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Old 20th October 2007, 01:54 PM   #1
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Default Question about "bloaty" bass with SET and KT88's

I have a set of back loaded horns w/ FE208E Sigma drivers. My first tube amp was a Fountek Altitude 3500 which was PP w/ EL34's. I am now using my SimpleSE with KT88's. It sounds great but seems to be a little bloaty with the bass. I don't like the excursion I am seeing on my drivers. I have switches installed for CFB/No CFB and UL/Triode. I like the sound most with Triode mode and no CFB. Does anyone have any recommendations on how I can correct this? Would a tube swap be in order? I have EL34's and I believe there are a number of other tubes that are "plug and play" with this amp.
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Old 20th October 2007, 02:34 PM   #2
Salas is offline Salas  Greece
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When you apply NFB, the cones move in a more controlled manner? If yes then its the poor damping factor that your alignment doesn't like, in the non NFB mode that you prefer. In such a case I would just damp the speakers with more wool and enjoy my preferable amp mode.
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Old 20th October 2007, 07:47 PM   #3
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It sounds great but seems to be a little bloaty with the bass.
Such is the nature of SE amps, and if you want to follow that path - get used to it. I personally can't as I like to listen to electric music and theres a lot of stuff going on in the deep bass region that I need to hear.

I have heard it said that a solution to this problem is to regulate your +B. This will tend to keep the current to the speakers more constant, and hence will help to control your driver on both the push and pull of the cone.

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Old 20th October 2007, 11:29 PM   #4
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Default Re: Question about "bloaty" bass with SET and KT88's

Quote:
Originally posted by awhite1159
I am now using my SimpleSE with KT88's. It sounds great but seems to be a little bloaty with the bass. I don't like the excursion I am seeing on my drivers. I have switches installed for CFB/No CFB and UL/Triode. I like the sound most with Triode mode and no CFB. Does anyone have any recommendations on how I can correct this? Would a tube swap be in order? I have EL34's and I believe there are a number of other tubes that are "plug and play" with this amp.
Sounds like underdamped woofers. You can improve the damping by lowering the Zo with gNFB. Running without gNFB leaves you strictly at the mercy of the components you use. Other than a VT with a smaller r(p) there isn't much you can do about it.
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Old 21st October 2007, 12:48 AM   #5
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Quote:
Originally posted by Shoog
I personally can't as I like to listen to electric music and theres a lot of stuff going on in the deep bass region that I need to hear.
do you listen to any minimal techno? here's a great mix by Franco Bianco I've found...

http://www.pinksilver.net/podcast/me..._-_320kbps.mp3
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Old 21st October 2007, 07:56 AM   #6
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Thanks very much for the replies.

I have listened to rock all my life but have recently started listening to more jazz of the fusion and acid type. Examples of what I am listening to are Oz Noy, David Fiuczynski, Ostric tentacles, etc. I should clarify that the bass is very 'tight' as would be expected from horn bass. It just seems that when I get the spl above 100db, I see alot of excursion and although I don't have any audible IM distortion, I can't imagine it is not there. Perhaps this characteristic is normal? The NFB seems to help but I hear a slight decrease in the higher frequencies as a result. I think this SimpleSE amp is a very good amp but it was a beginner amp and now that I am certain I like the sound of a tube amp better, I would like to move on to building a really good amp. I know I don't need alot of power so would the Paramour SET from Bottlehead be a good choice?
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Old 21st October 2007, 10:05 AM   #7
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I would say that the things you are experiencing with your SE amp, you will probably experience with most SE amps unless you go for a very low impedance output valve.

Before splashing any more money, take a look at the RH807 and RH84 designs. These employs a different kind of feedback which is simple to implement and can dramatically improve the bottom end without killing the overall sound. I would advise trying it with a 100K resistor before doing anything else.

Then I would advise you broaden your horizons and consider some of the top flight PP tube amps. Look at Gary Pimms Tabor amp (if you can find it). Also consider the Amnity or Raven amps. There is also a Magnaquest 2A3 amp which uses a centre tap choke as a phase splitter. Then there is the Vacuum State 300dba.

IMHO I believe any one of these amps can do everything that a SE amp can do, but considerably better. Remember that sloppy bass is almost a universal reservation about SE designs.

Shoog
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Old 21st October 2007, 11:32 AM   #8
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Bruce Rozenblit is often referring to this problem and states that it is a result of the efforts of avoiding FB. No or low FB increases Zout and lowers damping factor resulting in no control of the speaker cone.
This "phenomen" is often said to prove that tube amps can deliver deep bass, but is a quality flaw.

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Old 21st October 2007, 01:37 PM   #9
Salas is offline Salas  Greece
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Since I am familiar with both tube and speaker design, I maintain that the best way is to suit the speaker to non NFB amps, not vice versa. Its better to over damp the speaker in this occasion. As I suggested in post #2.
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Old 21st October 2007, 02:29 PM   #10
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You all gave possible causes.

May I add that, in an SE amp, the primary inductance of the OPT is usually lower than in a PP (the consequence of the unavoidable gap combined with a too small iron).
This causes heavy current peaks at lowest frequencies (specially if the power tube has a lo Rp) wich may produce B+ sag (as stated by Shoog in post #3).
This disturbs the driver and produce an effect near to "damped motor boating".
In this situation NFB makes things even worst !

Regulating the supply of driver stages alone may help.
Reducing link cap in the grid of the power tube sometimes proves satisfactory.

No need to deseperatly "push" basses in an OPT who will short'em producing desatrous side effects

Yves.
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