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Steve Cresswell 28th July 2009 10:50 PM

All DHT Phono Stage
 
Hi Guys

It has been a while since I posted on here but I have been busy building an all DHT phono stage based on this one from Dmitri Nizhegorodov's website:

http://homepage.mac.com/scress1958/..../3a5-phono.gif

Mine uses the Mullard DCC90, which is a direct equivalent to the 3A5 7 pin miniature battery DHT used by Nizhegorodov in his design.

I use a quad of 4000mAh NiMH recchargeable cells to power the filaments and a separate PSU for the three HT supplies needed.

Here's a pic of the inside of the phono section, which is built into an old DAC box:

http://homepage.mac.com/scress1958/..../phonodone.jpg

and here it is with its companion power supply:

http://homepage.mac.com/scress1958/....sucosmetic.jpg

It was a nice little project to do and a lot more interesting than the RCA variant I was planning on. It also sounds great :)

here is the lowdown on how it works from DN's site

http://www.dmitrynizh.com/3a5-phono.htm

I don't know whether anyone else has built it but it is definitely well worth building.

Steve

arnoldc 29th July 2009 12:54 AM

Wow, nice! Did you inorporate a battery charging circuit? Is it in the power supply enclosure?

Sheldon 29th July 2009 01:27 AM

Cool. I haven't built it, but I've got the parts and a design for a PS. I'm going to try a CCS for the filaments.

Let's see that supply.

Sheldon

Steve Cresswell 29th July 2009 02:20 PM

Quote:

Originally posted by arnoldc
Wow, nice! Did you inorporate a battery charging circuit? Is it in the power supply enclosure?
There's no charging circuit; I just rotate the quad of cells with a spare quad, charging the four rundown ones with a separate battery charger. It works well enough for it not to be a hassle

Quote:

Originally posted by Sheldon
Cool. I haven't built it, but I've got the parts and a design for a PS. I'm going to try a CCS for the filaments.

Let's see that supply.

Sheldon

A current source for the filaments sounds interesting.
My power supply is nothing special, just a CLCLC supply with dropper resistors and decoupling caps for the three stages,incorporated within the PSU enclosure. It supplies 140V, 120V, 80V and a 0V signal ground to the phono box. These go out on a captive four core cable, terminating in a four pin female plug at the other end. The phono stage has a four pin male receptacle as an entry point for the three supplies.

I would certainly bite the bullet Sheldon and build the phono stage. It is very quiet and drives my two stage C3g/46 monkey amp with no problems. The highish output Z does not seem to be a problem; not in my system anyway. It has a lovely delicate top end with plenty of punch and fast, deep bass. Midrange detail is excellent and imaging and soundstage width and depth are first rate.

Steve

Sheldon 29th July 2009 03:53 PM

Thanks for the description Steve. I will build it. But probably a project for this winter. I've got a few other things to finish first, including a mandolin that my brother and I started years ago.

Current pre is fairly low input impedance (50k), so I'll probably add some kind of buffer - maybe a source follower stage.

Sheldon

BTW, you should shoot Dmitri an e-mail. He'd appreciate your project.

Steve Cresswell 29th July 2009 06:17 PM

One thing I did forget to mention regarding the DCC 90 is that they can be a bit variable in gain and consequent hum sensitivity. I bought eight of them and have managed to find a combination that gives a strong centre image with a mono source and minimal hum pickup.
Whether the 3A5 is the same I obviously can't say.

Get two "loud ones on one channel and two "softer" ones on the other and they will skew the image very noticeably towards one speaker. Also they can be swapped within a channel between first and second/third stage, so that the least sensitive ones are in the first position and the most sensitive in the last two positions, this pays dividends in terms of noise reduction.

By exercising a little patience and getting the combinations right, it is possible to generate an enormous sound stage that disappears way into the distance and extends way outside the speaker boundaries.

An extreme right or left instrument is quite capable of appearing to come from over the corresponding shoulder whilst you sit eight feet away from the speakers. That is something I have never experienced in my room before.

These little valves are truly holographic in their presentation, if they are matched for gain as a set. Classical vinyl is stunning with this phono stage. Vintage Frank Sinatra and Dean Martin... lovely. Rock, Jazz, Soul... in fact anything you care to play sounds immediate and fresh. There's no hint of hardness to the sound and they are blindingly fast. A real street sleeper of a valve then. Quirky yes, but one that rewards careful setting up and matching with a stunningly good performance.

They are microphonic and sensitive to hum fields. As a result, they need to be kept well away from mains wiring, so the bottom shelf of a rack is out of the question. In fact they are far better off mounted inside a screened metal chassis rather than on top as I have done. They don't give a light show anyway. so poking them up where they can pick up interference is pointless.

Nizhegorodov has designed something truly special with this little blighter. I shall be emailing him to thnk him for sharing the design.

What I think I'll probably do eventually is to get a bigger case and mount the phono section inside.
Steve

arnoldc 30th July 2009 02:52 AM

Hi Steve, did you use the DCC90 due to unavailability of the 3A5? I'm wondering where in this part of my world I can source the 3A5...

rman 30th July 2009 03:58 AM

Wow that's a great project! I would love to build one of these.
But I think I better stick with what I have. "Tough economic times"
:bawling:
Still, that seems like a killer phono amp!

dsavitsk 30th July 2009 04:54 AM

Quote:

Originally posted by Steve Cresswell
One thing I did forget to mention regarding the DCC 90 is that they can be a bit variable in gain and consequent hum sensitivity. I bought eight of them and have managed to find a combination that gives a strong centre image with a mono source and minimal hum pickup.
Whether the 3A5 is the same I obviously can't say.

I've been playing with 3A5's lately, and they seem to be very hum sensitive. This is CCS loaded with a CCS filament supply. I haven't gotten far enough to try shielding, or taking any precautions really, but out of the box there is a lot of noise.

Steve Cresswell 30th July 2009 08:48 AM

Quote:

Originally posted by arnoldc
Hi Steve, did you use the DCC90 due to unavailability of the 3A5? I'm wondering where in this part of my world I can source the 3A5...

Quote:

Originally posted by dsavitsk


I've been playing with 3A5's lately, and they seem to be very hum sensitive. This is CCS loaded with a CCS filament supply. I haven't gotten far enough to try shielding, or taking any precautions really, but out of the box there is a lot of noise.

Hi Arnold
I used the DCC90 as 3A5s are pretty hard to find in Europe, but as the DCC90 is the direct European equivalent there was no problem and there are plenty about.

The noise issue is there with the 3A5 too then?
I think they and the DCC90 would benefit greatly from a sub chassis suspended on elastic with very flexible filament wiring, the tubes individually shielded with screening cans and the whole, mounted inside a metal box.

The first thing I'm going to try with mine is an earthed metal box over the top of them. The tubes barely get warm in use so heat removal is unlikely to be an issue. as they are, gain matched and in the socket that suits them they make very little noise, but I still think it is possible to do better, especially in terms of microphony.

Steve


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