Which of these two phono stages

[rman]

Hi all.

Any thoughts on the realitive merrits of theese two passive eq. phono circuits? I believe the 6c45p is the more linear tube than the 6dj8 but more prone to oscillation.

http://www.geocities.com/rjm003.geo/rjmaudio/diy_pho3.html


http://home.earthlink.net/~jeremyepstein/6c45riaa.html

Rolf.

 

[tubewade]

I haven't built or modeled either of these but they both look to me like they would do the job. I don't really know the 6C45P valve. In the course of searching for a great RIAA amp circuit I've come to really like Steve Benches RIAA #5 circuit. You might look at that one and read some of his comments before making your decision.

http://members.aol.com/sbench101/

No, I haven't built that one yet. I'm still using a 2 stage 12AX7 phono amp but plan on making a change in the future in order to reduce input capacitance for better results with some cartridges.

I presented a question similar to yours some time ago and got lots of great encouragement from some very knowledgeable people. That thread is here...

http://www.diyaudio.com/forums/showthread.php?s=&threadid=114096&highlight=

Best, Wade

 

[rman]

Thanks Wade. That Steve Bench design might be the one. It looks not too complex, and I belive he really knows his stuff. interestingly it fits with Miles Prowers comment that 6dj8's are best when used in cascode.

It would be interesting to hear how it compares to my Hafler 110's phono section (discrete all transitor), which I think sounds very good.

Thank's again.
Rolf.

 

[hotbottle]

I quickly scanned your replies and the the thread mentioned there, and I didn't see any reference to the one RCA published in their tube manual. It uses 12AX7s which I prefer to the 6DJ8s, and is nice and simple. However, it does sound very good. I use it myself.

http://www.bonavolta.ch/hobby/en/audio/rca.htm



Also, a commercial unit that many have reported sounds very good. The designer has a circuit on his website here

http://www.hagtech.com/images/cornet2schem.gif
http://www.hagtech.com/cornet2.html

 

[tubewade]

Hotbottle, I am glad you have some input for us on this subject. The first schematic that I referred to on the previous thread was Bob Danielak's implementation of the RCA circuit. That one appeals to me because of it's simplicity, however, using the high mu input valve such as 12AX7A, 7025, 6SL7 or 6EU7 results in a substantial input capacitance due to miller effect. In this circuit the input capacitance will be several hundred pico farads, enough to interact unfavourably with cartridge inductance. Sometimes one can live with this if your cable capacitance is very low and your pickup cartridge's inductance is on the lower side of normal. I have a variety of cartridges and the Ortofons give satisfactory results under these conditions but my Shure cartridges do not fare so well. They really would like to see less capacitance.

One other thing to considers is output impedance and drive capability. The RCA circuit will do if driving a very high input impedance through a very short cable. Otherwise, it really needs to be buffered.

Having said all of that, now I'll tell you that I am also using an unbuffered 12AX7 phono preamp (Harman Kardon) and it does sound pretty good. The reason for my earlier thread was to get ideas for something that will improve upon what I already have. The responses I received might be of benefit to anyone who is in a similar situation.

Warm regards,

Wade

 

[hotbottle]

Hi Wade,

Yes, I agree about input capacitance, but in practice, have not had many problems - even with Shure carts. Phono interconnects should always be low capacitance anyway. The second circuit I posted a link for, uses two 12AX7As in parallel on the input. Does this lower the capacitance or raise it?

On the subject of output drive, I have a version of this preamp that is "integrated" with a line preamp, so the output has no problems driving the short distance to the next valve input.

In standalone versions I have made, I add a cathode follower. There are some audiophiles who don't like cathode followers, but I cannot honestly say I hear any deterioration of the great sound this style of phono preamp produces.

 

[rman]

Hi again.

I have decided on the RJM audio 6dj8 project with the lclc tube
rectifier supply from the same webpage. I am wondering weather
to use one tube per channel, or one for input and one for output.
Is there a generally agreed upon way, is this one of those things
where some swear by one way, and others say the other way is best? I am thinking one tube per channel.

I picked up some Electro Harmonix 6922EH from a local supplier
today.

 

[hotbottle]

I prefer one tube for input, one for output, each channel using half a tube.

My reasoning is this. I may want to try tube rolling, and I may want to try a different brand tube for each stage. You can't do that if the input and output stage for each channel are wired into the one tube. For example, it's the first tube that must have super low microphonics. I'd rather be able to just swap one tube and test it, then not be forced to find another one. The tube for the second stages can be more microphonic, which may give you more choices to roll in the sound you like.

The usual objection to having left and right channel in one tube, is crosstalk. I have never found it to be a problem at all, and especially with tubes that have internal shields, I don't consider it an issue at all.

 

[peranders]

If you don't get any answers here you could always ask the guy here:
http://www.vinylengine.com/phpBB2/index.php
http://www.analog-forum.de/wbboard/index.php (german but english is OK)

 

[tubewade]

I prefer one tube for input, one for output, each channel using half a tube.

My reasoning is this. I may want to try tube rolling, and I may want to try a different brand tube for each stage. You can't do that if the input and output stage for each channel are wired into the one tube. For example, it's the first tube that must have super low microphonics. I'd rather be able to just swap one tube and test it, then not be forced to find another one. The tube for the second stages can be more microphonic, which may give you more choices to roll in the sound you like.

The usual objection to having left and right channel in one tube, is crosstalk. I have never found it to be a problem at all, and especially with tubes that have internal shields, I don't consider it an issue at all.

+1

 

[rman]

Thank's

One in and one out it is.

 

[rman]

Hi. P-A. (and all)

Tubewade and Hotbottle pretty much answerd my question.
I checked out that vinylengine link, fantastic phono specific
info! Found great stuff on cartridge loading. Also some great
reviews on my present phonostage. (Hafler 110 preamp). That
one is going to be hard to beat. A co-worker gave it to me because his friend was going to chuck it!

This 6dj8 project is just for fun. Does anyone know where I can
get a suitable Transformer? No luck fiding one yet. I need
about 625V C.T. to supply 20Ma. and heat the two 6922's and a
6X4 rectifier. (Indirectly heated 6 volt) I would like a nice small
transformer,peferably cheap.

Rolf.

 

[tubewade]

It may ease your search a little to consider using a hybrid full wave bridge, so you wouldn't need a 625 volt transformer with C.T., just a 310 volt one without C.T. It would also be safer to be free of the 625 volt possibility. You could still use your 6X4 as planned, and use semiconductor diodes on the negative side of the bridge. I would really recommend this type of power supply.

Wade

 

[hotbottle]

Coincidence! I did exactly the same on my preamp i.e. 6X4 with 2 silicon diodes to complete the bridge. As you say, it works well.

 

[rman]

Hi guys.

Hybrid bridge idea sounds interesting, might just try it.

 

[rman]

Hi. guys.

This project is starting to come together. I just need to be sure
of something for tweaking filter components. In a passive
filter like this I think the effective value of R1 is,
((Rp ll Ra of first stage)+ R1) ll Rg of next stage.
Is that right?

Rolf.