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In Reply to: RE: Difference between an AVC and a TVC posted by xaudiomanx on March 15, 2015 at 08:42:05
A no name out of Thailand.
It sounds great and test great. Very broad band.
It doesn't have a lot of inductance, only about 85Hy so you have to drive it with a low impedance source or the bottom end will get soft.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Tre'
Chip647 built me a 26 tube preamp and he suggested using it on the output of the preamp and leave the volume pot on the input of the preamp all the up. He thinks it would sound better. It is transformer output.
The 26 has a plate resistance of about 7500 ohms.
IMO and depending on the circuit, the output impedance of that stage would be to high to drive a AVC or a TVC.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I use the Bent Audio slagleformer modules (silver wound) on the output of my 26 preamp, in parafeed (4.3uF PF cap) and they work great! The slagleformers have about 150H inductance, which is plenty when fed from the mu output of the 26 tube plate CCS (Z of approx 1500R). Bass is deep and powerful, and flat to below 10Hz (lowest my RTA reads).
By the way, the laminations of the slagelformers can be restacked to have up to 190H inductance, but 150H works fine for me.
Yes, 150Hy has a reactance at 20Hz of almost 19k ohms and the mu output of a CCS has a low output impedance.
I do the same with a CCS loaded 5687 as the last stage of my phono pre through a 7.8uf cap. That and my CD player drive my AVC well.
Paul didn't state the step down ratio of his output transformer nor the value of the plate resistor, operating point, etc... so It's hard to know/calculate exactly what impedance he has to drive a AVC.
I guess my point was....an AVC is not a "one size fits all" deal.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The thing to watch out for on the single winding autoformers is that the load impedance that the source sees changes with every tap... so you will actually have say 23 different impedances that the tube (or cd player, etc) will be looking into.
so you might say that the "primary impedance" is not stable nor constant.
An avc will behave similarly to shunt type attenuator.
MSL
Builder of MagneQuest & Peerless transformers since 1989
As attenuation is applied the load impedance goes up. As long as it's not part of an EQ, isn't that a good thing as far as harmonic distortion is concerned?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I thought the preamp output transformers lower the output impedance to 600 ohms.
I misunderstood.
Yes, 600 ohms should driver the AVC OK.
Lower would be better.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It might be low enough depending.
I don't know what output transformer you have but if I guess a 15k:600 ohm
and I don't know what the circuit looks like but if you are parafeeding the output transformer then plate resistor on the 26 then it's in parallel with the plate resistance of the 26 so your stage's output impedance would be something lower than 15k ohm.
Even if the output transformer is series feed, the source impedance seen by the output transformer is lower than 15k. It would be the plate resistance of the 26.
If we use the plate resistance of the 26, shown in the data sheet showing a range from 7.3k to 8.9k, of 8k ohms then the output impedance of the output transformer will be more like 320 ohms.
But those are all just guesses.
If I had a complete schematic with voltages and currents, and if I knew which output transformer you were using, I could calculate the output impedance better.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre',
How could I get it lower?
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