Does anyone know the output impedance of the main (AO-29) amplifier in a Hammond M-100 series? I'm trying to split the output between the main speakers and a rotating mid-range horn at around 300Hz using a passive cross-over. For a transistor amp, cross-over design is easy, because the output impedance is effectively zero, but for a tube amp, one has to take the amplifier's impedance into account.</p>
Not sure about the AO-29 in the M Stefan but the AO-43 in the L series uses 2 x 16 ohm 12" speakers wired in parallel giving an 8 ohm power amp load. I'd suspect the AO-29 is the same.</P>
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Thanks. That's the same as in the M-100. However, that just tells me the speaker impedance, which is not necessarily the same as the amplifier's output impedance.</p>
The wiring diagram in the L100 service manual states 8ohm output from the T301 output transformer's secondary output. The only reference tothe transformer'sprimary input impedence in the diagram is a 4700 ohm resistor going into the primary side from the two power amp valves. </P>
Dunno if that helps at all?</P>
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The impedance is rather complex because, like solid state, feedback is used. This lowers the impedance.</P>
The best way to do this is to measure it by putting half load on it 16 ohm resistor and measure output voltage from a fixed note and gain setting. Then put a second 16 ohm resistor on it and again measure the voltage. I would not do the classic open circuit then load test for safety of the amp so do the 1/2 to full load test.</P>
You can do the math I am sure to calculate the effective series resistance presented by the amp. </P>
You can probably get an approximate value by taking the load impedance of the output tubes, transforming it down by the ratio of the output transformer and then multiply by the feedback ratio... Or just trust the output transformer matches the tube to the 16 ohm output.</P>
If we do that we canmultiply the 16 ohms by the feedback ratio to get close.</P>
The feedback ratio unfortunately is a factory volume setting that goes from a splitting resistor at the secondary of the output transformer.That will be frequency sensitive on top of it. The later models than the 29 have a more conventional feedback with a resistive dividervariable from about 1/30 to 1/100. Try the load tests suggested to measure the value. You could do an open to full load regulation test by keeping the <U>volume very low</U> for the test so as to not risk arcing the amp. Use 8 ohm load and oscilloscope to measure the peak voltage.</P>
Vload/Vopenckt = 8/(Rs+8) measure two voltages, solve for Rs</P>
The effective series impedance of the later M100 amps would be around 1/6 to 1/2 ohm.</P>
For the simple passive filters (usually just an L arrangement) the amplifier effective output impedance won't affect the result very much. The impedance of the speakers it drives will have impedance variations with frequency that will probably affect it more. The impedance of the amp will probably mostly affect the Q of the filter you use.</P>
If you know the ratio of turns of the output transformer, the impedance is transformed by the square of the turns ratio of course.</P>
Okay, I've determined the impedance of the amplifier output at middle C, and E, G, and B above middle C. It seems to vary from about 7.4 to 7.9 Ohms. Once I knew the amplifier impedance, I was also able to measure the speaker impedance (for the two speakers in parallel) over that range, and it increases from 8.4 to 11.2 Ohms. Then, I determined the impedance of the rotary speaker I'm using, and its impedances at those four frequencies are 10.7, 43, 18.6, and 10.7 Ohms respectively. So it appears that the rotary speaker has an impedance spike at around 330 Hz.</p>
That speaker was from a Yamaha BK-20 organ, which has a separate amp for the rotary speaker, with a high-pass filter on the amp's input. Unfortunately it is nowhere specified what the cutoff frequency is. I suspect that it's somewhere above 330Hz, so I'd probably better design for a 400Hz cross-over instead of my originally planned 300Hz one, and it looks like I do need to take the amplifier impedance into account.</p>
These are about what I would expect. The later amps with the better feedback should be down in the 1/2 ohm or so range. The 29 sure has a weird arrangement with that capacitor... more like frequency comp rather than linearity feedback.</P>
Speakers can be all over the map.</P>
Apparently the stock Leslie crossover frequency is 800 Hz</P>
just one of several articles searching Google for "Leslie crossover frequency".</P>
There is one statement to the effect that the crossover is not intended to be efficient. </P>
I think the 800 Hz is probably what should be used as the overall Leslie effect is what you are after. Having experimented with electronic leslie stuff, the rotating high end needs to be well above 400 or the result sounds "muddy". </P>
These articles mention for active filter a 12 Db/ Octave skirt which implies a fairly steep cutoff... Take heed that it is probably that for a reason...</P>
You won't get 12 DB with passive of course with simple filter...</P>
I wasn't trying to match the stock Leslie frequency so much as trying to keep the rotating horn from blowing. In the Yamaha organ from which this speaker (a 6" x 9" rectangular horn) was taken, there's a high-pass filter on the tremolo preamp, and the horn is referred to as "mid-range and treble". Unfortunately they never say where "mid-range" starts, although from looking at the filter schematic, I make it out to be around 300Hz. But, as I've discovered, the speaker seems to have a very high impedance at 300Hz, so that isn't going to work with a passive crossover.</p>
So, maybe I should take your advice and do the crossover at 800Hz. It'll be easier to design the crossover too, since the speaker impedances are probably pretty close to nominal at that frequency. The only problem with this is that unlike in a real Leslie, the lower frequencies won't be rotating at all, since I only have the one rotating speaker.</p>
12dB/octave is easily achievable with a 2nd-order passive crossover (one inductor and capacitor per speaker). </p>
Some of the Leslies don't rotate the low speaker.</P>
I have one of the RA-50 Yamaha combo Leslie like things that may have the same speaker you have. It is rotated and has a counterbalance weighton it.</P>
If you are concerned about blowing the speaker, you might want to put a fuse and maybe a low voltage MOV on it. </P>
If the speaker is from a RA-50 or is similar, searching for data on the RA-50might be fruitfull.</P>
I searched for the RA-50, and didn't find much info. Searching for the two-horn (plus fixed speakers) RA-100 returned more results. Here's a link to a picture of the innards of an RA-100, showing two of exactly the horn I'm using: Yamaha RA-100 Innards. Since you have an RA-50, do you have any specs on the horn?
Yep! The RA-50 has a single one which spins on axis perpendicular to the face of the unit. That is the exact same speaker with the counterbalance weight I mentioned.</P>
The RA-50 supposedly is 50 watts... I assume shared between the woofer and the spinning midrange...</P>
Frankly, I was disappointed in the sound from the RA-50 as it sounds too much as a point source and for lack of a better description, like a plastic coned speaker. I attribute that to the orientation of the spinning speaker too. I suspect the RA-100 is just two RA-50 in the same box...essentially... probably a common amplifier...</P>
In my Hammond, I mounted the speaker so that it rotates around the vertical axis, like the horn in a real Leslie. It remains to be seen how it sounds. I won't be hooking it up for a while, since my next order of business is to finish refinishing the organ so I can bring it into the house. (I took the "Leslie"-installing side trip because that involved cutting an extra hole in the speaker board, which I wanted to do before refinishing the speaker grill cloth.)</p>
I'll probably write all this up with pictures on my web site when I'm done.
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