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Old 5th July 2003, 09:04 AM   #1
mig-ru is offline mig-ru  Russian Federation
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Default first attempt at phono preamp, will this work?

having nothing to do this evening, i decided to attempt a phono preamp design with a passive filter. im pretty sure some things are wrong/missing. any suggestions?
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Old 5th July 2003, 01:04 PM   #2
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Default Circuit looks OK I think.

Never used the tubes you have. Biasing components are quite different from ECC series etc.
It is a relatively simple configuration. Even if it does have any problem all you will have to do is change component values. Should be OK.
Cheers.
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Old 5th July 2003, 01:25 PM   #3
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You might have a bit of a problem with gain, especially at the low end (common problem with passive). I haven't used that particular tube; what's the mu and plate resistance? Do you have any data on voltage noise density?

Also, a small resistor on the input of the first grid (say 1K if you're using MM) can be a phophylactic against RF.
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Old 5th July 2003, 04:05 PM   #4
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Hi,

6N1P is close to a 6DJ8 except for heater current according to Svetlana.

While I don't agree with that it can replace the 6DJ8 occasionally if you're not as picky as I am.

Cheers,
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Old 5th July 2003, 04:35 PM   #5
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The output resistance of the first valve is a significant proportion of the series resistance in the RIAA network. To put it another way, equalisation accuracy will be critically dependent on ra.

You have a very heavy load on the first valve, so its gain will be significantly less than mu, and passive RIAA attenuates by a factor of ten. The signal presented to the second valve will probably be about double the output from the cartridge. Noise in the second valve will be very significant.

Sorry to !$$ on your firework.
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Old 5th July 2003, 06:55 PM   #6
mig-ru is offline mig-ru  Russian Federation
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Quote:
Originally posted by EC8010
The output resistance of the first valve is a significant proportion of the series resistance in the RIAA network. To put it another way, equalisation accuracy will be critically dependent on ra.
What do you mean by this? As in R3 and R8 in parallel represent the initial resistance value for the filter, and not R8 alone?

Quote:
Originally posted by EC8010
You have a very heavy load on the first valve, so its gain will be significantly less than mu, and passive RIAA attenuates by a factor of ten. The signal presented to the second valve will probably be about double the output from the cartridge. Noise in the second valve will be very significant.
I tried squeezing as much mu as possible when doing the load lines while still staying within plate dissipitation limits. Wouldn't changing Ra just change the Ia for the most part as long as B+ is constant?

I want to get this going, so please help out further. I used this site for the filter section http://www.kabusa.com/riaa.htm . I havent found too much info on the actual theory behind the equalisation.
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Old 5th July 2003, 07:01 PM   #7
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Quote:
Originally posted by fdegrove
6N1P is close to a 6DJ8 except for heater current according to Svetlana.
I think the 6N1P is a little beefier, no? Wa for 6N1P is 2.2W, while 6DJ8 is about 1.5W, from what I remember.
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Old 5th July 2003, 07:26 PM   #8
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Hi,

Quote:
I think the 6N1P is a little beefier, no? Wa for 6N1P is 2.2W, while 6DJ8 is about 1.5W, from what I remember.
Sure, I have no quibbles with your choice.

Rather that I disagree with many vendors marketing it as a direct equivalent of the 6DJ8 except for heater current.

The datasheet as given by Svetlana isn't even accurate if you compare it to other sources.

This is one of those tubes I'd put put on a plotter to find out more about it.

Hope that clarifies it,
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Old 6th July 2003, 01:28 AM   #9
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Hi,

Quote:
What do you mean by this? As in R3 and R8 in parallel represent the initial resistance value for the filter, and not R8 alone?
A few issues here:

- the passive RIAA correction as shown is loading down Rp heavily by your component choice.

- while doing the math on V1 you didn't take the load of the RIAA correction into account it seems.

- both halves of the tube are set up the same way, you have considerable insertion loss from the passive RIAA correction but V1 has the same high gain now...hence the gainscaling is wrong, hence the remark from other members about the possible ( real IMHO) noiseproblem from V2.

IOW, I'm not adding anything other than what EC8010 hasn't said already just expressing it in a different way.

Cheers,
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Old 6th July 2003, 01:53 AM   #10
mig-ru is offline mig-ru  Russian Federation
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I'm sorry I guess I didn't express myself clearly. I didn't really understand what EC8010 meant, that's what I was wondering about. Should I take the resistance of the filter in parallel to the plate resistor of V1 to figure out the load? What should I do about the noise, grid stop resistors? Like I said I don't know much about RIAA equalisation, much else any equalisation. Thanks for the help.
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