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B1 Buffer Preamp
B1 Buffer Preamp
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Old 28th February 2014, 01:26 AM   #3931
exwiso is offline exwiso  Australia
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Quote:
Originally Posted by omishra View Post
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.
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
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Old 28th February 2014, 01:34 AM   #3932
exwiso is offline exwiso  Australia
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Oh and another thing...

Can I use 1 Ohm, 5 Watt in R1?

The original schematic gives 1 ohm, 3 watts.

David
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Old 28th February 2014, 01:48 AM   #3933
wintermute is online now wintermute  Australia
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B1 Buffer Preamp
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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Last edited by wintermute; 28th February 2014 at 01:49 AM. Reason: add with respect to the pot.
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Old 28th February 2014, 03:04 AM   #3934
exwiso is offline exwiso  Australia
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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
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Old 28th February 2014, 08:05 AM   #3935
wintermute is online now wintermute  Australia
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B1 Buffer Preamp
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/prea...-buffer-preamp

an excerpt below:

Quote:
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.

Tony.
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Old 2nd March 2014, 10:28 AM   #3936
cwtim01 is online now cwtim01  Hong Kong
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Default 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.
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Old 2nd March 2014, 11:22 AM   #3937
AndrewT is offline AndrewT  Scotland
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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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Last edited by AndrewT; 2nd March 2014 at 11:49 AM.
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Old 2nd March 2014, 12:08 PM   #3938
bogdan_borko is offline bogdan_borko  Serbia
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Quote:
Originally Posted by wintermute View Post
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.
If you`re already doing it on verro board, try floathing cascode on the the upper jfet. It clearly sounds better.
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Old 2nd March 2014, 12:55 PM   #3939
Sky63 is offline Sky63  Czech Republic
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What about this:

8 pieces of LSK170 ULTRA LOW NOISE SINGLE N-CHANNEL JFET, $15.76
8 pieces of LSK170 ULTRA LOW NOISE SINGLE N-CHANNEL JFET
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Old 2nd March 2014, 02:26 PM   #3940
cwtim01 is online now cwtim01  Hong Kong
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Quote:
Originally Posted by AndrewT View Post
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.
Quote:
Originally Posted by Sky63 View Post
What about this:

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