Transformer-driven SEPP output stage.

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For the sake of simplicity the attached cct does not show bias stuff etc. It appears to me that both upper and lower fet would have the gain of a source follower because of the negative feedback applied to the lower end of the transformer input winding. There is no unequal gain and output impedance problem like some othe SEPP's. I haven't tried it out myself yet but I think I might in the next fey days. If anyone is tempted to ask why don't I just use a complementary pair, well I just like using N-channels exclsively because you can get BIG ones.

Has anyone ever taken this aproach commercially, or experimented with it? More likely in tube days I expect.
 

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Circlotron said:
There is no unequal gain and output impedance problem like some othe SEPP's. I haven't tried it out myself yet but I think I might in the next fey days. If anyone is tempted to ask why don't I just use a complementary pair, well I just like using N-channels exclsively because you can get BIG ones.

Has anyone ever taken this aproach commercially, or experimented with it? More likely in tube days I expect.

hmmm, similar designs are used for RF amplifiers, where harmonic distortion is just filtered out... ...not sure if it'll be any better than complete solid state designs. My guess is probably not. I don't believe that transformers are the most linear devices that can be had. ;)

That said, my dad did build one like this as a PA amplfier once, seems to me that it didn't sound all that good. (Of course, biasing was over simplistic on that amplifier).

I'd be interested in hearing what you think of the amp, if you build it.

-Dan



Seems to me the big problem may be the low frequency signals... ...you may need a rather large transformer.
 
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The input impedance of the Mosfets is very high, so
a small transformer will be up to the job - it will just
need the appropriate # of turns.

To the extent that you match and/or degenerate the
Mosfets, this circuit will work very well; basically it's
a classic.
 
Nelson Pass said:
The input impedance of the Mosfets is very high, so
a small transformer will be up to the job - it will just
need the appropriate # of turns.


Yes, it is true that the transformer need not supply a lot of power.

Maybe I'm wrong, but I thought that the transformer must be designed so that it cannot saturate at low frequencies. Assuming a small core you'd need a lot of turns on primary to get in the range of 20Hz. I guess this is what you mean by "appropraite # of turns"? (Something like a million turns of #80 copper ;))

Also, won't you want even more turns than the minimum needed to keep core in a more linear region of the BH loop?

-Dan
 
Sch3mat1c said:
Depending on the impedance, you'd only need 500-1000 turns I'm guessing. Fine guage such as #36 would probably be adequate. Wouldn't the HF response be reduced by DCR as well as leakage inductance?

Tim

Yeah, you'd probably want to go with a ferrite core to get any HF response from the thing.... ...lets play the number game on this problem... :) (This is coming from my head, so bear with any mistakes I may make :)

Given we can calculate the number of turns for the primary of a given transformer as: Np = e*10^8/(B*f*Ac*4.44)
Where Np = Number of turns Primary
B = flux density in Gauss
f= frequency in Hz
e = input voltage in volts.
Ac = core ares in cm squared.

Assume that the transformer is a 1:1 ratio. 10 volts to drive the mosfets would be needed, so the primary voltage would also need to be 10 volts. If using ferrite, assume a Bmax of around 2000Gauss (to be safe, of course). Also, assume that the transformer is small, like a small audio matching transformer, with an Ac of about .2cm squared (I just measured a small audio transformer I had laying around).

If we say that we need to go down to 20Hz, with the above transformer and voltage and Bmax we would need 28150 turns! Not a small number! Then add the two secondaries of the same 28 thousand turns each, ouch! :(

Ofcourse, we could give up a little top end (maybe not) and go with a permalloy core... ...giving us an allowable 5000 Gauss (Again, staying in the linear region). That would reduce the number of turns to around 11250, a little bit better. :)

Now, if we used... ..say, a permalloy core, and increased the size of the transformer to around 1.25cm Squared (Similar to PC power supply switching transformer in size), we could get down to around 1800 turns at 20Hz. That's a little more reasonable. If we quadrupled the size of that tranformer (Ac = 5cm squared), we could get away with 450 turns. That's a even better yet! :) :)

Hey this was fun! Maybe I should build one now! Actually I've been thinking of building one for 145Mhz...

-Dan
 
Run what you brung...

I have an ETD49 style N87 core right in front of me and it has a centre pole diameter of... lets see... 16mm. How many turns would it take to get down to 30Hz at least?

/Circlotron - can wind really good SMPS transformers but can't do the hard sums. :scratch:
 
Re: Run what you brung...

Circlotron said:
I have an ETD49 style N87 core right in front of me and it has a centre pole diameter of... lets see... 16mm. How many turns would it take to get down to 30Hz at least?

/Circlotron - can wind really good SMPS transformers but can't do the hard sums. :scratch:

How about 1800 turns, for Ferrite...
And around 750 for permalloy or similar...

-Dan
 
Yes, it is true that the transformer need not supply a lot of power.

Maybe I'm wrong, but I thought that the transformer must be designed so that it cannot saturate at low frequencies. Assuming a small core you'd need a lot of turns on primary to get in the range of 20Hz. I guess this is what you mean by "appropraite # of turns"? (Something like a million turns of #80 copper ;))

Also, won't you want even more turns than the minimum needed to keep core in a more linear region of the BH loop?

-Dan
May consider a power toroid transformer for investigatiion. How ideal? It'll get the DIYer quickly in the groove. For example; a 25 VA made by TRIAD VPT24-1040. Two independent primaries [115VAC ea] and two independent secondaries [24VAC @ 1 A ea]. May work around the problems above; as a woofer amp or better.
 
Keep in mind that the MOSFET´s input is a big capacitance, an then, it will resonate in parallel with magnetizing inductance of secondaries, and that, both upper and lower resonances may be of different frequencies because of different capacitances in this circuit. If the resonance is of high Q, then the stage may oscillate. The conventional half bridge auto-oscillating is made of similar way, adding a saturated transformer. And finally, these resonances can cause phase bumps and or several distortion. Extremely, can cause the gate isolation to be destroyed if the signal oscillating goes to more than 30V in each secondary.
 
May consider a power toroid transformer for investigatiion. How ideal? It'll get the DIYer quickly in the groove. For example; a 25 VA made by TRIAD VPT24-1040. Two independent primaries [115VAC ea] and two independent secondaries [24VAC @ 1 A ea]. May work around the problems above; as a woofer amp or better.

I agree - I tried using power toroids, and it looks like anything north of 20-30VA gives you a VERY flat freq response at the low end down to 5Hz and decent THDs too. At the top end, depending on tx and source impednace of the driver, you can get good, average, and mediocre results.

I posted some results here: http://www.diyaudio.com/forums/pass-labs/215958-ticle-zen-amp.html#post3088213

issue regarding driving the input tx remain unsolved as yet ....
 
Keep in mind that the MOSFET´s input is a big capacitance, an then, it will resonate in parallel with magnetizing inductance of secondaries, and that, both upper and lower resonances may be of different frequencies because of different capacitances in this circuit. If the resonance is of high Q, then the stage may oscillate. The conventional half bridge auto-oscillating is made of similar way, adding a saturated transformer. And finally, these resonances can cause phase bumps and or several distortion. Extremely, can cause the gate isolation to be destroyed if the signal oscillating goes to more than 30V in each secondary.
Thank you Osvaldo for raising this caution flag regarding the use of power torroids in an F6 application.
 
Have a couple of pairs of these type transformers salvaged from late 60's early 70's 8-track units. Cores are ferrite EI with primary dc resistance 124ohm inductance 1.35H/ leakage 7.55mH, secondary1 5.6ohm 49.8mH/leakage 54uH, secondary2 5.4ohm 49.5mH/leakage 52uH. Frequncy response was flat from 20-20k using soundcard test setup.
 
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