Impedance multiplier hybrid parafeed - 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 20th June 2012, 05:17 AM   #1
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Default Impedance multiplier hybrid parafeed

Here is a generalization of the anti-triode concept. It's a hybrid with a tube voltage amplifier followed by a true current amplifier. The current amplifier is not a simple voltage follower, but a power stage with a defined current amplification ratio. A defined fraction of the output current is handled by the tube stage and the rest is supplied by the output transistors. I've scaled this so it can use a PA matching transformer as a parafeed OPT.

This allows the actual speaker load line (or OPT primary load line) to be scaled up in impedance to the voltage amplifier current range. The tube sees a constant fraction of the output load, allowing the tube to respond to the speaker in classic SET fashion, but at a higher output power level. Thus the characteristic is imparted by the tube, with the output power scaled up by the current amplifier stage. Another way to think about it is an impedance multiplier which scales the output impedance up by some defined ratio so the plate of the tube sees a scaled version of the actual loudspeaker load.

I built a simple op amp controlled circuit to test the idea.

The first stage is a SE totem pole anti triode with the idle plate voltage arranged to be near the midpoint of the power supply voltage so as to allow for symmetric swing. The anti triode is a 2:1 current booster or a 2X impedance multiplier.

The output node of the anti triode is directly coupled to the output stage such that a 3 way current split is achieved between the tube, the anti triode MOSFET Q1, and the output stage. Current is split according to the ratio of resistors R6 to R4 & R5 in parallel, here about 10:1 resulting in a 5K bootstrapped load at the anti triode output node, which results in a 10K plate load on the triode, scaled up a total of 20X from the 500 ohm OPT primary load.

Another way to think about it is that the triode is responsible for the characteristic transfer function and plate resistance by driving a defined fraction of the load. The MOSFETS handle most of the current while letting the tube work into a scaled version of the speaker load.

The output impedance is approximately the plate resistance divided by 20 or about 50 ohms, giving a damping factor of about 10.

The op amps are CCS powered and track the output voltage. The op amp feedback is arranged to provide a precise signal current split at all times.

Bias is servo stabilized at 100 mA on each side separately using the 1.2V drop across a diode connected darlington transistor and the 12 ohm source resistors.

The output stage operates in class A for most of the signal swing. The full signal output voltage with a 325V power supply is 100V RMS, which allows this amplifier to drive a 100V PA matching transformer directly. I use UTC LS-33s because I have them but there are others on the market; some are of really nice quality and have full range response.

The prototype is scaled to 20W power output, which is 100V RMS into 500 ohms. Selecting the proper taps on a 20W matching transformer for the speaker impedance will result in approximately 500 ohms load. 60uF coupling cap gives an Fc of about 5 Hz. The circuit could be scaled to use 70V matching transformers as well, with lower B+ and higher idle current.

Next step is to do some measurement of the distortion spectrum at various power levels.
Attached Images
File Type: png ImpMultHybrid.png (20.9 KB, 304 views)
File Type: jpg amptop.jpg (44.5 KB, 288 views)
File Type: jpg ampboards.jpg (93.9 KB, 275 views)

Last edited by Michael Koster; 20th June 2012 at 05:21 AM.
  Reply With Quote
Old 20th June 2012, 05:36 AM   #2
diyAudio Member
 
Join Date: May 2007
Looks elegant. What will happen if you instead of the antitriode output of the imput stage take the signal from the source above the two 91ohm resistors? I suspect distortion will be a lot lower that way with a lowish 5 kohm load.

Last edited by revintage; 20th June 2012 at 05:45 AM.
  Reply With Quote
Old 20th June 2012, 06:39 AM   #3
diyAudio Member
 
Join Date: Feb 2010
...Is this a PA amplifier ?
  Reply With Quote
Old 20th June 2012, 04:10 PM   #4
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Quote:
Originally Posted by 0feedback View Post
...Is this a PA amplifier ?
No.

It's a full range hifi amplifier with a power frequency response of 10 Hz to > 50 KHz and a SET harmonic structure with no distortion cancellation of any kind.

Using a matching transformer doesn't necessarily relegate it to PA use, if you take care to use a good full range transformer.

Also is just in the range to get 20W out of a 220/44V power toroid if you scale back to 70V output with a little higher idle current.

Last edited by Michael Koster; 20th June 2012 at 04:20 PM.
  Reply With Quote
Old 20th June 2012, 04:16 PM   #5
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Quote:
Originally Posted by revintage View Post
Looks elegant. What will happen if you instead of the antitriode output of the imput stage take the signal from the source above the two 91ohm resistors? I suspect distortion will be a lot lower that way with a lowish 5 kohm load.
Right. The Zout gets really low in that case as does the distortion. I can also accomplish the same thing by increasing the current gain of the output stage. I can dial in different amounts of SET-ness i.e. load line steepness.

Last edited by Michael Koster; 20th June 2012 at 04:21 PM.
  Reply With Quote
Old 21st June 2012, 02:19 AM   #6
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Michael, nice circuit.
Does it behave well on power up? (R23,R24 controlling that I guess). Could this be biased for class AB? (each impedance converter working half time) Maybe could just use a CCS for Q5 (instead of the anti-T), since the rest of the circuit is already impedance multiplying.

It would be interesting to also adapt the circuit for a conventional P-P setup into a CT'd power toroid OT.
__________________
I want a Huvr-Board!!

Last edited by smoking-amp; 21st June 2012 at 02:22 AM.
  Reply With Quote
Old 21st June 2012, 03:29 AM   #7
diyAudio Member
 
Join Date: May 2007
Quote:
The Zout gets really low in that case as does the distortion.
Correct. A quick check in a russian 6C45 Ua/Ia shows ideal, symmetric swing, low 2H conditions with a flat loadline(CCS/no load). As soon as it gets downloaded symmetry fails severely. 10k should give at least 2% 2H at full swing. What Ua/Ia-curves do you use?
  Reply With Quote
Old 21st June 2012, 06:13 AM   #8
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Quote:
Originally Posted by smoking-amp View Post
Michael, nice circuit.
Does it behave well on power up? (R23,R24 controlling that I guess). Could this be biased for class AB? (each impedance converter working half time) Maybe could just use a CCS for Q5 (instead of the anti-T), since the rest of the circuit is already impedance multiplying.

It would be interesting to also adapt the circuit for a conventional P-P setup into a CT'd power toroid OT.
It does power up OK, but I have been powering up the heater first with a separate supply.

I want to play with the SE harmonic signature at varying levels as well as validate the AT behavior wrt 2H, impedance, and clipping.

CCS and mu-follower configurations are easy to do by just changing resistors around. I will be interesting to compare AT and CCS with 2x the output stage current ratio, also to compare mu follower with CCS at a high output ratio.

It should work in switching mode but is not happy at low idle currents and likes to oscillate. I need to experiment with that some more.

Last edited by Michael Koster; 21st June 2012 at 06:41 AM.
  Reply With Quote
Old 21st June 2012, 06:14 AM   #9
diyAudio Member
 
Join Date: May 2007
Looking forward to your findings!
  Reply With Quote
Old 21st June 2012, 06:25 AM   #10
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Quote:
Originally Posted by revintage View Post
Correct. A quick check in a russian 6C45 Ua/Ia shows ideal, symmetric swing, low 2H conditions with a flat loadline(CCS/no load). As soon as it gets downloaded symmetry fails severely. 10k should give at least 2% 2H at full swing. What Ua/Ia-curves do you use?
heh heh ...

I get 2% 2H at somewhere around 2W output with the 10K load and 12mA idle current. But this is baseline to look at AT vs CCS. I have curves below.

the fft plots are 10mW, 100mW, 250mW, 500mW, and 1W fundamental power, zero db is 1W

I only show these to 1W because my interface adds too much H3 above about 3 volts input and I don't have my scaler set up yet. Anyway it's a baseline from which to compare CCS and mu follower, and various current ratios.
Attached Images
File Type: jpg 6C45P-loadline.jpg (89.9 KB, 201 views)
File Type: png impmult-10mW.png (97.4 KB, 194 views)
File Type: png impmult-100mW.png (97.2 KB, 32 views)
File Type: png impmult-250mW.png (99.1 KB, 26 views)
File Type: png impmult-500mW.png (99.3 KB, 21 views)
File Type: png impmult-1W.png (100.7 KB, 20 views)

Last edited by Michael Koster; 21st June 2012 at 06:28 AM.
  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
Recommendation plz: an inexpensive tube (or hybrid) can amp for hi impedance cans Wikkid Headphone Systems 8 6th September 2010 02:06 PM
Voltage multiplier argonrepublic Power Supplies 27 25th February 2010 02:05 PM
to parafeed or not to parafeed that is the question nhuwar Tubes / Valves 63 28th February 2007 12:29 PM
My wacky idea for a hybrid horn loaded woofer/planar widerange hybrid loudspeaker. Spasticteapot Multi-Way 0 30th November 2006 04:16 AM
Parafeed Brett Tubes / Valves 14 7th April 2003 12:53 PM


New To Site? Need Help?

All times are GMT. The time now is 06:59 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