[PRR]

> Wow--this amp sounds...

BoyElroy: this little plan has also come up on HeadWize forums, and Chu Moy may add it to his collection of articles.

Obviously you are the world's leading expert on this plan (being the only one to have built it). May we quote some of your comments on it? Do you want to co-author?

-PRR

 

[BoyElroy]

Hi PRR--

It's very funny that I'm the only owner in the world of a working PRR-Sezkeres! Its such a simple amp to build, I think a lot of people would enjoy making it (if not for the sound, then for the low cost!).

I just checked out the thread at Headwize and I too, am curious what other people think of the amp. It has extremely detailed highs and excellent bass on my Grados--plus, it's easily "tunable" through the addition and choice of input coupling caps.

On a technical matter, do you think that what oscillations there are would be attenuated by increasing the gain of the opamp; substituting a 15k-47k ohm resister in place of the 5k ohm I'm currently using?

Please feel free to use whatever comments you like. I do plan on comparing the Gilmore to the PRR-Szekeres--just as soon as I get some sleep....zzz.....

I am flattered that you'd offer me co-authorship of a review, but I'm afraid I'm more of a lay consumer than a designer or engineer. I'll leave all that to you and other qualified folks at Headfi/Headwize. I'll try to contribute what I can through posting some subjective comments and the like. I'd like to see where you'll take this design...it comes so close to the old "straight wire with gain" model...

 

[BoyElroy]

Well, spent last night listening to the PRR-Szekeres (P-S) with a wide variety of material and in two different configurations; one with no gate resistor and the other with a 100 ohm gate resistor. The drain resistor was an 8 ohm 20 watt non-inductive wirewound and I used a constant current source with a 5 ohm bias resistor.

I did not use any input/output capacitors and the power supply was a +/- 16.40 Vdc unit based on Kevin Gilmore's ultra-regulated design. The dc voltage at output throughout the evening was basically fixed at -.006 vdc on the left channel and .000 on the right channel!

All listening was done on a pair of Grado 325's and the signal was fed from;

1) MP3's from my PC played through a Soundblaster Live!
2) Plextor CD/digital out-->Soundblaster Live!
3) Analogue out from a Theta/AA DTI 2.0/DDE 3.0 digital frontend

With the 100 ohm resistor in place, the P-S had a really, really smooth presentation with non-fatiguing highs and very good, tight bass. There were many times where I'd just stop what I was doing and say to myself, "wow, that sounds really great! I never liked that song before...".

I don't want to give the impression that the P-S didn't sound detailed, but the midrange on this baby kicks so much #$#% that you don't really want more high end info. The presentation is balanced and extremely enjoyable to listen to. The bass was tight and very clean. The U2 Mango remixes made the Grados vibrate like nobody's business and I kept lifting the earcup off my head to make sure that driving beat wasn't my neighbors banging on my door.

Again, I don't want to say that the upper frequencies were "polite", but they just didn't stand out -- again, its the liquid midrange that's the real deal here--it's something special.

The P-S with no gate resistor is a totally different beast. The high end on this version will open up so much detail that your ears will be begging you for mercy on badly recorded/low bit rate encoded tracks. The highs are so high that I kind of suspect some degree of ringing past a certain freq. The bass and midrange are very clear and transparent--maybe even a bit more so than the 100 ohm version, but its the detailed highs that will grab your attention. On some tracks this P-S will literally have you hearing things for the first time on tracks that you thought you knew intimately. Background vocals, mumbled words, etc., become clear in a way here that's downright funny. You'll be like, "Ohh...so that's what that word was...I've been singing it wrong for 18 years..."

On the other hand, the high freqs. can get a bit tizzy on tracks that already have a large amount of treble/treble distortion and believe me, you will notice this excess energy. While listening to Yo La Tengo's studio and live versions of "Cherry Chapstick" I had to dive for the volume pot as the guitar chords began the distort. There is a LOT of treble information here! On the other hand, the ambience of the Hoboken venue where their live version was recorded will have you turning your head to see if some audience members voice isn't really someone standing behind you with a knife/baseball bat/frying pan. I kept doing that till I got too freaked out and turned down the volume...

The bass, again, is punchy, powerful and extremely clean--this version of the P-S is a real hot rod. On some tracks, it will blow you away with the level of detail and transparency. On other tracks, you will hear every single treble note and probably hear it for a while, as well.

The P-S with the 100 ohm resistor, on the other hand, has a very good level of high freq. detail with that killer midrange. Wow. Talk about smooth. smooth, smooth. And liquid. As in drip, drip, drip--you can hear the raindrops fade into silence, wrapped in their raindrop wetness...And the bass, through the Grados, will slap you upside your head and toss you around the room. This is easily my favorite Szekeres version (again, not that the other designs are 'not good', but that this version just hits all the right notes for me) and oddly enough, is probably the easiest one to build.

 

[BoyElroy]

(taken from a parallel Headwize thread)


Hi PRR-
I'm sorry to hear about your friend...I hope things work out okay for him. Maybe working with these circuits and all this electrical design stuff will help you from stressing out too much

.

So to give you some food for thought, I'm pretty certain that there is some sort of oscillation/ringing happening here. I rebuilt my original Szekeres the other day with all new premium parts as part of my comparison review and there's a noticeable difference in "listenability" at higher volume levels. Specifically, when the PRR-Szekeres reaches a certain level, it feels as if there is some pressure (?) build up that makes it uncomfortable to listen to for extended periods. I know that when I mis-align my speakers, for example, I get a similar sort of nodal peakiness that has a similar effect.

If this is some sort of oscillation, do you have any suggestions on where to go from here? I currently have the IRFZ 46N mosfets instead of the IRF510's thinking that since they have a lower voltage rating, they might ameliorate the oscillation, and they do sound different, but they have a higher input capacitance than the 510's so I'm not sure what's happening here...

The PRR-Szekeres is a funny animal, after more extended listening, I get the sense that on some tracks, the amp sounds terrific, and then on others, it seems to suffer a bit of harmonic disonance. I think its a very good idea for you to build one yourself and troubleshoot this issue (if indeed, there is one in the first place!)

Again, best wishes to your friend and thanks--

 

[PRR]

> about your friend...I hope things work out

Thanks. He looks good today, but will go under the knife in the morning to find out what is really wrong and hopefully cure it. IMHO things look good, but it will be an "interesting" day.

Cross-posted from HeadWize DIY: "Opamp to Source Follower bias configuration":

Try this variation:

From DC to 50KHz, the MOSFET is under strict op-amp control. This gives low distortion and output impedance, and strict DC control. Above 100KHz, the op-amp just gives a nice steady drive voltage and the MOSFET works like a simple cathode follower.

Rgate is necessary (as you found) and now may be 47 to 1,000 ohms. Don't go outside this range. Inside this range, there will be some interaction with the op-amp's internal output impedance, so there may be slight effect with different chips. 100 or 220 ohms looks best to me for most situations. I'd be curious if you hear any change with other values. (I think you should not.)

C5 and R4 are new. These are the key to MHz stability. These values should be good for most situations. Changes here should have little to "no" effect in the audio band, but large effects at radio frequencies.

I have changed the values of the feedback resistors so C5 and R4 don't load the op-amp badly. Gain turns out the same.

The LF411 op-amp and IRF150 MOSFET were just what I have in my simulator. I don't think you can even buy an LF411 today, and the IRF150 is far bigger than this amp needs or wants. Use a pretty-good (not fantastic) op-amp with GBW of 10 to 30MHz and less than 10nA input bias current.

IDC is an "ideal" current source. A LM317 would be used in real life. 0.2A should be plenty in most headphones, and output Z or linearity is not an issue. And this current directly sets the total heat to be gotten rid of. However if you expect to clip the amp with LOUD levels in low-Z phones, be generous. If the amp runs out of current on negative peaks, the op-amp goes crazy trying to urge the now-cutoff FET into reverse current, and recovery is messy for a microsecond after each clip. A big 30 to 50 ohm resistor will also work, but with more heat for less output.

V11, V12 are ideal signal generators; Rsource is to make them un-ideal. In real life your input jack or pot goes here. Rsource is not really needed in most cases, values from zero (short) to 2K or so would be typical good practice.

I have shown 12V power supplies but this is not critical. Stay way over +-6V or you'll get gross clipping at small levels. +-20V is far more than a 32 ohm phone needs, just more heat. For Hi-Z-only duty, you might want +-15V to +-20V supplies (watch op-amp rating!) and a lower current in IDC (say 0.1A to 0.050A).

I show 1,000uFd power capacitors. If your power supply is very good and very close, these are not needed. If in doubt, use 1,000uFd or more very close to the amp. While it is Class-A, it has huge signal currents in the power supply and the supply needs to resist changes.

-PRR

 

[BoyElroy]

Hi PRR--

My apologies for taking so long to get back to you. I was out of the country for several days for work and my laptop died on me...arrrgghh!!!!! Its amazing how frustrated I felt from not being able to access the web for even a few days. Man, what did people do before the www?

In any case, I was v. happy to come back and see your latest design. I'll get to work on it over the week end and see if I can get it finished by Saturday so that I can spend Sun. listening to it. Famous last words, eh?

I hope things went well with your friend and thanks again for all your work!