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Old 30th May 2012, 05:21 PM   #11
kevinkr is offline kevinkr  United States
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I took the liberty of editing the thread name so that its subject is a little clearer.
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Old 30th May 2012, 05:28 PM   #12
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Kevin
Thanks for your input. After I get the pre-amp powered up and hear what
it can do I may make the changes you suggest. I have been told that this
pre-amp sucks to the max so I dont want to spend anymore money than I
have to for now.

thanks
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Old 30th May 2012, 05:31 PM   #13
rongon is offline rongon  United States
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About 15 years ago, there was a commercially available circuit board for the RCA RIAA preamp (from Handmade Electronics, I believe). I stuffed one of those, put it in a nice chassis with power supply, and added another tube socket for a 12AT7 cathode follower, to help with the output impedance problem. The result sounded quite pleasant.

--
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Old 30th May 2012, 05:38 PM   #14
smbrown is online now smbrown  United States
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Bruce, I'm not sure I'd say it "sucks to the max" I personally really enjoy this stage and have built a number of them over the years. My favorite uses 6SL7s (or 12SL7s) instead of the 12AX7, but this would not work on your board since the SL7 is an octal tube. At any rate, if you use it within the limits give (like next stage needs 250K or greater input impedance) you may be surprised just how much it doesn't suck.
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Old 31st May 2012, 12:04 AM   #15
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Quote:
Originally Posted by rongon View Post
About 15 years ago, there was a commercially available circuit board for the RCA RIAA preamp (from Handmade Electronics, I believe). I stuffed one of those, put it in a nice chassis with power supply, and added another tube socket for a 12AT7 cathode follower, to help with the output impedance problem. The result sounded quite pleasant.

--

I also built the Handmade kit. Designed by Gordon Rankin, it was a nice implementation. Mine is on the sideline right now while I come up with a new power supply for it.
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Old 31st May 2012, 12:49 AM   #16
Ivey is offline Ivey  United States
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Default TUBECAD PHONO AMP, MOD RCA PRE-AMP

Bruce:

Here the layout of a modded RCA clone. My great grandfather copied it from Tubecad. I listen to it for years now. It sounds outstanding.

I include the article and the pcb layout.

I hope this will help. I ask you great grandfather and he stated that good metal poly caps can be used for the RIAA.


Joe
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File Type: pdf TUBECAD-B.pdf (10.7 KB, 283 views)
File Type: pdf TUBECAD-C.pdf (17.2 KB, 213 views)
File Type: pdf TUBECAD-D.pdf (9.3 KB, 187 views)
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Old 31st May 2012, 12:58 AM   #17
Ivey is offline Ivey  United States
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Here is a copy of the article and schematic.


Joe
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File Type: pdf RIAA_Preamps_Part_1.pdf.pdf (95.9 KB, 334 views)
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Old 1st June 2012, 03:16 AM   #18
cerrem is offline cerrem  United States
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Without getting to in depth.... Here are a few basic to strive for to make this simple RIAA stage pleasurable to listen to...

1. RIAA ACCURACY....this is pretty much trivial transfer function...just basic math to figure the proper values for POLES and ZEROES.... The bigger question is if you even care...since a sloppy RIAA curve will make some audiophiles go balastic while other listeners can be very happy and don't mind.... Keep in mind that MANY pressing were cut with sloppy RIAA curves to begin with....

2. HEADROOM.... are the tubes biased nicely??? Will a loud passages cause clipping and go to the rail??? The worst part of clipping is that the RIAA caps will get charged up to the rail, then they need "time" to recover...not good...
In the old days they used "1812 Overture" as the transient overload test record... then you ask yourself...is it my pre-amp??...or is it the cartridge not aligned properly?? Oh maybe the recording was distorted??? Or maybe all the above??

3. NOISE FLOOR......What you can do to minimize noise.....carbon comps for the added warmth...or metal films for the cold and sterile truth.....

4. VOLTAGE REGULATION.... I prefer good voltage regulation for linearity....but with that comes NOISE from the regulator/ reference.... A reg with a low noise floor is key to a good RIAA stage and this RCA phone stage is no exception...it will make it come alive...

Have fun with this phono stage....It may not be the best design...but having good supporting circuitry can make all the difference....

Chris
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Old 2nd June 2012, 01:59 AM   #19
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To all who got involved, Thanks for your input.
The last of the missing components showed up in todays mail.
I finished stuffing the board, constructed a crude voltage doubler power supply and fired up the pre-amp. It worked without a single problem, and sounded much better than I expected. It is very quiet considering the power supply I used for testing. Now I will build anothr board, get a chassis and
construct a real power supply to make a stereo unit.
Just for fun, I tried several tubes to hear how it would sound.
First was a 12AU7, (not very loud but worked). Next was a 12AT7, (sounded better and just a little more volumn), then a 12AX7 (loudest and best sound).
I also put the Russia 6n6P It worked pretty nicely also. I'm staying with the
12AX7. If anyone would like to built a pre-amp, you can use my PC board layout, it works well.
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Old 2nd June 2012, 12:29 PM   #20
rongon is offline rongon  United States
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Yes, definitely stay with 12AX7!

An RIAA preamp is no place for "tube rolling." The 12AU7 and 6N6P are radically different from 12AX7. Nowhere near the same amplification factor, plate resistance or transconductance. The frequency response of the preamp will be greatly altered by putting in such different tubes. It would deviate widely from "flat" response (adherence to the RIAA characteristic). It's no surprise that you wound up preferring the 12AX7 in this preamp. That's what it's designed for, so of course that is what will work best in that circuit.
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