Do the autoformer volume controls such as:
http://www.megathon.com/home_theatre_custom_install_volume_controls.htm
do the same thing as a normal potentiometer ? Can I use one as the volume control in a passive pre-amp.
I am an old (30 years?) Alps pot from an old amp, set up to shunt a resistor between my cdp and some wave av-8 monos.
It doesn't quite sound the same as when I had a cheaper (yamaha) cdp hooked up without the pot. (the yamaha had volume controls, the nad cdp dosen't)
Has anyone tried to use an auto former like the ones in the link above? how much attenuation should I use in a passive preamp between the cdp and the amp.
Thanks, I really hope this isn't a stupid question.
Adam
http://www.megathon.com/home_theatre_custom_install_volume_controls.htm
do the same thing as a normal potentiometer ? Can I use one as the volume control in a passive pre-amp.
I am an old (30 years?) Alps pot from an old amp, set up to shunt a resistor between my cdp and some wave av-8 monos.
It doesn't quite sound the same as when I had a cheaper (yamaha) cdp hooked up without the pot. (the yamaha had volume controls, the nad cdp dosen't)
Has anyone tried to use an auto former like the ones in the link above? how much attenuation should I use in a passive preamp between the cdp and the amp.
Thanks, I really hope this isn't a stupid question.
Adam
adharvey said:
well the simple answer is yes.... they are three terminal devices, they do not behave exactly the same way as a pot, but they are close.
the autoformers you refernced can work very well at line level under the proper circumstances, and like any other device can fail miserably when put into the wrong situation.
the speaker level controls you mentioned are cheap ($15 is a more common price without the high end packaging) and designed for power on an 8 ohm speaker. this means they have limited primary inductance and you need a low source Z to drive them, but i have heard of many situations at line level where people have had great results driving them with the low Z-out of a CD player or transformer output tube stage.
the good news is their power handling at 8 ohms buys them headroom in voltage so higher output sources tend to fare pretty well with them, but its a case by case ears on experience... there are no rules.
the only other real issue is the attenuations are pretty limited and coarse... 10-12 steps 3-6db per step with less than 40db of max attenuation... if you have a proper gain structure this isn't an issue... and i find 3db of autoformer attenuation to be much less to my ears than 3db of resistive attenuation... even with 3db steps level matching with mono autoformers is often a task for me, but with mono davens and 2 db steps its easy... still don't understand why.
dave
While these will work you will be unhappy with the response that you get from the transformers. They may claim a response down to 20hz but what they don't tell you is how many db down you will be at 20hz. If you purchased a high dollar VC with much better transformers the response will be better but by this time you would be spending enough money to warrant purchasing a nice stepped controls.
This type of control is meant to maintain a constant load on an amplifier/s. Most are switchable from 2,4, or 8ohm in order to best match loads. Just my .02
Joe
I will look for a link on making a control and post it.
This type of control is meant to maintain a constant load on an amplifier/s. Most are switchable from 2,4, or 8ohm in order to best match loads. Just my .02
Joe
I will look for a link on making a control and post it.
Some of those controls actually did give specs on response with dB limits. But these are not the right sort to use at line level. Yes, with enough "stuff" around them, one could make them work, after a fashion, but why? There are high quality autoformer volume controls out there (expensive, alas) and plain old ordinary potentiometers which are much more suitable for line-level attenuation.
For $1.49, a Radio Shack-sourced Alps potentiometer will outperform most of those designer controls. For $25, you can get even better pots, and throw a buffer in. Don't think about autoformers until you're ready to part with $150+.
For $1.49, a Radio Shack-sourced Alps potentiometer will outperform most of those designer controls. For $25, you can get even better pots, and throw a buffer in. Don't think about autoformers until you're ready to part with $150+.
http://www.siteswithstyle.com/VoltSecond/12_posistion_shunt/12_Position_Pure_Shunt.html
Try this...
Like SY pointed out there are much better ways to go. The Radio Shack controls do deliver very good performance for the money. If your adventurous check the site I posted. It will provide step by step instructions on fabrication of a control.
Joe
Try this...
Like SY pointed out there are much better ways to go. The Radio Shack controls do deliver very good performance for the money. If your adventurous check the site I posted. It will provide step by step instructions on fabrication of a control.
Joe
I'm using a pair of autoformers as my linestage. They were built by Dave Slagle. I'm not sure if he's still making them or not.
I paid $250 for a pair of autoformers, and I think they're significantly better than any active preamp I could have built for the same amount of money. My total cost was about $300 - $350, because you'll need a pair of rotary selector switches (one for volume and one for source selection), RCA connectors, chassis, wire, solder, etc.
Here's a picture of my unit in its current incarnation (it's been through a few enclosures):
I paid $250 for a pair of autoformers, and I think they're significantly better than any active preamp I could have built for the same amount of money. My total cost was about $300 - $350, because you'll need a pair of rotary selector switches (one for volume and one for source selection), RCA connectors, chassis, wire, solder, etc.
Here's a picture of my unit in its current incarnation (it's been through a few enclosures):
An externally hosted image should be here but it was not working when we last tested it.
Sowter makes a autoformer for controling line level volume. It can be found here:
http://www.sowter.co.uk/attens.htm
http://www.sowter.co.uk/attens.htm
> Can I use one as the volume control in a passive pre-amp.
No.
These are wound for use between a loudspeaker amp and a loudspeaker. Load impedance about 8Ω, source impedance very much lower. Probably the inductive impedance of the unloaded autotranny dips to 8 ohms at 20 Hz. Your CD deck's output impedance is more like 100Ω. Output level will be low and possibly distorted, and bass will be even lower.
Also as Dave says: the minimum useable attenuation setting is probably higher than you want as a main volume control. Steps may be larger than we'd really like too.
There are line-impedance tapped autoformers, but not at the $20-$60 level. Sowter has nice 10K:10K cores for £86.63, but you will have to find a 2 pole 24 position switch.
The main advantage of a transformer over a resistor attenuator is that it does not waste power. If you feed a 100 watt amplifier into an 8Ω pot to reduce speaker level, the pot burns up. With the autotranny, the transformer action reflects a higher load to the amplifier, drawing less power, and delivers 90+% of that power to the speaker. It runs cool.
At the 0.000,1 Watt levels that lines work at, overheated pots are not an issue. And pots are linear to DC and to high voltages and frequencies.
> What exactly is the z out of a cd player, and where would I find this out?
You could measure it. But the fact is: you never want to come close to loading most modern outputs in their actual source impedance. They have a rated load impedance, and you don't want to go lower than that. For hi-fi box line outs, usually 10K, though some will happily drive 2K or even 600Ω.
The internal impedance may be as low as 47Ω, but if you load them in 47Ω you will get half-output and probably gross distortion. The low output Z gives authoritative drive into long cables and no change in output level between 10K and 1Meg loads, but is not the load that the box can drive.
No.
These are wound for use between a loudspeaker amp and a loudspeaker. Load impedance about 8Ω, source impedance very much lower. Probably the inductive impedance of the unloaded autotranny dips to 8 ohms at 20 Hz. Your CD deck's output impedance is more like 100Ω. Output level will be low and possibly distorted, and bass will be even lower.
Also as Dave says: the minimum useable attenuation setting is probably higher than you want as a main volume control. Steps may be larger than we'd really like too.
There are line-impedance tapped autoformers, but not at the $20-$60 level. Sowter has nice 10K:10K cores for £86.63, but you will have to find a 2 pole 24 position switch.
The main advantage of a transformer over a resistor attenuator is that it does not waste power. If you feed a 100 watt amplifier into an 8Ω pot to reduce speaker level, the pot burns up. With the autotranny, the transformer action reflects a higher load to the amplifier, drawing less power, and delivers 90+% of that power to the speaker. It runs cool.
At the 0.000,1 Watt levels that lines work at, overheated pots are not an issue. And pots are linear to DC and to high voltages and frequencies.
> What exactly is the z out of a cd player, and where would I find this out?
You could measure it. But the fact is: you never want to come close to loading most modern outputs in their actual source impedance. They have a rated load impedance, and you don't want to go lower than that. For hi-fi box line outs, usually 10K, though some will happily drive 2K or even 600Ω.
The internal impedance may be as low as 47Ω, but if you load them in 47Ω you will get half-output and probably gross distortion. The low output Z gives authoritative drive into long cables and no change in output level between 10K and 1Meg loads, but is not the load that the box can drive.
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