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Inexpensive IC based phono preamp for LOMC
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Author:  mwhouston [ 10 Jan 2018, 16:23 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I've now built a few tube power amps, Black Lace (6V6) being one and two tube HP amps, Sanctum and Sanctorum with all carbon 1W resistors. I think "smooth" is the best way to decried the sound. Also an early Oatley K272A kit with sub-miniature tube and chip HP amp uses carbon where the later uses metal. It also has the "smooth" sound.

I think where so many build with metal, using carbon, may give your gear another point of difference. I haven't tried a MC head amp with carbon yet. Could be interesting.

Author:  phono guy [ 15 Feb 2018, 12:23 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hello, I'm a hobbyist learning electronics. I want to build an integrated amp with phono stage using Bruce's phono/preamp design and a LM3886 gainclone such as this:
http://www.circuitbasics.com/design-hi- ... er-lm3886/
For someone like me that has never done this, the info in that link is a treasure trove. I have a few questions. The build on that link is ready as it is for an input signal of 1V, but provides formulas to change it.
In your phono stage Is the gain of a typical MM cartridge enough to feed the LM3886 amp without the your preamp section? I looked at opamp gain formulas and used this link:
https://www.allaboutcircuits.com/tools/ ... alculator/
I used the values R1 0.3Kohms
R2 357.1Kohms -> I added 3 resistor, 100R+330K+27K
R3 0
R4 47Kohms
V1 0.004V
V2 0
Vp 12V
Vn -12V
And the calculations give me Vout -4.761V and gain non-inverting 1191. Is this ok? My cartridge is an Audio Technica ML440b, according to the tech specs its output is 4 mV.

What resistor values should I use, 0.25w is enough? All resistors use the same watts power?
Volume control: what's the difference in behavior between a 10k,50k or 100k volume pot?

Thanks.
Joel

Author:  gofar99 [ 16 Feb 2018, 16:02 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, It should be sufficient to drive a chip amp directly. You need to remember that the gain is at certain frequencies and is not linear in a phono preamp. The typical RIAA equalization curve (like employed in the preamp) is such that there is a mid band gain of about 40 db (100X) and a low frequency gain of 20 db more (10X more =1000) and a 10 db cut at high frequencies (1/10X). So yes the gain of the circuit is about 1000 at low frequencies, 100 at mids and only 10 at highs. This compensates for the way the music is recorded on the record.

I use 1/2 watt resistors mostly. For the IC preamp 1/4 watt is OK.

The difference in the various control values is related to what levels the input signals are and what value of resistance is needed to match both the source of the signal and the device it is going to. For solid state gear it is frequently as low as 10K and as high as 100K. For tube gear it is usually no less than 100K and can be up to 1meg or more. The higher the value the more prone circuits tend to be to hum and noise pick up from nearby sources. Lower ones however require the source to be able to provide more current than high impedance ones. A rather general rule is that solid state gear is current oriented and tube gear is voltage oriented (lots of exceptions to both though). I personally find that 100K is a pretty good value for most circuits.

Good listening
Bruce

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