Wanted to post briefly in praise of the Tribute Autoformer Volume control. I've been sceptical about passive preamps for a while but something about the simplicity made me want to give them a try. It was complicated by the fact that everything up to the power amp inputs in my system is fully balanced (phono is fully balanced valve stage). Pieter supplied me with a beautiful 4 way AVC.
Having built it in a crummy old box, with a 4 way selector switch and tried it I was blown away by how much better this was. And it wasnt competing against junk, but a handcrafted balanced valve line stage!
In my view well worth the money.
Having built it in a crummy old box, with a 4 way selector switch and tried it I was blown away by how much better this was. And it wasnt competing against junk, but a handcrafted balanced valve line stage!
In my view well worth the money.
What's a 4 way AVC?
My system is all balanced. I've used balanced passive switched attenuator (John Broskie) until recently, and have just built up a diy TVC. A TVC is easy to set up for balanced use, really its no different than single end except that you don't ground either end of the secondary. Never really thought about AVCs though.
My system is all balanced. I've used balanced passive switched attenuator (John Broskie) until recently, and have just built up a diy TVC. A TVC is easy to set up for balanced use, really its no different than single end except that you don't ground either end of the secondary. Never really thought about AVCs though.
The Tribute balanced AVC is 4 autoformers on a single spindle. Left/Right x +/- phases so 4 way.
The Autoformer is like a transformer but only one coil. The secondary is 24 taps.
Read lots about them and the way they present such an easy load to the source, but also instead of wasting signal as heat it changes volts for amps. So instead of sounding worse at low volumes it sounds in theory better. Ok not sure if go that far, but I can't quite believe how much of my system I've been masking.
The Autoformer is like a transformer but only one coil. The secondary is 24 taps.
Read lots about them and the way they present such an easy load to the source, but also instead of wasting signal as heat it changes volts for amps. So instead of sounding worse at low volumes it sounds in theory better. Ok not sure if go that far, but I can't quite believe how much of my system I've been masking.
I'm a relative newbie in terms of the electrical background - but that makes sense. Sounds excellent.
Thanks Mick, I now get it.
The Tribute AVC is well regarded and I'm sure it sounds great. But for balanced operation, is the added complexity worth it? My TVC has one transformer per channel, I dont need a separate transformer for each phase. You are using 2 coils per phase, I assume that this doubles the cost, more or less.
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Obviously if you want the signal to stay balanced then you need an AVC (or TVC) per phase so 4 total. If you're switching back to single-ended anyway then yes you just need 2. I had planned to do that, in fact I have a stereo AVC from Pieter which I bought before I went balanced (must find a use for that). Yes this doubles the cost of the volume control, and the switch (which must be 4-way also) and the wiring. The 4-way AVC was less than £700. As I'm building it myself I'm prepared to spend that kind of money to make that much difference. I suspect a balanced passive AVC would be silly money pre-made - I much prefer to build myself from the best components I can find. It's an incredible hobby.
Once I experienced balanced audio I took a decision that I want to retain the balanced signal for as long as possible. At present after the passive pre the signal goes to my PX25 power amp where it's taken back to singe ended at the transformer inputs. However I do plan to build a power amp which operates as balanced as far in as possible up to the single ended 300B/PX25/2A3. Which probably just means the input driver stage, but more research and thinking needed (and advice, which I'll be back for lol).
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Mick, my system is balanced all the way - my phono-preamp is balanced in from the cartridge and balanced out. My Dac is balanced out. My power amps are fully balanced from input to output. Yes balanced is the way to go. My TVC is also balanced but it does not have one coil per phase. Only one coil per channel. A transformer cares not if the input (or the output) is single ended or balanced.
My question was not around the complexity of balanced operation - in fact balanced operation has enabled me to simplify my system and sounds great as well. My question was why the cost of a 4 way AVC - I simply use 2 transformers and its all balanced.
This is what i bought
33 Step Transformers Volume Control passive preamp
$350 in Aussie dollar including postage. I've had them for 2 weeks and very pleased so far. By the way, that's about GBP218. Cheap as chips.
Perhaps my inexperience but I don't get it. Surely to volume control left (+ and -) and right (+ and -) you need 4 AVCs (or TVCs) ?
Perhaps the AVCs are different to the TVCs. With the Tribute, each of the 4 AVCs has IN, OUT and Ground. XLR pins 2/3 go to the INs (via the switch) and I've commoned the grounds for each channel across the inputs, the AVC and the XLR out.
Perhaps you're saying you connect the + and - of the input across the transformer primary, and similar with the secondary ? Hmmm ok I think I see. I wonder if a stereo autoformer (which I have spare) would provide a balanced volume control in that way ? I think I'd need more experienced heads than mine to comment on that.
By the way I'd be interested in your power amp design if you're happy to share ?
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If there's an advantage to transformers, that's not it. A 20k potentiometer will have a Johnson noise of about -118dBV at its center, not exactly significant.
So is this how you're connecting ? (1 channel shown and no switch for simplicity)
Attachments
Yes, +ve and _ve inputs across the primary and secondary. No centre tap.
Here's my power amp. I've been using 4 of these amps (each channel is bi-amped) for about 15 years now. Still happy with the sound and haven't felt the need to update. Note that the current source in the output stage really is 9amp. You need lots of heat sink. I haven't blown an output transistor for more than 10 years, but bridge rectifiers, which have their own heat sinks, blow up every year or so.
http://www.diyaudio.com/forums/solid-state/212424-balanced-class-fet-amplifier.html
Thanks. Pieter at Tribute was adamant that balanced stereo needed a 4-way, but I understand what you're saying. I doubt his motives are purely cynical/commercial so I think I'll pose a private question to him.
Re power amp, I'm strictly a valve man but thanks.
Speaking to Pieter and to more experienced friends yes I see how this works.
The Tribute stereo (2-way) control can accept 7VRMS so as long as the voltage across the two phases doesn't exceed this then it should be ok. There's also the increased indutnace : 150H of single coil inductance is some 18k at 20Hz (becoming much higher at higher frequencies), with 2 coils not a problem but possible effects from running it across the 2 phases.
Some of this is beyond my experience and knowledge level, so I'm going to build it and try it out. One suggestion was to give the output a 0v reference as per the diagram attached.
The Tribute stereo (2-way) control can accept 7VRMS so as long as the voltage across the two phases doesn't exceed this then it should be ok. There's also the increased indutnace : 150H of single coil inductance is some 18k at 20Hz (becoming much higher at higher frequencies), with 2 coils not a problem but possible effects from running it across the 2 phases.
Some of this is beyond my experience and knowledge level, so I'm going to build it and try it out. One suggestion was to give the output a 0v reference as per the diagram attached.
Attachments
Just for clarity, I realise the above could imply the suggestion came from Pieter - it didn't.
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