Power supply for amps

[frankwas]

Hi Everyone!

This is my first time posting in this forum! I would like to know if anyone can please help me determine a few things about two power supplies that I want to build for two amplifiers.

The first amplifier is this one:
http://www.kemo-electronic.de/en/bausaetze/b086/index.htm

I was thinking of using a toroidal transformer, a bridge rectifier and two electrolytic caps. But I have no idea what the specs must be. On the site it sats this amp needs 24 to 40V. But what must the VA rating of the transformer be and is it possible to power two of these amps from one psu? Lastly, for this amp, how many RMS watts does it put out? Cause I need to power a 40W rms speaker off one of them.

The second amp:
http://www.vellemanusa.com/us/enu/product/view/?id=522104

They recommend a 2 x 25VAC output toroidal transformer. But they say this reduces power by 15% (15W rms)! I want to know which transformer would you guys recommend I use to get the full 100wrms? I would also, like the first amp, power two amps from one psu.

If anyone can point me in the right direction, or even give me a design or anything I would really appreciate it, cause I don't want to build an amp that is underpowered, and I'm on a really tight budget, but am willing to buy the right components for the jobs, just not unneccasary ones :D .

Thanks all!

Francois.

[MP]

Hi frankwas, Welcome to Electronics-lab!
The power amp in your link can run on 25 to 30 volts. This transformer only gives you 25 volts. That is why it is not maximum spec. Most amplifiers need a bipolar supply. Note that this one gives you 2 X 25 volts. This means it is a 50V center-tapped transformer. You want +/-30V, or 60V, center-tapped, provided the power transistors can handle it. If you know which power transistors are used, you should look at the data sheet. You want to make sure error in your mains connection does not take the transistors above the maximum spec.
Also, note that the amplifier will only use what is needed in terms of amperage, so if you allow for some headroom in the way of amperage, you will be ok. This is a common mistake in making homebrew amplifiers. If you do not have enough amperage, it does not matter how many volts are at the secondary of the transformer.
Hope this gets you on your way.

MP

[audioguru]

Hi Francois,
Welcome to our forum. ;D
The Kemo amplifier kit has hardly any spec's. It has half the power supply voltage of the Velleman kit so its output power could be only one-quarter as much.

Even the Velleman amplifier kit doesn't have detailed spec's. They don't say its bandwidth nor its distortion at its rated power. They say its distortion only at 1kHz at 10W output.

A 30VAC transformer for the Kemo kit produces 40.4VDC at its rated power. The amplifier output will produce about 35V peak-to-peak into an 8 ohm speaker at clipping which is low distortion. 35Vp-p is 12.4V RMS which makes only 19W into 8 ohms. It might make 32W into 4 ohms. I talk about real RMS watts, not "music power".

With a dual 30VAC transformer, the Velleman kit might make 70W into an 8 ohm speaker and might overheat with a 4 ohm speaker.

With an 8 ohm speaker, both amplifiers are about 70% efficient. So each channel of a Kemo amp needs a transformer rating of 27VA, and each channel of a Velleman amplifier needs a transformer rating of 100VA. You can use a single transformer for two amplifiers.

[frankwas]

This is why you guys are called "Electronic Gods"!!!!

Thank you for your replies! I'm starting to see a clearer picture. Let me be more specific. I am going to use 2 amplifiers per channel in my system, so that is why I need to know the rating for 4 amps. Sorry about that. I think that I am going to go with the Velleman because it looks like it puts more power out. I will be using 4 ohm speakers throughout the system.

Let me just make sure I understand. If I build four of those velleman amps, and I buy two 200VA, 2x30V transformers, like this one: Transformer, toroidal, 225VA, 0-230V pri, 50/60Hz, 2x0-30,  (they don't have 200VA) or i buy one of these ones: Transformer, toroidal, 500VA, 0-230V pri, 50/60Hz, 2x0-30V sec, i can power all four amps (two each on the first transformer, or all four on the second one?).

The thing is the power supplies for these amps are built onto the kit. If I run all the amps in parallel, (i mean as in from one or two transformers), will it work?

Thanks in advance!

[audioguru]

Two 225VA transformers or one 500VA transformer will be fine.
It might be best to make two stereo amplifiers, each with a 225VA transformer.

[frankwas]

Thanks for the reply!

I made a very crude sketch of how i am going to connect the two amps at a time. Could you  please have a look and tell me if it looks ok?

Thanks.


P.S. I know the sketch is not up to standard!  ;D

[audioguru]

In the Velleman amplifier kit, the terminal numbers for the transformer wires are 3, 4, 5 and 6 and are labelled "AC".
I don't know if the wires from the transformer are correct, without seeing its datasheet.

[frankwas]

Hi GURU!

I just numbered it to show that i am connecting 1 with 1 in my diagram. The colourcoding is correct. I went to the shop and checked on the transformer. I need to make sure of something. If I connect two of these amps on one 300VA transformer 30-0-30 2x5A, will the transformer be string enough to pull both amps without being pulled dead?

TA.

[audioguru]

If I connect two of these amps on one 300VA transformer 30-0-30 2x5A, will the transformer be strong enough to pull both amps without being pulled dead?

Each Velleman amplifier delivers 70W to an 8 ohm speaker.
The amps are about 70% efficient so each one needs 100W from the transformer.
Only 200W is needed so about 100W will be wasted and not used.

[frankwas]

Ok.

Thank you for the clarity. The guy (like any normal salesman) tried to sell me a transformer for each amp! Where I live that is R400 x 4 = R 1600 which is about $200! Then I don't even have one amp!!! So, I have changed my mind again, please tell me what you think of ESP's amp. I am still gonna build two of the velleman amps, but for my other speakers, I wana build the ESP amp with his recommened PSU. Please let me know what you think and if you have any recommendations.

[frankwas]

I also want to add, I found the datasheets for the transformers if you would like to see them?

[audioguru]

The LM3876 is an excellent amplifier IC with about 56W into 8 ohms output, when used with a positive and negative 35V supply.

[frankwas]

Ok. Then I am going to go for that one. But is there a possibility to bridge these IC's or AMPS to give me a 100W amp? Or do you perhaps know of an amp design that is not too difficult to build that has about 100RMS?

TA.

[frankwas]

Sorry, I forgot to mention that in ESP's desing notes, he says you can use a LM3886 IC that willl give 68W power.

[audioguru]

you can use a LM3886 IC that willl give 68W power.

Read the datasheet.
It gives a little less power to an 8 ohm speaker with the same power supply as the other IC, or 68W to a 4 ohm speaker with a positive and negative 28V power supply, or 38W to 8 ohms.

National has an application note that talks about paralleling and bridging their amp ICs for much more power output:
http://www.national.com/an/AN/AN-1192.pdf

[frankwas]

Ok. But I am going to use 4 ohm speakers. I am busy reading through the datasheet.  It's very complicated.   
I don't understand. Do I do this modification after the two amps are built as in the shematic I posted? I was looking at figure 3 on page 8 of the application note. They say at +-35V the amp at bridged mode puts out 100w. Will the quality still be good?

[audioguru]

In the application note you must determine which IC they are talking about. Only the LM3886 can drive a 4 ohm speaker by itself. The other ICs need to be paralleled to drive 4 ohms.
They also show that when bridging amplifiers, it is the heat dissipation that limits how much power they can deliver, since an 8 ohm speaker seems to be only 4 ohms to a bridged amplifier.
Most of these amps cannot drive a 4 ohm speaker when bridged.

The applications note shows two bridged LM3886 ICs with a positive and negative 25.5V supply, driving an 8 ohm speaker to 100W. The ICs will melt if they try to drive a 4 ohm speaker when they are bridged.
It shows two LM3886 ICs in parallel with a positive and negative 35V supply, driving a 4 ohm speaker to 100W.
The quality from these ICs is excellent. They need the pcb to be designed a special way or they oscillate.

[frankwas]

Ok. Thanks for clearing that up for me. Ok then. I will just biuld a amp with a single LM3886 ocnfig and the two velleman kits. I really appreciate all your input Audioguru! You have really helped me a lot! Do you perhaps know of a site where I can get amp schemes that might have 100W amps that are not too difficult to build?

[RDL2004]

Before you go spending a lot of money, you might want to spend a few days reading posts here:

http://www.diyaudio.com/forums/

Check out the Chip Amps forum in particular.

[audioguru]

The LM3886 with a dual 28V power supply and driving a 4 ohm speaker with 68W is about as easy a circuit as it gets, and works extremely well.
I doubt that you will hear any difference in loudness between the 68W amp and a 100W amp. Your hearing's response to loudness is logarithmic, so 10 times the power sounds only twice as loud. Most people can barely hear a difference in loudness of twice the power.

[frankwas]

Thanks once again for all your replies guys!!!

I think I have finally decided what I'm gonna do. I am going to build the two velleman kits, and then two LM3886 amps. I am going to go with Rod Elliots design and his power supply. But in his notes he says that his power supply is only good enough for one dual opamp. Does he mean that it will only be string enough for a single LM3886?

Another question I have is heatsink ratings. Is lower ^oC/W better or how does it work? I am going to use fans on my heatsinks anyway, so will these values be that significant?

If Rod's psu design can pull two 3886's then I can really save a lot of money cause I can get a 160VA, 25-0-25 toroidal transformer for really cheap.

TA!!!

[audioguru]

I am going to go with Rod Elliots design and his power supply.

The LM3876 is designed to drive an 8 ohm speaker, not the 4 ohm speaker you said you will use.

But in his notes he says that his power supply is only good enough for one dual opamp. Does he mean that it will only be string enough for a single LM3886?

An LM3876 is a power amp, not a little opamp. His circuit uses a couple of 680 ohm resistors feeding 15V zener diodes to power a dual opamp for a preamp or something.

Another question I have is heatsink ratings. Is lower ^oC/W better or how does it work? I am going to use fans on my heatsinks anyway, so will these values be that significant?

Yes, the lower is the degrees/W number of the heatsink then the cooler will be the amplifier ICs. The datasheet and the application note for the amplifier ICs shows how to calculate a heatsink's rating and even gives recommended numbers. Heatsinks have a degrees/W rating without a fan and with a fan having a certain airflow.

If Rod's psu design can pull two 3886's then I can really save a lot of money cause I can get a 160VA, 25-0-25 toroidal transformer for really cheap.

A 25-0-25 transformer makes about positive and negative 35VDC. Each LM3886 amp gives 50W to an 8 ohm speaker with that power supply. Aren't you using 4 ohm speakers? The amps are about 70% efficient so two amps need a total of 143W which is 143VA from a transformer for 8 ohm speakers.

[frankwas]

Ok.

But in the LM3886's datasheet it says it puts out 68W on 4 ohms at 28V. Will my IC be damaged if I use the 25-0-25 160VA transformer with two 4700uF caps and a 400V bridge rectifier as in the diagram?

I am really confused right now.  ???

[audioguru]

But in the LM3886's datasheet it says it puts out 68W on 4 ohms at 28V.

Correct. With a positive and negative 28VDC supply, that is made with a 21-0-21 transformer.

Will my IC be damaged if I use the 25-0-25 160VA transformer with two 4700uF caps and a 400V bridge rectifier as in the diagram?

It would try to protect itself if the volume is high. It might be destroyed or just sound very bad. When it sounds bad then it might blow a tweeter.
The transformer would be overloaded if it supplies two amplifiers having 4 ohm speakers.
The applications note shows how to parallel two LM3886 ICs to drive a 4 ohm speaker from a positive and negative 35VDC supply from a 25-0-25 transformer.

[frankwas]

Oh ok. I understand.

But let me ask this, it might sound very strange: I'm only going to power, or need, 40W rms out of each amp and probably not constantly. Won't that give the transformer a little room to breathe? Or would you recommend that I get another transformer?