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Line amp/AVC/TVC thoughts

165.225.68.49

Posted on February 23, 2017 at 23:18:09
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
I have a phonoamp Zout of 2K Ohms, and a Power amp Zin of 100K Ohms/400pF (millercap) and need no voltage gain at all.

Have tryed with a 100K Ohms passive resistivie ladder attenuator. And even though the level (and HF-extend) where acceptable, I felt it a bit non involving and slow, compared to an solid state active buffert.

Now I am not sure if I would go for a a tube line stage (like Ale´s 1P4L) or Dave Slagle $200 AVC?
Anyone been there and have any thoughts?

 

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RE: Line amp/AVC/TVC thoughts, posted on February 24, 2017 at 06:42:25
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
If you like the sound of solid state and CDs you may want to stick with active. My experience with tvc and avc is they sound like real music with the tone and body of real instruments. Now I did try a 26 or other DHT preamps so I can not give you a comparison there.

For 200.00 bucks I would give Dave's a try. It will give a feeling of weather you like caps/resistors/transistors etc or iron. You can already guess which camp I am in.

Enjoy the ride
Tom

 

RE: Line amp/AVC/TVC thoughts, posted on February 24, 2017 at 07:42:37
Tre'
Industry Professional

Posts: 17260
Location: So. Cal.
Joined: February 9, 2002
Adjusted to -6db (the worse case) a 100K Ohms passive resistive ladder attenuator has an output impedance of 25k ohms.

The low pass filter created by 25k ohms driving into 400pF has a -3db point of 16kHz.

I know you said "the level (and HF-extend) where acceptable" but I disagree WRT the HF-extension. A -3db point of 16kHz will shift the phase all the way down to 1.6kHz

BTW don't forget to add the cable capacitance that will make the numbers even worse.

I think AVCs are great but I'm not sure your phono pre's output impedance of 2k ohms will properly drive one.

Ask Dave.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Sure, posted on February 24, 2017 at 10:03:31
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
Well. I'm usually play music at 1/10 of a 100k resistive ladder, but I got your point.
This is definitely what I am hearing.
By saying that I know my system is a bit overgained, and having a less gained (read Miller capacitance) input tube would solve. But this is a to big project right now.
But if it's all about the -3dB roll off point, how much headroom is needed for?

Btw, From what I have reading, Dave's AVC is a 20H loading, but can easily be restacked up to 10 times higher. This would make it an easier load with fairly low z out.

 

low frequency rolloff issues, posted on February 24, 2017 at 10:41:13
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
Most sources (your phono preamp is a good example) have an output coupling cap.

Its very common for such units to have a particular output impedance, such as the 2KOhms of your phono section. But that is likely at 1KHz and its likely not the same at 20Hz where its probably a lot higher (maybe 10-20Kohms). This is often due to the size of the output coupling cap.

Driving 100K this is probably not an issue, but when the passive control is introduced, it can be. Imagine adding 25K to whatever output impedance you already have. This is why you might notice that the bass is best only when the control is at full volume. YMMV due to the resistance of the control and the actual 20Hz output impedance which varies widely from source to source.

So in addition to the rolloff that Tre' mentions, you have another rolloff to contend with. With its attendant phase shift (bass rolled off with phase shift, which reduces impact of what remains).

In short you can make passives work, but quite often an active line stage will work better- not as dry, sterile and often more involving. Tubes do the involving thing even better...

 

RE: Sure, posted on February 24, 2017 at 11:45:26
Tre'
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Posts: 17260
Location: So. Cal.
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"But if it's all about the -3dB roll off point, how much headroom is needed for?"

some say it doesn't matter but if you want 20kHz phase shift free, the -3db point of the filter would need to be at 200kHz.

"...Dave's AVC is a 20H loading, but can easily be restacked up to 10 times higher."

the reactance at 20Hz of 200Hy is 25k ohms.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Sure, posted on February 24, 2017 at 11:45:32
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
Well. I'm usually play music at 1/10 of a 100k resistive ladder, but I got your point.
This is definitely what I am hearing.
By saying that I know my system is a bit overgained, and having a less gained (read Miller capacitance) input tube would solve. But this is a to big project right now.
But if it's all about the -3dB roll off point, how much headroom is needed for?

Btw, From what I have reading, Dave's AVC is a 20H loading, but can easily be restacked up to 10 times higher. This would make it an easier load with fairly low z out.

 

AVC, posted on February 24, 2017 at 12:17:32
sser2
Audiophile

Posts: 2571
Location: Pittsburgh USA
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You can try Atlas AT10. It is autoformer attenuator with eleven 3dB steps. Nominally it is 600 Ohm, but its high inductance (77H at 60Hz) makes it suitable for higher impedances. 40dBm. Frequency response is 20-20,000 1dB. The sound is superb, what you would expect from a good TVC. Well-made, with high quality silver-plated contact switch. It is ridiculously cheap, you can buy a surplus one for 10 bucks on eBay.

Since it is mono, you will need two of them. The transformers can be taken out and grafted onto a two-gang switch, like Russound ALT126R or like, for a single knob control. Or, for a fancier setup, you can use Elan VSE-100 relay switch with infrared remote. These things are also dirt-cheap, yet quality made.

I am sure that Dave's AVC is superior to AT10. But this would be kind of entry level TVC setup that can be later upgraded with Dave's transformers.

 

RE: Line amp/AVC/TVC thoughts, posted on February 24, 2017 at 13:06:15
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
Info. loss due to impedance mismatches has plagued 'philes for a very long time.

The long established 1:10 rule is a good guide. Your phono preamp needs to work, at a minimum, into 20 Kohms. The 100 Kohm attenuator was/is fine. However, that 100 Kohm attenuator needs to work into (sic) 1 Mohm and the cabling between the attenuator and the power amp must be extremely low capacitance. Substantial cable capacitance here will result in HF info. loss. :>( That 400 pF. mentioned could be an issue too.

You found a SS buffer to be, at least, tolerable. Try a tubed buffer. A single ECC99 set up as 2X cathode followers should do the job. As long as the signal source is DC offset free, a bipolar PSU will allow you to hold the cap. count inserted into the signal path by the buffer down to 1. :>) 10M45S CCS loading of the CFs is another refinement to consider.


Eli D.

 

RE: Line amp/AVC/TVC thoughts, posted on February 24, 2017 at 19:06:27
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
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Tre' IIRC I saw a post you made somewhere about you using the 33 step TVCs offered on ebay, and stated the inductance. If so can you tell me what the inductance is?

 

I measured 80Hy in the secondary....., posted on February 24, 2017 at 19:24:39
Tre'
Industry Professional

Posts: 17260
Location: So. Cal.
Joined: February 9, 2002
I use the secondary as a autoformer.

I've never tried them as transformers.

There's not enough inductance (for my use) in the primary. Besides, I can't think of a good reason to use them that way. (maybe if I were fighting ground loop hum?)

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I measured 80Hy in the secondary....., posted on February 24, 2017 at 19:40:25
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
I wonder if that would be enough for a phono stage with 1K output impeadance?

twystd

 

RE: I measured 80Hy in the secondary....., posted on February 24, 2017 at 21:01:25
Tre'
Industry Professional

Posts: 17260
Location: So. Cal.
Joined: February 9, 2002
80Hy has a reactance at 20Hz of 10k ohms.

I'm driving mine with about 400 ohms.

It takes a good size coupling cap.

I'm running a 4.7uf in parallel with a 2.2uf for a total of 6.9uf.

6.9uf into 10k ohms has a -3db point of 2.3Hz, not that there's a whole lot of 23Hz on most of my records but I like to cover all the bases/basses :-)

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I measured 80Hy in the secondary....., posted on February 25, 2017 at 10:17:19
dave slagle
Manufacturer

Posts: 5429
Location: NYC
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6.9uf into 10k ohms has a -3db point of 2.3Hz, not that there's a whole lot of 23Hz on most of my records but I like to cover all the bases/basses :-)


you really cannot extrapolate the impedance of a reactive load @ 20hz down to 2.3hz.

spice says your phase shift @ 20hz is -3.6º and -140º @ 5hz.... and if left undamped I'd watch out for tonearm resonance :-)

(in defense of the autoformers... I generally find that spice shows a much worse case than actually measures in circuit)

dave

 

RE: I measured 80Hy in the secondary....., posted on February 25, 2017 at 12:26:33
mqracing
Manufacturer

Posts: 4323
Joined: June 29, 2000
Dave wrote:

"(in defense of the autoformers... I generally find that spice shows a much worse case than actually measures in circuit)"

Spice (psud, etc) are limited tools... that don't take into account, for example, the various core losses of an inductor.

Data retreived especially concerning resonances should be consumed with a grain of salt. They will almost always be overstated nor is the Q of the resultant complex impedances is not captured in these programs.

MSL



Builder of MagneQuest & Peerless transformers since 1989

 

RE: I measured 80Hy in the secondary....., posted on February 26, 2017 at 08:06:58
Tre'
Industry Professional

Posts: 17260
Location: So. Cal.
Joined: February 9, 2002
"and if left undamped I'd watch out for tonearm resonance"

Would a larger cap help or hurt?

"if left undamped"

There is a 100k ohm resistor from selected tap to ground. Any help?


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I measured 80Hy in the secondary....., posted on February 26, 2017 at 17:35:32
dave slagle
Manufacturer

Posts: 5429
Location: NYC
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Changing the cap value moves and damps / undamps the resonance. Here are a few values (green is always the first value swept (3.5u))





Your 100K load helps damp it a bit but you really need a lower value to show a difference.





Now even though 5K looks nice and is still 10X the source Z to keep the tech heads happy, you now have the same phase shift increase that the guy in the original video complained about. (just at the other end of the spectrum)

dave

 

Thanks (nt), posted on February 26, 2017 at 20:02:43
Tre'
Industry Professional

Posts: 17260
Location: So. Cal.
Joined: February 9, 2002
.
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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