How to bias 2SK170BL in Boozehound Labs phono pre? - diyAudio
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Old 27th August 2013, 04:51 PM   #1
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Default How to bias 2SK170BL in Boozehound Labs phono pre?

I'm in the process of building a Boozehound Labs phono preamp (Boozhound Laboratories: JFET Phono Preamp Kit).

I have matched 2SK170BL fets that I'd be keen to use. I'm also keen to use separate 18V regs for each fet with a shared 24V psu.

Given the Boozehound Labs schematic below, and incorporating the following changes, how can I calculate the optimum values for R4 & R11?

- R6 & R7 replaced with 18V regs
- R2 = 47Kohm
- 2SK170BL with 9ma IDSS

Click the image to open in full size.

Many thanks in advance.
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Old 27th August 2013, 10:05 PM   #2
Itsmee is offline Itsmee  England
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I would keep R6 & R7 and only use one (shunt type) regulator to power the circuit, CMMR will be worse if you two regulators.

R2, 47K, no problem, you may also want to add a small value capacitor in parallel, check the loading instructions of your cartridge.

In the Boozhound Laboratories: JFET Phono Preamp Kit instructions, re-read the section titled 'options for getting more gain'; changing the value of R4 will alter the gain of the stage and the output impedance resulting in an incorrect RIAA equalisation.
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Old 27th August 2013, 10:18 PM   #3
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Thanks Itsmee.

I read somewhere that R4 & R11 should be changed if 2SK170BL fets are used instead of the supplied GR grade. I don't remember the rational and can't find the original link. I also lack the basic knowledge required to confirm this myself :-)

To look at it another way, will I be fine with the standard resistors if I use the BL grade fets?

Thanks for the reg recommendation. I assumed that a separate reg per fet would be best (probably naively).
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Old 28th August 2013, 08:18 AM   #4
Itsmee is offline Itsmee  England
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No problem using the BL grade, build and enjoy,
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Old 30th August 2013, 12:32 AM   #5
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I found the link that discussed circuit changes with BL grade fets:

[JN] Le Pacific type FET phono pres?

Thorsten Loesch wrote the following, commenting on 2SK170 based Hiraga Pacific phono preamps:

J-Fet's must be tightly selected (<0.1mA Idss difference), or you get
problems with balance, if your currents are much of the target 5mA you
need to change the Drain resistors or supply voltage.


The orginal design needs 5mA Idss Fet's, basically meaning you need GR
Grade Fet's, which are hard to find, you cannot directly use the
common "BL"Grade Fets.


Most "BL" Grade 2SK170 are around 8-9mA Idss and need to have reduced
drain resistors or increased Supply voltage. Shoot for 12-15V across
the J-FET. So with 8.5mA and 2.4K Drain load you need around 32V
Supply voltage, if using 24V the drain load needs to be dropped to
around 1.5k, which looses around 3dB gain per stage, compared to using
5mA Idss devices and 2.4K, so 6dB overall.


Given that I'll be using approx 9ma IDSS matched BL grade fets, the best solution appears to be to keep the resistors as per the Boozehound Labs scheme and increase PSU voltage to achieve the optimal voltage over the fets. This avoids changing the gain of each stage.

Is this the best way forward? I'll ensure all electrolytics are happy with >24V.
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Old 30th August 2013, 11:40 AM   #6
AndrewT is offline AndrewT  Scotland
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With the higher bias current and the higher Vds, the BL grade will operate at much higher temperatures than the GR grade.

How will you handle that?
__________________
regards Andrew T.
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Old 30th August 2013, 06:48 PM   #7
kcray is offline kcray  Indonesia
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Hello,

I simulated the circuit, the result is:
(Vpwr:24V, Rload=47K)
BL Grade: Idss=12mA, Id on both Fets is about 4.3mA and total gain is about 40 dB
GR Grade: Idss=5.5mA, Id on both Fets is about 2.4mA and total gain is about 36 dB

so don't worry too much, just build it and listen.

--

Last edited by kcray; 30th August 2013 at 06:54 PM.
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Old 20th October 2013, 01:46 PM   #8
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Thanks all for the advice. I've finally got the BHL preamp up and running, and it sounds great. I added a point-to-point B1 buffer to reduce output impedance (I'm using a Music First TVC preamp so needed to bring it down). One external 24V supply feeds two MC7818CT regulators (one for the BHL board, one for the B1). As I'm temporarily using a laptop sumps charger, I added a small RC filter upstream of the regs. Lots of Russian caps in there - K40y for the passive filter, Teflon for BHL coupling, silver mica and KBG pio in the B1. Much more dynamic and warmer than my old Clearaudio Smart Phono with a less aggressive (softer) top end. One record that wasn't listenable before (bad pressing of Massive Attack's Protection) due to excessive sibilance is now very tolerable. I've got a DL103R and Lundahl LL9206 transformers upstream of the BHL.

One thing I have been amazed at is how quiet the preamp is. I need to crank the MF volume control to hear background noise. At normal volumes I can't hear anything 1ft from my Tannoy LRMs.
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Old 20th October 2013, 01:49 PM   #9
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Old 27th October 2013, 06:50 PM   #10
Hikari1 is offline Hikari1  United States
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Are those big silver mica caps? I replaced the caps in my boozehound...Jantzen silvers on the output, REL for coupling between gain stages and a .1 Vishay 1837, a .033 Wima and a .001 Silver mica in the RIAA. Way different sound. Top end is totally different. Those K40y caps are extremely warm and rolled off in the top end. I can see how some would like that sound though. I am running mine off batteries, which is kind of a pain. I may try a regulated supply in the future.

I am interested in those big russian silver mica caps in large values. I'd like to try them in the RIAA section. Can you describe how they sound?
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