Pultec HF Boost is acting like a bandpass filter
« on: November 15, 2020, 05:58:52 PM »
Just finished a Drip EQ1 with Vintage Windings Inductor and Sowter Transformers.  I'm having an odd issue that I was hoping somebody might be able to point out a clear place to start investigating.

The HF Boost section is acting like a bandpass filter.  Essentially, it reduces the gain of all frequencies except for the the selected Center Frequency.  I believe that this how the filter circuit should work however there's no subsequent makeup gain after the filtering, which is the problem. So the result is a reduction of all frequencies other than the center...doesn't translate as a boost without the makeup gain.

Additionally, the HF Boost knob exhibits the same filtering behaviour (across the entire frequency spectrum) even when the unit is bypassed.

Makeup gain is present with LF Boost/Atten and HF Atten.

If you happen to have any spontaneous insights to this specific behaviour, I really appreciate your feedback.

« Last Edit: November 15, 2020, 07:52:26 PM by earth tones »


abbey road d enfer

Re: Pultec HF Boost is acting like a bandpass filter
« Reply #1 on: November 15, 2020, 11:43:19 PM »
Just finished a Drip EQ1 with Vintage Windings Inductor and Sowter Transformers.  I'm having an odd issue that I was hoping somebody might be able to point out a clear place to start investigating.

The HF Boost section is acting like a bandpass filter.  Essentially, it reduces the gain of all frequencies except for the the selected Center Frequency.  I believe that this how the filter circuit should work however there's no subsequent makeup gain after the filtering, which is the problem. So the result is a reduction of all frequencies other than the center...doesn't translate as a boost without the makeup gain.Additionally, the HF Boost knob exhibits the same filtering behaviour (across the entire frequency spectrum) even when the unit is bypassed.
That seems to indicate the load impedance is too low, like that would happen if you tested directly with earbuds. What are you connecting the output to?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Pultec HF Boost is acting like a bandpass filter
« Reply #2 on: November 16, 2020, 12:09:05 AM »
My testing setup is Pro Tools Signal Generator white noise > Audient id44 converters > balanced XLR patchbay > Pultec I/O > balanced XLR patchbay > Audient id44 converters > Pro Tools

gyraf

Re: Pultec HF Boost is acting like a bandpass filter
« Reply #3 on: November 16, 2020, 02:39:59 AM »
..you should REALLY ask Drip, as it's their product - it's always MUCH easier to support a design that you know well because you made it..

/Jakob E.
..note to self: don't let Harman run your company..

Re: Pultec HF Boost is acting like a bandpass filter
« Reply #4 on: November 16, 2020, 05:55:58 PM »
..you should REALLY ask Drip, as it's their product - it's always MUCH easier to support a design that you know well because you made it..

/Jakob E.

Yeah, that's definitely right. The downside to using a PCB layout from somebody else is troubleshooting. No schematic to reference.

Re: Pultec HF Boost is acting like a bandpass filter
« Reply #5 on: November 17, 2020, 11:52:31 AM »
Issue resolved...wanted to post an update for any other DIY amateurs out there.  Don't forget to start with the most basic tier of troubleshooting.  I disassembled my Pultec and went over every solder joint with a magnifying glass under bright light.  There was a tiny solder bridge between terminals of the potentiometer controlling the HF boost.  Cleaned up and the unit operates as expected. 


 

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