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PostPosted: 30 Aug 2016, 16:06 
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Hi all,

Let's say, for the sake of argument, that my project is something like Rod Elliott's 3A, a 60-100w stereo amplifier. Now let's say that I need a 300-400 VA transformer to make this work, even though the full capacity of the transformer would not often be used. I could get by with a 32-0-32 transformer which would result in a little less than +/- 45v under full load. 100w into 8 ohms is something like 28.3v RMS. My intended output devices are going to be TIP35/36C.

Antek sells transformers like this for not much money. I could probably get one for less than $50. I also don't much care for the look of toroidal transformers (if I can help it). I don't mean to be so picky but I'm wondering where I might be able to find such a transformer with the rated output capacity (two 32v windings with 4.8A each should be enough).

I know that A/V receivers for home theater generally have EI transformers. Maybe I should find one at a thrift store and steal one from there. Could be cheaper than buying a fresh one. Still, I'm looking to see what other amp-builders do. Does everybody simply use toroidal transformers?

Ed

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PostPosted: 30 Aug 2016, 21:22 
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Would a Digikey 237-1934-ND fit the bill? It's $42 :confused:

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PostPosted: 31 Aug 2016, 02:14 
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I looked at it. A mentor once told me that I need to size my transformer by taking the total output power (100w x 2) and then doubling it. So, a 400 VA transformer should be adequate for that size of amplifier. However,

I ran some numbers and it seems like those numbers would not work. Here is what I mean...

- Let us say I want a 25 WPC amplifier into 8 ohms. That means 14.14v RMS and 1.77A RMS into 8 ohms.

- Now let us say that the peak voltage is 20v and peak current is 2.5 A.

- Since bipolar transistors aren't going to give me a flawless, rail-rail swing with zero losses, let us assume I need a +/- 25v supply. I probably could get by with 22v rails but I want some extra voltage to be sure.

- The transformer need only produce the average value of the peak current, where the capacitors will handle the peaks themselves. The figure is 2 / pi, or 0.6366 x Ipk. For this value I get 1.59A per channel, or 3.18 for a stereo pair at full volume. Let's assume I need 3.3A to power the input circuitry, pilot LEDs, and other things that make our number easier to deal with.

- A transformer into a bridge rectifier isn't 100% efficient. That is, the RMS current in the secondary is something like 1.613 times the DC current if using a capacitor input filter. So, let's say our AC requirement is 5.4 A. Because I'm sure this amp won't be used at full power constantly, let's be a little cheap and say I need a secondary with 5A of capacity.

- Now I need a transformer that can supply the total voltage at the rated current. 25v / sqrt(2) = around 18v. This means I need a 36v CT transformer with 5A of secondary current. If I were powering a resistive load with this transformer, it would be burning 180 watts. That's almost double the amount I found by using my mentor's method (25wpc ~ 100 VA transformer).

So, were I to buy an 18v transformer with dual secondaries, it seems to me that I'd need much more power capacity than I'd rightly need. This also seems to suggest I'd need a transformer of around 800 VA to get two good channels of 100w each. It's very doubtful I'll need that much but that's what the numbers suggest.

Ed

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PostPosted: 02 Sep 2016, 02:13 
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Will you seriously be running at 100W RMS continuously?

The more usual view is that the higher watts [if I put it that way] are used to deal with peaks - not the normal listening volume.

If that is true, it is somewhat over-kill to have a power supply that is able to supply such power continuously.

That is one reason why rather large capacitors are used on high-end system.


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PostPosted: 02 Sep 2016, 04:57 
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acomeau wrote:
The more usual view is that the higher watts [if I put it that way] are used to deal with peaks - not the normal listening volume.


Nope! That's why modern amps suck compared to vintage.

Look at a vintage Kenwood, Marantz, Sansui, etc. If the specs say 100W/ch. RMS, they mean it.... the supply is capable of at least 400WRMS (class-AB ~ 45% efficient).

Here nowadays you have modern HTiaB claiming 1500W total power with an at-best 150VA transformer and caps that wouldn't have the delta-V/delta-T to reserve a transient more than a microsecond.

Or worse, a SMPS - for non class-A, their loops aren't fast enough to handle the demand and you get a sag-out. We have a couple of JVC in the store with SMPS and we can't give them away :bawling:

That's why vintage amps hold their value 8-)

Ed - I'd use more than one transformer then if you're going to use EI core... keep the losses lower and things running cooler.

Cheers!

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PostPosted: 02 Sep 2016, 19:31 
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Geek wrote:
Nope! That's why modern amps suck compared to vintage.

Look at a vintage Kenwood, Marantz, Sansui, etc. If the specs say 100W/ch. RMS, they mean it.... the supply is capable of at least 400WRMS (class-AB ~ 45% efficient).

Ed - I'd use more than one transformer then if you're going to use EI core... keep the losses lower and things running cooler.

Cheers!


Nope, that is an argument for DIY, not for vintage alone.

By more than one transformer, I suspect you mean more two independent PSU.....one for each channel.

SMPS is off topic ,,,, more pertinent is toroid core versus EI


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PostPosted: 02 Sep 2016, 22:04 
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acomeau wrote:
SMPS is off topic ,,,, more pertinent is toroid core versus EI


Take the comment as a whole, it will make sense.

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 06 Sep 2016, 08:55 
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Joined: 18 Aug 2015, 07:38
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Hammond comes to mind but I have not searched their site. I like the looks of toroid. It looks like money saved to me.


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PostPosted: 06 Sep 2016, 13:12 
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Download the 5C08 catalog... gives the relevent line :D

http://www.hammondmfg.com/5ccat.htm

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 06 Sep 2016, 15:01 
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acomeau wrote:
Will you seriously be running at 100W RMS continuously?

The more usual view is that the higher watts [if I put it that way] are used to deal with peaks - not the normal listening volume.

If that is true, it is somewhat over-kill to have a power supply that is able to supply such power continuously.

That is one reason why rather large capacitors are used on high-end system.


!!!WOW!!!

A power-supply should be able to give the full rated power of the amp, plus a reasonable safety margin (min. 20%) all day, all night, and into the next morning. Otherwise, one is asking for trouble, and deserving of it fully when it comes. Capacitor banks exist to provide the peaks, which can often exceed the RMS ratings by 200%, or more. In the case of one of my amps, the peak can be as much as the RMS x 5 for one second without clipping.

Any system that 'cheats' on the power-supply via using extra large caps is emphatically, specifically and absolutely not "high end" at any price. And, in fact, if it claims an RMS output even 1% larger than the capacity of the power-supply, it is a fraud.


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