Zero Feedback Impedance Amplifiers

[Susan-Parker]

Views and News

Hi everyone.

I see that this thread which I started on the 8th of October has passed the 28000 views and is now 6th out of 5890 threads for number of views in the solid state amplifier section (and seventh for number of replies).

It is a matter of some surprise to me the interest and level of discussion that has occurred and it is very pleasing to hear of people's progress with their versions of my design.

I do my best to answer questions as fully as possible but I am aware that in all the replies there must be some which I have inadvertently overlooked.

As it is the last few weeks I have probably been spending up to 50% of my available "work" time directly or indirectly on matters relating to the amplifier. And as I previously mentioned I have invested in a new sound card and software.

So a big thank you for your interest.


Good News

Despite apparent reservations about the use of inductors/transformers in audio amplifiers "I'm not in love with transformers, or any other part that has distortion, but sometimes they have their uses." (14th October 2004) see:

http://www.diyaudio.com/forums/showt...741#post491741

The one and only Mr Nelson Pass has now implemented these in his latest Zen amplifier series "Zen Variations 7: More fun with Son of Zen and SuperSymmetry".

http://www.passdiy.com/pdf/zen-v7.pdf

Inductive Loading (from page 6) has the Zen-V7T and for which Mr Nelson Pass now plots distortion graphs up to 30 watts (at 1KHz about 3%).

At last someone is mentioning the effective additional power rail generation "Because the coupled inductors can swing much higher
voltage than the supply – in an idealized case, they would peak at twice the supply."

I am sure Mr Nelson Pass will add about the preference of using bi-filar or multi-filar windings to get close coupling between windings.

Mr Nelson Pass also shows a version, the Zen-V7E, for driving electrostatics on page 12.

(As an aside I note that electrostatic loudspeakers were in use before 1930!)

Mr Nelson Pass comments that he finds the sound of the Zen-V7T "lively and musical".

So all in all most excellent as I hope that this will encourage people to have a go at trying out transformer based amplifiers given that you can build mine and if you so desire change to the Zen topology with a few dabs of the soldering iron and a handful of different components.

And if you already have a Son of Zen version you can try out a bifilar wound toroid transformer very quickly.

Enjoy.

Best wishes,
Susan.

[peranders]

Susan, you are obviously smarter than I because I can't figure out how you have made these searches about top ten list of views? How did you do?

Anyway has anybody tested Nelson's transformer amp stuff? I haven't heard much about Neslon's transformer amps.

[Susan-Parker]

Hi peranders,

Originally posted by peranders
Susan, you are obviously smarter than I because I can't figure out how you have made these searches about top ten list of views? How did you do?

At the bottom of the page for the Solid State threads there is a search line with various options to present the listed topics in different orders.

Showing threads 1 to 25 of 5891, sorted by [views/replies/etc.] in [decending/asending] order, from [the beginning/last year/etc.]

Just select the version you want and click on the GO icon.

Hey presto!

I have not included the Announcements and the Permanent entries as they don't move

Best wishes,
Susan.

[peranders]

I like complicated solutions as some of you already know. "Less is more" is not my style more like "more is more". So far (sorry Nelson) I have had no desire to build (but I don't mind listening if the opportunity comes) any Pass amps but this extremely simple amp is something which have caught my interest, just becuase of it's simpleness. Maybe I'll built a Zeus amp. I have all the stuff already.

[mikelm]

Originally posted by Susan-Parker
there must be some which I have inadvertently overlooked.

Hi Susan

I was wondering if this applies to my post 416 re b2 spice modeling of transformers and one or two other things...

I would be interested to hear you thoughts

mike

[peranders]

Originally posted by Circlomanen
DO NOT OMIT THE GATE-SOURCE VOLTAGE PROTECTION!!!!!
I Played realy loud and all of a sudden it was quiet. Both fets was fully open and it had 28 ampere current running through the transformer....

No good.

Me:
allrightythen....I was playing Ricky Martin and my amp developed some kind of allergy.( )

His amp is not optimal and we are discussing this at Hififorum.nu. My suggestion is that the transformer needs some interfacing towards the mosfets. Maybe the capacitance's of the mosfets together with the properties of the transformer that are casusing troubles?

[Upupa Epops]

To peranders : Don't take guys nice toy - you know, nine devices - it is losing faith.

[peranders]

Sometimes simpleness has it's price but I'm sure this amp topology can be made to withstand even Ricky Martin. If you don't know how to destroy an amp you don't know probably not how to fix it.

If you do have destroyed an amp you probably realize that something must be done.

[Susan-Parker]

Hi Mike,

Yes, sorry. My weekend has been quite busy and I am still playing catch-up. I was updating my website to make the audio stuff more accessible and domestic duties overtook me yesterday.

Originally posted by mikelm
Hi Susan,

I have just read through this thread with great interest and anticipation – This is just the kind of amp that appeals to my luddite tendencies.

Thanks. I have tried to keep the audio path as simple and direct as possible.

Outside the path that carries the audio I have no issues with as much complexity as might be desired.

Temperature tracking mosfet bias circuits with a (PIC/Atmel) processor and ADC/DAC setup is fine as far as I am concerned (as long as it is properly screened).

My power protection circuit (still finishing this off) has almost as many parts as the actual amplifier.

For those that want to know what this possibly cryptic and leading comment might be about here is a quick overview of what I have in mind.

The basics for this the circuit uses yet another mosfet in the power rail which is connected up as a form of semi-regulator and it has an SCR on the gate to ground which is triggered by a sense circuit from a load resistor in the supply rail (after the storage capacitor(s)).

In normal operation the mosfet supplies a semi-regulated voltage at a level set by a zener less the gate/drain drop i.e. a 37 volt zener would regulate out to around 33 volts using IRFP140/150 type devices.

The advantage of this is that one can have a little higher supply voltage which allows for mains variations and some of the power what would otherwise get dissipated in the driver mosfets gets to go into the regulator part.

When the over-current condition is triggered the SCR which is across the zener fires and clamps the mosfet gate to ground. (One also had to take care of the normal gate/drain zener protection for this to ensure that the mosfet gate does not go out of bounds.)

Power is therefore removed from the driver mosfets and remains off until the amplifier is turned off and on again (after a pause to let the main caps bled out).

I did not notice yet if you had an answer Sowter on the o/p transformers. I think would like the bi-filar version. As I would probably use them as chokes. Mind you the toroids also sound interesting. I wonder if anyone makes Bi Filar wound toroids these days.

It would appear that many of the standard main toroids now have bifilar wound secondaries "to get better matching when used in parallel".

Otherwise you can wind your own bifilar choke easily enough. Buy the 200VA transformer kit and two reals of enameled wire from RS. Strip off the mains windings from the bobbin and re-wind. I don't know what quality of steal is used for the RS transformer laminations - I have asked but they didn't know.

I have been speaking with Sowter but a they don't have the smaller transformer size in stock I am looking at a version using the 120 size laminations. However I have to wind one up myself to ensure that it all works as I intend and I have not yet had the time to do this.

Transformers from Sowter will be well over a hundred pounds each, so if you can do this yourself or use a toroid then that would be a cheaper option to get you going.

To recap the "Rules"

1. Use a transformer of four times the mains VA rating for the number of watts you want to drive.

I.e. For 50 watts audio use a minimum of a 200VA transformer.

2. For an off the shelf toroid transformer using the secondaries only chose the secondary voltage to roughly match that of the power supply rail you are using.

E.g. If you have around 34 volts DC supply then use a 2 x 30 Vac toroid.

For those of us using RS (they do do credit card orders) the following would be suitable:

RS # 422-5348 ( 225 VA, 2 x 30 Vac, 3.75 Amps per winding.) at UKP 22

or for an extra three pounds

RS # 257-5231 ( 300 VA, 2 x 35 Vac, 4.28 Amps per winding.) at UKP 25.50

The latter might be preferable if you were using higher bias levels.

Both of these have dual 115/230 Vac primaries, which give a little more flexibility, however the single 230 Vac primary versions are four or five pounds cheaper.

B2SPICE...

I have been modeling an amp with some similar features for a year or two in b2spice. I always manage coupled chokes and transformers by using inductive coupling – it’s a bit hit and miss as you have to adjust the inductance to effectively change the number of windings. Also leaving the inductive coupling at 100% can cause problems... as can perfect symetry – almost perfect works OK !

Here is one version of my design – not yet built cos I got stuck on the chokes !

I got stuck with the simulation as well. In practice the circuit isn't as hair trigger pass/fail as the simulation might suggest.

However good matching and coupling between windings is important, hence the multi-filar winding.

I will be very interested to try both your and my designs using as similar components as possible to see what audible effect the dreaded feedback has !!! As you can see this version has heavy feedback but I have simulated versions with source degeneration and higher open loop gain

As you have probably noticed I am not keen on feedback amplifiers to drive speakers. In my opinion one can use feedback all the way through, but not for the very last stage that is driving the reactive and inconsistent load of the speakers.

My amplifier topology gives under 1% distortion even at 35 watts. It would only need very light feedback to drop this right down, but would be a step away from the simplicity of a few bits of wire and a mosfet or two.

However AB comparisons are always good, and simple circuits that encourage experimentation can only lead to greater understanding and hopefully enjoyment.

I would certainly be interested in what you do find with this

Great thread....

thanks

mike

Thank you.

Best wishes,
Susan.

[Susan-Parker]

Hi peranders ,

Originally posted by peranders
Sometimes simpleness has it's price but I'm sure this amp topology can be made to withstand even Ricky Martin. If you don't know how to destroy an amp you don't know probably not how to fix it.

If you do have destroyed an amp you probably realize that something must be done.

Which is why I encourage the use of cheap IRFPXXX parts to start with as well as a sensible quick blow fuse in the power rail AFTER the smoothing caps.

Blowing up a couple of IRFP140s is irritating but wiping out 50 pounds plus of matched pair lateral mosfets would bring me to tears

Also there is not uncertainly as to where the failure is, and it is quick to repair (we are all using fuses or a current limiting bench PSU aren't we).

And yes, I did blow up several sets of devices in the early days so I do have the T shirt with a picture of an IRF150 in a TO3 case upside down with bent pins and a wisp of magic smoke rising

Best wishes,
Susan.