My tubes4hifi ST120 amplifier measures significantly different than a Parasound Lamp V3. I have attached the measurements. Above 2KHz, the use amp is 2db quieter than the Parasound. Is this showing that my tubes are worn? I recently noticed things sounding dark and I never thought that before. Measurements in the picture.
That is interesting, but an electrical measurement of just the amplifier would be more clear.
Are both channels the same?
Are both channels the same?
Good question. I did only measure one side of the amplifier. I'm wondering if it is just that the tubes are getting older. It just recently started sounding darker and the 2db difference is a lot. I will measure the other side when I have a chance. Also, I'm not familiar with how to measure just the amplifier. Can I do that with REW and a Scarlet interface?
Kind of odd that the difference is ~constant above 5kHz.
A high frequency roll-off in the tube amp would have an increasing difference from the top curve.
A high frequency roll-off in the tube amp would have an increasing difference from the top curve.
Yup and I think this is a recent development. My speakers just started sounding a bit duller so I did the measurements. The problem is that the speakers are DIY and I just redesigned the crossover so I thought that I just made a mistake but they measure really well with the Parasound so I don't think it has anything to do with them. The crossover is really simple 2nd order with one resistor in the tweeter circuit. They sound great with the Parasound. I'm suspicious that I either have a bias or tube problem.
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I can confirm that both sides of the amplifier measure the same. I've had the tube amp for 2 years. I use it 2-4 times a month. Maybe 150 hours on the tubes. Maybe more. I really don't know. I'm suspicious about them though.....
I was incorrect about the crossover. It is a 2nd order circuit with baffle compensation on the woofer and an L-pad on the tweeter. Here is the impedance plot. The tube amp is on the 4 ohm tap so it should handle the 4 ohm load on the tweeter, right? I don't think this looks like a difficult load to drive.
Attachments
Quite possibly the speaker impedance at high frequencies is loading down the amplifier output,
and by voltage divider action the highs are decreased (rather than being rolled off with an RC filter).
Do you have a schematic for the amplifier? Does it have overall feedback, or not?
and by voltage divider action the highs are decreased (rather than being rolled off with an RC filter).
Do you have a schematic for the amplifier? Does it have overall feedback, or not?
Only 1 ohm of output impedance will cause a 2dB drop on a 4 ohm speaker. Your new crossover needs the solid-state amplifier.
Ed
Ed
If this is the total output of amp + speakers ( that's my guess ) it shows that the speakers has lower impedans at the treble
area and that st120 has too little NFB to cope with it. Also confirmed by the 45 hz resonance peak.
No it does not show anything about the tube performance. It shows the amp's ability to perform into various loads.
You could try to use the 4 ohm tap.
I think the basic question is, is this an acoustic measurement, measuring the speaker output?
If so, you cannot just conclude things about the amp. You are measuring the combo amp and speaker, and since speakers have much more variation, you can't really say anything about the amps.
Jan
It's -probably- sound pressure measurement (the vertical axis is SPL).
It shows the amplifier (OPT, same tap?)+crossover+loudspeaker drivers ... +++ the ROOM.
If you measure two device (amplifier) with correct measuring set (microphone stand + calibrated microphone, same distance between microphone and loudspeaker -driver- center), under the same conditions (same absorbent surfaces -the human body acts like this!!-) the results might be the same.
If any high frequency absorber differs, the difference (BTW 2dB is "nothing" in this case) would be occurs.
It shows the amplifier (OPT, same tap?)+crossover+loudspeaker drivers ... +++ the ROOM.
If you measure two device (amplifier) with correct measuring set (microphone stand + calibrated microphone, same distance between microphone and loudspeaker -driver- center), under the same conditions (same absorbent surfaces -the human body acts like this!!-) the results might be the same.
If any high frequency absorber differs, the difference (BTW 2dB is "nothing" in this case) would be occurs.
Agreed. It is 'nothing' in this case. Even being yourself in a slightly different place for the two measurements can make more difference.
This is a totally unsuitable way to compare amps.
I understand the OP has a soundcard, it is a mystery to me why he doesn't just measure the amps.
Jan
This is a totally unsuitable way to compare amps.
I understand the OP has a soundcard, it is a mystery to me why he doesn't just measure the amps.
Jan
The OP has noted a sound difference when using 2 amps on the same speakers. He wonders why.
My guess is that the amps react differently on the speakers impedance variations, the st120 is shy on
NFB and could very well act "soft" for variations in impedance. Increasing the NFB would make the st120
be less affected by the speaker.
My guess is that the amps react differently on the speakers impedance variations, the st120 is shy on
NFB and could very well act "soft" for variations in impedance. Increasing the NFB would make the st120
be less affected by the speaker.
That only reinforces the need to measure the amp, not the acoustic speaker output. That's daft.
Apart from that 'nfb is the cause' being totally speculation. Or a guess, as you say.
I am always amazed when people suggest cures before they even know what the actual problem is. Must be clairvoyance.
Jan
Apart from that 'nfb is the cause' being totally speculation. Or a guess, as you say.
I am always amazed when people suggest cures before they even know what the actual problem is. Must be clairvoyance.
Jan
1 ohm output impedance is not unusual for a tube amplifier. The speaker's impedance varies too widely for flat response with such an amplifier. Measuring the amplifier would confirm this.
Ed
Ed
It's not a guess. I have myself analyzed the vta70 ( very similar) and observed this problem. Correcting the NFB corrects the amp.
And it's not unknown that speakers may sound different with different amps, amps will differ in their ability to copy with strange loads.
Totally agree here. I am on the 4 ohm tap, but the amp cannot handle it very well. It really means that this amp needs a speaker that doesn't drop below 6 ohms anywhere, even on the 4 ohm tap. I'm going to use a Class D amplifier with these speakers.
I have measured the speakers with 3 different amplifiers (Class D Hypex, Parasound Class A/B and St120 Tube). The Class D and Class A/B measure the same. The tube amp struggles with anything that dips below 5 ohms on the frequency response. I also measured the amplifiers on a different speaker. The tube amp does the opposite on that speaker......the magnitude above 3Khz is increased, which is responding to a different tweeter circuit where the impedance is rising. This is showing me that the tube amp is not driving varying impedance loads accurately and would be much better suited to a speaker that has limited varying impedance and never dips below 6 ohms. All in all, I am moving to Class D or Benchmark amplification.