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Carbon film vs metal film resistors in a stepped attenuator?

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G

Member
Joined 2002
Bruno Putzeys said:
I wouldn't reject good carbons out of hand. I've got no idea what you mean by "little rebel", but unless you know the type to be *bad* you should give it a serious try.

That is the moniker that Ohmite gave them I guess. I'm an instrumentation tech and here in the shop we have a little variety pack of these resistors in a little plastic shelf unit. We never use these as we do not do board level repairs. So they just sit there. I was just wondering if anyone had any experience with the sound of carbon film resistors in a stepped attenuator. Here is the link to the attenuator I'm talking about.:

http://www.siteswithstyle.com/VoltSecond/12_posistion_shunt/12_Position_Pure_Shunt.html

I going to use Grayhill switches to build these.
 
diyAudio Senior Member
Joined 2002
Hi,

Or should I go ahead and get tyhe Holcos?

While the old style Hocos are good sounding resistors, they're not what I'd consider totally neutral.

I designed a number of attenuators using the H4 series and wasn't too pleased with the result. They sounded rather dark and rounded, in short not my cupper.
Later I replaced the resistors of the most often used positions with Vishay S102 bulkfoils and this was much better.
Although much more costly than Holcos, especially 10 to 15 years ago, the bulkfoils are much more transparent sounding than the Holcos.

As for carbon films, I don't know the Ohmite series you have but I think they're worth a try; their lowish self-inductance makes them quite suitable to the task and modern carbon films are often good sounding resistors, cheap too.

If you're using the solder lug type from Grayhill, soldering out different resistors later on is actually a cinch so trying new Rs should be a straightforward job.

Cheers,;)
 

G

Member
Joined 2002
Maybe I will use a tant resistor in the series position and the carbon films in the "step positions". I would guess it is going to be an improvement on a film pot either way. What attenuation range would you suggest Frank where the bias voltage on the input stage is about 1.7v? -20dB through-50dB seems to be the fave for this particular attenuator.
 
diyAudio Senior Member
Joined 2002
Hi,

Maybe I will use a tant resistor in the series position and the carbon films in the "step positions".

What type of attenuator are you considering?
Both positions are important but I'd not bother to put a tantalum resistor in an attenuator. Odd as it may seem these tants seem to sound their best when some current is running through them...
Keep in mind you'll need the better part out of the E96 series if you want some precision...

If you want to find out what the resistors really sound like, try using a single type throughout. As said before you can easily change them later on.

What attenuation range would you suggest Frank where the bias voltage on the input stage is about 1.7v? -20dB through-50dB seems to be the fave for this particular attenuator.

Not sure why you bring up the bias voltage of the tube here but here's what I do:

- you'd really want a range from 0 to say -60dB to infinite attenuation (a mute switch is what I use for the latter).
- assuming your switch has 23 usable pos. (one is no att.) you can make the first and last steps rather crude in operation, say 3 to 4 db each.

-it's important to know what range you most often use now and to what that corresponds to in dB. That's where you'd want the finest steps, say 1 to 1.5 dB each.
This is important as a change of speakers with totally different effeciency can make for a very considerable change in where you'd like the finest increments.
You can measure this by measuring the resistance of the pot you're currently using and convert that to dB. If you don't have the formula, I'm pretty sure it can be found on Voltsecond's site.

- All steps added up should give you the full range of say - 60 dB and of course the total impedance should be the same as the pot you're replacing.

-Use the best solder you can get and make sure not to overheat the soldering lugs or...The lubricant will go up in smoke.
There's probably plenty of info on this part of the process on the forum already.
Either way, your craftmanship is very important to the endresult.

Hope this helps,;)
 

G

Member
Joined 2002
fdegrove said:
Hi,



What type of attenuator are you considering?
Both positions are important but I'd not bother to put a tantalum resistor in an attenuator. Odd as it may seem these tants seem to sound their best when some current is running through them...
Keep in mind you'll need the better part out of the E96 series if you want some precision...

If you want to find out what the resistors really sound like, try using a single type throughout. As said before you can easily change them later on.



Not sure why you bring up the bias voltage of the tube here but here's what I do:

- you'd really want a range from 0 to say -60dB to infinite attenuation (a mute switch is what I use for the latter).
- assuming your switch has 23 usable pos. (one is no att.) you can make the first and last steps rather crude in operation, say 3 to 4 db each.

-it's important to know what range you most often use now and to what that corresponds to in dB. That's where you'd want the finest steps, say 1 to 1.5 dB each.
This is important as a change of speakers with totally different effeciency can make for a very considerable change in where you'd like the finest increments.
You can measure this by measuring the resistance of the pot you're currently using and convert that to dB. If you don't have the formula, I'm pretty sure it can be found on Voltsecond's site.

- All steps added up should give you the full range of say - 60 dB and of course the total impedance should be the same as the pot you're replacing.

-Use the best solder you can get and make sure not to overheat the soldering lugs or...The lubricant will go up in smoke.
There's probably plenty of info on this part of the process on the forum already.
Either way, your craftmanship is very important to the endresult.

Hope this helps,;)

The reason I brought up the biasing is a normal CD player will put out 2 volts. Since the grid voltage is -1.7v I obviously can't have 0dB attenuation without clipping the input stage. I've attached a diagram of the circuit in rough terms. The switch is a 12 step 1 pole Grayhill series 71 (I like dual mono controls too). R12 is where I planned on putting the tantalum resistor.
 

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G

Member
Joined 2002
Peter Daniel said:
Odd as it may seem, I was never impressed with sound of tantalums. I never liked the way they cut off top end. The midrange and bottom end is very nice though.

I'm taking here about input series resistance.

Well I definately don't want to cut off the top end. I guess Caddock is the way to go. Or just use carbon films all the way. Thanks Peter.
 
diyAudio Senior Member
Joined 2002
Hi,

The reason I brought up the biasing is a normal CD player will put out 2 volts.

2 Volts RMS means also 6 Volts P2P.

Since you're using a shunt with the shunt element variable, you could make the series resistor larger.
Or you could install an attenuator network straight at the CD input of the preamp.

Either way the series resistor(s) could/should be Caddock or your prefered type for both if you'd like to be consistent.

Cheers, ;)
 
Vishay source

Hi G,

If you're interested pursuing the Vishay option, I'd give Texas Components a try. They make an unencapsulated version of the Vishay S102K which, because having shed the usual rubber and plastic shell, sounds a touch more transparent than its encapsulated brother. Pricing is not out of the ballpark either .... I think their resistors sell for about US$6 each, and they fill orders of any quantity (they like to say you must order at least one resistor). Their website is here and a link to their unencapsulated "audio" resistor can be found here. Arbie is your contact person. Just note that one must be careful handling these resistors as the leads can easily break away from the ceramic substrate, which is the first step in the lead actually breaking off the bulk metal foil resistive unit. Cheers.

Tom
 
diyAudio Senior Member
Joined 2002
Hi,

Gavin,

While taking a second look at the attenuator diagram I noticed you sacrificed pos. #1 for the muting of the signal input.

In practice I don't think this is very useful and by doing so you lose a useful position for attenuating, you only have 12 to begin with.

Which in turn made me notice that all other positions of the atten. are occupied by dividers.
This implies that there's no position foreseen with no attenuation at all.
You may want to change that as it may be useful with sources having a rather weak output and for the occasional fault finding.

The muting can easily be taken care of by a simple DPDT switch if you take the hot signal wire at the output and run it to the switch, shorting the output to ground when the mute is activated.

In this way the signal doesn't even see an extra switch contact in normal operation and you gain a position on the attenuator.
The output of the preamp is muted iso of the input and so will be the input of the amp.

Also keep in mind that not all tube circuits appreciate this kind of variable g' and may well show an altering frequency response depending on atten. position.

Cheers, ;)
 

G

Member
Joined 2002
Hi Frank,

I realize it's not perfect but with my disposable income situation being bad right now it will have to do. It will attenuate either from -1dB to -40dB or from -21dB to -60dB by adding a SPDT switch. VoltSecond says he has gotten good feedback on the design so I'm going to give it a shot. I have attached a accurate diagram of the circuit below. I took your advice about the mute. I will place it at the output of the preamp. Do you think the 75 ohm resistor looks right in that position? I have a nice scientific calculator but I have not tried to calculate decibels (on my calculator) in so long that I am just going to wing it. Thanks for the reply.
 

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diyAudio Senior Member
Joined 2002
Hi,

Do you think the 75 ohm resistor looks right in that position?

For the mute switch you mean?
If so, you don't want any resistor in that position at all, you just short the signal straight to ground at the output of the linestage, not the output of the attenuator.

As there is no crosstalk to speak of in a DPDT switch you can simply fit it to the center of the frontpanel of the linestage and take the signal for L and R channels to the top solder lugs of the switch, the bottom lugs to the star groundpoint.
Closing the switch will effectively short the output to ground.

Cheers,;)
 

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G

Member
Joined 2002
fdegrove said:
Hi,



For the mute switch you mean?
If so, you don't want any resistor in that position at all, you just short the signal straight to ground at the output of the linestage, not the output of the attenuator.

As there is no crosstalk to speak of in a DPDT switch you can simply fit it to the center of the frontpanel of the linestage and take the signal for L and R channels to the top solder lugs of the switch, the bottom lugs to the star groundpoint.
Closing the switch will effectively short the output to ground.

Cheers,;)

What I did was add the 75 ohm resistor in place of the mute. I don't know what kind of step that is in dB but it does give me one more step in the attenuator. I will add the DPDT switch in the output stage. Thanks.
 
diyAudio Senior Member
Joined 2002
Hi,

I don't know what kind of step that is in dB but it does give me one more step in the attenuator.

I see...You had me confused as it said MUTE a bit higher on the drawing.;)

Anyway, I'm sure I have the logarathmic conversion tables and formulas here somewhere in project a folder.
I'll dig them up and scan them for you later on next week.

Cheers,;)
 
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