Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
How do you choose output transformer impedance?
Hello,
What things should be considered to choose the best output transformer for a given tube and situation? If low output impedance is important for good speaker damping and control, why won't all amps just use output transformers with the biggest possible primary winding impedance? That would give the biggest output transformer impedance ratio and lowest output impedance of the amp. Since it doesn't seem to be this way, what other considerations are there? Tube type, Plate voltage, etc? If I wanted to make a class A push-pull power triode amp with a 300B output tube, run at 400V Vp, what would be the best output transformer primary impedance to use, secondary impedance is going to be 4ohms. Thanks! -at |
#2
|
|||
|
|||
But consider surge on valve anodes and OPT when the amp is "underloaded"
(the load's impedance is higher than the amp's output one)... It's not like the transistor (whew!) amp. If you connect i.e. 16 Ohm speaker to 4 Ohm amp output, the amp's likely to be damaged. -- Pozdrawiam, Krzysiek Słychań |
#3
|
|||
|
|||
"Krzysiek Słychań" wrote in message ... But consider surge on valve anodes and OPT when the amp is "underloaded" (the load's impedance is higher than the amp's output one)... It's not like the transistor (whew!) amp. If you connect i.e. 16 Ohm speaker to 4 Ohm amp output, the amp's likely to be damaged. -- Pozdrawiam, Krzysiek Słychań So, do you mean, that if I use 4ohm speakers, the output impedance should be at least 4 ohms? Then why do some people say that the output impedance should be at least 10 times lower than than the speaker impedance for good control of cone motion? That would mean that with a 4ohm speaker, the Zo should be 0.4ohms. I've also hear that the output impedance must be less than one ohm for best performance. And they don't speak about SS amps, but tube ones. If I understood it correctly, you mean that if the speaker impedance is higher that the amps output impedance, the amp is likely to get damaged? That would mean, that with an amp with a single ended 300B tube with Rp 700 ohms and a output transformer that has primary impedance of 4000ohms and secondary 4ohms, the amp is likely to get damaged, since the output impedance of the amp is now 0.7ohms with a 4 ohm load. Then to raise the output impedance to safe 4 ohms, the output transformer primary impedance should be 700 ohms. But people use higher impedances on the output transformer primary, and the amps don't blow... or do they? Or did I misunderstoond something? -at -at |
#4
|
|||
|
|||
AT,
I have a few suggestions for you. 1) Buy two books. #1: Audio Reality by Bruce Rozenblitz. Has quite a few useful tube projects in addition to practical theory. Then find a copy of the Radiotron Designer's Handbook." it's available on CD. Has more info than you ever wanted to know about tubes and tube designs. 2) Find some reprints of articles by Norman Crowhurst on line. Excellent info answering just the kinds of questions you are posing daily on R.A.T. Best regards, Jon |
#5
|
|||
|
|||
"Jon Yaeger" wrote in message ... AT, I have a few suggestions for you. 1) Buy two books. #1: Audio Reality by Bruce Rozenblitz. Has quite a few useful tube projects in addition to practical theory. Then find a copy of the Radiotron Designer's Handbook." it's available on CD. Has more info than you ever wanted to know about tubes and tube designs. 2) Find some reprints of articles by Norman Crowhurst on line. Excellent info answering just the kinds of questions you are posing daily on R.A.T. Best regards, Jon Thanks. Ok, I'll try to find them. -at |
#6
|
|||
|
|||
"Krzysiek Słychań" wrote in message ... But consider surge on valve anodes and OPT when the amp is "underloaded" (the load's impedance is higher than the amp's output one)... It's not like the transistor (whew!) amp. If you connect i.e. 16 Ohm speaker to 4 Ohm amp output, the amp's likely to be damaged. -- Pozdrawiam, Krzysiek Słychań Wouldn't that quadruple the impedance of the OPT primary? A 5000 ohm OPT primary would became 20kohm. That would make the output impedance of the amp four times lower. So, where's the safe margin? I've heard that for best speaker control, the output impedance should be lower than 1ohm, or 10 times lower than the speaker impedance. If it was 10 times lower, with a 4 ohm speaker it should be 0.4ohms. A single-ended class A amp with a 300B tube with Rp 700ohms and a OPT primary 4000 and 4 ohm speaker the Zo would be 0.7ohms [7004000:4)]. That's lower that 1 ohm but not 10 times lower. To get 10 times lower Zo, the OPT primary should be 7000ohms. That would give Zo of 0.4 ohms. Or have I misunderstood something? Are there some problems? What things should be taken into account when deciding upon a OPT and it's primary impedance? -at |
#7
|
|||
|
|||
"at" wrote in message
... What things should be considered to choose the best output transformer for a given tube and situation? Maximum power, or nearby with pentodes, minimum distortion. Triodes, just max. Po (since min. distortion comes at infinite Rl, which is rather useless). Reason there's one "best load impedance" is because the tube will only draw so much current at a given voltage and signal level. Same applies to transistors as well, because they have the same curves as pentodes, but it isn't thought of as such because they have such a contrasting design culture. Output impedance has absolutely nothing to do with it; that can be corrected with negative feedback. The "perfect voltage source/source impedance" Zo model works great for anywhere but when either voltage or current levels are pressed to their limits for the amplifier. Tim -- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms |
#8
|
|||
|
|||
at wrote: Hello, What things should be considered to choose the best output transformer for a given tube and situation? If low output impedance is important for good speaker damping and control, why won't all amps just use output transformers with the biggest possible primary winding impedance? That would give the biggest output transformer impedance ratio and lowest output impedance of the amp. Since it doesn't seem to be this way, what other considerations are there? Tube type, Plate voltage, etc? If I wanted to make a class A push-pull power triode amp with a 300B output tube, run at 400V Vp, what would be the best output transformer primary impedance to use, secondary impedance is going to be 4ohms. Thanks! -at Learn to do load line analysis, and you will be able to calculate all the impedances you will ever want. At the moment, you show you know almost nothing and have a steep learning curve to climb. But you could start by reading a few books. And when you try to build an amp, get back to us about your problems. But I can tell you that transforming audio voltages follow the same impedance rules mentioned in the basic information in the theory books which you should read and study. There is quite a lot on the WWW, so try doing a google search.... Patrick Turner. |
#9
|
|||
|
|||
Many considerations:
- the higher the load, the lower the available power (you get V swing but not A swing, and being the power VxA...) - the higher the primary/secondary ratio, the more difficult is to wind a good OPT. A 2K5 OPT is much easier to make (and therefore often better sounding) than a 10K. - a rule of thumb says that triodes work best into a load 2-3 times their plate resistance (ie, a 2A3 with a plate R of 800R is usually used with a 2K5 OPT) If You want to be more precise, You need the anode curves of the tube You chose and a ruler. Choose a working point, then draw many loadlines and see what happens. Usually a higher load is needed the more You "push" the tube towards its limits (high V at working point). But if You want to be sure, just find the original D/S of the tube You prefer and stick on to recommended values for ALL parameters. After all, these leaflets were written by professionals: they must have had some sound reason to tell that ie. a 6V6 needs 8K a-a in PP, isn't it? Would You play the same "it-seems-to-me" way with the braking pads of Your car? By the way: the 300B needs a helluva driving voltage (some 250V p-p). I'd start from something simpler. Ciao Fabio "at" ha scritto nel messaggio ... Hello, What things should be considered to choose the best output transformer for a given tube and situation? If low output impedance is important for good speaker damping and control, why won't all amps just use output transformers with the biggest possible primary winding impedance? That would give the biggest output transformer impedance ratio and lowest output impedance of the amp. Since it doesn't seem to be this way, what other considerations are there? Tube type, Plate voltage, etc? If I wanted to make a class A push-pull power triode amp with a 300B output tube, run at 400V Vp, what would be the best output transformer primary impedance to use, secondary impedance is going to be 4ohms. Thanks! -at |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
on topic: we need a rec.audio.pro.ot newsgroup! | Pro Audio | |||
Altec 15356a Line Transformer | Pro Audio | |||
Artists cut out the record biz | Pro Audio | |||
What output impedance should tube tuners phono preamps | Vacuum Tubes | |||
rec.audio.car FAQ (Part 2/5) | Car Audio |