balanced input stage - Page 3 - 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 2nd June 2004, 05:31 AM   #21
diyAudio Member
 
Majestic's Avatar
 
Join Date: Mar 2004
Location: Melbourne
Default Re: balanced input stage

Another interesting thread. (You guys rock! )

Without wishing to take any of the attention away from Sonix's last posting, which seems to be waiting for commment; I am really curious about Sch3mat1c's ( - hmm, must be an audio h4ck0r! ) point about how the first schematic can offer both an unbalanced and balanced inputs.

Quote:
Originally posted by Sch3mat1c
WTF, that first schematic has an unbalanced input...but no way to balance the signal when using it... omgHUH??
Quote:
Originally posted by sonix
Click the image to open in full size.
Given this schematic actually comes from Svetlana themselves, I'm assuming they wouldn't have made such a fau-paxs?

So how does this work??
I've heard it's possible to theoretically balance a signal with just a couple of resistors, but never seen it done before. Is that what Svetlana are doing here? But then if it was that simple, why all the bother with imperfect phase splitters in our PP amps?!?
__________________
check out my avitar and go easy on me ok? :-)
  Reply With Quote
Old 2nd June 2004, 06:14 AM   #22
zobsky is offline zobsky  India
diyAudio Member
 
zobsky's Avatar
 
Join Date: Mar 2002
Location: Dallas, Tx, USA
sonix, .. you still haven't answered the question "why do you want a balenced input stage, .. for phase splitting in a PP amp or some other reason" , .. budget?

the responses and direction of this thread can vary markedly depending on the response.

PS. SY,.. i remember bothering you endlessly on this topic a few months ago, .. i will (eventually) get to build that 6bg6 PP amp, .. as time permits. your suggestions on balenced jensen input trannies are not forgotten (yet)

thanks, V
__________________
"Any fool can know. The point is to understand" - Albert Einstein
  Reply With Quote
Old 2nd June 2004, 06:51 AM   #23
sonix is offline sonix  Germany
diyAudio Member
 
Join Date: May 2004
@ zobsky:
my preamplifier has exits XLR and I would like to have therefore entries XLR at the end amp. i like XLR
  Reply With Quote
Old 2nd June 2004, 08:27 AM   #24
diyAudio Member
 
Majestic's Avatar
 
Join Date: Mar 2004
Location: Melbourne
Quote:
Originally posted by sonix
...
i like XLR
Same here. I'm really fed up with RCA jacks.
I feel like using XLR's for all my normal unbalanced connections even!
__________________
check out my avitar and go easy on me ok? :-)
  Reply With Quote
Old 2nd June 2004, 08:46 AM   #25
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Default Re: Re: balanced input stage

Quote:
Originally posted by Majestic
I am really curious about Sch3mat1c's ( - hmm, must be an audio h4ck0r! )


Quote:
point about how the first schematic can offer both an unbalanced and balanced inputs.
Given this schematic actually comes from Svetlana themselves, I'm assuming they wouldn't have made such a fau-paxs?

So how does this work??
The way I see it, the neon lights are either on all the time, to regulate cathode voltage, or they are there merely as a precaution to ensure it starts right and the voltages end up somewhat correct. The former doesn't make any sense in a balanced circuit. I guess it's because it's a direct-coupled circuit.. but I've made something more complicated (three 12AX7 triodes and half a 6SN7 in a circuit similar to Fred Nachbaur's quasi op-amp) without any parts to assure startup conditions. Basically it can't latch into another operating point because of designed-in stability, and I don't really see this one doing it either. But whatever, I'm droning on again. So anyway, the circuit is all balanced, as you might guess. I don't know what the output has in the cathode circuit but the other two stages (preamp and driver) are LTP. The negative supply voltage allows a vague approximation of a CCS for the cathode circuits of the tubes ('SL and 'SN), and cascading two will ensure relatively good rejection. (Think of it as a wrench on the end of a pipe. You can apply torque at the end, but if you don't push back on the pipe it'll be pulled out by the same force. If instead you balance this force by pushing with another wrench, there is no common mode force and the pipe stays put.)

Any resistance in the cathode circuit of a diff. amp (LTP) reduces the 'pushing' of the circuit to balance the force on that pipe. Ideally you use a CCS (pentode or transistor) so the circuit balances itself by sensing the unbalanced force and pushing back with another wrench of its own (in this case the other tube). If you have a balanced input then input controls both the wrenches. With unbalanced signals, the circuit gains control of the other one (indeed, note the switch that nulls input to that tube, putting things back on the behavior of the circuit instead).
But the cathode resistor is equal to the plate load resistor on the preamp stage so I doubt balance is very good there.

Anyway I've babbled on long enough here, you get the idea.

Quote:
I've heard it's possible to theoretically balance a signal with just a couple of resistors, but never seen it done before. Is that what Svetlana are doing here? But then if it was that simple, why all the bother with imperfect phase splitters in our PP amps?!?
I don't think so, unless you want to do tricky floating power supply kludges (I've seen such a phase splitter design before). It always involves an active or active-ish device, tube or transformer. Resistors can't invert a signal. However, you can certainly get the common-mode junk from a balanced line using three resistors. Fine for de-multiplexing a stereo signal but no good for general audio use where common mode rejection means lower noise pickup off the cable run.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 2nd June 2004, 09:34 AM   #26
benny is offline benny  Australia
diyAudio Member
 
benny's Avatar
 
Join Date: May 2004
Location: Melbourne, Australia
Send a message via MSN to benny
Default Re: balanced input stage

Quote:
Originally posted by sonix
hi,
i need a balanced input stage, like this:
Click the image to open in full size. , for a 300b se amp like this:
Click the image to open in full size..
is this possible?
thanks
sonix
Hi,

to make this simple, i take it all you want somthing similar to the seccond schematic, only a ballanced design which you showed in the first schematic??? right???

if this is all you want, then yes, this is perfect for what you want. if the output from your source is already ballanced as would seem logical from the XLR output you say your preamp has, then this idea works fine. The amp is baisically just two seperate identical sides, right from the input. I don't know what all the discusion of needing input tranny's was about, but if your source is already ballanced, unless you need impedance matching, then you won't need an i/put tranny or anything... just plug your ballanced source in, and it should be fine... simple.

If you want to use the unballanced input, then maybe this is where an I/p tranny would be good, for use as a PI, and this would now give you a ballanced signal.

That's making it as simple as possible for you as i can, to save any confusion... hope your project turns out good for you whatever you decide to do.

cheers
  Reply With Quote
Old 2nd June 2004, 09:37 AM   #27
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
I wondered about those neons too, and came to the conclusion that they are there to prevent Vhk limits from being exceeded when HT is applied before the valves have warmed up. Used as thermionic Zeners, they would destroy any self-balancing action that the circuit might have.

EDIT: Balanced does not mean impedance matched, even though traditional telecomms used 600R.

For a proper explanation of balanced audio, have a look at the papers by Bill Whitlock etc at Jensen Transformers site.
__________________
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 2nd June 2004, 09:59 AM   #28
sonix is offline sonix  Germany
diyAudio Member
 
Join Date: May 2004
@benny
Quote:
to make this simple, i take it all you want somthing similar to the seccond schematic, only a ballanced design which you showed in the first schematic??? right???
it's right, but i would like to use only one 300b. is that possible and would it work?
  Reply With Quote
Old 2nd June 2004, 10:03 AM   #29
benny is offline benny  Australia
diyAudio Member
 
benny's Avatar
 
Join Date: May 2004
Location: Melbourne, Australia
Send a message via MSN to benny
Quote:
EDIT: Balanced does not mean impedance matched, even though traditional telecomms used 600R.
who said ballanced meant impedance matched???

i know i didn't. i said:

Quote:
if your source is already ballanced, unless you need impedance matching, then you won't need an i/put tranny
this is saying, if the source is already ballanced, why use an input transformer.... unless of course, the impedance is too high and needs to be bought down... then this is a good reason to use an I/p tranny. I didn't say a ballanced input means the impedance is automatically right.

I will go and read those papers though, nothing wrong with reading as much as possible... always good to learn something new.

cheers
  Reply With Quote
Old 2nd June 2004, 10:25 AM   #30
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
I misinterpreted your punctuation to be a reference to the 600 Ohm legacy that's proving so hard to shake off. My apologies.
__________________
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

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
Balanced input stage beneficial for unbalanced turntables? leadbelly Analogue Source 37 28th April 2008 01:34 PM
Balanced line stage- input or output attenuator nobody special Pass Labs 4 7th April 2006 01:18 PM
input switching design for balanced line stage ijomojo Pass Labs 3 23rd January 2005 02:02 PM
Balanced Input Stage Bukitoo Solid State 5 11th March 2004 03:03 AM
balanced diff. input stage Altaic Solid State 15 21st July 2002 10:07 PM


New To Site? Need Help?

All times are GMT. The time now is 05:46 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