Decibel scaling question

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mlvp

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Decibel scaling question
« on: 29 Nov 2003, 09:12 pm »
Hi John,

I'm trying to understand the decibel scale of the NOH...
I read that the dB scale is 10 log (Pout/Pin), where
Pout and Pin are output and input power, respectively.
Derived from that is a dB scale for voltages, which
works out as 20 log (Vout/Vin).

Now a perfect transformer should not absorb any power,
so Pout = Pin... can I still assume that the attenuation
factors for the NOH follow 20 log (Vout/Vin)? (Or, in other
words, -6 dB for each halving of the voltage?)

Michiel

John Chapman

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Decibel scaling question
« Reply #1 on: 30 Nov 2003, 12:14 pm »
Hello!

Yup.

Thanks!

John Chapman
www.bentaudio.com

mlvp

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Decibel scaling question
« Reply #2 on: 1 Dec 2003, 09:24 pm »
Thanks,

I was afraid of that.

My system has excessive gain, so I need approximately
12 dB more attenuation than the TX-102 can provide.

Of course, I could try to compensate for that by using
some resistor attenuation (L-pad?) on the input of the
NOH or the amplifier.

What is your first reaction to that? Adding resistors to
a TVC seems like throwing the baby out with the bath
water.

The best option is probably to add another transformer/
autoformer. I looked around already, but the number of
different transformers is overwhelming. Could you give
a ballpark figure for something like impedance, for
instance?

Michiel

John Chapman

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Decibel scaling question
« Reply #3 on: 1 Dec 2003, 10:01 pm »
Hello!


This comes up quite a bit - usually with horn speakers. We have an easy solution. We make a special tx102 for these cases with taps down lower than usual. Often we start at -60 then work on up either stopping early (since you'd never make it to unity gain anyway) or go right up to 0db and stretch the taps appart as we get to the higher levels.

There is no charge for the special trannie and we work out with you the best range for the tx102 to be made.

Thansk!

John

mlvp

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Decibel scaling question
« Reply #4 on: 3 Dec 2003, 11:31 pm »
That's even better!

Does this have any side effects like a different
frequency response or a lower maximum input
voltage (my CD player puts out 4.4 V)?

A few questions about the tap spacings... the
standard spacing is:

17 x 2dB
4 x 3dB
1 x 6dB

Is there a reason for the bigger steps at the lower
end, like a decreasing sensitivity of the ear at lower
levels, a smooth transition to mute, or the way the
secondary windings must be laid out? The 6dB
step in particular seems hardly practical.

Is it possible to go from -60dB in 2dB steps until
the taps run out? Besides, is 23 taps the maximum
amount of taps available? A few extra (unused) taps
would give peace of mind about future use with low-
powered tube amplifiers.

By the way, I am surprised about the specially-wound
transformers. Perhaps it should be obvious, but to me
at least, it wasn't. Why not advertise this option on
the web page?

Michiel

John Chapman

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Decibel scaling question
« Reply #5 on: 4 Dec 2003, 03:10 am »
Hello!

No side effects - it is not that big a change really. The large core tx102 is fine to well over 10V input so that cd player level is no problem either.  

We now have a few more taps. Latest tap arrangement is -52 / -46 / -43 / -40 / -37 / -34 / -32.......0.

The +6db is not the big jump it seems to be as in almost all system it is never used really so it just becomes a really low volume changing to a slightly higher really low volume.

We can put the taps anywhere you'd like. Most switches have 23 or 24 steps so that limits a bit how many we use (as does the fact it would get nuts to try and put a bunch more taps inside the trannie!). The trick becomes to choose the taps so that you'll be running in a zone with modest sized steps - this is not hard to do and it is why we came up with the versions that go lower down. To do this we cheat a bit and stretch the taps down at the very bottom so that we can make better use of the taps in the operating zone. If someone is going to be actually using the pre-amp way down low in volume then we make the steps smaller down low. If you wanted a few extra taps we could fit them in I'd guess but re-wiring a level control switch is a bear of a job so it is not too likely you'd do it later....

I usually go through a rough estimate of the volume of a system and we can tell pretty close where you'll operate the pre-amp. Rather than 'advertise' it I only bring it up if it looks like it is needed.

Thanks!

John

mlvp

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Decibel scaling question
« Reply #6 on: 4 Dec 2003, 09:36 pm »
Hi John,

Thank you for your answers, they are much
appreciated.

I'll do some homework to come to grips with
how the numbers work out in practice (like,
how big is a 3dB step to my ears?), as well
as some pondering over the system I want
to own over a couple of years.

It will take some time (I want to get it right,
as I hope to enjoy it for a long time), but as
soon as I have made up my mind, I'll get
back to you by e-mail.

Michiel

John Chapman

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Decibel scaling question
« Reply #7 on: 4 Dec 2003, 10:48 pm »
Hello!

Thanbks again for the thread. It's been good with lots of good info. Look forward to hearing from you!

Thansk!

John

gonefishin

Decibel scaling question
« Reply #8 on: 18 Feb 2005, 02:44 am »
Hi John,

  I know you don't offer the NOH kit any more, but I sure still value your advice.
 
   I'm currently in the middle of some amplifier changes.  As of now, I've got a 25 Watt AKSA amp running my midrange/treble.  I've got a 100Watt version AKSA on order.  Because this is a kit I'm sure it will take some time to finish the project and make a case for it.  So I'm a ways off from changing my amplification over.  

   So for right now I have the AKSA 25 Watt amp running the mid/tweet and an IRD SS amp running the TAD woofer.  The AKSA has a ton of gain.  Instead of adding a pad or resistor attenuation to the midrange/treble.  I choose to continue the same set-up I had when I used my tube amps on top.  I needed to "add" some gain to the SS IRD woofer amp...so I have an in-line Foreplay preamp acting as a gain stage before the IRD amps.  This way I'm not mucking up the midrange/treble.  (If you remember...this was actually on your suggestion.  This has seemed to work out well for me )

   My problem?  The gain in the AKSA.  Level one on the NOH preamp is too loud for a starting position.  I've read your review about the sonic differences between the different versions (I have MKII) and I really would like to stay with the MKII that I have...but I need more attenuation.

   What do I do?  I don't want to add resistors to the NOH  :cry:


     thanks, *sniff...sniff*
 dan

John Chapman

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Decibel scaling question
« Reply #9 on: 22 Feb 2005, 03:37 pm »
Hello!

I'll need to check with S&B to see what they have. Only thing that has come to mind so far is another trannie in-line to knock the signal down. About how much lower would fit your system gain?

 One other thought is to see if there is a mod for the aska to knock down the input sensitivity. A resistor divider right at the input to the aska (I know it seems evil....) would be so darn cheap and easy it would be an idea to try that as well just to see if you can hear a difference.


Thanks!

John

gonefishin

Decibel scaling question
« Reply #10 on: 22 Feb 2005, 11:29 pm »
Thanks for the reply John.  I had asked about adjusting the gain of the amp itself...which I was told that there is very little adjustment that could be made before starting to alter the characteristics and voicing of the amp.  

  I'm at work today...but I would guess that another three steps (or at least two) below my (now) lowest position could work.  (did that make sense???)

   You know...I suppose reducing the input sensitivity may be a good idea.  But I'm getting such amazing resolution (from top to bottom) right now with the Bent/AKSA/EdgarHorn TAD combo that I'm not sure how much I'd want to stretch it.  I'm hoping that this may be a final evolution to my main system (except maybe a 10Y or 45 amp)...but I'd really rather see if I could get things set up the way they're supposed to be first.  Then, if solutions can't be found...I'll pursue the other avenues further.  I know this can be a bit of a pain...and I apologies.  


 Would it be possible for S&B to modify the transformer and add any additional taps?  Do they still offer or have any of your mkII?  or something else that would be low (or preferably no) compromise?

 
 
Quote
The trick becomes to choose the taps so that you'll be running in a zone with modest sized steps - this is not hard to do and it is why we came up with the versions that go lower down.


   Should I see where I'm at first...and how the bottom should be set up...as if I were (gasp) ordering a new one?  Then I may be able to at least see where I'm at.

   I know your a busy man...and I appreciate any and all input you can help me with.

  thank you,
        Dan

John Chapman

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Decibel scaling question
« Reply #11 on: 23 Feb 2005, 01:00 am »
Hello!

No problem at all with the questions.  3 or 4 steps lower would be about 8db lower so you'd need about an 8 db reduction in level or so.  S&B could easily make some MK II's with low taps - I did not want to go there in my earlier reply since they are so darn expensive! Looking for a more cost effective answer.  If you had a friend who wnated the current unit or something like that maybe the new ones could make sense.

Scope it out a bit to get a good handle on it first and then we'll make a plan

Thanks!

John