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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

Author:  mwhouston [ 19 Mar 2018, 20:35 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Yet another MM "Mimic" phono preamp. This time with some polies (4.7uf) on the output. Also one poly in the EQ stage. Enclosure Is cast Al with red epoxy paint. Stunning colour.

Sounds great with my new Garrott K3 MM cart in new Rega P3.

Forgot I can't load bigger images here. Will upload some soon.

Author:  gofar99 [ 19 Mar 2018, 21:09 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Mark, I have a few I built out there as well. They all have poly caps in the RIAA network and the best sounding ones have Auricap XO 3.0uf /200V output caps. A bit hard to come by and a bit pricey but nice sound.

Good listening
Bruce

Author:  mwhouston [ 19 Mar 2018, 21:16 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi Mark, I have a few I built out there as well. They all have poly caps in the RIAA network and the best sounding ones have Auricap XO 3.0uf /200V output caps. A bit hard to come by and a bit pricey but nice sound.

Good listening
Bruce


Evidently 3uf is big enough.

Author:  mwhouston [ 20 Mar 2018, 02:28 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Just an image of the inside. I pickup the brass engraved label (Decal) tomorrow and then you will see the outside complete. Once more, silver pated fine copper wire for hookups, heavy duty gold plated RCAs, silver solder (WBT 4%) the smallest piece of vero-brd (strip brd) I have ever used and mil spec chip socket with an OPA2134. 1/4W carbon resistors, poly caps in the EQ and output stages complete this simple build.

Attachment:
P3193678 (800x600).jpg

Author:  mwhouston [ 24 Mar 2018, 18:15 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

For more detail of my latest build; http://retro-thermionic.blogspot.com.au/2018/03/mimic-carbon-ii.html

This has to be the best sounding Mimic so far. Carbon resistors and polies in the EQ stage and output. Looks good with the Red Delicious Epoxy Enamel paint and big gold plated RCAs.
Attachment:
P3223681 (800x600).jpg
Attachment:
P3223682 (800x600).jpg

Author:  Geek [ 25 Mar 2018, 08:01 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Beautiful! :D
Wish I had the time to paint cases like that (have the skill, just no time :( )

Author:  mwhouston [ 25 Mar 2018, 12:43 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Geek wrote:
Beautiful! :D
Wish I had the time to paint cases like that (have the skill, just no time :( )

Really it is a quick process. We are talking minutes not hours. Drilling the enclosure takes more time. 15mins prep (sanding). 5mins prime. Next day 5mins top coat. Then four days to dry. All out of a spray can which can last for years. Paint is Epoxy Enamel. Dries tough.

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