[Roger A. Modjeski]

Development of the EM-7 push pull 10 watt amp has led me to some interesting thoughts on the topic of transformer winding resistance and its effect on damping (output impedance). Output impedance and damping factor are the same thing, just stated differently. I like to get a damping factor of 5 to 10 on my amps. Many SE amps without feedback are as low as 1 and a pentode amp with no feedback can be as low as 0.1. Yes damping can get less than one. At that point the amp is a current source rather than a voltage source and the output will vary directly with speaker impedance which will certainly be a different sound, one which was likely very far from what the speaker designer had in mind. A damping factor less than 5 can cause noticeable frequency response differences on many speakers while one above 10 will play most speakers with little difference. Damping factors from 20 up make no difference at all as the output impedance is now so small as to vanish.

In reading spec sheets from several makers I see no mention of damping factor, regulation or output impedance. This rather important aspect seems to be ignored, perhaps because the answer is not so good. Achieving good damping without feedback, which seems to be the main selling point of these amplifiers, is no easy task. In making a survey of output transformers that DIYers are using for non-feedback amps I find that rarely is any mention made of insertion loss or winding resistance. These affect the damping of the amplifier greatly. In feedback amplifiers one can reduce the output impedance very easily with feedback. In non-feedback amps one finds that damping is quickly lost in the transformer with no way to recover it. Hense, I am working on some very low resistance transformers.

Just to get a handle on how this affects what you hear lets look at some simple numbers. Damping factor is simply the load impedance divided by the output impedance. So a damping factor of 10 on a 8 ohm speaker represents an output impedance of 0.8 ohms. There is also another way to look at damping as l oad regulation, which is what I usually measure. To measure this you simply put a constant sine wave through the amp loaded at the impedance of interest (say 8 ohms) and then disconnect the load and see how many dB the output goes up. Since 1 dB is about 10% that implies a damping factor of 10. It also directly tells us what will happen to the frequency response of a varying load much more directly than output impedance in ohms.

Load or output regulation tells us something more interesting than damping. Using it we can more easily predict how un-flat an amplifier's output will be into a widely varying load like an electrostatic speaker where the impedance can vary from 100 ohms to 1 ohm over the audio band.  Someone brought me an OTL to listen to some years ago and we hooked it up to my QUAD 63s which range from about 50 ohms in the bass to 4 ohms in the treble. The amplifier had a damping factor of about 1 or a regulation of 6dB. Which is an 8 ohm output impedance. Another way to look at that is to imagine a zero ohm amplifier with an 8 ohm resistor in series with the hot lead. If we know the regulation is 6 dB that tells us that when the output can climb no more than 6 dB into an open circuit, which 50 ohms is close to and the output will also fall 6dB into a load of 4 ohms. This is indeed what I heard. The bass was up 6 dB and the treble down 6 dB thus altering this exceptionally flat speaker into a very unflat sound. Interestingly enough the maker of this amplifier had sold one to a QUAD 63 owner who just loved it. There is indeed no accounting for taste.

In the previous example and in the case of most cone speakers the impedance peak in the bass is quite narrow and the result is one note bass. The speaker will literally ring at that frequency and you basically get a pitchless bass and a constant thump thump thump.

Now if an amplifier had 1 dB regulation the output can rise no more than 1 dB no matter how high the impedance peak is. If the speaker is a reasonable load it likely not go below 4 ohms and the output will then fall only 1 dB making the total unflatness 2 DB which is not so bad. A lot of SET, SEP and non-feedback amps will have low damping/poor regulation and many makers do not even state these characteristics. No wonder listeners hear great differences with these amps. I would not want to make such an amplifier and therein lies the challenge. How does one get low output impedance without feedback? I leave this question open.

[steve f]

I just don't get how people can be so against correctly applied negative feedback. In the bronze age of solid state amps, the 70's, there were amps with lots of feedback. Usually much more than any circuit required. Those days are gone. Why deny a designer a valuable tool for amplifier correction?

[Ericus Rex]

Good info, Roger.  If an amp with a damping factor of 20+ can work well with virtually any speaker (if I read your post correctly) what are the reasons for you to not shoot for 20+ instead of 5-10?

[Roger A. Modjeski]

Good info, Roger.  If an amp with a damping factor of 20+ can work well with virtually any speaker (if I read your post correctly) what are the reasons for you to not shoot for 20+ instead of 5-10?

To get damping that high and still have good stability is almost impossible unless you want to be a Mc Intosh.

[sonicboom]

Not too familiar with Mac amps beyond what I've heard at shows/demo's.  With your statement above, are you saying that their amps utilize high levels of NFB and are therefore unstable to some extent?

Regarding the low resistance trafo designs you're looking into, I would assume that this would pertain to the transformer's primary DCR being kept low.  At the same time I would think that you'd try to keep inductance high for better tube loading and thus lower distortion.  Would this be correct?  If so, this would mean a bigger core that would allow for a heavier wire on the primary while keeping the number of turns relatively high.

I would be interested to learn your thoughts furthering this topic as your analysis on output impedance seems spot on thus far.

[Roger A. Modjeski]

Not too familiar with Mac amps beyond what I've heard at shows/demo's.  With your statement above, are you saying that their amps utilize high levels of NFB and are therefore unstable to some extent?

Regarding the low resistance trafo designs you're looking into, I would assume that this would pertain to the transformer's primary DCR being kept low.  At the same time I would think that you'd try to keep inductance high for better tube loading and thus lower distortion.  Would this be correct?  If so, this would mean a bigger core that would allow for a heavier wire on the primary while keeping the number of turns relatively high.

I would be interested to learn your thoughts furthering this topic as your analysis on output impedance seems spot on thus far.

Mac amps tend to be stable because of the rather complex output transformer and driver it requires. Those amps have more driver tubes than most except for ARC amps which are complex for no good reason at all. Frank McIntosh was interested in very low distortion and high damping both of which were achieved with large feedback factors.

You are correct the primary and secondary DCRs must be low. Inductance should always be within reason but if one goes too far on that for the bottom end one suffers at the top. Primary inductance determines the lower frequency limit and leakage inductange determines the high frequency limit for low power bandwidth. Core saturation determines the maximum power at low frequencies.  For a particular winding scheme the ratio of primary inductance to leakage inductance is pretty much fixed.

You are correct that a larger core is necessary to reduce the winding resistance and these transformers are relatively large for their power output.

[Ericus Rex]

You are correct the primary and secondary DCRs must be low. Inductance should always be within reason but if one goes too far on that for the bottom end one suffers at the top. Primary inductance determines the lower frequency limit and leakage inductange determines the high frequency limit for low power bandwidth. Core saturation determines the maximum power at low frequencies.  For a particular winding scheme the ratio of primary inductance to leakage inductance is pretty much fixed.

There's more useful information in this paragraph than in any of the electronics books I've read so far.  Thank you Roger.  It's been said before but I'll say it again here; I would love to see you write a book on tube circuit design.  Not only for your vast knowledge on the subject but also for the simple and straightforward way you present complex material.  Any plans?

[Roger A. Modjeski]

I do think about writing a book often. Not sure how to publish it. I tend to like printed material I can hold in my hand. Given the time it will take to do a nice job I would like to sell copies in print or electronically. I would like some feedback on this from my readers as to format and pricing.

[Ericus Rex]

I'm very happy to hear this, Roger.

I would prefer an actual printed book.  I need to write helpful tips in the margins in order to keep things straight in my brain and Kindles don't allow that.  As for pricing, I, for one, would pay whatever you're charging for the book.  I imagine most others who do tube diy would also be willing to pay whatever you ask.

I can't help you any with the printing.  Perhaps if you post a new topic with the question of printing in the subject line you'd get the answer since all of AC would see the post and not just us MR fans reading this thread.

If you want to go large scale there are distribution companies who sell on Amazon who could help you.  I think this was the path Bruce Rozenblit took with his latest book.

[airhead]

For what it is worth, the specs on (at least some version of) the NYAL Futterman OTL give a damping factor of 50 at 8 ohms and 100 Hz.  Of course, this amp is known for using very large amounts of feedback.  Sounds good to me!

[cheap-Jack]

Hi.

I do think about writing a book often. Not sure how to publish it. I tend to like printed material I can hold in my hand. Given the time it will take to do a nice job I would like to sell copies in print or electronically. I would like some feedback on this from my readers as to format and pricing.

FYI, John Broskie publishes ALL his design brainchild on-line for zip money.
Plus he also replied tons of emails from his zillion royal readers daily FREE!

A silly question: why you want to charge yr e-readers money?

Just let yr readers print out hard copies. Save yr cost of paper publishing & distributing!

c-J

A technical question for you: - a pentode O/P power amp WITHOUT any global feedback is NO good. Why?

[Ericus Rex]

Hi.
FYI, John Broskie publishes ALL his design brainchild on-line for zip money.
Plus he also replied tons of emails from his zillion royal readers daily FREE!

A silly question: why you want to charge yr e-readers money?

Just let yr readers print out hard copies. Save yr cost of paper publishing & distributing!

c-J

Dear C-J,

Why don't you do whatever it is you do for a living for free?  And since you're into using others as examples to follow; there are lots of members here on AC that aren't annoying.  Why don't you give it your best effort to be  more like them?

[/mp]

imagine a zero ohm amplifier with an 8 ohm resistor in series with the hot lead.

Kindly allow me to flaunt my antediluvian formal physics education terminating with a C+ in college E & M. In your gedankenexperiment, what difference does it make whether the resistor is attached in series to the hot or cold lead? Aren't the combined elements of a complete circuit seen by the amplifier the same regardless of the permutation of the elements in the circuit? I.e. 8 ohms + x ohms = x ohm + 8 ohms.     

[tubegroove]

I do think about writing a book often. Not sure how to publish it. I tend to like printed material I can hold in my hand. Given the time it will take to do a nice job I would like to sell copies in print or electronically. I would like some feedback on this from my readers as to format and pricing.

Amazon has a self-publish service that I believe works well.  I guess they will have some sort of revenue share with the author.  I believe it also allows the production of hard copies on demand once the customer pays for it (saves upfront publishing costs).
I would be ok with an electronic copy - I am sure there must be software available to make margin notes etc...

[Roger A. Modjeski]

Amazon has a self-publish service that I believe works well.  I guess they will have some sort of revenue share with the author.  I believe it also allows the production of hard copies on demand once the customer pays for it (saves upfront publishing costs).
I would be ok with an electronic copy - I am sure there must be software available to make margin notes etc...

Indeed it does not matter which lead the resistor goes in, put half in each lead, it's all the same. Electronic folks tend to put things in the hot lead and leave ground alone as a reference. The point is for the listener to hear this one effect on its own without introducing another different amplifier which may add its own colorations.

[cheap-Jack]

Hi.

Dear C-J,

Why don't you do whatever it is you do for a living for free?

Take it easy, buddie. No hard feeling.

Hopefully you don't mean to tell us you would make a living on selling the books of yr designs.
John Broskie has been doing it FREE for his passion as he makes money somewhere else.
I design-build my audios, e.g. tube phonostages, power amps, 4N pure silver audio ICs & power cords
for many years simply as a hobby to supplement my passion for quality music. 
For fun yes, for a living? NO!

I never fancy I'd use it to buy a living as I am much better & easier money in my electrical engineering industrial career.

c-J

[poseidonsvoice]

I do think about writing a book often. Not sure how to publish it. I tend to like printed material I can hold in my hand. Given the time it will take to do a nice job I would like to sell copies in print or electronically. I would like some feedback on this from my readers as to format and pricing.

Contact Bruce Rozenblit & see how he did it.

Anand.

[Roger A. Modjeski]

The book I've been thinking of is not of specific designs but a teaching tool for those who want to understand how amplifiers (and other things) work but find most books too technical. My teaching career, which I have been doing on the side, involves explaining difficult technical topics in simple language.

[Ericus Rex]

The book I've been thinking of is not of specific designs but a teaching tool for those who want to understand how amplifiers (and other things) work but find most books too technical. My teaching career, which I have been doing on the side, involves explaining difficult technical topics in simple language.

I welcome this approach and would happily pay your asking price for it.  I, for one, realize your time and expertise are very valuable.

[richidoo]

Thanks for the informative post.

I would be very interested in reading your book. I have a few books on tube audio design but I am still a bit intimidated. I am bored playing with IC chips, would like to start experimenting with tubes. I have Rosenblatz' old book, it is well written, but outdated compared to his current designs. I have not read his new book.

I have read your invitations here on AC over the years for interested audiophiles to come visit you, work with you, learn about tube design, attend your community college classes, etc. Your commitment to education is commendable. It would be great to have your vast knowledge and innovative approach to tube amp design archived for future audiophiles.

There are some rare audio engineering books that sell on Amazon Marketplace (used) for >$100. Elektor publishes new tube design books for >$50. I think a simple small paperback containing your specific design theories and goals for layman readers would sell well at $25. A comprehensive study on tubes and design theory would sell at <$50. Your reputation and notoriety are worldwide.

There are many small companies that publish books for DIY authors in small batches. You can sell them from your own website, or on Amazon, where you can contract them to fulfill orders. Here is the first one listed on Google, not necessarily best for you, but they are easy to find. http://www.e-booktime.com/paperback.html

I would be honored to be one of your proof-readers.