Implementing JLM99V opamp in active eq

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stitchy
Posts: 7
Joined: Wed Jul 02, 2014 10:20 am

Implementing JLM99V opamp in active eq

Post by stitchy »

Hello!
I acquired a couple of active EQ boards that were originally designed to sit in the feedback
circuit of a tube eq.
I'd like to implement them in a discrete opamp active EQ circuit instead.
I have a pair of JLM99V amps, JLM PS kit, and 1:1 600 input and output transformers for balancing.

Boards:
Image
Image
Schemo:
Image
Here is the basic hookup I was given with the boards:
Image

Questions:
-Would the simple circuit design presented for hookup be all that was needed?
(for instance, in the tube EQ, "C" passes to ground through a 8.8uf cap)
-Any other considerations for hookup of the opamps?

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Joe Malone
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Re: Implementing JLM99V opamp in active eq

Post by Joe Malone »

stitchy wrote:Hello!
I acquired a couple of active EQ boards that were originally designed to sit in the feedback
circuit of a tube eq.
I'd like to implement them in a discrete opamp active EQ circuit instead.
I have a pair of JLM99V amps, JLM PS kit, and 1:1 600 input and output transformers for balancing.

Questions:
-Would the simple circuit design presented for hookup be all that was needed?
(for instance, in the tube EQ, "C" passes to ground through a 8.8uf cap)
-Any other considerations for hookup of the opamps?
See attached diagram.
Need to add the 2 x 470uF 16v Non Polar caps so the volt or two of DC on the output and -in of the JLM99v doesn't make any noises on the cut and boost pots.
Make sure the input transformer is never discounted as it is the only referance to 0v ground that holds the opamp +in at 0v for this to work properly.
You may also need to add the 2 x 47pF if you see any oscillation at full cut or boost.
C should just go directly yo 0v ground. No 8.8uF needed.
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Joe :-)
JLM Audio
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stitchy
Posts: 7
Joined: Wed Jul 02, 2014 10:20 am

Re: Implementing JLM99V opamp in active eq

Post by stitchy »

Hi Joe!

Makes sense, cheers and thanks!
470uf fixed cap suggestion?
I don't know of any film caps that large in value...

CHEERS!

-s

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Joe Malone
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Re: Implementing JLM99V opamp in active eq

Post by Joe Malone »

stitchy wrote:Hi Joe!

Makes sense, cheers and thanks!
470uf fixed cap suggestion?
I don't know of any film caps that large in value...

CHEERS!

-s
Since C used to work with 8.8uF to 0v ground then the 470uF going to B could be that value or 10uF and should work fine. Use non polar electro cap and if you want it to be like a film cap bypass it with one.
Joe :-)
JLM Audio
Capturing Audio without Injury

stitchy
Posts: 7
Joined: Wed Jul 02, 2014 10:20 am

Re: Implementing JLM99V opamp in active eq

Post by stitchy »

Output coupling as well?
Though I would imagine the higher the uf value, the lower the frequency load dependent?
"When low frequency oscillation is not a threat, I generally go by this rule of thumb Keep R x C = .1
1uF for 100k"

If this is true, then ~120uf would work for 600 Ohm output primary load?

Image

Cheers and thanks!

-s

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Joe Malone
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Re: Implementing JLM99V opamp in active eq

Post by Joe Malone »

stitchy wrote:Output coupling as well?
Though I would imagine the higher the uf value, the lower the frequency load dependent?
"When low frequency oscillation is not a threat, I generally go by this rule of thumb Keep R x C = .1
1uF for 100k"

If this is true, then ~120uf would work for 600 Ohm output primary load?

Image

Cheers and thanks!

-s
It could work fine with the small cap to the output transformer as well but you have to watch that as you approach DC the output transformer goes from 600ohm to its dc coil resistance which will be very low. So the small value cap may roll off the low end faster than you think and cause frequency and phase problems. If you have the test gear to check this then start with the small values. Using the larger values technically can improve the noise floor if the rest of the circuit is very quiet but since there are inductors this will probably not be the case. You can use a large value tant between -in and B as long as you have it the correct way around for what ever dc is present and you watch with a scope that there are no long reverse dc voltage pulses on the cap during power up and down. You may also need to resistor load the input transformer secondary to keep it flat under the varying eq load. There are a lot more things to get right before having to think about the very tiny tiny improvement film caps have over bypassed non polar electros :-)
Joe :-)
JLM Audio
Capturing Audio without Injury

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