B1 Buffer Preamp
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analog_sa
diyAudio Member

Join Date: Aug 2002
Location: Cascais
Re: Input Impedence

Quote:
 Originally posted by Diomedian The effects I'd notice if input impedance went too low would be just more noise, is that right?

It's in fact the opposite. Assuming you are talking of source impedance in parallel with attenuator.

Diomedian
diyAudio Member

Join Date: Nov 2004
Location: Halifax, N.S.
Re: Re: Input Impedence

Quote:
 Originally posted by analog_sa It's in fact the opposite. Assuming you are talking of source impedance in parallel with attenuator.
Alrighty - either I'm asking the wrong questions, or the questions don't make any sense, or my assumptions are wrong, or... some combination. :-) Please treat me like I don't know anything, because this is a reasonable approximation of actual knowledge.

1) It is desirable for the buffer to have high input impedance? This makes it easier for the source to drive - yes?

2) The input impedance of the B1 is determined solely by the pot? This is because the source sees only a 25K resistor (the whole length of the pot)?

3) On the B1 side of the pot, it sees a 1K in series with a variable resistance (some part of the pot), and a variable resistance to ground (the rest of the pot) paralleled with output impedance of the source?

4) If I replace the pot with a 2K resistor, (that is to say the circuit now looks like input -> 1K -> 1uF -> 2sk170, and input -> 2K -> ground), the input impedance of the B1 is now 2K, is that correct?

5) Doing as in question 4) - this is the equivalent of a shunt-type attenutator set to min volume (assuming 2K is the smallest value used by the attenuator)? Er... or is that max volume?

6) Would a ladder-type attenuator be a better idea for this circuit since the shunt-type gives such a low input impedance?

7) Finally, is there a reason a linear taper pot was used, and not a log scale? Was it simply cheaper, or does this circuit for some reason require a linear pot?

Or... should I just stop asking all these silly (and possibly terribly insignficant) questions and just go test it for myself? hehe

Thanks in advance, and sorry for all the noise on this thread...
-d

 6th July 2008, 06:30 PM #173 analog_sa   diyAudio Member     Join Date: Aug 2002 Location: Cascais 1. Yes. 2. 25k in parallel with 1M. Still 25k. 3. Yes. 3. Yes 4. No. There is also a resistor in series with the input which generally determines the worst case scenario as seen from the source 5. As in 4. Max volume is determined by series resitor and max value of shunt; if shunt is open circuit there is no attenuation. If shunt is a pot attenuation is set between pot value and series resistor. Min volume is set at min value of shunt (0 ohm) 6. ??? 7. Log pot would work just as well Your questions are not silly. If you want to see silly i can point you to a few threads to compare Do you intend using a rotary switch or a pot for the shunt? If using a pot you are likely to lose some voltage gain at max setting across the shunt. You can calculate this depending upon the resitor values and see if it's acceptable. Most people use shunt controls as a good compromise between quality and cost - a really nice series resistor and a cheap pot sounds significantly better than a cheap pot on its own.
 6th July 2008, 07:17 PM #174 khundude   diyAudio Member   Join Date: Sep 2006 I think the "ladder" and "log" are the same, no? This is were a series of relays is used so that they are not in the chain.
juma
diyAudio Member

Join Date: Nov 2005
Location: Berlin
Quote:
 Originally posted by khundude I think the "ladder" and "log" are the same, no? This is were a series of relays is used so that they are not in the chain.
No, it's not the same, it's not even something you can directly compare.
"Ladder type attenuator" refers to topology, to way how the attenuator is built, ladder-like.
"Log" refers to attenuation characteristic, it represents mathematical function of change of resistance from potentiometer's/attenuator's wiper to ground and number of degrees when you turn the knob.
Example:
a) We have 10k log. potentiometer/attenuator . We turn the knob to 12 o'clock position. Resistance from wiper to ground is 500 Ohms.
b) We have 10k linear potentiometer/attenuator . We turn the knob to 12 o'clock position. Resistance from wiper to ground is 5k.

Oh boy, I wish I could put it clearly in less words
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Diomedian
diyAudio Member

Join Date: Nov 2004
Location: Halifax, N.S.
Quote:
 Originally posted by analog_sa 6. ??? Do you intend using a rotary switch or a pot for the shunt?
Thanks for all the replies analog_sa!

To rephrase #6, I was thinking that since using a shunt-type attenuator causes input impedance of the buffer to vary with the volume setting, would it be better to use a ladder-type attenuator (not in shunt mode) that doesn't cause the input impedance to vary?

If I use a shunt at all, it would be with an attenutator.

thanks again,
-d

 6th July 2008, 09:01 PM #177 supernet   diyAudio Member     Join Date: Dec 2001 Location: Europe Should I expect higher noise and THD figures, if I use LM 317 regulator with CRC filter for PSU instead of discrete one?
Zen Mod
Official Court Jester
diyAudio Member

Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Quote:
 Originally posted by supernet Should I expect higher noise and THD figures, if I use LM 317 regulator with CRC filter for PSU instead of discrete one?

use stacked 317s , with both heavy bleed and mucho uFs on output
( in that case with all necessary diodes for chip protection)
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supernet
diyAudio Member

Join Date: Dec 2001
Location: Europe
Quote:
 Originally posted by Zen Mod use stacked 317s , with both heavy bleed and mucho uFs on output ( in that case with all necessary diodes for chip protection)

OK.

I will use two separate regulators for each channel. I was thinking to use 2X4700 uF 35 V before LM 317 and 100 uF at the outout. For the buffer, I will probably use two 4700 uF caps / mono channel instead od 15000 uF.

Should this be OK?

Zen Mod
Official Court Jester
diyAudio Member

Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Quote:
 Originally posted by supernet OK. I will use two separate regulators for each channel. I was thinking to use 2X4700 uF 35 V before LM 317 and 100 uF at the outout. For the buffer, I will probably use two 4700 uF caps / mono channel instead od 15000 uF. Should this be OK?

when I wrote stacked , I meant as in attached schm . - cascaded 317 , with ref leg of input one connected to output , not to ground ;

that way input one is prereg for second 317.

heavy bleed - I meant - put resistor load on output of reg

in fact - you can pretty easily use exact schmtc
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