Thanks Dave. I owned the BENT with your AVCs and stupidly sold it. I will take a look at some of the folks you mention. The Experience Music looks interesting with its volume balance function. Thanks. |
Hey,
John still sells the naked units and OEM's them to Experience Music, Purity Audio and a few other high end manufacturers have proprietary versions. Artemis Labs, Aprilsound and Thomas Mayer all use a version of the autoformers in their top level designs and Silbatone has them wound with high purity silver.
We are working with Paul at Front Panel Express to offer up a plug and play chassis for both the remote and manual autoformer modules To fill the gap John left when he stopped selling finished units and Jeffrey Jackson and Myself offer finished units to match the aesthetic of our soon to be released Emia phono stage.
dave |
Dave, how many companies make an AVC using your transformers? Is John C still selling commercially available units? |
Hey,
The Z-out is a function of three things. Source impedance driving the inductive volume control, step down ratio and DCR of the coils.
In the case of an autoformer with a 50 ohm DCR the Zout is as follows. (numbers quickly read of a log graph so are approximate)
1K @ 0dB
505 @ -3dB
252 @ -6dB
76 @ -12dB
22 @ -18dB
8.2 @ -24dB
2.8 @ -30dB
1.5 @ -36dB
.58 @ -42dB
The input impedance at low frequencies is the parallel combo of the primary inductance and the reflected load. and in the case of the autoformer above it will be down 0.1dB @ 11hz
dave |
Haven't seen any other published spec's.
As I remember, Zout was something like:
1400 ohm @ 0 dB
140 @ -3
14 @ -6
1.4 @ -9
and so on, down the 24 steps, except the steps weren't exactly linear (logarithmic) and they never rated for the option of gain. Seen worse numbers from tube line stages but that's more of the exception than the rule.
I'm feeding less than 10Kohm (parallel active high pass/low pass) but since my volume setting never gets above two-thirds (CDP starts to skip with bass notes), impedance and roll-off is not an isssue.
Zin was the opposite and 2 Mohm (JFet territory) at 0 dB sounds familiar. Forget the minimum but that's hardly an issue against the Fletcher-Munson curve. However, capacitance of associated equipment and cables becomes an issue, which, for us laymen, is the least understood part.
|
S&B removed their website that showed this. For a while the URL I bookmarked still displayed that page even though S&B removed it from view on the public site. However, now the URL is dead too. That was a very informative chart. Don't know why S&B felt the need to remove that page. |
I have used an Aragon 8008bb amp with an EVS attenuator which is fixed resistor. Im not going to sell either piece as that Aragon sounded better then my Levinson and the EVS has a transparency none of my 12ax7 or 6sn7 pre amps could match. But; and a big but; together they were meh. Always moving the sub crossing frequency and dealing with room bass nodes. After discovering impediance matching I bought a Bent silver tvc and found bliss. Two months later I gave up on the low sensitivity rig and now its speaker to set amp impediances Im dealing with. |
For what it's worth, I ran a Sonic Euphoria passive line stage for years with an Aragon 3002 amp, and never had a problem with an impedence issue, always had as much volume as needed with that combination. I don't recall the input impedence on the Aragon, but the combination worked very well for me.
Regards,
Dan |
Ngjockey,
Thanks so much for that information! I have looked high and low for output impedance on TVC's to no avail. All I ever got was a nonspecific "lower than resistor based designs." Is that true for all TVC's, or only the S&B's? |
For the record, TVC's have an output impedance below 100 ohms for all but the highest 2 or 3 steps. In fact, the first half is usually under 1 ohm. S&B removed their website that showed this. |
I really appreciate the information. I just ran across a NAD C162 preamp for sale. Based on the info that I got and my previous experience with NAD equipment, I went ahead and grabbed it.
Thanks. |
Yes, an amp with a higher input impedance would be preferable. There is a rule of thumb that the input impedance of a component should be at least 10 x the output impedance of its upstream component. I'm about 38 years removed from my last physics class, so I defer to someone else to provide you with the math. Failure to observe the 10 x rule can result in attenuation of the signal at frequency extremes.
Since I like to use passives, I tend to stick with amps that have ~ 50-100 K ohms where impedance mismatch is not an issue. Then the only consideration is making sure you have sufficient gain. |
Just for purposes of understanding, would you say that an amp with a higher input impedance would be better? Why?
Thanks again. |
I don't think I would run a passive into an amp with an input impedance of 22K ohms. I love passives, especially TVC's, but I think that is pushing it. |
