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  »  New  Ultimate HF output transformer?..  Leads to further air core heads...  Melquiades Amplifier  Forum     7  71465  07-14-2005
  »  New  Fun with transformers?..  Re: An amplifier is not a subject but a service.....  Melquiades Amplifier  Forum     7  65421  10-21-2005
  »  New  Entry level DSET Melq?..  Look outside the Lundhal if you need more current to dr...  Melquiades Amplifier  Forum     10  74342  01-25-2008
02-03-2005 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,540
Joined on 05-28-2004

Post #: 1
Post ID: 634
Reply to: 634
An ultimate transformer for narrow bandwidth?

It happens that I lately become heavily engaged a project that covered me from music-listening and the Morons-insulting person into a soldering and a constantly questionsasking Moron. Well, it is life.... :-(

I lately have some reasons to think about optimum requirements for SET’s out transformer if the amp handles juts a narrows bandwidth. For instance let pretend that we have 2 monoblock and each of them drive: LF <100Hz and HF >100Hz. So, would it be possible to optimize a specific transformer to push an envelop of an each reproduced range?

So, Chris, Thorsten, Dima, Joe or whoever “get” it, could you come up with some, preferably radical, ideas?

I wonder if an oversized trans with a lot and lot of iron in its core, with huge inductance of primary, with few turns and thick weirs will “do” on the LF amp. If for instance, the amp that does not handle anything under 1000Hz would be benefited if to make it with an air core?

Did anyone ever try it or hear about such an attempts? Are any seeds of an engineering wisdom that you might spread?

Thanks,
Romy the Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
02-04-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 2
Post ID: 635
Reply to: 634
Re: An ultimate transformer within a narrow bandwidth?

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Allo Romy,
Big topic this! I will write something suitably detailed when I get a chance this weekend, but you're right - the transformer is key and one that is optimised for the task in hand is absolutely the way to go and something that not many people seem to get. Particularly in the midrange and treble, there are things a narrowband device can do that would be impossible in a full range design.

The good news is that as far as transformer design goes, the 6C33 makes it easier than any other valve...

Anyway, more later...
02-07-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 3
Post ID: 639
Reply to: 635
Re: An ultimate transformer within a narrow bandwidth?
Alas, not much time to deliver the promised treatise...
So, tantalisingly, I will suggest this; for the range 1k up, you would have no problem getting a small mumetal OPT made suitable for the 6C33/S2 combination. Actually, I have something similar but designed for VV52 (300B on anabolics), which I was able to directly compare to a respected full range OPT based on Silicon steel.

No contest; the mumetal device took the sound to a whole different level; it was so much more delicate, ethereal and composed (not unlike an S2 or my ribbon tweeters in their range) that I rapidly came to the conclusion that full range OPTs and non-exotic core materials are a waste of time if one is after the ultimate.

Sowter in the UK can make you a pair to try - I can help you spec if needed.

Other possibilities include amorphous which I have only heard so far driving ESLs... that was the usual, iron based Metglas alloy but the new kid on the block is an exceedingly costly Cobalt based material (amorphous and not to be confused with the Permendur type stuff offered by MQ from time to time). A friend has directly compared both amorphous types in a PP 2A3 amp and said the amorph cobalt is astonishingly clear sounding; space between notes/instruments, easier discrimination etc. This was the same winding geometry/bobbin, core size, just different core material.

Which brings us on to the cheapest and (possibly, for this application) best core material: air. For a 6c33 at 1kHz upwards, in purely engineering terms, it's quite doable. I have some bobbins that measure (-3db) 600Hz to 50 or 60k when driven from 600 ohms; the 6c33 will give you a lower LF cutoff. Also, these have an enormous ratio of almost 100:1, again, the 6c33 into Vitavox S2 is a less compromised design so you may get more top end as well.

Haven't listened to them yet - no time to build the blasted amp at the moment, but in terms of frequency response, they certainly "work". I expect the sonics should be something to behold.

I will dig up details of the bobbins and windings etc...

For LF, I would probably be inclined to go for the biggest core I could afford on the iron based Metglas, overspec'd to keep the flux low (less core distortion) and high primary inductance (less tube distortion). Hell, maybe even go to a *monster* core on silicon-steel and accept a low treble rolloff.

Sorry this is so rushed; I still hope to write something more structured covering sizing of the core, core selections etc...I will also leave you with this snippet: interwinding insulation material may be as important as the core/winding geometry in the final determination.
02-08-2005 Post does not mapped to Knowledge Tree
slowmotion


Oslo, Norway
Posts 60
Joined on 07-22-2004

Post #: 4
Post ID: 641
Reply to: 639
Re: Dedicated trafos

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Hi all

This also a subject I am very interested in exploring.
Any pointers would be very welcome.

cheers Wink
02-13-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 5
Post ID: 651
Reply to: 641
aircore transformer design

chaps,
I found the design details; it was posted by on the Joenet a few years ago.
Basically, it involved making a vertically sectioned bobbin; start with a 70mm diameter cylinder of whatever material (I used white nylon cos it was available and cheap; teflon or something like maple may be the way to go), 78mm length.

Then cut 17 grooves in, each 3mm wide, separated by about 1.6mm of material. Groove depth of a few mm.

Primary is 9 sections of 34 swg wire, about 200-250 turns each, secondary 8 sections, 22 gauge, wound to the desired turns ratio (about 18 for the original design). All sections wired in series, with alternate winding of Primary/Sec/Primary/Sec etc

Now this gave about 200mH of inductance, which is about right for a tweeter application with low Z tubes. DC resistance (DCR) of primary was about 200 ohms, which is probably on the high side for a 6C33, so may want to experiment with thicker windings and more/deeper sections, although more turns will degrade top end response. I think with the 6c33 you may get away with it though.

Now, given that the 6C33 has such a splendidly low internal impedance, it seems a shame to go and wind a high DCR transformer. Particularly for bass duty - which is why I think an enormous core with many turns (!) of thick wire is the way to go. You need the turns for the inductance and to keep the core operating at a low flux level at LF (less core and tube distortion).

The flux due to an ac signal component is proportional to (primary ac voltage) / (core cross sectional area * number of turns * frequency).

more later....

02-13-2005 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,540
Joined on 05-28-2004

Post #: 6
Post ID: 653
Reply to: 651
Q: aircore transformer
Chris,
 
may I ask you about this air core transformer for HF? When it decays at the bottom then how sharp is it? Is it usable right up to the roll-of or it should to be "given a space"? Also, did you do any listening evaluations of this air-core mumbo-jumbo that you might share? 

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
02-14-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 7
Post ID: 656
Reply to: 653
Re: Q: aircore transformer

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Alas, no time for anything at the moment, the listening valuation must await construction of the amps (and speakers...)
Also, it is a very custom application - it is designed to drive an array of ribbon tweeters. Normally these have matching transformers on the back of them, to give an 8 ohm load from the ribbon, but I'll be ditchng those and connecting the ribbon elements in series and driving direct.

So the transformer is something like a 3k:0.5 ohm device and I won't have any real reference point. Well, I suppose there's the one ribbon running with the mumetal OPT. Guess this conflicts with the Romy dictum of "thou shalt not fix what you are not sure is broken", but hell, my gut tells me this is the way to go.

Anyway, the rolloff at LF will probably just be 1st order (the equivalent circuit looks like a simple LR filter). For a 6C33 it's complicated by the fact that the winding resistance is comparable to the internal impedance of the tube (thinking about it, you would want a low DCR transformer for the 6c33 otherwise you might end up dropping 50V across the winding...) and that resistance is distributed throughout the winding - not immediately confident that the rolloff is still exactly 1st order.

What I will do when I get some time spare is take some measurements using a 100ohm source; this will then tell you what the response of a similar design in Melquiades might look like.

There is no reason to stop you running the OPT in its lower rolloff; actually, in that respect its better than a cored device. Distortion will increase as the tube sees a lower load (think of very low LF - all the tube sees as the load is the primary winding resistance), but unlike a cored device, there is no additional distortion from the core itself overloading. So as long as you have plenty of "power headroom" in the amp output stage, you should be alright. With my planned ribbon amps, this is a deliberate feature; the amp will be bandwidth limited by an LR filter at the input (actually a tiny little input special input transformer I wound myself on some incredible cobalt amorphous material, -3db is in the MHz range with just a single layer winding) - this will give the primary 1st order rolloff at 1.5k or so. Then, the OPT stage will have another rolloff at ~600Hz which will give the ribbons added protection. Because the 1.5k cut is before the output stage, this means the amp won't see much signal in the lower LF range, thus the tube won't be too stressed. Oh, the input transformer means I can hang the driver stage on a negative supply and direct drive the output valve. No coupling caps... I suspect you could use the same approach on Melquiades.

Now, as to why I think the aircore approach is interesting; in any cored transformer, you have eddy current losses. These increase with frequency; basically, the changing magnetic field induces currents in the core, which sap the signal.

This is why cores are laminated and (theoeretically) the thinner the better; this increases the resistivity of the core (the lamination have a thin layer of insulation applied). I say theoretically, because I know people who have listened to 2 almost identical devices differing only in lam thickness and preferred the thicker stuff.

Anyway, it's one big advantage of amorphous, and why very high frequency transformers are made of ferrite (lots of tiny particles glued together - high resistivity).

Thankfully, the losses also reduce with flux, which goes inversely with frequency, otherwise we'd get not much power out of transformers at all at HF.

Nonetheless, the losses are still there. Also the permeability of the core is a function of frequency, all of which combine to produce something not quite ideal. I dislike the idea of a loss that varies with signal.

With the aircore, the only "dynamic" loss mechanism is dielectric, but most of this can be mitigated by using a thin walled tube for a bobbin, or even having a stack of self supporting coils.
Further, there are less potential mechanisms for low level dynamic loss.

Anyway, must run, I will see if I can get those measurments sorted this week and also have a think about how one might optimise the aircore design to be more suitable for the 6C33. I for one am curious as to whether the existing top end rolloff of -1db at 20kHz (with 600 ohm source) will improve.

cheers
cv

PS I listened again to that ORTF Scherchen Mahler #5 you recommended and it suddenly clicked; dunno why I didn't quite get it first time. It's terrific- many thanks for the tip.



02-16-2005 Post does not mapped to Knowledge Tree
Thorsten


United Kingdom
Posts 65
Joined on 12-06-2004

Post #: 8
Post ID: 657
Reply to: 634
Re: An ultimate transformer within a narrow bandwidth?

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Hi,

 Romy the Cat wrote:

I lately have some reasons to think about optimum requirements for SET’s out transformer if the amp handles juts a narrows bandwidth. For instance let pretend that we have 2 monoblock and each of them drive: LF <100Hz and HF >100Hz. So, would it be possible to optimize a specific transformer to push an envelop of an each reproduced range?


Absolutely. Making a transformer for < 100Hz would mean to use a huge core and a fairly large airgap to get more inductance at a given standing current. This makes winding the transformer with low parasitic capacitance and low leakage inductance a problem that is increasingly hard to solve as size increases (though I believe S&B has a solution to that problem now).

Equally, if you make sure the Amplifier for > 100Hz does NOT RECEIVE any input below 100Hz (or at least an input that falls off progressively), meaning a line-level highpass or one build into the interstage coupling, then you can make the transformer relaively small with relatively few turns as it does not need to handle LF anywya, which makes getting low parasitic capacitance and low leakage inductance much easier.

 Romy the Cat wrote:
If for instance, the amp that does not handle anything under 1000Hz would be benefited if to make it with an air core?


Yes, air core transformers can be made. Funny, I'm working right now on a pretty crazy speaker project where I want to use an aircore autoformer to act as both crossover choke and matching device for a ribbon tweeter.... ;-)

There was a website that covered Aircore transformers for Valve Amp's, no idea of the link though, it was Eytalian....

Ciao T


"It is to Madame Justice that I dedicate this concerto, in view of the holiday she seems to have taken from these parts." V
02-16-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 9
Post ID: 658
Reply to: 657
Re: An ultimate transformer within a narrow bandwidth?

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T wrote: "Absolutely. Making a transformer for < 100Hz would mean to use a huge core and a fairly large airgap to get more inductance at a given standing current. This makes winding the transformer with low parasitic capacitance and low leakage inductance a problem that is increasingly hard to solve as size increases (though I believe S&B has a solution to that problem now)."

Fairly sure it was a typo, but the bigger the airgap, the *lower* the inductance, all else being equal.
We need a large airgrap for a 6C33 due to the high current. The advantage is that the airgap linearises the BH characteristic of the core, so that it's perm is more constant (generally a function of level and frequency, particularly at LF).

If you confine the transformer to LF, you don't really care as much about parasitics (capacitance and leakage inductance) as these are things that affect the top end.

T's next point above >100 Hz was spot-on in my book.

cheers
Chris
02-16-2005 Post does not mapped to Knowledge Tree
Thorsten


United Kingdom
Posts 65
Joined on 12-06-2004

Post #: 10
Post ID: 659
Reply to: 658
Re: An ultimate transformer within a narrow bandwidth?
Hi,

 cv wrote:
Fairly sure it was a typo, but the bigger the airgap, the *lower* the inductance, all else being equal.We need a large airgrap for a 6C33 due to the high current. The advantage is that the airgap linearises the BH characteristic of the core, so that it's perm is more constant (generally a function of level and frequency, particularly at LF).


Nope, not a typo. Rather incomplete explanation. A much larger core with a larger (than common) airgap will still give a lot more inductance and will do so with better linearity.

The key "problem" (actually, I am not sure in conventional systems with Speakers having limited LF extension it is a problem at all) with SE Output transformers and LF is that at at LF the insufficient inductance and lack of core linearity lead to distortion much increased on the midband. We can fix that for an active system by making a transformer designed to not increase to at least down to 32Hz, at a price.

Ciao T

PS, my point of the problem not being one is best summed up as the "MaxxBass" effect, feed a 32Hz fundamental note to an Amp that will distort heavily at 32Hz (not enough primary inductance) and feed the signal to a speaker having not much output below 60Hz (Minimonitor?) and the subjective result will be one with a clear midrange and a subjectively unbelievable (for the size speaker) LF output. Take away the distortion below the speakers cutoff and the sound becomes "bass light" and "anemic".


"It is to Madame Justice that I dedicate this concerto, in view of the holiday she seems to have taken from these parts." V
02-16-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 11
Post ID: 660
Reply to: 659
Re: An ultimate transformer within a narrow bandwidth?

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Allo T
Ah, I think we are agreed on all counts.
The suggestion of low bass via the deliberate introduction of its harmonics is something that has been done with guitar amps since way back, even if it wasn't the original intention...

cheers
cv

02-24-2005 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,540
Joined on 05-28-2004

Post #: 12
Post ID: 676
Reply to: 660
An ultimate bass transformer?

Chris,

Since I got that mind-boggling result with “Melquíades” (will finish the thread about the Melquíades sound) I went for an “ultimate solution” for my installation: a “Super Melquíades” amp. It will be dual chassis (amp+PS) 3-chennal amp to milliamp my horns by one amp per side.

When I asked Dima for an absolute craziest specification for LF channel he come up with something ridicules like .5A, full power down to 10Hz at –1dB and so on. In other words he proposed some kind of monster that will not run over 3Khz but would be an endless emperor at the bottom octave.

My transformer maker looked at this said “Hm…” and then decided to try making it. I spoke with him today and he said that he almost met already the Dima’s specifications, furthermore…. he so love the result that he is planning to convert it into a commercial product.

Considering that phenomenal bass that “Melquíades” do now with it’s 9H of primary and with 21Hz of cut off I wonder what “Super Melquíades” will be doing with a transformer the handle a full power down to 10Hz, having a lot of spare current in gap and with the inductance of primary over 15H? I really would like to here THAT sound. If everything turns out to be OK then I will do in a month.

Rgs,
The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
02-25-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 13
Post ID: 677
Reply to: 676
Re: An ultimate bass transformer?

This will be very interesting indeed... that is an enormous airgap and will make for a very linear and unstressed hunk of iron. You may wish to apply similar tactics to the power supply (low DCR inductors) if you want to be really surprised...

Which reminds me, I must find time to measure that aircore.

Look forward to hearing all about it - I'm quite enjoying this vicarious little journey.

cheers
cv

02-25-2005 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,540
Joined on 05-28-2004

Post #: 14
Post ID: 678
Reply to: 677
...apply similar tactics to the power supply...

 cv wrote:
You may wish to apply similar tactics to the power supply (low DCR inductors) if you want to be really surprised...

What do you mean, Chris?

I usually use my input chokes 4-5 times of critical inductance and twice of rated current, so it is very far form saturation. For instance the Melquíades’ output stage sucks 200mA and it uses input indictor of 10H and .5A. It has DCR of 53R. I might use 1A rated choke with lower DCR but what would be an effect. As I understand the chokes present inductance at a given current. So, when I “undercurrent” the chokes I’m loosing the effective inductance in them. Perhaps I might for my 200mA load try 20H choke at 1A but what sonic benefits I would get if I go there?

Also I would like to learn one more thing. I am planing to have a transformer for the B+ of the output stage rated at 1A. It will give me a nice spare current for 3 output stages even if I run all MF and HF hannels with dual anodes. My question is: would it be advisable to have the LF channel running it's own B+ supply? Theoretically it is class A operation and the current draw is constant but still something suggest me that I might consider this options. What would you your take on it? A separate supply with over-current transformer and a huge indictor in the filter?

Rgs,
Romy


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
02-25-2005 Post does not mapped to Knowledge Tree
cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004

Post #: 15
Post ID: 679
Reply to: 678
Re: ...apply similar tactics to the power supply...
Well, I have little comparative experience of this cos the required chokes get huge and expensive, but some people I trust swear by low DC resistance chokes; they feel that this parameter is so important that they will sacrifice inductance to achieve it. Ie nothing to do with the inductance or current handling, (at least, not directly). I heard a system which utilised low DCR chokes that had a very unstressed and "non-frenetic" (or is it unneurotic?) sounding bass - didn't draw attention to itself in a bad way.

As for giving each LF channel its own supply, yes, definitely worthwhile. Class A is only constant current in PP! In SE, the current draw will be anything but constant. The last thing you want is the bass channel demands modulating the supply rails on the MF and HF channels... and a low DCR for the power supply inductors will mean that the 6C33s have a more steady supply to draw on. Think of it as bringing some of the benefits of regulation without the baggage.

If it becomes impractical, you can run everything through 1 big choke first (with low DCR) and make sure each channel has some good filtering after that.

Some guys I know run separate supplies for each *tube*, no less... whether this is worth doing really depends on the circuit topology and type/loading scheme of the input tube...I suspect that with 2 stages and that little *** valve upfront, little will be gained.
cheers,
cv
Page 1 of 1 (15 items) Select Pages: 
   Target    Threads for related reading   Most recent post in related threads   Forum  Replies   Views   Started 
  »  New  Ultimate HF output transformer?..  Leads to further air core heads...  Melquiades Amplifier  Forum     7  71465  07-14-2005
  »  New  Fun with transformers?..  Re: An amplifier is not a subject but a service.....  Melquiades Amplifier  Forum     7  65421  10-21-2005
  »  New  Entry level DSET Melq?..  Look outside the Lundhal if you need more current to dr...  Melquiades Amplifier  Forum     10  74342  01-25-2008
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