Preamp and impedance question

The output impedance of tube amplifier with a transformer is a function of the (output impedance of the amplifier section multiplied by the turns ratio) ^2  (squared).  The 4 ohm tap has less total turns, so it has lower output impedance (higher damping factor).  On the other side, changing the tap, changing the turns ratio, reflects a different load back to the amplifier, and coupled with the transformer, that can have some impacts on the flatness of the frequency response at the ends of the audio spectrum and the phase. All depends on the overall design.

In terms of selling the amplifier, "clones", especially something that is a production clone, are of limited resale value, so keep that in mind when looking at what you spend. The transformer is a huge part of the implementation. You are building something "like" a Luxman mq-300, but certainly not a Luxman mq-300.

You can ask Spatial audio if they can supply an impedance graph/chart. There really is no excuse for not being able to supply one. From there, you could make a decision at least on this speaker. They claim 8 ohm, and there is no reason to doubt that. Rather large mid-bass, so the impedance could dip, but 8 ohms is still likely best for overall performance.  Different speakers, of course, will behave differently.

If it only dips to 7ohm, then it will be just fine but the speaker is only part of the equation. The crossover will have an influence too. I don't know what they speaker does at the frequencies for the subwoofer. There will be some influence from the circuit for the mid. I expect there is some loading at low frequencies, but would be highly unlikely to be much less than 8 ohms.

EPDR is not a meaningful term for this topic. That is a term more applicable to solid state amplifiers operated in the linear region and is most applicable to a discussion on thermal dissipation, not drive capability or optimization for frequency response and THD.

"And that if it say -50 degrees in the bass, which is not uncommon, combined that with the "5ohm" of what the manufacture "says" the speaker is, "could" represent a combined EPDR ("equivalent peak dissipation resistance") load even down to 2ohms depending on what the -phase angle is."

This "idiot" obviously knows a heck of a lot more about amplifier design than you do. Do you even know what EPDR means? It is a measure of the voltage delivered / current delivered at a given frequency. Why does that matter? .... It matters because that comes into play for power dissipation in an amplifier in the linear region. Keep in mind, we are talking real music here, not continuous pure tones, so low EPDR only increases the dissipation a bit dependent on music material. EPDR raises the power dissipation at the device level, and for BJT devices, that can lead to BJT secondary breakdown. This is from a Toshiba app note.

Secondary breakdown is a failure mode in bipolar transistors in which negative resistance (current concentration) occurs under high-voltage and high-current conditions. Current concentration causes local heating, resulting in a small hotspot. The impedance of the hotspot decreases, causing further current concentration. This cycle called thermal runaway leads to device degradation and destruction.

MOSFETs and Tubes do have secondary breakdown effects, but their safe operating areas and secondary breakdown mechanisms are different and hence they don’t experience failure in the same way a BJT amplifier will from short term thermal events such as those that could be induced by low EPDR. Amplifier power dissipation at a given power output will go up with phase angle (assuming not pure class-A), but you don’t get the catastrophic failure of a BJT amplifier.

EPDR does not mean your amplifier has to delivery higher peak currents. An EPDR of 2 ohms, does not mean the current drawn is higher than an impedance minimum of say 3 ohms. What it means is the amplifier power dissipation is a maximum at the EPDR minimum. It would be rare that it would be at the impedance minimum (if at all), so the impedance minimum does represent the maximum current delivered by the amplifier.

Having a 4 ohm tap isn’t going to make your amplifier more robust to a lower EPDR than the impedance peak. It is quite meaningless within the framework of impedance matching of the amplifier, or the output impedance of an amplifier. EPDR just tells you for a given speaker what the potential maximum power dissipation will be in your amplifier output devices, assuming a particular amplifier architecture.

The phase element, if Georgehifi is relating it to EPDR is not relevant. It relates to thermal dissipation in output devices which tubes don't have an issue with.

Why did you make a post, delete it, then create a reply to it like this?  I don't understand.

Post removed Jun 08, 2020georgehifi7,214 posts06-08-2020 4:58pm

Post removed Jun 09, 2020

That's right.
Clean up aisle 4.

@georgehifi , Maybe you could explain to gte357s, exactly how this EPDR is "taxing" a tube amplifier knowing that being frequency based, and real music, the time at any of these EPDR minimums is very short.

@gte357s , changing the tap from 8 to 4 ohms will have minor changes on the frequency response, distortion, and damping primarily of the woofer (which also shows as frequency response). There are likely as many who would like the 8 ohm tap as the 4 ohm tap, but of course best to use the one you like.

@georgehifi , all you did is regurgitate what you already said, without in any way relating EPDR to a tube amplifier, except in the way you used it, it is wrong. EPDR is not impedance and-does-not impact frequency response w.r.t. output damping factor. That is not how it works ... at all.

How does EPDR specifically impact a tube amplifier??  This is a discussion on tube amplifiers. Just saying "It could not drive" is at best an opinion based on your friends listening preference, but without any hard data ....     EPDR does not impact current delivery, it impacts power dissipation in the amplifier. Do you refute that? The highest current will be delivered at the impedance minimum. Do you refute that?

@atmasphere may wish to jump in based on the comments made about his amplifier.

Savvy how? I am sure I can match them in patents and AES papers. Really doesn't matter, since they would all agree with ME, not with you. You don't even realize that you have not made any arguments to support your claim and have done nothing to show the impact of EPDR on a tube amp. You just keep repeating the talking points that illustrate either a lack of understanding of EPDR or of amplifier architecture and operation. I really couldn't care less that what you believe, but when you come on here and give bad information, I will point it out.  I called out to @atmasphere as he appears to have a solid understanding of amplifier operation.

Here is what @atmasphere said,

**Any** 60 watt OTL will be uncomfortable with a load like that (see JA's comments in the review at the link). This is a great example of how *not* to do it- you always want a tube amp to play loads for which it is intended!

I don't know atmasphere's OTL specs, but normally they have a somewhat high output impedance. The huge resonant spike with voltage drive on the JBL would cause a significant level change at that frequency.

We also noted that particular customer was going through power tubes faster, which is no surprise since a lower impedance will cause more of the power generated by the output section to be dissipated in the output section- and that's hard on tubes. We finally got him to install a pair of ZEROs and then the tubes stopped failing, plus he had more power. The Wilsons traditionally have been fairly efficient so none of this should come as any surprise. The national sales manager at Wilson (John Giolas) had our amps for several years.

Note this is --average-- impedance, not EPDR minimums, which will increase the instantaneous power dissipation in the output section of a tube amplifer (exactly as I said) based on music content, which does not have much impact on tube life at all (also what I stated). 

Here is another point, at the same volume level, whether you use the 4 ohm or 8 ohm tap, the instantaneous power dissipation in the amplifier output section will be almost exactly the same at the EPDR minimum.  Then again, I believe the MQ-300 is ClassA, in which EPDR does not have any meaning.

I have not even once changed what I have wrote about EPDR. I have addressed your specific arguments in every case, while I have yet to see you make even one deep technical analysis w.r.t. the discussion  you raised on EPDR.  I have also used my own words, while you have only linked to articles.  Would you like some Maple Syrup with those waffles? Perhaps it will go well with those sour grapes.

georgehifi7,226 posts06-10-2020 1:21amJust keep waffling on,

I think for someone to make this statement, they have to understand how EPDR impacts an amplifier. By all indications from what you have posted, you do not.

Taken on board Ralph, with a yawn.
But for anyone to say EPDR (combination of impedance and -phase angle) "does not matter", is dreaming.

George,

Everyone reading this thread can see that I and atmasphere talk in very specific terms and in detail when we talk about EPDR. You are not fooling anyone but perhaps yourself.

Since you are such an expert on EPDR, tell everyone, specifically, what was inaccurate about atmasphere's post about tube amplifiers and EPDR. Here, I am reposting the relevant section:

Tubes are a bit different from transistors in a number of ways as we all know :) One of the ways they are different is the Safe Operating Area (SOA). With tubes you can exceed the SOA without damage to the tube if the tube is allowed to cool off afterwards. A tube dramatically overtaxed, such as in a loss of bias, can turn cherry red from heat, but if allowed to cool off, and the problem corrected, can continue to give normal service.