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Tube amp frequency response?

post #1 of 13
8/31/09 at 8:21pm
Thread Starter 
Has anyone ever try to measure the frequency response of a tube amp and use RMAA to see the specs?

I'm curious as to how the FR of a tube amp changes relative to different tubes used and how much they deviate from a flat FR.
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post #2 of 13
9/1/09 at 2:43am
Quote:
Originally Posted by moonboy403 View Post
Has anyone ever try to measure the frequency response of a tube amp and use RMAA to see the specs?

I'm curious as to how the FR of a tube amp changes relative to different tubes used and how much they deviate from a flat FR.
Here's a wealth of testing on the millet hybrid tube amp:

Ti Kan's Millett Hybrid Headphone Amplifier

I've noticed that the differences in THD/IMD distortion patterns and differing noise floors are usually higher than the differences in FR across the amps - that's where I would focus if you're trying to figure out what causes audible differences between tube amps.

http://www.head-fi.org/forums/f6/twi...71/index2.html (scroll down a bit)

I have to find my link repository of them. I'll get back to you with some more if I find it.
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post #3 of 13
9/1/09 at 2:58am
Thread Starter 
Thanks for the info, but I'm hoping for an all out tube amp as opposed to hybrids. However, I'm pretty surprised at the relatively flat FR of the hybrids and perhaps a tube amp will deviate more from a flat FR.
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post #4 of 13
9/1/09 at 4:19am
I know Little Dot claims 10Hz-100KHz @-1dB for my MkIV, which would be a pretty darn flat FR curve. But I also see a much narrower response from higher-end tube amps, which makes me wonder if there isn't something else at play.
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post #5 of 13
9/1/09 at 4:28am
Thread Starter 
Quote:
Originally Posted by Mehve View Post
I know Little Dot claims 10Hz-100KHz @-1dB for my MkIV, which would be a pretty darn flat FR curve. But I also see a much narrower response from higher-end tube amps, which makes me wonder if there isn't something else at play.
Here's the quoted manufacture spec of the Manley Stingray tube amp I'm currently using:

Frequency Response: 15 Hz - 40 kHz, -1dB

Which specific tube amps are you talking about?
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post #6 of 13
9/1/09 at 11:46am
the MkIV fr wouldn't be including the too small output coupling Cap bass corner with 32 Ohm Cans - really need to see loaded output response
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post #7 of 13
9/1/09 at 3:09pm
Quote:
Originally Posted by moonboy403 View Post
Here's the quoted manufacture spec of the Manley Stingray tube amp I'm currently using:

Frequency Response: 15 Hz - 40 kHz, -1dB

Which specific tube amps are you talking about?
Yeah, those are the kind of specs I usually see with tube amps. But if you check out LD's page for the amp, you can see the 10-100KHz specs. I agree it sounds a little odd.
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post #8 of 13
9/1/09 at 5:25pm
Thread Starter 
I usually don't trust quoted specs. Moreover, it really doesn't matter if the amp can extend to 200khz since

1) I've read that no mic can record beyond much over 40khz
2) We can't hear anything past 22khz.
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post #9 of 13
9/2/09 at 7:35am
Quote:
Originally Posted by Mehve View Post
I know Little Dot claims 10Hz-100KHz @-1dB for my MkIV, which would be a pretty darn flat FR curve. But I also see a much narrower response from higher-end tube amps, which makes me wonder if there isn't something else at play.
Its also probably a computed, not measured curve though, so how closely it corresponds to reality depends on how they came up with the numbers. But it is rather simple to know. connect it to your computer and run rmaa. As a side benefit, you will get additional information about its performance.
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post #10 of 13
9/2/09 at 1:24pm
Quote:
Originally Posted by moonboy403 View Post
Has anyone ever try to measure the frequency response of a tube amp and use RMAA to see the specs?

I'm curious as to how the FR of a tube amp changes relative to different tubes used and how much they deviate from a flat FR.
Actually, it depends a lot on the topology used. Depending on the tube amp at hand there might be a change.. or none at all when tuberolling.

@sanderx: you've got a computer able to measure signal up to 100khz ? I want yours Better would be to hook up a signal generator (a good one) and a scope to the amp and see if there is a serious attenuation at 100khz.
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post #11 of 13
9/2/09 at 3:07pm
Thread Starter 
I know a lot depends on the topology used, that's why it's best if someone got some measurements.
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post #12 of 13
9/9/09 at 12:37am
A tube amp frequency response up to 100kHz is completely believable IF the manufacturer is fully trustworthy.

BEHOLD the specs of the hopelessly ancient (released in 1959) Heathkit W5 mono, all-tube (including tube rectification) 25 Watt power amp:
Frequency response: 5-160kHz +/- 1db

WITH the full set of measurement curves (as no one number or curve will suffice) and a detailed description of the test setup, all listed in the assembly manual
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post #13 of 13
9/12/09 at 5:50pm
Quote:
Originally Posted by 00940 View Post
@sanderx: you've got a computer able to measure signal up to 100khz ? I want yours Better would be to hook up a signal generator (a good one) and a scope to the amp and see if there is a serious attenuation at 100khz.
For negative results you usually need a lot less than for a positive confirmation. And seeing that it is indeed flat to 96kHz (something new computers increasingly can tell you out of the box) is a fairly good result as it is. especially if it was with a load and included the low frequencies.
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