I have been searching for a good design for a tube preamp. My phono preamp has a 300 mV output, so I have been thinking of somewhere around 12db to 18db gain (to provide a bit of flexibility in making a power amp next). I have found a few designs that look great, but either have too much or too little gain.
It dawned on me that I really only need that amount of gain on 1 input (for the phono). The other sources have higher outputs (although the manual for my Marantz CDP specifies 600 mV, which seems odd). Could I put a step up transformer on one input in connection with a preamp that otherwise has a gain of 6db?
The phono preamp has an output impedance of 150 ohms. The preamp inputs will feed into a 100k pot.
Would a transformer work for this application? If so what would I look for (turn ratio and impedance, etc.)? Would it be possible to get a 6 to 10 db gain in this manner?
Thanks in advance for any thoughts.
Jazzzman
It dawned on me that I really only need that amount of gain on 1 input (for the phono). The other sources have higher outputs (although the manual for my Marantz CDP specifies 600 mV, which seems odd). Could I put a step up transformer on one input in connection with a preamp that otherwise has a gain of 6db?
The phono preamp has an output impedance of 150 ohms. The preamp inputs will feed into a 100k pot.
Would a transformer work for this application? If so what would I look for (turn ratio and impedance, etc.)? Would it be possible to get a 6 to 10 db gain in this manner?
Thanks in advance for any thoughts.
Jazzzman
You could use a 1:1 line transformer with the windings connected in series to make an autotransformer.
The input goes to the center tap and bottom. The output is from the top and bottom, so there would be
+6dB gain, something like this.https://sub.allaboutcircuits.com/images/02145.png
An example of a transformer, if the phono can drive it. http://cinemag.biz/output/PDF/CMOQ-4.pdf
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Yes, can you post the schematic for your phono, and the circuit following the transformer.
The phono preamp is a Vista Audio Phono1ACL. Boris, the designer, told me that the output has an impedance of 150 ohms. He tweaks the gain according to request. I had asked for 40 or 41 db (I do not recall which) and 47K input impedance.
The amp I am thinking of is likely to be Thomas Mayer's 6GL7 single stage, 6AH4 single stage or, if I could figure out how to do it, a variation of Ruffrecords 6sn7 mu follower using a lower mu tube like the 6GL7 or 6AH4.
Here is a link to the schematic on the phonopreamp: High End Phono Preamplifier
Here is a link to the 6GL7 single stage schematic:
VinylSavor: The Octal Line Preamplifier, Part 1 : Signal Section
Here is a link to the 6AH4 single stage (you have to scroll about half way down to the Sunday June 10, 2012 article entitled "Making of a 6AH4 line stage: circuit") (I am not going to go with the TVC on the output, however):
VinylSavor: 6AH4
Thanks for your help!
The amp I am thinking of is likely to be Thomas Mayer's 6GL7 single stage, 6AH4 single stage or, if I could figure out how to do it, a variation of Ruffrecords 6sn7 mu follower using a lower mu tube like the 6GL7 or 6AH4.
Here is a link to the schematic on the phonopreamp: High End Phono Preamplifier
Here is a link to the 6GL7 single stage schematic:
VinylSavor: The Octal Line Preamplifier, Part 1 : Signal Section
Here is a link to the 6AH4 single stage (you have to scroll about half way down to the Sunday June 10, 2012 article entitled "Making of a 6AH4 line stage: circuit") (I am not going to go with the TVC on the output, however):
VinylSavor: 6AH4
Thanks for your help!
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Thanks, JM. That is an interesting thought, I might try something like that in a phase 2. I do not want to overly complicate this initial project. I could include an extra input jack in my preamp and after using the autotransformer for a while proceed to phase 2 and then compare the sound of the phono through the autotransformer with it through the extra Op Amp.
Any of these circuits should work fine with a 1:2 step up transformer.
It's best to have it in a shielded enclosure and away from noise sources.
As an alternative, the phono circuit's gain could be increased by 6dB.
Instead of the four 3.32k feedback resistors for the op amps, use 4.99k.
That increases each op amp's gain by sqrt 2, so two op amps in series
will increase in gain by 2, or 6dB.
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I think it should work well for this purpose, and it has wide bandwidth.
Not sure of the cost, but you could check with CineMag on the pricing,
and see what model they would suggest.
If it were me, I'd try the resistor change first. If that works well enough,
and it should, then you're done.
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