This is a phono preamp module out of a Sansui amp.
A few questions regarding its possible enhancement...
First off, in testing, it sounds fantastic, with a claimed +/- 0.5dB accuracy to the RIAA specs.
That spec, I don't really want to mess around with.
However, what I want to possibly do is to be able to give it a small +3dB "boost" of it's output.
I don't want to change the IC either, it sounds fine.
I also want to (if possible) lower it's already fine output noise a tad.
Residual hiss can only be heard with no signal if ear is near speaker, but not big worry.
Would changing the value of the 100 ohm feedback resistor R6 or R7 be a way to give the output a boost? - and if so, would it make a difference in the RIAA spec?
OR... would the addition of a simple NPN transistor at the output be more suitable?
Thoughts?
A few questions regarding its possible enhancement...
First off, in testing, it sounds fantastic, with a claimed +/- 0.5dB accuracy to the RIAA specs.
That spec, I don't really want to mess around with.
However, what I want to possibly do is to be able to give it a small +3dB "boost" of it's output.
I don't want to change the IC either, it sounds fine.
I also want to (if possible) lower it's already fine output noise a tad.
Residual hiss can only be heard with no signal if ear is near speaker, but not big worry.
Would changing the value of the 100 ohm feedback resistor R6 or R7 be a way to give the output a boost? - and if so, would it make a difference in the RIAA spec?
OR... would the addition of a simple NPN transistor at the output be more suitable?
Thoughts?
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I don't see any information on the M5214L on the web. It may be a hybrid IC without enough open-loop gain to get more closed-loop gain.
Ed
Ed
The Mitsubishi M5214L seems to be a 7-pin SIL single op amp. I've been unable to locate a data sheet so I can't comment on the gain capability.
The big issue is what is the drive capability of the Mitsubishi M5214L. In the current configuration, the device output load is the LR6 and LR7 in series(100 +100 ohm).
R7 should be like 100ohm / sqrt(2) ~ 71ohm.
C8 should be 220uF * sqrt(2) ~ 330uF
Beside the gain icrease, this will (very) slightly change the FR as a side effect. But the change will probably be neglectible (depends on the OLG of the opamp)
C8 should be 220uF * sqrt(2) ~ 330uF
Beside the gain icrease, this will (very) slightly change the FR as a side effect. But the change will probably be neglectible (depends on the OLG of the opamp)
I would leave the circuit as is, and just add a operational amplifier gain stage at the output, on a small board. This way it will be easy to compare the sonic results to stock output (just add a jumper on the board to bypass the gain stage), and maybe a trimmer can be used to make the gain variable and select the best one. SMD parts may help to save space, if required.This modification is also easy to remove in the future. The board may be soldered in place of the coupling capacitor or the nearby resistor.
Only at very high frequencies. There isn't much HF content in the records anyway
Ok, hit me with a 3dB boost opamp that runs off the +/- 15 supply.
I've got a load of TL072's around, some 55xx's too.
Most opamps will deliver +3 dB gain if provided with the proper feedback network. If you have NE5532 available, use it. Just set up a plain non-inverting gain stage with a gain of 1.414 = sqrt(2). Don't forget to add decoupling caps by the supply pins for the opamp.
Tom
Tom
And increase R6 to keep R6 + R7 the same, that would at least give the same frequency response if the op-amp were ideal.
Whatever you do, the output noise will slightly increase rather than decrease.
It isn't... Lose the bypass cap to lower gain. Or increase Rk, or voltage divider on input, or output?
WOT probably has the parts to try it in his junk box.
Well, it appears they made a small change to the schematic that I showed earlier!
Those two resistors (R6 and R7), marked as 100 ohm were actually 120 ohms!
Must have been a mid-production change.
Anyhow, I replaced R7 with a 75 ohm, and the 220uF with a 470uF.
Bingo!... got the boost I was looking for, and the tonal quality is perfect to my ears!
What I do, when I do not want to touch the pre-amp (components values... OP Amps...), is... I play with the (+ and - 15V in your case) power supply rails. Give it a nice low noise, wide bandwidth power supply... and then fine-tune the sound to your liking by playing with 10uF / 50V decoupling caps.... try smaller uF values (4.7uF) - and maybe higher voltage... try different types. I still have my favourite stash of Black Gate N (and in particular the Nx) ones.... which are unbelievable... So, together (power supply and local decoupling), you should be able to achieve what you want... I know, the above won't change the actual frequency response curve... but, the PERCEIVED change in sound will get you to where you want to be.
And, of course, if you want to play with the actual RIAA response... you can, knowing that you have (already) provided the best voltage rails to that pre-amp.