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PostPosted: Sun Mar 05, 2006 12:21 am 
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Yes I know...there is that other project I should finish....
However, I could not resist to post something which came to my mind after mulling again over Pedja's AD844 I/V, and also that "Supersymmetric" circuit Nelson Pass proposed with these so-called Current Feedback Op Amps.

Attached, on the top you find one channel of a very simple circuit, which could be used as a preamp.
To reduce the gain of the amp, in order to be able to run it open loop, a resistor must load pin 5, the TZ-node of the AD844. So I had the idea to use the volume potentiometer to accomplish this. And this way, the pot is "isolated" from the input as well as from the output.
The output buffer of AD844 is quite beefy and should drive even longer cables without problems. Max. output current is ?50mA.
And this diamond buffer runs open loop....do you read this, Jocko?

On the bottom, the same circuit, but in a balanced version.
Staying with the philosophy of such an amp, an open loop voltage regulator should be used, however they have a highish output resistance. This balanced version would be less demanding, as the supply current fluctuations of both amps will cancel each other (more or less).

A quad volume potentiometer would be needed in such balanced pre, or a nice relay volume control.

What do you think?
Would it be worth to buy some of these not so cheap AD844's? (as I have still to buy some parts for that other project... :? )

Ciao, Tino


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PostPosted: Sun Mar 05, 2006 3:25 am 
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We have been using a circuit like this for several years in our less expensive products. The sound is *far* beyond normal op-amps (but still not as good as zero-feedback discrete circuits).

One problem with your circuit is that the input offset current will cause problems. Another problem is that when you load down pin 5 with low resistor values (such as the 5k you have shown) then some problems crop up. I will leave it to you to find solutions to these problems.


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PostPosted: Sun Mar 05, 2006 5:09 pm 
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Thank you for the hints, Charles. I'm afraid that I will not see a solution to the problems on a theoretical way, so I have to buy these parts.

Regarding the offset, I think at injecting/sinking current with a ccs.

Regarding the other problem, I'm a little disappointed as I thought to place a pot at pin 5 loading the internal mirrors and having a buffered volume control.
If it is a current limit problem of the mirrors......I do not know their maximum current though. In the AD846 data sheet, which is a similar part, they claim maximum 1mA. So I could increase the resistor at the inverting input(s) to avoid that a signal of say max. 5V will saturate the mirrors even when pin 5 is connected with a very low resistor to ground. Although I believe that AD844 will allow higher currents in the mirrors.
Unfortunately, I can't really find out this from the datasheets. Maybe I'm just not bright enough...

Tino


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PostPosted: Tue May 09, 2006 4:43 pm 
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Tino,

this is an interesting idea. The AD844 is one of my most favorite opamps for audio. Few days ago my friend gave me an interesting hint - to use opamp output from a compensation pin (exactly as you did), to omit internal OA output diamond buffer and to use external discrete buffer. I am very curious how this would sound and I am gonna try it with AD829 and AD744.

Cheers,
Pavel


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PostPosted: Tue May 09, 2006 5:42 pm 
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Tino,

How about a buffer after the volume control............more isolation from the load? Looks like a great idea though. I might have a discreet vesion somewhere in my files from the Japanese publication MJ. I will post it if I can find it.

Jam


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PostPosted: Tue May 09, 2006 7:17 pm 
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Tino, you could probably use an 100k pot on that position.
Also, you should use a higher value resistor on the input (R1), use 1K, and it should be right at the input of the chip, so, after R2.

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"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein


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PostPosted: Tue May 09, 2006 7:22 pm 
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jam wrote:
Tino,

How about a buffer after the volume control............


Yes - a buffer after the volume control improves the sound, a pot can have low value. I once made something like this:

http://web.telecom.cz/macura/refpresch.gif

The buffered pot (from both sides) was a great thing.


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PostPosted: Tue May 09, 2006 7:30 pm 
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pma wrote:
Tino,

this is an interesting idea. The AD844 is one of my most favorite opamps for audio. Few days ago my friend gave me an interesting hint - to use opamp output from a compensation pin (exactly as you did), to omit internal OA output diamond buffer and to use external discrete buffer. I am very curious how this would sound and I am gonna try it with AD829 and AD744.

Cheers,
Pavel

Pavel,
Been there done that. Did not like it.
Gone discrete.
:mrgreen:

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PostPosted: Tue May 09, 2006 7:33 pm 
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Elso,

thanks for sharing your experience :mrgreen:


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PostPosted: Tue May 09, 2006 7:51 pm 
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pma wrote:
Elso,

thanks for sharing your experience :mrgreen:

It's hard work and I tried many circuits. Basically it similar to Sonnya's circuit.


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PostPosted: Tue May 09, 2006 7:57 pm 
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Looks very good. I will see through the circuit.


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PostPosted: Tue May 09, 2006 8:04 pm 
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pma wrote:
Looks very good. I will see through the circuit.

The discussion was here:
http://www.diyaudio.com/forums/showthre ... highlight=

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PostPosted: Wed May 10, 2006 1:32 am 
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Nice looking schematic - the jfet opamp. The choice of R1 and R2 looks to be quite high, are the values correct?

jh

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PostPosted: Wed May 10, 2006 2:42 am 
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Ah, back when DIYA had more than chip amps and home made interconnects. Sorry, back to topic.


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PostPosted: Wed May 10, 2006 5:59 am 
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Jim Hagerman wrote:
Nice looking schematic - the jfet opamp. The choice of R1 and R2 looks to be quite high, are the values correct?

jh

Jim ask Jocko! [joke]
If I remember well the values were discussed in the original thread.

Nice looking doggy, yours?

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