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bahrens

Tube phono preamp
« on: May 07, 2009, 09:15:09 PM »
Hi!  A friend of mine has asked if I would be able to build him a tube phono preamp.  I've built several stompboxes as well as a Dumble ODS clone, so I'm quite comfortable with the build but I was hoping to get some discussion going as to what might be a good circuit to use.  My friend wants to spend 200-300, which seems fairly reasonable (at very least if not factoring in labor) provided I don't get too fancy with the enclosure.  I've gotten through most of what I've been able to search on this forum and have done some searching elsewhere, but I feel like a lot of the info out there is tainted by audiophoolery and I trust this board a lot more than I trust the hi-fi crowd. 

The only constraints for the build are that it not cost more than $300 and that it use only tubes in the signal path, with said tubes running at proper high voltages.


SSLtech

Re: Tube phono preamp
« Reply #1 on: May 07, 2009, 09:56:25 PM »
Either buy one... ...or sell him one with op-amps in it, and tell him to look at a picture of a glowing tube when he listens to it.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Freddy G

Re: Tube phono preamp
« Reply #2 on: May 07, 2009, 10:25:32 PM »
PRR wrote:
Use #18 wire for rock-n-roll, #16 wire for classical.

bahrens

Re: Tube phono preamp
« Reply #3 on: May 08, 2009, 01:22:37 PM »
Either buy one... ...or sell him one with op-amps in it, and tell him to look at a picture of a glowing tube when he listens to it.

Keith

Your response is probably at least somewhat tongue in cheek but if he wanted to buy one he would, and I'm not going to charge him money to build something other than what he asked for.  He knows full well that a good transistor based pre would sound great and do just fine, and that there are commercially available products available.  He wanted tubes, specifically, and wanted it hand built to his aesthetic specs (Wood & metal with tubes exposed).

http://www.classixaudio.com/

It looks like these are based on the Marantz 7C, is that right?  That looks like a pretty simple approachable circuit.  I don't really want to buy a kit but I'll have to check out the 7C schematic and figure out a layout.  I've got a bunch of G-10/FR4, eyelets, terminal strips, etc so I'll do my own boards and layout. 

keefaz

Re: Tube phono preamp
« Reply #4 on: May 08, 2009, 01:38:38 PM »
Maybe have a look at Steve Bench phono tube circuits:
http://greygum.net/sbench/sbench101/
Note that you will need to add a line stage, maybe Jackob's SRPP from gpultec circuit ?

pstamler

Re: Tube phono preamp
« Reply #5 on: May 08, 2009, 02:18:51 PM »
If you want to build from scratch rather than a kit, I'd recommend a preamp with passive EQ between two flat tube stages. Each stage could be, say, a 6SL7 voltage amplifier direct coupled to a 12AU7 cathode follower. Since those are both dual tubes, each channel would use two tubes.

At the risk of blowing my own horn, several years ago I wrote an article for audioXpress about passive EQ networks, which included info on using them with tubed amplifying stages. It's in the December 2001 issue.

Peace,
Paul

Re: Tube phono preamp
« Reply #6 on: May 08, 2009, 04:28:09 PM »
Maybe have a look at Steve Bench phono tube circuits:
http://greygum.net/sbench/sbench101/
Note that you will need to add a line stage, maybe Jackob's SRPP from gpultec circuit ?

This one is a nice one:  http://greygum.net/sbench/sbench101/Preamps/RIAA5.gif
I pilfered the 1st stage as is and tried it as a mic amp front end.  Not too shabby.   


Paul's suggestion of a passive eq between gain stages is good advice too.  Paul did a very nice design for a magazine a few years ago.
Maybe he can tell you which back issue of the magazine you could buy.


Borbely has published a couple.  Some //'ed low noise J-Fets, cascoded by a valve is his general method.  P.R.R. would probably advocate against cascodes unless you can help it though  ;D

A phono pre is similar in gain/noise requirements to a good mic amp.  Although, it's arguable that we need as stringent a noise spec given the dynamic range of vinyl.  I don't know myself, I've done it a few ways and do like the black silence before I put the needle down but I'm also perfectly happy with something that's noisier but sounds 'glorious' when it's actually amplifying music...


For non tube but a decent pre - the ONO from N. Pass is quite nice.  Although I'd probably still adapt it and use straight gain stages with passive eq.


My $0.02

Edit:  should read all of the posts.  Paul already gave the issue containing his design.  It's a good 'un
« Last Edit: May 08, 2009, 04:30:02 PM by Winston O'Boogie »

Jonte Knif

Re: Tube phono preamp
« Reply #7 on: May 08, 2009, 04:37:29 PM »

The first question is MC or MM? I hope not MC, because then it gets critical. You need low noise/microphonic tubes, or MC step up trannie. But then it gets expensive. Put K170 jfet as the first stage and don't tell him :) Seriously, if you want a decent S/N ratio with MC-cartridges you got to do some research and tube swapping. But with MM it's much easier. High output MC could do also.

The lowest noise tubes like 417A, 6C45Pi, C3g, D3a and the like are either too expensive or microphonic or both. Perhaps you could start with 2 parallel connected ECC88 or 6DJ8 to keep the project manageable.

When you look at tube data sheets you get the picture of the lowest noise possible with the tube type. Higher transconductance means lower noise, but _only if the tube is well made_ 12AY7 as suggested in the previous schematic is in theory not the lowest noise tube, but good ones can beat bad 6DJ8.  And you do get about 60dB S/N ratio. Is that adequate, I don't know.

Just my opinion.

Re: Tube phono preamp
« Reply #8 on: May 08, 2009, 04:48:17 PM »

Put K170 jfet as the first stage and don't tell him :)


That's what a lot of commercial units do.  Or, as Borbely, cascode with a 6922 or?


...12AY7 as suggested in the previous schematic is in theory not the lowest noise tube, but good ones can beat bad 6DJ8.  And you do get about 60dB S/N ratio. Is that adequate, I don't know.



Parallel sections with give you a bit less noise but will up your capacitance.  Hence, I guess, the cascode in the schematic.  Although you could use the capacitance as part of the RIAA.

60dB?  I don't know either.  I've been happy with something like that myself but some folks seem to spend more time with their ear up against the speaker, listening to the noise,  than they do sitting in a chair listening to music.

Just my opinion.

Good ones though. 
« Last Edit: May 08, 2009, 04:53:05 PM by Winston O'Boogie »

Re: Tube phono preamp
« Reply #9 on: May 08, 2009, 05:05:04 PM »
I've been working on the SBench RIAA5 for a few years now  :o  Kiira also highly recommended this design, which is why I built it.

At first I had a bad newbieish grounding layout and suffered from really bad hum.  I thought the issue was the power supply, so I ripped the power supply apart and started from scratch.  Back when it was up and running, it sounded absolutely glorious on top of the mmmmmmmmmmmmmm....   Every year or so, I'll post a question or two about it here, with the result being that my power supply redesign is still half baked, so ehhh, I'm still not done.  ;)

Anyway, NewYorkDave helped me out a long time ago with a simple 6SN7 CF line amp stage he tacked on to the end.  If you are interested, I can post what I've got so far.

-Chris


Re: Tube phono preamp
« Reply #10 on: May 08, 2009, 05:25:02 PM »
Please do!
Any other observations about this SBench RIAA5 build?
I been looking for a good/proven riaa pre for a long time.
Might as well just build this one!

keefaz

Re: Tube phono preamp
« Reply #11 on: May 08, 2009, 06:41:15 PM »
it sounded absolutely glorious on top of the mmmmmmmmmmmmmm.... 
Just curious, what tube did you use for V2 (labeled 12AY7 on schematic) ?
Steve Bench did make a note on the right of schematic that he used GE 5 Stars 6072A, I don't know if it would make a big difference though... Did you try to cover it with a tube shield connected to ground ?


 

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