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Old 20th June 2011, 11:31 PM   #521
johnm is offline johnm  United Kingdom
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Quote:
Originally Posted by Mooly View Post
Yes start with R317, just working on the one channel. Be prepared to experiment a little increasing the value in small steps and always retesting with the bias on minimum after each change.

I would tend to leave the PSU caps as designed, changing the values changes time constants and with some SPSU's that can be critical for correct and safe start up.
Will do Mooly and - as always - many thanks for all your time/help. Very much appreciated indeed

Quick question about the 47uF caps, C205/255 which you mentioned were well and truly in the signal a while back. Would it be OK to use a 68uF film cap for these two positions? I just happen to have a pair here, and after the success of replacing all those 'back-to-back electro/film bypass' combinations in the signal path with just a single film cap I'd like to get rid of those two electrolytics too

Lastly, I'm thinking I'll need to do something to attenuate my CD source with this amp. The Aux1/Aux2/Tape1/2 inputs are 150mV sensitivity & 50K impedence. I don't really want to muck about with changing all the inputs, so I guess I could build a potential divider behind the Aux1 phono sockets? How would I work out suitable resistor values to achieve a better match between source and pre-amp? Ta!

Last edited by johnm; 20th June 2011 at 11:37 PM.
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Old 21st June 2011, 12:36 AM   #522
johnm is offline johnm  United Kingdom
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Just found this thread:

Inline Attenuator Help

Think the -15.6db attenuation (post #2) should do fine for the Aux1 input on my amp.

Last edited by johnm; 21st June 2011 at 12:38 AM.
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Old 21st June 2011, 06:40 AM   #523
Mooly is online now Mooly  United Kingdom
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If a 68uf will physically fit it will be fine.

Attenuators... very much trial and error believe it or not, at least in order to get the setting just right. It's what feels right as you listen and turn the volume control. 2 volts rms from CD vs "150 mv" that was common back in 70's is a difference of 22db. However CD is fixed to the 2v rms maximum, it can never exceed that level.

CD is a 16 bit technology. 16 bits gives 65536 separate possible combinations. Players are designed (the "unnofficial" standard) that the 65536th level, which is maximum, equates to around 2 volts RMS (2.8v peak). That value can never be exceeded in playback. In practice the program material is well below that level to allow for the loudest peaks that come along... assuming a good recording that isn't compressed

With old sources such as cassette/reel to reel etc that absolute limit doesn't apply and it was acceptable to allow peaks to go well over the nominal "max" 0db level. If you didn't then the quieter parts would be buried in the noise.

So what all this means is that although the numbers can appear to be calculated out exactly (and they can), in practice it often won't give the desired "feel" to the volume control range.

The answer is to rig up a 10k or 22k pot (or presets) and find the setting that works best for you. Then just measure the resitances and substitute fixed resistors.
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Old 21st June 2011, 02:53 PM   #524
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Thanks Mooly - managed to find a couple of 47uF MMX film caps (which the Naim guys rave about when used in the feedback positions) in the parts box so used those. I don't know if it's placebo or not but they seem to have had a very positive effect on the sound. Lovely well balanced sound now which seems to handle all facets of the frequency range equally. Lovely and grain free on vocals too - like a very good valve amp in fact. I think getting rid of all the obvious electrolytics in the signal path in the control & EQ sections has really paid off handsomely, even though it was a bit of a pain to do. I'll get some pictures up later should anyone else fancy doing the same to their amp.

Thanks for the explanation regarding CDs playback levels - all makes a bit more sense now I'll see if I have some spare pots. Apologies for yet another 'newbie' type question, but how would I wire this up to the phono socket(s) to test? Would I still need a second resistor to create the 'L' pad?

The amp is sounding so good now - can scarcely believe the design is pushing 32+ years old!
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Old 21st June 2011, 05:06 PM   #525
Mooly is online now Mooly  United Kingdom
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Easiest way to experiment is perhaps to use an old phono lead. Just cut it in half and wire the preset pot across the half that goes to the CD player. The other half goes to the amp with the centre lead conected to the wiper of the preset and the grounds of the two halves connected together... like this

Once you have it set right just measure the resistances and pick the nearest values
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File Type: jpg Preset.jpg (16.5 KB, 131 views)
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Old 22nd June 2011, 08:41 AM   #526
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Ah that's great - many thanks Mooly!
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Old 22nd June 2011, 11:38 AM   #527
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Be aware that by using an attenuator like that you may be increasing noise. Whether you'll hear it is another matter. Personally I would go for reducing input sensitivity but that might be difficult depending on the circuit.
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Old 23rd June 2011, 12:53 PM   #528
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Hmmm perhaps I should look into altering the input sensitivity within the amp itself then - I'd rather do the job properly or not at all.
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Old 23rd June 2011, 02:30 PM   #529
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Hi John, to alter the input sensitivity you either have to use an attenuator as outlined or alter the actual gain of the preamp section of the Sony. If you did the latter then the "sensitivity" of the phono stage would be altered too, because that feeds into the main preamp... you would be reducing the gain that much the phono stage wouldn't drive the amp to anything like normal levels. It would also have to be done with a scope and generator to ensure the opamp was correctly compensated with running at a lower gain... it alters the whole character of the amp really.

I feel a simple resistive divider really is the best solution, the "cutting a phono lead" enable you to get the values exactly how you want, then just fit them to the input socket you use for CD etc. The noise contribution of the attenuator is half of a quarter of nothing when you compare it to the noise of the Sony.
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Old 24th June 2011, 09:59 AM   #530
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Thanks for your reply Mooly - OK I'll give the phono lead attenuator a whirl and see how it goes.

Just got myself a Technics SP-10 Mk.II so two projects on the go now - arghhhhh! And there I was telling myself the Hi-Fi madness would end once I'd gotten the Sony amp project finished
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