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Old 31st October 2020, 06:49 PM   #481
mcandmar is offline mcandmar  Ireland
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Switched out the biasing resistors for the SiC boards during the week.

As expected it did clean things up a bit, measured results where a little surprising to be honest, and bordering on the limits of my test setup for frequency response and noise floor. I just wish a had measured the harmonic content before and after as i believe that would have been far more telling.

Musically, there is a noticeable improvement in clarity, dare i say the old setup was a tad smoother and more romantic sounding, i suspect there was more harmonic content in there making it a little more warm and cuddly.

I can absolutely see why someone would prefer it that way, and i will admit i wasn't immediately sold on them, it took a few days of listening to come to the conclusion that the SiCs are right, and that they are staying. I will be using this amplifier for everything, Music, Vinyl, TV, Netflix, Youtube videos you name it so the cleaner option is the better choice for me.

Still deeply impressed with this pre amp, you know you are onto something good when you cant stop feeding music through it
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Old 31st October 2020, 07:11 PM   #482
mogliaa is offline mogliaa  United Kingdom
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01A question
Glad you like it. The SiC bias arrangement was a great thing to land on. Iím very happy abs havenít found anything to sound better on the cathode of this preamp. Who knows, we may find it In The future!
Ale
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Old 31st October 2020, 07:17 PM   #483
andyjevans is offline andyjevans  United Kingdom
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Quote:
Originally Posted by mcandmar View Post
Switched out the biasing resistors for the SiC boards during the week. Dare i say the old setup was a tad smoother and more romantic sounding, i suspect there was more harmonic content in there making it a little more warm and cuddly. I can absolutely see why someone would prefer it that way.
How flattering - that's me! Romantic, smooth, warm and cuddly. I always knew it.

I'm back on filament bias. For me smooth does it. My 01A stage is driving my EL12n outputs in my current main amp.
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Old 31st October 2020, 08:45 PM   #484
mcandmar is offline mcandmar  Ireland
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Quote:
Originally Posted by mogliaa View Post
Glad you like it. The SiC bias arrangement was a great thing to land on. Iím very happy abs havenít found anything to sound better on the cathode of this preamp. Who knows, we may find it In The future!
Ale

Some active circuitry perhaps? I experimented with biasing a small preamp tube using LEDs, and a TL431 shunt regulator. The circuit had a lot better high frequency response with the 431 vs the LED, but the regulator itself is inherently very noisy which created an audible hiss on the audio output. There is something there though worth exploring...
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Old 31st October 2020, 08:49 PM   #485
mcandmar is offline mcandmar  Ireland
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Originally Posted by andyjevans View Post
How flattering - that's me! Romantic, smooth, warm and cuddly. I always knew it.

I'm back on filament bias. For me smooth does it. My 01A stage is driving my EL12n outputs in my current main amp.

...filament bias seems to sounds much better to me too.
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Old 13th January 2021, 05:27 AM   #486
Flikoman is offline Flikoman  Croatia
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What device would you recommend to use for source follower? What mosfet? IXTP or something else?

Thanks
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Old 13th January 2021, 07:47 PM   #487
nashbap is offline nashbap  United States
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Quoting Ale from his blog

"Hi Tomislav,
I use the STF3LN80K5 which has a plastic package which can be bolted directly on the chassis, protection diodes between G and S and more importantly very low Crss (0.1pF):
https://www.st.com/resource/en/datasheet/stf3ln80k5.pdf
You canít ask for more!"
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Old 13th January 2021, 08:15 PM   #488
mogliaa is offline mogliaa  United Kingdom
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Indeed, thanks Nash for sharing. Cheap and excellent MOSFET for the job. Also as a plus it comes with protection diodes inside the dye which reduces the overall part count for the follower

Btw, for the ones who love the DHT sound and want a low gain stage due to their systems requirements, here is a good example to look into. I love the sound of the 01a stage:

01a Low Gain DHT preamp (finished) – Bartola(R) Valves

You can implement effectively any medium mu DHT with low current and high rp, whilst driving successfully any low impedance amp (eg solid state for example)

Cheers
Ale
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Old 13th January 2021, 11:25 PM   #489
nashbap is offline nashbap  United States
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Ale, I suppose there is no issue changing the value of R3 in the divider to get an output gain of ones choice keeping R2 at 100k. For example, R3 at 100k should yield gain of 9db, roughly half of the otherwise 18db.
Thanks. nash
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Old 14th January 2021, 05:18 PM   #490
mogliaa is offline mogliaa  United Kingdom
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Yes, the higher R3 the better to provide signal headroom to the Source Follower as the circuit is DC-coupled and we are looking to avoid extra complexity.

If R3 is lower, there is a way to make it work by adding a PMOS to uplift the tail of R3 to get a higher DC level to allow the SF to operate well when swinging larger volts. However, is worth looking at this in detail as may not be needed if we are using the stage as buffer or pre-amp of the common signal levels (e.g. 2V).

Other than that, I think is a great solution for using a DHT and provide a low gain stage with a great performing (and sounding) stage which is a mu-follower. A similar approach is taken with a SUT at the input and then a step down transformer as anode load. This way you can get the best out of the DHT stage whilst reducing the impact of the microphonic noise etc. This is something I discussed with my friend DHTRob who led me this path and played with the UX-120, which is a lovely sounding valve but greatly microphonic.

Enjoy it as much as I do!
Cheers
Ale
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