[JohnFerrier]

Finished at midnight, showing at Adire Audio this afternoon.



JF

 

[was ist los?]

Any more details or pics? I'm guessing it uses boutique parts.

 

[JohnFerrier]

Well, I'm interested in finding somewhere I can have some measurements made: THD profiles and noise. If time permits, I'll try Magnolia HiFi or perhaps one of the local audio equipment mfgs. Because of another thread I was involved with, I’m bent on finding someplace with an Audio Precision System Two Cascade.

Anyway my Boutique Follower is a pure class A design using super-matched complementary JFETs. Meaning devices matched closer than dual devices (actually much closer). I have no offset adjustment potentiometer and am not using a servo. However, because of careful matching the offsets are well below 1mV and as the batteries discharge to the dispose level the offset creeps up to 2mV. The only capacitors used (all polypropylene) are across the power supplies, made up of eight 9V alkaline batteries. The ground and power planes are constructed of 0.032” sheet copper insulated with electrical grade Teflon. All signal lines are in the air and wired with silver (17AWG in most cases). Active devices are heat sunk with copper refrigerator tubing and solid 6AWG copper wire. Silver impregnated epoxy is used to permanently glue devices into heat sinks and for electrical connections to ground/power planes (a 350 watt soldering iron just didn’t work...but did that sheet copper get hot : ). Besides copper and silver, there is a lot of steel: the enclosure and a secondary lining. The top and bottom covers include vent holes (offset to permit only high angle penetration) that allow for a bit of a chimney effect. The follower weighs in at a hefty 10 pounds. I'll admit, there is some excess (including the use of wires doubled back on themselves to avoid "cable directionality"...but that is another story).

My design is a derivative of Erno Borbely’s Super Buffer. I departed from Borbely after encouragement to try removing global feedback. I find listening tests stressful, but I liked to listen to what I heard on the bread board with global feedback removed. The immediate effect for me was that the music lay soft and loose. It was not tense or electronic sounding. Simultaneously, it does not have a problem with sibilance. I'm not an expert at all so can't really provide much information, but an easy way to try an amp without global feedback is to not include the output stage in the feedback loop. I remember someone posting a two opamp circuit with the BUF634 outside the FB loop. Maybe that person had in mind to isolate the transducer load from the feedback loop.

I'm really glad to now be on the listening side of this project.


JF

 

[ppl]

Hi John Good Work I tried using the output stge outside the Feedback loop in my early Head Amps. LT-1122+LT-1010 and while this affords great input to output isolation it also leaves the gross distortions unretoutched and thus can mask the real performance inherrent in the Gain stage. dont be shocked to find THD in the 0.1% Light loads to 0.8% normaly loaded. use the Free RM to test your amp and get an idea if you really need the accuracy of Audio Precisions THD analizer. http://audio.rightmark.org/
http://www.valuetronics.com/manufact...udioPrecision/
http://www.poulpetersen.dk/apwutils/ukaputil.htm
http://mixonline.com/ar/audio_audio_precision_portable/
measurements available from this Lab
http://www.libermansound.com/audiopr...aps2c_over.htm
http://www.audioprecision.com/index....&id=1000000551

 

[JohnFerrier]

Hi Phil,

Thanks for your posts. I'm not after super THD numbers. The idea of a low 10khz / 1khz THD ratio (< 3) appeals to me though.

I'll admit it seems hard to improve on op-amps, but I figured if I didn't build a discrete now, I probably never would. The one area that I think discretes outperform op-amps is noise. Borbely's Super Buffer is presuably 260nVrms (input referred). I believe that is half of an OPA637. Regardless, both inaudible.


JF

 

[ppl]

Hi John
A low THD ratio between 1K and 10 is also my preference as my ear picks up non linear Distortions more so than i like and this has resulted in my almost complete dissatisfaction with most solid state gear and that’s is why i first off now Build rather than Buy and also why i like Multi-loop. I truly hate what an uncontrolled Ring HF response dose to Brass percussion. A 5 dB peak at the HF cutoff makes cymbals sound splashy and snare drums sound like sandpaper. Sibilance is also magnified by this.

operating the output stage open loop will if properly designed not ring at cutoff and so the High THD will not be as audible as one might first think since THD will be constant vs frequency the ear will preceive the rising THD vs frequency as stronger harmonic content than is really present and such things as sibilance and grain will be disproportional louder than in reality.

Congratulations on your new amp design hopefully it will provide you with years of Musical enjoyment

Disclaimer
The above is merely my opinion on my perceived perception of this issue and YMMV

 

[JohnFerrier]

No noise or THD measurements, yet. I have measured the frequency response: DC-530khz…

And here is a schematic.

Besides the 22uF polypropylene caps, there are 0.1uF polypropylenes in parallel supplying each of JFETs (not shown on schematic…). Note: the capacitors are in the "right" part of the circuitry...hehehe... : ).

The 1k/333 ohm resistors are necessary because the circuit otherwise oscillated with the volume at the lowest levels. The two values differ because the input capacitance of the JFETs differ…The Elma stepped attenuators are not make before brake (and that makes sense to me) so the 1M ohm resistor.

The 2sk170/2sj74s are low noise parts (and low noise is the salient feature of this design...as far as I know, this is the lowest noise design there is...). Paralleling four of them ought to divide the noise by two and, of course, provide more class A output. My circuit is running at ~35-40mA.

I enjoy listening to it a lot (and need to make another trip to Costco to purchase more eight packs of 9V batteries...my low noise power source...).


JF

 

[tangent]

Ummm, I suppose you can get away with the high impedance of 9V alkalines with a class-A design. I mean, it's not like load-modulated rail ripple will be a problem. Still, I'd think a look at NiMHs would be worthwhile.

 

[JohnFerrier]

Quote:

Originally Posted by tangent Ummm, I suppose you can get away with the high impedance of 9V alkalines with a class-A design. I mean, it's not like load-modulated rail ripple will be a problem. Still, I'd think a look at NiMHs would be worthwhile.

Hence still all the capacitors in the PS and the use of parallel batteries... Problem I have with NiMH is that they last 1/4 the duration of alkalines. And changing batteries (for charging or otherwise) is a 20 minute operation. With NiMH I'd have to change every other day rather than on the weekends... Also, at one time I saw graphs indicating that alkaline batteries are low noise (but then it's all relative...). I've not seen a comparision of alkaline v NiMH noise...I would think though that with more ampacity that alkalines are lower noise/lower impedance... And my guess is batteries better AC/DC convertor + Jung's Super Regulator...the other alternative...


JF

 

[ppl]

John I got to hand it to ya you covered all the often over looked details. You might wish to also look at the attached

 

[JohnFerrier]

Hi Phil!

Thanks for the post. I'm actually kind of happy, because I think that in the end I found something that was a bit unique... Measured distortion will be higher, but a listening test revealed sound I enjoyed.

Here, again, is one of my humble starts from late October...

At the time, I liked the simplicity of the emitter follower. So, I guess it makes sense that I ended up with another simple (perhaps somewhat elegant) design.

Take care!


JF

 

[ppl]

Since you are in my are your arange to bring it to a meet. This topology for som unknown reason is attracting my attention in spite of my first initial rejection. BTW what do ya think of my Discrete component cascode op amp. ?

 

[JohnFerrier]

Quote:

Originally Posted by ppl Since you are in my are your arange to bring it to a meet. This topology for som unknown reason is attracting my attention in spite of my first initial rejection. BTW what do ya think of my Discrete component cascode op amp. ?

Yes, I'd like to do that. I don't know how my low noise source follower compares to other amplifiers.

I like your discrete op amp a lot and have not seen anything exactly like it. (I just copied it to my _ppl folder...) The only thing I can add, since I see your use of an LED, is that recently at diyaudio someone had data that Red LEDs (and even more so IR LEDs) are lower noise than other colors (and zener diodes). I had heard that LEDs are low noise, but hadn't seen data. Kind of off track, but the only real thing I can contribute...


JF

 

[tangent]

Quote:

And changing batteries (for charging or otherwise) is a 20 minute operation

Build a trickle charger into the circuit. (Can't fast-charge 9Vs, due to small cell size.) An LM317 and a resistor, and you have a very rugged and tolerant constant current source. Some wires and a jack, and you're done. A bonus feature is that you can run the amp from wall power while you charge the batteries.

Quote:

I would think though that with more ampacity that alkalines are lower noise/lower impedance...

I don't know about noise, but NiMH has alkaline beat in impedance. A NiMH 9V is about 1 Ohm, while an alkaline is more like 2 Ohms, and the alkaline's impedance starts rising about 1/2 through its lifetime, while the impedance of a NiMH cell doesn't start rising until it's almost depleted.

In applications where the current varies, this lower impedance will probably dominate the noise issue: the resultng current-modulated ripple will probably be higher than any inherent cell noise. In your class A app, I wouldn't be able to guess.

 

[JohnFerrier]

Quote:

Originally Posted by tangent Build a trickle charger into the circuit. (Can't fast-charge 9Vs, due to small cell size.) An LM317 and a resistor, and you have a very rugged and tolerant constant current source. Some wires and a jack, and you're done. A bonus feature is that you can run the amp from wall power while you charge the batteries.

This likely is my best longer term solution. Thanks Tangent!


JF