LM4780+ 2 Way Plus Servo

jackinnj · 6th November 2009, 08:59 PM

I am thinking that perhaps the GBW of the LM4780 will allow the elimination of a quad opamp, or perhaps just use 2 duals. I have some small 2-ways and want to biamp them so am going to have some boards made --

peranders · 09:14 PM

R13? not right

Aren't the DC servos a bit slow?

LT1014 too slow for audio, only 20 dB feedback. Only 0.2 V/us which is worse than µA741.

Redshift187 · 6th November 2009, 11:01 PM

R13 and R14 should be 20.5k, not far off.

DC servos are supposed to be slow, or else they affect the audio.

What is the purpose of U1A, U1D, U2A and U2D? It looks like they're filters, but what for?

jackinnj · 6th November 2009, 11:31 PM

Quote:

Originally Posted by peranders

R13?

Aren't the DC servos a bit slow?

LT1014 too slow for audio, only 20 dB feedback. Only 0.2 V/us which is worse than µA741.

1) 20.5K/1K OK

2) I won't use the LT1014 --it's just the IC I clicked on MultiSim -- could be National LME49740.

3) The other opamps are high-pass/low pass a la Linkwitz-Riley.

I have drawn it without buffer, and just one filter opamp.

gfiandy · 7th November 2009, 07:23 PM

Hi, I spent quite alot of time working on using power opamps as filters. It is fraught with problems as the gain bandwith product is limited. The very large currents and voltages at the output lead to problems with parasitic feedback.

It takes a long time to get this type of circuit working well so unless you really need to save the cost I would recomend keeping the extra opamps it will work better and be much more predictable.

Do a carefull noise analysis of your servos. The 2.2M resistors will be generating alot of Johnson thermal noise, now theoretically the caps filter this off but I am not sure what will happen at the negative virtual earth input of the opamp and you have massive gain here and I think the potential for noise to get amplifed. However I am not sure on this.

Regards,
Andrew

jackinnj · 7th November 2009, 09:19 PM

Thanks, Andrew, that's why I asked the question (s).

jackinnj · 8th November 2009, 01:28 PM

Here's a bode plot of the servo itself -- the response is down 56dB @100Hz if I've drawn everything correctly!

peranders · 8th November 2009, 02:35 PM

You must take into account temperature variations which contribute to the offset. The -3dB level should be in the area on 1-5 Hz.

You make also a mistake here. You must include the LM4780 (or an opamp) also. The LM4780 will make the response slower.

jackinnj · 8th November 2009, 08:00 PM

With an 8MHZ GBW virtual opamp as a surrogate for an LM4780, the f3 is 4.6Hz and the output of the servo itself is down ~30dB @100Hz.

peranders · 8th November 2009, 08:23 PM

Did you have the same circuit as with the one you have drawn? Did you connect the input signal to the surrogate opamp?

6th November 2009, 08:59 PM	#1
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	LM4780+ 2 Way Plus Servo I am thinking that perhaps the GBW of the LM4780 will allow the elimination of a quad opamp, or perhaps just use 2 duals. I have some small 2-ways and want to biamp them so am going to have some boards made --

6th November 2009, 09:07 PM	#2
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	R13? not right Aren't the DC servos a bit slow? LT1014 too slow for audio, only 20 dB feedback. Only 0.2 V/us which is worse than µA741. __________________ /Per-Anders (my first name) or P-A as my friends call me Last edited by peranders; 6th November 2009 at 09:14 PM.

6th November 2009, 11:01 PM	#3
Redshift187 diyAudio Member Join Date: Aug 2008	R13 and R14 should be 20.5k, not far off. DC servos are supposed to be slow, or else they affect the audio. What is the purpose of U1A, U1D, U2A and U2D? It looks like they're filters, but what for? __________________ Tyler

8th November 2009, 02:35 PM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	You must take into account temperature variations which contribute to the offset. The -3dB level should be in the area on 1-5 Hz. You make also a mistake here. You must include the LM4780 (or an opamp) also. The LM4780 will make the response slower. __________________ /Per-Anders (my first name) or P-A as my friends call me

8th November 2009, 08:23 PM	#10
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Did you have the same circuit as with the one you have drawn? Did you connect the input signal to the surrogate opamp? __________________ /Per-Anders (my first name) or P-A as my friends call me


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7th November 2009, 07:23 PM	#5
gfiandy Audio Engineer diyAudio Member Join Date: Jul 2003 Location: Cambridge UK	Hi, I spent quite alot of time working on using power opamps as filters. It is fraught with problems as the gain bandwith product is limited. The very large currents and voltages at the output lead to problems with parasitic feedback. It takes a long time to get this type of circuit working well so unless you really need to save the cost I would recomend keeping the extra opamps it will work better and be much more predictable. Do a carefull noise analysis of your servos. The 2.2M resistors will be generating alot of Johnson thermal noise, now theoretically the caps filter this off but I am not sure what will happen at the negative virtual earth input of the opamp and you have massive gain here and I think the potential for noise to get amplifed. However I am not sure on this. Regards, Andrew

7th November 2009, 09:19 PM	#6
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	Thanks, Andrew, that's why I asked the question (s).

8th November 2009, 01:28 PM	#7
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	Here's a bode plot of the servo itself -- the response is down 56dB @100Hz if I've drawn everything correctly!

8th November 2009, 08:00 PM	#9
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	With an 8MHZ GBW virtual opamp as a surrogate for an LM4780, the f3 is 4.6Hz and the output of the servo itself is down ~30dB @100Hz.

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