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choke coupled outputs

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Posted on April 20, 2015 at 09:34:08
mqracing
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 Several weeks ago while checking out some research materials on the net I ran across a short article showing a choke coupled output stage (power amp output level)... so I should have bookmarked it so I could find it again...

if anyone has any articles, schematics, notes, etc on using a choke output in lieu of a conventional two winding transformer for output stages at the power amp or preamp level that you can pass along I'd be most appreciative.

MSL



Builder of MagneQuest & Peerless transformers since 1989

 

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RE: choke coupled outputs, posted on April 20, 2015 at 10:15:42
Triode_Kingdom
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 Was it an autoformer, or a straight choke with a blocking cap?

 

RE: choke coupled outputs, posted on April 20, 2015 at 11:52:26
mqracing
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 straight choke with a // blocking cap...

but if you have other references (say for autoformer)... that's would also be good to read.

MSL



Builder of MagneQuest & Peerless transformers since 1989

 

RE: choke coupled outputs, posted on April 20, 2015 at 15:27:29
Al Noakes
 Transcendent SE OTL / Minibeast uses this topology , I think they even did this with 300Bs . I did a breadboard a few years ago with 3 x 6C41 with a Hammond 660mH choke cap coupled to the speaker . Worked well , needed feedback . Got hot... I reckon an autoformer would be much more attractive

...x

 

Autoformer, posted on April 20, 2015 at 16:31:27
Triode_Kingdom
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 I thought I had saved some files on the autoformer thing a few years ago, but a quick search of my drive didn't locate anything. Wish I could remember the source. I'll dig deeper when time permits.



 

RE: choke coupled outputs, posted on April 20, 2015 at 20:15:51
elblanco
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 OT, but

 

Choke coupled is going to be inefficient, posted on April 21, 2015 at 08:23:14
Ralph
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 And if you are talking about an output section, efficiency is pretty important. Otherwise you will be using a lot more tubes to do the same job without anything else to show for it.

Another thing to consider is that the chokes will define a low frequency cutoff. Its hard to imagine how they would be able to outperform a conventional transformer-coupled circuit or an OTL.

 

RE: Choke coupled is going to be inefficient, posted on April 21, 2015 at 10:06:43
dave slagle
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 Why would choke coupled be inefficient?

why would a choke defining a LF cutoff be any different than a transformer?

dave

 

RE: Choke coupled is going to be inefficient, posted on April 21, 2015 at 10:35:16
Ralph
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 Unless the chokes are pretty big, the cutoff is likely to be higher.

They have to dissipate the energy from the voltage drop across them which means that energy is not available to the speaker. The more energy developed by the output section that is dissipated by the output section, the less efficient.

Normally chokes are nice to use in a power supply because the DC voltage drop is less than that of a power resistor. So they are good at knocking out power supply noise without loosing a lot of power supply voltage. But this property does not work to your advantage in designing an output section. You want some voltage drop across such a device if you are going to make a voltage swing that is translated into power.

Additionally, the choke is not offering any impedance conversion, so that has to be done by the power tubes themselves.

As an OTL manufacturer, I'm very used to what that last statement means. I'm pretty sure that if I built an output section using two triode sections, I could make more power with my Circlotron circuit than one could do with the same power tubes and a set of chokes!

If one used an output transformer, the coupling to the speaker load would be much more efficient as the transformer would allow for impedance conversion. So any way you look at it, a choke coupled output section will be inefficient.

 

RE: Choke coupled is going to be inefficient, posted on April 21, 2015 at 10:47:04
dave slagle
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 They have to dissipate the energy from the voltage drop across them which means that energy is not available to the speaker.

so does the primary of a transformer.

Additionally, the choke is not offering any impedance conversion, so that has to be done by the power tubes themselves.

tap the choke for your output and you have impedance transformation.

dave

 

Tapped? Then its an autoformer nt, posted on April 21, 2015 at 11:24:25
Ralph
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 -

 

RE: Tapped? Then its an autoformer nt, posted on April 21, 2015 at 11:46:29
dave slagle
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 Inductors by nature do not dissipate AC power so I fail to see where the inefficiency comes into play.

dave

 

RE: Tapped? Then its an autoformer , posted on April 21, 2015 at 13:19:47
Ralph
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 It will be dissipating variable DC power, not AC. The output of course is AC.

 

RE: Tapped? Then its an autoformer , posted on April 21, 2015 at 13:29:27
dave slagle
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 How is variable DC not AC?

Granted there will be some copper losses from the energy stored and released by the inducance but I'd hardly call that inefficient and the same exact thing happens in the primary of an output transformer. Obviously the ideal solution is to do away with the transformer altogether.... oh now I get it.

dave



 

One way to use an autoformer is with a Circlotron output nt, posted on April 22, 2015 at 08:56:19
Ralph
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 -

 

Thank you very much..., posted on April 22, 2015 at 10:55:20
mqracing
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 there are some really great articles if you go back to the index.

A lot of reading, studying, and thinking available on the site.

MSL



Builder of MagneQuest & Peerless transformers since 1989

 

RE: Tapped? Then its an autoformer , posted on April 23, 2015 at 03:42:41
Al Noakes
 An autoformer is definitely do-able but I wonder if the 'secondary' DCR tap can be made small enough to dispense with the coupling cap , maybe even using thicker wire . Obviously there would be a small amount of offset . What do you think ?

Al

 

Inductively-Coupled Cathode Followers, posted on April 23, 2015 at 06:13:31
Triode_Kingdom
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 I'm not a big fan of these direct-coupled output schemes. However, I think low power aficionados (15W or less) have long overlooked the benefits of the inductively-coupled cathode follower. Rather than using huge numbers of parallel tubes to achieve a match, it's much more reasonable to use, say, two 6AS7s and a matching transformer. The transformer would not only require a much lower turns ratio than anode-coupled designs (i.e. improved frequency response), but by virtue of the reduced impedance transformation, less driving voltage at the grids of the followers. I believe followers in this configuration could be coaxed into producing as much power as their anode-coupled counterparts, and they would do it in a much more linear fashion. Unfortunately, the rush to produce ever greater output levels in the '50s and '60s completely overwhelmed any desire to research this topology. To the best of my knowledge, no one has ever made the requisite transformers for this service. Really a shame.

 

RE: Tapped? Then its an autoformer , posted on April 23, 2015 at 07:20:28
dave slagle
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 I did it with the 6C33 for a NY noize. As I recall I had about a quarter of an amp of current and the secondary section had a 0.1 ohm DCR for a 25mv DC offset across the load. I really expected major driver offset and issues from the DC but didn't notice any with my lowthers.

Unfortunately the amp sounded like 6c33's so I moved up conventionally coupled GM-70's for the next year.

dave

 

RE: Tapped? Then its an autoformer , posted on April 23, 2015 at 11:36:36
mqracing
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 Unless I've misunderstood your question... there's a pretty darn good reason to you cannot do what your thinking about...

if you have an autoformer in the output stage of a power amp (i.e., a power tube is driving it) which is hooked up to a speaker load at some tapped point... you would want to use a blocking cap to keep the dc off of the voice coil of your driver, heh?

Or have I gotten your application all wrong?

MSL



Builder of MagneQuest & Peerless transformers since 1989

 

RE: Inductively-Coupled Cathode Followers, posted on April 23, 2015 at 11:39:44
mqracing
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 One of our OEM's employs (if I understand his topology correctly) an "inductively coupled cathode follower" at the small signal level.

So there has been some contemporary interest in and work being done
in this area.

MSL



Builder of MagneQuest & Peerless transformers since 1989

 

RE: One way to use an autoformer is with a Circlotron output nt, posted on April 23, 2015 at 11:47:57
mqracing
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 I was just reading up on the Circlotron schema... very interesting.

I wonder if the circlotron would be a candidate for use with a special
impedance matching autoformer such as the Peerless A-262-E or an adaptation of this design to meet a different set of impedances.

MSL



Builder of MagneQuest & Peerless transformers since 1989

 

RE: One way to use an autoformer is with a Circlotron output nt, posted on April 23, 2015 at 11:54:24
mqracing
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Peerless catalog page with specs on A-262-E

be sure to scroll down to bottom of the second page



http://www.technicalaudio.com/pdf/Electronics_Catalog_Extracts/Peerless_div_Altec_transformer_coil_1960_REM_24.pdf



Builder of MagneQuest & Peerless transformers since 1989

 

RE: choke coupled outputs, posted on April 23, 2015 at 12:01:33
mqracing
Manufacturer

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Joined: June 29, 2000
 Was talking to Kevin Carter (Lundahl importer) and asked him if he was familiar with any circuits or any references for choke loading an output stage. Kevin told me that the Sr. Mr. Lundahl (founder of) had designed and offered such a device many years ago.

As relayed to me--- it would be Lundahl part number 1621 but you must go to the Lundahl.se website to look up that part number and see the published specs for it.

Thanks Kevin.


MSL



Builder of MagneQuest & Peerless transformers since 1989

 

RE: Inductively-Coupled Cathode Followers, posted on April 24, 2015 at 05:50:05
Triode_Kingdom
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Location: Central Texas
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 "One of our OEM's employs (if I understand his topology correctly) an 'inductively coupled cathode follower' at the small signal level."

Do you know of any manufacturer who has employed a transformer-coupled, cathode follower *output* stage? There's at least one schematic at the Bonavolta audio site, but I don't recall ever seeing this topology in a commercial amplifier.


 

RE: Inductively-Coupled Cathode Followers, posted on April 28, 2015 at 19:45:25
Tre'
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 I run a circlotron circuit with 4 6as7s using a transformer with a 325 ohm primary.

It sounds much better to me than 8 6as7s without a transformer. The transformer provides a much more horizontal load line for the output tubes even though there are only half of them.

Tre'
Have Fun and Enjoy the Music
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