Pass B1

Audiolab 7000 Series - Now available at HiFiMART.com
Hi Sachin,

What will be the output impedance of the B1 pre? Sorry if I missed this info somewhere in the thread!

Thanks,
APK
 
apk said:
Hi Sachin,

What will be the output impedance of the B1 pre? Sorry if I missed this info somewhere in the thread!

Thanks,
APK
Click to expand...

Hi Anand,
B1 output impedance is 50Ohm to 1K(very low).Its a high quality buffer with volume control can be matched with anything.In simple words its an impedance matching device.

"If you put a buffer in front of a volume control, the controls 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.
The thing here is to try to make a buffer that is very neutral. Given the
simple task, its pretty easy to construct simple buffers with very low
distortion and noise and very wide bandwidth, all without negative
feedback." - Nelson Pass

http://www.passdiy.com/pdf/B1 Buffer Preamp.pdf

Regards,
Sachin
 
sachu888 said:
Hi Anand,
B1 output impedance is 50Ohm to 1K(very low).Its a high quality buffer with volume control can be matched with anything.In simple words its an impedance matching device.
Click to expand...

Thanks Sachin.

My power amp has a slightly lowish input impedance of 22K ohms, and hence I was wondering if the B1 pre will be a good match. Seems from your post that there should not be any concern from an impedance matching point.

Regards.
 
sachu888 said:
Hi Anand,
B1 output impedance is 50Ohm to 1K(very low).Its a high quality buffer with volume control can be matched with anything.In simple words its an impedance matching device.

"If you put a buffer in front of a volume control, the controls 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.
The thing here is to try to make a buffer that is very neutral. Given the
simple task, its pretty easy to construct simple buffers with very low
distortion and noise and very wide bandwidth, all without negative
feedback." - Nelson Pass

http://www.passdiy.com/pdf/B1 Buffer Preamp.pdf

Regards,
Sachin
Click to expand...

And guys this is what rocked into recent Pune HFV meet. So read there howit sounded. :D it was mine DIY double decker pass b1 before and after volume control.
 
omishra said:
And guys this is what rocked into recent Pune HFV meet. So read there howit sounded. :D it was mine DIY double decker pass b1 before and after volume control.
Click to expand...

Hi Om,

Any particular reason that you went for 2 B1s stacked together? Of all the possible B1 combinations, which one do you like the best?

- B1 only
- B1+LDR
- B1+B1
- B1+SSP

Am sitting on the fence now.....trying to make up my mind. Also trying to figure out which is the best possible combination of the B1.

Thanks,
APK
 
apk said:
Hi Om,

Any particular reason that you went for 2 B1s stacked together? Of all the possible B1 combinations, which one do you like the best?

- B1 only
- B1+LDR
- B1+B1
- B1+SSP

Am sitting on the fence now.....trying to make up my mind. Also trying to figure out which is the best possible combination of the B1.

Thanks,
APK
Click to expand...
SSP sound is clean but OPAMPs change it. Please note that SSP is amplifying with little gain, then OPAMPs harmonic character added to output which further amplified by power amp.

B1 is equally clean but transparent mean no change in signal, no harmonic added as its not amplifying anything.

Now why 2 buffers?
Consider source-> volume control-> power amp.

Your source makes electric current through volume control from which you select a portion for power amp. Volume control is connected to power amp then power amp sees variable impedance of volume control. Thus it can not get max required current and mismatch has it added noise and distortion. [Imagine voltage dip in house mains when you added current sucking load.]

Now consider, Source-> buffer1 -> volume control -> buffer2 -> poweramp

Buffer1 poses large impedance thus very lower current drawn from source, means it not loaded. now buffer drives current in volume control

Buffer2 after volume take changing impedance of volume control and gives constant low output impedance. This means maximum drivability or current available for power amp. This volume control changes does not change impedance for source as well as power amp. Both see constant impedance irrespective of volume level.

So,
Source-> buffer1 -> volume control -> buffer2 -> poweramp
made best combination. First I tried with SSP then I chose b1 as both the buffers. Even two SSPs could be used but within limits of OPAMPs.
 
Last edited:
Sachin, I would like to back out as I might not be able to spare time for this project :(. Hopefully will be part of future batch.
 
omishra said:
Now why 2 buffers?
Consider source-> volume control-> power amp.

Your source makes electric current through volume control from which you select a portion for power amp. Volume control is connected to power amp then power amp sees variable impedance of volume control. Thus it can not get max required current and mismatch has it added noise and distortion. [Imagine voltage dip in house mains when you added current sucking load.]

Now consider, Source-> buffer1 -> volume control -> buffer2 -> poweramp

Buffer1 poses large impedance thus very lower current drawn from source, means it not loaded. now buffer drives current in volume control

Buffer2 after volume take changing impedance of volume control and gives constant low output impedance. This means maximum drivability or current available for power amp. This volume control changes does not change impedance for source as well as power amp. Both see constant impedance irrespective of volume level.

So,
Source-> buffer1 -> volume control -> buffer2 -> poweramp
made best combination. First I tried with SSP then I chose b1 as both the buffers. Even two SSPs could be used but within limits of OPAMPs.
Click to expand...

Thanks Om for the explanation. Sounds very interesting. Just one question though......if you use the following configuration:
source -> volume control -> buffer -> power amp, then doesn't the buffer convert the high impedance from the volume control to low impedance and feed the power amp. Wouldn't this acheive the objective of feeding a low impedance to the power amp. So why do we need 2 buffers, one before the volume control and one after?

Thanks,
APK
 
apk said:
Thanks Om for the explanation. Sounds very interesting. Just one question though......if you use the following configuration:
source -> volume control -> buffer -> power amp, then doesn't the buffer convert the high impedance from the volume control to low impedance and feed the power amp. Wouldn't this acheive the objective of feeding a low impedance to the power amp. So why do we need 2 buffers, one before the volume control and one after?
Click to expand...

See the bold marking. It answers your question. I used LDR based volume control which is lower 13K ohm, normally for volume control it should be 25K-50K ohms. I wanted benefit of LDR but same time wanted to hide its impedance behind buffer1. Source could see only buffer and not the volume control.

So if your volume control is >= 25K ohms impedance then you can avoid source buffer.

I answered another FM below question.
Q. I have no clue about what mono/stereo pot/LDR etc means. How does it impact me?
[OM] Those are different volume controls. For 2 channel single volume control operated with single shaft is stereo pot. Separate volume control is called dual mono, you need to adjust left- right individually and separately. LDR based volume control is voltage controlled volume control LDR changes volume while with the knob you change only control voltage. You can start with dual mono and later jump to LDR. But LDR based volume control has low impedance which many source does not operate, and you need another buffer before it.
Click to expand...
 
Last edited:
@Sachu888, just sent you PM.

Pls add me to the GB for Pass B1. I want the stereo pot option.

- Ravi
 
Hi,
Final list of participants for B1 Gb

1.TheCoolestOne
2.Sann
3.Svaze (+extra Power supply kit)
4.ruenigma
5.denom
6.Murli N.
7.Santy
8.Atharva
9.NGD
10.Hemantwaghe
11.Vikoma
12.apk
13.keith_correa

Please reply if I missed someone.Gb is only for Pass B1 and Power supply kit,no hardware (Case,RCA,wiring or any kind of switch) will be provided.Participants also have to manage a 18-0-18 transformer.
Omishra can arrange R-core transformers.
Note: B1 kit will have a good quality Stereo Pot.
Regards,
Sachin
 
Last edited:
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