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Help me to choose a DIY amp!

post #1 of 9
Thread Starter 
Hello

I am about to build an amp and I would like to ask you what amp will be better partner to my Grado phones in terms of sound quality (3D soundstaging, low and articulate bass and extended, but not crude highs) and resistance against interference:

1. Richard Murday’s JFET-MOSFET driver (the scheme looks good to me)
2. Greg Szekeres’ class A MOSFET driver (the original one or the AC/DC coupled versions?)
3. CMOY’s opamp based amp?

I'll only use the amp at home so portability is not important to me. Big and hefty parts sound better. Does the JFET version of Szekeres amp works better compared to the original one?


Please DIYers, help me with your opinions! Thank you in advance and Happy Holidays!
post #2 of 9
The Cmoy with the Apheared 47 mod is supposed to be a very good match. It's designed for driving low impedance / high sensitivity headphones like Grado's
post #3 of 9
The original szekeres amp is an easy build and sounds damn good IF you have source equipment which has enough drive voltage.Most , if not all home CD players fulfill this requirement.
But try using the amp with a portable cd player and it will sound thin and weak.
Same thing with other source equipment.In my system I have the szekeres amp hooked up to the record output of my preamp.All of the equipment hooked up to the preamp input has more than enough output to drive the amp directly.
But I also added front panel jacks for
1-direct output in parallel with the rec output but before the rec out mute switch
2-an output AFTER the gain stage but BEFORE the volume control and output mute
#2 allows a connection to the amp when used with low output devices.
So I would suggest building the original szekeres amp,try it with your system and if you feel you neeed more volume then add either the jfet stage or preceed the anp with a simple op-amp gain stage
post #4 of 9
Hello,

I believe Szkeres' amp is good idea as well. If you are driving Gradoes, you DO NOT need voltage gain. (But you can have it.) But JFET version is not necessary more difficult to build. However, JFET version is built with very distict PSU using very very large capacitor. This is very good idea, but for beginners, this tactic is not recommended. Very Large capacitance can cause large surges which can easily blow parts. So use simple regulator circuit if you are not confident with your skills.

Note that Szkeres' amp has a minor disadvantage. The current source for both Original Szkeres and JFET Szkeres amps are R4 which is a power resistor. Unfortunately, this slows the MOSFET down. In the past, I have added a constant current supply in the place of R4. I found my upgraded amp sound much faster. I learnt at school this semister that adding this CCS improve the slew rate of this type of amplifiers.

CMOY amp does not have this problem since its output stage is complemental pair. Similarly, SDS have complemental pair as well.

Tomo in ICE!! (Darn it why do I have to live in this cold place!?)
post #5 of 9
Thread Starter 
Thanks rickcr42.

You say that JFET stage increases the gain of the amp or I'm wrong? Mr. Murdey sais that the JFET stage is just a better input stage for the MOSFET (optimises its input impedance). He sais also that JFET version of Szekeres amp sounds more musical.

Has anybody ever tried to build the JFET-MOSFET driver and could explain the sonic difference?
post #6 of 9
Thread Starter 
Tomo, could you give me more info about that constant current supply?

And did you make some changes in schematics of your Szekeres amp for better sound?
post #7 of 9
I believe Tomo's changes are on the Addendum page of the Szekeres article at headwize:
http://headwize.com/projects/szeke1a_prj.htm
post #8 of 9
Hello,

Yes, the details on modifications are already posted in the addendum of the article.

The easiest way to put it is that ... MOSFETs are like a variable resistors here. This means that current that goes thru the MOSFETs will be changing also. This will make transient response poorer because if there is no current, there is no signal even if there is voltage. (More so with AC coupled Szkeres.**)

Also, the constant current source poses relatively larger impedance for Mosfets. This makes the operation more linear.

These are basic theoretical reasoning behind my upgrades. I hear more details in high end with these which proves my reasonings.

Tomo
post #9 of 9
Hello,

JFet inputs are superior to Mosfet inputs. Simply because mosfets are not that clean. However, if you do not need gain, the residual noise will be well under the level of hearing so it would not matter. So following Mr. Szkeres' spirit of simplicity, I would go without JFETs.

However, if you do need gain, most optimal solution is to use JFET gain stage. This will be very low noise and have very nice clean gain. Also some boast that these actually taste good too.

Tomo
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