Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

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rjpjnk wrote:Note:  I no longer need a buffer for the phono stage because I was able to modify the Schiit Loki EQ input impedance.

In the mood to explore, I opened up the Loki to have a look at its circuits and saw that that it used a 475R resistor followed by a 47.5K shunt resistor to ground at the input to the first SS device (transistor?). I swapped the 47.5K resistor with a 220K and now the input impedance is 220K and the performance seems unchanged. Other than the fact that these components are extremely tiny SMDs it was a simple mod.

The Loki now seems to work perfectly in the PAS 3x tape loop. It now presents the impedance the phono stage is expecting and hence no longer cuts the bass response 6dB when I connect it.

I wonder what the consequences are...

Hi rjpjnk, just wondering if you have any longterm observations on running your PAS 3 with the modification to the Loki. I am thinking of going the same route,
using the PC 6 phono board to drive the Loki. Thanks in advance.

keego

keego38 wrote:

rjpjnk wrote:Note:  I no longer need a buffer for the phono stage because I was able to modify the Schiit Loki EQ input impedance.

In the mood to explore, I opened up the Loki to have a look at its circuits and saw that that it used a 475R resistor followed by a 47.5K shunt resistor to ground at the input to the first SS device (transistor?). I swapped the 47.5K resistor with a 220K and now the input impedance is 220K and the performance seems unchanged. Other than the fact that these components are extremely tiny SMDs it was a simple mod.

The Loki now seems to work perfectly in the PAS 3x tape loop. It now presents the impedance the phono stage is expecting and hence no longer cuts the bass response 6dB when I connect it.

I wonder what the consequences are...

Hi rjpjnk, just wondering if you have any longterm observations on running your PAS 3 with the modification to the Loki. I am thinking of going the same route,
using the PC 6 phono board to drive the Loki. Thanks in advance.

keego

Hi keego. No issues at all! The Loki works perfectly with its new higher input impedance and it is a super simple mod if you have a very tiny soldering iron. Best of luck!

BTW, this mod also allows the Loki to be used with older VTA phono preamps like the PH12 that do not have an output buffer.

Rich

Thanks for getting back Rich. Tell you what, last week i bit the bullet and lifted a secondhand Loki in near mint condition off ebay. I couldnt believe how tiny it was when it arrived! and then when I opened it up to do your mod, those tiny resistors were nearly invisible to me..

However, with the help of a strong magnifying glass and a steady hand i soldered in the 220k resistors.
It works a dream and it sounds amazing! I only use the phono board feeding a 250k Alps pot, which also acts as the power amp load, so I have no other tone controls. Now I can fine tune all those LP's that have no bottom end! About 60-70% of my collection sound great flat, so I built a tape loop circuit so I can completely bypass the Loki if desired. Sounds very transparent in circuit though..

Thanks so much for this mod., I'm totally happy now! System is Thorens TD 150, RB300 fully modded, Denon DL 110, PAS 3 phono stage, lots of mods,Lux KMQ7 power amp and Rogers LS3/5a from 1978 with new Falcon FB110 matched drivers. Oh, and Schiit Loki! It'll see me out!

Paul

Glad to help Paul. That Loki is a real nice tone control, and it really does completely bypass it's circuits when you set the switch. However, even in bypass mode the input impedance is still in the circuit! They never even mention that, but fortunately it was obvious from the circuit board. This is unfortunate because someone might put this in line with a tube amp and think it is having no effect because the bypass is selected. Wrong.

With you mod the impedance will be a constant 220K under any conditions.