Phono Frustration

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izzy wizzy
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Phono Frustration

Post by izzy wizzy » Sun Mar 03, 2019 9:33 pm

This hobby can bring great joys but then at other times, huge frustration. I need to vent before going back to the man cave for more solder fumes.

I thought after 10 years I should re-build the phono stage as after much mucking around, the layout was looking a bit second hand and I could do better now. So spent quite a bit of time laying it out again, 3D modelling and so on. Put all my hard won experience into it once again, same box, same circuit with the same parts and ....

For the first time I can remember in building these things, it hums, doesn't go loud enough, is noisy and terribly microphonic to the point the whole thing is unusable. DC voltages seem mostly right and consistent between channels. So I guess I must not have stuck it together right. Just when you think you're going to bathe in wonderous sounds, it's quite the opposite. And I detest fault finding. It's very tempting to just rebuild it how it was.

Anyway, back to the shed :evil:

Cheers,
Stephen

simon
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Re: Phono Frustration

Post by simon » Sun Mar 03, 2019 9:49 pm

Nightmare!

RhythMick
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Re: Phono Frustration

Post by RhythMick » Sun Mar 03, 2019 9:55 pm

Feel for ya Steve, been there. You were one of those helping me through the last one. All I can say is there has always been light at the end of the tunnel and usually the end result is much better for it.

Probably best to rest though, attack it again when you're fresh. If you're anything like me the answer will come to you at 3am. Keep a pad and pencil by the bed.

Chin up.

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IslandPink
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Re: Phono Frustration

Post by IslandPink » Sun Mar 03, 2019 10:34 pm

There must be something in the air, Stephen. I got the left OB speaker more or less finished yesterday, and got the system working in stereo for the first time in years. It sounds quite poor. I have no idea why - back to the drawing board. Good luck with your investigations.
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

simon
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Re: Phono Frustration

Post by simon » Sun Mar 03, 2019 11:29 pm

Nightmare!

izzy wizzy
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Re: Phono Frustration

Post by izzy wizzy » Mon Mar 04, 2019 8:05 am

So as some of you must have been thinking ... well my wife admitted to this afterwards ... that I must have left something really obvious out. While feeling sorry for myself, rumaging around in the shed pouring over the old boards wondering how could it all have gone wrong, I noticed some N cell batteries and thought oh yes, I put new ones in, that's why they're here. Then slowly it dawned, no I didn't. I only have one set. Oh oh ....

Dumbass me left out the battery bias on the first valve yet the voltages on it were what I was expecting. I can't quite get my head around that when G1 isn't grounded and it made sound. There's a lesson in there somewhere and it might also have something to do with why biasing can lead to microphonics.

So after my rants here, I really hope sticking the batteries in returns me to happiness. Also hoping for my wife to stop ribbing me about leaving the batteries out.

Thanks for listening.

Cheers,
Stephen

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Re: Phono Frustration

Post by RhythMick » Mon Mar 04, 2019 8:24 am

Can't believe you left the batteries out dude...

:o

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pre65
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Re: Phono Frustration

Post by pre65 » Mon Mar 04, 2019 12:05 pm

Hands up those who have not done anything just as silly.

I sure have. :oops:
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Re: Phono Frustration

Post by RhythMick » Mon Mar 04, 2019 12:16 pm

pre65 wrote:
Mon Mar 04, 2019 12:05 pm
Hands up those who have not done anything just as silly.

I sure have. :oops:
Not me not a chance.

Not today anyway... Yet.

simon
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Re: Phono Frustration

Post by simon » Mon Mar 04, 2019 12:53 pm

This sounds like a bit of a result. Who wants a complex solution when a simple one will do :-).

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Re: Phono Frustration

Post by Andrew » Mon Mar 04, 2019 6:47 pm

izzy wizzy wrote:
Mon Mar 04, 2019 8:05 am


Dumbass me left out the battery bias on the first valve yet the voltages on it were what I was expecting. I can't quite get my head around that when G1 isn't grounded and it made sound. There's a lesson in there somewhere and it might also have something to do with why biasing can lead to microphonics.


Cheers,
Stephen
Is it possible the valve was operating in grid leak bias?
Analogue, the lost world that lies between 0 and 1.

RhythMick
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Re: Phono Frustration

Post by RhythMick » Mon Mar 04, 2019 7:00 pm

Wouldn't that need either a very big Grid resistor or very high Grid current, or both?

izzy wizzy
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Re: Phono Frustration

Post by izzy wizzy » Mon Mar 04, 2019 7:08 pm

Andrew wrote:
Mon Mar 04, 2019 6:47 pm

Is it possible the valve was operating in grid leak bias?
Not without the grid connected to ground as it stands on the battery. The screen bias was setting the condition. And that leads me on to thinking about the whole bias thing.

I had a theory that the bias point can exacerbate microphonics if the grid (G1) sinks or sources current. I could get 6SL7s dead quiet if there was no grid leakage and the D3a in triode, I could get the same. Not so lucky in pentode but after this, with G1 flapping in the breeze, the microphonics were stupid bad. The bias point was being soley set by G2 so it makes me wonder, what my G1 bias is doing? And if it's having little effect, then maybe that is leading to a more microphonic situation that could be achieved with a bit of messing about ot better study of the curves. With the 6SL7, I thought dragging the anode down too low led to it being more microphonic. How this circuit without the battery even made a noise will also be something I'll put to the panel when I get some time :)

Cheers,
Stephen

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Nick
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Re: Phono Frustration

Post by Nick » Mon Mar 04, 2019 8:16 pm

I think we would need to see the actual circuit to have a chance of an idea.

I can imagine that the grid bias could affect the physical condition of a grid so could alter the level of microphonics. There will be a electrostatic force on the grid if there is a potential difference between it and the surrounding electrodes.
Resistance isn't futile it's V / I.

izzy wizzy
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Re: Phono Frustration

Post by izzy wizzy » Mon Mar 04, 2019 8:31 pm

phonoMk24.jpg
Here it is but imagine no battery.

It hummed like crazy until I grounded the input connector and then it was quiet, well it didn't hum, was a bit hissy, super super microphonic and lower in volume relative to what it would have been with the battery.

Cheers,
Stephen

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