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Old 4th February 2005, 10:37 PM   #21
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Hi there..... for those interested..Since it has been published.........D Self wrote an interesting article in the J Audio Eng Soc Vol 35; No 11; 1987 Nov........ <Ultra low noise Amps and granularity distortion>. I don't have anywhere fast enough web connex to retrieve the material.
The servo loop biassing and multiple input tranny low noise building approach is technically the bestway...for a very low front end input...

rich
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Old 4th February 2005, 11:01 PM   #22
mcs is offline mcs  Denmark
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Default Re: The best phono preamp on op-amps?

Quote:
Originally posted by Soundadvice
The circuit on the lm833 data sheet seems to have alot going for it in comparison to the `one op amp types` normally found in most mixers,integrated amps,not to mention some other manufacturers data sheets ! I have not built this yet , but even for the not very mathematical amongst us the gain can easily be increased at the second stage,the only downside is that from the figues provided and the ones I calculated for a different first stage gain,it always seems to come up with-E96 values of resistors.
I have just built one of the LM833 app. note preamps, and it sounds just fine to me. I used OP275 op. amps., as they were the only low noise type I had. You can see a picture of my almost complete board here: http://stiftsbogtrykkeriet.dk/~mcs/Phono1_PCB_small.jpg

I did manage to get all the resistors in E-24 values and all the caps in E-12. The schematic with my values is here: http://stiftsbogtrykkeriet.dk/~mcs/Phono1_sch.gif

If anybody wants the PCB drawings (or a PCB) let me know.

Best regards,

Mikkel C. Simonsen
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Old 5th February 2005, 12:32 AM   #23
EUVL is offline EUVL  Europe
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Default RJM Phono Preamp

Hi,

I built two of these about 1 year ago, one for MC and one for MM. The MC version has 40dB gain per buffer stage, battery powered. Don't claim they are best ever, but sound nice for the complexity and effort. Beats most commercial ones at the US$200 price tag.

http://www.geocities.com/rjm003.geo/...io/pho_a6.html

Opamps I used are AD797 for front, OPA637 for the rear, both biased in class A with a 2mA constant current diode between output and -ve rail. RIAA capacitors are MKP1837 1%, values recalculated from the Lipwitz paper.

Also built the version 3 of the Rod Elliot "Phono Preamp for All', which came originally from S. Bateman. Have more gain than the one above and actually sounded better for some cartridges. But I like the Buffer-Filter-Buffer configuration, rathar than RIAA on the feedback loop.

The next I am going to build would be JFET Class A, balanced output, no feedback, also buffer-filter-buffer. But many other projects planned before than (including turntable).


Patrick
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Old 8th February 2005, 09:50 PM   #24
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What kind of transformer should I get for the power supply for an audio circuit like the phono stage? I've been searching on the net about transformers but couldn't find anything clear. Any suggestions? thanks.
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Old 8th February 2005, 09:59 PM   #25
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For phono stage ? 5 - 10 VA, sec. winding 2*18 V AC.
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Old 8th February 2005, 10:00 PM   #26
bucurb is offline bucurb  Romania
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Thumbs up lm833 phono

Mikkel,


You did very nice work. Your phono preamplifier looks absolutely great. Could you post your pcb design, please?
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Old 8th February 2005, 10:33 PM   #27
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Quote:
Originally posted by djaleksei
[B]What kind of transformer should I get for the power supply for an audio circuit like the phono stage?
Hi there.....I doubt you will find the ideal transformer you are looking for....a low flux EI type(or toroid).....no-one really makes them anymore although a custom one-off wouldn't come cheap.
The drawback with the AES & R.I.A.A eq is it requires +18dB bass boost at 50Hz and poor layouts with bad wiring layout will find the stray 50/60Hz from elsewhere and not from the pickup. Ideally the juice should come from within the amp you are using via additional pass regulators so common mode intererence signals are cancelled out as part of equipment case and wiring. I found some time ago that a separate power supply put in a box just for mag or MC pickup eq's will create interference charge capacitive injection clicks through transformer windings or via earth returns. Remember ic regulators have a good ripple rejection spec....(better than discrete) and this is a good starting point....however these require short pcb layouts, thick tracks and short earths to obtain the full ripple rejection. So many designs go wrong due to excessive pcb track length.
I'm not a great believer in interwinding screens although for safety compliance they maybe required.
If you have to design with discretes....avoid toroid transformers.....high magnetising flux can create problems with ajacent wiring. Any transformer has to be some distance away from sensitive input circuitry.

rich
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Old 9th February 2005, 12:36 AM   #28
mcs is offline mcs  Denmark
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Default Re: lm833 phono

Quote:
Originally posted by bucurb
Mikkel,

You did very nice work. Your phono preamplifier looks absolutely great. Could you post your pcb design, please?
Thanks! You can find PCB files here (PDF): http://stiftsbogtrykkeriet.dk/~mcs/Phono1_PCB.zip

All the files are mirrored for printing on transparancies.

Regarding the transformer. I use a 12V AC wall wart transformer placed about 25cm away from the unshielded preamp board - and I have no hum problems...

Best regards,

Mikkel C. Simonsen
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Old 9th February 2005, 01:36 AM   #29
EUVL is offline EUVL  Europe
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Default Transformer for Phono

My 2 cents' worth :

Don't waste time (and money). Just get yourself 4x 8.4V 250mAH NiMH's and done. Sufficient for 10 hours for most opamp circuits.

I have tried them all -- remote Cap Multipliers, LM317's, Jung Super Reg, Current sourced shunt regulator, .....

You saw what I am using after all that.


Patrick
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Old 9th February 2005, 10:49 AM   #30
cedus is offline cedus  France
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I think this will be very difficult to do something better than Thorsten Loesch phono preamp. http://www.users.nac.net/markowitzgd/phonopre.html

I built one 2 weeks ago using exactly the components he recommends and 2 1.2 AH 12V lead batteries and it is just wonderful. The sound is great ! It makes wonders with my modfified TD 125 MKII.

Give it a try and I am sure you will not be disappointed.

And go for batteries. I can't see any reason not to use it. The sound is clearly better than with AC and it does not cost more.
It costs me around 30 for the 2 batteries and the charger and for the value that is the best investment you can make.
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