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Old 2nd June 2011, 04:17 PM   #1
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Default Gain and volume pots - newbie question

I've just swopped some gear for a tube preamp that sounds wonderful. However the volume is very high in my system - it's fitted with a stepped attenuator which only gives me 3 clicks from zero to maximum tolerable volume.

I was just wondering if there's a way to get more pot travel by fitting a higher resistance value pot? In general I prefer a film pot anyway to a stepped device as stepped pots never seem to have the 'right' value available, I always seem to want something in between.

I'd rather not go down the route of resistors on the power amp inputs as I'm not sure how linear this method is - though perhaps some really high-quality components are the answer? I wasn't impressed with some attenuator plugs I bought from ebay some years ago.
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Old 2nd June 2011, 04:33 PM   #2
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I think what you want isn't so much pot travel as being continously variable. To that end, I don't see why you can't just replace the stepped attenuator with a regular pot. And I'd think you wouldn't need a higher resistance value than you have currently with the stepped attenuator.
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Old 2nd June 2011, 04:51 PM   #3
kevinkr is offline kevinkr  United States
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Stepped attenuators generally have significantly better electrical performance and channel to channel matching than even good pots. Your issue relates to having far too much gain preventing the proper operation of the attenuator. Good ones may have step to step gain change resolution of 1dB or so in the region where they are used most - provided that the system gain structure is well thought out. As an aside 1dB is generally regarded as the smallest perceptible change in loudness so a resolution of 1dB should cover all situations. It should be noted at the top and bottom of the range it is not unusual to have 2dB or 3dB steps on 24 step designs.

Need to figure out how much gain your new pre-amp has and how much the power amplifiers have as well.

Line stages with modest gain may be used effectively with power amps having high gains (old Leaks, Radfords, etc..) and lower gain power amps may be used effectively with pre-amps have higher levels of gain.

Perfectly linear attenuation networks can be added internal to the amplifier right at the input jack - if the amp has a lot of gain this is probably a good way to go. Depending on the amplifier you have it may be possible to modify it to reduce its open loop gain and then adjust the closed loop gain such that the feedback margin remains constant, but overall amplifier gain is much lower.

Would help to know a whole lot more about the actual equipment you are running.
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Last edited by kevinkr; 2nd June 2011 at 04:55 PM.
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Old 2nd June 2011, 06:01 PM   #4
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Thanks guys, appreciate the input. The preamp is an Audio Note M2 Phono, which as part of various mods has the variable feedback switch disabled (I'd rather keep it no-feedback anyway if possible). 23db gain on no-feedback so quite high.

The amps are VAC PA60c monoblocks, 30w triode/70w pentode, used in triode, with a single VAC PA90 PSU. These look a little like the current model VAC Auricle Musicblocks, but aren't the same as they as the old amps have much chunkier output transformers and the said off-board PSU. Fantastic amps and a great match for the speakers. They are pretty old and I can't find input sensitivity numbers anywhere and even VAC weren't forthcoming with much info when I contacted them a while back. I suspect they are moderately input-sensitive but nothing so severe as, say, a Leak Stereo 20.

Speakers are Tannoy 15in Monitor Gold dual concentrics with custom xovers and DIY GRF cabinets, which take the sensitivity quite high to something like 97-98db.

Sources are MC cart + matched step-up into the M2 Phono stage, and a fairly standard output voltage DAC, so nothing really to be done there. I had been using them hapilly with a Pass DCB1 so I know I don't **need** any gain, though I must admit the sound through the M2, for all the lack of adjustability and hair-trigger sensitivity, is markedly superior. (The Pass has previously seen off a number of valve preamps, this is the first I've put in the system that's beaten it).
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Old 2nd June 2011, 06:07 PM   #5
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And just to add, my goal would be to get exactly that sort of 1db per click in the ideal range, without compromising anything in terms of sound.

Perhaps I should also mention that amp has been giving me some ground loop problems and I have a Xitel ground loop box between preout and inputs - removing it adds hum and is the same with every preamp. I had been thinking of adding some higher-quality 1:1 transformers (UTC A20 or equivalent) in the circuit, though perhaps there is a transformer option to lower the input a notch (or maybe some resistors judiciously placed on such an arrangement? I wouldn't know where to put them tbh).
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