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All DHT Phono Stage

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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:

An externally hosted image should be here but it was not working when we last tested it.


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:

An externally hosted image should be here but it was not working when we last tested it.


and here it is with its companion power supply:

An externally hosted image should be here but it was not working when we last tested it.


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 said:
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

Sheldon said:
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
 
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.
 
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
 
Steve Cresswell said:
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.
 
arnoldc said:
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...


dsavitsk said:


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
 
dsavitsk said:


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.


Try backing off the filament voltage slightly . I'm very surprised any of these battery valves would be quiet enough let alone in a three-stage phono . I've used battery types including the 30 , 3A5 , 49 , 3B7 and 1LE3 and found all to have hum problems that could only be cured by wrapping foil around the bottlle or using a screening can . Microphony is another issue I have had , it always seems to be the low power filament types that have poor microphony , type 30 special is the only acceptable battery type I've found to date

316A
 
316a said:



Microphony is another issue I have had , it always seems to be the low power filament types that have poor microphony , type 30 special is the only acceptable battery type I've found to date

316A

Hi

The low power filament valves will probably have the finest wire in their filaments and as a consequence the filaments are going to be much more susceptible to vibration than the thicker wired higher power types.

I have just made up a makeshift screening can from an old tin can connected to the chassis with a clip lead and this has removed the last vestiges of hum so that it is inaudible even at maximum volume with no input.

Need to find something a bit more elegant now and apply a bit of damping to the chassis while I'm at it.

Common sense tells us that these battery valves should not work in a phono stage, but work they certainly do and properly fettled mechanically they give a superb sound. I reckon Dmitri ought to be congratulated for thinking outside the box as, it were. I have built several phono stages using the usual suspect valves but none has been as good as this one. Whether that says about my building skills than the tubes in question I wouldn't like to guess :)
Steve
 
Steve Cresswell said:


Hi

The low power filament valves will probably have the finest wire in their filaments and as a consequence the filaments are going to be much more susceptible to vibration than the thicker wired higher power types.

I have just made up a makeshift screening can from an old tin can connected to the chassis with a clip lead and this has removed the last vestiges of hum so that it is inaudible even at maximum volume with no input.

Need to find something a bit more elegant now and apply a bit of damping to the chassis while I'm at it.

Common sense tells us that these battery valves should not work in a phono stage, but work they certainly do and properly fettled mechanically they give a superb sound. I reckon Dmitri ought to be congratulated for thinking outside the box as, it were. I have built several phono stages using the usual suspect valves but none has been as good as this one. Whether that says about my building skills than the tubes in question I wouldn't like to guess :)
Steve

I'm well aware of the pitfalls of DHT's , I've built up quite a considerable stock (close to 300) and have been experimenting almost constantly for the past five years . Currently using thoriated drivers such as 15E , 3C24 and 10Y , battery types are on the back burner as are any oxide filament types apart from output valves .
If you need to damp the chassis , you would be better off getting some damping sheet , cutting into squares and applying to the underside of the chassis . A bit of extra weight may also help , one of the best ways to implement damping is to add large blobs of blu tac to the top of the chassis and once you find the offending areas , use the damping sheet underneath . I find adding trampoline suspension makes matters worse , with the 3A5 , you should find some sprung screening cans . You should also consider backing off your filament voltage , this is what I usually find to be the prime source of hum and microphony

IMO an ECC83 common cathode or ECC88 cascode stage followed by a type 46 etc output stage would be a preferable solution

316A
 
Rather than spring the chassis, I've had good success mounting the tubes to a fairly thick sandwich of aluminum and copper, then using pieces of sorbathane (sp?) to set it on the main chassis.

The filament voltage can sometimes be manipulated to good effect. I've done that to reduce distortion in my 801A amps. I can see where it might help with hum. However, in this case it may not be so easy, as the voltage across the filament sets tube bias.

Sheldon
 
Well it turned out that I didn't need any fancy suspensions for the valve sockets. All the problem turned out to be was the top of the metal case ringing and passing its vibrations through to the DCC90s.

Damping the case top on the inside with a sheet of Blu-Tack did the job of suppressing the microphony.

As Sheldon says I can't back off the filament voltages as it is these that bias the tubes in this design. Re the ecc83 or ecc88 cascode stage....been there and didn't like it, far too clinical and "hi-fi" sounding for my liking. The ecc88 cascode was the worst offender, the ecc83 a bit warmer but still as boring.

I do however run 46s in my power amp. They are direct driven by triode strapped, choke loaded C3gs in a DRD/Monkey configuration.

This DHT phono stage does what none of the others I have had could do; it plays music. Couldn't ask for more really.

Steve
 
ikoflexer said:
Thanks for sharing Steve, I've had this phono on my "todo" list for some time and it's very inspirational to read your experience with it.

Thanks for the positive comments

All the tweaking is now done and below is a picture of the phono stage sitting between my amp and Lenco GL78 turntable. Tin can screening removed for the photograph ;)
The PSU box can be seen bottom right of the pic.

An externally hosted image should be here but it was not working when we last tested it.


Steve
 
Very nice.

For shielding the input tube on my current phono stage (from Steve Bench design), I used some short pieces of copper tubing. They are attached so that they are electrically connected to the chassis.
 

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