Which transformer for 3886 chipamp.com kit?

[Schlumpfpeter]

Hi!

I feel kind of stupid for asking this question, because I got all the information I theoreticaly need to make I choice by myself, but all those additional numbers and values I see at mouser are completely confusing me.

The quest:
I need to find a fitting transformer for my stereo chipamp.com 3886 kit at mouser.com (I am ordering additional parts there, so it would come handy to buy those transformers there as well).

The information I got:
"For the transformer, a transformer with dual secondaries producing between 18vac and 25vac is recommended. One DIYer has used a 2x30vac transformer without issues, but 2x25vac should work best for a wide variety of loads. One can choose a variety of VA ratings and rail voltages. Keep in mind that after rectification, the rail voltages are somewhat higher than the non-rectified AC secondary rating of the transformer. The secondary voltage averages to 1.4 * the AC voltage, minus diode losses. The transformer regulation is also a factor, dependent on the size and regulation characteristic of the transformer in question. Suffice it to say that a commonly used 18V transformer results in approximately 25V rectified, while a common 22V supply produces around 34V rectified."

So in my naiv state of mind, I went to the mouser website and looked for a transformer with those specifications.... and know I do not even know where to start describing the confusion in my head

Can you guys please point me out to a transformer at mouser that would work for this particular case?

Thank you very very much,

SP

 

[AndrewT]

For 8ohms load use a maximum of dual secondary 25ishVac.
For 4ohms use maximum of 22Vac

For EU mains voltage you need a 230:0-25,0-25 Vac and >50VA per channel. I prefer a minimum of 160VA, some go as high as recommending 300VA.
Look up Rapid, RS, Farnell, CPC and dozens of other EU retailers. They all have suitable transformers.
Why use a USA retailer for EU? For a start they operate on 110/120Vac instead of 220/240Vac.

 

[tomchr]

For 8ohms load use a maximum of dual secondary 25ishVac.
For 4ohms use maximum of 22Vac

I concur. That'll give you roughly ±28 V for 4 Ω and ±35 V for 8 Ω. That'll ensure that the LM3886 operates within its limits.

>50VA per channel. I prefer a minimum of 160VA, some go as high as recommending 300VA.

Depends on the application. For sine wave reproduction you need:
2 x 22 VAC @ 121 VA per channel for 4 Ω operation.
2 x 25 VAC @ 101 VA per channel for 8 Ω operation.

For music signal (14 dB crest factor), you need:
2 x 22 VAC @ 51 VA per channel for 4 Ω operation.
2 x 25 VAC @ 44 VA per channel for 8 Ω operation.

I recommend aiming for somewhere in between these two extremes. So 2 x 22 VAC @ 160 - 200 VA for a stereo amp intended for operation down to a 4 Ω load.

For US buyers, I'd go with an Antek AS-2222 (2 x 22 VAC, 200 VA) transformer. For buyers elsewhere, I suggest the Hammond 1182N22 (2 x 22 VAC, 160 VA). You can get the Hammond transformer at Mouser.

Tom

 

[jvhb]

TTSA0200 - Transformer AUDIO TSA200VA - voltage to 50V - Shop Toroidy.pl

Very good transformers from Poland.

- and a nice german one:
http://hifituning24.de/transformatoren/ringkern_vergossen_v-rkt-ln/160va-2x22v.php

 

[torgeirs]

Is the crest factor VA numbers the effects of Intermittent operation described in page 4 in:
http://www.noratel.com/fileadmin/content/downloads/school/Transformer.pdf ?

 

[6L6]

To directly answer your question:

I need to find a fitting transformer for my stereo chipamp.com 3886 kit at mouser.com

160VA -

http://www.mouser.com/ProductDetail...wUzoUXIIvySPCJQuQgm7bfHYgJfRMAPQOsze2gpyF4Q==

225VA -

http://www.mouser.com/ProductDetail...zoUXIIvySPCJQuQgm7b/xabF2qHrDymY%2bXk9OHOKA==

That said, especially being in Europe, I'd get the Toroidy or the one from hifituning24 that jvhb linked to.

 

[davym]

I buy direct from Airlink, good prices, options for screening & with a low pound rate - might be worth considering. Standard Range Toroidal Transformers: CM0225222: 225VA 230v to 2x22v

 

[tomchr]

Is the crest factor VA numbers the effects of Intermittent operation described in page 4 in:
http://www.noratel.com/fileadmin/content/downloads/school/Transformer.pdf ?

You can find my write-up on the crest factor in the Thermal Design section of my Taming the LM3886 pages. I expand on the implications for the power supply in the Power Supply Design section.

Tom

 

[asuslover]

Multicomp Torroid Power Transformer 25V x 2 - 5A | MCTA250/25 (MCTA25025) | Multicomp
they have 15% off coupon online and they have free shipping deals (sometimes).
These toroids are actually noratel brand.

 

[Schlumpfpeter]

Guys, you are awesome. Thank you very much for all the advices.

My profile flag is a bit misleading, as I am actually German, but I live in the UK, but obviously still in Europe.

I think I will go with the Polish manufacturer JHVB did mention, the product seems rock solid and quite cheap for what is mentioned. They want 20 Euros for the delivery to UK, but I also need two additional transformers for the Myref Freman edition I am going to build, so at the end the additional delivery costs will pay out.

Mouser will provide the myref parts and the Toroidy guys the transfomers.

Thanks guys!

 

[torgeirs]

You can find my write-up on the crest factor in the Thermal Design section of my Taming the LM3886 pages. I expand on the implications for the power supply in the Power Supply Design section.

Tom

In the sheet I do not understand the: Peak Output Voltage/Vout(peak) for crest factors < 2. Peak voltage is constant with crest factor

This is variable voltage analysis as i understand it. In the example i refered to, the voltage is constant or zero and they did a time x VA analysis.

I read this as that with crest factor of 14 db, 25 lin, the VA can be reduced by root(25) = 5. What factors have i forgot?

The other aspect of VA is how much energy the transformator can deliver. That is heat + sound. In home systems the sound is < 1% of enegy.
The rest will be heat in amp, crossover and speaker elements.
How much enery can your system dissapate without amp and speakers distorting to much. Many times this is less than the VA values stated in this thread.

 

[tomchr]

The crest factor is the ratio of the peak output amplitude to the RMS value.

You can find the power dissipated in the output stage by integrating the instantaneous output power over a full cycle. This gets rather hairy for anything other than a sine wave.

You are correct that the peak output amplitude does not change as the crest factor changes. However, I find it to be a reasonable approximation of reality to assume that an amp operating with a signal having a 14 dB CF is operating at an RMS output voltage of Vout_max/5. Thus, it is possible to calculate the power dissipated in said amp by calculating the power dissipated when it's delivering that output swing. No?

If you have a better way of doing the math I'm all ears.

The other aspect of VA is how much energy the transformator can deliver. That is heat + sound. In home systems the sound is < 1% of enegy.
The rest will be heat in amp, crossover and speaker elements.
How much enery can your system dissapate without amp and speakers distorting to much. Many times this is less than the VA values stated in this thread.

Dude. What are you smoking?!

A 100 VA transformer can deliver 100 VA under the conditions specified in the data sheet.
An LM3886 running on ±28 V rails will deliver just shy of 40 W to an 8 Ω load (sine wave). Under these conditions, it will draw 58 W from the power supply, thus, requiring a 87 VA transformer. It will draw 58 W from the power supply and deliver just shy of 40 W to the load regardless of whether you use a 100 VA transformer or a 10 kVA transformer.

Tom

 

[torgeirs]

Well, I have never smoked my transformer. I have not even noticed that it is hot, so somehow I must put to much money into it.

You are stating the absolute max values of the lm3886.
With an isolated IC and a 3W/C heatsink maybe 30 watts thermal and 45 watts elecrical is max

This should be considered when choosing a transformer.

And if your speaker crossover has low awg coils that distort at 1 watt, no 200VA transformer is needed. Just my two cents.

I just wonder if a crude tumb rule is root(crestfactor linear)*(Powersupplyvoltage^2)/impedance speaker

Then I disregard the Total Quiescent Power Supply Current and Output Dropout Voltage.

The result is not that far away from the sheet with the your good approximations when
Total Quiescent Power Supply Current << Iloadmax
Output Dropout Voltage << Supply voltage
crestfactor > 6 db

 

[torgeirs]

The crestfactor between 6 and 20dB it chould be in last post.
At some point the losses in the trafo and the "Total Quiescent Power Supply Current" dominates the power needs.

A tried and tested construction is the Genelec 1030A.
It has 2 amps at 80W and 50W. Transformer is 100VA.
http://www.genelec.com/sites/defaul...s models/2-way loudspeakers/1030A/ds1030a.pdf
I would use that as a starting point for a 2 ch everyday 40W amp.

 

[tomchr]

With an isolated IC and a 3W/C heatsink maybe 30 watts thermal and 45 watts elecrical is max

I'm not sure where you get your numbers from. In my experience (see this thread) the 40 W of power dissipated in the LM3886 is about right, assuming a reasonably sized heat sink.
I have no issues meeting the data sheet specs (38 W, 8 ohm; 68 W, 4 ohm @ ±28 V and 50+ W into 8 ohm on ±35 V rails) of the LM3886 even with the isolated LM3886TF package.

Tom

 

[torgeirs]

Sorry, the 3 C/W heatsink was wrong. Should have been 3C/W total.
The 30 watts is from a linear seminar I atteded 20 years ago. Se pictures enclosed.
Their example is with a nonisolated package.

Isolated chip is about 2 C/W. Do not know if that is with case to heatsink C/W or not.
Nonisolated is about 1.5 C/W with grease.

Your post shows that to meet spec a Large heatsink is needed. Maybe some use a medium. Then a smaller trafo can be used as lesser watts can be output anyway.

 

[FatCookies]

I don't want to create a new thread so I'll stick my query here:

I am currently running a stereo LM3886 amplifier, with a +/-25V 225VA transformer and am considering adding a centre channel - should I try and source a transformer that can deliver more current, or will the existing transformer suffice for three channels?

 

[6L6]

I'd be happy using the existing transformer for 3 channels.

 

[asuslover]

I guess you'll use 3 channels for tv(movies) only , so the 225va should be enough, when you switch to music, you'll use 2 channels only, again the 225va is great.

 

[AndrewT]

A 225VA transformer is suitable for 112W to 225W of total maximum output power.
Your two channels just fell outwith that range: 50W+50W = 100W, but you have proved it works !
Three channels coming to 150W is within that range.