Passive Preamp for Tiode ST-70

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Passive Preamp for Tiode ST-70

Postby LaserBoi » Tue Jun 01, 2010 1:19 am

I have a Triode Electronics ST-70 amp. Input impedance of 470K and sensitivity of 1.3v = full output. I am using a Cambridge Audio iPod dock which has an ouput of 1.7Vrms. I have enough gain and I want to add a passive preamp (volume control) box to the front end. I do not want to modify the amp. I simply would like to know what value of pot or stepped attneuator would be the best. I am thinking 10k or 20k, I have tested using a 50k cheap pot but I think there is a little roll off in the high end so I think I should go down some.

Thanks.....Mike ~~
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Postby Geek » Tue Jun 01, 2010 3:30 am

Hi, welcome!

Most people I know use 100K, but if you can hear a rolloff with 50K.....

What cables (if any) are you connecting the pot to the amp? If "audiophile" stuff, that can have HUGE capacitances, contributing to the rolloff.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby LaserBoi » Tue Jun 01, 2010 3:43 am

I am using good quality cables. 1.5 meters long. Not the 200 dollars ones or anything. I am in Hong Kong so they are about 30 dollars a pair. I may be fooling myself into thinking I hear some rolloff. I have a 20k stepped attenuator and a 100k one as well. I was thinking of trying the 20k in a fairly standard circuit "in" through the attenuator grounding the other end and using the wiper out, and trying the 100k in a shunt type Haller setup with 10k and the 100k as the shunt. I can also try the 100k in the standard ciruit type as well. Then I guess just listen to them and see if I hear any loss in the high end or not.

Oh and thanks for the Welcome,
Mike~~
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Postby Geek » Tue Jun 01, 2010 6:23 am

Hi,

Could it be the pot then?

One doesn't usually need "golden ears" to hear the difference between an Alps and an Alpha ;-)

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby LaserBoi » Wed Jun 02, 2010 7:21 am

Yes it could be the pot. it is just a cheap Log 50k to test with. I will check it out withthe stepped ateenuators I have and that will most likely tell me. On the DACT website there is a tool to show rolloff using different value pots with input impedance and capacitance. Using a 100k pot and 470k input it doesn't take very much capacitance to start having significant rolloff and most passive preamps (should be called volume controls) that are pre made use 10k or 25k in them even the ones that cost a few hundred dollars. So I would think 100k would be a little high. That is why I am so confused about what the best value would be. So I really appreciate all the input I can get.

Mike~~
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