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Old 7th January 2007, 07:56 AM   #1
psymo is offline psymo  United Kingdom
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Join Date: Jan 2007
Default Help me complete my first gainclone.

Hello everyone, Iíve just finished building an Audioselector LM3875 kit and its sounds absolutely outstanding! I just have two questions before I put the amp into a case: -

1. I am currently using an Alps 20K pot for the volume control. Would a stepped-attenuator make a significant and worthwhile improvement, I was thinking about a Dale Vishay from Hificollective, these are available in 10K, 50K or 100K, which would be best? or do you know a better alternative available in the UK.

2. I have used CAT 5 cable for the input wiring and was considering changing this for Teflon insulated solid silver wire (0.5mm), is this worthwhile or am I becoming totally obsessed?

Thanks in advance,

Pete
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Old 7th January 2007, 08:16 AM   #2
AndrewT is online now AndrewT  Scotland
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Location: Scottish Borders
Hi,
obsessed, who, us?

Silver plated copper wire may offer a slight advantage over plain copper wire or tinned copper wire.

Solid silver sounds way over the top.

Teflon insulation may also offer a slight advantage.
But again it's down to cost/benefit.

Stepped attenuator do offer slight advantage, but MUCH MORE IMPORTANT is to sort the filter conditions from source to receiver when you decide to use a passive attenuator between source and receiver. It is not easy to get this right (or even close to right over a restricted volume adjustment range) and it will dominate your result.

re CAT5,
did you strip out the individual twisted pairs? or keep the four pair intact?
Have you tried CAT5 wound as multiple pairs for speaker cable?
8*four pair for bass, 4*four pair for mid, 1*four pair for treble? maybe a few less pairs for shorter cables <2m.
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Old 7th January 2007, 10:38 AM   #3
psymo is offline psymo  United Kingdom
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Join Date: Jan 2007
Hi Andrew, thanks for your prompt response.

I think Iíll ditch the silver wire idea then, I thought this mignt be an overkill!

Are you able to advise on a Ďstarting pointí design/component list for a stepped attenuator? If it helps, my source is a Dennon 3910 or sometimes a Marantz 5400OSE and my speakers are PMC FB1+ transmission lines.

With regard to the CAT 5, I stripped individual pairs for this, is this OK and yes, I am using these speaker cables here which I found to be brilliant. Funnily enough, it was whilst researching DIY speaker cables and interconnects that I came across and became interested in gainclones.

Cheers

Pete
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Old 7th January 2007, 11:00 AM   #4
AndrewT is online now AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hi,
there are a few stepped attenuator calculators in the Forum.
for ladder (one pair in path at a time) and for series (Lpad)
The problem is the price of obtaining more than 12 positions.
I just bought a triple knob version (aikido FP stepped attenuator) from Glassware on the Tubecad site. This mixes ladder and series to obtain 36 steps from 3 6position switches. not built yet so no feedback, but it looks good.

Warning.
the attenuator should be about 10 times the source impedance.
and the receiver impedance should be about 10 times the attenuator.

1k0 source > 10k attenuator > 100k Zin.
or
100r source > 1k0 attenuator > 10k Zin. IF the source can drive 1k0!!!!
or
anywhere in between.
If you make the ratios closer, the filters from the capacitances end up cutting the audio and the log law of the attenuator goes haywire.
If you get it about right the sound quality is superb.
Inverting topology in the chipamp makes the Zin very low.
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