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step-up transformers connected in 'balanced'

the thin end of the wedge

step-up transformers connected in 'balanced'

Postby tresaino » 02 May 2009 07:59

Some of you may remember the thread from last year on why there are only so few balanced phono preamps available on the market: https://www.vinylengine.com/phpBB2/viewt ... no&start=0. Well, the same is obviously true for step-up transformers – an even smaller niche market compared to phono preamps. Allow me to start with a quote from the previous thread:
tresaino wrote: What I'm currently exploring is whether we can change step-up transformers from RCA to balanced (both in and out). Some people say that step-ups are also inherently balanced and therefore this is not a problem, however it has been said before that cartridges are 'differential' but not a true balanced device - as for that one would need a center tap on the cartridge coil.

Steerpike_jhb wrote: Absolutely you CAN! The only thing you need to pay attention to is to ensure symmetry throughout. A cartridge is as balanced as you can get. Centre tapping just causes problems and asymmetry.

It has taken me a year to get there, what you see/don’t see in the crappy box on top of my preamp are Lundahl LL1931 step-up transformers, connected in 1:16 and in ‘balanced’ mode.

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I actually suspect the Lundahls are not connected in truly balanced but rather in differential mode, as I can see no third wirings on the XLR connectors or going to the ground tap on the back of the box. Useful information about the LL1931s and how to connect them can be found on their website: http://www.lundahl.se/pdfs/datash/1931.pdf

My original plan had been to rewire the Bent Audio step-up transformers from RCA to balanced mode, but John Chapman from Bent Audio responded that on the TX103 secondaries there were only 2 (and not 3) wires, therefore he could not rewire for a “true balanced” mode. He was obviously aware that cartridges do not have 3 wire balanced connections and are ‘differential’ but not a true balanced device, however he said he still felt better if the step-up transformer was connected in true balanced rather than in differential mode. The story went on and he was supposed to build a new pair of MUs in balanced mode for me, but in the end it didn’t work out and we lost contact, so I started to explore different avenues and ended up with Lundahls.

I won’t comment on the quality of sound here, as other people compared the Bent TX103s with the Lundahl LL1931s before me. Except to say that they are indeed extraordinarily transparent and that I am not looking for anything better.

What I want to comment on is the connection in ‘balanced/differential' mode and the resulting absolute, total silence – no hiss or hum whatsoever! Absolutely nothing, there is no difference in noise if I switch from the CD input to the phono input on the preamplifier. I have been playing with step-up transformers for quite some years by now, I love them but have to admit that all of them gave me at least some hiss or hum, particularly when I cranked up the volume or if the step-up ratio was on the higher side. Not any more, these Lundahls are as silent as it gets.

I don't think this was ever discussed here properly, but step-up transformers connected in balanced/differential mode are a small revolution to my ears. :wink:
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Postby tresaino » 02 May 2009 08:03

d to the g wrote:So, did you use the ground wire as your common ground? Can you give us the schematic that you used for connections?


I gave the camera away to a friend for the weekend, however I still have an old picture on my computer, it shows the awful soldering job that the shop had done from where I bought the Lundahls. The transformers have in the meantime been resoldered, but we kept exactly the same connections that you can see here.

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A comment on the picture: you will notice that on the “XLR in” side three wires do indeed go towards the transformers, however from them to the “XLR out” connectors there are only two wires. You may also notice that originally there actually was one ground wire to the ground tap, we cut if through as a friend of mine thought it made no sense - it was only wired on one of the two transformers! Anyway, the trannies remained silent as a mouse also without this one-sided ground wiring, and the new soldering was finally done without this extra ground wire.

I’m not a specialist on technical issues. Have they been wired correctly, as specified in the schematics on the Lundahl website? You can compare by clicking the link to the schematics that I provided in my posting above.
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Postby tresaino » 03 May 2009 07:59

Was the hum noticeable at normal listening levels single ended? I mean, between cuts or before and after the music, or did you have to turn it up a lot for it to hum?

sorry I screwed this up initially as the thread should have been launched here and not in the turntable/arms section. To your question, with step-ups in standard RCA the hum was usually noticeable between cuts or in relatively quiet moments, as you say. I did not have to turn the volume up a lot to hear it. I realize that step-ups can be relatively silent also in conventional mode, but the problem simply disappeared now. For info, I believe in my setup the difficulty with noise has to do with the inverter that I’m using to drive the motor of the EMT 930 – the little Siemens inverter is hidden on the floor behind the records.

Aren't transformers inherently balanced and have no need for a third wire?

Dave, I think they are - just like a phono cartridge.

From what was said in previous threads I understood they are “differential” rather than “true balanced”, but I hope the electronics specialists in this forum can add something to the soup.

I believe this sentence from the Lundahl datasheet above is critical: “If phono preamp is unbalanced connect pins 9 and 8 to input ground”. In other words if you have a balanced phono stage, which is my case, pins 8 and 9 should NOT be connected to input ground. I believe this is why there are only two wires coming out of the transformers, not three, as in the picture.

Please compare this to Bent Audio’s John Chapman assessment above: “He was obviously aware that cartridges do not have 3 wire balanced connections and are ‘differential’ but not a true balanced device, however he said he still felt better if the step-up transformer was connected in true balanced rather than in differential mode”. Chapman did not say why he felt so, but from a non-specialist perspective I would say that Bent Audio and Lundahl have a different way at looking at this. Who can shed some light on this?
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Postby soulmerchant » 03 May 2009 22:01

I think one of the best things you did was to get away from those terrible rca plugs. Any residual humm would then only likely be from:

1. Phono amplifier - often due to poor grounding plan on circuit board. :roll:

2. Lack of shielding somewhere.

The statement about the 'balanced' phono amp is correct. If your phono amp is really balanced then you could get away with floating the transformers. They would need no ground connection. A lot of mic transformers work this way. BUT - they would still need good shielding, and that must be separately grounded to avoid hum.

Nice to hear about someone likes the Lundahls. I was always afraid to buy them because of their small size (guess I'm a bit 'old school'). The best transformers I have ever heard were very BIG...
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Postby tresaino » 04 May 2009 10:51

Quick reactions in sequence. Dave, I provided the link to the Lundahl datasheet in my first post above, please take another look. And John Chapman’s quote comes from an email exchange, not from a Bent Audio article.

To : I did move single ended transformers around all my life :lol: , and did indeed find some spots were I was almost hum free. Almost. In balanced the problem simply disappeared – a huge advantage. As you know, finding the best position for a single ended transformer can be difficult, as the placement of electronics and cable length affect one’s choices.

a phono cartridge is an inherently balanced device, just like a microphone. It's a matter of hooking it up in the right way. A suitable ground must be used for the third wire. I think the same would apply to a transformer.

soulmerchant wrote: If your phono amp is really balanced then you could get away with floating the transformers. They would need no ground connection. A lot of mic transformers work this way. BUT - they would still need good shielding, and that must be separately grounded to avoid hum.


And to both and soulmerchant: this is the area that I still don’t understand fully… what do you mean when you say a “suitable ground” must be used for the third wire? And to Soulmerchant, is shielding separately grounded according to the wiring suggested by Lundahl? My impression is they are currently floating as no ground wire follows from the transformers to the XLR connectors, probably another proof that my phono preamp is indeed really balanced.

Any further views and advice from you would be welcome.

PS: having total silence with step-up transformers has another huge advantage, I’ll come back to this.
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Postby tresaino » 04 May 2009 21:08

Dave you are right, if it doesn't hum no need to worry. Soul to our pieces, ahem, peace to our souls!

And to , let me just add a piece of information from an email exchange with the phono preamp designer, from some time ago (not sure I shared this before). He had talked to a friend in the professional world, who confirmed to him that "a balanced generator - such as a cartridge - does not need central tapping, because this is done correctly in the differential input – in our case the phono preamp. However, in order to work properly, the generator/cartridge must not have any connection to the ground of the arm or of the turntable, nor to any metallic screening parts of the cartridge. Therefore, all the cable from the cartridge to the preamplifier must have the active + and – leads independent from the ground and from the screening of the cable." So floating all the way through should be valid, also in case there is an additional step-up transformer in the chain.
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Postby tresaino » 05 May 2009 13:15

I have re-read all your responses, thanks for the advice to all of you. I think at some point I will at least try to completely disconnect the ground wire coming into the transfos, and then listen if there’s any difference in terms of silence. Sounds weird, I know..

Soulmerchant, I don’t know whether size always matters with step-ups. Since your are “old school”, think about the tiny transfos built into the headshell of a SPU G/T. Otherwise you have my sympathies, particularly when it comes to turntables and tonearms. :D

Another comment: discovered that there is a new Lundahl step-up available now, the LL1933. Slightly bigger and heavier, otherwise same step-up ratios of 1:8 or 1:16. The main difference seems to be the nickel lamination core with the 1933, while the 1931 has an uncut amorphous cobalt core. The company says “for those who prefer a low distorsion, linear magnetization curve”. However the frequency response is about the same, not sure one can hear a lot of difference..

Finally, here’s the other important advantage of a balanced/differential setup: since there is no hum in the first place, you can increase the volume on your normal preamplifier and still avoid hum. This means that the relationship between the gain of a cartridge and the step-up ratio of a transformer is much less critical than in conventional single ended setttings.

An example: I have a Fidelity Research FR1 Mark II, it has a very low output of only 0.1 mV. Some people believe this cartridge needs a step-up transformer with a ratio of at least 1:30 or better 1:40 to perform properly. But since there is no hum also at much higher volume levels, I can easily compensate for the low output of the cartridge by moving the volume knob on the preamp to the right. The FR1/II together with the Lundahls set to 1:16 delivers only 1.6mV into the preamp, but this is no longer a problem. I tried the FR with different step-up transformers in the past, and never managed to play it noise-free at normal listening levels. Now it works. 8)
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Postby tresaino » 05 May 2009 20:17

Peanuts. I think we're talking around 350 euros per pair for the 1933s, and some 40 less for the 1931s. Think about the inflated prices of some Japanese vintage stepups or, even worse, some new stuff available out there that goes for 5 to 10 times higher.
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