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77.43.10.3
This is my 'summer time' amplifier (with much less waste heat than my previously published Stonehenge amplifier). It has been entirely built with surplus HW purchased at radio-ham fairs/e-bay auctions(overall expenditure for parts of about 100 Euro, tubes included).
Key Features:
- 'zero impedance' 300V HT power supply (that is, the amplifier is directly powered by rectification of 220 AC mains, with double isolation chassis and input isolation transformer)
- novel PP topology with automatic zero current imbalance (of the 70 mA bias current)
- output toroidal AC mains transformers for minimum flux leakage/stray capacitance and low cost
- output residual hum lower than 5 mV peak
- 6 dB overall NFB
- no PCB/all direct wiring
The distortion at rated power is almost entirely second harmonic (1%, see measured output spectrum).
ecc230
Follow Ups:
~!
"The Sound of One Hand Clapping is Crashing by Design" HKM
Output Spectrum, 4W @ 1KHz
ecc230
Let's try again.
ecc230
Schematic
ecc230
I see disadvantages but I don't see the advantages of your circuit over a normal push pull circuit.
But hey, if you like it.......
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
No adjustments of PP bias currents, low components count, no trimmers, 'eternal & perfect' balance of output currents (as the transformer primaries are in series), no need for tube matching, no HT transformer, modest increase of second order distortion whith tube ageing.
Last but not least, you can use economic AC mains transformers (the same as for linear power supply) instead of expensive audio transformers (because of the perfect balance of the bias current).
Does a DIYer ever needs more?
ecc230
"Last but not least, you can use economic AC mains transformers (the same as for linear power supply) instead of expensive audio transformers (because of the perfect balance of the bias current)."
You're not mentioning the downside of off-line operation; for example:
What's the voltage test/isolation rating on your input transformer? Most of the low level transformers I own are rated/tested at 500VDC. What kind of isloation would be required for agency approvals if you were going to make this as a product, and how would that impact the cost? How would it impact the signal quality aspect of the input transformer, e.g. coupling, inductance, voltage headroom, frequency response in circuit?
What other construction is necessary to double isolate the exposed case and user controls, and how does that impact cost?
Cheers,
Michael
I used this output transformer (2 X 115V - 36V (that is, with 2 X 18V in series):
http://it.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=5404968
The reflected load is then 75 Ohm (in each half primary section), corresponding to the theoretical maximum power transfer.
For feedback I had manually added an auxiliary coil through the center of the toroidal transformer (about 100 turns); the actual output of this coil was 1/8 W.R.T. the 36V output, enough for the purpose.
For the input transformer I recommend something like this one:
http://it.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2106447
If you wanted to create an industrial product the safety problems are the same as with old TV sets (nothing new).
ecc230
Seems a bit wasteful but I suppose the power supply is almost free ;)
You can obtain good performance from toroids running single ended or push pull . Balancing push pull , all that is required is a bypassed CCS in each cathode . For single ended I've used a 50VA 230:9+9 using one of the secondaries as an offset winding . The offset winding is driven by a high impedence DC source such as a choke or ring of two CCS (chipregs are no good) . This can also be used to power the filament , in my case I ran a 6B4G at approx 40mA , the offset being achieved by the 1A filament supply in conjunction with the 25:1 turns ratio . The 50VA toroid could take around 3-4mA before saturating . Alternatively valves with greedier filament such as 2A3 can be catered for by adding a tertiary winding
Al
"No adjustments of PP bias currents, no trimmers, 'eternal & perfect' balance of output currents (as the transformer primaries are in series), no need for tube matching..."
Only in terms of saturation of the output transformer. The current, in the output tubes, will still need to be matched if you want the signal waveform to be right.
Also, your output tube cathode bypass caps are 100% in the signal path.
With normal Class A PP that cap is not even needed. (although most put them in anyway)
"Last but not least, you can use economic AC mains transformers (the same as for linear power supply) instead of expensive audio transformers (because of the perfect balance of the bias current)."
Again, only in terms of the output transformer not saturating because of DC.
How good the frequency response of that transformer is has to do with more than just core saturation.
As for "no HT transformer".
1. It forces you to add an input transformer. BTW if you are going to do that anyway, why not make it the phase inverter? A transformer with a center tapped secondary will be a better phase inverter than one built with a tube.
So, input transformer and a 12at7 diff amp to the output stage.
2. It looks scary to me. I like the main rail to be isolated from the wall.
One last question. Why 6as7? There are much more linear tubes you could use.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
This circuit has been made possible only because of extensive LTSpice simulations. I would have never tried its construction if LTSpice had not been available for simulations. The circuit performs exactly as LTSpice model predicts (notwithstanding the crude and ultra-economic construction, see the photo, and I hope nobody gets horrified).
Experimenting with LTSpice model (if you send me an e-mail at cellai@space.it I will be glad to deliver the file) will help to answer all 'what if questions' you may have, and to figure out the inner working of the circuit.
Safety issues they are the same as for any vintage TV set.
The reason for 6AS7 choice is because I have many of them available (and I can directly purchase in Russia/Ucraina lots of Svetlana NOS items for 7 Euro/each (plus shipping).
Just for fun I have created correspondent designs with 6L6 and EL 84.
If you limit your choice to tubes with less than about 300V anode voltage (and to no more than 8K plate to plate reflected impedance) you can play with this topology and plenty of commercial AC mains 'output transformers'.
ecc230
Maybe you have noted this is a kind of "parafeed" amp with all signal going through the big caps parallell to the output tubes. One could simplify it with only one cap from midpoint to ground but it might worsen PSRR.
We did a variation of this "novel technology" about a year ago at diyaudio with a PMOS as upper element(modulated CCS) in a 6B4G amp. Still in the workshop to be built.
About mains-toroids as OPTs I can say they work fine having built at least one with 6B4G and 6L6 as anti-triode. Current-balancing is critical though, as you have to stay within a few mA.
Brgds
Lars
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