Wanting a little gain

[FallenAngel]

Hey guys,

So I have a modded Zhaolu D2.5A DAC with a discrete output stage and a balanced JISBOS buffer that drives my headphones. I'm thinking that I might want a little extra gain from this setup so I'm looking for a simple (and preferably physically small) gain stage to add. Tubes are VERY welcome, especially if they run very low voltages.

Suggestions highly welcome, thanks!

This is what it looks like inside, just to give you an idea of how "physically small" I'd like it to be.



Thanks!

 

[nikongod]

If memory serves, there is a regulated +12/-12V source in the zhaolu.

The gain stage from the original millet hybrid with low voltage car radio tubes could be worked in with a couple caps.

If you are willing to use the +24V(ish) from before the filters, you can of course buy a few more volts. A plate CCS on a 12au7 or 6dj8 should get you an acceptable operating point.

incidentally, what is the offset voltage from the + to - outputs of the DAC chip? just measure across the "dac" side of the caps.

 

[FallenAngel]

Your memory is "almost" right, the voltages are regulated +/-15V and unregulated +/- 25V.

I forget what the voltage offset off the DAC was, I think 2.5V, but I'll have to double check.

My only concern is that I might be overloading the transformer (which I can turn 90 degrees for a little more room to work with).

Thanks

 

[nikongod]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Your memory is "almost" right, the voltages are regulated +/-15V and unregulated +/- 25V.

even better if you decide to do the tubes.
The unregulated negative voltage will hurt you, but if you can find 40V to use that opens many windows.
Quote:

I forget what the voltage offset off the DAC was, I think 2.5V, but I'll have to double check.

That sounds like the voltage from output to ground.
I mean from output to output.

If its low enough from output to output that you would be happy if it were a cmoy (20-30mV or less), a 600:10K ohm step-up transformer with the primary floating will also buy you a bunch of gain without using more power from the power transformer. Dunno how you feel about transformers, but they can be cool. A pair of Edcor (not the best from what I hear: have not tried this brand myself) will set you back $25 or so. You could of course spend much more on some other band which should sound like sex.
Quote:

My only concern is that I might be overloading the transformer (which I can turn 90 degrees for a little more room to work with).

Something to note is that the same transformer is also used for the upgraded headphone amp boards, and (im assuming you have a passive output on the dac) you no longer have the load from the original output stage. Im not sure how much power the jisbos's are using, but you may still have some to spare.

on that note, nothing leaks noise as bad as an over-loaded r-core transformer

 

[digger945]

How much power do you have to spare?

 

[cetoole]

Why not mod the JISBOS for a little gain? Might not be the easist thing to do directly on the PCBs, as it involves adding a couple resistors to each board, but shouldnt be too complex.

 

[FallenAngel]

nikongod:
I'll have to check on signal/signal voltage tonight, that was measured signal/ground.

digger945:
I believe the 4x JISBOS draws around 180mA under full load. Working backwards, there should be at least a couple of hundred mA to use.

cetoole:
There are a couple of threads about JISBOS with gain
HeadWize: DIY Workshop > Little discrete portable amp (a bit less stupid than first time )
http://www.head-fi.org/forums/f6/por...s-gain-282313/

I will be going through the schematics in more detail to see if this can be implemented, might certainly be the best (easiest) option.

 

[NelsonVandal]

Yes, something like this. No need for tubes. What good could they do?

Sorry for the low res pics. Head-fi can't handle .png that well. Higher res at: JISBOS WITH GAIN eSnips Folder

 

[MisterX]

Quote:

Higher res at: JISBOS WITH GAIN eSnips Folder

There was a schematic on that page?

From the "related folder" on the right:

Sorry, got a little distracted there.....

 

[FallenAngel]

Your schematic has quite different values than the stock JISBOS (which I already have), any suggestion on the quickest and easiest way to use the stock JISBOS values but add gain to it?

 

[cetoole]

Yup, that is what I was thinking Nelson. Why add another stage when you already have the gain to spare? Probably easiest to do it under the board.

 

[FallenAngel]

This being the JISBOS schematic, it looks like I can add 2 resistors (R13/R14 in NelsonVandal's schematic) between P1 and C3, while cutting that trace.

Good thing I have an extra set of JISBOS buffers around to play with.

Any recommendation on values and whether I should still keep C3 in the picture?

 

[cetoole]

Yeah, basically. You could also do it without cutting the trace by soldering R11 and R12 on the bottom, and just airwiring the feedback, instead of connecting that side to the board. As for the values: Linear Technology - LTspice IV Downloads and Updates

 

[NelsonVandal]

JISBOS with gain has only a little over 50 dB OLG. The output impedance is just below 3 Ohm. The distortion will be a bit high driving low Z phones, and I guess it's more suitable to drive high Z phones. It's fairly easy to do this tweak and find out if you like it or not. I find JISBOS both unity gain or with some gain to sound a bit grainy and warm but frank, dynamic and fun. It can be improved by the use of CFP input stage, cascoding or differential input stage, but then it would be another amp.

Nice frequency and phase response.

Open loop gain and phase response.

Not the fastest amp, and a slight overshooting, but even the "standard" JISBOS simulates this overshooting, but it's faster and not as easily seen without magnification.

Noise is a little bit high but not irritating in any way.

 

[NelsonVandal]

Distortion 1 V output 300R load

... and 30R load