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-   -   3:1 Line Input X-former ? (http://www.diyaudio.com/forums/tubes-valves/60714-3-1-line-input-x-former.html)

valveitude 13th July 2005 03:29 PM

3:1 Line Input X-former ?
 
Well, Sy & EC8010 threw a much needed frosty glass of reality in my face by pointing out the input capacitance issues of my Spud farm (Tater Patch) amplifier idea:bawling: . Turns out that a source impedance of around 3.3k (10k if driving a single tube) is about as high as you would want to go without compromising the HF extension.

I am hoping that the “collective” here could recommend a line transformer with a 10k:3.3K, or thereabouts , ratio. Now, I don’t need the normal bottom extension of a full range transformer since I’m rolling it off at 100hz anyway, but the highend rolloff is important.
Any and all suggestions would be appreciated .

Thanx,
Casey

EC8010 13th July 2005 04:15 PM

I'm afraid I'm going to do it to you again. Transformers don't like capacitive loads; it makes them ring. You could always use SY's "Heretical" line stage - and it would be able to drive cable capacitance, so you could have it handy to the listening position but put the power amplifiers near the loudspeakers.

valveitude 13th July 2005 04:47 PM

Quote:

I'm afraid I'm going to do it to you again.
Lovely

Quote:

Transformers don't like capacitive loads; it makes them ring
:headbash:

Quote:

You could always use SY's "Heretical" line stage
Actually, that did come to mind. I'll have to re-read that thread. If I follow through with this amp, its becoming clear that some form of follower will be needed.

Thanx...I think;)
Casey

SY 13th July 2005 06:45 PM

Re: 3:1 Line Input X-former ?
 
Quote:

Originally posted by valveitude
Well, Sy & EC8010 threw a much needed frosty glass of reality in my face by pointing out the input capacitance issues of my Spud farm (Tater Patch) amplifier idea:bawling: . Turns out that a source impedance of around 3.3k (10k if driving a single tube) is about as high as you would want to go without compromising the HF extension.

"Duuuude, you're harshing my buzz!"

I'm in accord with EC. Put a follower on your amp's input and all will be well. If you're not afraid of putting in a negative rail, you can use the CCS topology from the Heretic, then direct couple into the grid of the problem child tube. You'll have to readjust the biasing a little to account for the 1.5-2V of offset, but IIRC, you had the cathode resistors bypassed anyway.

PRR 14th July 2005 04:21 AM

Ideally, you get 1/9th the capacitance, but have to put in 3 times the voltage. If you figure the reactive power, it hasn't changed (no surprise). The voltage step-down forces you to put more gain in the tube before it, which generally leads to a higher-Mu tube. Since Gm for a given cathode size and grid-winding precision is fairly constant with Mu, you end up with a higher plate resistance. Which means less power output. It is at best a break-even game, and tends to be a downhill slide.

The transformer adds winding capacitance, and its leakage inductance adds to your output impedance, shifting the roll-off from 1-pole towards 2-pole.

What you have done is eliminated the driver from the power amp, and minimized the preamp. You can't do both. If your preamp could drive the IHF standard load, 10K||1,000pFd, you would have no great difficulty driving your Spud. Or if you used a conventional 12AX7 input power amp, 1Meg||100pFd, you could use a more minimal preamp than what the IHF standard load suggests. But you've chopped too much.

Keep the preamp output impedance down around 1K and you can drive about anything in the hi-fi world. Violate that, and you limit your options. The Spud may not be your last amp ever; do you want to be forced to build a new preamp to go with your post-Spud amp?

Cathode follower is indeed the simplest cleanest fix. (Well, a HV BJT or MOSFET is simpler, but not allowed in this forum.)

Mouser has 10K:2K transformers that will take several volts at 100Hz for about $1.50 each.


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