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Old 13th May 2012, 02:29 AM   #1
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Default phono line stage project

hi everyone,

disliking my yamaha amplifiers electronic phono stage (sounds ok, what you would expect, but the gain is very low) i was thinking of bypasing it internally and building a valve stage to replace it.

the only thing is that i am on a budget, so im looking to use 12ax7 long platers which i have here, then change to some genalex gold lions when i have the spare dosh as i have been told they are a huge step up from the average ax7 and are well worth it (opinions please!).

transformers are not a problem for me as i have loads of them and can make my own (have done before)

i just need some info on power supplies

i was looking at this one from audio innovations:
http://i31.photobucket.com/albums/c3...n/ai500-ph.gif

it looks simple enough, however there is no supply schematic- i have built a few small amps before but am not sure about how to go about one for something so sensitive as a phono stage- my questions are:

what size capacitors to use in the dc heater stage to stop humming, and also there has been debate about weather to go ac or dc on this, i think dc is the way to go with a phono stage but again please advise

what is the best way to go for the hv supply rectifier ez81 or bridge diodes and also the right sized capacitors for these? i hear it will be more punchy with solid state?

also what voltage should i drive the ax7s to for pleanty of gain- 300v?

thanks in advance for your help!
oh and my player is a garrard sp25 mk3 with a magnetic cartridge


mike
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Old 13th May 2012, 02:56 AM   #2
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300 V is fine, I would have two 330uf of capacitance with a 1K resistor between. use the bottom schematic, but change the plate resistors to 220K. as far as the heater circuit, just run a strait ac transformer (12V) and tie the center tap of the 12ax7's together and take a .1 uf poly cap (250V is fine) and go from the the common tap to ground.
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Old 13th May 2012, 03:13 AM   #3
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thank you dave ill give it a try!
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Old 13th May 2012, 03:27 AM   #4
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JMO, steer clear of the "reissue" GECs in a phono preamp. THE current production 'X7 variant to use is the Sovtek 12AX7LPS, which is clean and quiet.

I've uploaded a tweaked version of the "classic" passive EQ RCA circuit. Several people have built it, with considerable success. As you found a lack of gain in the SS phono setup, substitute a 1 mA. constant current source (CCS) for the 2nd gain block's 100 KOhm load resistor. Any well filtered PSU of about 275 VDC is quite sufficient. Use an individual LR8 regulator in each channel to lock the rail at 250 VDC.

Don't forget a DC heater supply. Simple and highly effective is Greinacher ("full wave") doubling 6.3 VAC with 2X Schottky diodes and following the doubler cap. stack with a 7812 3 terminal regulator.
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File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 804 views)
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Old 13th May 2012, 03:53 AM   #5
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thanks, always good to have more info!
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Old 10th August 2012, 11:38 PM   #6
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finally i have time to get back to it....

i have built the 1989 version of this schematic, however there is almost NO bass.... and im unsure why... all of my component are within 10 percent of the values in the original schematic, which should make little difference, im using a stanton a600 cartirdge through this....does anyone think i need to inrease the capacitance from 51nf up?

thanks in advance

http://www.drtube.com/schematics/ai/ai500-ph.gif
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Old 11th August 2012, 12:42 AM   #7
kevinkr is offline kevinkr  United States
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What is the input impedance of the input on your amplifier that you have this connected to? For good bass response with that 12AX7A based design you are going to need something like an input impedance of 1M or so.. You could add a 6DJ8 based cathode follower to improve drive capability into low impedance loads.

This design is very similar to one of my very early designs - with some care this is a decent performer.

I don't see a 51nF cap anywhere in this design so I am puzzled by your comment as well as the comment about 10% tolerance parts, cumulatively this can result in several dB or worse of response error in the RIAA equalization. I use 1% resistors throughout, and tightly matched resistors and caps in the RIAA. (Better than 1%) This design is sensitive to the tolerance of the first plate resistor as well as the actual operating point of the tube. (Anything that effects rp will affect the response to some extent depending on design)

Take a look at SY's article "His Master's Noise" and my "Muscovite" thread for possible insights into the level of attention to detail needed to get a design right. Might want to look at the Russian 6S3P, 6S4P as potential fodder for a phono pre..
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Old 11th August 2012, 07:12 AM   #8
kes321 is offline kes321  Hungary
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Quote:
Originally Posted by preludefan View Post
finally i have time to get back to it....

i have built the 1989 version of this schematic, however there is almost NO bass.... and im unsure why... all of my component are within 10 percent of the values in the original schematic, which should make little difference, im using a stanton a600 cartirdge through this....does anyone think i need to inrease the capacitance from 51nf up?

thanks in advance

http://www.drtube.com/schematics/ai/ai500-ph.gif
Hi preludefan,

my opinion is that the RIAA circuit is not Ok.
here's a link how to calculate a passive RIAA network: Phono Stage Design, you may found a lot of similar calculation, if you look around on net.
The 1987 version is similar to one of its Audio Note's RIAA circuit, you should choose to Rg 1.82Meg then, the 1989/1991 version could be used if you put a cathode follower between the first ECC83 stage and RIAA network, plus change the 47k to a 37k resistor, htis is how I calculated it. Personally just did the some similar mistake, the difference is that I get the high's sound terrible, I'm on the way to correct it.
regards: kes
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Old 11th August 2012, 11:48 AM   #9
Cookj is offline Cookj  Belgium
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I think the 51nF cap should be 51pF!

Regards

John
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Old 11th August 2012, 08:18 PM   #10
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Quote:
Originally Posted by preludefan View Post
finally i have time to get back to it....

i have built the 1989 version of this schematic, however there is almost NO bass.... and im unsure why... all of my component are within 10 percent of the values in the original schematic, which should make little difference, im using a stanton a600 cartirdge through this....does anyone think i need to inrease the capacitance from 51nf up?

thanks in advance

http://www.drtube.com/schematics/ai/ai500-ph.gif
you will get no bass if you forget to bypass the cathode resistors. OR in the 1989 version of this circuit you must use a 47K Ohm resistior (yes, 47K) in the signal path or you will lose a lot of low frequency response...

Also, the 51pF input cap isn't really so necessary IMHO... the first 0.47uF coupling capacitor can bre reduced if you have any rumble problems though.

I hate to be the one to mention this, but 10% matching is not really good enough for phono - especially try to get the RIAA components to 2% or better (i.e. trim resistors by adding small resistors in series or very large ones in parallel).

Since you are on a budget I would suggest checking the first two things though (also check that your coupling capacitors are really 0.47uF).

regards
ian
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