Output Transformers

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Hi there,

What is the model of the two OPTs ?

Can you PM the builder informations ? I'm searching an builder too.

Thank you

 

SACThailand makes "Silk" brand transformers that are suppose to be really good. They aren't really known but they do post measurements of their transformer compared with others, including AudioNote. The prolific DIY speaker builder Troels Gravesen is a big fan of them which is enough of an endorsement for me. Here are a few links for you to see.

http://sacthailand.com/Transformer_Output.html
http://sacthailand.com/Transformer_TestOutput.html
http://www.troelsgravesen.dk/TRAM.htm

I wish I could make a better recommendation between Lundahl and AudioNote but I haven't heard either. I hope my links help though.

 

You should ask @dsavitsk. His builds involve a lot of transformers, he even uses transformers for outputs of his solid state designs! I have seen him use Electra-print and Luhndahl and I wouldn't be surprised if he's tried AudioNote as well.

 

I've been on Glenn's list for almost a year now and still haven't had my number called.

I happened to pick up a used Glenn OTL a few weeks ago and have really been enjoying it. I have really cheap tubes in it right now, but have better one's inbound. My specific OTL unit has a larger transformer than what he typically uses, along with speaker outs, a driver tube voltage selector (6v, 12v & 25v) and a 4 pin XLR headphone out. It has enough balls to effortlessly power my HE-6, which surprised me quite a bit. It's a lovely sounding amp - very dynamic, effortless and noticeably more resolving than my Ragnarok, even with cheap tubes (Yggdrasil as a DAC).

This said, I am tempted to buy another one if get to the top of the queue at some point. I'd like to configure my current setup for orthos and try the Luhndahl transformer/C3g setup with the HD800, as there is a very small amount of transformer hum that can be heard if no music is playing and the room is silent. Not a deal breaker by any means, as the price I paid for the amp was very low, but it would be an additional refinement.

His 300B amp intrigues me as well. What headphones are you planning to use with it? I should maybe consider having him build one of those for me, rather than another OTL.

 

No, just some Raytheon 6as7g's (until better tubes arrive), as the intended use is actually for my HD800. I don't know the specs on the transformer on my exact unit, and I should probably contact Glenn about it. It would be interesting. I just plugged in the HE-6 for fun, expecting nothing. I certainly didn't think it would even run the HE-6, let alone run it well. The previous owner told me that 6336 would be ideal for orthos, so if I ordered another amp, I could move to 6336, but for the moment I'm concentrating on voicing it for the HD800.

It would be interesting to hear his 300B amp. Have you heard someone else's build already? Best of luck finalizing the build choices. I'm sure the result will be worth the effort and wait. Glenn does great work.

 

As far as other builds go, I think the 300b by Tom Christiansen would be a really good option.

http://www.neurochrome.com/300b-set-amp/

Someone at headfi even made a version for headphones. He really goes all out making regulators for everything.

 

I have not used AN OPTs. Their reputation has always been such that they were a bit overpriced for what they are, but I have no idea if that is deserved. If nothing else they are hard to get in the US.

I love Lundahls and use them all the time. I also use a lot of Cinemags these days. I have definitely used a lot of Electra-Print iron in my life, too, but sometimes getting what I want from Jack can be tricky.

More important that the manufacturer, however, would be to use the right sized and wound transformer. Unfortunately, OPTs designed for speaker use will not generally work as well for headphones as purpose designed ones as there are too many compromises. To that end, take a look at Sowter as they make OPTs designed specifically for headphones.

 

Hi MoatsArt,

I didn't listen to these output transformer, I didn't build any amp.
I have just read some topics at diyaudio.

AudioNote seems overpriced. Lundhal has solid reputation. But I think the best option is to make custom output, Silk can build OPT with several impedance. Also Automatic electric europe can do very good custom OPT and if you use interstage transfomer they can use Finemet material.
O Netic can build excellent OPT.


Thanks you for the information.

 

I'd say having 8 ohm and 32 ohm secondaries is a great option. If you have two 8 ohm windings then you can wire them in parallel to get 8 ohms or in series to get 32 ohms. As a bonus the 32 ohm configuration has a center tap which means you can have a balanced output which many headphones can use. Most speakers are 8 ohms anyway so you don't have to worry too much about 4 ohms and 16 ohms. The 32 ohm tap should be fine with high impedance headphones as well because this is a 300b amp that outputs 8 watts. That means even with the 600 ohm T1 headphones, you should have more then enough power to spare.

 

8 and 32 Ohm windings on the same OPT is probably not the best idea. Assume your speakers are 90dB @ 1W and you want to listen at 90dB. That means you need 1W of power which is 2.83Vrms on the output. Keeping the volume knob at the same place, that means you will be at 5.7Vrms on the 32 Ohm tap. Into 32 Ohm headphones like Grados that are 100dB @ 1mW, that's 130dB. That will make you deaf very quickly. Thus, for phone listening you have to turn the volume knob way down, which throws out signal. What that means is that when listening to headphones, you are adding gain, then attenuating all of that gain away, then reamplifying through the headphone amp. Gain is expensive in terms of noise and distortion, and you have basically created a worst case scenario.

That's just the problem with using the same OPT for both speakers and phones. In general, speaker OPTs are just a poor choice for headphones. They have to be built for high DC current, high signal levels, and high signal (AC) current. To do that, they need large cores, large airgaps, and thick magnet wire. All of those things degrade the signal which is the tradeoff for more power handling. It is why you don't see high quality 100W, or 1000W OPTs -- they basically can't be made. No matter how good a speaker OPT may sound with phones, a properly made one will sound better.

Take a look at the Lundahl LL1689. It is a superb transformer that will work for many headphone amp scenarios.

 

What is the right way to do it then? How does the EC Balancing act work for balanced outputs?

Is there a reason you couldn't use the 8 ohm tap for sensitive headphones like Grados and then 32 ohms for headphones that need power/voltage like the Audeze LCD, HiFiMan HE-6 and high impedance headphones(HD800, T1, etc)?

The impedance ratio is the square of the turns ratio of a transformer. So if you have a 4:1 transformer then the impedance ends up being 16:1. For an 8 ohm secondary this means that your transformer is 128 ohm : 8 ohms. Say your transformer is an 8192 ohm: 8 ohm transformer. Taking the square root would yield a turns ratio of 32:1. For a 8192 ohm : 32 ohm transfo the same calculation would give you 16:1 or 32:2 which is the same as saying 32:1+1. This basically means if you have two 8 ohm windings in series then you get a 32 ohms secondary while in parallel you get 8 ohms. Someone more knowledgeable than me can correct me if I am wrong.

 

Well the center tap in the 32 ohm winding doesn't have to be grounded and I fail to see how that is fundamentally different then one long 32 ohm winding.

But more importantly, I thought it was a safety hazard to leave the secondary ungrounded with high voltage tube amps. Are there ways to be sure that the secondary is sufficiently insulated from the primary and still have an effective transformer?

Edit: Is it acceptable to have a resistor between the tap and the ground? That way there is a path to ground in case of a short but shouldn't interfere with AC operation?

 

Transformers do not have an impedance. They have inductance. When we say that a transformer has, for instance, a 5K primary, what this means is that it has sufficient inductance such that it provides a 5K impedance at a frequency we care about. Impedance = 2*pi*F*L, so a 5K primary at 20Hz has ~40H of inductance.

If you load the 8 Ohm tap with a 32 Ohm load, that reflects as a 20K primary, but if we only have 40H, we will lose bass below 80Hz. One solution is to wind the transformer with sufficient inductance that it can be used as a 20K:32, but you run into several problems -- first, you need a ton more wire which raises the DCR of the transformer adding impedance to the system. Second, by adding all of those turns, you also increase the interwinding capacitance which rolls off the treble. It is also likely you'll need a larger core which reduces fidelity (and increases the amount of wire needed.) So in the attempt to make your transformer meet the specs you need, you made it perform considerably worse.

The second typical solution is to use resistors in parallel with the load. For instance, an 11R resistor in parallel with a 32 Ohm phone will appear to be an 8 ohm load which will reflect correctly. Additionally, it will soak up that extra gain which makes it seem like a good solution. Unfortunately, what it also does is make the amplifier work a lot harder to drive the more difficult load. That adds distortion and noise (and expense).

This is one of the reasons that parafeed and pushpull are good for headphone amps -- it is easier to wind good transformers (since they don't have the standing DC that eliminates inductance). The whole point of the single feed circuit is that it reclaims some efficiency since the transformer can swing above the power supply. But you trade off other things for the benefit. And with headphones where that efficiency benefit is not that important, it does not make a lot of sense.

It is inherently unsafe. It is an old audiophile canard that grounding the secondary creates some sort of sonic detriment. This is simply not the case. Transformers are pretty safe, but they can and do fail.

 

Well what about if I loaded the 8ohm winding with a 32ohm load of a parafeed transformer? Since the choke and transformer are in parallel then the resultant load would not be that much more then the ideal 5k even with a primary impedance of 20k. Would it still result in a higher bass cutoff?

 

I cant see how so called balanced headphone output is going to make a bit of difference. weather the transformer
is grounded in the middle or one end.
Balanced is for long runs of cable to stop hum and noise pickup the cable on a headphone is about 1 meter that is not going to pick up hum.
And I don't know of any balanced headphones or speakers except for electrostats.

 

There's no such thing as a "balanced" transducer in the first place, as balanced is a term that refers to circuit and transmission line aspects. The headphone driver is simply modeled as a load with two terminals. You can drive it with a differential signal (both ends varying with respect to the amplifier ground) or you can ground one side and drive it with a SE signal.

Estats are more complicated in that they require a bias voltage, but there's nothing "balanced" about them. Even if you were to drive only one end it would still respond but not necessarily net you the same level of performance. Estat amps drive their loads with a differential signal that varies with respect to the bias voltage ground, which might seem like semantics at this point but again is an important distinction.