2. amps-preamps
  3. 153702-525

Looking to Try a Passive Preamp

I am looking to try a passive preamp in a system using a Audio Mirror Tubadour III DAC having an output impedance of 1.5K - 2K ohms  and a BEL 1001MK5 amplifier having an input impedance of 22K ohms. I am interested in this approach due to having a problem with EMI and RFI interference with my tube based active  preamp which may become a subject of another thread herein.

I own a Schiit Audio SYS passive. My concern is, given it's input impedance of 10K ohms and it's output impedance of 5K ohms may make it not the best fit. I understand that the input impedance of the SYS and Bel amp are added to give the total impedance that is 'seen' by the DAC, the combined 32K+ impedance due to SYS/amp/2pairs interconnects and that should be OK.  I am concerned about the output impedance of the SYS which to my limited understanding seems too high. Am I correct on any of this? 

If I am correct in my concern with the SYS I am interested in finding another passive pre having input/output impedance better suited to my system. Suggestions sought.

Thanks in advance for any help. Mike
 
mesch
8 responses Add your response
dbxrecord
29 posts
 
According to the DAC manufacturer, there are alternatives to the low output 5977 tubes , and they recommend 6N1P, 6DJ8, ECC88, 6922 and 6H23P which can provide higher output.
They considered the 5977 the best sounding, but did not state what equipment they used for that subjective opinion. Perhaps try a set of the other tubes to see if if they solve your issue. 
mesch OP
3,481 posts
 
I have always known the output impedance of the Jolida Fusion to be 600ohms. One reason I paired it with the BEL. 
rff000
23 posts
 
I own the Jolida Glass FX DAC III and I was curious about the 47K output impedance figure because my preamp lists 13K as the input impedance and the audio sounds OK. Well, I just called Jolida and they said that the actual output impedance is 600 ohm, i.e. less than even 1K, let alone 47K! So my input of 13K is 21.7 times the output, which is over twice more than the recommended 10 times figure.

I asked why 47K is on the website even now and all they could say is that it must be a typo.
mesch OP
3,481 posts
 
Yes, the output impedance for the DAC seemed high to me. This is the information I received from Vlad at Audio Mirror. Also the output impedance for the SYS seems high. Is this so?

As to input impedances, am I right that the input impedance 'seen' by the DAC would be the sum of the input impedances for the DAC and AMP? 

Also the BEL input impedance its 27K ohms not 22K ohms. Did not catch this in my edit. Thanks George.

I am enjoying the DAC. My Jolida pre has an input impedance of 100Kohms so the DACs output is no problem. I was enjoying the pre until the RFI/hum problem. Will be calling Jolida to to see what can be done. I replaced the Jolida with the preamp section of my NAD C356BEE. This combination provides silent background. 
holycannoli
2 posts
 
someonenew
8 posts
 
georgehifi nails it again. Bravo!
georgehifi's avatar
georgehifi
10,312 posts
 
mesch
jmolsberg



No good for a passive with up to 2kohm dac output impedance and his power amp at 27kohm input impedance.
He needs an active preamp best would be a Schiit Freya with "passive" "tube" or "solid state" selectable outputs. Ideal because all with low gain.
And if the OP ever changes the dac to one with a nice low output impedance the the Schiit passive mode could be used to great effect.
https://www.schiit.com/products/freya-1

Cheers George
jmolsberg's avatar
jmolsberg
467 posts
 
Does that output impedance seem pretty high for the dac? 
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