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Old 22nd September 2007, 12:25 PM   #1
ben106 is offline ben106  Switzerland
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Question s5electronics tube amp kit damping factor

Hi,
Does somebody know the damping factor of the new K-8LS stereo tube amp from s5electronics ?
Because on the website I read that the output impedance is 8ohms, so the damping factor would be 1 with an 8ohms speaker. That seems pretty low, but it may be possible.

Thanks for the help

Ben
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Old 22nd September 2007, 07:56 PM   #2
Jeb-D. is offline Jeb-D.  United States
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That is misleading, I'm pretty sure they mean that it is meant for an 8ohm load. Not that the Zout is 8ohms. I'm using one of their amps driving a 3-way speaker, which it handles well. So, I couldn't imagine the Zout being that high. Damping factor is usually pretty much an ignored spec with the tube crowd, since it is usually pretty poor (which does not mean it sounds poor!). Actually I pretty much ignore most specs supplied by manufactures. It seems like they pretty much make them up anyways. Plus, some poor spec amps sound really good and some super-spec amps sound like poop. For a tube amp, ideal is around 1ohm, but as long as the output impedance is below 2 ohms it's all good IMO.
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Old 23rd September 2007, 10:42 AM   #3
ben106 is offline ben106  Switzerland
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Thanks for the reply !
On their website they say : Output impedance : 8ohms, Speaker impedance : 4 to 8 ohms. That's confusing..!
I sent an e-mail to s5electronics but they haven't replied yet.

I want to know the damping factor because I'll build a pair of speaker boxes, and I'll chose the speakers depending on the damping factor.
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Old 23rd September 2007, 04:49 PM   #4
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The DF could very well be 1.

DF is the ratio of load Z to output Z, nothing more.

As the output impedance of the amp goes LOWER the DF goes HIGHER.

The use of negative feedback lowers the output impedance.
In practice typically the highest DF that you will see using tubes and transformers for the output is DF=~10. That depends on the amount of negative feedback in the circuit.

In the case of a tube amp with Zero Negative Feedack, and using the usual plate connection of the output tubes, the DF is going to be very close to 1.

The only thing you want to avoid with a tube amp is a speaker with overly high "Qtb" in terms of the alignment of the woofer in the cabinet as that may lead to a very wooly and out of control bass character.

Because a tube amp passes maximum power only at ONE impedance - all others, higher Z and lower Z being lower in power (and speakers being a not constant Z) - tube amps (even with feedback) tend to subjectively sound somewhat different than solid state amps, even when the DF issue is not a factor in comparison... just a point to take into account.

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Old 23rd September 2007, 09:53 PM   #5
Jeb-D. is offline Jeb-D.  United States
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Most S-5 kits do use negative feedback. But they are running the output tubes in Pentode mode. So who knows, maybe DF is 1. Though I'd be surprised considering it sounds quite balanced driving a 3-way system. Where most hi-Z out amps are known to fall short, do to the highly reactive load.
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Old 23rd September 2007, 10:12 PM   #6
jnb is offline jnb  Australia
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I would have expected more global NF with a pentode output stage. Triodes seem more capable of holding their own in this position.
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Old 24th September 2007, 12:31 AM   #7
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I did not say it was DF = 1, I said it could be as low as 1.
It is likely higher, up to about DF=10 depending on the specific amp.

Not sure what you mean by "high-Z" output amps Jeb-D, all tube amps have an output impedance that is usually within a factor of 10 of the nominal output impedance. That means a lower Z than the output impedance of unity to 10x lower. There is a transformer involved here and an impedance transformation between the plate of the tubes and the load that is rather much a constant.

If the "reactive load" is still similar to the output impedance of the tube amp, then the tube amp should not have any problems with it.

I think what you mean is low impedance loads, which sometimes are due to reactive dips in the impedance curve of a given speaker - like with certain ML ESLs??

jnb, as you increase the global NFB, the gain drops, and there is a practical limit to the output Z drop that you can achieve, so not sure what the significance of your comment is?? Unless this is just a general comment on the schematic?

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Old 24th September 2007, 05:05 PM   #8
ben106 is offline ben106  Switzerland
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Thank you all for your posts !

If nobody is sure, I'll measure the output impedance of my amp when I'll get it !
If I'm lucky, s5 will give me an answer.. (they don't seem to be sure about the output impedance, and didn't give me an answer about the damping factor).

I'll post here again when I get a result !

Ben
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Old 25th September 2007, 08:38 PM   #9
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Default Total Noob Question

Sorry to go off on a tangent in this thread, but I have a quick question. To put this in perspective, I am a total newbie and this will be my first tube amp attempt.

Question: What is the difference between the newer K-8LS and the previous K12G model?

Believe it or not I cannot find much information on the internet that goes into much detail. Obviously the L model uses different tubes as well as incorporates an additional two. (Though I don't know what the extra two do) In addition S-5 now offers add on gain and tone control modules, which I don't plan on using anyways.

I have read they changed the design because the tubes used with the G kit are getting scarce. There seem to be plenty on ebay though.

If anyone can quickly sum up the pros and cons of the new amp design, I'd appreciate it. More specifically, the G is $156 and the L is $192. Am I getting a better amp for that money, or would it to be wiser to stick with a tried and true design that many people have released modification how-tos and the like for?

Thanks in advance for any insight!
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Old 26th September 2007, 09:42 AM   #10
ben106 is offline ben106  Switzerland
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I bought an L amp because I live in Switzerland, and it is able to use 230V (insted of 110V).
I tried the amp yesterday, maybe my speakers are not enough efficient (JBL Xti 100, 91dB/W/m I think), but the volume is really low (I tried with a preamp, it was a bit better, but clipping happened if I turn the volume too loud).
On the website they say : 20Hz-20kHz, but I don't know if it is at -3dB, -6,... When I listened to it, there were no bass (in my opinion).

Phiberopticz : I would say, you should take a K12G model if you have 110V, as this amp is tried.

s5 said that for the K-8LS, the output impedance is 16ohm, for the K-16LS, it is 8ohm. I measured, and got approx 14ohm for the K-8LS (at 1kHz). So the damping factor is 0.5.

Ben
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