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Danieln

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  1. I modified and measured the first one in order to compare the results side to side. It positively corrected the midrange shooting. I'm very careful to measure things since a few db can easily be lost using hearing only. But this mod left no doubt in the hearing test department :-) I then modified the second one and I'm now very happy with my Amp/ LaScala combo. Note again that my amp has a 20 Ohms output impedance. First step for you is to measure your amp output impedance.
  2. This is a followup to my previous post "La Scala impedance curve vs Tube Amp" Having designed and built a pentode based SEP amp, I discovered that it's high output impedance is not a good match to the AL crossover. I also read it is one of the worst crossover used in the LS. My first goal is to have a constant impedance network and secondly to made the midrange a true passband. I completely dismantled the AL cross over and ended up with a big collection of parts, mainly from the infamous notch filters. Looking at this kit, it became obvious that a redesign was possible. A lot of the inspiration was from discussion and schematic revolving around ALK work. So the end product is mainly inspired from the ALK universal. A first order filter for the lower crossover. A combination of second and third order for the high transition. i modeled and Spice simulated many versions of the design. I ended-up with the following: 1) A 1.25 mH inductance to the woofer 2) A 38uF going to the midrange filter. These two first elements defines the lower crossover. 3) A 0.3mH, 2uF and 10 Ohms connected to T3a (-6db) 4) A 2.4uF, 0.2mH, 8uF tweeter filter (Including protection diodes) These two elements defines the higher crossover. Simulations shows a lower crossover at 450Hz @ -3.1Db and a high crossover at 5900Hz @ -2.8db. As a proof of concept, the AL provided all the parts to assemble this design. Use two 2.5mH in parallel to built the 1.25mH Use the 30uf + 8uF to built the 38uF Modify the AL .5mH and .125mH to .3mH and .2mH (By basically transferring the right amount of wire from one to the other coil) So I built this thing, installed and measured its performance. I measured the low transition at 440Hz @-2.9db and the high transition at 6000Hz@-2.88Db....basically matching the model. Of course, this is just an ugly proof of concept. Better component can be used.
  3. Deang, thanks for pointing to some solutions. I read a lot about ALK work, and yourself and decided to do my own investigation. i will start another thread more appropriate to the subject.
  4. Nice and simple. Since you are operating in triode mode, you will not experience the impedance shift I described above. Back to the main subject... Anyone skeptical can do this simple experiment: I just did connect a 20 Ohms resistor in series to the left channel of my solid state amp. Not to the right channel. I used me Heresy which have the original impedance shift crossover. Listening in alternance to left and right, it is very obvious that the bass/mid mix is very shifted to the midrange using the resistor. This simulate a tube Amp high output impedance. A bit later, i will measure the AC voltage using 200 and 2500 Hz tone. Dan
  5. Very nice built wdecho. I can see you also use Edcor transformer which are bargains. I will consider your advise concerning painting end caps. I am not a big fan of this blue .... Do you run any feedback? By the way, my 807 are Russian :-) I turn on the Amp in the morning and close it at night. 12 to 15 hours per day for the last 10 months. No sign of deterioration yet. I run them at 75% of max plate dissipation, and feel they will last few years @ 3000 hours/year.
  6. This is my home made mono block. I built two of them. I have been playing with tubes since I was 13 years old, and now 57. I have a big collection of tubes, and one big criteria of this built was how the tubes looks. I love how the mid 20's to early 30's globe tube looks. So I choose the 224A tetrode (Used as a triode) as the input tube, and the 227 as the driver. Output are the famous WW2 807, which is a derivative of the 6L6. I used two in parallel single ended class A, for a total of about 14 watts. I will post the schematic a bit later.
  7. Thanks for your reply Maynard, As i stated, i actually measured to confirm my calculations. First the amp output impedance was measured by driving a 10 Ohms, followed by a 20 ohms resistance. 10 Ohms yielded an output of 4.0 Vpp while 20 Ohms yielded 6.0 Vpp. Little maths give you a 20 Ohms output impedance. Second, i drove a 200 Hz tone from the tube amp to the LaScala. Measuring input voltage to the speaker gave 1Vpp. The same volume setting at 2000 Hz gave 3.5 Vpp, which is even more than what I predicted by calculation. Third, I ran some sweep scan from 100 to 15000 Hz and observe the midrange input voltage jumping . The same experiment using a solid state amp gave an almost completely flat response, as expected from a low output impedance source. All of this is consistent with the the theory, which is a simple Ohms law in fact: The voltage source from the Amp is divided between output impedance and the speaker according to the speaker impedance curve. I will come back with more detail about the Amp Dan
  8. Hello, my name is Daniel and this is my first post here. Couple of weeks ago, I bought a pair of black 1988 LaScala for a very good price. All drivers are original, cabinet in fair condition that I plan to re-veneer in the next few months. CrossOver is the infamous AL type. I read a lot of very good information here, and quickly get interested in the crossover. I have a Tube Amp based on the RH-807 design. It is basically a class A pentode based single ended output stage. The Amp have very little feedback, around 6db I figured. One very important characteristic is the output impedance that I measured at a huge 20 Ohms. In fact, any pentode based amplifier, including push-pull without a lot of feedback will exhibit a high output impedance. In theory, the output impedance is the tube plate resistance reflected on the secondary of the output transformer. A typical plate resistance value for a 6L6 based tube is around 30 kOhms. With a 5k to 8 ohms transfo, the plate resistance (output impedance) is 30000/625= 48 Ohms. Considering the 6db feedback, it fall to 24 Ohms, not very far from the value I measured at 20 Ohms. Now, why bother??? This 20 Ohms resistance is in series between the load (Speaker) and the source (The Amp) Consider the lower frequency where the speaker presents itself as a 6 Ohms load. For this case, the voltage at the speaker input terminal will be: Vin = Vsource (6/26) = .23Vsource Consider the mid frequency where the speaker presents itself as a 30 Ohms load. For this case, the voltage at the speaker input terminal will be: Vin = Vsource (30/50) = .6Vsource It can be concluded that the midrange will be 2.6 time more driven than the bass (.6/.23). a 8.3db bump. At first, I was skeptical about this. Then I measure the input signal to the La Scala from the tube Amp and effectively proven right. It also explains very well why anything midrange was screaming at me :-) I then used a more modern transistor Amp with a <1 Ohms output impedance, and everything was normal (speaker Input Voltage and sound). The 1 Ohms output impedance does not absorb any significant voltage from the source, whatever the speaker impedance. Conclusion is, output impedance matters a lot when using speakers presenting a non constant impedance with frequency. Any pentode based design without feedback needs to pay attention. Triode based design are much less affected due to a lower plate resistance, but will still exhibit the bump if no feedback is used. Solid state get a free pass :-) The next step was to fix the problem, which will be detailed in a next post
  9. Hello, my name is Daniel and this is my first post here. Couple of weeks ago, I bought a pair of black 1988 LaScala for a very good price. All drivers are original, cabinet in fair condition that I plan to re-veneer in the next few months. CrossOver is the infamous AL type. I read a lot of very good information here, and quickly get interested in the crossover. I have a Tube Amp based on the RH-807 design. It is basically a class A pentode based single ended output stage. The Amp have very little feedback, around 6db I figured. One very important characteristic is the output impedance that I measured at a huge 20 Ohms. In fact, any pentode based amplifier, including push-pull without a lot of feedback will exhibit a high output impedance. In theory, the output impedance is the tube plate resistance reflected on the secondary of the output transformer. A typical plate resistance value for a 6L6 based tube is around 30 kOhms. With a 5k to 8 ohms transfo, the plate resistance (output impedance) is 30000/625= 48 Ohms. Considering the 6db feedback, it fall to 24 Ohms, not very far from the value I measured at 20 Ohms. Now, why bother??? This 20 Ohms resistance is in series between the load (Speaker) and the source (The Amp) Consider the lower frequency where the speaker presents itself as a 6 Ohms load. For this case, the voltage at the speaker input terminal will be: Vin = Vsource (6/26) = .23Vsource Consider the mid frequency where the speaker presents itself as a 30 Ohms load. For this case, the voltage at the speaker input terminal will be: Vin = Vsource (30/50) = .6Vsource It can be concluded that the midrange will be 2.6 time more driven than the bass (.6/.23). a 8.3db bump. At first, I was skeptical about this. Then I measure the input signal to the La Scala from the tube Amp and effectively proven right. It also explains very well why anything midrange was screaming at me :-) I then used a more modern transistor Amp with a <1 Ohms output impedance, and everything was normal (speaker Input Voltage and sound). The 1 Ohms output impedance does not absorb any significant voltage from the source, whatever the speaker impedance. Conclusion is, output impedance matters a lot when using speakers presenting a non constant impedance with frequency. Any pentode based design without feedback needs to pay attention. Triode based design are much less affected due to a lower plate resistance, but will still exhibit the bump if no feedback is used. Solid state get a free pass :-) The next step was to fix the problem, which will be detailed in a next post.
  10. I bought them 2 days ago :-) 1988, black, fair condition. $800 CAD. Much bigger in person than on pictures! All drivers ok, I plan to modify the infamous AL crossovers, and refinish the cabinets. Dan
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