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Old 20th April 2012, 11:40 AM   #7531
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Or maybe cascoding the second stage before the output follower?
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Old 20th April 2012, 03:51 PM   #7532
AndrewT is offline AndrewT  Scotland
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The digital sources have a maximum output level. CDP for instance is usually around 2.2Vac.
Vinyl does not have a maximum. The output voltage depends on the speed that you move the generator.
For 5cm/s the output of an MM is usually in the range 2mVac to 7mVac.
But that is NOT the maximum.
Maximum could be 10dB, or 20dB, or more above that 5cm/s, typical speed, chosen for sensitivity comparisons in the specifications. 25cm/s is common in high energy recordings and I believe that nearly 40cm/s is possible.
What if you drop the cart carrier accidentally, or the cart jumps a track, or jumps over a boulder in the track.
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Last edited by AndrewT; 20th April 2012 at 03:55 PM.
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Old 20th April 2012, 04:06 PM   #7533
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In order to play my LP collection I think 5mV is not a bad starting point.
When there eventually is a recording with a high mean level it can be adjusted by attenuating the volume; on the other hand total gain of the phono preamp should be enough to cope with the majority of recordings, and I'd like the output signal of the phono preamp to be comparable with the signal of my internet radio and CDP.
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Old 20th April 2012, 05:29 PM   #7534
Salas is offline Salas  Greece
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Quote:
Originally Posted by pieter t View Post
Or maybe cascoding the second stage before the output follower?
I did that and I did not like it. You may try, but the overload is going to onset in the first stage partially too, especially at the treble.
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Old 20th April 2012, 05:39 PM   #7535
AndrewT is offline AndrewT  Scotland
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Pieter,
you are not listening.

The maximum digital level is ~2.2Vac.
After adding a +50dB RIAA stage the MM cart output maximum level is ~25cm/s * 1V/cm/s * 300 (~+50dB) ~=7.5Vac

It is not an extra gain stage to bring up the vinyl source to match the CDP output that you need.
It is extra attenuation for recordings that for you are a bit loud. I think you will find that a +50dB RIAA stage combined with a MM cart that outputs ~5mVac @ 5cm/s will generally give more SPL that an equivalent CD recording feeding direct into your pre-amp/buffer.
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Old 20th April 2012, 09:01 PM   #7536
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Quote:
Originally Posted by AndrewT View Post
Pieter,
you are not listening.

The maximum digital level is ~2.2Vac.
After adding a +50dB RIAA stage the MM cart output maximum level is ~25cm/s * 1V/cm/s * 300 (~+50dB) ~=7.5Vac

It is not an extra gain stage to bring up the vinyl source to match the CDP output that you need.
It is extra attenuation for recordings that for you are a bit loud. I think you will find that a +50dB RIAA stage combined with a MM cart that outputs ~5mVac @ 5cm/s will generally give more SPL that an equivalent CD recording feeding direct into your pre-amp/buffer.
Andrew,
I do listen .
Still I think a 50 dB gain preamp is the way to go and reach the level of digital sources.
We have a turntable here with two cartridges: AT440MLA and Grado Gold (4 and 5 mV resp. 5cm/1kHz).
Preamp is a Cambridge 640P; this one has a gain of 39 dB at its MM input.
When we listen through the AT cartridge the output level at 1kH/5cm is 4 x 89 (39dB) = 356 mV.
When we switch from LP to CD we have to turn down the volume 10-12 dB on average to match volumes; indeed there are LP's with higher recording levels and then the difference is smaller, but once more, on average (the majority of LP's) the difference is around 10 dB.
Therefore I think that a 50 dB gain preamp will bring levels pretty close together.
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Old 20th April 2012, 10:59 PM   #7537
Salas is offline Salas  Greece
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As I wrote originally if there is no line gain stage that usually has dB to spare, it can be needy in a passive pot situation directly to a 1VRMS amp or more. Unfortunately I don't recommend modding the gain stages in this kind of phono directly, but to add gain after. Maybe change the output buffer to some 3X scheme of choice with good Zo.
About the 30 Ohm spec of its standard buffer you are right, for BL should be ~60-70 Ohm. Looks like I had noted max spec for V type Fets that I had measured in a special build. Its an inadequate note. Should read ''under 100 Ohm with average IDSS JFETS''.
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Old 21st April 2012, 08:46 PM   #7538
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Found a bad joint - problem solved

In trying to find my problem I changed the transformer to 25Vac. This is giving me 40V into the LM317, which is only just warm dropping to 38.3V. The IRFs are bolted to the case and are cool. Now I'm getting a steady 28V out from the shunt.



Now to build another then connect to the RIAA circuits.
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Old 21st April 2012, 09:00 PM   #7539
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It seems that the voltage before the LM317 is dropping to 39V under load. Will this be a problem if I expect it to give me 38V out?
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Old 21st April 2012, 09:07 PM   #7540
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Quote:
Originally Posted by chalkandtalk View Post
It seems that the voltage before the LM317 is dropping to 39V under load. Will this be a problem if I expect it to give me 38V out?
Yes, a LM317 does not feel happy with some 1V of voltage drop; it has a 1.25V reference so ....
I wondered how you get 40V DC out of a 25VAC transformer (25 x 1.4 minus diode drop...), or maybe you have a high power transformer which does not have to deliver its current?
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