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DHT Headphone Amp/Preamp Combo
DHT Headphone Amp/Preamp Combo
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Old 14th January 2019, 10:47 PM   #1
Gungnir is offline Gungnir  United States
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Default DHT Headphone Amp/Preamp Combo

I originally built a DHT headphone amp, which a started another thread about last year, but the amp was noisy so I took it apart.

My new idea is to build an all DHT preamp that can also function as a headphone amp. I have a huge supply of parts. But there are a few other hurdles.

I currently own Sennheiser and Byerdynamic headphones 300-600 ohms, but want to purchase a pair of Hifiman 400i, which are 32 ohms.

My original plan was to use 71a, but now I think I want to use the 47 tube in triode mode (maybe even pentode) for the output..

On the driver side, I have 1:4 input transformer to 01a or 26. I plan on either a ccs or plate choke capacitor coupled to 47. I also have a pair of LL1660 10ma that I could use as interstate transformer.

I plan on filament bias for the 01a with Coleman boards and cathode (or fixed) bias on the 47, also with Coleman regulators.

I am waiting on a quote from Rod for his fixed bias boards

I will probably try and somehow rig the input transformers off chassis so that I can omit them when using the amp as a preamp

The OPT will be LL2765, which can be configured for 32,150, or 600 ohm. The issue with this is only one configuration can be used at a time and I want it to serve duty as a preamp and any impedence headphone, so I think 150 ohm would be the choice.

I really want to put all this in one big 17"x10"x3" chassis

Does this all sound doable or am I doomed to build two separate amps? Any suggestions are welcome
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Old 15th January 2019, 06:59 AM   #2
hpeter is offline hpeter  Slovakia
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you should put up some idea drawings on paper first

LL2765 is a good start, in 600r mode its fine for driving poweramp.
but LL1660 inside pre, as interstage.. is like hunting pidgeons with rpg . mosfet gyrator will do the job.
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..BLOG.. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000 // 4-125a test
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Old 15th January 2019, 02:39 PM   #3
Tjj226 is online now Tjj226
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I was actually looking at doing the exact same thing.

Im thinking a choke loaded 6j5 into a 71a. I am using the same output transformer and I am also using the rod coleman regulators.

I also have a monstrosity of a switch that will allow me to switch between headphone mode and preamp mode. In headphone mode, the 6j5s will be in the circuit providing the voltage swing needed by the 71a. In preamp mode, the 6j5s are out of the circuit and the 71a can act just as a single preamp stage.

I am also contemplating using a 45 at lower voltages.

I also agree that the 1660s should probably be left for something else. I am using 2 hammond 156cs for the choke load on the 6j5 at ~5ma. They are super cheap, super simple, and they have fairly decent sound.
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Old 15th January 2019, 02:55 PM   #4
Gungnir is offline Gungnir  United States
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I think the main issue that I am having is being able to drive 32 ohm head phones and a power amp. 600 ohm out is easy for my Senns and Beyers, but not so much for the Hifiman. Also, I really want to keep this all DHT. Also, the gyrator seems to add more complication than I would prefer, but CCS is doable, I also have four 90H plate choke that I could double up or maybe connect the 1660 as a plate choke?
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Old 15th January 2019, 04:18 PM   #5
hpeter is offline hpeter  Slovakia
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1660 can be as choke, - leave secondaries open
a i said, put some drawing, then people will come to discuss.
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..BLOG.. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000 // 4-125a test
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Old 15th January 2019, 06:03 PM   #6
hpeter is offline hpeter  Slovakia
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Click the image to open in full size.
DHTRob - weBlog Search
how about this ?
not sure but it could drive +300r phones.
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..BLOG.. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000 // 4-125a test
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Old 15th January 2019, 08:20 PM   #7
Gungnir is offline Gungnir  United States
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Not enough gain for headphones, I'll try and draw up a schematic tonight of what I have in mind.
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Old 18th January 2019, 02:16 PM   #8
hpeter is offline hpeter  Slovakia
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gainwise yes, but powerwise it should be good.
i think that small transformer as parafeed. not 100% sure about 32r, though....

i posted some uk dhts with about mu30. thats the highest i have seen
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..BLOG.. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000 // 4-125a test
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Old 18th January 2019, 02:55 PM   #9
hpeter is offline hpeter  Slovakia
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how to use gyrator (some info on my blog)
Click the image to open in full size.

i expect gain ~28-30x + 2~2,5x +0,3x(in 600r mode) = 16,8~22,5x more like 18x , i think

amp:
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..BLOG.. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000 // 4-125a test
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