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Old 19th March 2003, 04:12 PM   #21
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The circuit below shows a (simplified) equivalent circuit of a real transformer.
Rc is the series resistance of the primary.
Ll is the leakage inductance of the primary.
Lp is the inductance of the primary (the ideal transformer has infinite inductance).
Rp represents the losses in the core.
Cd is the capacitance of the primary winding.
The secondary will also have series resistance, inductance, leakage inductance and all these parameters can be measured with respect to the secondary windings as well.

With this definition of terms out of the way:

Large transformer often sound better. The larger cores do not saturate nearly as easily during the peak current demands. Jocko and I have measured this and noticed a subjective correlation between transformer size and sonics. Even for low current circuits like preamps, large transformers tend to sound better. A small resistor in series with the secondary windings to limit peak current often improves the sound of smaller transformers, which tends to confirm the saturation theory. This can be seen looking at the transformer's secondary voltage waveform with a scope.

Paul McGowan discusses the topic of large transformers in a Stereophile interview. The subject is on the first page of:
http://www.stereophile.com/showarchives.cgi?222

Toroidal transformers can sound fine but they do not tolerate DC and low frequency garbage that tends to make them saturate and mechanically buzz. It is not that hard to ADS couple the primary windings with back to back filter caps shunted by diodes to limit reverse bias voltage on the capacitors to a diode drop. Jeff Rowland was doing this 10 years ago as I recall. Toroidal transformers are also designed for high flux density for greatest efficiency which makes them more sensitive to the previously mentioned. I know of one designer that has his custom transformers made with lower flux densities.

Different size and type transformers have different leakage inductance for the secondary windings. The combination of this inductance with rectifier capacitance can form a high frequency resonant circuit creating RFI noise that your amplifier may not like. The use of 1000uF caps with no high frequency bypassed can make this problem even worse since this resonant circuit may be at a frequency above the point were the 1000uF cap becomes inductive again. Paralleling the secondary winding with a capacitor will form a lower frequency resonance with a high Q that can helpful or harmful dependency on the resonant frequency. It is possible that this cap may also provide increased diode switching noise since diodes are discharging the stored charge in the cap as they turn on (and off as well, from the reverse recovery currents) This low impedance cap provides a much lower source impedance that the leakage inductance and winding resistance of the transformers secondary windings and will allow higher diode currents. Winding techniques and core shapes can also provide different degrees of coupling of RFI noise on the AC line to your amplifier/preamplifier circuits and form resonant circuits that ring when excited by RFI from the AC line or diode bridge.

In the course of designing an amplifier with a 60 Hz oscillator to create a clean 120V power source for low level circuits, I found something interesting. The various step up transformers used on the output of the amp sounded different. This was even though the transformer was outside the amp's feedback loop for and did not effect the stability of the amp. The best sounding transformer in this set up was an output transformer from a 200 W Conrad Johnson tube amplifier! The higher bandwidth and better core material may be a factor since the charging current for the diode bridge and capacitor supplies being powered, contain frequencies much higher than the 60 Hz of the AC line voltage.

I don't suppose it surprising that different transformers sound different in light of these and other factors. There are circuits that can minimize these problems whose design should be optimized for the particular transformer used.

It's excitation current not "excitable current"

http://pemclab.cn.nctu.edu.tw/W3elem...er/tsld022.htm
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Old 19th March 2003, 04:58 PM   #22
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fred, i'm impressed - again.
thanks again for bein' so kind to let your knowledge get to us....!
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Old 19th March 2003, 05:54 PM   #23
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Default Re: Re: What we need most in an Audio Power Transformer

Koinichiwa,

Quote:
Originally posted by Peter Daniel


Would it be advantageous then to use 115/230V transformer with primaries in series with 115V? Providing of course that secondaries meet the load requirements in such setup.
I would think so.

I have not had the chance to extensively test such setups due to 240V AC being standard here and 220V/230V in Europe.

When I had Mains transformers custom made for my large PA Amp's in east germany in the 1980's (on LL cores) we found that with around 20 - 30% more turns the transformers ran with much less vibration and heat and seemed more efficient. Given the cost of doing that in copper in those days it was a major piece of budget even in these Amp's.

For people outside the 100...120V Mains voltage area the requirement would remain for dual transformers, using dual transformers with 115V+115V primaries would allow a simple change of voltage between continents AND it would allow also the balancing out of parasitic leakage capacitances to the chassis.

Using then dual winding full wave rectification is simply a question of specifying the correct higher secondary Voltage. As an example use 35V+35V to give appx 18V+18V AC in the "low flux" connection, just right for gainclones.

Sayonara
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Old 19th March 2003, 06:03 PM   #24
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Fred Dieckmann

Ditto! Welcome aboard Fred. Okay, it seems like you and Kuei are in agreement on the lower flux core density as being the primary quality to look for in a good sounding transformer. I will restate the order of importance as I understand it so Kuei, feel free to correct me:

1. lower flux core density

2. DC blocking ability

3. low leakage (measured by reading the Aq)

Furthermore, these attributes, in the order presented, will be relevant in evaluating all types of transformers and are universally accepted as the way to a good power supply.

If we are all in agreement, we can begin assessing a weight of importance to each of these attributes and we can begin exploring economic ways to correct their deficiencies. I am quite ethusiastic about what will be discovered in this thread because of the eclectic nature of its readers and the multiple real world experiences of our members. I will let a few days go by for you guys to think about other factors that you may have left out. One that comes immediately to my mind is the wire gauge used to construct the transformer. This may be obvious, but its the kind of thing that shouldn't be assumed as a constant. I'm sure there are other things that we haven't considered and I expect a few days of rumination will bring them out.
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Old 19th March 2003, 06:07 PM   #25
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Koinichiwa,

Quote:
The transformer in a SS amp will make up 30-60% of its cost and is probably the best place to start.
Sure. I merely pointed out that doing it "right" (I am investigating this currently in detail WRT Valve Amplifier transformers) is quite costly and stops you from being able to use "commodity" Transformers.

Yes, you get a better transformer, but at a very significant cost increase. I think most people rather will add a handfull of components worth a few bucks and double or quadruple the number of used "commodity" transformers as this will still be cheaper than have the theoretically near ideal part made.

I think having a simple universal "conditioning" PCB (or at least a suitable circuit) available which can take the needed diodes and capacitors to block DC, the suggested series RC snubber on the primary to reduce and damp ringing, the suggested common mode LC filtering (good standin for not having electrostatic screens) would be of quite good value to a lot of DIY'ers.

Then perhaps comparative testing between different parts in critical positions (especially CM Chokes and small filter Cap's to go with these) would be helpful too.

Then one can also look at the rectification issue and try the various options using bridges, full wave rectification, seperate sections or centertapped and so on.

The result would perhaps be a "best practice" FAQ for handling the way power gets into our gear. This way a lot of expensive power conditioners can become reduntant, simply because gear is engineered to work well from the start.

Sayonara
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Old 19th March 2003, 09:08 PM   #26
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DC blocking ability is not a function of the transformer but is achieved but a capacitor circuit in series with the primary winding

"3. low leakage (measured by reading the Aq)"

This is not a feature on which to base merit as larger transformers can have larger excitation current due to lower primary inductance. Low flux density though, can be a good thing and leads to less distortion and mechanical hum and buzz on noisy AC lines.

I have used the split primary 220 volt connection run on 120 volts for several high bias amps and I think it is worth doing. The transformers have less mechanical hum and buzz in this mode. There no reason not to try using two transformers with both the primary windings in series and secondary windings in series. Make sure the secondaries are wired with the correct polarity by measuring the open circuit voltage before applying a load to the secondaries. The wrong polarity will not hurt the transformers but will give a very low voltage. This will double the VA rating of the single transformer with the trade of twice the winding resistance. This can be potentially beneficial by lessening the peak diode current in the rectifiers. I have seen small resistors added to the secondaries in even supplies for power amps. These resistors are used in the output stage power packs
sold at: http://www.schuro.de/preisl-ntke.htm These very good designs for diode bridge/rectifier cap circuits other than the fact one should add series resistors to the snubbers caps. I recommend looking at the http://www.ub-elektronik.de/download/ntke.pdf document.

A lot of these techniques can be use to get the desired voltage from surplus transformers. Some transformers split the primary into 100 and 20 volt windings to use with 100 volt or 120 volt mains differing connections of these windings can give an option of several secondary voltages. I have even used the two 100 V windings in series and added the 20 windings in series with the secondaries. You must observe the polarities though and connect series windings for highest voltage. I often drive one of the windings with a signal generator to get the turns ratio I want between primary and secondary you can adjust to output level of the generator to get 11.5 on the primary and multiply the measurement secondary voltage by 10 to determine the voltage when the primary is driven by AC. This lets you use lower voltages that the signal generator can produce and lets you measure things without using lethal voltages.

MAINS VOLTAGES CAN KILL. USE THE GREATEST OF CAUTION AND DO NOT WORK WITH THESE VOLTAGE IF YOU DO NOT FEEL YOU ARE QUALIFIED.


*And, gentle Puck, take this transformed scalp
From off the head of this Athenian swain;
That, he awaking when the other do,
May all to Athens back again repair
And think no more of this night's accidents
But as the fierce vexation of a dream.

A Midsummer Night's Dream- William Shakespeare
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Old 19th March 2003, 09:51 PM   #27
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The main disadvantage of running primaries in series with 120V is lowering rated power of the transformer. It has nothing to do with a core but with the gauge of secondary wire. The ga. is calculated to deliver current in accordance with expected voltage on secondaries, so running secondaries at half voltage requires increasing the gauge of the wire to achieve the same power rating, yet the gauge doesn't change and effectively one can expect lower power because secondaries have hard time with delivering enough current.

I got that info from Plitron technical dept. when I was investigating that, when building Aleph X. So it is advisable for that setup to either get higher than required power of the transformer or as for bigger gauge on secondaries.
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Old 19th March 2003, 10:15 PM   #28
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Fred Dieckmann
Quote:
This is not a feature on which to base merit as larger transformers can have larger excitation current due to lower primary inductance.
Well, yes but it can still be valid if we use the value as a percentage. For example; to judge two identically rated transformers that have different readings in Aq. If we agree that excitable current can effect the sound of a transformer, then we must allow for the possiblility that less Aq will sound better than more. I don't know this to be true, it very well may be vice versa or a non issue but I think that it deserves exploration. Lets say one 1kVA toroid has Aq=200mA and another has Aq=50, why couldn't these be compared and evaluated?
Quote:
DC blocking ability is not a function of the transformer but is achieved but a capacitor circuit in series with the primary winding
I don't know that this is true. There may be some transformer manufacturers that build the cap into their transformer winding. The point is that it is an issue with transfromer performance so it was included. Why haven't you commented on the wire gauge used in the construction of the winding? If we are going to use an amp that uses a total bias of 20A, what is the optimum thickness of its winding wire to get the best sound?
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Old 19th March 2003, 11:00 PM   #29
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Default X-formerfiles.

Hi,

Quote:
There may be some transformer manufacturers that build the cap into their transformer winding. The point is that it is an issue with transfromer performance so it was included. Why haven't you commented on the wire gauge used in the construction of the winding? If we are going to use an amp that uses a total bias of 20A, what is the optimum thickness of its winding wire to get the best sound?
While windings do have capacitance I don't know of any xformer with a built-in cap.
I would not make too big an issue out of it though.
Normally and unless you specify otherwise, the manufacturer uses a chart to define AWG with respect to VA.
Other factors he'll keep in mind are enamel voltage insulation for HV xformers, temperature and humidity insulation.

Obviously for a 20A rating you're not going to pick a wire thickness that can just about carry that amperage, if you want the xformer to run relatively cool you pick the next AWG size.

There are many more factors coming into play such as core quality, mechanical stability and so on.

To me, a good xformer is cool running, doesn't exhibit any mechaincal noise, is able to respond quickly to current demand and is basically self-regulating.

A decent powerxformer manufacturer is a major key to success, be that commercially or personnally.
While I would agree that a good transformer is expensive, if however sound quality matters than it should matter to you too.

From experience, I never heard of an underrated xformer sounding better than an overrated one, unless perhaps they were of greatly differing quality.

Lets never, ever forget that an amp is nothing more than a modulated PSU. (I'm simplifying for simplicity's sake).

Strange as it may seem I often find that tubeamp PSU xformers are far better made than the xformers used in solid state amps, probably because the tubeamp xformers are often made the old school way.
Also, keep in mind that good core material for an OPT is NOT what you want for a powerxformer, quite the contrary and this is the problem of most toroids...they're bandwidth lets all the PSU crap through, in both directions.

So what's needed is a very poor bandwidth xformer, in fact if it wouldn't let through more than a couple of hundred Hz we'd be fine.

Cheers,
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Old 20th March 2003, 12:11 AM   #30
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"So what's need is a very poor bandwidth xformer, in fact if it wouldn't let through more than a couple of hundred Hz we'd be fine."

I don't think I agree. I believe the frequencies of the charging current waveform are higher than this. Limiting the amount of EMI trough the transformer would be nice but the high leakage or high frequency core losses or leakage inductance to achieve this can bring a whole set of new issues to deal with. On my power amp oscillator set up to generate clean AC, the tube output transformer sounded better than the transformers design for 60 Hz.

"problem of most toroids...they're bandwidth lets all the PSU crap through, in both directions."

This is a function over the core geomentry which have lower leakage inductances. It is not a function of the core material.
IE transformers have greater bandwitdhs than you would suppose. Alot of what come through from the line is RF coupling
though interwind inf capacitance. Spilt bobbins minimize this capacitance but have higher leakage inductance than transformers with the secondary windings wound over the primary windings.

"that excitable current can effect the sound of a transformer"

Once again the term is "excitation current" and it is a function of the primary inductance and core losses. This current in the primary also changes with load current and the primary currents due to changing load currents with signal are much higher than the unloaded excitation current. Two transformers that have different excitation currents have different designs and several other parameters that are also different. It really doesn't make sense to talk about the excitation current in isolation from the other design parameters, since changing it, changes several other design parameters at the same time.

"The main disadvantage of running primaries in series with 120 V is lowering rated power of the transformer. It has nothing to do with a core but with the gauge of secondary wire. The gauge. is calculated to deliver current in accordance with expected voltage on secondaries"

Losses in transformers are also related to core losses. The gauge of the secondaries (and primaries) determines the power loss due to IxIxR or so called I squared R losses. These power losses are a function of current not voltage. Don't forget we are talking quality not quantity here. Some additional transformer losses are not a big deal and we are most often are not running a transformer at maximum capacity. Transformer designers are trying to design for the maximum efficiency for a given core size and getting them to design for low flux density and the resulting advantages is a foreign concept to them. My designer friend had a major battle trying to get transformers with low flux density designed since the most important goal is high efficiency. Just as in class A amps efficiency is not the most important factor. What is a disadvantage for efficiency maybe an advantage for sonics, just like in amplifiers.
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