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Old 20th July 2003, 03:26 PM   #21
SY is offline SY  United States
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Commercial tube amps and preamps using switchers would include the various Berning designs, starting with the late-'70s TF-10 preamp. The TF-10 had quite a good reputation. Mine had a pretty good load of ultrasonic noise on the output, but otherwise worked well.

The supplies in these were pretty complicated, since the chip implementations were some years in the future.
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Old 20th July 2003, 04:03 PM   #22
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They now have to meet EMI regulations
Somethign that one should be very wary of, w.r.t. EMC / EMI compliance of both COTS and some well-known audio manufacturer's use of SMPS.

The EMC regulations are covered by various European EMC standards, and include a conducted emissions test, which measures noise from the equipment, conducted onto the mains, and a conducted susceptibility test, covering the sensitivity of the equipment to outside disturbance.

The standards comprise of limits, which vary in a frequency-dependant manner but which start at 150kHz!.

This means the noise levels at audio frequencies have no defined limits and are therefore open to severe abuse. Companies using SMPS technology can quite legitimately conduct high levels of noise, at audio frequencies, onto the mains, which can pollute the feed to other items.

There are exmples of very expensive items manufactured by well-known manufacturers doing just this. These items can (and do) have a marked deleterious effect on the sonics of other items using more conventional PSU's connected to the same mains supply.

Personally I feel it would require great efforts to acheive the same levels of performance in an SMPS as one could more readily acheive in a linear one. My suggestion is it would require the sort of efforts that are applied to the better class D and similar types of amplifier.

Andy.
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Old 20th July 2003, 04:18 PM   #23
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Hi Jeffrey J;

>>>...I was trolling the "Tubes" section here because I'm about to design and build my first tube amp; a modest project, befitting my experience with such devices - a headphone amplifier. Probably a 6DJ8/E88CC cascode input driving a concertina splitter driving two push-pull White cathode followers, but I'm not quite sure on the topology just yet. However, I am very open to suggestions at this point (I have 2 6DJ8, 2 6SN7 and 2 12AU7A tubes to work with, hint, hint)....<<<

Okay, why not take a look at the Optimised Morgan Jones Headphone Amp over at Headwize.com for a start. Given the complement of valves you have to work with, I'd keep the White Cathode Follower stock, with 6DJ8's at the optimised operating points. Then, I'd use a 12AU7 in place of the 6DJ8 input valve, one section per channel. I don't have the operating point data on it at my fingertips, so you'll need to find Vp, Rk, etc on your own there. But with a gain of around 17 to 19 on the 12AU7 you'll only lose a little voltage gain from the 6DJ8 and frankly it won't hurt that design a bit. Try it without feedback first and see what you think. I ended up adding a feedback loop to mine but it was more a case of needing less gain and being too lazy to redo the amp for a lower gain input valve.

One big suggestion would be to dedicate one 6DJ8 to the role of the 'top' of the cathode follower and the other to the 'bottom' of the cathode follower. This way you can use 2 separate 6.3V supplies, one of which can be floated up to the same potential as the cathode on the 'top' of the cathode follower. You need to make sure that you do not exceed the heater/cathode voltage specs. I've had troubles that way (live and learn....) and won't make that mistake again.

Good luck on your amp and all the best,
Morse
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Old 23rd July 2003, 12:58 PM   #24
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Originally posted by Johan_Ch

...
Do any one know of any resources on the net that would help in the construction of a switched power supply?

/Johan Ch
Poor guy - you started the thread but seem to have gotten lost along the way.

There are some excellent resources on SMPS design, but if you are just starting out, I highly recommend National Semiconductor's line of "Simple Switchers." These are chips that combine pretty much everything needed to implement a wide variety of SMPS topologies in one device (typically a 5 or 7 pin TO-220 package). Their web site also sports a very nice simulation engine that will spit out a full design given the input and output parameters. It's unbelievably good, if a bit limited in range (don't ask it for Vo > 200V!)

Online Power Supply Designer

Even if it can't design exactly what you want, it is still an excellent tool to show you how changing specific input or output parameters affects the rest of the circuit.

Unitrode used to be the absolute best source of info on SMPS design, but since they got bought out by TI, the quality and quantity of information available from them has practically disappeared (TI is notorious for the poor quality of their datasheets as compared to their peers).

Linear Technology makes some interesting, if a bit overpriced, chips similar to National's, and Maxim has long been a player in the lower power DC-to-DC conversion field.
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Old 23rd July 2003, 02:07 PM   #25
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Hi,

you may also want to check Power Integrations application notes. http://www.powerint.com/appnotes.htm
They are excellent for beginners, although they deal mostly with flyback topology. But flyback would be good topology for high voltage outputs up to 100W, and also has decent cross regulation between outputs.

Best regards,

Jaka Racman
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Old 23rd July 2003, 05:58 PM   #26
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Quote:
Originally posted by jeffreyj
Unitrode used to be the absolute best source of info on SMPS design, but since they got bought out by TI, the quality and quantity of information available from them has practically disappeared (TI is notorious for the poor quality of their datasheets as compared to their peers).

Linear Technology makes some interesting, if a bit overpriced, chips similar to National's, and Maxim has long been a player in the lower power DC-to-DC conversion field.

Most of the Unitrode PDF's can still be found on TI's website. As a long-time TI observer, it appears to me that they have made an about-face wrt the hobbyist market -- you can see this in the DSP and Microprocessor arenas where they are starting to sponsor contests once again -- they have also gone to greater lengths to expand communications with users -- I would say that in this respect they are catching up to Nat Semi and Analog Devices.

Linear's chips are expensive -- and distributed through Digikey which is no bargain -- but I don't find t his to be a problem relative to the other inputs -- mostly time in my case. (You can buy directly through Linear's webstore, up to a couple hundred pieces, however.)

Nat Semi AND On-Semi have PCB layouts for switchers right on their websites, btw.
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Old 24th July 2003, 02:00 AM   #27
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Johan Ch,

I forgot to include a reference in my last post about a National Semiconductor application note that has some useful and easily applied advice on laying out pc boards for switch mode power supplies:

AN1149

***

Jackinnj,

While it is true that TI has retained the collected wisdom contained in Unitrode's PDFs, it is no longer so easily and obviously accessible as it was when Unitrode was just Unitrode. I'm sure this has everything to do with TI's customarily Byzantine data organization and not any overt or covert effort to put their acquisition in it's proverbial place

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Old 29th January 2004, 04:43 PM   #28
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This is kind of an old thread but I will throw some new wood on the fire.

It appears one of the bigger reasons SMPS are not used much with tubes is that they are not available off the shelf and that they are a very difficult project for the DIYer.

There is now a commercially available off-the-shelf SMPS for smaller tube applications. I discovered it when I was doing research and looking at schematics for a tube preamp or headphone amplifier.

It is available from lighthouse electronics. Do a search in google for "smps tube" and you will find it.

Some of the stats include:

Power Rating: 30 Watts
Line Input Voltage: 90 - 130 VAC
Plate Voltage: 275 VDC/40mA
Filament Voltage: 12.6 VDC/1000mA
Regulation: 0.8% or better
Switching Frequency: 55kHz

Power entry filters and enclosures are supposed to eliminate EFI/RFI

As a rookie I am intrigued by the idea of having a fully assembled and tested power supply at a reasonable cost. It would allow me to concentrate my attention on the amplifier circuit. I am hesitant about other factors mostly the whole solid state thing. Is the switching speed high enough? Is the regulation enough? Will the amp sound 'liquid' or 'flat'?

What do you think?
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Old 29th January 2004, 10:46 PM   #29
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Hi Jeffreyj.....

Now to the difficult one..

I believe there is a Conrad Johnson tube amp uses a SMPS unit but perhaps..... not....sure which model type.

The purpose of using SMPS is to better the conventional performance and spec compared to using a normal mains tranny +cap rect input system.

My 350W tube amp uses a hard switching PFC boost converter f=70Khz B+ 485V 2A ...with 650VA mains tranny and low volt sec as isolating.....The 97% eff is real bonus esp for hi power fixed bias op......too many other advantages to list......

As with all SMPS, aspects of EMI filtering on ajacent equipment cause S/N related problems. Dealing with chassis induced charge-injection-noise from semi's and radiated crap from cables are the worst contenders. Bear in mind typ tube amp min S/N level spec of -60dB down from full o/p; and also the Hi Fi tuner probably has a stereo sensitivity of 1-5uV limiting + good AM rejection are very low levels.
The only reliable design concept is to put all into biscuit tin within the amp metal chassis with cap feed throughs, ..I found it took more space up than using a mains tranny plus the guts that go with it.

Any DIYer really wanting to make a high/low wattage SMPS won't escape the need for some test gear and experience. You'll going to need both... unless one has access to either..... there's no way out.

<< If you design a PSU with discontinuous SMPS flyback topology......you will first hear it on the tuner......in front of the signal >>. Design with continuous or average current mode.

Those wanting the Unitrode data books (reprints) best contact Magnetics div of Spang who I believe have reprint rights.

Juggling the frequency v.s core size, Pout, voltage, is all part of the magnetic design process.

Vīva SMPS!

rich
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Old 29th January 2004, 11:23 PM   #30
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Quote:
Originally posted by richwalters
Hi Jeffreyj.....


The only reliable design concept is to put all into biscuit tin within the amp metal chassis with cap feed throughs, ..I found it took more space up than using a mains tranny plus the guts that go with it.

Vīva SMPS!

rich
The "biscuit-tin technique" is used and described by Linear TEchnologies on their site.

I wouldn't be surprised if they used one of those older TEK medical application scopes -- although it looks like they feature a TEK 545 !
Jack
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