i know many times....output potentiometer

[Stefanoo]

Hello everyboy,

i know this subject has been considered/covered many times.
I have researched on the forum but I couldn't really find an answer to my questions and so i try to post them here hoping that somebody will be kind to answer them.


The subject is the output potentiometer on the Aleph P1.7

The service manual shows a 8 steps output shunt type attenuator.
The value for the resistor in serie is 100ohm whilest the shunt resistors from output to ground are values within the range 61ohm - 8kohm.


Does this mean that the input impedence of the attenuator is within the range 100ohm+61ohm=161ohm - 100ohm+8kohm=8,1Kohm?
Is the calculation correct? If so, is it not too variable impedence?

If i say, for example, that i am using a 3Kohm LADDER type, in this case,since the LATTER type has a steady impedence the 3k value would make sense to me.
Conversely if i use a Shunt type, as is the case of the P1.7, that fix value would start making as much sense to me as before.
The impedence would vary a lot along the different steps.

If i conversely consider the output impededence of the shunt type show on the aleph P1.7 service manual it is something within the range 37ohm - 98ohm.
(this time is the parallel of the 100ohm resistor.

So when i say that the stepped attenuator, for example, is 3Kohm,am i actually referring at the input or output impedence?


Lat question can anybody help me to figure out the output impedence of the aleph P?
The service manual specify an impedence within 0-800ohm.


Since i have to build the output potentiometer and i have to buy the actual value that i will use, i woul ike to figure out what value would be the most suitable.
I have found so far on the forum 3K to be the best value



Well... hope these questions are not too doomed for you guys.

[Nelson Pass]

Originally posted by Stefanoo
The subject is the output potentiometer on the Aleph P1.7
The service manual shows a 8 steps output shunt type attenuator.
The value for the resistor in serie is 100ohm whilest the shunt resistors from output to ground are values within the range 61ohm - 8kohm.

Does this mean that the input impedence of the attenuator is within the range 100ohm+61ohm=161ohm - 100ohm+8kohm=8,1Kohm?
Is the calculation correct? If so, is it not too variable impedence?

The output shunt to ground is in parallel with the output impedance
of the circuit, which is about 2 Kohm. So with all relays wide open,
the output impedance is about 2 Kohm, and it gets less as the
volume setting goes down.

[jupiterjune]

Well,
I can tell you that I use a 2200 ohm or so stepped attenuator with 11 active positions on my BOSOZ. It works just great! It is a D4 rotary switch by Electroswitch, which I got from Mouser. If you were ordering resistors, you might consider adding one to the order.

JJ

[Stefanoo]

Originally posted by Nelson Pass


The output shunt to ground is in parallel with the output impedance
of the circuit, which is about 2 Kohm. So with all relays wide open,
the output impedance is about 2 Kohm, and it gets less as the
volume setting goes down.

thanks Mr. Pass.
Let me recap it for the sake of the clarity.

Taking in consideration that output impedence of this circuit is about 2Kohm, the total output impedence with the resistors from output to ground being the range of 0, 61ohm,..,8Kohm will be within the range 0 - 1.5Kohm, correct?

That value has to be doubled as a single ended output (so about 3kohm) since the out - and out + will shunt the input stage of whatever power amp connected to it.

Am i wrong by saying that it would be better to have a lower output impedence like for instance 1kohm?
If so, is it not appropriate to halven the value of the maximum resistor ( the res on the last step) for 8k downt to 4k, lets's say?


...hhhuuummmm.....but then by keeping the same value of resistor on the gain control i would, in this way, just lower the volume at the output, i think, of about 1db....mmm.....

[Stefanoo]

The table of attenuation that comes out from the regulation shown o the service maual with - 100ohm res and shunt output to ground resisors - is

R1=100ohm
Shunt Res Ratio attenuation(dB)
---------------------------------------------------------------
61,9 0,382334775 -8,351123995
124 0,553571429 -5,136526663
249 0,713467049 -2,932521597
499 0,833055092 -1,586525535
1,00E+03 0,909090909 -0,827853703
2,00E+03 0,952380952 -0,423785981
4,02E+03 0,975728155 -0,213423259
8,06E+03 0,987745098 -0,107102339

It seems weired to me...am i making soe imstake here?
When i turn the volume up to the last 3 steps,thought, i barely hear the difference...which is not wat i wish.

I'm trying to figure out the best combination for R1/R2 to reach a good volume regulation.
I am planning on using a 24 positions 4 decks Elma switch therefore a shunt type.
Is there any specific reason of why the R1 (the resistor in serie) would have to be 100ohm and not higher?
Maybe because this allows a lower shunt resistor at the maximum volume position (8kohm) and also a lower impedence swing along the different steps?

If i would like to have -2db -4db -6db as attenuation on the last 3 steps i would need resistors on the order of 300ohm 200ohm 120ohm is it correct?
In this steps thought the output impedence would be desiderably low, correct?
Since the last step (8k) is rarely used, the output impedence due to the 2k output impedence of the circuit, vould be preatty much stable, correct?

Just an aside note:

Searching on internet i came across to a webpage where it was explained that a good regulation would have more distance on the first steps and finer by going up and therefore something like this:

-62 -53 -46 ..... -12 -10 -8 ..... -2 0

does it sound good?


One last question:

Since i would like to have a fixed resistor on the gain section instead of the trimmer (i have listened to the pre on both configurations: trimmer and fixed equivalent resistor and the difference is clearly audible)....so the question is.....

how much would an average output voltage have to be ...(in average since i know that it depends upon the power amp used)...

For instance
with an average input signal source capable of 2Vpk-pk (which is the average output value for standard Cds) should the output voltage at the pre, at maximum volume, be around 5V ...or is it too much?


tks for the attention.

[Nelson Pass]

Originally posted by Stefanoo
For instance
with an average input signal source capable of 2Vpk-pk (which is the average output value for standard Cds) should the output voltage at the pre, at maximum volume, be around 5V ...or is it too much?

I don't know, since I don't have access to your system. I would
suggest that you try different values and see what works best.

[Stefanoo]

got it.
So it has to set on the actual system.

Ok i'm tring to find good low, very low, value metal film resistor for the output potentiometer.
The vaule that i need are

on the left the attuenuation level and on the right side the output to ground resistor value. the serie resisto is 100ohm:
Atten Rshut
------------------------
53 0,224374426
46 0,503711773
41 0,899265652
37 1,432776044
34 2,035883532
32 2,576607896
30 3,265543203
28 4,146132201
26 5,276314482
24 6,734491107
22 8,628683001
20 11,11111111
18 14,40241013
16 18,83390465
14 24,92601748
12 33,54498311
10 46,24752956
8 66,1425342
6 100,4760238
4 170,9713864
2 386,2116094
0,2 4293,136699



where can i find these low value resistor?
Peter suggested mme to go for the dale sold from mouser....but what type? there are a lot and i would like a good quality res.
I was actually thinking of using Welwyn resistors but i don't know where to find them thought.
Any advice?

[jupiterjune]

Stefanoo-

I had to use several different types for the 2200 ohm attenuator I built. I started with Vishay/dale RN's. Then I started filling in missing values with Vishay CCF's. For some of the really small values, I had to use Vishay power resistors, I think the model was 'PR0'. All bought from Mouser.

If anyone has a good source for all the low values listed by Stefanoo, I'd be glad to hear it too.

JJ

[Stefanoo]

well i don't like that!
I would rather like to have only one brand.

I was thinking of Welwyn resistors but i can't find a dialer for this resistor's brand?
Does anybody have a clue onn where to find them?

I'll try to open a specific thread for this....

[Stefanoo]

Originally posted by jupiterjune
Stefanoo-

I had to use several different types for the 2200 ohm attenuator I built. I started with Vishay/dale RN's. Then I started filling in missing values with Vishay CCF's. For some of the really small values, I had to use Vishay power resistors, I think the model was 'PR0'. All bought from Mouser.

If anyone has a good source for all the low values listed by Stefanoo, I'd be glad to hear it too.

JJ

after long researches.....the only decent/affordable price are:

Welwyn 0.5W from Newark
The value starts from 1ohm...and they seems to be decent resistors.
I don't expect anything in particular....but the shouldn't be too bad....
that is the only thing that i can come out with for now.....everythingelse seems to be muchmore expencive...