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Old 22nd January 2011, 05:09 PM   #851
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Originally Posted by Vinylsavor View Post




What could be done is to provide some switching at the output between 9:1 for headphone and 4.5:1 for linestage

Best regards

Thomas
Very doable; actually I made some line output transformers for this application with very good results. Even 1 VRMS into the 300 ohms HD600 will be very very loud. Nice tricks can also be done to make the transformer as a volume control as well (very good for load and noise!).
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Old 22nd January 2011, 06:32 PM   #852
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At some point it becomes an idea to use two stages in the preamp - a 26 direct coupled to a low Rp tube like a 6B4G, 2a3 or 71A. Makes the output transformer a hell of a lot easier to manage.

Andy
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Old 22nd January 2011, 06:57 PM   #853
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If the preamp is merely for line level it's being a separate unit is already questionable, adding another tube to get more drive would be a nice example of over-engineering IMHO.
Way more sense would make to drive an integrated line level input amplifier with an inductive volume control. These controls are very well able to drive long interconnects; something like this www.tribute-audio.nl/products/signal/inductive volume control.htm
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Old 22nd January 2011, 07:20 PM   #854
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Hi Pieter,

but how do you ensure that the inductive volume control is adequately driven? I found inductive volukme controls to sound best when driven from a low impedance source. Preferably a transformer coupled one.

This is the reason why a stand alone linestage makes sense IMHO to provide a high impedance to all possible sources and to act as a buffer to drive an inductive volume control nicely

Thomas
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Old 22nd January 2011, 07:43 PM   #855
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Hi Thomas,

In the meantime a bit OT..., but when I developed my autoformer volume control, I tested it with source impedances up to 3k, which is pretty "safe" I think. That asks for quite some inductance. My autoformer has an inductance of about 150H, that is an AC impedance at 20 Hz of almost 20k, going up with frequency of course. At the same time, using advanced core material, coil DC resistance could be kept very low at some 30 Hz. Bandwidth depends on volume position but with a 2k source impedance it nevers falls under 200 kHz. Already some 8 years ago I visited Dave Slagle and we did some comparisons, showing that the quality was very much on par, using pretty different materials. In the meantime there have been some improvements, and Dave's will not be the same now I guess, but these autoformers are excellent options for volume control.
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Old 22nd January 2011, 07:44 PM   #856
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coil DC resistance could be kept very low at some 30 Hz
should be 30 ohms of course....
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Old 22nd January 2011, 07:48 PM   #857
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Hi Pieter,

I have not heard yours yet, but I heard other TVCs which claimed to work well with highish output impedances. I never liked them.

As you read in this thread some DIYers don't really care a lot for low Zout even linestages with 7k Zout are not uncommon. I've seen Phonostages with even more.

To be universally usable a linestage should have very high input impedance and low Zout. That's what I'm doing with my commercial linestages since I don't know what customers use them with.

Different with my own stuff where I have everything under control.

Thomas
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Old 22nd January 2011, 08:01 PM   #858
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Hi Thomas,
There is a difference between transformer volume controls (TVC's) and autoformer volume controls.
Feedback from listeners always confirm the autoformer type to be more transparent and dynamic.
I do care very much for correct impedances between elements of a chain. When output impedances are (too) high I always find the sound stage to suffer; with correct impedances the sound stage is broad and deep. Therefore I wound for a high inductance with the autoformer; interface with normal source equipment seems OK, but yes not everything can be controlled. When something is wrong it points to something weird with the source.
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Old 12th February 2011, 05:54 PM   #859
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Quote:
Originally Posted by pieter t View Post
Very doable; actually I made some line output transformers for this application with very good results. Even 1 VRMS into the 300 ohms HD600 will be very very loud. Nice tricks can also be done to make the transformer as a volume control as well (very good for load and noise!).


Are we talking here of using the same 9:1 Lundahl OT, with headphone and line out hooked up differently at the seconday?

IIRC, the 1669 is wired something like 9 : 9 / 1 : 1 : 1 : 1, tho I don't really know what that means.
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Old 12th February 2011, 10:14 PM   #860
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E.T.,

No, I was talking of my own transformers (there seems to be more than Lundahl).
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