El D and myself are designing valve synthesized FM tuners.
A link to what we are doing is here:
Re: 100% Tubed FM Tuner
Although our approaches are slightly different I would like the unit to have both phono and a headphone output. Sadly the MPX decoder is SS, a LM4500A chip and a op-amp MPX post filter. The LM4500A should have good rejection of the HD channel interference used on some US transmissions. The MPX filter can drive the phono output and a volume control for the headphone amp. I would like the headphone amp to be at least part valve (SS CCS OK).
Requirements are:
HT 100V as that is what the rest of the circuit runs on.
Could use 120V smoothed but unregulated.
THD < .5% when driving 32R cans
Voltage gain ~3x
Output clipping ~4Vppk @ 32R
Small space 1-1/2 valves per channel.
The low HT makes the design more difficult. A small OPT is required - any recommendations/suggestions/ideas/simulations would be much appreciated.
A link to what we are doing is here:
Re: 100% Tubed FM Tuner
Although our approaches are slightly different I would like the unit to have both phono and a headphone output. Sadly the MPX decoder is SS, a LM4500A chip and a op-amp MPX post filter. The LM4500A should have good rejection of the HD channel interference used on some US transmissions. The MPX filter can drive the phono output and a volume control for the headphone amp. I would like the headphone amp to be at least part valve (SS CCS OK).
Requirements are:
HT 100V as that is what the rest of the circuit runs on.
Could use 120V smoothed but unregulated.
THD < .5% when driving 32R cans
Voltage gain ~3x
Output clipping ~4Vppk @ 32R
Small space 1-1/2 valves per channel.
The low HT makes the design more difficult. A small OPT is required - any recommendations/suggestions/ideas/simulations would be much appreciated.
Well ... since you've drunk the Koolaid already (SS op-amp MPX post-filter), why not get your 3x gain off that, and then drive a conventional OTL cathode follower?
3 valves, 2 triodes each, would seem to deliver the 1/2 W of OTL quite easily. No 'valve amplification color' of course, but who cares.
Pretty awesome simple to design, too.
Don't forget the current steering resistors in the cathode cross connect though.
You'll still be dissipating some 25 W of anode heat, 4 W each triode, I think.
... for both channels.
GoatGuy
3 valves, 2 triodes each, would seem to deliver the 1/2 W of OTL quite easily. No 'valve amplification color' of course, but who cares.
Pretty awesome simple to design, too.
Don't forget the current steering resistors in the cathode cross connect though.
You'll still be dissipating some 25 W of anode heat, 4 W each triode, I think.
... for both channels.
GoatGuy
Umm that's a lot of heat just for headphones I was think something more efficient hence the transformer. Its more difficult to get the gain in the MPX filter as its a modified sallen and key. I only need <100mW for headphones. Yep a valve MPX is tricky to make perform as modern SS and the coils difficult. The 10.7MHz IF we are borrowing toko 10k as they seem fine at 100V (measured at 400V primary to secondary).
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OK. Well, one of my favorites remains the G-to-1 (gain-to-one) RA to RK gain stage. in your case, 3x, so RA maybe of 16 kohm, and RK of 6.8 kohm, itself split to give the desired midpoint VA.
You know, RK1 = 1.4 kohm and RK2 = 5.4 kohm. The to-grid bias point is taken from the junction of course, thru a convenient 50 kohm to 220 kohm resistor. And of course, the usual series 220 to 560 ohm oscillation suppression resisstor as well on the grid.
The advantage of this is that it has a seriously more linear gain due to cathode degeneracy. (i.e. no capacitors down there!). But with the split RK it also has quite a bit of dynamic range. Take the gain-at-the-anode thru a usual coupling capacitor, to the 'output' stage(s). Pentode is OK, or ganged-together-triodes. Use a parafeed perhaps to a small output transformer; I like modern well-made low-power power transformers for this. Your step-down is what, 4,000 to 10,000 ohm primary to 32 ohm secondary, or sqrt( 125 to 300 ) = (11 to 17) to 1. A 115 VAC to 9 volt secondary is a pretty darn good fit.
And as you say, it only has to do about 3 VRMS into the cans.
Just saying...
GoatGuy
You know, RK1 = 1.4 kohm and RK2 = 5.4 kohm. The to-grid bias point is taken from the junction of course, thru a convenient 50 kohm to 220 kohm resistor. And of course, the usual series 220 to 560 ohm oscillation suppression resisstor as well on the grid.
The advantage of this is that it has a seriously more linear gain due to cathode degeneracy. (i.e. no capacitors down there!). But with the split RK it also has quite a bit of dynamic range. Take the gain-at-the-anode thru a usual coupling capacitor, to the 'output' stage(s). Pentode is OK, or ganged-together-triodes. Use a parafeed perhaps to a small output transformer; I like modern well-made low-power power transformers for this. Your step-down is what, 4,000 to 10,000 ohm primary to 32 ohm secondary, or sqrt( 125 to 300 ) = (11 to 17) to 1. A 115 VAC to 9 volt secondary is a pretty darn good fit.
And as you say, it only has to do about 3 VRMS into the cans.
Just saying...
GoatGuy
Do you have a sketch that would be most excellent, I did not think of using a power transformer much cheaper solution. I can easily get one and measure the response. Toroidal or EI. I was thinking ECC88/6DJ8 for low voltage. I think a small one 1.5VA. The no load current is usually 1/10 of full load which places the primary inductance in the sort of 80H area which sounds good. What about a LTP?
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