Balanced DHT-Preamp/Dac-Output anyone ?

[Blitz]

I have done some search but have not found a lot on this, so allow me to open a thread. I would like to build a new DAC-output-Stage for my Buffalo III Dual Mono. The Ess9018 generates per channel 32mA. It works currently into a 20Ohm resistor generating 640mVpk.

My power amp has a diff. pair as input (http://www.triodedick.com/monobill_2/monobill_2_schema_versterker.GIF)...so I would like to see what a fully balanced, DHT-solution would be like.

Has anyone done this already and can share some schematics / experiences ?

I guess I would see a design like in the Karna http://www.nutshellhifi.com/Karna.gif

...Only question is if I could simply copy the middle part with the 45 or which changes to be made...but overall I like the idea of

- fully balanced
- triodes work as diff. pair with a current source
- dacs are directly connected to the grids of the triode (the voltage offset will be neutralized by the current source)
- triodes work in pp into an interstage transformer (1:1?) maybe I could use this one as it still sits on my shelf http://www.monolithmagnetics.com/si...stage transformer balanced bifilar prelim.pdf
- a gain of 10 minimum would be ideal.
- it will drive directly into the input tube section of the power amp, so 100k input impedance.

I guess not ground-breaking stuff, but I have not seen it realized so many times either...so if you can give me some hints, highly appreciated.

Current questions I have is around the gain achieved as I saw some designs with 5:1 IT etc. These dht triodes ar as well oftenly not high in amplification factor...so, some practical experience of someone realized this kind of design would be very nice.

 

[thoglette]

I presume (as you mention the Karna) you're familar with Lynn Olson's comments on the M24 Monarchy DAC, which might give you some idea where to start.

The M24 sidesteps the slewing problem by using a passive I/V converter—which cannot slew—followed by a ferrite RF toroid combined with the Miller capacitance of the 6DJ8 SRPP circuit. This two-pole low pass filter quiets down most of the RF coming from the converter, and it doesn't hurt the 6DJ8 was originally designed as a preamp tube for FM and TV front ends. (Don't try to get RF through an op-amp, they're not designed for that.) The zero-feedback tube sections are always in Class A, of course, and have hundreds of volts of headroom. No wussy 15 volt op-amp supply rails here.

If you feed a 6DJ8 SRPP into a phasesplitting transformer, similar to the early Raven versions.

I believe (having not done it) your DAC can provide a balanced output already so you could go straight into PP 6DJ8s

See this project

 

[Blitz]

These are two excellent hints....I did not know that page, thx a lot.
Yes, my dac provides 32mA current ouput per channel...the colleague in your link used a very small i/v resistor of 1 ohm and a 1:54 transformer. wow.

...so in theory his output impedance would be like (1+1)*54*54, right ? Around 5k...which would work with my input impedance of 100k.

I am using currently a 20ohm resistor and tubes, but with capacitors in the path...so in theory his solution might outperform mine....need to explore...I habestill some Lundahl iron like ll1674 and ll7903 from other projects...so will try this first before designing an expensive tube output stage again...

Great advise !

 

[andyjevans]

i've used the Hammond 124B as a differential plate choke - it's 49% nickel. Works very nicely. I used it with 4P1Ls. I'd be interested to try filament bias with this instead of a CCS.

http://www.hammondmfg.com/pdf/5C_124_126.pdf

 

[Blitz]

I am just looking atthe marvelous projects of Thomas Mayer:

VinylSavor: The Ultimate Line Preamplifier

There is a nice picture of his selection of iron...obviously no schematic as the man does this for a living...

...nevertheless, I wonder: He uses IT and anode chokes ...? With a one stage design (four tubes ? That surprises me a bit...wouldn's be the primary if your IT your anode choke at the same time ? Obsiouly he found a way to get rid of any cap in the signal path...so not even the cap between ct of the IT and Cathode like in the Karna is allowwd I guess...

But I do not really understand this yet...

 

[Sodacose]

The Raleigh Audio Rakk DAC might be of interest to you:

Active output

That's differential parafeed for a DAC output. Again I don't think exact values are given.

 

[Blitz]

Funny....I actually have one in my cellar from old Rakk Dac times...I tried it at that time withthe Rakk Dac II, but preferred a Borbely Fet output board...but might be that the coupling caps were not that great, I will get the dust from it and try it again...they never published a schematic, so was not even sure what kind of a design it was....