32V CT 200VA Toroidal Power Transformer from parts express used for Single Supply? - diyAudio
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Old 26th February 2009, 03:23 AM   #1
tresch is offline tresch  United States
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Default 32V CT 200VA Toroidal Power Transformer from parts express used for Single Supply?

Hey fellas!

I'm currently absorbing all sorts of fun electrical knowledge and starting to put together a shopping list for my first chipamp. As I mentioned in my intro thread, I'm doing an experiment on building a small set of desktop full range speakers as-cheap-as-possible. I need an amp to drive them, so I'm building that, too!

The CHR70s don't need much power, so I'm going with the LM1876 for the chip.

Now, the speakers are cheap, at $70 for the pair, but that's still a significant chunk, relative to a decent set of multimedia speakers, say, from Klipsch, and I'm trying to do this (for science!) as close to $100 bucks as possible. Throwing a $50 transformer certainly doesn't help! I realize that I'm trying to make something that sounds awesome, but, I'm also playing around to see how much I can do for as little cost as possible, so here's the question:

32V CT 200VA Toroidal Power Transformer
http://www.parts-express.com/pe/show...number=129-075

This transformer has been mentioned in at least one other thread, but wasn't the real topic of the thread and I wanted to get some more opinions about it, to see if it would be a reasonable choice. This is not a dual secondary or a center-tap transformer. It actually has THREE secondaries at different current ratings and voltages. It's a replacement part that was designed for a specific purpose, and it's EXTREMELY CHEAP.

What I'm looking at specifically is that it has a 36v secondary (according to the description and user comments) that's rated at 4amps. Since I'm running 4ohm speakers with the LM1876, I was looking at 18+18 transformers, but with this thing, I could wire the chip up in a "single supply" circuit with the 36volt secondary, and it should give me about 140VA, or about 70VA per channel, which, for the little LM1876, should be plenty.

I'm rather new to this, so my question is: is this a horrible idea? are there big drawbacks to the single supply way of doing things? Are you as curious as I am and want to see how well it works?

it's $20 instead of $50! That's a pretty huge price difference...
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Old 26th February 2009, 05:21 AM   #2
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Try mpja.com or apexjr.com. Both have excellent tranny selections at prices similiar or lower then PE. I bought a 24vct 10 amp unit from marlin jones for 17.50 plus shipping.
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Old 26th February 2009, 08:09 AM   #3
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The CHR70 has a 4 Ohm voice coil.

With 4 Ohm loads the rail voltages for the LM1876 should not exceed 20 V. A 2*18 V transformer will give you something like 24-25 V rails after smoothing that will sag to around 21-22 V under load.

200 VA is very generous for a maximum 2*20 W amplifier. You could save some money, if you chose a smaller transformer. 80-120 VA are enough.

Even 20 V already means an effort for heatsinking the T-package. The TF-package cannot run with that voltage and 4 Ohm speakers at all. Go for 2*15 V with the T-package (~19 W into 4 Ohm) or 2*12 V with the TF-package (~11 W into 4 Ohm).

With a 2*18 V transformer look for 8 Ohm speakers or a different IC.
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Old 26th February 2009, 08:35 PM   #4
tresch is offline tresch  United States
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I was shooting for the 18 volt number because that seems to be what people tend to recommend for 4ohm loads, namely, the gainclone PS for beginners article. After some examination of the datasheet for the LM1876, it does look like more than 20 volts is going to be a bit much.

Also, what about the LM1875? similar specs, but I'm sure there's plenty of subtle differences between the two chips and how they handle power and such, just based on the very different package... What seems to be the consensus around here as far as the 1875 vs the 1876?

as for transformers, this fella: http://www.mpja.com/prodinfo.asp?number=7846+TR would certainly suffice, no? 2x12v, plenty of current. Would 12volt be enough plenty? that would give me something like 15 volt DC under load. Would be nice and safe, I suppose, but the speakers will handle 22rms and the amp at 15 volts would only be putting out about 15. I'd like to push these speakers a little to see what they can do, at least. Maybe I should bump up to a bigger IC? 3875 perhaps?
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Old 26th February 2009, 09:25 PM   #5
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Quote:
Originally posted by tresch
I was shooting for the 18 volt number because that seems to be what people tend to recommend for 4ohm loads
...for the LM3875 or LM3886

Quote:
Originally posted by tresch
Also, what about the LM1875? similar specs, but I'm sure there's plenty of subtle differences between the two chips and how they handle power and such, just based on the very different package... What seems to be the consensus around here as far as the 1875 vs the 1876?
There is none. The LM1876 is rarely used. The power dissipation issues are very similar for 1*LM1876 vs 2*1875.

Quote:
Originally posted by tresch
as for transformers, this fella: http://www.mpja.com/prodinfo.asp?number=7846+TR would certainly suffice, no? 2x12v, plenty of current.
How many channels do you want to feed off that?

Quote:
Originally posted by tresch
Would 12volt be enough plenty? that would give me something like 15 volt DC under load.
Depends on many parameters.
- How loud do you want to listen?
- How efficient are the speakers?
- How difficult is the load they pose?
- What type of music are you listening to? Highly compressed MTV program or extremely dynamic classic at originaly levels?
- etc.

Quote:
Originally posted by tresch
the speakers will handle 22rms and the amp at 15 volts would only be putting out about 15. I'd like to push these speakers a little to see what they can do, at least.
The rms handling is quite insignificant, because it is a steady signal. Music is dynamic in nature. And the difference between 15 and 22 W is only 1,6 dB, so hardly noticeable. Human hearing works kind of logarithmic, so you need ten times the power to perceive a signal double as loud.

It is widely agreed that an amplifier with too little power endangers the tweeters, because it will start to clip early. On the other hand an amplifier with too much power will be able to drive the woofers into their mechanical limits, which are often much lower than the electrical limits. So it is a matter of common sense to turn down the volume, when the speakers start to sound strange in either case.

If you want to push it a bit, go for the LM3886 with 2*18 V transformer 120-160 VA.
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Old 26th February 2009, 10:44 PM   #6
tresch is offline tresch  United States
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okay pacificblue, I'm gonna go for the +-15VDC option. Only question then is current. Is there concrete math out there that dictates how little current I can get away with before it starts affecting the sound?

http://www.mpja.com/prodinfo.asp?number=7845+TR
This is a 12-0-12 4A transformer. Would I be correct in stating that this would be a 96VA transformer, then?

Sorry I'm asking a lot of questions, but I've assembled most of my knowledge so far from searching through threads and reading tutorials, and a lot of the knowledge there is very subjective. People say things like "you want at least 100VA per channel" when talking about a 50 watt per channel 3876 builds, and such. So, is a general good rule about twice the input VA as final output power? If that's the case, then at 2x20 (or more realistically 2x15) the 96VA transformer should be just fine
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Old 26th February 2009, 10:44 PM   #7
tresch is offline tresch  United States
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Thanks for all the responses. I see now that the LM1876 is a dual channel chip. I don't know how I missed that the first time around.

You'll have to excuse me as I tend to ask a lot of questions right off the bat, especially when encountered with a lot of subjective information. Of course, the nature of music is, as you said, very dynamic and subjective, and this is all part science and part art, which it's very much part of the draw.

Based on the knowledge on this thread Noob Question -.- it seems that, with a 24V (center tapped) transformer, 4A should be plenty for running two channels.

My next amp, I think, will be a 2x LM3886 setup, so when I build that I can hook it up to these guys and see what kind of difference it makes. For now, I must try and restrain myself!
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Old 27th February 2009, 07:33 AM   #8
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Quote:
Originally posted by tresch
Thanks for all the responses. I see now that the LM1876 is a dual channel chip. I don't know how I missed that the first time around.

Based on the knowledge on this thread Noob Question -.- it seems that, with a 24V (center tapped) transformer, 4A should be plenty for running two channels.
Yes.

Quote:
Originally posted by tresch
My next amp, I think, will be a 2x LM3886 setup, so when I build that I can hook it up to these guys and see what kind of difference it makes. For now, I must try and restrain myself!
Yes, you can.

Quote:
Originally posted by tresch
Only question then is current. Is there concrete math out there that dictates how little current I can get away with before it starts affecting the sound?
Ohm's law U/R=I, the output voltage divided by the speaker impedance. But don't bother. Use the sum of the output power of all channels in W and buy a transformer that has a power rating in VA greater or equal to that.

Quote:
Originally posted by tresch
People say things like "you want at least 100VA per channel" when talking about a 50 watt per channel 3876 builds, and such. So, is a general good rule about twice the input VA as final output power? If that's the case, then at 2x20 (or more realistically 2x15) the 96VA transformer should be just fine
General rule is that you need at least as much VA as you have output power. If you listen at high levels, the transformer should have more power. This is mostly due to the fact that it only recharges the smoothing capacitors for about a third of each AC half cycle. So it will have to deliver about three times the amount of current that is actually going into the speakers for short moments. This can lead to the transformer going into saturation, which again leads to THD that affects the rails. Therefore some people recommend a VA rating up to three times the output power. If the difference can be heard, depends on the quality of your sound system. Twice the output power is a good compromise.

Quote:
Originally posted by tresch
http://www.mpja.com/prodinfo.asp?number=7845+TR
This is a 12-0-12 4A transformer. Would I be correct in stating that this would be a 96VA transformer, then?
Exactly.

Try not to use transformers much below 100 VA. Their regulation is bad, i . e. the voltage at no load is much higher than nominal. From ~100 VA upward regulation is below 10 %. 96 VA should be okay.
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Old 28th February 2009, 06:02 AM   #9
tresch is offline tresch  United States
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awesome! thanks! Very helpful. I'll be buying and/or order the parts this weekend, and I'm going to try and document the whole process from the point of view of a newbie as I go.

I my CHR70s arrived yesterday. Handsome little speakers! (shiney too) Can't wait to see how they sound!
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Old 28th February 2009, 09:58 AM   #10
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Quote:
Originally posted by tresch
I my CHR70s arrived yesterday. Handsome little speakers! (shiney too) Can't wait to see how they sound!
They got promising reviews here in Germany already.
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