CCS in tail of differential phase splitter - diyAudio
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Old 27th January 2004, 08:52 AM   #1
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Default CCS in tail of differential phase splitter

Hi,

I just joined the diyAudio forum. My question is about the desirability and preferred type of CCS in the tail of a differential phase splitter. I am building a PP amp, of my own design, and the input stage will be a 6SL7 diff. amp (LTP) which will split the phase. There will be a negative rail of -100v which will enable me to use 47k in the tail of the LTP with the grids at ground potential. I think this should result in the 6SL7 drawing ~ 1mA per side and I'm planning to use 100k plate resistors.

47k seems a reasonably high value for the tail resistor, which I hope should mean that I wouldn't need to offset the unbalance of the LTP by using unequal plate resistors. However, I wonder if I would get noticeably better results using a CCS in the tail, instead of the 47k resistor? I know a CCS is theoretically the best solution but would it really make that much difference if the tail resistor is already 47k? I'm afraid of introducing unwanted noise, which could mean that a CCS would solve one problem only to cause another.

If a CCS is the best solution, would it be better to use SS or a tube? If SS, what device would be best (NPN transistor + zener, LM317, FET etc?) If a tube (probably a pentode?), would it make much difference which one I used? I guess it would have to be one that can operate at fairly low plate voltage (<100v) and current (2mA). The 6AU6 comes to mind but I don't know how noisy it is.

Thanks in advance for any advice,

Ray
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Old 27th January 2004, 10:49 AM   #2
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i have used both c4s and enhancement mosfet ccs on a diff stage and always found it to be better. I prefer solid state than tubes as they offer a higher impedance more towards an ideal ccs. However many people would use tube to maintain an all tube approach. Also a tube approach would require a higher voltage i.e the 6au6 aproach needs atleast more 150volts as the screen is biased normally at 100-150 volts.
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Old 27th January 2004, 12:16 PM   #3
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Default Go for it!

Any phase splitter has large enough signals for noise not to be an issue. Even better, your proposed differential pair will reject noise in the tail because it is common mode.

Whether you use SS or valve, your CCS will need a negative rail because the cathode of the 6SL7 won't be very positive. You could use SS (but not 317) from your -100V rail, but a 6AU6 could easily achieve a tail resistance of 4,700k rather than 47k and fits the requirements very neatly.
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Old 27th January 2004, 01:37 PM   #4
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I've done it both ways, and don't see a huge performance difference, though the CCS does spec out a bit better (I used simple one or two bipolar circuits). You'll still end up having to slightly unbalance the plate resistors to get proper AC balance, so the real advantage is in PS rejection.
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Old 27th January 2004, 02:43 PM   #5
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Originally posted by SY
You'll still end up having to slightly unbalance the plate resistors to get proper AC balance,
I don't agree. Since the CCS has near-infinite resistance, it forces the signal current current to swing backwards and forwards between the anode circuits with no loss, so perfect AC balance is obtained by equal value anode loads.
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Old 27th January 2004, 05:24 PM   #6
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EC: That's what the theory says. But I've not always seen that to be true with real circuits using lowish gm triodes- I've always gotten some imbalance. I'll rig up a breadboard circuit tonight with a 6SL7 and a 6SL7 and do some quick measurements.
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Old 27th January 2004, 05:43 PM   #7
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SY: I tested a 6SL7 differential pair phase splitter a while ago, and although I wasn't explicitly checking for balance, nothing shouted at me that it needed investigation. What was reinforced, however, is the need to load both anodes equally. Even the capacitance of a x10 oscilloscope probe was enough to unbalance it and bring the HF -3dB point down from 105kHz (probe on both anodes) to 70kHz (probe on one anode).

I await your measurements with interest...

Incidentally, substituting a (Loctal) 7F7 reduced distortion by 18dB...
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Old 27th January 2004, 05:52 PM   #8
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EC; Yeah, I got burned on that one myself some years ago when I was puzzled about why my split load inverter was so unbalanced at higher frequencies.
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Old 28th January 2004, 04:37 AM   #9
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Thanks for the helpful replies. I'm tempted to take the pentode route, because it's a simple solution, provided that it can work properly with the -100v negative rail (I suppose I could use a voltage doubler if 100v isn't enough). I can easily provide a separate heater supply to avoid cathode-heater voltage problems and sticking an extra tube socket in there would be no great problem.

Does anyone know if the 6AU6 is a noisy tube and, if so, would there be a better choice?

Thanks,

Ray
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Old 28th January 2004, 01:05 PM   #10
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Ray asks...

Quote:
Does anyone know if the 6AU6 is a noisy tube and, if so, would there be a better choice?
I'm using 6AU6's as the input tubes in my 47P-P differential power amplifier. Thay are nice and quiet in this application. They should be quiet when used as a CCS also.

The 6AU6 has reasonable transconductance and high plate resistance so should make a fairly good CCS.

Gary

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