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Szekeres vs. PRR SOL amplifier

post #1 of 21
Thread Starter 
Hello,
I would like to build single ended class A amp. Szekeres was always on my list, but meantime I have remembered another mentioned project from old Headwize forum; the PRR designed SOL. I was looking for the schematic and finally found it. What is the difference between them? I know that Szekeres is a buffer, but what about SOL? Maybe someone from the forum built it and share sound quality.....
http://wiki.faust3d.com/wiki/index.php?title=Image:PRR-SOL.png
http://headwize.com/?page_id=31

Thanks for the help.
post #2 of 21

The Szekeres has its output taken at the source (common drain or source follower), the SOL has its output taken at the drain (common source).

 

The Szekeres is a buffer i.e. it has no voltage gain (gain <=1).

 

The gain of the SOL is -(gmRD/[1+gmRS]).

 

w

post #3 of 21

I remember starting that thread years ago (under a different user name). After my first haphazard test build, I eventually worked out the noise/hum problems by rebuilding it with a better power supply. The amplifier worked well afterwards. The output impedance of this amp is relatively high though. I had planned on doing a pcb layout but never got around to it.

 

Here's an archive.org link to the original thread.

 

http://web.archive.org/web/20070630143743/http://headwize.com/ubb/showpage.php?fnum=3&tid=6717&&action=seekpost&pid=57397#57397

post #4 of 21

They are simple enough circuits, build both and let us know.  From my experience they would both sound musical, but since I haven't built either as standalone circuits with those parts I can't really say for sure.  For an amplifier, I would target the SOL (or rather something else).

 

The only 2 things I would do different to the SOL is to put a Jfet, in common drain, in front of the mosfet to keep the input impedance high, and a CCS on the drain of the mosfet.  This circuit, I have built (with different transistors), and it sounds wonderful, with some negative feedback.  In fact, I think it's a Zen amp variant.  It won't be the best measuring amplifier, but it will be enjoyable to build and simple enough to learn on, and it sounds good.

 

Edit: in addition to what wakibaki stated above, the Szekeres only amplifies current output.  The SOL amplifies voltage and current.  There is one more mode of operation for a mosfet, which amplifies voltage but not current.  You can do some research for that.


Edited by holland - 9/7/13 at 6:46pm
post #5 of 21
Thread Starter 
Thank you very much for all answers.
I will start with SOL, but which version of published schematics should I choose: one from wiki with 45 supply voltage or Headwize one with 12 volts? I am a beginner, but somehow I would lean towards 45Volts even if that would require bigger heatsinks. But, if you would suggest 12 volts version what trimmer, present in that design, should adjust: voltage present at the output equal to 1/2 of supply volt or.....something else?

Holland;
I have an interest in audio DIY hobby for a long time. I said I am a beginner cos I have never learn how to design circuit.... From that period of time I noticed two sides of active CCS opinions: for and against. I think Pavel Macura, PMA from diyaudio forum, is for active CCS, but many prefer passive. My time is limited, but I will try later your advice with active CCS. I want to build the passive first and make it to work before further experimentation.

One more question... about power supply. I know that recommended one for SE Class A amps is regulated. How much current the amp consumes? I think about transformer power ratings for suggested option: 12V or 45V.

Thank you again for the help !!!!!
Edited by Foton - 9/8/13 at 10:06am
post #6 of 21
Quote:
Originally Posted by holland View Post
 

...put a Jfet, in common drain, in front of the mosfet to keep the input impedance high...

 

The MOSFET gate is virtually an open circuit, so if we're talking about this circuit:- http://wiki.faust3d.com/wiki/index.php?title=Image:PRR-SOL.png, then the input impedance is 10M||22M = 6M875, which is a pretty high input impedance in anybody's book...

 

@Foton - I can't find a Headwize schematic for the SOL, only the Szekeres, but you might want to consider that the Szekeres variant shown here:- http://wiki.faust3d.com/wiki/index.php?title=DIY_MOSFET_Amplifiers_Overview will run from a 12VDC 250mA or better wallwart, whereas, if you want 45V, you're almost certainly going to have to build a PSU. The other disadvantage of the SOL is the very large (high wattage) resistors you will be forced to buy, whereas in the Szekeres variation you'll get away with 2 100R 600mW in parallel for R6, the rest all 600mW.

 

w

post #7 of 21
Thread Starter 
OK, sorry for obvious questions. I have posted questions last night without spending time today on reference to headwize. It looks like 45V supply voltage schematic is for Hi-Z headphones. I will go with this one then. I also see there reference to PSU with minimum 30VA trafo. What is interesting, PRR does not recommend regulated PSU (if there is no much AC voltage variation). He does mention details of non-regulated supply, but no schematic. I do not know how to convert his words into... circuit. Can anyone point me to schematic or help with one?
This is what PRR said in Headwize about PSU details:

"There is no reason to regulate the supply unless you have extreme wall-voltage variation. You only need to trim ripple (this plan has no ripple rejection) AND slow-down any wall-voltage variations to a sub-audible rate. And fat caps are cheap and simple. For 2*0.2A load, assume at least 400uFd and then go wild, 4,000uFd. Add an R-C filter for really smooth DC. We don't want to lose much more than 10% this way, say 4V of drop at 0.4 Amps is 10 ohms. Another 4,000uFd will leave the rail VERY clean, and muffle a sudden 120v-110V utility drop to a sub-audible non-thump."
post #8 of 21
Thread Starter 
post #9 of 21

OK, thanks.

 

w

post #10 of 21
Quote:
Originally Posted by Foton View Post

OK, sorry for obvious questions. I have posted questions last night without spending time today on reference to headwize. It looks like 45V supply voltage schematic is for Hi-Z headphones. I will go with this one then. I also see there reference to PSU with minimum 30VA trafo. What is interesting, PRR does not recommend regulated PSU (if there is no much AC voltage variation). He does mention details of non-regulated supply, but no schematic. I do not know how to convert his words into... circuit. Can anyone point me to schematic or help with one?
This is what PRR said in Headwize about PSU details:

"There is no reason to regulate the supply unless you have extreme wall-voltage variation. You only need to trim ripple (this plan has no ripple rejection) AND slow-down any wall-voltage variations to a sub-audible rate. And fat caps are cheap and simple. For 2*0.2A load, assume at least 400uFd and then go wild, 4,000uFd. Add an R-C filter for really smooth DC. We don't want to lose much more than 10% this way, say 4V of drop at 0.4 Amps is 10 ohms. Another 4,000uFd will leave the rail VERY clean, and muffle a sudden 120v-110V utility drop to a sub-audible non-thump."

 

Capacitance multipliers, or CRCRC filtered.  Or you could use a 3 pin LM317HV or TL783 type circuit.  The image below and this http://www.ti.com/lit/ds/symlink/lm317hv.pdf .  You can do the math for resistor values and pull it together.

 

 

Quote:
Originally Posted by wakibaki View Post
 

 

The MOSFET gate is virtually an open circuit, so if we're talking about this circuit:- http://wiki.faust3d.com/wiki/index.php?title=Image:PRR-SOL.png, then the input impedance is 10M||22M = 6M875, which is a pretty high input impedance in anybody's book...

 

Sorry, misunderstood and I was also referencing mods which the OP might not do.  Since I mentioned 3 different things, all lumped together, but the 2 important ones were the JFET and negative feedback.

 

The JFET would really be needed with negative feedback for interactions with the source, to keep the input impedance high.

 

The JFET is to drive the gate capacitance of the MOSFET.  Though I never built a Szekeres or sim'd it, I read about it, and it suffered from HF roll off due to interaction with the gate capacitance and the pot.  The gate sees the source's output impedance, the volume pot, and the resistor that's in front.  The Szekeres is really meant to be driven by a preamp and not driven by a volume control directly.  The PRR-SOL is configured the same feeding the MOSFET.  There isn't any reason to doubt that there would be a different effect due to the gate capacitance interaction.  Or perhaps I misread the circuit, but I don't think I did.


Edited by holland - 9/8/13 at 4:23pm
post #11 of 21
Thread Starter 

In above mentioned Headwize link, Tomo talks about regulated PSU and use of LM317:

"

5. If I use an LM317 based power supply, am I getting too close to the regulator's limit?



Two of these circuits will draw almost 1.3A continuously, which is already close to the LM317's limit. A LM338 would be a better candidate with its 5A rating."

 

And this is in reference to 12 Volt PSU. Can I still use LM317? This is interesting PSU for class A amp which uses LM317 and is capable of delivering 2A:

 

 

 I did find two PSUs: one with use of LM317 and another one from ESP website:

http://www.redcircuits.com/Page123.htm

http://sound.westhost.com/project15.htm

 

I am not sure, which one to use: with 45V supply voltage ( recommended output of transformer by PRR is 48V) It is above the input limit of LM317 ( which is 40V). I did not check LM338 , yet.

Can I still go with LM317 with the PSU listed in the first above link? Or the ESP one would be better....


Edited by Foton - 9/8/13 at 5:04pm
post #12 of 21
Quote:
Originally Posted by Foton View Post
 

In above mentioned Headwize link, Tomo talks about regulated PSU and use of LM317:

"

5. If I use an LM317 based power supply, am I getting too close to the regulator's limit?



Two of these circuits will draw almost 1.3A continuously, which is already close to the LM317's limit. A LM338 would be a better candidate with its 5A rating."

 

And this is in reference to 12 Volt PSU. Can I still use LM317? This is interesting PSU for class A amp which uses LM317 and is capable of delivering 2A:

 

 

 I did find two PSUs: one with use of LM317 and another one from ESP website:

http://www.redcircuits.com/Page123.htm

http://sound.westhost.com/project15.htm

 

I am not sure, which one to use: with 45V supply voltage ( recommended output of transformer by PRR is 48V) It is above the input limit of LM317 ( which is 40V). I did not check LM338 , yet.

Can I still go with LM317 with the PSU listed in the first above link? Or the ESP one would be better....

 

Use a regulator for each board, ala mono blocks, or a high current one.  Most 3 pin regulators I've come across work the same, and are almost drop in replacements.   LM317HV handles higher voltages (57V).  You can also float a regulator if you need.  TL783 handles up to 125V.  3V drop is a bit low if you get mains fluctation.  I would look for 5V for "better" regulation with these 3-pin regulators.

 

To be honest, I don't think it matters that much which way you go.  The ESP one will be fine and low noise.  You can serialize cap multipliers for even lower ripple.  You can drop a zener in it to make it a "regulator" as well (it doesn't regulate well), but if you put 2 of those (with zener) in serial with 5V drops in between each (source is some 60V), you can get really low ripple.  3 pin regulators are not that great at ripple reduction, at higher currents or voltages.  You can always update the PSU later if you need to.

 

You can also use a laptop switching power supply as an interim solution.

 

Personally, a simple series voltage regulator with additional filtering is what I would use, that is ESP like with a zener voltage reference.  Not saying you should do this, just what I would do.

post #13 of 21

SOL, 600R source...

 

 

 

Szekeres variant, same conditions, 12V supply

 

 

 

w

post #14 of 21
Thread Starter 

Hello Wakibaki,

Thank you very much for graphs. Hmmm, that looks interesting.

OK, as per recommendation I will go with dual PSU and LM317HV regulators. I bought transformers today, TRIAD F7-36. Is there any reason to use IRF610 transistor instead of IRF510? I have both types and obviously I would like to use better choice.

post #15 of 21

I don't think there is any reason to use IRF610. You don't need 200V VDS. IRF510 has a lower rDS(ON), it has lower total gate charge, so, without knowing anything else about their respective linearities, I'd go with the IRF510 as it'll be more efficient and have better HF response.

 

w

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