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Old 16th August 2004, 07:45 PM   #1
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Default Carbon film vs metal film resistors in a stepped attenuator?

I have a bunch of Ohmite 1/4 watt "Little Rebel" carbon film resistors would these be acceptable in a stepped attenuator? Or should I go ahead and get tyhe Holcos?
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Old 16th August 2004, 08:04 PM   #2
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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.
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Old 16th August 2004, 08:38 PM   #3
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Quote:
Originally posted by Bruno Putzeys
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/VoltSe...ure_Shunt.html

I going to use Grayhill switches to build these.
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Old 16th August 2004, 09:49 PM   #4
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If you're going to do a shunt attenuator, use the best resistor you can find in the shunt position. The other resistors are not as important. If you were to build a series attenuator, I'd stay away from the carbon films because of noise issues.
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Old 16th August 2004, 09:52 PM   #5
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Hi,

Quote:
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,
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Old 16th August 2004, 11:47 PM   #6
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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.
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Old 17th August 2004, 12:21 AM   #7
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Hi,

Quote:
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.

Quote:
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,
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Old 17th August 2004, 01:33 AM   #8
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Quote:
Originally posted by fdegrove
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.
Attached Images
File Type: gif series shunt attenuator (small).gif (10.9 KB, 434 views)
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Old 17th August 2004, 01:48 AM   #9
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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.
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Old 17th August 2004, 01:51 AM   #10
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Quote:
Originally posted by Peter Daniel
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.
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