Monica USB: Lack of "speed" and "timing"?

[ole]

Since we at first discussed DAC's here...
I will soon test a 24bit/192KHz DAC. I have also ordered some different OP-Amps to test it with.
I am 99.8% sure that I will be disappointed, but I want to try it.
I will upgrade the power supply, cards, capacitors, everything I find possible.....
The Op-amps I think will be the ones I end up with is 4x OPA627:
http://www.datasheetcatalog.org/datasheet/BurrBrown/mXqxttt.pdf
They need to be set in via adapters to make them dual. (Original is NE5532, that stinks)
-Jonas

I agree, you will probably be disappointed.

A while ago, I played with several different opamps as buffers. The NE5532 isn't bad at all when used properly (beware, it's not unity gain stable). Somewhere I read that it has a "sonic foot print like a yeti". I think that pretty much sums it up, but it is actually quite nice. The OPA627 may have better specs, but the sound isn't as pleasant.

Ole

[assendor]

Hello Ole,
thanks for the information.

It is amazing how many opinions there is when it comes to those different components.
It probably differs enormously what circuit they are in, how they are loaded, voltages and so on??
I have no real experience in the Opamp field, when it comes to sound. (Other than that A/B amps really are big Opamps.)

I did find something interesting:
http://www.passdiy.com/pdf/B1%20Buffer%20Preamp.pdf

Probably better than any opamp as a buffer?
Got to try it. Then I most certainly do get the absolute best out of that voltage output 24/192DAC.

Hey....!!! Martin, this Buffer here could be superior to lower the output impedance of Monica3 ???
This circuit are supposed to have a output impedance of 50ohm :) :)
Mr Nelson Pass writes this:
"The input impedance of the Gate is exceedingly high ? many millions of ohms, and
the output impedance at the Source pin is about 50 ohms."

-Jonas

[Paul N]

Hi Jonas

That circuit does look nice, and it's ideal for Monica! Why so? Because the input needs to held at one half the supply voltage- the same as Monica's I/V board's output (set by the trimmer pots)! So we don't need the circuit's bias arrangement, if we directly couple Monica- via R102/R202- to the first FET's gate- so no coupling caps needed (the BGs in Monica would be reused on the circuit's output). So we lose C100, R103, R2, R3 etc (and the pot, obviously), and all powered by Monica's existing supply! In fact all we need are R102, and the two FETs, as R104 is optional; though R105 good practice, as it grounds the output at dc, avoiding connection thumps etc (my Monica already has this).

So we go from an output impedance of 5k to 50R (plus ?whatever you make R104/204), with the addition of a single FET in the signal path. A FET type that I've used as a gain stage in another application in the past, which sounded excellent. Has to be worth a try- and not only with low input impedance preamps...

What do you think Yeo? Might make Monica even better- in all applications?

Paul

[yeo]

hi paul

you are always one step ahead!

i like the circuit jonas pointed. but still looking at it as itself and then you come up with the brilliant idea of coupling to monica directly!

i got to build this!

yeo

[assendor]

Hello Paul,

That looks superior. You didn't even need time to think it trough before you had that answer popping up :) I am amazed  :o
Just a few questions so that I, or we don't fry something when there is no need to  :D

If this B1 Buffer Preamp is powered by 24volts then, you also have to power Monica3's Gain-stage with the same voltage,
am I correct?
So when you then Bias the output of the Gain-stage to half it's supply voltage, this is also the half supply voltage at the Buffer Preamp circuit?
Did I get this Right?

Also some other thing Paul,
Mr.Nelson Pass writes this:
"All of the transistors are N channel JFETs. The stock parts are 2SK170?s,
LSK170?s or 2SK370?s, and you can use substitutes having Idss between
than 5 and 10 milli-Amps and transconductance numbers from 5 to 30 milli-
Siemens."
I am asking then,
The 2N3819 That is used in Monica's Gain-stage, has Idss=min"2mA" Typ"10mA max"20mA" (Should be OK?)
and Fw Transconductance Gfs=min"2mS" Typ"5.5mS Max"6mS" at 1KHz. (What about this? ))
Does this mean that if you test a lot of them, and match them up it is possible to find some that is above 5 this will also work fine?
Do you know what happens if that Transconductance Number is too low? (Isn't this the Amplification factor voltage VS Current?)
Is it better if it is up against 30milli Siemens, or is it best if it is closer to 5milli Siemens.??

I do have 2N4416 also laying around in numbers, but I do not know if I can use them either here?

I know it is easiest to just drive over to my friends house and get a hand full of those 2SK170, but just out of curiosity it would be interesting to hear why this and that...  ;) ;)
That is the best part of DIY, to test different stuff than it say in the "recipe" :)
But Of course, it is NO FUN when it doesn't work.

I was studying the FIG 3  Distortion Waveform, It is so nice that you almost could wish for more of it :) :)
0.0007% and almost Clean 2nd harmonic, that is not bad. I don't know if I have ever seen anything better without any feedback.

-Jonas

[Paul N]

Hi

Before I start I must thank martinbls for starting this thread, and Jonas for spotting Nelson Pass's circuit; also Glenda 17 who also raised the possible need for a buffer on Monic'a I/V output

Jonas- yes to your first two questions. From the specs you quote, 2N3819 seems suitable. No expert on FETs, but 2SK170 is a low noise type, originally intended for movin coil cartridge amps etc- i.e. a low level audio application, which has to be a good thing.

I had to try my own suggestion-

I had some 2SK170 (10 of them). They were all marked "BL" so broadly in the same Idss range, but even so I was able to match to pairs quite closely. I installed the circuit- hardwired- on the back of my I/V boards (easier to do as I only use half of each board- would be harder to do with Monica 3?s onboard I/V stage, or a fully populated off board I/V pcb)

So operating at 24V- each channel?s I/V + buffer has it?s own PH shunt reg. I fitted the gate R (1k) very close to the FET, and also threaded a small ferrite on the resistor?s lead. (As regular readers will know, my modified I/V board had been a bit twitchy at rf, so I thought it wouldn?t do any harm). I didn?t use the ?safety? R on the output (1k in the original design), though I will probably use say 470R here eventually. I reused the super E-ed BGs as the output caps, and grounded the output with 220k. Both resistors carbon film (I?ve found bog standard C films beat all metal films on sound- only tantalums sound better, IMHO). It feeds my preamp which has an input impedance of ~87k (Before I fitted the buffer, I grounded the output caps with IM- time constant a bit high, but didn?t want to lower the preamp?s effective input impedance any more than necessary)

WOW!

WOW WOW WOW??

Increased clarity. Firmer more powerful bass. More dynamic. If you think the ?sixth diode? mod was good, you have to try this!

It sounded startlingly good form the start, and can only improve with run in. Only one possible downside. Last night- I was very tired, so it might be my imagination- I noticed after a couple of hours that the sound was a bit greyer than before. Tonal accuracy and correct instrumental  timbre is very important to me, so this was a worry

I left it running overnight, and this morning it sounds fine- very fine! Might have been illusory- I?m suspicious of any sort of follower (don?t know why NP described circuit as negative feedback free- followers are 100% feedback)! It might also be the BGs- not sure I got the pairs so the dc across them was the same polarity as before

Thanks everybody

Paul

[Paul N]

HI

Re 2SK170. This was recommended to me by Paul Hynes and I've ben one as the output gain stage (not configured as a follower) of my Tuner's stereo decoder for a while

Also see

http://www.diyaudio.com/forums/showthread/t-50364.html

Seems 2SK170 is highly thought of. Still seems to be freely available too, though not from the big outlets, here in the UK at least

Paul

[yeo]

hi paul

you are too fast for us here!

well, i believe nelson pass refers it as "no global negative feedback". local feedback seems ok.

i got to get these transistors soon!

yeo
ps: i have lots of 2n3819. may try this first.

[assendor]

That was quick Paul... You really must have liked that Mr.Nelson Pass B1-Buffer design, and you are fast!!
I wouldn't be caught up in a Gunfight with you man  ;D

   Well it has a potential here then, that is super.
It is really great that Martin asked these questions in the first place, or this would have never been thought of. (Oh well..)

Even if it tends to be working more or less perfectly in different applications, it is pretty obvious that it can make Monica deliver
her full potential in more low impedance demanding systems.

I am really anxious to hear what yeo thinks of the result with this buffer implemented Paul's way?
Hey yeo, If it works fine with those 2N3819 please let us know..

Some Thoughts: (Oh NO  ;) )
I was thinking about the Feedback thing.
I know that Mr.Pass doesn't always fancy "No feedback" designs. I think I have red that it depends on the result, some say.
Probably all music, of newer date anyway, possibly have been mixed and tuned using systems with a lot of feedback for playback.
I do not know what to think or believe other than what sounds best, is the best to me. So if it needs a little local feedback I am totally OK with it, as long as the signal result is the way it was meant to be.

   I have also red something about the difficulty to get the correct "room feeling" or "Sound image" with NO Feedback designs.
It is obvious that this are opinions, not hard facts. Also if you have feedback in too many steps well... A/B sound :) :)
My system as is now, has Absolute NO Feedback at all. It sounds very good to me.
So it will be interesting to add some feedback to see what happens. Like if I put in this circuit we are discussing here
"the B1-Buffer Preamp".

   Sound reproduction is so complicated, it seems impossible to ever be able to understand it properly.
That is among the things that makes it such a fantastic DIY Hobby. You never end "There".

-Jonas

[yeo]

hi jonas

2n3819 works very well!

thx again to paulN for his suggestion. excellent tweak!

i'm supposed to be working today but the excitement is just a bit too much to bear!  ;D

http://diyparadise.com/web/index.php?option=com_content&task=view&id=141&Itemid=27

yeo

[ole]

Thank you, thank you, thank you all, especially PaulN. I just love this minimalistic buffer idea and the way it fits perfectly into the existing circuit, no extra power supply, no added coupling caps. And dirt cheap. And it works just fine with 35V, too.

I didn't have time to try it earlier, but today I bought a couple of 2SK170 and matched them with their Idss as suggested in the Nelson Pass article. The BGs are now off board soldered directly to the terminals, freeing up space for the two JFETs. They fit in nicely.

My amp has 100k input impedance, but still it greatly benefits from this. Yeo already said it, everything is better. It is. Indeed.

Thank you, thank you, thank you  8-)

Ole

[martinbls]

People, I can?t believe what I?m reading here...

I WANT TO TRY THIS!!

At night from thursday to friday I finished my Monica, well build into a nice chassis, very carefully wired and everything works very fine. Then, on friday, I noticed Yeo?s link to the new site with the FET-tweak! Boy, now I was too fast!

But I need little help here!! If I got things right, from the whole Pass B1 Buffer Preamp Yeo ONLY used the four FETs, correct? So the only thing Yeo did was adding the two 2N3819 FETs per channel to the output circuit of SS I/V gain stage and then Monicas output impedance has changed to a lower impedance, am I right? No need for other components such as capacitors or resistors, right? No need to adjust anything, because trimpots are already adjusted to 12 volts, right?

Ok, but he tried it on the seperate SS I/V, so what about Monica USB? To which pins on the pcb the two FETs have to be connected? Are there any connections on the pcb that have to be interrupted?

Please guys, help! I will disassemble my Monica to check this out!

And, just to not forget the fun of building things, a picture of my assembled Monica.

Martin

PS: This morning I bought some 2N3819 FETs, so I?m ready to mod! Please let me know how to!

[Dino Jakarta]

 ;D ... ?;D Haiyaaah... We all brotherhood of Renegade DIY Soldiers here !! ?;D ... ?;D

Whattacoincidence... yesterday I sent e-mail to Mr Pass asking his opinion on the possibility of implementing his B1 Buffer Preamp to my gainclone amp AND....
Today in this very afternoon of Jakarta humid heat... SO SHOCKED to learn that Y'ALL kind souls here have been intensively discussing Mr Pass design.... Haiyaahh....

My fault... my fault... :P ?Naughty me not visiting this forum frequently...
Gotta get my solder gun ready now...!!!

PS: Martinbls... ? hope it's not too late for us...

[Dino Jakarta]

Brief report... :-[
No luck for me..... haiyaah.... :-[   hummin hard :-[
 .... dunno why... checked all the groundings seemed OK...
(Better redo it tommorrow lah... coz its already 11.42 PM now my time... )

[assendor]

Hello Dino,

I could be asking a really stupid question here now, but Humming hard.. ?? If so, have me excused please :)

Did you remove the two(Four) Black Gates, before you soldered everything like yeo did, like on the Picture?  :-?
If you did, Did you remember in the excitement of the night, to re-mount them in signal path before your preamp after the B1-Buffer?
 ;)
You know how it is, if you are in a hurry hurry situation, and it is late, the brain is really not totally "available anymore"...

Just had to look really hard at the images myself, to see how it possibly is done.
HAVE to do this, have to try it, have to NOW..

Well what did I just say about late, It is 22:50 here now, Sleep , Tomorrow that is, Then I'll do it. But can I wait.. YES! HAVE too..  :-[

-Jonas