Danieln Posted February 1, 2018 Share Posted February 1, 2018 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. 3 Quote Link to comment Share on other sites More sharing options...
Danieln Posted February 1, 2018 Author Share Posted February 1, 2018 Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted February 2, 2018 Share Posted February 2, 2018 This is a relevant discussion. If you re-post it in the Talkin' Tubes section of the forum you are likely to get some activity. Here, it will just get lost. I'll join in when it's posted there. Maynard Quote Link to comment Share on other sites More sharing options...
JohnA Posted February 2, 2018 Share Posted February 2, 2018 ALK Engineering produces a constant impedance crossover network for the La Scala. He will build them, or you can download the schematic and build your own. There are others here that will also build you a pair. They sound very good. The Type AL crossover is the worst put into a La Scala. Better sound in in your future. Quote Link to comment Share on other sites More sharing options...
Danieln Posted February 2, 2018 Author Share Posted February 2, 2018 Thanks Maynard, i just did Quote Link to comment Share on other sites More sharing options...
Deang Posted February 3, 2018 Share Posted February 3, 2018 On 2/2/2018 at 12:29 PM, JohnA said: ALK Engineering produces a constant impedance crossover network for the La Scala. He will build them, or you can download the schematic and build your own. There are others here that will also build you a pair. They sound very good. The Type AL crossover is the worst put into a La Scala. Better sound in in your future. The download section has been removed. All he has out there is the schematic for the Super AA - which will work just fine. However, it’s been demonstrated that the swamping resistor does reduce available power from the amplifier. At any rate, an easier and more straightforward approach is to simply add a 15 ohm resistor in parallel with the K-55, and change the 13uF capacitor to 26uF. 1 Quote Link to comment Share on other sites More sharing options...
Danieln Posted February 4, 2018 Author Share Posted February 4, 2018 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. Quote Link to comment Share on other sites More sharing options...
Olomana Posted February 5, 2018 Share Posted February 5, 2018 I have a older jolida jd 1000 RC amp, I am wondering what type of amp design this has and whether I have the same problem? Quote Link to comment Share on other sites More sharing options...
Danieln Posted February 6, 2018 Author Share Posted February 6, 2018 I looked-up the spec of your amp. It is pentode based push-pull, ultra linear, They also state less than 5 db feedback. I dont know if this is in addition to the ultralinear, which is a kind of feedback in itself. The best is to take some measurement before doing anything. it looks like a quality instrument, and i would be surprise if the designers didn't control its output impedance (which is not the load impedance (4 or 8 Ohms).) Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.