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In Reply to: RE: Maximum Output posted by mqracing on January 29, 2015 at 01:37:08
is there a table or formula I can look at?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
for an explanation of the differences btwn small
signal response and large signal response see:http://www.magnequest.com/tech_article2_voltsecond.htm
for further discussion of the effects of L vs "reflected impedance"
see:http://www.magnequest.com/tech2.htm
Builder of MagneQuest & Peerless transformers since 1989
Edits: 01/29/15
"Phase Shift Desired
Multiplier
5 degrees
11.43
10 degrees
5.67
15 degrees
3.73
30 degrees
1.73
45 degrees
1
Here is an example of how to use the above multipliers. Say you have a 5,000 ohm nominal primary impedance and you desire only 15 degrees of phase shift at 100 hertz.
So at 100 hertz we would want (by referencing the table above) an inductive impedance of 3.73 times 5,000 ohms.
Solve for the following;
18,650 equals 6.2834 times 100 times L
L is the unknown variable. In this case L must be 29.6814 henries."
So if the 5K is changed by up tapping to 10k the required inductance for a desired phase shift will double.
So for a -3db point (phase shift of 45 degrees) L = reflected impedance / (6.2834 * frequency). (added to my spreadsheet)
If I double the reflected impedance by up tapping the output transformer the -3db point goes up an octave.
3200:8 with 25.5Hy will do 20Hz at -3db. Connecting a 16 ohm speaker will reflect 6400 ohms and the -3db point will be 40Hz. To maintain 20Hz at -3db would require 51Hy.
Thanks Mike.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre:I didn't check your calculations or anything but I think
you got the picture.Just using a small signal model will make some crudballs
look like their ready for prom night...the tougher test.. and the more realistic... is the large
signal test.I found the following text on a Lundahl spec sheet...
to explain large signal vs small signal....FPL= large signal
FRL= small signalWe define Power Low Frequency Limit, FPL, as the frequency where wLP = RLOAD. (The reactive impedance of
the transformer equals the primary load impedance). At FPL, the output power is reduced to 50%.We define Response Low Frequency Limit, FRL as the frequency where a (small) output signal is reduced with
-1 dB due to finite primary inductance. FRL = w / p, if you solve w in
wLP = (RLOAD in parallell with RANODE).
In the FPL when the power is reduced by 3db the phase angle of the load impedance will be 45 degrees. Instead of your loadline being a straight line it's going to begin to look like a NFL football (i.e., ellipse). Your tube generated distortion is going to increase dramatically and that isn't even taking into account that the transformer borne distortion will also be going up at the same time.Plate curves assume a linear with frequency unchanging
pure resistance is loading the anode of the tube. If your instead loading the anode with a transformer... nothing could be further from
the truth. Now you must take into account the characteristics and functioning of the transformer... cause now we have reactive components in a complex relationship with "pure" resistances.Now the effective loadline can look more like a football or beachball.
And this elliptical loadline changes shape with frequency... so that your tube now has to drive an everchanging loadline....and it aint going to be pretty when having to do all that work.So instead of focusing on the "easier" small signal response... looking at and evaluating the large signal response (the decidely tougher test) will be much more predictive of how well your tube and circuit will perform.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Edits: 01/29/15 01/29/15 01/29/15
"It does not matter if your speaker does not go down to 31.25 Hz or not. If you are sending low frequency information to the amplifier, the amplifier will make the plate voltage move. If the plate voltage moves, the tube must spend bias current to charge and discharge the primary inductance as well as the actual primary load impedance. If the plate voltage or current "clips" because of low frequency plate excursions, the sound will suffer. "
"With 27 H, the load line is nasty. With a 27 H primary if there is any low frequencies making it from the preamp to the output transformer, I would expect the sound to be a bit nasty too. "
The practical implications for me; I just placed the high pass filter cap back in series with the input to my SET amp that drives my midrange (and tweeter) that only plays to 200Hz.
Thanks again Mike.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That's a very important point that Voltsec made--- cause a lot of people will initially say I don't care what my amp is doing down at 50 or 40 or 30 hertz... cause my speakers only go down to X... but as Voltsec points out by then the damage is done....Your response--- using a high pass filter--- it's generally regarded as a taboo approach in hi end circles--- but I think the approach has a lot of initial merit to it. Frank Van Alstine in the mid eighties started using a high pass filter on the input of his ST-70 mod/circuit cause (his reasoning) it did not allow the circuit to be swamped with frequencies which it could not handle.
Or spec and use a transformer with really, really good capabilities in this respect (i.e., large signal goodness).
And to bring this back to "creative tapsmanship"-- i.e., ratioing transformer impedances-- don't do it--- your not likely on the vast majority of transformers to have enough "surplus" L to keep the loadline from becoming a beachball. And your tube really won't be all that happy with that.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Edits: 01/29/15 01/29/15 01/29/15 01/29/15
"Your response--- using a high pass filter--- it's generally regarded as a taboo approach in hi end circles--- but I think the approach has a lot of initial merit to it."If this was a full range amp I might regard it as taboo as well but this is a bi-amped system with a SS amp driving a pair of 15" JBL 2231s using an active 12db per octave low pass filter at 200Hz.
The JBL 2118J midrange speakers roll off on their own and are -3db at 200Hz.
I think keeping some of the LF out of the tube amp with a cap should help the issue that Voltsec points out and I believe this would be true even if my tube amp's OPT had "full" inductance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/29/15
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