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Old 14th September 2020, 11:49 AM   #1
EUVL is offline EUVL  Europe
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Default A Simple Discrete Current-Mirror IV Converter, la AD844

.
This was supposed to be a private project, and I was not planning to publish.
But it is amazing to see people still talking about the AD844 as IV.
That is even though Pedja has published a very similar but discrete circuit quite some 15 years ago.
https://www.diyaudio.com/archive/blo...f-iv-stage.pdf

The problem with the AD844 is that it lacks bias.
The -Vin node has a Zin of 50 ohm, which implies a bias of about 0.25mA.
Using a 0.25mA-biased push-pull input to convey (say) 1mA current signal can only mean that it has to work in Class AB,
i.e. no longer push-pull as long as signal is > -6dBFS.
And yes, you can improve this by stacking 4x or 8x AD844s.
But it is a clumsy and no-longer-elegant solution, apart from expensive.

Just for fun, we did something more similar to a discrete-but-simplified AD844 than Pedjas.
It has a bias of 2mA, i.e. equal to 8x AD844 in parallel.
Performance is not comparable to the CEN IV, but it uses fixed rails and has an integrated emitter follower buffer.
Bandwidth & distortion were measured with 2mApp DAC current (e.g. PCM1704), and Riv is 2.8k for 2Vrms output.
For the likes of PCM1796, you can increase bias to 3mA and it will couple with 4mApp without much loss in performance.
For more DAC current, you will have to stack as well, or use through hole components for even higher bias.
We want to use SMD because you can get really-low-noise Toshiba dual BJTs that are easy obtainiums.
And they are monolithic, ie. perfect for thermal tracking.

A lot more fun than stacking AD844s.


Patrick

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Attached Files
File Type: pdf CM IV Description Public 200913.pdf (568.1 KB, 243 views)
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Old 14th September 2020, 08:28 PM   #2
MarcelvdG is offline MarcelvdG  Netherlands
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What types of transistors did you use? And what values for the other components?
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Old 14th September 2020, 08:38 PM   #3
diyiggy is offline diyiggy
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Very nice, how it compares vs the bulker discrete published by Pedja Rogic and made from 550C/560C bjt family please ? And vs an Opa 861 instead of the AD844 ?


Thnak you for sharing, plus the tiny board is very handy
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Old 14th September 2020, 09:06 PM   #4
miro1360 is offline miro1360  Europe
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I have a feeling this design will sounds great
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Old 14th September 2020, 09:06 PM   #5
EUVL is offline EUVL  Europe
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> What types of transistors did you use?
As mentioned in the article, low noise dual NPN and PNP from Toshiba, in SOT23-5.

> And what values for the other components?
Not obvious enough from the photo ?

> how it compares vs the bulker discrete published by Pedja Rogic and made from 550C/560C bjt family please ?
You can compare the THD plots at 1kHz 2Vrms output.
And the bandwidth figure (measured at 2Vrms) will give you a very good idea on the slew rate.


Cheers,
Patrick
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Old 14th September 2020, 09:09 PM   #6
EUVL is offline EUVL  Europe
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>I have a feeling this design will sounds great

I am sure the one published by Pedja (linked in post #1) also, be it somewhat different.
A real shame that it does not get more attention.

The nice thing here is the thermal tracking and matching of the dual devices.
And I have lots of them left over from the discrete opamp project :
One Last Attempt at Discrete Opamp in DIP8 Footprint




Patrick
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Old 14th September 2020, 09:19 PM   #7
EUVL is offline EUVL  Europe
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Just discovered a slight error in the PDF file, now rectified.
See attached.


Apologies,
Patrick

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File Type: pdf CM IV Description Public 200914.pdf (567.5 KB, 172 views)
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Old 14th September 2020, 10:24 PM   #8
miro1360 is offline miro1360  Europe
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Someone could try it with AD1862 DAC
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Old 15th September 2020, 11:25 AM   #9
EUVL is offline EUVL  Europe
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It is so simple that I am sure you can breadboard (or vero) this with through hole components in half an hour, for a first impression.
Just need some BC327/337 to start with.




Patrick
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Old 15th September 2020, 11:29 AM   #10
diyiggy is offline diyiggy
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Thanks Patrick,

Very brillant to have also putted headroom enough for pps 1206 case...my modest tweaking experience showed me it is a game changer vs C0G...at least when the music is playing
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