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Old 24th May 2012, 12:09 PM   #1
NZkeith is offline NZkeith  New Zealand
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Default Tube Pre-amp for both CD and Phono inputs

Hello there yet again Folks. I am asking for some input from those that have so much more knowledgeable than myself. I really appreciate the way that ALL the Folks out there are so willing to share their information with those that are less experienced. Just what a great forum diyAudio is... MANY THANKS...

OK now down to the nitty-gritty... I have attached a part-circuit of just what I have at present and this amplifier is for CD input only at present. The input tube is a single ECC83 ( 1/2 per channel ), driving a single 6V6 in Class A mode. I am using a single 50K log pot on the input at this stage.

What I imagine for the additional PHONO input is that I would have to use a full ECC83 per channel now, to get more gain for the phono input. The cartridge output requires 47K impedence and has an output of 4.5mV. ..... 2 RCA inputs with a switch between 'CD' and 'Phono' would be my idea ????

Any help with a SIMPLE circuit would be very much appreciated and I look forward to hearing form anyone that is willing to help....

Many thanks yet again....

Keith, New Zealand
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File Type: jpg 6V6 input.JPG (41.6 KB, 313 views)

Last edited by NZkeith; 24th May 2012 at 12:14 PM.
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Old 25th May 2012, 04:26 PM   #2
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I'd suggest that you'll rename this thread, or start a new one, titled: "Looking for a phono stage". Better still, search this forum for "phono stage". There have been quite a few threads on the subject. Basically an independent phono stage should work properly with any pre-amp.
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Old 25th May 2012, 06:17 PM   #3
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Greetings from FixitLand!

<quote>OK now down to the nitty-gritty... I have attached a part-circuit of just what I have at present and this amplifier is for CD input only at present. The input tube is a single ECC83 ( 1/2 per channel ), driving a single 6V6 in Class A mode. I am using a single 50K log pot on the input at this stage.

What I imagine for the additional PHONO input is that I would have to use a full ECC83 per channel now, to get more gain for the phono input. The cartridge output requires 47K impedence and has an output of 4.5mV. ..... 2 RCA inputs with a switch between 'CD' and 'Phono' would be my idea ????

Any help with a SIMPLE circuit would be very much appreciated and I look forward to hearing form anyone that is willing to help....</quote>

It's not as simple as just adding one stage to get more gain. A phono stage requires certain equalization as well, and usually two tube stages are used. Further, there are two general ways to do that equalization -- passive (lossy network) and active (network in a feedback loop). The output of a typical phono stage feeds a line-level input, so you basically need two tube stages (could be one tube, such as a 12AX7/ECC83) ahead of your existing preamp.

I threw together a pretty-good-sounding phono pre using a 6BH6 pentode and a 6GH8 pentode-triode tube (both fairly cheap tubes to obtain, versus a 12AX7 which can be spendy). It's set up for standard RIAA EQ plus settings for 78-rpm records.

Take care,
--
J. E. Knox "The Victor Freak"
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Old 26th May 2012, 03:03 AM   #4
NZkeith is offline NZkeith  New Zealand
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Thanks a stack for your personalized help, it is much appreciated.... Yes, it is very frustrating when knowing just what one wants, but do not have the skill... At over 60.... not much chance of catching up now is there ... Click the image to open in full size.

Attached is a pic of the little amp as I have it now, suppose there is much that is not totally correct .... But in the end it does sound great on CD, but as requested... A bit of CD AND MM Phono would really be nice....

Thanks yet again....

Keith ..... NZ

IMG_3929.jpg

IMG_3941.jpg

It's not as simple as just adding one stage to get more gain. A phono stage requires certain equalization as well, and usually two tube stages are used. Further, there are two general ways to do that equalization -- passive (lossy network) and active (network in a feedback loop). The output of a typical phono stage feeds a line-level input, so you basically need two tube stages (could be one tube, such as a 12AX7/ECC83) ahead of your existing preamp.

I threw together a pretty-good-sounding phono pre using a 6BH6 pentode and a 6GH8 pentode-triode tube (both fairly cheap tubes to obtain, versus a 12AX7 which can be spendy). It's set up for standard RIAA EQ plus settings for 78-rpm records.

Take care,
--
J. E. Knox "The Victor Freak"[/QUOTE]

Last edited by NZkeith; 26th May 2012 at 03:08 AM.
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Old 26th May 2012, 03:40 AM   #5
rongon is offline rongon  United States
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That's a nice looking little amp! I like its rounded edges style.

I'm no expert (far from it), but maybe that's a good thing for the purpose here. It's not easy to make a tube-based RIAA preamp that's quiet enough to be really enjoyable, and I'm sure it would be especially difficult to incorporate an RIAA preamp into an already existing amplifier. If the RIAA preamp has its own power supply and chassis, it will keep the noise that comes from the bigger amp's power transformer and higher current output tubes away from the very low level phono signals.

You might consider constructing something very simple and easy to pull off, but that still performs well. Eli Duttman gussied up the old RCA RIAA preamp circuit. That uses one 12AX7 tube per channel. I think you can get good 12AX7's for $10 or less each. Tube RIAA(something easy) -- The advantage of a 12AX7-based circuit is that it will draw little current, so a very small (and inexpensive) power supply is all that is necessary.

I built a stock RCA RIAA preamp years ago, except that I added a 12AT7 cathode follower to its output. My brother still uses it. It sounds very nice, sort of "vintage" warm, but cleaner. It's quite enjoyable.

There's a similar circuit that uses 6DJ8 (ECC88) instead of 12AX7, by RJM Audio. RJM Audio - Tube passive phono preamplifier -- That one is also very simple. You could replace the 500R resistors and 100uF 10V capacitors in the 6DJ8 cathodes with a red LED and 1N4148 diode in series to get the +2.5V cathode voltage. This circuit will require a higher current power supply than the 12AX7 one will.

I'm sure you'll get lots more ideas from others. You have a lot of options to choose from.

--
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Old 26th May 2012, 05:34 AM   #6
NZkeith is offline NZkeith  New Zealand
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Thanks Guys,
One learns something every day..... I did a bit of a search on " RIAA Equilization " and came across these article.... found them very well explained..... RIAA phono equalization article by Don Hoglund. ...... Constant-amplitude vs. constant-velocity, in layman's terms Trust that it will be a bit of an eye opener too..... Plenty more out there to boggle the mind .... Magic thing this " Mr GOOGLE "

Thanks for all the input and trust that my minor input might help others too

Best wishes....

Keith NZ

Quote:
Originally Posted by rongon View Post
That's a nice looking little amp! I like its rounded edges style.

I'm no expert (far from it), but maybe that's a good thing for the purpose here. It's not easy to make a tube-based RIAA preamp that's quiet enough to be really enjoyable, and I'm sure it would be especially difficult to incorporate an RIAA preamp into an already existing amplifier. If the RIAA preamp has its own power supply and chassis, it will keep the noise that comes from the bigger amp's power transformer and higher current output tubes away from the very low level phono signals.

You might consider constructing something very simple and easy to pull off, but that still performs well. Eli Duttman gussied up the old RCA RIAA preamp circuit. That uses one 12AX7 tube per channel. I think you can get good 12AX7's for $10 or less each. Tube RIAA(something easy) -- The advantage of a 12AX7-based circuit is that it will draw little current, so a very small (and inexpensive) power supply is all that is necessary.

I built a stock RCA RIAA preamp years ago, except that I added a 12AT7 cathode follower to its output. My brother still uses it. It sounds very nice, sort of "vintage" warm, but cleaner. It's quite enjoyable.

There's a similar circuit that uses 6DJ8 (ECC88) instead of 12AX7, by RJM Audio. RJM Audio - Tube passive phono preamplifier -- That one is also very simple. You could replace the 500R resistors and 100uF 10V capacitors in the 6DJ8 cathodes with a red LED and 1N4148 diode in series to get the +2.5V cathode voltage. This circuit will require a higher current power supply than the 12AX7 one will.

I'm sure you'll get lots more ideas from others. You have a lot of options to choose from.

--
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