfire.com/magic2/hq-audio/tutorial.htm) kX Project (http://www.kxproject.com)

[Jan]

Hi, I just extended the power supply part of the "toolbox" pages, for AMP1. Please expand the tree at the left hand side of this page.

Also, you can probably not find a ready built power supply, but would need to build one. The schematics are on the "toolbox" pages.

[locky]

Hi Jan,
I have now received the AMP 1 kit. Thanks for the prompt delivery. I am in the process of trying to source the bits for the PSU. I have so far located the following and wondered if you could confirm that they will be suitable/make any comments before I order them -

Transformer -
Either...
1. Toroidal transformer
primary 230V
secondary 2 X 30V 300VA (2 X Vsec @ 150 VA each, suitable for
series/parallel connection).
I am assuming that the 2 outputs from the transformer can be combined to form 300VA before being fed into the rectifier?

Or...
2. Toroidal transformer
primary 230V
secondary 2 X 24V 330VA

Any thoughts on what would be the most suitable between these 2?

Rectifier -
200V 25A single phase bridge

Capacitors -
2 X 50V 10000uF Panasonic compact aluminium electrolytic

5V 50mA supply for the chip -
I am still working on this. Whilst there is some very helpful info on the transformer, the rectifier bridge and capacitors on the website link, there isn't much explanation on the 5V supply. Is it possible to insert a section on this in the power supplies link? I would find this very helpful.

Thanks again for the great service!

[Jan]

Hi,

An AC voltage that is rectified will have a DC voltage that is 1.41 times higher than the AC voltage. So you should aim for a transformer with an AC voltage of 2x18 to 2x24 volts. These would give 2x25VDC and 2x34VDC volts respectivly. You need a dual coil transformer for the positive, ground, negative supply. You use one coil for positive to ground and one coil for ground to negative. That is, you would have the two seconcadary coils in series, tapping the ground in the center. So in your case, the 2 X 24V 330VA should work well. They should give you about +/-34 volts, minus about one volt loss in the rectifier. Check the specified primary voltage is the same as the voltage in your house. Your mains voltage may be higher than the transformer is specified for and idle voltage is a bit higher than the full load voltage, which is usually what is given in the specs. You do not ever want to go above +/- 36V.

The rectifier should be fine. They are relatively cheap so you migt as well overrate them.

Capacitors is a big topic for debate in audio DIY circuits. I would say 10.000 uF is enough for normal domestic use. Some would claim you should have more, maybe 20.000uF or more. About 10.000 to 20.000 uF would be typical for a commercial high end amp in this power range. Bulk caps do two things: filter out mains ripple, buffer energy for transients in the signal. The transient effect is basically important when you play at high volume. Basically, the bigger caps the better but benefits become harder and harder to notice as values grow. Also, a high ripple rating is good for performance and life of the capacitors. Standard 10.000 uF caps may be rated 3A ripple, while a high grade cap may be rated 15A. High grade caps also have a price tag. For an amp that is expected to be used at high power for extended times, get good caps! So what caps to use is a question of what you can spend and the expected use of the amp.

For the +5V I recomend looking in a datasheet for a 7805 chip. It will give you some ideas. Just a few days ago, there was something in a parallell topic in this forum.

I will try to come up with something better for the +5V supply in the "toolbox" pages in the future.

[bearman]

Jan,

If I were to use a small 6v 300ma DC transformer tied to the mains power, and hook it to just the 5v regulator portion of your schematic, would that work? Just about everyone has an old 6v or 12v DC transformer sitting around.

I did a small test board and get 5.01v pretty constant. Actually fluctuates between 5.00 and 5.01 a little.

[Jan]

Hi,

Yes, you can use a separate transformer for the +5V. Some high end amplifiers use separate internal transformers for preamps/low level signals just like this. I would recomend a voltage regulator / stabilizer like indicated in the schematics, just like you have done. You may find a +5V that is stabilised, but do not expect good results with these. The +5V supply must be groundend to AGND/J2.

[bearman]

Along those lines.....If the 5v PSU takes a few seconds to get to full 5v and the 30v PSU gets to full voltage quicker or vice versa, is that going to be a problem too?

I'm still awaiting my 2x22v transformer so I can't see if they match up too well.


Thanks for all of your help Jan.

[Kenk]

quote:Originally posted by jan
[br]Hi,

You may find a +5V that is stabilised, but do not expect good results with these. The +5V supply must be groundend to AGND/J2.


Hi Jan, can you please explaine why a seperate 5v power supply it not good? As I am a new to building amp, I am trying to understanding this.

Thanks,

Ken

[Jan]

Hi,

I was referring to the wall socket "battery charger" type PS supplied with various electronic equipent. While some may be good, most are made on very tight budgets and with looser technical demands than we would want. For example for charging the battery of a portable tool, demands may not be high and therefore the components are probably not the best. What you want for an audio power supply is something that is as free of voltage ripple/noise as possible and responds without voltage over/undershoots on changes of the current. What I ment was that you might not get this from the +5V stabilized supply that you have. But there are probably good ones as well. I would suggest trying. First test; put a voltmeter on it and measue.

[locky]

Hi Jan,
I now just about have all the bits for the 30V psu, but am still a bit confused about the circuitry to take a bridge from this to a 5v supply (The top right hand part of the power supply diagram). Any chance you could give me a little more explanation on this? It seems to be a combination of resistors, capacitors and a diode? What is '7805'?
Thanks yet again...

[Jan]

Hi,

The 78XX are a series of standard voltage regulators circuts. The last two digits are the volatge so a 7805 is 5V, 7812 would be 12V etc. They also come in a negative version, 79XX, where 7905 would be for minus 5V etc. The 78XX and 79XX regulators are all in 3 terminal packages, where one is the input, one is the ground and one is the output. The 7805 are available from many different manufacturers and manufacturers may have slightly different names for their series, like L78MXX or similar. You can also find versions of these families that are very fast response, low dropout (low minimum voltage input to output), high voltage (high voltage input to output), high presicion etc. A search in the web for 7805 and "voltage regulator" should give you thousands of hits.

These regulators all have one thing in common; you put a voltage on the input and you should get a fixed output voltage. For a 7805 you should get 5.00 V or very close to this. For audio amplifiers, the stability is even more important than the actual voltage value; you should have the same voltage, regardless of input voltage variations or variations in current drawn through the regulator. In real life, voltage regulators do this more or less well. For AMP1 any value between 4.9 and 5.1 V should be OK as long as it is stable.

The 7805 requires a few periferal components to work well:

- R1 will lower the input voltage to the regulator a bit. The 7805 are usually rated for an absolute maximum input voltage of 35V. The resistor will also take some of the heat dissipation. All power (UxI) from 30V to 5V (=25V) and some current, for AMP1 about 50 mA, will have to dissipate somewhere. In this application the dissipated heat is 25V x0.05A = 1.25W, shared between the resistor and the 7805. At 50 mA, the resistor would have a voltage drop of 47*0.05=2.5V and a heat dissipation of 2.5^2/47=0.13W. The resitor will help remove some ripple on the input to the regulator, especially if you also add a bulk resistor of about 1000 uF or more in parallell to C3 (not shown in the schematic).

- C3 and C4 are there to absorb voltage overshoot spikes, for example caused by inductance in the PCB traces. These small caps should be placed as close to the 7805 chip as possible.

- D1 protects the 7805 in case the input voltage is higher than the output, for example if the input is de-powered while the output capacitor C5 is sill charged. Without the diode, the 7805 could possibly be damaged by reverse voltage. For a 7805 with +5V out, the diode can probably be left out. For higher output voltages 78XX the protection diode is definitly recomended in case C5 is anything bigger than 10 uF. I would put the diode there or consult the manufacturers data sheet to see what they recomend.

- C5 improves transient response; if the current drawn from the 7805 output changes rapidly, the cap buffers some energy, helping to keep the voltage stable. Without this capacitor, the over/undershoots would most likely be bigger. You will find very varying recomendations for the size of this output capacitor. Some prefer to have a small value, some a very large.

The 7805 are available in different current/power ratings and different packages. A commonly found version is for maximum 1.5 Amps in a TO220 package. They cost around $1 to $2. The important issue when selecting the voltage regulator is that it should be capable of handling both the maximum current and the maximum heat dissipation. For our example, 30V in, 5V out, 50 mA, a TO220 package should handle this well, even without a heat sink. For higher currents, you should probably add a heat sink.

[locky]

Hi Jan,
Sorry for another newbie question but I have now wired up the transformer to the rectifier bridge and am getting some unusual readings... I have a 2X24V transformer with the 2 windings centre tapped. This is giving me 2 X 28.5 (57vac) which I presume is a little high due to the fact that it is idle and the input voltage probably higher (more like 250v than 230v). This bit makes sense to me. However, when connected to the bridge rectifier (200v 25A) and tested the opposite side, the readings are 2 X 26V (52VDC)... I was expecting the readings to be higher. Am I missing something or is this reasonable?

[locky]

A few more details for you...The transformer has 2 black wires on the primary side, connected to 230vac. 4 wires carry the 2 secondaries - RED/YELLOW and BLUE/GREEN. I have connected and grounded the YELLOW and BLUE. The Red and Green are connected to the AC connectors on the rectifier. The bridge rectifier is a single phase brudge, rated 25 amps and 200 volts.
Thanks in advance...

[locky]

I have now sorted the problem out - I was taking readings before having the caps in place. I'm now getting 39.5vdc with the caps in place which makes sense if the ac voltage is 28v. This, however, presents me with another issue - what's the best way to reduce this to a safe 30v? As it stands the voltage is clearly over the 36v cut-off point. How do I best reduce it?
Thanks in advance - and thanks also for the additional info on the 5v supply on the PSU link which has been very useful.

[Jan]

Hi,
First a reply to an older question;
If the +5V takes a few seconds to reach its correct value at power up, no problem. It just means unmuting will be delayed some at startup.

Reducing from 39.5vdc to below 36V can be done with a number of diodes in series. A slightly more fancy solution is to use voltage regulators, i.e. build a regulated power supply. I am not familiar with any rated at above 5A, so you would have to do some searching, or use a few in parallell. Can anyone out there suggest something?