|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
128.194.160.53
In Reply to: RE: FWIW posted by Ralph on February 19, 2014 at 08:11:18
"A Circlotron amplifier will function without filter caps in the power supplies. I've seen people talk about how the filter caps act as coupling caps, but if the amp works without them, seems to me maybe that isn't quite the case. Just say'n :)"
I suppose it depends what "function" means. Without power supply capacitors, the two supplies for the output stage are producing full-wave rectified sinewaves. This means the supply voltages are varying between (say) 150V and zero volts 120 times per second. That means there will be significant intervals of time during every 1/120 th second period when the amplifier is incapable of outputting any audio signal. This will inject a huge 120 Hz "modulation" onto the audio output.
In any case, the logical point is that whatever current passes through an output tube necessarily also passes through the power supply that connects to it. So the audio signal passing through the tube will necessarily pass also through the power supply. There is literally nowhere else that it can go. (As in the schematic at the top of the page; the wires from the output tube anodes connect to the power supply and nothing else.)
The discussion of what might happen if the power supply had no capacitors in it is an amusing one in its own right, but not really relevant in practice, I think. When one does use a capacitor, the audio signal passes almost exclusively through it as it passes through the power supply. Apart from anything else, for the majority of the mains cycle the sinewave voltage from transformer secondary is less than the voltage stored on the capacitor, and so the rectifier diodes are non-conducting. That leaves only the capacitor as the route by which the audio signal can pass.
Chris
Follow Ups:
The Futtermans I have seen employ an *actual* coupling cap in addition to the 'coupling caps' in the power supply. So we can look at the Circlotron has having 'one less'.
Another method of sorting out if a coupling cap exists is to examine the DC capabilities of the circuit. If a coupling cap is present the circuit will have a timing constant that will prevent DC response. We don't see that in the Circlotron- the output is capable of holding DC levels indefinitely, in fact requires a DC offset control system due to the lack of a coupling cap. So I've always seen this as a 'cake and eat it too' sort of thing; if you have a coupling cap you don't have DC response. This is of course a pragmatic viewpoint, I am sure stricter definitions exist!
"Another method of sorting out if a coupling cap exists is to examine the DC capabilities of the circuit. If a coupling cap is present the circuit will have a timing constant that will prevent DC response. We don't see that in the Circlotron- the output is capable of holding DC levels indefinitely, in fact requires a DC offset control system due to the lack of a coupling cap. So I've always seen this as a 'cake and eat it too' sort of thing; if you have a coupling cap you don't have DC response. This is of course a pragmatic viewpoint, I am sure stricter definitions exist!"I think an important point in the discussion is the relative timescales of the period of the audio signal, on the one hand, versus the timescale of the "pumping up" of the smoothing capacitor every 1/120 of a second. Or, in other words, the relative frequency of the audio signal under discussion versus the mains frequency. But first, it should be emphasised that regardless of the relative frequencies, the audio signal *will* go through the power supply, since there is no other route from the anodes of the output tubes than through the power supplies. This is just basic conservation of charge. The only matter for debate could be how much passes through the smoothing capacitor and how much through the rectifiers and power-transformer secondary.
If we consider an audio signal with a frequency that is large compared to 60Hz, then there will be many oscillations of the audio signal between the periodic replenishment of the charge on the smoothing capacitors, which happens every 1/120 of a second. During much of the time between the replenishments, the instantaneous power-transformer secondary voltage will be less than the voltage across the capacitor, and so the rectifier diodes are non-conducting and the *only* route for the audio signal to follow is through the capacitor. It is in series with the loudspeaker. In this regime, the audio path is clearly through the smoothing capacitor, and it is playing a role much like any other coupling capacitor.
At the other extreme, if we consider an "audio" signal that is very low in frequency compared to the 60Hz mains (and the DC response you mentioned is the limiting case of this), then there will be many replenishments of the charge on the capacitor during one cycle of the very low "audio" frequency. In this regime, the power supply with its smoothing capacitor is behaving rather differently from a pure coupling capacitor, since the recharging process from the power transformer is now playing a big role during the course of a single very low frequency "audio" cycle. That "replenishment" process doesn't occur with an ordinary coupling capacitor.
In practice, the various parameters like the current-supplying capability of the transformer, and the value of the smoothing capacitor, are chosen so that there is very little "sag" of the power supply DC voltage under the load demanded by the amplifier, and there is very little ripple on the DC output level from the supply. Under these conditions, it is therefore the case that the rectifier diodes are non-conducting for the majority of the mains cycle, and that therefore the power transformer is completely "out of circuit" for the majority of the time. Since the fluctuating current (the "audio signal") passing through the output tubes must necessarily pass through the power supply, it follows that it is necessarily passing overwhelmingly though the smoothing capacitors, and not through the rectifier diodes and power transformer secondary winding. (This is especially clear cut for the higher audio frequencies. One could debate a bit more about the "near to DC" extreme case.)
Chris
Edits: 02/20/14
So, if we used batteries instead, would there be coupling caps?
The circuit would work the same way.
"So, if we used batteries instead, would there be coupling caps?"
Maybe the term "coupling" is in any case not the best one to describe the role of a PS capacitor, since "coupling capacitor" carries the connotation of a capacitor between one stage of amplification and the next. The PS capacitor is in the audio path, and to that extent it has features in common with a coupling capacitor.
If batteries are used instead, then we now have the audio signal passing through them. Presumably if the batteries are essentially "ideal" (maybe like a set of car batteries in series?), they will have extremely low impedance and there will be no significant interference with the audio signal. However, given the way that passions are aroused by discussions of the pros and cons of different capacitor types, I could easily imagine analogous debates over batteries, whether Sears is better than Pep-Boys, etc., etc.!!!
I see Lew's point about whether the cathode bypass capacitors would negate the benefits of putting in cathode resistors for current-balancing purposes. My suspicion would be that any resulting unbalance for the AC audio currents would be a relatively minor issue, and the main concern is to ensure some degree of stability and balance of the quiescent DC currents through the tubes, which can be quite large and dissipate a significant amount of power. I know from my own experience that if one isn't careful, then with paralleled output tubes it can be possible for one of them to hog most of the current, and hence dissipate a lot more power than the others.
Chris
ChrisMr.Karsten says to you : "So, if we used batteries instead, would there be coupling caps?"
And you responded to him that you have responded , but I ask you now : What`s happen with car ( lead acid ) batteries when they during exploatation become discharged ? .
I think than they need some battery Charger to again become Charged !,( and usually simple battery charger consist from one main trans. and SS rectifier ).Any way I want to tell you next , only by using just my simple as possible sentence & words without Any intention to discredite you or anything simmilar !!! :
Batteries are just DC E.energy power Source Storage Units , same as Elko`s in convetional Amp PSU , further stored E.energy from batteries are partialy wasted (discharged) in power tubes , generating un-usefull heat, and finally some of that stored DC E.enegry is discharged on the load(speaker) , converting that stored E.energy in sound , also in un-usefull heat too , that generated sound is only Usefull Work that we actally need .
And yes ! all of this 3 device (charged battery , power tube ,load(speaker )are connected in series !
Finally I want to say that any serious E.Engineer look in this way how that E.machine (Amp)work , audophiles and amateurs don`t , they use term`s something like : audio-path , audio that or this , and so on and so forth , bla , bla , bla .....Best Regards !
__
Enlightened Evolution-Astral Projection
Edits: 02/20/14
"And you responded to him that you have responded , but I ask you now : What`s happen with car ( lead acid ) batteries when they during exploatation become discharged ? ...."
I'm not sure I'm catching the point you are making, but it seems anyway we are in agreement that the power supply (battery in this case) is in series with the speaker and tubes. The AC audio current that passes through the loudspeaker passes also through the battery. If you want to call that the "audio path" or not is just a matter of preference for words, but the facts are presumably not in dispute.
Chris
Chris
If we connect that same Amp ( basically one black box who amplify something ) which we usualy use in home for reproducing of music signal on the load of table shaker , which is usual tool in airplane industry , and excite that Amp with different input signal in range from 20Hz - 20Khz , same as is in audible freq. range , can we say that inside of PSU`s Elko`s pass some table shaker signal ? , I think no !
__
Enlightened Evolution-Astral Projection
"If we connect that same Amp ( basically one black box who amplify something ) which we usualy use in home for reproducing of music signal on the load of table shaker , which is usual tool in airplane industry , and excite that Amp with different input signal in range from 20Hz - 20Khz , same as is in audible freq. range , can we say that inside of PSU`s Elko`s pass some table shaker signal ? , I think no !"Banat, now I'm really puzzled where this discussion is going! I can't see that there is any new logical issue that arises for shaker tables that is in any way dfferent from the situation with loudspeakers and audio signals. They are exactly equivalent, as far as the point under discussion is concerned. It makes me suspect that the argument is more about peoples' preferred set of words to describe the same unambiguous and undisputed set of facts.
But just in case, let me pose three questions that have yes/no answers to you:
1) Would you say the shaker table signal is passing through the wires from the amplifier to the shaker table?
2) Would you say the shaker table signal is passing through the output tubes in the amplifier?
3) Would you say the shaker table signal is passing through the power supplies in the amplifier?
Chris
Edits: 02/20/14
ChrisI see no point to respond you on that 3 question !, only by giving yes or no answer , since I have allready to much clear&loud responses here , I don`t want to open new marathon of questions & answers , and it is little beat to late hours by me here on this side of planet Earth .
Best Regards !
__
Enlightened Evolution-Astral Projection
Edits: 02/20/14
nt
Maybe the term "coupling" is in any case not the best one to describe the role of a PS capacitor, since "coupling capacitor" carries the connotation of a capacitor between one stage of amplification and the next. The PS capacitor is in the audio path, and to that extent it has features in common with a coupling capacitor.
That's always been my opinion.
With regards to Banat's circuit (seems I saw that proposed on DIYAudio a few years back) it might be a good idea to use resistors that are more like 100 ohms. That would make the caps smaller and the bias would not be self bias as it would still need some bias voltage, but not nearly so much. :) Each approach has its own set of problems...
because, as you say, that would reduce the required size of the bypass capacitor. I calculated in my head that to achieve a -3 db point at 2Hz with a 30-ohm cathode resistor (so response would be flat at 20Hz), I'd need ~3000uF bypass caps. I'm sure one can get by well with a higher -3db point, however.
With my amps, that's a lot of new parts to buy and install (3 cathodes times 4 tubes per monoblock), if I were to do it. 24 new resistors and bypass capacitors, in total.
If you install such cathode resistors, you will want to increase the power supply voltage accordingly.
just after posting. 100R per cathode would drop around 13V per cathode, if I bias the whole 7241 at around 400mA, which is typical. Thankfully there is no real need to go that high in resistance.
Chris thanks............i can see with the 4ea 6C33 OTL..an the need for them to be fused....that 4 more caps may be the way to go...ezey to pull
if one feels there a down side....sound wise..i would think these should also have a smaller .1mf cap ...Ah...yes the never ending story of the OTL...an the ESL...
So it gos........
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: