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Old 25th Jun 2018, 2:02 pm   #1
martin.m
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Default Push pull audio output transformers

In my younger days I constructed many single ended valve amplifiers using parts from scrap radios. I fancy making an amp with a push pull output stage to play my 78rpm records through. The problem is a lack of a suitable output transformer. I don't want to buy an expensive new component (if such items are still available). I have heard suggestions that a 250-0-250 vintage mains transformer might work. Has anyone tried this and how would it be connected up? Could the outputs of two single ended amps be combined to have the same effect as a push pull circuit? Any advice would be gratefully appreciated.
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Old 25th Jun 2018, 3:53 pm   #2
John10b
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Default Re: Push pull audio output transformers

No problem obtaining new transformers, look at RS typical price £30.
I tend to look on eBay, I have been pleasantly surprised at what’s on there.
Cheers
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Old 25th Jun 2018, 3:55 pm   #3
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Default Re: Push pull audio output transformers

The main benefits of a push-pull circuit come from there being no static magnetic field in the output transformer core (the primary halves are wound so that their static fields cancel and their out-of-phase alternating currents generate alternating fields which add up in phase). You can really only achieve this with proper push-pull. Two single ended amps won't do it.

People have used mains transformers as valve output transformers, but they're often wound pretty crudely and if there has been any optimisation of stray inductances and capacitances then it won't have been done with audio in mind. In all likelihood phase-shifts across the audio frequency range will be large which means you won't be able to use much negative feedback to reduce any other shortcomings in the amp. If you did you'd risk turning the whole thing into a power oscillator, perhaps capable of destroying the speakers you've attached to it as well as the amp itself.

That said, if you don't want real hi-fi then you could give it a go. You don't want a 250-0-250 secondary though. You want to use a split, or centre-tapped, mains primary (0-120, 0-120 or 0-120-240) for the valve side and a relatively low voltage secondary for the speaker side. A 240V primary with a 9V secondary would give a turns ratio of 26.67:1. The impedance transformation goes like the turns ratio squared, so an 8ohm load on the secondary would be transformed back to about 5.7kohm between the ends of the primary. That might suit some power valve pairs quite well. Of course the proper way to do this would be to choose your valves first, check their data sheet to find the recommended anode-to-anode load impedance and then calculate the other way to find what the secondary voltage needs to be to give a good match to your speaker impedance.

Obviously the power throughput, the DC current capacity of the primary windings and, if you're thinking of very high voltage valves (I'm guessing you aren't) the insulation strength of the primary all need to be considered too.

Cheers,

GJ
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Old 25th Jun 2018, 5:19 pm   #4
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Default Re: Push pull audio output transformers

GrimJosef's post is really helpful!

A couple of comments... using two 120V primaries in series, these will be optimised for 120V AC. If you use a valve amplifier with 250V HT, you can expect 220V (peak) across each half, or 155V RMS. That won't hurt the transformer, but if you are interested in bass response, the transformer will probably run into saturation at full power, at lower than 65Hz (I assumed the mains transformer is on the verge of saturation at 120V 50Hz. Small ones are often designed to be so). So you might want to limit the low-frequency input into the transformer.

Can two single-ended stages be combined? Well, yes they can, but as GJ says, you don't get the benefit of DC flux cancellation. Nevertheless, some useful distortion reduction can still be had, and obviously, power is increased. Just be careful of matching the speaker - if the original amplifiers were for 15 ohm speakers, connect in parallel and use this to drive an 8 ohm speaker; if they were intended for 3 ohm, connect in series and drive an 8 ohm speaker. The matching will be pretty near either way.
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Old 25th Jun 2018, 5:20 pm   #5
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Default Re: Push pull audio output transformers

Mains transformers can work OK as push-pull output transformers: in the past I've used a 350-0-350V one as a transmitter modulation-transformer [original primary-winding feeding an 807, two 6V6 as the push-pull] and it worked just fine.
In such applications lack of HF response is a good thing [you don;t want to waste precious power on any modulation frequencies above 3KHz].
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Old 25th Jun 2018, 11:53 pm   #6
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Default Re: Push pull audio output transformers

Thank you all for your advice. The subject is not as straightforward as I thought. Looking on the RS website I found a 12W push pull valve output transformer for £28 and a 25W version intended for EL34s for £44. Very reasonable, so I don't think it's worth messing round with substitute transformers. Vintage valve amplifiers by Leak and Quad go for crazy money today so I thought I would have a go at making my own.

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Old 26th Jun 2018, 12:22 am   #7
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Default Re: Push pull audio output transformers

If you haven't done it before then you'll certainly learn a lot making your own, and you might well have fun too ! Matching the performance of a Leak or Quad amp in good condition will be tough though without pretty good quality transformers.

In 1963 the list price for a single Quad II power amplifier was £22-10-0. Inflation based on the RPI converts that to just under £460 today (2018) https://pbs.twimg.com/media/DgiXTP4XkAA0SrM.jpg. If you're prepared to shop around a little £460 should get you a nice used original Quad II. So today's prices really aren't crazy money. They are actually quite close to what Quad IIs cost when they were in production.

Of course you could buy the modern version of the same amp brand new. But that would cost you a lot more than £460.

Cheers,

GJ
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Old 26th Jun 2018, 10:21 am   #8
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Default Re: Push pull audio output transformers

A significant fraction of the parts cost of a good valve amplifier is the output transformer. Cheaper transformers have limited bandwidth and so severely limit how much feedback can be applied, hence increasing distortion. Not a problem for playing 78s, but if you want hi-fi then an expensive OPT is unavoidable.
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Old 27th Jun 2018, 1:53 am   #9
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Default Re: Push pull audio output transformers

Yes, a £30 transformer from RS won't have the same performance as the ones that Quad and Leak used but I just enjoy making things. My main amp is a Leak Delta 30 but I fancied something a bit more "old fashioned" to play 78rpm records with, preferably using International Octal valves ( I have plenty of them). Thanks again for all your advice.
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Old 27th Jun 2018, 6:48 am   #10
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Default Re: Push pull audio output transformers

HHMMM can I add my two bobs worth?
Japanese amplifier manufacturers HATE negative feedback!
Especially if the valve is 100 years old.
To me that would suggest ANY transformer with the required ratio would be ample.

Quad and Leak made extremely capable amplifiers ( AND transformers) for their time, BUT would fail abysmally with modern "DOOF DOOF" bass lines from CD's, a little like the 17 year old with his modern car, that has a used one gallon/4 litre PAINT tin strapped to the end of the exhaust pipe!!

If I may be so bold as to suggest a modest Push Pull pair of 6AQ5's, that will furnish about 7 watts of TRUE RMS power, being derivatives of the superb 6V6/6L6/807 family of valves.

Power is subjective!!. I have seen people on this website post pictures of their houses. Neighbors are at LEAST 2 meters away. I have a seven watt per channel stereo amplifier that I built a few years ago. Wifey chases me regularly around the house with a broom if I crank it up too much.

I offer a single channel of PP amplification, ideally suited to your application, output transformer, second hand, BUT made by HMV ( His Masters Voice) in Australia, about the beginning of 1970. It's TINY but very capable as I listened to MANY Beatles LP's as a kid on a "Stereogram", using these transformers. IF you want it, email me and its yours free.

Joe
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Old 29th Jun 2018, 10:55 pm   #11
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Default Re: Push pull audio output transformers

Not to take anything away from Joe's generous offer, there would be something to be said for trying out the RS PP transformers.

There is an amp builder in South Africa who is considering trying them (RS output transformers, D29A09F) in some stereo Mullard 5-10 builds for his customers. You might look at this thread and the one on his build there: https://www.avforums.co.za/index.php/topic,72459.0.html

Last edited by qualityten; 29th Jun 2018 at 11:01 pm.
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Old 30th Jun 2018, 1:41 am   #12
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Default Re: Push pull audio output transformers

I have had acceptable results using RS output transformers in my recent ECL80 amp project. This was using one of the small multi-tapped ones because of the low power output and it was in single ended mode, but I did notice that it looked like it may be able to be configured in push-pull mode, as the primary has a centre tap. Max current is 40mA so that limits power, but they are cheap. Sound quality was quite acceptable for general use.
Alan.

Last edited by Biggles; 30th Jun 2018 at 1:41 am. Reason: typo
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Old 30th Jun 2018, 6:57 am   #13
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Default Re: Push pull audio output transformers

For any valve audio amplifier I make these days, I mostly use Hammond output transformers. AES stocks them, there are at least 71 types, many are direct replacements for vintage Fender & Marshall types but others are generic in a large range of power options for push pull, or single ended use and they are very reasonably priced for the quality. They tend to be very well made and finished products.

https://www.tubesandmore.com/product...=Type%3DOutput

(I could also strongly recommend the Hammond range of universal mains input voltage & frequency power transformers. These run a very low core flux and low magnetization current, being highly suited to all audio, radio & TV work and they have got me out of trouble in vintage TV restorations too, where the original transformer's radiated field affected the CRT's beam).
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Old 30th Jun 2018, 6:57 am   #14
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Default Re: Push pull audio output transformers

They should work well!! I dont think they are Hi-Fi however!!
19 Henries primary inductance will definately be short on bass. at least at louder levels.
With sensitive speakers I think they will work OK.

They are however very much larger than the freebie I offered.

Joe
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Old 30th Jun 2018, 12:10 pm   #15
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Default Re: Push pull audio output transformers

I agree with Joe, the RS (OEP) transformers are not exactly hifi, probably more suitable for guitar amps. The primary inductance at < 20H is low and it is extremely unlikely that the primaries and secondaries are wound in sections (interleaved) meaning that the frequency response will be poor. Hence the low prices.

A quality output transformer will have a primary inductance of 60H+, to give good LF response and have multi-sectioned interleaved windings, to extend HF response.

The 'frequency response' curves given in the RS data sheets are rather meaningless - showing the 'response' to be virtually flat from 2.5Hz up to about 8KHz which, of course is only possible to achieve if the transformer is driven from a low impedance source (probably 50 Ohm from a signal generator), not the high impedance of the valve anodes that will actually be driving it..

If you want anything like HiFi at a reasonable cost, I would also recommend the Hammond transformers that are available from several outlets in the UK. Be careful if buying direct from the USA, you could get stung with a massive 'customs clearance' bill for duty, VAT & inspection charges. (Been there, got the T shirt!)

Adrian
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Old 30th Jun 2018, 4:37 pm   #16
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Default Re: Push pull audio output transformers

I think it all comes down to whether the OP wants a cheap and cheerful amp to play records on, or whether to spend quite a bit of extra money for something approaching Hi-Fi quality. I said in an earlier post that I used the cheapest RS transformer in a little low powered stereo amp and I was perfectly happy with the performance. The frequency response was about 50Hz to 20KHz at the -3dB points into a dummy load. Now I know it would probably fail miserably on an expensive test jig, but is that really the point? I was happy with it.
Alan.
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Old 30th Jun 2018, 6:47 pm   #17
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Default Re: Push pull audio output transformers

There's been some success in using multi-tapped "100V Line" transformers as push-pull outputs.

http://home.alphalink.com.au/~cambie...mp/6an8amp.htm

http://www.vk6fh.com/vk6fh/ECONOMICA...0AMPLIFIER.htm


These transformers have the big advantage of being cheap - and unless you've got 'golden ears' I suspect you wouldn't be able to hear the difference between one of these and an 'audiophile' transformer at 10x the cost.

The OP is requesting something through which to play 78RPM records, after all.... extended frequency-response either at the bottom or top-ends is probably rather less than desirable in this instance!
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Old 2nd Jul 2018, 8:44 pm   #18
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Default Re: Push pull audio output transformers

Hi Martin, you have a PM,

Ed
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