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-   -   TVC before Input Transformer? (http://www.diyaudio.com/forums/tubes-valves/110331-tvc-before-input-transformer.html)

dre7 19th October 2007 12:38 AM

TVC before Input Transformer?
 
Hi:

Is there any reason against using a transformer/autoformer volume control before an input transformer? Opinions?

resident 25th October 2007 03:56 PM

Hi Andrew,
Let's bring this thread in life! ;)

As I can understand you would like to use a passive TVC preamp and your amp has an input transformer.
I don't thing there is any problem.......
If you would like to know the value of the pot you must post the exact specifications of the transformers. So the experts over here will help!

dejanm 25th October 2007 04:21 PM

no problem ... use the opportunity to connect them with balanced connection ... and that's it ...

dre7 25th October 2007 04:51 PM

Thanks guys!

I believe the situation is more using an input transformer in general and how to size and place a pot to make it a TVC.

The amp is 600:80k input => 6s2s => 1:1 interstage splitter => PP6B4G. The amp sounds fine and quiet when a 100k pot. is placed across the secondary of the input transformer. What's concerning me is the reflected impedance to the source. I use a CDP through a DEQ. The output impedance is around 100 ohms.

In a perfect world (ha!) I'd like to get the input impedance of the amp at least around 10k, but using megohm value pots to do so isn't very satisfactory. I tried a 2M, and it was terribly noisy.

Options I've considered: I see Sakuma amps that use an input placing a 100 ohm pot on the primary side and loading the secondary with a 500k resistor. I've also heard of guitar amps that place a resistor across the input jack to establish the impedance of the amp.

I'd really appreciate any help you can offer.

resident 25th October 2007 09:44 PM

I think it's better to place the pot at the input (across primary) but I'm not sure for that. But that's not a TVC, even if you place the pot at the secondary.....
You need a trans(or auto)former with a few taps at the secondary to function as stepped attenuator. Say 12 taps (or much more) and then with a selector schedule you can choose how much attenuation you want.

dejanm 25th October 2007 09:59 PM

Normaly, the pot will be connected with 24 (or more) secondaries of the transformer. These secondaries have the function to adapt the output impedance of TVC, depending of the level of attenuation. If you look at the Music First site (they use S&B transformers) you will find typical values for output impedance depending on the attenuation.

The input impedance of the TVC is defined by primary of transformer(s) and if the output impedance of the source is about 100 ohm, that is not a problem at all.

The input impedance of the power amp can be critical. If it is 10K and more, than again - no problem ... I tried my TVC with Pass Labs X150.5, which has on RCA inputs 11K input impedance and it was working without any problems.

dre7 25th October 2007 10:25 PM

Quote:

But that's not a TVC, even if you place the pot at the secondary.....
I agree. It's more a transformer substituting for a preamp stage.
Thanks for the input, I have more to think about here. Ultimately, I think I'm going to go with a PEC pot, so I want to be sure of the value beforre comitting. ;)

Thanks again.


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