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Volume Question

B

Bob King

Active Member
I don’t think specific brands have a influence to my question- I have a 35 WPC stereo tube amplifier; global negative feedback is taken from the 8 ohm tap of the output transformer. One set of speakers are 8 ohm with 93 dB efficiency, the other set are 4ohm with 93 dB efficiency. The 8ohm speakers operated from the 8 ohm tap are noticeably louder than the 4 ohm speakers operated from the 4 ohm tap. Same thing occurs with another 4ohm set of speakers of lower efficiency are tried.
It seems like the entire output level drops as the hum and noise is at lower volume also, and the volume control has to be advanced maybe 15 degrees for the same volume. Both channels are the same and sound quality is excellent.
Doesn’t seem to happen on another tube amp that I have......and have never noticed this with guitar amps.
Anyone have an opinion about this ?
 
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so its louder when connected to the 8 ohm tap, but you have to turn the volume control up further to get there? Not entirely sure I understand what you are saying.
 
Example is with 8 ohm speakers on the 8 ohm tap, the volume control is at 9 o’clock for normal volume. With 4 ohm speakers on the 4 ohm tap, volume control is at 11 o’clock for the same volume. Feedback from the 8 ohm tap in both cases.
 
Bob King said:
Example is with 8 ohm speakers on the 8 ohm tap, the volume control is at 9 o’clock for normal volume. With 4 ohm speakers on the 4 ohm tap, volume control is at 11 o’clock for the same volume. Feedback from the 8 ohm tap in both cases.
Click to expand...

2 different speakers rated at the same sensitivity doesn't mean they really are the same.
Have you tried the "4 ohm" speakers on the 8 ohm taps?
 
so its a voltage thing. The 8 ohm tap puts out more voltage at a given volume knob position than the 4 ohm tap does, so technically you're getting more power when connected to the 8 ohm tap no matter what the speaker load actually is. Voltage is approximately 40% higher at the 8 ohm vs the 4 ohm, depending on exactly how accurately they placed the tap.
 
gadget73 said:
so its a voltage thing. The 8 ohm tap puts out more voltage at a given volume knob position than the 4 ohm tap does, so technically you're getting more power when connected to the 8 ohm tap no matter what the speaker load actually is. Voltage is approximately 40% higher at the 8 ohm vs the 4 ohm, depending on exactly how accurately they placed the tap.
Click to expand...
Kinda what I was thinking, that the ratio of voltage in the NFB circuit and the lower voltage at the 4ohm tap are combining to lower the volume. And I get that the manufacturer stated efficiency may not result in the same volume. I like the sound and that the floor noise seems reduced. So I will live with it.
 
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opnly bafld said:
2 different speakers rated at the same sensitivity doesn't mean they really are the same.
Have you tried the "4 ohm" speakers on the 8 ohm taps?
Click to expand...
I have not tried that, which would verify if it indeed is a voltage thing or relative inefficiency of the 4 ohm speakers.
 
6DZ7 said:
Which is why you don't want to put a 4 ohm speaker on the 8 ohm tap.
Click to expand...

I generally don't rely on too many manufacturers specifications, so I (and others) say just try it.
Edit: See post below for additional points.
 
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tube amps aren't excessively bothered by impedance mismatches, though at high power levels and depending on the specifics it can abuse the tubes a little bit more. Unless pushed to their limits though, for the most part they don't care if you put a 4 ohm speaker on the 8 ohm tap or vise-versa. They may not meet specs for distortion and power output though.

I have 6 ohm speakers which in theory are wrong for everything. Some amps they work better on the 4 ohm tap, others they do better on the 8 ohm tap.

then you get the whole impedance curve thing, so that "8 ohm" speaker may be higher or lower through most of the band. A lot of mfg don't give you that plot.
 
8 Ohm speakers are rated for sensitivity at 1 watt - so it's 93dBa @ 2.83V into 8 Ohms. Some 4 Ohm speakers are rated for sensitivity at 2.83V... but that's 2 watts into 4 Ohms, so actual sensitivity is 3 dB less. Gotta read the fine print.
 
I have a pair of Onkyo speakers that have some really oddball efficiency rating. Its not the usual 1watt at 1 meter thing, its measured at 50cm for unknown reasons. 98db though, but I think that translates to 92db at 1 meter.

Nothing quite like lying with misleading specs :)
 
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