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and now for something completely different part 3


kevin gilmore
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10 hours ago, kevin gilmore said:

newest version of the current feedback amplifier with the servo not in the audio circuit so that you can use cheaper opamps. But if you run 30v, you still need 30v capable opamp. Like opa551.  what the noise difference if any, I don't know yet.

 

Hi Kevin:

Other than removing the servo from the signal path, what other benefit does this added current mirror offer? 

And why would the SS Dynalo need it? Isn't the servo on the various Dynalo versions already out of the signal path?  

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I consulted with Kevin and there is an option to replace the servo with a trimpot similar to what was done on the original Dynalo.

I use a 10K trimpot (10K - 20K should work for supply rails between 24VDC - 30VDC) with the wiper wired to pin 6 of the servo OP socket and the two outer legs to V+ and V-.   With no input connected, I adjusted the output DC offset to 0V and it only drifts about 3mV from cold start to fully warmed up. 

offset trim pot.JPG

Edited by mwl168
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On 12/2/2017 at 10:28 AM, ti5002000 said:

Nice!

@mwl168 did you soldered directly a wire to the wiper pin?

No, take a closer look at the pic I posted; I soldered a 24AWG wire to one of the outer leg of the trimpot then plug the trimpot directly into pin 5 , 6 (out), 7 (V+) of the OP socket and insert the 24AWG wire into pin 4 (V-) of the socket. The socket holds the trimpot tight and it's totally reversible. 

Pin 5 of OP27 is NC but is offset trim for OPA445, it's not used for this amp so the pin 5 of the OP socket is not connected to anything on the PCB. 

 

Edited by mwl168
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4 minutes ago, kevin gilmore said:

pin 5 on the opamp is not a NC, so if you want to do this and put a jumper from pin 4 to pin 5 on the back of the board cut off pin 5 of the opamp, or bend out of the way.

the best servo of course is no servo at all as long as everything is stable with temperature

Thanks Kevin. I posted my edit the same time you responded and I stand corrected. 

I got pin 5 as NC from the OP07 and OP27 datasheet but as you said, pin 5 is not NC on OPA445 and OPA551. 

 

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12 minutes ago, mwl168 said:

No, take a closer look at the pic I posted; I soldered a 24AWG wire to one of the outer leg of the trimpot then plug the trimpot directly into pin 5 (NC), 6 (out), 7 (V+) of the OP socket and insert the 24AWG wire into pin 4 (V-) of the socket. The socket holds the trimpot tight and it's totally reversible.  

Understood Thanks!

9 minutes ago, kevin gilmore said:

pin 5 on the opamp is not a NC, so if you want to do this and put a jumper from pin 4 to pin 5 on the back of the board cut off pin 5 of the opamp, or bend out of the way.

the best servo of course is no servo at all as long as everything is stable with temperature

Yes, i will try this and see what is the result.Thanks

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I tried this myself yesterday, though I bent the leg on the trimpot and soldered a jumper wire to it, so nothing in pin 5 of the dip socket.

Seems to work fine, offset drifts some but nothing bad at all, probably in the +/- 3-4mV range while I was watching it. Listened to a cd with protection circuit in place and no issues.

 

BTW, Kevin, this is a very very nice amp! Congrats!


Sent from my iPhone using Tapatalk

Edited by Pars
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