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Any reason I have not seen a fixed bias phono stage???

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Posted on December 9, 2014 at 08:03:04
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Not battery bias, which I have played with, but a good old voltage divider with a pot or two so you can change the bias on the fly.

Thinking, besides the extra hardware, you could really nail the bias down and get rid of cathode resistor and cap.

Is this another example of status quo or is the a problem in a phono stage I am missing? Now that it is transformer interstaged I can inject in straight in the transformer like the rest of my amps.

Thanks Tom

 

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RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 09:03:56
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 Depending on the design, fixed bias on the first stage could create problems when it results in bias voltage applied to the coil of the cartridge, or the secondary of any step-up.

Other then that, I'm always in favour of fixed bias. Indeed eliminating at least a resistor. Bypass caps I don't ever use. Fixed bias also has the benefit of not raising Zout of the stage, which could be beneficial when using LCR's.

I moved from battery bias to filament bias in my phono stage. Yes, it introduces noise, but results are fabulous. I might try using Schottky diodes in the cathode once. If this results in the same sonic results while loosing the noise, then that would be great.

NC

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 09:40:27
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 The issue probably has to do with the need to provide an extremely clean DC voltage. A manufacturer would be hard pressed to justify the cost of the additional PS compared to a simple cathode bypass cap. For DIY, it's possible to do, but then a coupling cap has to be inserted to protect the cartridge. You might consider the 5755 for the first stage. It's a zero bias triode, so it doesn't require either a bypass cap (the cathode is grounded) or a coupling cap. I haven't heard this tube yet, but it's the next thing I want to try for a phono pre.

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 10:11:55
drsx
Audiophile

Posts: 369
Location: around
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Zeners can be pretty noisy. Try a LM4040 instead. Or, even better, use a LM4040 to bias a PNP or P-FET as the shunting element (like here although we used an LED).

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 10:16:04
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 A schottkey diode should work great.

 

Its impractical?, posted on December 9, 2014 at 10:25:36
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 If a voltage exists at the grid of the tube, it has to be blocked from the cartridge to prevent damage to the coil or magnetic field of the cartridge.

Coupling caps have their uses, but generally minimizing their use is often seen in a good light.

Of course a bypass cap is a coupling cap too, but there are a lot of design variables here so its uncertain if one is even needed.

For example, if you are running the EQ in the feedback loop, you won't want a bypass cap.

I don't use bypass caps in my circuits as they are fully differential so the cathode circuits get a CCS instead.

Since you are running transformers using differential amplifiers would be a good move as you could reduce noise even further...

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 10:51:38
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 A variation on the battery bias theme might work well. Take the 6SC7, with its common cathode connection. Place a small NiMH cell or 2 in the line between the cathodes and ground. Use 1 triode of the twin in each channel. Without a cap., there is no DC offset in the grid circuit to create problems with a cart. or SUT.

Charge the NIMH part(s) before use. The 2 mA. or so trickle of regular use will keep things in order.

BTW, I like grid leak biased 2nd gain blocks. As pointed out by Thorsten Loesch, the huge (20 Mohm) grid to ground resistor VERY lightly loads the EQ network.


Eli D.

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 11:14:32
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 So I should be able to fix bias the second and third tube because we do not have to worry about the cart or sut?

Thanks Tom

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 11:23:24
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 As long as you avoid bias on the cartridge it should be fine. Bias on the SUT might be acceptable.

The bias supply needs to be VERY quiet though.

NC

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 11:24:52
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Interstages like low rp tubes but thanks for the idea. Maybe a non interstage phono in the future. The great thing about doing this as a hobby is we can go so far out of the box. I do not have to sell it to anyone, just enjoy the sound that on the albums.

Thanks Tom

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 12:41:14
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 "Interstages like low rp tubes but thanks for the idea."

A little "sand" will allow you to have your cake and eat it too. The ZVN0545A enhancement mode MOSFET exhibits tiny capacitances. Even WIMPY high RP/low gm types, like the 12AX7, easily drive the little FET. DC couple (a complete "no brainer") ZVN0545A source followers to the wimpy triodes and your interstage trafos will be fed by a low impedance source.


Eli D.

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 13:34:43
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Absolutely
Also the ZVN0545A has no effect on the sonics being a unity gain part.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Counterpoint SA-5, posted on December 9, 2014 at 18:00:41
garymuffley
Dealer

Posts: 445
Location: Maryland
Joined: March 29, 2006
 The Counterpoint SA-5 used adjustable fixed bias in the phono and line stages.

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 9, 2014 at 22:31:42
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 For all of the reasons given by other inmates plus one big one, sound quality.

I'm sure I've tried most things there is to try in phono stage design and IMHO you can't beat Schottky diodes for bias. Makes life simple because you don't have to bypass and they sound great!

SiC Schottky will give you approx 0.8V and standard is more like 0.2V at the sort of current one might typically use. Stringing them in series does not seem to compromise sound so you should be able to get the voltage you need quite easily.

Naz

 

Do you recommend a specific manufac part number?, posted on December 10, 2014 at 07:45:50
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 I can find many CREE 15A TO220 packages but no small SIC schotky diodes. What are you using? Thanks, M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 10, 2014 at 13:30:33
Vincin
Audiophile

Posts: 200
Joined: October 14, 2005
 hi, is there a minimum DC current required to pass through these diodes to keep them linear. Any issue with only 1-2ma?

 

Cree CSD01060, posted on December 10, 2014 at 13:49:31
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 TO220 or TO252 packs. 4A, 600V at 25 deg C.

A really good sounding and robust Schottky.

Naz

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 10, 2014 at 13:56:49
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 The published data is difficult to read at very low current but IME their knee is quite sharp and dynamic resistance quite low. I have used them down to 0.8mA with no noticeable difference compared to 10-15mA other than a very small forward voltage drop reduction.

Naz

 

RE: Cree CSD01060, posted on December 10, 2014 at 17:42:13
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Hey Naz I have some csd04060's look really close on data sheet but I do not know what I am looking for. What do you think, the tubes are at 8ma and up?

Thanks Tom

 

RE: Cree CSD01060, posted on December 10, 2014 at 17:51:32
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 Hey Tom,

At a glance they are almost the same and will be fine. In fact any Cree will most likely exhibit very similar characteristics and will sound great.

Best part about them is they have virtually no sonic signature of their own ... nothing gets lost from top to bottom and nothing is added. Can't beat them for Phono stages IMO and I've done some fairly extensive testing in this department.

Naz

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 11, 2014 at 11:21:19
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 Technically speaking, a good LED biasing a triode is considered fixed bias, as the bias voltage won't change noticeably with varying current draw.

If you make a negative voltage supply, you'll still have an electrolytic cap in the signal path.

The notion of changing the bias voltage on the fly ought to have very limited use in such a circuit, as bias voltages and load resistances will have to be very carefully chosen for the circuit to work properly.

 

Electrolytic?, posted on December 12, 2014 at 01:54:43
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 If you use fixed bias, its good practise to add a high quality film cap to ground as (bypass) close to the tube.This avoids the a AC path via the last cap of the bias pus which is often located physically far away.

Also, make sure the DC path connects (via the pot) to ground close to the tube to make the current as small as possible. You can also add a small rf choke.

Like with all things audio, fixed bias can be implemented poorly or very good.

NC

 

Thanks, What is wrong with a 1n5818, posted on December 13, 2014 at 07:32:09
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 Have you tried a smaller schotky? Did you compare it with a LED? Thanks again. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Thanks, What is wrong with a 1n5818, posted on December 13, 2014 at 15:03:31
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 Tried a number of crees and and I couldn't pick a difference. Even tried 5817 with only 0.2V drop but equally good sound. All Schottky diodes sounded much better than standard silicon diodes for some reason and also better than the LEDs I tried though there was some variability in these that I didn't hear in Schottky.

If you try please let us know what you think.

Naz

 

RE: Thanks, What is wrong with a 1n5818, posted on December 13, 2014 at 15:31:29
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 Thanks NAZ! I sure will. Waiting for parts. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 13, 2014 at 21:30:21
Vincin
Audiophile

Posts: 200
Joined: October 14, 2005
 Hello,
Just to report the result of the Cree diodes mod. I tried them in the phono, DAC buffer, driver stage in power amps in place of the Lithium battery grid bias. The range of current is from 2ma to 12ma.
The diodes are staying! they also make life easy in term of wiring and connection for the cathode circuit.
Thanks.

 

RE: Any reason I have not seen a fixed bias phono stage???, posted on December 14, 2014 at 01:51:46
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 Excellent.

I also used battery bias before Schottky. I preferred Grid battery bias with Lithium to Cathode with rechargeables but (as you said), Schottkies really made life simple and do not lose out in any way.

Naz

 

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