3:1 Line Input X-former ? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 13th July 2005, 03:29 PM   #1
diyAudio Member
 
valveitude's Avatar
 
Join Date: Apr 2005
Location: Naches,WA
Default 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 . 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
__________________
Experience is a wonderful thing. It enables you to recognize a mistake when you make it again.
  Reply With Quote
Old 13th July 2005, 04:15 PM   #2
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
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.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 13th July 2005, 04:47 PM   #3
diyAudio Member
 
valveitude's Avatar
 
Join Date: Apr 2005
Location: Naches,WA
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


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
__________________
Experience is a wonderful thing. It enables you to recognize a mistake when you make it again.
  Reply With Quote
Old 13th July 2005, 06:45 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Default 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 . 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.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 14th July 2005, 04:21 AM   #5
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Line output as line input transformer? bambadoo Tubes / Valves 7 22nd January 2008 06:44 AM
Can I split line level input? dublin78 Chip Amps 15 14th February 2007 03:43 PM
line input impedances? Dominique Parts 5 5th August 2004 11:19 AM
line output to xlr mic input bogusblackman Everything Else 7 28th February 2004 12:32 PM
What's the best cap 'TYPE' for DC filter at line input. Brian Guralnick Parts 14 11th October 2003 12:08 PM


New To Site? Need Help?

All times are GMT. The time now is 10:53 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2