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Old 31st August 2008, 07:56 PM   #1
Elias is offline Elias  Finland
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Question Using mains transformer as output transformer

Hello,

I'm thinking how well would a toroidal mains transformer work as an output transformer. You may need to modify the secondary winding to get the correct transform ratio but there should be not much problems in that because in toroid the secondary is on top and extra windings easy to remove. (I've modified toroid earlier also but for different purpose)

So the starting point could be for example 230V/12V 120VA toroid which are easy to find and quite cheap too.

How will the distortion level be in these kind of toroids? The power rating should be enough for <10W output, right?

How about bandwidth? I would guess that because the big and heavy toroid is initially designed for 50Hz, it could support low frequencies quite ok? I don't know what will happen at 20kHz though?

Which one would be better actually, a toroid or 'square' iron core mains tranformer for this purpose?

Please share your experience


- Elias
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Old 31st August 2008, 08:16 PM   #2
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Hi Elias

Search some old threads, there is already a lot of discussion on toroidals as output transformers.

More specific on your question regarding the 230/12 transformer. This is about 20:1, or an impedance relation of 400:1. A speaker of 8 ohms on the secondary giver 3k2 on the primary, which is a nice value for a lot of valves out there. But a single primary implicates that it can only be used in either SE or parafeed. The application in SE will probably be disastrous: the DC current will saturate the core...the application in parafeed may provide better results. A better option is a 110V+ 110V primary: connect both in series, apply the B+ to the CT and the extremes go to the plate of a PP stage The low voltage secondary goes to the speaker). Still better (technically speaking) would be tubelab's suggestion: get yourself an isolation transformer (220V to 220V, or 220V to 110V+110V, etc), connect the windings in serie for a CT winding (take care that each halve has the same amount of turns) and use this as a primary. The secondary you have to wind yourself, but as you said, that is relatively easy to do by hand. The advantage is that you have more inductance on the primary!

Erik
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Old 31st August 2008, 08:52 PM   #3
Jaime is offline Jaime  Uruguay
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You can do this
http://www.audiocostruzioni.com/a_d/...o-2a3/mad2.htm
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Old 1st September 2008, 11:20 AM   #4
Elias is offline Elias  Finland
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Hello,

simple SE amplifier I have on my mind. Core saturation of ferrite core could be an issue. Thanks for pointing that out Erik. However I have no measured data to show how much DC a toroid can take before saturation.

The idea in the Jaime's link can have merit. At least it avoids DC.

Image copied from the link:
Click the image to open in full size.


Well, still wondering about the sound quality of toroidal transformer..

How about core hysteresis, which can be high in ferrite cores? That cannot sound very good?


- Elias
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Old 1st September 2008, 11:28 AM   #5
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Hi Elias

Bad tongues say a toroidal can take very little DC (with the exception of the SE versions developed by amplimo/plitron, but I do not know which technique they used...) before saturating, and this is due to their perfectness (valve amplifiers goes deeper in the theme).

People from this list used toroidals as output transformers (again, in PP or parafeed) and most (or all) were quite impressed by the positive results: good bandwith, blabla. As for the sound quality, that is subjective: some people would not even consider toroidals as they do not look like 'traditional' trafos. I myself also used a toroidal once, and it sounded fine...but then, it was one of my first projects and I did not compare to a good quality EI trafo.

As said before, do a search on toroidal. Pay special attention to posts by Shoog, as he has done several designs using toroidals, from input to IT's to output.

Here is a recent thread showing an amplifier were a toroidal could be used as well
Spud-Assist: Totem-Pole Current-Mirror PP Hybrid

Erik
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Old 1st September 2008, 06:59 PM   #6
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> Here is a recent thread showing an amplifier were a toroidal could be used as well
> Spud-Assist: Totem-Pole Current-Mirror PP Hybrid

I'm using a 25VA Amveco mains toroid in a test amp. I used
that one because it was the largest one I had and I thought the
primary inductance would be better than the 15VA ones.

I'm using it in a parafeed circuit very similar to Diego's, using a
MOSFET as an anode load with active complementary current drive
First description I know of is at:
http://www.diyaudio.com/forums/showt...005#post638005

Anyway, the transformer's performance as a parafeed audio output
is excellent. It has no finite inductance effect down to the Fc of 8Hz
set by my parafeed cap, and the upper Fc is about 40 KHz (-3db).
Attached is a scope photo of a 10KHz square wave.

These results are using the primary at 2K ohms impedance.
I don't know what happens if you go up to higher impedance but
I guess it may affect the HF performance somewhat.

The sound is fine. I haven't done a direct comparison but this amp
sounds like a good SET. It's nice and detailed, seems to have great
frequency range, and I don't think the transformer is negatively
affecting the sound, i.e. it's doing what a transformer should do.

I would not hesitate to use these in an inexpensive project to
check it out for yourself. If they saturate at 25VA at 60 Hz, they
should be good for a few watts down to 20 or 30 Hz. I think there
is a lot more info on this on other threads here as cited.

Cheers,

Michael
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File Type: jpg amveco-10khz-sq.jpg (43.9 KB, 2070 views)
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Old 2nd September 2008, 10:41 AM   #7
Elias is offline Elias  Finland
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Hello,

It's good to hear Michael that you've had good results with toroids. Seems promising for me too.

Actually at this point I would try to find some way to avoid semiconductors in the signal path. I've done silicon amps before and now I'd like to try something different. I'm thinking building a simple SE without exotic and expensive components, a sort of starting point for me.

I know the active loading will work, but at first I'll try to keep it passive.

I came accross this old idea using DC bias curent at the secondary to compensate magnetic field from primary DC current. Looks good and simple on paper. Can make it adjustable if you like to try different levels of magnetisation of the core.

Anyone tried it in practise?
Attached Images
File Type: png toroid.png (20.7 KB, 2122 views)
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Old 2nd September 2008, 11:04 AM   #8
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Hi Elias

At least one member (I can't remember who) reported good results (very extended bandwith) using the technique of DC bias current in a unused winding.

Erik
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Old 2nd September 2008, 12:40 PM   #9
alexg is offline alexg  Philippines
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I finished a PP KT88 using Talema toroidal tranformer as OPT. 120VA, 2X 115V primary, 2X 12V secondary (paralleled). I unwound some of the secondary so that it will give me 10V, with 8 ohm speaker load, primary Z is 4.2K.

I also used an autoformer (center tapped choke) to do phase inversion (just like how Jack Elliano executed a couple of PP amps on his site).

At first I was getting very attenuated bass and I thought it was the toroidal tranformer, but later found out that it was the combination of the capacitor and the autoformer phase inverter, forming a low frequency filter. When I increase the size of the coupling capacitor to 3.3uf, wow!

Am using 400VDC B+, paralleled 6n1p as driver (22K plate load, 500R cathode resistor bypassed with 100uf) , the output tubes triode strapped and cathode biased using a common 250R resistor and bypasedd using 220uf cap.
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Old 2nd September 2008, 12:57 PM   #10
316a is offline 316a  England
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I've used 230:9+9 30VA and 50VA units SE with good results . One secondary is used for output to the speaker , the other is used to offset the DC in the secondary , for this a high impedence DC source is used . I fed the filaments of a 6B4G (6.3V/1A) with DC using a 280mH isolation choke and placed one of the secondaries in series with this . The transformer is 25:1 , so 1A of filament current offsets approx 40mA of anode current . Requires a variable fixed bias supply for optimum results but worked ok with a cathode bias network also . Takes a bit of tweaking to get right , there is a window of approx 2mA either side of the 40mA figure before audiable saturation occurs .

cheers

316a
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