Posted by strieb (A) on July 30, 2008 at 04:42:05

In Reply to: RE: Questions about the TU-875 posted by vkung on July 29, 2008 at 20:29:23

Thanks Victor. Glad to hear that the pot is an Alps. The push selector — fine by me. But, I am not any clearer about the output impedance issue. If it were not stated as an Ohmeric value, wouldn't I still need a output current value as well in order to know the impedance?

Perhaps I am missing something here, but I am not aware of what that could be. In order to try and match components — even in ballpark terms — I think we want to know impedance values. If someone has a TU-875, perhaps they could meter it for me and write it up here on the forum.

I think that you had better be prepared for more questions like this one to come along in the future. As a distributer of these kits, potential customers are going to expect information and guidance. — Lorne
Sonic Lizard

• RE: Questions about the TU-875 - Caucasian Blackplate 01:29:56 07/31/08 (1)
  In Reply to: RE: Questions about the TU-875 posted by strieb on July 30, 2008 at 04:42:05

  The ST-70 has very high input impedance, around 470k. You could likely run a cap coupled 12AX7 preamp into the thing, so I wouldn't be too worried. By the way, what kind of tubes does this unit run?
  
  

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  • RE: Questions about the TU-875 - vkung 01:35:37 07/31/08 (0)
    In Reply to: RE: Questions about the TU-875 posted by Caucasian Blackplate on July 31, 2008 at 01:29:56

    3 X 12AU7
    
    

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• RE: Questions about the TU-875 - vkung 22:39:40 07/30/08 (1)
  In Reply to: RE: Questions about the TU-875 posted by strieb on July 30, 2008 at 04:42:05

  Unfortunately we have not measured the output impedance of TU-875, but it is estimated to be several hundred ohms. If the input resistance/impedance of Dynaco ST-70 is higher than 10kohm, then it should be acceptable to use it with TU-875. I am using TU-875 to drive my Audio Research VS-55 and VT-100 with very good results.
  
  

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  • RE: Questions about the TU-875 - strieb 08:12:47 07/31/08 (0)
    In Reply to: RE: Questions about the TU-875 posted by vkung on July 30, 2008 at 22:39:40

    Victor: Thanks for your prompt and forthright reply.

    The preamp that was originally designed to complement the ST-70 was the Dynaco Pas 2 /3 series. Two R's in the Pas are in parallel with the input impedance of the ST-70 ... thus ... R1 (62K) + R2 (510K) + R3 (470K) in // equals a load of (nominally) 50K Ohms.

    Perhaps the best way to procede goes like this: build the TU preamp. Then meter the output impedance. In accordance with whatever the result may be, the assembler can do adjustments by adding in R's — either in series or in parallel so as to bring the TU close to presenting a load that is a tenth of the input impedance of whatever amp is being used. AFAIK, a 10:1 ratio is the usual target. Cheers ... Lorne
    Sonic Lizard
    
    

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    Edits: 07/31/08

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