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Phono pre-amp Question

So I'm looking at this schematic and I realize there is a learning opportunity. Here is the question, How do I pick the input and output transformers? I think I understand the input transformer. You pick it on the basis of the MC cart? So if the cart wants a certain Z load you pick the Input transformer to match? Now the output transformer, here is where I'm confused. The Tube-Cad blog says that the output transformer needs a 1:14 step up, so would that be another input transformer? I ask because most of the ones I see step down to drive speakers. Also what about the z presented to the amp? I would want to keep the Z as low as possible? Thanks for helping me work this out :D
 

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athos56 said:
So if the cart wants a certain Z load you pick the Input transformer to match?


Not exactly. The impedance is of secondary importance. First pick up the step-up ratio based on what the cart outputs. Then adjust the 10k to match the desired loading.

The output impedance with the 1:14 seems too high to make much sense. It is probably just a proof of concept rather than a real phono stage but some sort of output buffering would be very desirable.

I don't remember seeing this article, so there may be more than meets the eye here. Link?
 
Re: Re: Phono pre-amp Question

analog_sa said:



Not exactly. The impedance is of secondary importance. First pick up the step-up ratio based on what the cart outputs. Then adjust the 10k to match the desired loading.

I see, that is very helpful.

It is probably just a proof of concept rather than a real phono stage but some sort of output buffering would be very desirable.

Yeah, your probably right.

I don't remember seeing this article, so there may be more than meets the eye here. Link?

Here it is. http://www.tubecad.com/2007/09/blog0120.htm

Thanks for the help, I saw the low part count and went hmmm. :D
 
Deleted my post after reading the article.

Yeah, the output impedance will not be particularly low with the proposed step up, which is undoubtedly available somewhere.

MC input transformers with the required gains are easy to source, Sowter makes several suitable models, but they are very expensive.

Given the expensive iron employed I would not use the 12AX7A, its modest gm makes for higher than necessary noise levels although the gain provided by the input transformer will probably swamp this. Still I would use something like a pair of D3A, and that might allow the ditching of the 12AU7A CF or replace with 5687?

Search for some of the Thorsten Loesch phono designs here. (SP?) They're very good and carefully calculated designs.

Some cheap, simple phono stages are on my site as well. OK for MM and HO MC cartridges. These are very old designs I did in the late 1980's, but perform well with good parts, and can be built very cheaply.
 
So was this something that could actually be built and used or a proof of concept? kevinkr, I looked at the posts you linked and have added them to my list of things to ponder. There was also this other phono-preamp on the site which used an opamp, which looked interesting.
 

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athos56 said:
So was this something that could actually be built and used or a proof of concept? kevinkr, I looked at the posts you linked and have added them to my list of things to ponder. There was also this other phono-preamp on the site which used an opamp, which looked interesting.

The transformer coupled design could be built, and I am sure there are similar implementations out there. Were I to do it I would ditch the cathode follower stage in favor of a pp driven output transformer driven by something like the 5687 - you could get >20dB out of this stage and low output Z as well. I would use a fully differential path with the MC input transformer driving a pair of D3A in turn driving a 5687 driving a push pull plate to line transformer. (1+1:1?)

To paraphrase from somewhere else: "Friends don't let friends use an op-amp in the audio path of an otherwise tube design" :D

To let all that tube goodness out :) you really need an all tube topology, while the hybrid will be very quiet the end result will not be tube sound. There are plenty of designs out there that have excellent performance and are completely tube based - which will be simpler and require fewer power supplies, and will not suffer from the compromised performance of all but the very best op-amps out there.