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Phono preamp 'headroom' capability....how important?


catman

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G'day all, looking at quite a few popular DIY phono stage designs of late, many (based on op amps) seem to be taking the option of full battery supplies and whilst this does have definite advantages in terms of noise etc, one aspect worries me. 

 

In some examples that I've seen, two 9 volt batteries are configured as a simple split rail power supply giving plus and minus 9 volts to the op amp.  Whilst this obviously works, I wonder how badly compromised the headroom figures are at such a low working voltage. 

 

I recall reading a good analysis on another forum (AK), which found that whilst the preamplifier worked fine at 9 volts per side, signal headroom capability was in fact severely restricted and even distortion figures were somewhat compromised. 

 

Yet those who use this approach don't seem to complain!  For what it's worth my DIY power supplies have always been configured for at least 15 volts per side which I'm advised is a ideal working voltage for op amp circuitry.  Any thoughts on this?  Regards, Felix. 

Edited by catman
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A properly designed and configured pair of regulators will provide not only superior noise performance to batteries, but allows much greater flexibility. Although there are quieter regulators, you really can't beat an LM317/LM337 for performance for the Dollar. Unless you do a REALLY bad job at building regulators, it is incredibly unlikely that the noise performance will be any worse than the surface noise on even the very best LPs.

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Hi Felix,

 

I use a single-ended battery supply on my 'Muse' JFET-based phono stage ... so 0v / 24v (nominal: fully charged is >25v).  24v is obtained by using 2 x 12v SLAs in series.

 

So that is more headroom than simply using +/- 9v batteries ... as well as providing a lower output impedance to the power supply (than tranny batteries).  If you look at the +/- voltages at the output of the 1st & 2nd gain stages, you get (this is for the 46dB MM version):

* 1st stage Drain voltage set at ~12v

* input 5mV peak-to-peak

* output 220mV p-t-p

* signal after RIAA circuit is 20mV p-t-p

* 2nd stage Drain voltage set at ~12v

* input 20mV p-t-p

* output 1,060mV p-t-p.

 

So in terms of headroom, let's say with a +24v DC rail, the Drain at each gain stage can happily swing +/- 9v from the nominal setting of 12v.  This is 18v p-t-p.  Hence, considering the 1st gain stage only:

* an output of 18v p-t-p is 80x the normal output

* which means it can take a 400mV input signal

* which I suggest is an excess of headroom! :D

 

Now considering the 2nd gain stage:

* an output of 18v p-t-p is 17x the normal output

* which means that the phono stage can cope with an 85mV input signal

* which I suggest is definitely a more-than-acceptable amount of headroom. :)

 

So IMO a 24v battery supply is not deficient in any way, in terms of headroom.

 

 

Regards,

 

Andy

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