B1 Buffer Preamp

Re: Re: B1

stein2 said:


B1 is a buffer stage, no gain in signal. If you think but not sure if you need gain, take a single pot, add some coax with terminals at both ends and try with passive volume only between your source (CD for example) and power amp. If you get enough output from source to fully drive the power amp, B1 should do. If you can go with the single pot all the way and feel that the amp lacks output power, then, some gain in the control/preamp unit would help. In that case, you might consider NS10 with a proper, modern shunt PSU... like some of us did


I suppose I should have mentioned that the Goldpoint is a series stepped attenuator type passive "preamp". What I'd like to know is what the advanatges of an active buffer stage over just a good quality passive would be. I'm quite happy without an active preamp so far, but I'm not sure what I'm missing by not having a buffer. I was considering trying to squeeze the B1 into the Goldpoint by leaving the PS caps in the PS.

sparky
 
Official Court Jester
Joined 2003
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vaistinu konza!

juma said:



Ma jok, to je lokalna fora - Munze Konza (Zemun Zakon) ! Vaistinu Konza (Zakon) ! (tako se pozdravljaju i odpozdravljaju zemunci na satrovackom) :D
A dal'je tuzemstvo ili inozemstvo - ostaje da se vidi !:D

Anyway, I don't think that noise scheme will work here - it's quite different from Broskie's implementation in Aikido preamp. Furthermore, B1 doesn't need it - it has a very good intrinsic PSRR already. I know what would make you happy - nice, hot, glowing 6SN7 as WCF :D


me modestly ( as always) thinks that even B1 will benefit from that sort of noise injection , but - it will be audible , that's pretty questionable ;

interesting you're mentioning Broskie's site - even if it's certainly treasure , I always had difficulties reading it , just because I simply can't make mental map of site .... too many things in one place , I resume ;
hehe - I'm probably used to old , hard copy based , reading ; what's sometime limiting to me is fact that PC screen have just one page .....

"injecting noise" have many faces ..... ;) - first one , known to me , was in reading and constructing tube stab ( too back in time and space ) , where upper resistor ( in resistive divider , connected from output rail to ground , feeding error amp first grid) was deliberately bypassed with small cap ;
then I have thoughts that main reason is for decreasing gain of that stage in/on higher frequencies , but later I discovered beneficial influence of that cap even on lower frequencies ;

interesting - seems that all electronics is made of simple blocks and tricks ..... rare ones are capable to mix them in sorta unusual way ;

that's why this forum is known as Papaboard ........ not under any other name

:rofl:
 
Buffer is "buffering" between two stages thus allowing optimal impedances/loads to occur at output of the previous stage to the following stage. For example, a cathode follower has gain of 1 or less but if well made almost infinite input impedance and very small output impedance which allows a fairly long cable to be used from the preamp to the power amp and therefore making it easier for the preamp to deliver the right kind of signal to the next stage. Passive attenuators usually only create mess at the input of the power amp. For example, F5 has no capacitors at the input, only one 1k in series and a 100k in parallel, adding just a passive attenuator, the input impedance of the amp varies with the change on the pot position... That can although does not have to necessarily ruin the performance of the amp.
 
bubba177 said:
i have built a B1 but have need of a B2-can anyone doodle out a schematic for me? thanks bubba


Hah!:wave: Where in MO are you, bubba?

Does anyone have a BoM in excel or some other format with columns for price, qty, part# and such for Mouser or Digikey? I'm lazy, what can I say:usd: What's the approx parts cost using the Master's original parts (using perfboard). I searched for it, but couldn't find anything.
 
umm what? lol, I didn't mean a BOM & parts cost estimate for the B2, I know bubba was joking. I searched for "list", "parts list", "BOM", and "cost" and couldn't really find what I'm looking for. I've read the Mr. Pass' ETM article but I'm a very visual person and like my parts lists organized like this ...I mean I know I could go thru the article and make one but I figured someone had already done it, plus I should be doing more important things right now, like my Differential Equations take home test :crying: ugh:dodgy:
 
pinkfloyd4ever said:
I searched for "list", "parts list", "BOM", and "cost" and couldn't really find what I'm looking for. ...I mean I know I could go thru the article and make one but I figured someone had already done it, plus I should be doing more important things right now, like my Differential Equations take home test :crying: ugh:dodgy:

I think most folks just use existing parts on hand, there are so few. Plus you are not going to find the JFETS needed at Digi-key or Mouser, not the Toshiba ones at least. If you you non-boutique parts including coupling caps, it will be maybe $10. tops plus the enclosure, pot, switch, jacks, t-x, JFETs and board. What you use for the latter group of parts will depend on how fancy, or not you want to make it, and will determine over 90% :xeye: of the cost. Those choices will be personal and different for most.
 
Re: B1

sparky6 said:
Hi all,
What would be the advantages over passive?

Thanks...

Maybe none, maybe a lot. It all depends. In an attempt to explain the impedance issue (Z), here you go: For the purpose of understanding what is going on with the Z's, you can view the output Z of your preamp and the input Z of your poweramp as resisitors; series resistors in fact. You know that voltage will be dropped across a series resistor, right?
Now if the largest value of those series resistors is the output Z of the preamp (passive or active, doesnt matter) the largest amount of voltage (your precious signal!) will be dropped across that resistance. The smaller part will be dropped across the input Z of your poweramp. That doesnt sound like a good idea, does it?
Let's say, as an example in rough figures, that your preamp outputs 10V and has an output Z of 1k and your poweramp has an input Z of 100r, what will happen? Well, you will nearly throw away 9 volts in the preamp and only 1V in the poweramp.
This is where the buffer and impedance matching comes in. The B1 has a very low output Z.
The figures are just for illustrative purposes and very approximate, and the explanation is rather simplistic, but what...
So, do you need one? Build one and you will find out.

:)
 
Following on from Steenoe's explanation.
If the interconnect cables and next stage have some capacitance, then this forms a low pass filter with the output impedance of the source.

eg. Rs=1k0 (output impedance), Capacitance of cables 200pF/m and 2.5m long along with 680pF at the input of the next stage, gives C=1180pF.
The F-3dB of the Low pass filter is 135kHz.
Many builders/designers suggest that this filter should be between 160kHz and 500kHz to minimise it's effect on the audio band frequency response.

If a 100k passive pot is added between the two active stages the Rs, for the next active stage varies upto 25k. This reduces F-3dB to 5400Hz. Nothing short of a disaster in reproducing the sound in anything close to High Fidelity.
 
AndrewT said:
If a 100k passive pot is added between the two active stages the Rs, for the next active stage varies upto 25k. This reduces F-3dB to 5400Hz. Nothing short of a disaster in reproducing the sound in anything close to High Fidelity. [/B]


Who uses a 100k pot in a passive pre?

10k should be max! With this you have a good match for input and output imedance if your source is lower than 1k and your amp higher than 25k!

What many people ignore is the fact that output impedance of a passive pre (e.g. a 10k pot) is significant lower than 1k in the most used range of -40 to -20dB!
 
Tolu said:
What many people ignore is the fact that output impedance of a passive pre (e.g. a 10k pot) is significant lower than 1k in the most used range of -40 to -20dB!
wrong.
with Rs=1k0, the 10k pot will have a total source resistance as presented to the previous stage of 11k.
The output impedance of that 10k pot @ -20dB will be ~1k0//10k0~=910r. That is not significantly lower than 1k.
It is the value I used in the example calculation in para 2 of my post.

If a 100k pot were to be used the Rs @-20dB will be~9k01. That is 9times worse than my example and yet many builders adopt this 100k pot value simply because they are not aware of the compromises that are being foisted upon them.
So much for informed decision making.

If you want to get involved in the hobby of amplifier "design" you MUST be prepared to do the arithmetic.
If not then build exact copies of reputedly good designs.
Willy nilly swapping of components is not design.
 
theory refresher

First of all, I'd like to say thanks for the refresher to Stein2, Steenoe, AndrewT and Tolu - I truly appreciate your time in bringing me up to speed.

One question remains though about the rated input Z of the power amp ->25K. I'm a still a bit confused by this number in the specs. In your eg. you're using values of 100 for input Z ????

I was advised to choose a value close to this for the series stepped attenuator.

Now at about 1 o'clock (which is as loud as I'd want to go) I have 22k5 for Rs and 2k5 to ground (-20 dB). The interconnects are short ~.7m - so what would would be a rough calc for the LP cutoff?
 
Well, In electronics, I wouldn't normally call a 9% improvement significant.

Andrew, The way I see it is that you want to lower the source resistance as seen by the load of the preamp. With the B1 this resistance is 1k. With a passive pre, this resistance is dependent on the volume setting. With a 10k pot it is a value between 0 and 2k5.

At -20dB it is 900 ohm. This means that for volume settings between 0 and -20dB the source resistance (as seen by the load, a power amp for instance) is lower with a passive pre.

My subjective assessment of B1 concurs with these calculations, i.e. it sounds undynamic with 1k output resistance in my system.

Thanks,

Abo
 
Re: theory refresher

sparky6 said:
First of all, I'd like to say thanks for the refresher to Stein2, Steenoe, AndrewT and Tolu - I truly appreciate your time in bringing me up to speed.

One question remains though about the rated input Z of the power amp ->25K. I'm a still a bit confused by this number in the specs. In your eg. you're using values of 100 for input Z ????

You are welcome.
The figures were chosen for the purpose of showing the overall picture of how the impedance issue between two circuits works. It was not a real life example. Illustrative purposes, means just that. Anyway, with so few parts involved i wouldnt shy away from just building it, and see if works out as hoped for.

:)
 
Linear taper / no shunt?

Please forgive my simple-minded question.
The recommended pot for P100 / P200 is a 25k linear taper. My previous understanding is that 1) a log taper will give a better usable range for audio, 2) a linear taper can be substituted if a fixed resistor is added (about 5-10% of the pot) as a shunt between the input and the wiper. Most importantly, This would "restore" the usable range and secondarily the shunt resistance may provide some benefits (perhaps when used in a so called "passive preamp" configuration).

So my question is why does the design of the B1 call for a pot with a linear taper and why is no shunt used (at least I was not able to identify an equivalent)?

Alternatively, does the 25 k linear pot provide enough usable range to be used as a volume pot?

Forgive what is probably a simplistic question.