B1 Buffer Preamp

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A

AndrewT

R.I.P.
Joined 2004
spiri said:
But only if distortion does not matter.
A peak signal current about 10% off the IDSS is acceptable, then the distortion increases.
Click to expand...
Of course distortion matters.
All SE amplifiers (without global NFB) distort if driven to near clipping.
That's why I strive for the Source to have a clipping voltage at least 10dB above the maximum input signal to the power amplifier.
A B1 running on 18Vdc has a maximum unclipped voltage of 6Vac.
A power amplifier with a maximum power sensitivity of 1.5Vac will leave the B1 with an unused 12dB of overhead.
i.e at absolute maximum output from the power amplifier the B1 is passing a 1.5Vac signal.
The current passing the B1 is a completely different story. It depends on the impedance of the cables and load/s connected to the B1.

If the Idss of the B1 jFets were 8mA and the bias of the jFets were 7.9mA (since the voltage is 9Vds, not 10Vds) then the maximum unclipped output current capability will be around 7.8mA.

1.5Vac (2121mVpk) into a 20k load resistance is 0.1mApk output current into the non reactive element of the load.
Allow 1mA of peak current for the capacitive elements of the load gives an absolute maximum output current of 1.1mA.
That is 14% of the B1 current capability.

Music does NOT stay at absolute maximum peak transient power all the time.
Replayed at -20dB relative maximum power and the voltages (and currents) will be one tenth of the maximum values.

That takes the output current from the B1 down to 1% of capability as an average for normal music listening.

Now that I have explained the B1 operational currents and how they relate to bias current what does your comment mean?
A peak signal current about 10% off the IDSS is acceptable, then the distortion increases.
Click to expand...
 
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E

exwiso

Member
Joined 2012
omishra said:
Output of Pass B1 can drive any power amp (IMO). You need not worry driving power amp of input impedance 100KΩ.
You can connect most sources to Pass B1 buffer while you use volume control pot of 25KΩ above.
Click to expand...

Power was not my concern. The output of the CDP is such that I reckon you could put a brick in the B1 circuit and it could still deliver.

My dilemma is purely focused on sound quality. Many people on this thread have said that as this is such a simple circuit with so few components, the quality of the component and/or matching can make an appreciable difference. This sounds reasonable to me. Joy!

Back to the attenuator. Found a (very) Useful Information page at Goldpiont

Detailed Information
This seems to say 50K into valves and 25K into SS... but 25K is good for both and is a good choice for a passive pre. Add in the original design value was for 25K and I guess it's 3 to 1 for 25K. (+1 wintermute above).

Then again they start out by saying that for a 100K input impedance, use a 100K, 50K, 25K, or even a 10K level control and that the rule of thumb is to match the input of the power unit, which in this case is 100K.

I am thinking I might just get some cheap China pots say 25, 50, 100K of the same make and see if one sounds better than the others. Might work, might not; and if not, no major damage done.

David
 
wintermute

wintermute

Just another Moderator
Joined 2003
Paid Member
Hi David,

I may be wrong here, but I think you are worrying about something you need not :)

Yes if you are using a passive preamp, you will need to consider your amps requirements impedance wise (with respect to the pot).

However the point of the buffer (B1) is to buffer the amp from the volume control/source. So I think you can forget about what the amp requires from a volume pot point of view.

Using a 5W resistor instead of a 3W resistor is actually a good idea (provided there are no space issues) as it will generally run cooler.

Tony.
 
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E

exwiso

Member
Joined 2012
Hi Tony
This is all good news. Like I said at the outset, I've never tried anything like this before. Plug and play has been the limit to my game.

So maybe I need to check that I really understand what I am doing here

1. I need a volume control between source and power amp.

2. I took the B1 to be like something between a passive and normal pre with the best of both and none of the bad

3. I think what you are telling me is that the B1 is a volume control with a buffer between it and the amp so that the properties of the volume control, but not the results, are isolated from the amp.

David
 
wintermute

wintermute

Just another Moderator
Joined 2003
Paid Member
Yes that is pretty much it! I'm not sure if you have read Nelsons article on the B1 or not, but it relates the musings of some people that the effect of a passive pre (ie pretty much an attenuator between the source and the amp) causies suckage... ie it has a tendency to suck some of the life out of the music...

Towards the end of the article he declares "no suckage here" ;)

It is available here --> https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp

an excerpt below:

Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.
What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.
I suppose if I had to floor the accelerator to drive 55 mph, maybe I’d think the life was being sucked out of my driving. Then again, maybe I like 55. Nice and safe, good gas mileage…
Is impedance matching an issue? Passive volume controls do have to make a trade-off between input impedance and output impedance. If the input impedance is high, making the input to the volume control easy for the source to drive, then the output impedance is also high, possibly creating difficulty with the input impedance of the power amplifier. And vice versa: If your amplifier prefers low source impedance, then your signal source might have to look at low impedance in the volume control.
This suggests the possibility of using a high quality buffer in conjunction with a volume control. A buffer is still an active circuit using tubes or
transistors, but it has no voltage gain – it only interposes itself to make a low impedance into a high impedance, or vice versa.
If you put a buffer in front of a volume control, the control’s low impedance looks like high impedance. If you put a buffer after a volume control, it makes the output impedance much lower. You can put buffers before and after a volume control if you want.
Click to expand...


Tony.
 
cwtim01

cwtim01

Member
Joined 2012
2SK170 substitutes

Hi,

Just spent 3 weeks to read this entire thread and would like to try building a B1, and probably a DCB1 later also. However, I found it extremely difficult to find real 2SK170 (I got 40 from an online source, but turns out they're all fakes as the Idss are all over the places) so I would like to know if the following parts are suitable replacements:

2SK932-22 (Idss 7.3-12mA, Pd 200mW, NF 1dB, Yfs 25/50/-, Vgds -15V, Vgs_off -0.2/-0.6/-1.4)
http://www.onsemi.com/pub_link/Collateral/EN2841-D.PDF

BF862 (Idss 10-25mA, Pd 300mW, NF 0.8nV/sqrt(Hz), Yfs 35/45/-, Vgds -20V, Vgs_off -0.3/-0.8/-1.2)
http://www.nxp.com/documents/data_sheet/BF862.pdf

Both are SMT parts, but I can find SOT-23 to through hole adapter so this is not a problem.

Thanks and regards.
 
A

AndrewT

R.I.P.
Joined 2004
bf862 is smd.
The parameters are a good substitute for the k170.
They run @ Idss with 10Vds so will be warm. Might be good to add a copper or aluminium flag to the case and keep big trace areas to cool the lead outs.

two channels of the B1 needs 4 off k170 (either lsk170 or 2sk170)
two channels of the DCB1 needs 6 off k170 (either lsk170 or 2sk170)
The other transistors in the DCB1 are BJT, or do not need to be low Vp, high gm, devices.

I wonder if the DCB1 would be better at +-8Vdc, when using the SMD BF862 to help reduce temperatures?
If so, the BF862 version could run on 12+12Vac transformer.
 
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bogdan_borko

bogdan_borko

Member
Joined 2005
wintermute said:
22K is the recommended maximum from Salas for the DCB1 I'm going to use a 25K pot, as I thought that was the upper limit I had seen posted at some point. 20K sounds like a good choice :)

I'm currently doing a verro board DCB1. Hopefully won't be too long before it is finished!

Tony.
Click to expand...

If you`re already doing it on verro board, try floathing cascode on the the upper jfet. It clearly sounds better.;)
 
cwtim01

cwtim01

Member
Joined 2012
AndrewT said:
bf862 is smd.
I wonder if the DCB1 would be better at +-8Vdc, when using the SMD BF862 to help reduce temperatures?
If so, the BF862 version could run on 12+12Vac transformer.
Click to expand...

Sky63 said:
What about this:

8 pieces of LSK170 ULTRA LOW NOISE SINGLE N-CHANNEL JFET, $15.76
Click to expand...

Thanks both. LSK170 is indeed an option, but the cost of 8 pieces of LSK + shipment is probably enough to get 50 pieces of BF862 locally so I'll try the latter. Hopefully I can find a few matched pair towards the 10mA side, otherwise will try lowering the DC voltage as suggested by Andrew.
 
D

didiet78

Member
Joined 2007
KatieandDad

KatieandDad

Member
Joined 2011
AndrewT said:
bf862 is smd.
The parameters are a good substitute for the k170.
They run @ Idss with 10Vds so will be warm. Might be good to add a copper or aluminium flag to the case and keep big trace areas to cool the lead outs.

two channels of the B1 needs 4 off k170 (either lsk170 or 2sk170)
two channels of the DCB1 needs 6 off k170 (either lsk170 or 2sk170)
The other transistors in the DCB1 are BJT, or do not need to be low Vp, high gm, devices.

I wonder if the DCB1 would be better at +-8Vdc, when using the SMD BF862 to help reduce temperatures?
If so, the BF862 version could run on 12+12Vac transformer.
Click to expand...

I've never noticed the JFETs having a temperature problem.
 
cwtim01

cwtim01

Member
Joined 2012
Dennis Hui said:
cwtim01,

You should still be able to get some matched jfets from Spencer (FET Audio | Hi-End Audio Projects) and I think he's local to you.

Cheers,
Dennis
Click to expand...

Thanks Dennis, from the FET Audio site it seems Spencer is now in Singapore, though I'm not sure.

Anyway, for around the same price of 1 matched pair of 2SK170BL from Spencer, I just order 100 pieces of BF862 (plus adapters below). :D

12882861743_c3f1693e31_o.jpg
 
cwtim01

cwtim01

Member
Joined 2012
didiet78 said:
If you going to use BF862, try Juma version
http://www.diyaudio.com/forums/pass-labs/146310-bf862-preamp.html

Didiet
Click to expand...

Thanks Didier for the link, for the past 3 weeks I've been making quite some effort to understand how the B1 and DCB1 works, so will try to build them first.

The Juma version also seems very interesting, and the thread even contains the Gerber files so it's pretty easy to print a PCB (it quite cheap to print a PCB in China and ship to HK), so I might try that out later.
 
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