JLM99V as summing amp

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living sounds
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JLM99V as summing amp

Post by living sounds »

Hi Joe,

does the JLM99V make a good summing amp? I'd like to use it in the CAPI 2-ACA for this purpose. I would remove the resistor following the op amp, so there is a cap in place to prevent DC from magnetizing the transformer.

Thanks!

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Joe Malone
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Re: JLM99V as summing amp

Post by Joe Malone »

living sounds wrote:Hi Joe,

does the JLM99V make a good summing amp? I'd like to use it in the CAPI 2-ACA for this purpose. I would remove the resistor following the op amp, so there is a cap in place to prevent DC from magnetizing the transformer.

Thanks!
Yes JLM99v if you isolate the DC offset like you mention works well for this.

If you have a link to the schematic I can check it for any in and out problems for you.
Joe :-)
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living sounds
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Re: JLM99V as summing amp

Post by living sounds »

Thanks Joe,

here are the schematics. The channels are summed through 22k resistors.

Consolesumming.png

I would remove R14 to account for the DC offset at the output of the JLM99V. Signal path / DC blocking caps are bipolars, so no need to change anything there.

How would you describe the sound of the JLM99V in a summing application?
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Joe Malone
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Re: JLM99V as summing amp

Post by Joe Malone »

living sounds wrote:Thanks Joe,

here are the schematics. The channels are summed through 22k resistors.


Consolesumming.png


I would remove R14 to account for the DC offset at the output of the JLM99V. Signal path / DC blocking caps are bipolars, so no need to change anything there.
Perfect
How would you describe the sound of the JLM99V in a summing application?
At low levels very pure but adding warmth to the low end and silky high end as driven higher in level.

How many 22k summing resistors or channels will you have?
Joe :-)
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living sounds
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Re: JLM99V as summing amp

Post by living sounds »

Thanks Joe! I had a JLM99V in one of my BAs but it got destroyed accidentally. I remember it sounded better than all the other op amps I've tried...

The console has 32 channels + 8 returns, so 40 get summed to each side of the master bus in total.

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Re: JLM99V as summing amp

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living sounds wrote:Thanks Joe! I had a JLM99V in one of my BAs but it got destroyed accidentally. I remember it sounded better than all the other op amps I've tried...

The console has 32 channels + 8 returns, so 40 get summed to each side of the master bus in total.
Ok so a gain of 27 max which is easy. Also C15 will probably not be needed or should be smaller value like 22pF to 47pF
Joe :-)
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living sounds
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Re: JLM99V as summing amp

Post by living sounds »

Cool, thanks!

I forgot to add an inductor to the schematics. I added it between the input cap and the op amp's inverting input to reduce RF reaching the summing amp and increase stability. This shouldn't make a difference WRT to implimentation, I think.

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Re: JLM99V as summing amp

Post by Joe Malone »

living sounds wrote:Cool, thanks!

I forgot to add an inductor to the schematics. I added it between the input cap and the op amp's inverting input to reduce RF reaching the summing amp and increase stability. This shouldn't make a difference WRT to implimentation, I think.
Yes inductor is good idea as long as it is small and way out of band for rf rejection only
Joe :-)
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living sounds
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Re: JLM99V as summing amp

Post by living sounds »

Thanks Joe.

I use a 20uH inductor at the moment. Self-wound with a ferrite bead. I couldn't detect a change of the frequency response in the audio band.

BTW, what about the buffer stage after the fader? In the feedback path it has a 47k resistor with a 47pf cap on top. Would this be a good candidate to use a 99V, too?

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Re: JLM99V as summing amp

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living sounds wrote:Thanks Joe.

I use a 20uH inductor at the moment. Self-wound with a ferrite bead. I couldn't detect a change of the frequency response in the audio band.
Perfect. This inductor should be as close to the -in as physically possible.
BTW, what about the buffer stage after the fader? In the feedback path it has a 47k resistor with a 47pf cap on top. Would this be a good candidate to use a 99V, too?
Do you have a copy of this sections schematic?
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living sounds
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Re: JLM99V as summing amp

Post by living sounds »

Joe Malone wrote: Perfect. This inductor should be as close to the -in as physically possible.
Yes, that's where I put it. 8)
Joe Malone wrote: Do you have a copy of this sections schematic?
Sure:
ACA_Booster.jpg
Everything, including the dotted part, is actually there. As in the summing section I have used bipolar caps and will remove the resistor following the op amp (R8).
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Re: JLM99V as summing amp

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living sounds wrote:
Joe Malone wrote: Perfect. This inductor should be as close to the -in as physically possible.
Yes, that's where I put it. 8)
Joe Malone wrote: Do you have a copy of this sections schematic?
Sure:

ACA_Booster.jpg

Everything, including the dotted part, is actually there. As in the summing section I have used bipolar caps and will remove the resistor following the op amp (R8).
Yes will work fine. But I wonder why no one has recalculated all the resistor values in this whole design as the noise floor and crosstalk could be much improved without any real change to the circuit.
Joe :-)
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living sounds
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Re: JLM99V as summing amp

Post by living sounds »

Joe Malone wrote:
Yes will work fine. But I wonder why no one has recalculated all the resistor values in this whole design as the noise floor and crosstalk could be much improved without any real change to the circuit.
It's an exact copy of old API circuitry, so authenticity is the paradigm. :D

But I only care for getting the best sound out of it, so may I ask what you would change? Also, my console sums through 22k resistors, but that was not the case for the API console the mixbus was designed for. I tried different values for R5, but this audibly changed the sound in ways I ultimately didn't care for.

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