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PostPosted: 13 Nov 2011, 10:54 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
Hi!

I also did this one.

I finished three days ago, and have been testing.

At the first instance, I also felt the need for more output gain, but after a few tests and in comparision with my Sony TA-FA3ES amplifier (MM / MC) phono-stage. It is just about the same audio level (using headphones for accurate results).

This RIAA pre-amplifier has a beautiful sound, but to be honest, I can´t say it´s better from my Sony ES audio. I tried hard to give a 1st place for one of them, but they are really similar.

So, once more, this makes me a happy guy, because now I have a stand alone RIAA pre-amplifier which I can connect to my Oddwatts.

Oddwatts? Now that´s another history...

It seems that the pre-amplifier´s output is not enought to drive the Oddwatts to full output.

After many tries and more tests, I found out that the best option, and without messing up with the audio caracteristics, is to place a 1000uF low ESR electrolytic capacitor across the 50ohm resistor on the final stage (this was really the best option, from many many other modifications I tried).

The pre-amplifier has now a little bit more gain. I tried several vinyl records looking for a loud recorded one. On the loudest audio peaks, it never distorts the sound.

I also designed a simple PSU with RC filtering, which still keeps the pre-amplifier quiet, no HUM at all! I can post it here if somebody is interested.

There is just one technical doubt, which I don´t remember very well from those old school days...

Where are those formulas for transistor aplications that define the output gain with emitter bypass capacitors?


Have a wonderful Sunday!

Miguel


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PostPosted: 13 Nov 2011, 16:08 
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Joined: 06 Jun 2008, 18:23
Posts: 5151
Location: Australia
Good work. I'm interested in the PS. I still run mine on batteries but have thought about an external mains powered PS.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |


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PostPosted: 13 Nov 2011, 18:27 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
Well sure! :)

This PSU was built around those low profile PCB transformers.

I got one for less than 5Eur. It has 2 x 24vac windings @ 200mA, but of course you can go for less current.

So the windings are wired in series to get 48vac, that will offer you around 60vdc, and so this is a long way down to 24vdc, that will also reduce ripple naturally.

I decided for a double "pi" filter stage, with values shown, according to my calculations, ripple is only 88uV.

The resistor values were calculated along with most common capacitors values.

You only need to be carefull at C1 were the maximum voltage swing gets near 60vdc. So this is why the 1st capacitor is 470uF, because the idea is to place 2 typical 1000uF / 35v in series.

As for the other capacitors 50v is more than safe, and easy / cheap to get.

This PSU works like a charm. There is absolutely no HUM at all (even with headphones). And my transformer is not even shielded (it is potted).

Summary:

C1 - 470uF / 100v

C2 - 470uF /100v

C3 - 220uF / 50v

C4 - 220uF /35v

All resistors are 1/2W.
Attachment:
2sk170_riaa_psu.JPG

Please feel free to post any modification you would like to do, in case you got a slightly different transformer, and i´ll do calculations for you.


Enjoy your music! :)

Miguel


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PostPosted: 14 Nov 2011, 04:57 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Good work.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |


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PostPosted: 16 Nov 2011, 10:25 
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Joined: 31 Oct 2011, 09:51
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Is this pre-amp kit still available? I have been getting the old 404 when I try to track down boozhound labs online. Thanks.


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PostPosted: 16 Nov 2011, 10:35 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
Yes, it seems to be in stock.

Website is down, but 2 kits left on eBay!

http://www.ebay.com/itm/BHL-Audio-JFET-Phono-Preamplifier-kit-2sk170-RIAA-/230692541525?pt=LH_DefaultDomain_0&hash=item35b658d455

Cheers,
Miguel


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PostPosted: 23 Nov 2011, 16:45 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
Well, here are my final comments on this nice project.

Personaly I still find the amplifier sound to bright.

There is to much gain on high frequencies / lack of bass.

I have checked several times for any mistake, or wrong components, but everything is just alright.

I wonder what can I do here, to get some sort of flat frequency response, but without any harm to the RIAA curve.

It´s just a guess, but I believe that the "cure" is from C4 ahead... Why is C4 only 100nF? Is it the real impedance match from RIAA stage to Q2?

This circuit has much more to offer, I really think so!
Any suggestions?

Cheers in advance,
Miguel


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PostPosted: 23 Nov 2011, 17:00 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Try changing the cartridge load resistor (sits across the input) . Usually 47K try 22K and 100K. The BHL works extremely well in my system with Grado wood body reference cartridge.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |


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PostPosted: 24 Nov 2011, 14:34 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
I tried the changing the resistor values, but almost no effect.

I also played with some silver mica capacitors in parallel with the loading cartridge resistor. The 50pF is the one that seem to help a little bit.

It must be my turntable, it a crappy Kenwood plastic model P-29, offered by a friend of mine. But it´s in very nice shape, and he says he didn´t listen more than 100 hours at most.

I´ll try with a Rega P3 turntable that another friend of mine owns (maybe next weekend).

I´m using genuine Toshiba 2SK170 matched jfets, K40-9Y capacitors, Nichicon Muse KZ electrolytics, silver mica caps (5%), 1% Dale millitary specification resistors.

Have also tried several PSU´s, and the best one is by far the classic transformer / bridge with RC filtering. The SMPS has a little whistle in the background. I have also tried batteries which perform just as good as the transformer.

I have done my best, and have high expectations with another TT.

Cheers,
Miguel


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PostPosted: 24 Nov 2011, 16:14 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
A linear supply is the best. Even try batteries. I only change mine (AAs) every three or four months and I use my TT a lot.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |


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