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PostPosted: 27 Dec 2018, 13:44 
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This is a phono preamp taken from Max Robinson's Fun With Tubes site. My question is does it look like a good candidate for use with a technics sl5 with a MM stylus eps24cs with 2.5mv at 1khz output and Matt's marblewood amp? I have attached a copy of the scematic. I appreciate your opinions on this.


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PostPosted: 27 Dec 2018, 16:14 
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Your choices here are really about solid-state verses tube implementations and feedback verse feedforward equalization designs.

Since you seem to be asking about tubes, we'll place that one on the back burner. The circuit you have asked about is a feedback equalization design. This is the "high output" feedback circuit presented on the FWT website. This design does have a bunch of advantages associated with negative feedback. However, it is an unknown with respect to how well it does against the RIAA curve. I personally have never used this particular design, but if you intend to, I would recommend testing it carefully against the RIAA curve to make sure it doesn't unduly color the sound.

There is a feedforward design presented in the RCA tube manual which I have used in the past. In the RC-30 manual it is sample circuit 29-16 (shown below):
Attachment:
RIAA-Preamp.jpg

This uses in industrial 12AX7 and produces acceptable results. It tracks the RIAA curve acceptable well without being unduly complicated. Like any such high gain, small signal circuit, layout and shielding are critical to performance. I have used this circuit in the past and while it produces good results, it is sensitive to power quality and construction methods.

Of course there is also Bruce's most excellent "GrooveWatt" (here: http://diyaudioprojects.com/Tubes/RIAA-Phono-Preamp/). This is also a feedforward design but the SRPP input and output help it work particularly well. Also, Bruce is very particular about his designs and this one works well.

Now I'm going to return to that solid state comment above. We all know that tubes are great and we build with then whenever we can. However, this is one instance where solid state can serve us very well. I have build a couple of the "BoozHound" labs JFET RIAA preamps and I will say that the perform very nicely. (Here: http://diyaudioprojects.com/Solid/JFET-Phono-Preamplifier-Kit/ and https://boozhoundlabs.com/collections/all/products/bhl-audio-jfet-phono-preamplifier-kit-2sk170-riaa) This can be a great way to get your MM equipped turntable up and running with good performance and for a nominal cost. Then you will be free to tinker with the vacuum tube RIAA preamps at your leisure. I say this because it has been my experience that, unlike power amps, the high gain RIAA vacuum tube stages require some learning curve and rework before getting an acceptable level of performance. Repeated prototyping seem to be a minimum requirement. YMMV.


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PostPosted: 27 Dec 2018, 17:19 
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Thanks Matt for the much needed advice and suggestions. I will look them over and go from there. I greatly appreciate your wisdom on this. I thought the rca was intended to be used with a tone stack or buffer due to it's high output load of 220000 ohms. Im feeling my way through the dark on this. Maybe solid state is a better answer at this point. I will look at both some more.


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PostPosted: 28 Dec 2018, 17:52 
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Hi, (Thanks Matt -you saved me a lot of words). I really don't care much for either of the posted schematics (no surprise as I am really picky). Both tend to have issues with accurately following the RIAA standard and are prone to hum and noise. They will however work in non-critical applications. As Matt noted the one I posted is really a high performance project. It needs care in layout and building. When done properly it will deliver exceptional sound. Now wandering a bit OT, if you don't feel comfortable with the complexity of tubes the simple one IC phono preamp I posted a while back is easy to build, not fussy in layout etc and will provide a really high quality output. If you go for the tubes (my preference naturally) we are here to assist you with what ever one you build.

Good listening
Bruce

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PostPosted: 29 Dec 2018, 20:17 
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I'm kind of at a stall here at the moment. I'd be lying to say that I'm not really partial to tubes. Of course I grew up with them. Should have started this hobby 40 years ago. With that said my layouts are lacking and often I have done things over several times. So, it's a good learning experience. I have power trans, and most parts to go with either of the above plans and am willing to give one a go. Bruce yours is an excellent design and well worth the build. I might be better off building one of the others first due to the already available parts and the need to get the experience. If I go with the rca is the output load an issue? What to do,lol.


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PostPosted: 29 Dec 2018, 22:52 
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That's not the output impedance, that's the recommended load. You should be fine with either design as long as cables are kept short. You could add one more dual triode and make cathode follower output for the preamp.

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PostPosted: 24 Jan 2019, 01:45 
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gofar99 wrote:
Hi, (Thanks Matt -you saved me a lot of words). I really don't care much for either of the posted schematics (no surprise as I am really picky). Both tend to have issues with accurately following the RIAA standard and are prone to hum and noise. They will however work in non-critical applications. As Matt noted the one I posted is really a high performance project. It needs care in layout and building. When done properly it will deliver exceptional sound. Now wandering a bit OT, if you don't feel comfortable with the complexity of tubes the simple one IC phono preamp I posted a while back is easy to build, not fussy in layout etc and will provide a really high quality output. If you go for the tubes (my preference naturally) we are here to assist you with what ever one you build.

Good listening
Bruce

Ok it’s not tubes but it’s a giant killer. Bruce’s OPAMP based MM stage punches way above its weight. If you want a super clean and very accurate sound, this is what you should build. I have built and sold quite a few (thanks Bruce I was able to retire early) in many, many various component mixes even with Russian military PIO caps. I could write a book about my builds of this “Mimic” device it is so good.

Now back to tubes. Make it easy on yourself and buy the Greek kit from Radi0kit. It comes complete but you need your own PS. Not hard if you use the reverse step down Tranni trick.

The kits come in two forms; high end components or lower end components. You’ll knock-up the PCB in minutes. But the work is in PS and enclosure etc. I use a 1U 19” rack mount with tubes poking out the top. Made a few, sold a few kept one for myself. Mimic may beat it in peformance.

I’ll try to post some links.

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PostPosted: 24 Jan 2019, 01:48 
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viewtopic.php?f=10&t=6000&hilit=Mimic

https://www.ebay.com/itm/RADI0KIT-RK-31 ... 2094389487

http://retro-thermionic.blogspot.com/20 ... reamp.html

http://retro-thermionic.blogspot.com/20 ... reamp.html

http://retro-thermionic.blogspot.com/20 ... reamp.html

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PostPosted: 24 Jan 2019, 02:11 
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viewtopic.php?f=9&t=5226&hilit=Radi0kit

One guy bought a Mimic then a La Gitana (tube phono) amd liked the Mimic OPAMP based unit over the tube.

Another has a Chenook phono stage which cost him over $2K. He claims Mimic is almost better. All my Mimic run on two 9V batteries but for the guy with the Chenook he had me build an external PS. I use $50 LT discrete regs to ensure an absolute quiet PS.

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PostPosted: 24 Jan 2019, 12:05 
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Hi Mark, do I get royalties? Anyhow, yes the IC preamp is far better than you would expect and both easy and cheap to build. All mine use SMPS dual voltage DC to DC converters. I like them because they provide really quiet DC and are simple to use. 9 volt batteries are fine as well. Rechargeables with a relay to remove the charging voltage when in use would be a fine alternative too. I have sold a few as well, but really just encourage others to build them. I recently tried the Burson V5 ICs in one and had to add DC blocking capacitors to the inverting input to prevent serious dc offset on the outputs. It seems that they have some slight difference in the dc at the inputs that is amplified in the circuit. The basic circuit does not have dc offset protection and with some ICs will lock up the output. I use Burr Brown OPA 2134s with about 95% working fine. Most other IC types are OK too, but every now and then one from the same batch will misbehave. A QC issue I suspect. Hints...use good output caps, sockets for the ICs (in case one misbehaves), good grounding (single point star or buss) and clean DC. The end result will sound better than a large number of commercial products. BTW they can be configured to work with a tape head instead of phono cartridge.

Good listening
Bruce

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