input attenuation on AKSA 55

CButterworth · « **on:** 15 Mar 2004, 03:19 am »

Hello,

I have a working AKSA 55 (sans Nirvana). I built a passive preamp with an Elma selector switch and an Alps (Blue Velvet) 50K pot. However, the volume from the AKSA is so powerful, that I cannot (don't want to turn) the volume above 8 O'Clock on the volume control on the passive preamp.

My thought was to add some kind of attenuator between the selector switch and the pot to reduce the line-level signal before it enters to 50K pot. Would the simplest method simply be to place a resistor in-line with the +ve line? I have some mil-spec Vishay-Dale resistors that have a 1% tolerance and are 1/4W.

Or, would I somehow have to make a link through a resistor to -ve?

OK, so what sort of resistances should I try? I vaguely remember reading a discussion in the forum about the size of pots, and problems associated with impedences when using increased resistances. Maybe I'm mistaken.

Any ideas?

Thank-you,
Charlie

AKSA · « **Reply #1 on:** 15 Mar 2004, 05:13 am »

Hi Charlie,

Increase R8 from 2K2 to 3K3. This will bring gain back at the cost of feedback factor; but it will sound very good indeed.

Cheers,

Hugh

PSP · « **Reply #2 on:** 15 Mar 2004, 02:39 pm »

A general approach to attenuation is to build an L-pad or T-pad. I don't have time right this second to describe this in detail (do a search on L and T pads). For an excellent overview, see VoltSecond's web page (the examples are based on the Foreplay, but this works in general). http://www.siteswithstyle.com/VoltSecond/12_posistion_shunt/12_Position_Pure_Shunt.html Scroll down to section 7.1 and study the diagrams. There is an Excel spreadsheet that you can download... you input the impedances and the desired attenuation in db, and the spreadsheet will calculate resistor values for you.

As I understand it, the ideal place to put an L- or T-pad is right at the input of the power amp. IMO, every DIYer outght to know how to use these... it's simple, effective, and--if you use good resistors--has very little sonic penalty.

Good luck,
Peter

Malcolm Fear · « **Reply #3 on:** 15 Mar 2004, 02:59 pm »

I have turned a passive attenuator ito a shunt attenuator. You will attenuate the level and also improve the sound. You can experiment with resistor brands (Riken Ohms etc) and really make a difference.
Have a look through the threads on Valhalla mods for the GK-1.

DSK · « **Reply #4 on:** 15 Mar 2004, 10:53 pm »

Here's another option. Don't know what the impact is on sound quality ...perhaps you can search and find better quality ones.

http://www.electronicsplaza101.com/electronics/search/res/r35397.html

kyrill · « **Reply #5 on:** 15 Mar 2004, 10:56 pm »

Hi
This might help you Look for the 50k attenuator
http://www.goldpt.com/pre.html
but I would be curious to Hughs suggestion.
fb changes will have an (great) effect on perceived sound quality. And it is like I read that you may expect a beter sounding amp by using his suggestion

CButterworth · « **Reply #6 on:** 16 Mar 2004, 05:02 pm »

Thank-you for the replies and advice. I think that I will go with designing and implementing a T-pad network. This is very similar to the sketch sent to me by Malcolm and I will use the ALPS pot as a shunt.

Basically, to get a 20dB attenuation for the 50K pot and input impedences (assuming that the input and output is 50K - according to Hugh, the AKSA has 47K input impedence, so close enough). I would require R1 and R3 to be about 41K, and R2 to be about 10K.

What formula can I use to find the attenuation when I have the current Alps pot turned to the loudest that I can tolerate? This would give me an idea of what I want to set the minimum attenuation to.

Regards,
Charlie

PSP · « **Reply #7 on:** 16 Mar 2004, 05:56 pm »

Charlie,
The T-pad that you have designed should be "pretty close" to a 20db reduction across your entire attenuator range.

Unless you are using very fancy resistors (I use Xicon 1% metal films available from Mouser, $0.09 each for 1/4 watt or Roedersteins from Michael Percy if I'm being extremely picky) you could buy a set of resistors for a 20db pad and a 30 or 40db pad... for two channels you need six resistors, so it costs $0.36 for each pad you want to try. I always need to buy some extra parts to justify the shipping charges.... In DIY, you can never have too many resistors lying about...

Good luck,
Peter