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Old 12th June 2012, 10:10 AM   #1
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Question Newbie question about transformers

Hello,

Basically: Do valve-based designs necessarily have to use large wound transformers to generate the voltages used there-in and also to match the output?

I ask because the use of such transformers has always put me off designing with valves - the transformers tend to large & heavy, hence expensive, difficult to design, and difficult to source.

So, are there any standard 'modern' approaches to valve design, i.e. using switched-mode techniques and the such like?

Cheers,

Andy
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Old 12th June 2012, 10:56 AM   #2
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An OTL (output transformerless) Tube Amp with a SMPS wouldn't have any iron at all, so yes, it is possible. But that is kind of a challenge.

But the easiest (and autochthonous) way is to use conventional transformers. Big, heavy and maybe expensive, but simple passive components !

Each way to avoid transformers will lead to much more complicate active system.

The simple task of generating a HV supply usually needs just the Transformer, rectifiers and caps. A SMPS needs a controller, switches, active regulation, slope compensation, filtering etc...

The OPT is just an impedance matcher. If it's wound properly it can be regarded just by its ratio and you can ignore bandwith limitation and DC-R etc. Feedback is just an option in such a conventional amplifier.
Avoiding the OPT will make the whole amplifier much more complicated. Active global voltage feedback is a must to bring the output impedance down. But impedance isn't the whole story because the OPT also transforms current. But current isn't something which tubes are famous for. So you either need many of them in parallel or use some really big ones with all the associated drawbacks (more distortion (those tubes weren't designed for audio), gigantic heater current, etc.)

You get what I mean ?

'Modern approach to valve design' is kind of an oxymoron. Like Eco-friendly 400cci Hemi V8

'difficult to design' - No definitely not. Sheet of paper, pencil, curves - done. No feedback needed for basic operation, no four quadrant curves, no drive current (as long as you stay A1/AB1), no huge gate capacity etc...

'difficult to source' - Maybe some years ago. Thanks to the internet, you can get transformers from everywhere. Cheap ones from China or from your country like 1,2,3

Hope I cleared things up a bit :-)
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Old 12th June 2012, 11:16 AM   #3
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Quote:
Originally Posted by the_manta View Post
An OTL (output transformerless) Tube Amp with a SMPS wouldn't have any iron at all, so yes, it is possible. But that is kind of a challenge.


The simple task of generating a HV supply usually needs just the Transformer, rectifiers and caps. A SMPS needs a controller, switches, active regulation, slope compensation, filtering etc...

:-)

TRUE if you want a complex SMPS supply.

UNTRUE if you are happy with a simple flyback SMPS. Using the TL494N you only need a handful of components to achieve the HV.

Another way of achieving HV without a lot of iron is to produce a HV push-pull SMPS with post regulation. Not as efficient but easy to design and work with.

With modern speakers you will always need an OPT.

Someone may be able to come with an ES design that has the appropriately high input impedance needed or even a 4K LS.

Last edited by KatieandDad; 12th June 2012 at 11:20 AM.
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Old 12th June 2012, 01:56 PM   #4
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Thanks for the replies!

My goal is to make a zero gain amplifier, with a valve at is heart such that the 'sound' of a valve can be added to the signal. (Long story ).

I wonder if such a thing already exists. Any ideas would be grrrrreeeat!

Andy
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Old 12th June 2012, 02:41 PM   #5
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Quote:
Originally Posted by underwurlde View Post
My goal is to make a zero gain amplifier, with a valve at is heart such that the 'sound' of a valve can be added to the signal. (Long story ).

I wonder if such a thing already exists. Any ideas would be grrrrreeeat!
Consider building a tube based line stage (or buffer) and using a solid state power amp.

Do a search here for tube buffers.

Hollow state line stages/buffers typically require a power transformer but do not need output transformers. They can provide tube sound to the signal.

SY's heretical line stage comes to mind ( and there are many others)....

A Heretical Unity gain line stage

Last edited by boywonder; 12th June 2012 at 02:54 PM.
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Old 12th June 2012, 03:34 PM   #6
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OK, thanks for the pointer.

This seems a pretty simple approach I see around:

Click the image to open in full size.

Any thoughts on this application. Only need two low voltage supplies.

Andy
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Old 12th June 2012, 03:38 PM   #7
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It's not unity gain and it has a high output impedance, so doesn't seem to fit what you said you were trying to do. Questionable linearity and reliability as well. Chinese circuit?
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Old 12th June 2012, 03:40 PM   #8
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Quote:
Originally Posted by underwurlde View Post

So, are there any standard 'modern' approaches to valve design, i.e. using switched-mode techniques and the such like?
See David Berning's ZOTL circuits. Some brilliant engineering there.
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Old 12th June 2012, 03:43 PM   #9
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Yep, Chinese. One of those headphone amps which ppl seem to rave about it giving 'that' tube sound which is what I'm after really, plus the simple supplies.

Soooo, how to make it unity gain?

It won't be driving a load per se, no speakers or headphones. It will be placed in-line with other kit which will do driving / amping etc. Guess I wasn't too clear on that considering my original post.

I'm not averse to a little bit of gain btw.

I'll check those designs out now...

Thanks a-gain (pardon the pun, ho-ho),

Andy
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Old 12th June 2012, 03:49 PM   #10
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...headphone amp less the MOSFET output buffer that is...

Andy
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