and now for something completely different part 3

[kevin gilmore]

continuous improvement

basically trading one opa445 for a opa551, 4 transistors, 5 resistors and 1 cap

probably should do the same thing to the ss dynalo.

[jose]

1 hour ago, kevin gilmore said:

probably should do the same thing to the ss dynalo.

Oh no! please Dr. Gilmore... I just finished doing it . 

 

[ti5002000]

Guys( the ones that already built the amp) ,what do you think in comparison with ss dynalo?

[Pars]

Yes

Edited November 30, 2017 by Pars

[mwl168]

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?  

[kevin gilmore]

ss dynalo has the servo's in the same place as the cfa amp. the audio signal is nowhere near as much as that in the ksa5, but still the quality of the servo opamp can have some slight effect. I can possibly measure the difference, but don't know if I can hear the difference.

[ti5002000]

Guys can i use this https://pt.mouser.com/ProductDetail/Vishay-Dale/RN60D5600FB14/?qs=sGAEpiMZZMu61qfTUdNhG1jRaYe8f2sLpmQ2711SR7M%3d for the 560ohm resistors ?

It only has 250mw of power rating..Is that ok?

Thanks

[Pars]

Yes. Dale RN60s are military rated, and will handle twice the power that they are rated for.

[ti5002000]

Thanks Pars!

[Pars]

Here's a revised ltspice model with the revisions made (I think, someone check this as it was done vs. the gerber file).

The current mirrors get the offset down to ~10mV without the servo opamp connected.

CFP2 v1.10.zip

[kevin gilmore]

no that is not correct. here is the schematic, and I hope the board matches it

cfa2cmirror.PDF

[Pars]

Looks like you have the servo input going into the noninverting pin of the opamp? Is that right?


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[kevin gilmore]

current mirror flips the polarity, so you have to flip the polarity on the opamp.

the difference between the 2 servo's is something like 1mv at 7 volts peak to peak output. so very little difference.

Edited December 1, 2017 by kevin gilmore

[Pars]

Here is an updated spice file per Kevin's latest schematic (pls check). Still shows -51mV without the opamp. I guess the current mirrors purpose is just to take the servo out of the audio signal path, and not about offset without the servo?

CFP2 1.10.zip

[mwl168]

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. 

Edited December 2, 2017 by mwl168

[ti5002000]

Nice!

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

Edited December 2, 2017 by ti5002000
question

[mwl168]

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 May 15, 2019 by mwl168

[kevin gilmore]

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

[mwl168]

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. 

 

[ti5002000]

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

[Pars]

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!


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Edited December 4, 2017 by Pars

[ti5002000]

Will this work on a ss dynahi or dynalo?

[kevin gilmore]

ss dynalo already has the pot, just don't populate the opamps

Edited December 4, 2017 by kevin gilmore

[ti5002000]

Oh yes ,of course....Thanks Kevin...But my Dynahi without the opa445 is definitely a no go for me...

[jose]

Guys... you're killing me!