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Use of a Schiit Loki with Dynaco PAS 3x

I own a Schiit Loki mini EQ and am very happy with it for use with modern solid state gear.  However, I have a second system that is all vintage vacuum tube gear and the Loki input impedance makes it incompatible as is.

In my case, I want to use the Loki in the “tape loop” of a Dynaco PAS 3x preamp. Like many vintage tube preamps, the phono stage of this amplifier is unbuffered, and it is necessary to add a buffer circuit in order to include a modern equalizer in the mix. But I have found that with a very simple modification the Loki can work perfectly with this preamp, and I would like get opinions on this.

Here is the issue.

The Loki has an input impedance of about 50K. The PAS 3x phono stage (at its Tape Out jacks) wants to see an impedance of 225K or its frequency response will suffer. When I connect the unaltered Loki the bass response drops by 6dB at 20Hz due to the excessive loading of the Loki.

This is exactly in line with published test results as shown here:

http://www.audioregenesis.com/documents/PAS_Phono.pdf

I decided to have a little fun. I opened the Loki to examine its circuitry and found that the input to the first SS device consists of a 475R series resistor followed by a 47.5K resistor shunt to ground. This was indeed producing the 47.5K measured input impedance. As an experiment, I replaced the 47.5K resistor with a 220K resistor on one channel and it appears to work perfectly this way. I plan to modify the other channel to match.  With this higher input impedance the Loki can now be used with my Dynaco PAS 3x tube preamp with no loss in bass due to impedance mismatch, and I have not been able to detect any other change in performance.

I have swept the frequency response and both channels appear to perform identically. This resistor does not appear to be used as part of a bias setup. There was no measurable DC change on the input to the first 3 terminal device this network feeds.I do not know what the device is, but I assume it is a BJT. THere are two in parallel on each channel.

I would like to know if this is a good way to fix this problem, and if there is any technical downside to this modification. If this is not the proper place for a technical post I would appreciate a redirect.

Thanks so much!


rjpjnk
imhififan's avatar
imhififan
2,184 posts
 
Like cartridge loading, if sounded good, it's good.
atmasphere's avatar
atmasphere
11,806 posts
 
You already did the change- I think you're good to go.
Have you refurbished the PAS-3?? If its stock, it needs new power supply filter capacitors for both the high voltage and the filament supply! I would also replace the selenium rectifier with a silicon device. A while back I had a custom switch built up by Shallco to replace the volume control; I might still have some of those on hand. If you replace the tube rectifier with HEXFREDs, the output voltage is higher- thus allowing for regulation. I think we still have some regulator boards on hand for that as well. They fit right inside the preamp so from the outside it looks quite innocent :)
rjpjnk OP
1 posts
 
Yes, the PAS is already restored in every way I want at this point, including B+ and filament power supplies, and any out of tolerance components. I even added a MOSFET buffer and a second set of output jacks so it could drive a subwoofer without effecting the frequency response of the output. (The sub has a 10K input impedance which is way too low for a PAS to drive otherwise). Works like a charm and does not load the PAS power supply much. Everything about this PAS 3x works great, and it sounds great too, but it sure is picky about impedance matching its outputs: both phono stage and line stage.

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