Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

The good, the bad, and the ugly OTL amplifiers
The good, the bad, and the ugly OTL amplifiers
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 8th November 2006, 11:40 AM   #41
Burnedfingers is offline Burnedfingers  United States
Account disabled at member's request
 
Join Date: Nov 2002
SY

I appreciate your input. Someday I might try to design and build an OTL amplifier. I'm not thrilled with the Futterman design after pulling up several schematics and patents off the web. The square wave response was another reason. It just plain doesn't look right as its got a glitch on the side indicating a possible problem in the output stage design.
  Reply With Quote
Old 8th November 2006, 01:44 PM   #42
Brian Beck is offline Brian Beck  United States
diyAudio Member
 
Brian Beck's Avatar
 
Join Date: Dec 2004
Location: South Florida, USA
It seems to me that an intrepid OTL fan might consider designing and building a matching speaker design that is “OTL-ready”. In other words, one could concoct an amp and a speaker that work together as a system. The main goal would be to make a high impedance speaker. If you used multiple identical drivers connected in series, you could make a high impedance speaker that would be a far better match to an OTL amp. I’m thinking of something like a line array. Suppose, just for discussion, that you made a tall three-way design. Imagine twelve tweeters in a vertical column, next to twelve midrange drivers in a vertical arrangement too. If each tweeter and midrange driver were rated at 8 ohms nominal, you’d have a roughly 100 ohm speaker. Usually, with an array like this, the drivers are connected in series-parallel arrangement to set the total impedance to close to 8 ohms (or maybe 4 ohms). But here the strictly series connection works in favor of the high-voltage, low-current “desires” of an OTL amp. For the woofer section, you probably would not want twelve 12” drivers, but imagine a pair of drivers in series in a sealed enclosure, sized appropriately to create the desired damping. If the cross-over to the midranges could set to a fairly low frequency (which is feasible since there are so many midranges to share the excursion burden), then the woofers would be operating near their system resonance frequency, and the resultant impedance in the bass range can be very much higher than 8 ohms.

I don’t think designing a whole new speaker system just to justify the use of an existing OTL amp makes much sense, but if someone were considering building a speaker and an amp anyway, then this might be an option.
__________________
Brian
  Reply With Quote
Old 8th November 2006, 09:17 PM   #43
thoriated is offline thoriated  United States
diyAudio Member
 
Join Date: Nov 2003
Location: flyover country
My OTL's balanced driver and output stage combination appears to have an open loop impedance comparable to that of an inverted Futterman (e.g., with 8 6AS7's per channel and about 26db of ac feedback, I was getting a damping factor of about 40 into 8 ohms, before I added positive current feedback to bring that over 100) but it is a completely different topology than either a Futterman or Circlotron.

This OTL design is also DC coupled throughout (and has a ground referenced output). I have been satisfied with tube reliability, considering having had only a handful of output tubes fail in the 18+ years I have been running this amp, although I have replaced several due to falling transconductance with use over time. This is in light of the somewhat elevated output stage supply voltage of +/- 160 - 165 Vdc I am running these tubes at.

This amp behaves very well into loads as low as 2.5 ohms (the lowest I've tried), but you can tell over time that there is an increase of harmonic distortion (slight thinning of sound) at average levels of more than a few watts a side into such impedances.
  Reply With Quote
Old 8th November 2006, 10:08 PM   #44
ErikdeBest is offline ErikdeBest  Switzerland
mostly thinkerer
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
The good, the bad, and the ugly OTL amplifiers
Hi Thoriated

Sounds really interesting, I mean, high damping factor, DC coupling, etc! Can you show us a schematic of this amp?

Many thanks

Erik
  Reply With Quote
Old 8th November 2006, 10:14 PM   #45
Burnedfingers is offline Burnedfingers  United States
Account disabled at member's request
 
Join Date: Nov 2002
Yes, a schematic would be great.
  Reply With Quote
Old 9th November 2006, 04:59 AM   #46
thoriated is offline thoriated  United States
diyAudio Member
 
Join Date: Nov 2003
Location: flyover country
Hi -

I probably shouldn't post a schematic of the actual amplifier since I think the means by which I accomplished the DC coupling is patentable and I'm not quite ready to throw away the ability to profit from it if necessary (as I approach retirement age), but I can describe how I achieve the higher damping factor, which should be adaptable to at least some other OTLs. Basically, I have a low resistance current sense resistor (around 0.47 ohms in the case of my OTL) on the return leg of each output, and I tie the signal ground to the 'high' side of the current sense resistor. The actual output stage supply ground return is connected to the 'low' side of the current sense resistor. This allows me to sum the positive current feedback directly into the same feedback node as the negative voltage feedback by taking the current feedback off the 'low' side of the current sense resistor. With pot trimming and inverse open loop gain shaping of the current feedback circuit, I can increase the effective damping factor by up to ten times across the audio band with my OTL, so have achieved damping factors of 200 out to 20khz when trimmed (but my OTL has an open loop response down only 4db at 20khz which allows such a high damping factor to be achieved at higher frequencies).

One reason I like this approach of increasing damping factor is, besides the fact that it doesn't require any basic changes to the cicrcuit topology is that I am effectively reducing the overall negative feedback under higher current draw situations. I think this probably allows the best amplifier stability into load.
  Reply With Quote
Old 9th November 2006, 06:27 AM   #47
ErikdeBest is offline ErikdeBest  Switzerland
mostly thinkerer
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
The good, the bad, and the ugly OTL amplifiers
Hi Thoriated

Thanks for the explanation, can't say I understand it completely from the first read after just 5 hours of sleep and one coffee, but when time passes by...

I think you are right on the uniqueness of the DC-coupling thing in tube OTL's and it's potential for patent. Good luck... (kid feeling the sweet taken away...)

Erik
  Reply With Quote

Reply


The good, the bad, and the ugly OTL amplifiersHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
UGLY Looking Amp! - but good working halojoy Solid State 311 29th October 2014 06:58 PM
The Good. The Bad and the Ugly Car Amps boxcustom Car Audio 115 17th August 2011 01:34 AM
OPT -the good, the bad or the ugly ?? AuroraB Tubes / Valves 1 9th February 2007 11:07 PM


New To Site? Need Help?

All times are GMT. The time now is 10:33 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki