Harrison Ford Filter 500 series overtone problem

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Martan

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Nov 9, 2019
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Hi all,

I’ve built a functioning Harrison Ford Filter in a 500 format but it has some serious even order harmonic distortion (clearly audible) in both engaged and bypass modes. I have the switch and pots wires off board. I’ve reflowed all the joints, switched all the ic’s, changed grounding scheme from shared ground to separate audio ground, and ensured chassis ground. Any ideas what I should check? The off board wiring for the pots was neater before I realized the HPF and LPF needed to be flipped in the chassis (the problem was there before hand)
https://imgur.com/MIu4AN3
 

TwentyTrees

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Sep 17, 2015
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Does the filter work? Assuming that the distortion you mention is not there with a loopback test, the only things in circuit when the Harrison Ford Filter is in bypass are the line receiver and driver - INA134 & DRV134 on the schematic I've got. What level are you feeding the unit with? How much 2nd harmonic distortion are we talking about? Are all component values correct?
 

Martan

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Nov 9, 2019
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Hi,

Thanks for the reply. It’s being fed from Logic Pro with an i/o insert. Here’s a sine wave without the insert:
https://i.imgur.com/4z6RgLp.jpg
And then with the filter inserted:
https://i.imgur.com/So47BUq.jpg

It’s like an octave effect, really. I checked all values while populating, and the caps have been rechecked. I might have to remeasure the resistors, as they are the Dale type without bands and some have the values facing down :p. I do have a second build  that is ready to be wired up. Will see if it’s a fluke in the next day or two.

Thanks
 

Martan

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Thanks. Will do. I've been going off of the BOM in the support thread since the original schematic link was dead. I found this one that has been drawn on. Is it correct?

https://imgur.com/tgkg4YO

Marty
 

Martan

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abbey road d enfer said:
this and the excessive THD point to an incorrect load; not strictly a short, but something like a 10 ohm resistor instead of a 10k.

Thank you for the tip. I pulled a leg on every resistor and they all seem correct. All 10k or 390R. Double checked the caps as well. I tried different I/O cables as well as different slots in the lunchbox. Switching to a square wave instead of sine has the same effect but the harmonics change relative position. The ICs are DRV134PA, INA137PA, and TL074CN, but I think they all seem fine.
 

12afael

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the schematic is based on my original drawing. someone added those lines to the power supply.
you should have 6db of difference not 10db and the excessive load would explain the distortion.
I guess the drv134 should be able to drive 600ohms so unless you have it connected to a low impedance mic preamp it should be fine.

maybe you could try disconnecting the positive and negative output to see if having it unbalance give you some clues.

but yes I think is important to find where are you loosing signal.
Is one of the chips hot?

cheerz

 

Martan

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Nov 9, 2019
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Will check on the hot chips. I just ordered a test jig from capi to help me get at the board while in the lunch box. Thanks guys.
 

Martan

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Nov 9, 2019
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Update with CAPI test jig:

I got the test jig and tested some things I couldn't really before. Without chips there is +11V and -11V on the +/- input pads. Is that supposed to be there?

With an audio probe, In+ at the INA137 is much more quiet than In-. I lose some signal volume between the capi 500 pcb adapter and the input pads on the HFF.

Just for reference, the gear in between DAW and HFF is an apogee symphony and a BAE 500 rack. Going to disconnect In+ and check results but wanted to ask about that input voltage.
 

Martan

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Nov 9, 2019
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Next update:
DRV chip too hot to touch and now getting audio probe signal at pin 4 but no output at pins 1 and 8:(
 

12afael

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I got the test jig and tested some things I couldn't really before. Without chips there is +11V and -11V on the +/- input pads. Is that supposed to be there?

I don´t think that is right.
 

Martan

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Nov 9, 2019
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Next bit of info (trying to sneak a few minutes at a time):
The pin out for the drv chip is fine with no power applied, but continuity shows up on pins 3 (gnd) 5 (-16V) and 7/8 (output +) when energized (bare socket). I’m guessing a shorted electrolytic on the negative power rail, but haven’t dug in yet (weird that it would happen on two builds—-backwards cap?) edit-Just checked, they are not backwards
 
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