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GAK 25th February 2004 04:35 PM

dc phono
 
2 Attachment(s)
Hi
I'm thinking to build this phono. Any comments?
To build or not to build??
Is the schem correct?:confused:

fdegrove 25th February 2004 11:09 PM

Hi,

At first glance R 17 seems of.
I'd expect to see something like 1K there...

C9 1m ? Does that mean a 1000 F?

Cheers,;)

GAK 26th February 2004 08:06 AM

Hi Frank
I find the schem in the net . I don't remember where .
I like it 'cause it hasn't a coupled cap btw the first and second stage !
And I expect too much gain from this phono with the 6SL7.
But as a newbie I'm not sure if it's a good project .

R17 seems of . For me maybe it's 10K.
At the G1 on the second triode we don't have 0V.It's dc coupled with the first stage.So the resistor at the cathode must be higher than 1K. What do you say?
C9 it's 1000uF.I don't thing there's a problem to put something lower or to leave it as it is.

Frank Berry 26th February 2004 08:13 AM

The output impedance of this preamp will be extremely high.
You'll experience some loss of high frequencies if the output cable is anything more than a couple of feet long.

You might want to add a cathode follower stage to lower the output impedance.

Otherwise, the preamp looks pretty straight forward and should be a simple and fun project.

Happy soldering!

fscarpa58 26th February 2004 08:54 AM

2 Attachment(s)
Hi

It seems good as is.

but Rout >20k Ohm

Federico

Brian Clark 26th February 2004 09:22 AM

Quote:

I find the schem in the net . I don't remember where .
I like it 'cause it hasn't a coupled cap btw the first and second stage !

That's Bob Danielak's circuit :

http://www.geocities.com/bobdanielak/dc_phono.gif


Quote:

R17 seems of . For me maybe it's 10K.
At the G1 on the second triode we don't have 0V.It's dc coupled with the first stage.So the resistor at the cathode must be higher than 1K. What do you say?

The 100k is correct though 85k can be used to get more current through the stage but I wouldn't go any lower.

Brian.

fdegrove 26th February 2004 10:24 AM

Hi,

Quote:

That's Bob Danielak's circuit :
Yep...I thought I'd seen it before somewhere...

However DC coupling a phono stage seems very attractive but...
Unfortunately neither vinylrecords, cartridges nor tonearms are perfect.

These all exhibit a resonance frquency in the 10 to 15 Hz region.
Even whith a flat as a pancake record these resonances are going to drain the PS of the phonostage heavily.
What's even worse it's going to amplify this across the entire stage.

On the bright site, one could build it, monitor it it for while and if what I suspect does cause trouble a highpass filter can easily be added.

Due to the high Zo a buffer or at the very least a low Zo line stage is recommended.

Cheers, ;)

GAK 26th February 2004 09:34 PM

2 Attachment(s)
So I can't avoid the 3rd stage.:(
I don't want to use a line stage.I have an input volume at my amp.

Thanks Brian that you remember me Danielak's site.I went back and I see another phono stage with a cathode follower at the end.:)
But with three stages.Is there any solution not to use a 3rd stage??
I think with an interstage transformer I can do it.But what is better ,a 3rd stage or a step down xformer?
The other phono from Danielak is in the attached photo.

fdegrove 26th February 2004 10:23 PM

Hi,

Quote:

But with three stages.Is there any solution not to use a 3rd stage??
Well, in this case the RIAA correction is spread over two stages so the last one is a necessary evil.

Oh, and if you want a volumecontrol on this one as well the only place left is at the output...

Cheers,;)

ThorstenL 26th February 2004 10:23 PM

Re: dc phono
 
2 Attachment(s)
Konnichiwa,

Quote:

Originally posted by GAK
I'm thinking to build this phono. Any comments?

Well, as you ask, do you really feel that having a low quality 100uF Cathode capacitor is preferable to haveing a high quality, small value capacitor (say 0.047uF) between the RIAA and the 750k Resistor, in sonic terms?

Even better, if you actually increase the gridleak resistor of the second stage sufficiently you could use an even smaller value capacitor and completely drop the cathode bias resistor.

Also, a 6SL7 as output valve, very high output impedance, very bad load tolerance. I would do a simple, 2 Stage Phonostage more like this....

Sayonara


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