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PostPosted: 11 Nov 2019, 22:49 
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Hello everyone,

I'm working on a class AB push pull amplifier design. For reason of the widely varying load impedance of the output stage inducing substantial voltage sag in the B+ power supply (presumably), I've explored means of regulating the B+. Put differently, I don't like how high the output impedance of my power supply is. My current concept is to put in place a linear power MOSFET series regulator with the voltage reference established by a pair of VR tubes in series. This component of the design is in a state that I'm at least game to try it, although a substantial heat sink enters the scene for the power MOSFET ;) . Also, I'd venture a guess that this amplifier is about the upper boundary of power for this strategy to be viable (10-14W)

Basic design highlights:
-6V6 AB1 output stage, combination of fixed bias and cathode bias
-Concertina phase splitter, cathode biased
-Cathode biased 12AX7 input stage

However, after some casual internet research, I've seen a few cautionary notes about combining regulated B+ with certain bias strategies and a more common emphasis towards regulating pentode screen grid voltages than B+. The basic saying seems to go "regulate the B+ and have a fixed bias with regulated supply or regulate neither". I think I follow the logic behind avoiding a regulated bias voltage supply if the B+ is regulated, but in the converse, where B+ is regulated does the fixed bias voltage supply need to be regulated? Or more simplistically, can any of the stages be cathode biased if their plate voltages are regulated? I've got no issues with regulating the fixed bias component of the output stage. Rather, I'm wondering if I need to go with full fixed bias for the output stage and find a different means of biasing the concertina and the input stage (both cathode biased).

Also, will the ultra-linear configuration interact unfavorably with a regulated B+?

Thanks!


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PostPosted: 13 Nov 2019, 21:24 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Interesting, I have not heard of any issue with both being regulated. I don't see the need though. My questions are more of a general nature, why is the voltage sag an issue? Why do you connect it with impedance issues? Output transformers and typical loads do not generally cause this behavior. A really badly designed speaker system can have excessive impedance variations but that usually just cause it to sound badly. It might reflect as distortion in the amplifier because of the change in tube loading, but I have not seen it cause significant sag in the B+. I am making an assumption that the amplifier is for hi-fi use and not pro audio. In pro audio gear (guitar) sag is useful and often designed intentionally. In hi-fi use it doesn't normally cause any issues as peak power is only needed for a really short time as virtually no music has continuous full power demands. A signal generator can demonstrate it, but music is usually at the 5%-10% average power level. If you are having serious sag at those level of use then the power supply is deficient. It is too small for the application. Sometimes larger filter capacitors will help. You didn't specify if you use tube rectification of solid state. Tube rectifiers will sag under load to varying degrees. SS will not. If you are concerned about power supply impedance and are using tube rectifiers then the first thing I would try is replacing them with SS. It will significantly boost the B+ voltage as well so beware that it might exceed ratings of some components. 6V6 tubes vary depending on brand and exact type on how much voltage they will withstand and how much dissipation they can handle. If you swap tube rectification for SS I would want to verify first that you will not cook your output tubes. Also watch for the rating of capacitors. BTW if you go for VR tube voltage regulators I would caution that in some circuits they can generate electronic noise. It can be cured by proper designs and I would seek out such solutions first.

Good listening
Bruce

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PostPosted: 14 Nov 2019, 01:48 
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Joined: 23 Nov 2018, 19:41
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You’re correct, this is a hifi design rather than a guitar design. I think I should clarify that when I said “load impedance” I meant the power supply load; in this case predominantly the output stage plates.

“Why is the voltage sag an issue?”

Well... maybe it isn’t? I haven’t built the amp yet so I can’t say for certain. But the basic motivation for cooking up the regulated B+ was that at higher loads the current draw from the amp is enough to substantially change the B+ from the power supply once I roughly account for RC network and inductive components series DC impedances. My design goal was a system that would break 10 watts of audio power. I realize that that’s could well be much more power than I’ll draw from the system on the average listening session but it seemed like a nice target. I bet you’re right though, at 5-10% of full power I don’t think the sag I’m referring to will appear.

So I think we could agree that maybe I don’t need the regulated power supply. For grins though, how do you think a well-regulated output transformer center tap voltage supply would interact with an ultra linear configuration?

As far as VR noise, that’s a point well-taken. Unfortunately I don’t see that much good information about these devices so I’m all ears if you or anyone has experience with them.


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PostPosted: 15 Nov 2019, 21:37 
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Hi, I see no reason why having regulated B+ and U/L would not work. I have built a few amps with extremely robust power supplies that are class A P-P U/L and they behave fine. With fully class A operation the load on the PS is essentially constant (I actually use constant current sources in the cathodes) so it would mimic the behavior of a regulated supply in drain. They work fine.

Good listening
Bruce

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PostPosted: 17 Nov 2019, 14:29 
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" (I actually use constant current sources in the cathodes) so it would mimic the behavior of a regulated supply in drain."

An interesting point. Looking at the Poddwatt as an example, this CCS configuration is your use of a LM317 to set the bias voltage? Correct me if I'm wrong but at first glance it looks like this approach would work with AB1 operation as well?


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PostPosted: 17 Nov 2019, 16:20 
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Hi, Actually it can not do AB. AB requires the current to change with the output power demands. The CCS holds the current stationary to within a very few %. The voltage across it will vary in step with the output...but not the current. It forces the amp to remain at all times in class A mode. It will work on triodes, U/L and pentode modes. The Oddwatts all use U/L as that is what I consider the best ratio of input power / output power and distortion.

Good listening
Bruce

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