Median Compressor (Spirit of 1776) - Functional Testing - UPDATE

andrewsrea

Active member
As you can see from the pictures below, I have the Median Compressor populated and loaded in the enclosure for functional test. FYI: I am calling my build, the 'Spirit of 1776' - a little play on the UREI 1176 it borrows from. I had a few challenges so far:

- Reviewed the design, compared it to the UREI schematic and decided to build it stock, as it is a great design which could easily be used for bass and keys as well.

- I had accidentally selected VSSOP sized (smaller than a tick) OPA1678 IC's from Mouser, which they could not be used to the SOT-23 to SIP adapter I purchased from PedalPCB. I reordered SOT-23 size (checking the footprint specs to the adapter), which a dozen cost me $18 delivered vs. the $40 that (4) TSSOP / VSSOP adapters would have cost. I'll most likely throw away the VSSOP's, because it is unlikely I'll every have a use for them.

- For functional test, I've substituted TL072's for the OPA1678. As others have stated with their builds, you could definitely use TL072 and not spend the time and money on the OPA1678. I will be using the OPA's which I expect would have an ever-lower noise floor and transparent sound, however the TL072's had no noticeable noise floor, even with the 'Compress' (threshold) on full. The tone from them was nice and accurate.

- Surface mount MMBF5457 was my first attempt at the surface mounted FETS, which was a failure as I do not have a convenient way of auditioning and matching them. Therefore, they were never close to the targets. Luckily, I considered this might happen and had already socketed for T0-92 2N5457, which claimed to be NOS Fairchild components I bought from PedalPCB.

- Found a matched pair of these 2N5457 Fairchilds and got close to the schematic specs (Vgs = 1.43 for Q1) and found that setting resulted in symmetrical compression of a 1kHz sine wave on an oscilloscope.

To be honest, guitar compression as an effect has rarely been my thing (pumping & breathing, squishy tone, etc) and I have lots of experience with studio compressors (mostly subtle effects), which are used as gentile ways of getting an instrument or vocal to sit in a mix. That said, I really like this pedal compressor over a Ross, MXR (VCA type) or a DoD 260 (opto-resistor type). My favorite setting so far is the Compress at 8:30, Attack at 9:00, Release at 8:30, Blend at 3:00, Tone at 11:00 and Volume at 2:00ish.

Ready for paint and marking!


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Before you toss the VSSOP chips, my guess is that someone here who is used to getting boards made might be interested in them. This compressor is on my next order list. Nice, clean build!

Thanks, and yes, I will hold them for someone who could use them. I have been fixing large component items (mostly tube amps and SS) since the late 70's and even though I can solder like a pro, I cannot see myself soldering something smaller than a tick. I many have between 5 and 8 left.
 
Finally swapped the TL072 for the recommended OPA1678 and the impact is subtle. A tad more analogous translation (sound of the guitar in = sound of the guitar out), which can sound sterile. However, into a tube amp and using the blend on 90%ish, it sounds big.

The TL072's in the Median Compressor has a noticeably lower noise floor than standard pedal compressors. The OPA1678 was lower by about 6dB, but either you would have to crank the level and compression 100%, to hear noise (which I would never operate that way).

The pedal is inherently bright, which is great for dark amps. For a bright Fender Blackpanel type amp, I keep the tone at 9 o'clock which is a tad brighter than when the effect is bypassed.

In summary, this is a soft-knee studio grade compressor that along with its blend can make an instrument sound big and sustain longer, without anyone realizing it is compressing.
 
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