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Old 16th September 2004, 03:23 PM   #121
dqswim is offline dqswim  United States
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Thanks for pointing out the high dissipation of R3. 400mw is a bit much. If the value of R3 is raised to 33k (47K) the dissipation through the resister is reduced to ~119mw(83mw) allowing the use of a smaller 1/4 (1/8 th) watt resistors. These resisters will still pass 1.9ma (1.33ma) through the voltage setting Q4. I checked the data sheets for both the MPSA18 (“This device is designed for low noise, high gain, applications at collector currents from 1uA to 50 mA”) and the also the data sheet for the MPSA42 which seems to have a turn on current of under 0.1ma. I think staying over 1ma will still give adequate voltage stabilization.

When I simulated the circuit changing R3 to 33k(47k) also changes the value of R4 to 1.13k (1.15k) in this case.

I am unsure if a pot is needed to correct for variations between the zvp3310’s although I imagine it would help the current sharing through the 2sk389. I imagine that a pot across R4 would accomplish this or possibly a small drain resistor, neither of which will work well on the opamp boards This worries me a bit because I think small changes in VGS of the zvp3310 might greatly changing the current sharing through the differential pair. At the same time Nelson didn’t use any drain resistors in his DIY opamp article when he used the zvp3310a, maybe that is a good sign?

dave
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Old 16th September 2004, 06:27 PM   #122
dqswim is offline dqswim  United States
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Going over the opamp boards again I noticed the trim resistor R4 is there for exactly that reason and should be able to make adjust the current sharing.

Great that takes care of that worry, should be good.

-dave
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Old 17th September 2004, 12:14 AM   #123
BDP is offline BDP  United States
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dave,

Look at post #57 for what I came up with and the results. This was with +/- 15 volt rails.

BDP
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Old 17th September 2004, 01:18 AM   #124
BDP is offline BDP  United States
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Here is the op-amp with the zvp3310 and the pot installed. I'm only using one side of the dual op-amp.

BDP
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Old 17th September 2004, 02:55 AM   #125
dqswim is offline dqswim  United States
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I read through this thread (and post 57) awhile ago and just got a little confused on the ‘drain resistor.’ Now I’m pretty sure that I got it as R3 & R4 refer to the drain of the 2sk389 and not the ZVP3310.

The opamp on the towel looks good.

I am curious as to how you added a 100 ohm gate resistor to the opamp board. Did you just cut the trace on the front of the board between Q1 and the ZVP3310 and then bridge in the 100 ohm resistor?

Do you have any idea what kind of dc offset variation you would get with different zvp3310’s using the same R3 & R4? Or do you plan on using a 10k pot in all of the opamps you build up this way. That would make for a lot of pots when making the crossover. I need to read some more about the importance of the offset. All I can recall is that I think at unity gain the importance is diminished.

Another day or two and I should be getting a little gift from mouser, then the real fun begins.

regards,
dave
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Old 17th September 2004, 03:26 AM   #126
BDP is offline BDP  United States
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I installed the 100 ohm resistor in the gate/base hole and bent the gate and resistor leads and joined them together with solder. No cutting or drilling.

I have built three of these and only measured the first one for the actual drain resistance. I dont recall what the actual voltage was gate to source of the zvp3310, but I think it was around 3.3 volts. Since you are using a different CCS resistor the 1850 ohm I measured would be different. You could setup and measure the Vgs of a few zvp3310's for 10mA and design from there. A pot was easy and fit nicely. I plan on using these discrete op-amps for other projects with more gain so being able to adjust for zero offset is a plus.

BDP
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Old 17th September 2004, 09:51 PM   #127
dqswim is offline dqswim  United States
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Quote:
I installed the 100 ohm resistor in the gate/base hole and bent the gate and resistor leads and joined them together with solder.
That sure is a better solution than cutting traces.

I'll post what kind of variation I get between the zvp3310 devices when I get them and get a change to test them. I will also simulate what kind of change this would have on the circuit.

dave
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Old 22nd September 2004, 06:35 PM   #128
JohanH is offline JohanH  Netherlands
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Default And how does it sound?

I´m considering the MOX opamp as a buffer and in a 2way active XO.

Other than moamps remark that it outperforms several integrated opamps, I have not found any remarks on the MOX sound quality in the threads I´ve read.

Could any of you who´ve built it please post some comments on the sound of these opamps?

Thanks in advance,
Johan
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Old 4th October 2004, 08:14 PM   #129
Mark25 is offline Mark25  United Kingdom
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Default MOX X-over Q Question

could someone please explain to me why the MOX X-over is equiped with the ability to have a Q upto 2.4?

Any Q above 0.7 gives a hump in the electrical transfer charachteristic of the X-over

Thanks Mark
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Old 4th October 2004, 08:38 PM   #130
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Hello,

Many people think you need a LR response of the active filter to achieve the” perfect” system response. In fact IMHO the electrical response of the filter is not that interesting, because it is the total system response that you hear.

Changing the Q to achieve a “bump” in a HP filter in a SUB is one way of making the subwoofer player lower.

You don’t really know how the interaction between the different sections in a speaker performs, and therefore it’s nice to have the option of different Q values.

The mox filter is indented for prototyping only, the final filter is easily made a lot smaller on either breadboard or on any second order filter module.

\Jens
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