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Help with tube amp measuring with less treble.

ThatSoundsGood

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These are 2 measurements of the same speaker from the same spot. One is with a Tubes4hif ST120 and the other is a Parasound Zamp V3. Why does the tube amp have 2db less treble above 3KHz. I know that the Parasound measurement is more accurate since it measures the same as my Buckeye Class D amp. Are my tubes going bad in the tube amp and losing treble?
 

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Blumlein 88

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Without knowing the speaker, first guess is output impedance. Tube amps have high output impedance. If the speaker you are using has a low impedance in the treble it would cause a roll off vs the Parasound. Some tube amps are a bit soft in the treble too.
 
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ThatSoundsGood

ThatSoundsGood

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Without knowing the speaker, first guess is output impedance. Tube amps have high output impedance. If the speaker you are using has a low impedance in the treble it would cause a roll off vs the Parasound. Some tube amps are a bit soft in the treble too.
The Speaker is a DIY design with a Purifi 6.5" woofer and a Blesma T34B tweeter. The crossover is 2nd order. The tweeter circuit has a 12ohm resistor to bring it down to the woofer. I haven't done an impedance measurement yet on it so I will do that and report back. I wouldn't think that this crossover and driver combination would be a difficult load to drive.
 

kemmler3D

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The Speaker is a DIY design with a Purifi 6.5" woofer and a Blesma T34B tweeter. The crossover is 2nd order. The tweeter circuit has a 12ohm resistor to bring it down to the woofer. I haven't done an impedance measurement yet on it so I will do that and report back. I wouldn't think that this crossover and driver combination would be a difficult load to drive.
First off, nice build, I was thinking of going DIY and those drivers were very high on my list. Eventually ended up with LS60s but I'd still love to hear a Purifi + Bliesma build.

Second, was everything in the room (including you, but especially the mic) at the exact same spot when you took the measurements? Treble sweeps are very vulnerable to positioning.
 
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ThatSoundsGood

ThatSoundsGood

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First off, nice build, I was thinking of going DIY and those drivers were very high on my list. Eventually ended up with LS60s but I'd still love to hear a Purifi + Bliesma build.

Second, was everything in the room (including you, but especially the mic) at the exact same spot when you took the measurements? Treble sweeps are very vulnerable to positioning.
Oh yeah. Everything was exactly the same. The measurements were taken within the same minute.
 
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ThatSoundsGood

ThatSoundsGood

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I was wrong about the crossover. It is a 2nd order, with baffle compensation on the mid. The tweeter has an L-pad and 2nd order filter. Here is the impedance and phase measurement. It's interesting because the measurement gets bad around 4K or 5K on the tube amp and that is the lowest impedance part of the tweeter. But it's at 4 ohms and the tube amp is on the 4 ohm tap so it should be fine.
 

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Blumlein 88

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Looks to me like below 3 khz the impedance is always more than above 3 khz which fits with the idea the output impedance is dropping everything above it. Also the tube amp is up vs the other at 600-700 hz and at 50 hz which fits with areas where the speaker has its highest impedance. It is the output impedance of the tube amp. Why don't you measure the tube amp output impedance and see what it is? BTW, it may be higher impedance at higher frequencies due to less feedback on the tube amp too.
 
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ThatSoundsGood

ThatSoundsGood

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Looks to me like below 3 khz the impedance is always more than above 3 khz which fits with the idea the output impedance is dropping everything above it.
Then would you say that it is just a poor performing amplifier? It's 4 ohms above 3Khz and the phase angle is rising smoothly so I wouldn't think there is a problem. But the measurements also different around 100Hz which is the other spot that it dips to 4 ohms. Tube amps suck.
 
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Blumlein 88

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Then would you say that it is just a poor performing amplifier? It's 4 ohms above 3Khz and the phase angle is rising smoothly so I wouldn't think there is a problem.
I think it only means your tube amp has an output impedance of .5 ohm or more which isn't bad or good it is just so. With a different speaker you don't get this. With some speakers it might even be a plus. This issue is high frequencies into that speaker have low impedance and at low frequencies the speaker has a high impedance meaning the output impedance is a lesser factor on the low side. If speaker were 4 ohms all the way from top to bottom the FR would be even. It is not however, it has more impedance below 3 khz. If you like the sound don't worry about it.
 
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ThatSoundsGood

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I think it only means your tube amp has an output impedance of .5 ohm or more which isn't bad or good it is just so. With a different speaker you don't get this. With some speakers it might even be a plus. This issue is high frequencies into that speaker have low impedance and at low frequencies the speaker has a high impedance meaning the output impedance is a lesser factor on the low side. If speaker were 4 ohms all the way from top to bottom the FR would be even. It is not however, it has more impedance below 3 khz. If you like the sound don't worry about it.
But what speaker has a flat impedance? I like the sound of the speaker. It measures incredibly well on and off-axis. It sounds great with the Parasound or Buckeye Class D that I use to design them but the tube amp sounds so dark. It's clear that this tube amp struggles with loads below 5 or 6 ohms. Thanks for your insight on this. I would need a speaker that doesn't dip below 6 ohms on this amp. It's fine, I'm selling it and buying a Benchmark amplifier or another Buckeye.
 

Blumlein 88

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A Magnepan comes close to being purely resistive around 4 ohms too. Wrong approach in my opinion however. Not that Maggies are a bad choice. Just that transducers make the most difference. So find speakers you like and cater to them. If the tube amp does not do it for you with your speaker, get the amp that does. Sounds like it is a modern class D or AB design.
 
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ThatSoundsGood

ThatSoundsGood

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A Magnepan comes close to being purely resistive around 4 ohms too. Wrong approach in my opinion however. Not that Maggies are a bad choice. Just that transducers make the most difference. So find speakers you like and cater to them. If the tube amp does not do it for you with your speaker, get the amp that does. Sounds like it is a modern class D or AB design.
I forgot about the magnepans. Not a fan of those due to their frequency response being far from flat. My tube amp has always been fine with my 8 ohm (nominal) speakers. I know that they are a distortion factory but it sounds great. For these speakers (which measure spectacularly and are designed with the lowest distortion drivers) I will be moving either to a Class D or Benchmark amp. Cheers and thanks again for the help.
 

DVDdoug

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If speaker were 4 ohms all the way from top to bottom the FR would be even.
Only if the amplifier's output impedance is also resistive, or very-low relative to speaker impedance across the frequency range like a solid state amp.

In any case, changing speakers to fix an amplifier problem is the wrong solution...
 

terryforsythe

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I was wrong about the crossover. It is a 2nd order, with baffle compensation on the mid. The tweeter has an L-pad and 2nd order filter. Here is the impedance and phase measurement. It's interesting because the measurement gets bad around 4K or 5K on the tube amp and that is the lowest impedance part of the tweeter. But it's at 4 ohms and the tube amp is on the 4 ohm tap so it should be fine.
Speakers are a reactive load, and the Equivalent Peak Dissipation Resistance (EPDR) may actually be lower than the measured impedance. This may or may not be contributing to the cause the 2dB drop.

If you really have your heart set on using the tube amp and bringing up the treble, you could try adjusting the L-pad by increasing the value of the shunt resistor, though this may change your tweeter response a bit if the L-pad is on the output side of the crossover. As for me, I would just stick with a solid state amp and not worry about it.
 
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ThatSoundsGood

ThatSoundsGood

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Speakers are a reactive load, and the Equivalent Peak Dissipation Resistance (EPDR) may actually be lower than the measured impedance. This may or may not be contributing to the cause the 2dB drop.

If you really have your heart set on using the tube amp and bringing up the treble, you could try adjusting the L-pad by increasing the value of the shunt resistor, though this may change your tweeter response a bit if the L-pad is on the output side of the crossover. As for me, I would just stick with a solid state amp and not worry about it.
The EPDR is a bit lower than the measured Impedance since I can see that also in REW. I'm definitely ditching the tube amp. I realize that it has been changing the frequency response of every speaker I've put it on. When the impedance is low then it doesn't drive it well. When the impedance is high then it it increases that spectrum of the frequency response. These are not difficult speaker loads to drive at all. I knew that this was possible with a tube amp but I didn't know it could be this drastic. It's changing as much as 3db over a wide spectrum, which is not only easy to hear, but is destructive. No thanks. Hey @Buckeye Amps: Another order coming your way soon.
 
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