B1 Buffer Preamp

[johnm]

I know it does seem unlikely - I have read about LM317s being faked though on other electronics sites.

Perhaps I just was unfortunate and there was a faulty batch or something. Next time I'll be sure to order via Farnell or RS and not risk Ebay.

[sachu888]

Hi,
I am using Pass B1 with LDR Volume control.I must say that B1's performance is quite impressive. Most important observation is the quietness of the system even at high volume. Soundstage improved a lot. Instruments became more defined & very very clear.
I have noticed the bass sounded very thin with B1 using LDR volume control.The Bass-power was reduced drastically.I get a better bass response If I put my LDR volume control at max position and control volume from my Technics integrated amp.I would like to ask other FM using B1 regarding their experience.
My system: TT--> DIY Phono--> B1--> Integrated amp

Regards,
Sachin

[tinitus]

Originally Posted by sachu888

.....I get a better bass response If I put my LDR volume control at max position.....

maybe its what gives you lowest possible impedance

what input impedance do you have on your B1

[sachu888]

Originally Posted by tinitus

maybe its what gives you lowest possible impedance

what input impedance do you have on your B1

Thanks,but I don't know the input impedance.Can you tell me how to measure it?

Regards,
Sachin

[KatieandDad]

What is the impedance of your LDR volume control ?

The input impedance of the B1 is VERY high - predominantly the 1M0 resistor located at its input.

[sachu888]

[QUOTE=KatieandDad;3024183]What is the impedance of your LDR volume control ?

13k

Regards,
Sachin

[KatieandDad]

That's too low. It will be acting as a Low pass filter with the output cap of your pre-amp.

As an experiment try using a cheapie 25K - 50K Log pot to see if you get the same effect.

The B1 will work well with the pre-in connected directly to the B1 and the LDR attenuating its output. However you will get similar but different (that sounds alien) results because the 13K LDR will act then as a low pass filter with the 10uF output of the B1. Not nearly as noticeable though.

[sachu888]

What if I put LDr to output stage of B1 as Mr Pass suggested here http://www.dms-audio.com/system/files/B1-Preamp.pdf
"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." -NP
Does it make any difference.

Regards,
Sachin

[KatieandDad]

As long as the pre-in isn't overdriving the B1 the LDR will work fine at the output as I mentioned above.

One advantage of having the volume at the input is that all the input, SIGNAL and NOISE is attenuated by the volume control.

The B1, correctly implemented is silent, it will only amplify what is at its input.

[tinitus]

makes very little sense to place the att after a buffer, unless you have a buffer on both sides of the att

I know some of the earlier pass diy pream design had att's both in front and after
but also using special low impedance att (5k) on output
and power amp designed to match, etc etc

btw, Nelson did post a B1/LDR design

well, I think you can build with double sk170, with small changes (sj74 may be harder to find)