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In Reply to: RE: +2 posted by 91derlust on May 27, 2015 at 19:46:27
"Also, at least one EE and another enthusiast... have measured the current draw changing during playback."That's kinda funny. I'm pretty sure that tens of thousands of engineers have seen this, probably going back to at least the '30s. Anyway, as Paul pointed out, it can be caused by non-linearities in the output tube. Another potential cause is the fact that source material can contain significant content at frequencies to 20Hz and below. Most mechanical ammeters will respond to those frequencies if installed in series with the anode or cathode of the tube. Meter fluctuations in this regard do not indicate changing average current, only that the average must be calculated over a longer time. A third possible cause for changing meter readings is simply the fact that music isn't sinusoidal. Various patterns can cause ammeters to respond differently.
As an aside, this attempt by the output stage to "modulate" the power supply at the frequency of the source material is the reason the power supply must be designed to maintain the lowest possible output Z over frequency. Lcrit regulation, a longer-term characteristic, is only peripherally related to this. I personally consider Lcrit regulation to be one of the least necessary requirements to designing a high quality SET. In fact, if not done exactly right, it can seriously degrade sound quality. Chokes are essentially current sources, the antithesis of the voltage source that's needed to maintain steady B+ against instantaneous current changes.
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Buy Chinese. Bury freedom.
Edits: 05/27/15 05/27/15 05/27/15 05/27/15Follow Ups:
"That's kinda funny. I'm pretty sure that tens of thousands of engineers have seen this, probably going back to at least the '30s"Glad you had a chuckle. I'm not an EE - am I expected to know this, or is asking questions okay?
"Another potential cause is the fact that source material can contain significant content at frequencies to 20Hz and below. Most mechanical ammeters will respond to those frequencies if installed in series with the anode or cathode of the tube. Meter fluctuations in this regard do not indicate changing average current, only that the average must be calculated over a longer time. A third possible cause for changing meter readings is simply the fact that music isn't sinusoidal. Various patterns can cause ammeters to respond differently."
So, the fluctuations do occur in SET amps playing music, but should average over time. Are the fluctuations relevant then, or should a supply provide enough energy to handle these over the time taken for the averaging to occur? Or both?
"I personally consider Lcrit regulation to be one of the least necessary requirements to designing a high quality SET. In fact, if not done exactly right, it can seriously degrade sound quality."
Regarding the PS, care to provide some hints? I am listening!
Thanks TK.
Regards,
91.
Edits: 05/28/15
"Glad you had a chuckle. I'm not an EE - am I expected to know this, or is asking questions okay?"
I wasn't laughing at you, it just reminded me that we have a few members here who invented hi-fi. They're constantly "discovering" micro-minutiae that the rest of us should care about, often just by listening. :)
"So, the fluctuations do occur in SET amps playing music, but should average over time. Are the fluctuations relevant then, or should a supply provide enough energy to handle these over the time taken for the averaging to occur? Or both?"
The fluctuations must be averaged over time for the purpose of determining whether average current is really changing. However, they represent *instantaneous* changes, because they mirror what the source material is doing. In that respect, the power supply must be designed so that those instantaneous current changes don't create instantaneous voltage changes on the B+ rail. IOW, the PS must be able to hold it's voltage constant despite changes in average current, as well as the instantaneous current changes that result from amplifying music. The former is where regulation comes in. However, in a SET, average current changes are really miniscule, so I don't worry much about power supply regulation. The instantaneous current changes are a big deal though. Luckily, the supply only needs to exhibit a low Z output over frequency to deal with that. This is a reasonably simple goal to accomplish, and we have tools now that weren't readily available 20 years ago. The supply can be modeled in SPICE, and the simulation will let the designer "look" back into the supply to see how it will react to the amplifier's instantaneous current demands. LTspice (also known as Switcher CAD) is free to download online.
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Buy Chinese. Bury freedom.
Thanks for the explanation TK - it is really helpful to me. I think I need to get to know SPICE, at least so I can do what you suggest.
Yeah, I suspected you were not laughing at me. I hear what you are saying regarding the invention of Hi-Fi.
Thanks again.
91.
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