B1 Buffer Preamp

[rhysh]

Originally posted by marc brown


Ahhh, that makes sense. Were you thinking of putting the B1 board in the same chassis as your current preamp? I've started avoiding interconnects in my system where it's feasible. They're just another variable and expense in your system's sound.

No, i was going to use a diffrent chassis, hence the reason for the B1 in the first place. I didnt want to run a 50k signal over external interconnects.

[AndrewT]

The B1 buffers go at the source ends not the receiver ends.

[rhysh]

Originally posted by AndrewT
The B1 buffers go at the source ends not the receiver ends.

Indeed it does.

[rob lenk]

why
if its function is to buffer- high input impedance and relatively low output impedance - voltage transfer from the sources and mot modulated by the imput of the amp
inquiring minds wanna know
r

[pinkfloyd4ever]

there may be some confusion here?

I was told it should go like this:

source--> attenuator--> B1--> amp

[davidfmartin]

All,

I have been working on a B1 BOM while I wait for my board to arrive from passlab. I am a noob at this so please be gentle if I made mistakes. I have been sourcing parts mostly from Mouser and Digikey but welcome all suggestions from other vendors and will add them.

http://spreadsheets.google.com/ccc?k...etpEM1-8JYImCQ

Thanks!

[AndrewT]

Originally posted by rob lenk
why
if its function is to buffer- high input impedance and relatively low output impedance - voltage transfer from the sources and mot modulated by the input of the amp
inquiring minds wanna know
r

Most power amplifiers are designed to require a fairly low source impedance to perform at it's best.
Most source equipment is designed to send a good quality signal when the load impedance is high.

Inserting an interconnect between typical units causes conflict and adding a pot/attenuator makes this conflict even worse.

What we require is a source with a low output impedance, 10r to 500r is OK.
The power amps should have an input impedance in the range 20k to 100k.
Insert an attenuator and cables between the two upsets all this carefully arranged impedance matching, by adding capacitance and mismatching the source impedance.
Add a buffer after the pot and normality is restored.
But, where?
Low impedances with good output current capability can drive a power amp's high impedance input in parallel with the interconnect cable capacitance. A pot cannot do this. In steps the buffer to help out.
Fit the attenuator where it is convenient. Add a buffer to drive the cables and present the low source impedance to the power amps. Place the power amps beside the speakers. Ensure the source can drive the pot impedance in parallel with the cable capacitance. If not then add a further buffer to the source.

The buffer goes in the source not at the receiver.

[BFNY]

Originally posted by noelectrix
Another Pot/B1 related question.

For some reason I want to have the buffer in front of the Pot. Can I just leave out the pot, or do I have to substitute it with a 25k to 50 k fixed resistor between signal and gnd?
Im not shure and help is appreciated
stefan

I have built a version of the B1 where I used a 1M ohm resistor instead of the pot. It works fine and tests well.

[BFNY]

Originally posted by rhysh



But surley just becuase my volume control is in a seperate box, does not make a diffrence. The volume control would come before the preamp even if it was located internally.

Yes, why have you not put a volume control on the input to your existing preamp? I have never seen a preamp / linestage without any volume control as part of the design. Unless it is a dedicated low level phono preamp. How are you controlling the volume now? Is the volume with no pot about right for the loudest you listen, or is it WAY too loud?


if the the former condition, you will normally be listening with the pot in front at full volume, which eliminates the concept of large attenuation followed by large gain (which you want to avoid)

If so, the question, then, is whether you need the zero gain active buffering between the volume pot (that the b1 provides) and the existing preamp gain stage.

To answer this, you need to provide the nitty gritty details of the exisiting existing preamp input impedance, and length and type of cables proposed between B1 and preamp. Cables have typical capacitance of 30-80pf/foot.

[rhysh]

I have not put a volume control on the input, becuase i want to get the highest quality volume control possible for a certain budget, without confining it to one component. The preamp is still in the making, i am currently using somthing else as a preamp with a RK27 on the input. Not good enough.

I will be only using about 1ft of cable between the B1 and my preamp.

I can scrap the B1 all together, and just use the volume control but then i will be sending 50K over an interconnect, which is not a good idea...

Originally posted by BFNY


Yes, why have you not put a volume control on the input to your existing preamp? I have never seen a preamp / linestage without any volume control as part of the design. Unless it is a dedicated low level phono preamp. How are you controlling the volume now? Is the volume with no pot about right for the loudest you listen, or is it WAY too loud?


if the the former condition, you will normally be listening with the pot in front at full volume, which eliminates the concept of large attenuation followed by large gain (which you want to avoid)

If so, the question, then, is whether you need the zero gain active buffering between the volume pot (that the b1 provides) and the existing preamp gain stage.

To answer this, you need to provide the nitty gritty details of the exisiting existing preamp input impedance, and length and type of cables proposed between B1 and preamp. Cables have typical capacitance of 30-80pf/foot.