D3a Phonostage reloaded Assistance required

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Nick
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#61 Re: D3a Phonostage reloaded Assistance required

Post by Nick »

Mike H wrote:The master concurs.....

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I meant to Thomas's bit of course, you dont need telling you were right Mike :-)
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thomas
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#62 Re: D3a Phonostage reloaded Assistance required

Post by thomas »

Well I need a new calculator…. :)

I was out by a 'nought'. Looking at my back-of-envelope I can see where I went wrong, -15dB of 1.8v is actually 320mv, and not the 32mv I'd worked out…..

But, I believe things are looking much better. May I present Left and right channels, 1kHz at -15dB i.e. 1mv signal into the phonostage….

2nd harmonic seems to have calmed down a bit, at -100dB/ -95dB. 50Hz still an issue by the look of it, though…..

One thing I did notice, I had to keep still when I captured these, the slightest tap on the desk took the noise floor right up. It all seems rather alarmingly microphonic. I guess cute though my valves look sticking out of the top, I'll really have to enclose them for the next build.
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thomas
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#63 Re: D3a Phonostage reloaded Assistance required

Post by thomas »

….I've just reduced that -72dB 50Hz 'spike' by about 10dB to -80dB or so by the simple expedient of moving the power supply away from the phonostage by about 18 inches... :oops:


All good fun….
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#64 Re: D3a Phonostage reloaded Assistance required

Post by Mike H »

Ta-da! :D
 
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Dave the bass
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#65 Re: D3a Phonostage reloaded Assistance required

Post by Dave the bass »

Bluddy 'ell MrT, you a professhunooll or summat? :lol: Thats proper design/build/measure stuff right there.....You're bringing the Kent Massiff into disrepute!

Awethumne work fella, keep it up. I'm just about to finish of my most recent top secret build then I'll house my Hagerman iRIAA pcb , DL Audiotester and then start tugging on yer coat tails for guidance.
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Nick
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#66 Re: D3a Phonostage reloaded Assistance required

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Far from Shabby. You start to see how the dB scale means that hum, microphonics and noise appears where you don't expect it. Do you have any more gain on the input to the audio tester, going from the 2v to 200mv input moves it all up 20dB.
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#67 Re: D3a Phonostage reloaded Assistance required

Post by izzy wizzy »

Dave the bass wrote:Bluddy 'ell MrT, you a professhunooll or summat? :lol: Thats proper design/build/measure stuff right there.....You're bringing the Kent Massiff into disrepute!
Being part of the Kent Massif (well I live in Kent anyway), couldn't agree more DTB :wink:

Makes me want to go and get some proper testing gear. Great work!

cheers,

Stephen
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#68 Re: D3a Phonostage reloaded Assistance required

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Nick wrote:Far from Shabby. You start to see how the dB scale means that hum, microphonics and noise appears where you don't expect it. Do you have any more gain on the input to the audio tester, going from the 2v to 200mv input moves it all up 20dB.
Ooh thanks! Yes indeed….I fired up an am radio I'd dusted off, about 3 ft away, just to see it it still worked and my phonostage/test gear reacted to it :shock:

Sorry I'm not quite sure what you mean re more gain? Actually I did set the 200mv scale initially and then pretty quickly(!) realised I'd calculated wrongly, ie 320mv not 32mv, so went back to the 2v scale.

Actually I could drop the audiotester an extra -5dB, to -20dB input to the Millett, which should drop the voltage it sees to around 180mv so I could use the finer scale. That would work out that the phonostage only sees a 0.57mv signal, (assuming my maths is OK :? ) Hmmm…..
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#69 Re: D3a Phonostage reloaded Assistance required

Post by thomas »

izzy wizzy wrote:
Dave the bass wrote:Bluddy 'ell MrT, you a professhunooll or summat? :lol: Thats proper design/build/measure stuff right there.....You're bringing the Kent Massiff into disrepute!
Being part of the Kent Massif (well I live in Kent anyway), couldn't agree more DTB :wink:

Makes me want to go and get some proper testing gear. Great work!

cheers,

Stephen
Ta but just scratching the surface really! :oops:
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#70 Re: D3a Phonostage reloaded Assistance required

Post by Nick »

thomas wrote:Ooh thanks! Yes indeed….I fired up an am radio I'd dusted off, about 3 ft away, just to see it it still worked and my phonostage/test gear reacted to it :shock:
Its normally a case of the radio reacting to the amp. If the amp is oscillating its probably transmitting harmonics the radio will hear as a signal.
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#71 Re: D3a Phonostage reloaded Assistance required

Post by thomas »

Nick wrote:
thomas wrote:Ooh thanks! Yes indeed….I fired up an am radio I'd dusted off, about 3 ft away, just to see it it still worked and my phonostage/test gear reacted to it :shock:
Its normally a case of the radio reacting to the amp. If the amp is oscillating its probably transmitting harmonics the radio will hear as a signal.
Aha, this was going to be my next question! How to use an am radio to test for oscillation…. I think (hope) my test gear jumped at the radio yesterday merely because of microphonics when I switched it on…..

So I'm guessing I just put the radio near the phonostage, turn its dial, find a noisy bit and move it away or switch the phonostage off to see if its related…..or something?!
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#72 Re: D3a Phonostage reloaded Assistance required

Post by Nick »

Yep, thats about it. Dave has done this in the past.
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#73 Re: D3a Phonostage reloaded Assistance required

Post by Mike H »

I read that as Thomas is saying that the amplifier + test gear reacted to the radio, not the other way around.

In that case it's suggesting that the pre-amp is sensitive to RF pick-up. Not at all impossible. Next question then is do you want to try some low-pass filtering on the input? Not unusual for the input to be strapped to ground with a low value cap, mainly for the purposes of loading an MM cartridge.
 
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#74 Re: D3a Phonostage reloaded Assistance required

Post by thomas »

Mike H wrote:I read that as Thomas is saying that the amplifier + test gear reacted to the radio, not the other way around.

In that case it's suggesting that the pre-amp is sensitive to RF pick-up. Not at all impossible. Next question then is do you want to try some low-pass filtering on the input? Not unusual for the input to be strapped to ground with a low value cap, mainly for the purposes of loading an MM cartridge.
Well I've been playing it in situ last night and today, and waved the a.m. radio around it with little consequence so I think I have got away with those 300r cc's grid stoppers- for now, at least. I'll no doubt swap them all out for 1K's next time its opened up, just to be sure.

My listening experiences with it have gotten much more positive- I ditched the yellow 'lcr' labelled pp output caps for some wima 2.2uf pp's and the 'plasticky sheen' seems to have gone. The sound is far more fast, immediate, full range and naim like (in a good way…) than the Thorsten I've been comparing it with- is that, I wonder, largely to do with the much lower impedance riaa network (20K not >200K) ? I'm also pleased that as far as I can hear it really doesn't need a c.f. on the end of it. Anyhow, I think this is well worth spending some time me tweaking it….

My main reason for building it was to use some sut's and an mc cart that I've had for a couple of years and not really used, and that's my next test later :)
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#75 Re: D3a Phonostage reloaded Assistance required

Post by IslandPink »

I'm sure the 20uF RIAA helps - I found more transparency, tone and dynamics in going from 105k to 20k .
Bear in mind the output caps may not need to be as big as 2.2uF . If you aren't driving anything worse then say 100k into the next amp you can run under 1uF , for instance 0.68 or 0.82uF , in which case you can use film/foils like the Mundorf ZN ( oh, did I ever mention I'm fond of MUndorf ZN's ? :D ) instead of metallised film.
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