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6SN7 SRPP

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famousmockingbird

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Joined 2010
#1
Hey guys I am looking to do something with some 6SN7's I got laying around and I am thinking of giving my families ears a rest from loudspeakers so I want to build a headphone amp to drive some 32 ohm headphones I have.


I will draw a schematic out later but want to run it by the forum first.

I want to use a 400v HT. I will figure out how involved I want to get as far as regulated B+ and or heater supplies go later. But first I want to check on my calculations for cathode resistors, I came up 390 ohms for both resistors with the input triode fully bypassed. This should set them around -5v bias pulling 12mA.

Is this project worth doing? Or is there another way of utilizing my 6SN7's into a headphone amp?

Thanks
 
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#2
Do you plan to supply 32 ohms headphones directly from 6SN7 SRPP without output transformer ?
That is not going to work at all.
32 ohms load requires a lot more current than your SRPP can supply.
Simple calculation as an example: 20 mA over 32 ohms is some 13 mW only.
You can use 6SN7 SRPP but only with some OPT.
 
#11
Perhaps it would be better to place the output trasformer in the cathodes (using two independent primary windings, each in place of the respective cathode resistor). This arrangement has the problem that the bias current can't be set by chosing the cathode resistors. If the DC resistance of each primary winding is 390 ohms, this might work. Alternatively, the B+ can be lowered so that the cathode current sets itself at 12 mA.
 
#12
I simulated this 6SN7 SRPP with Hammond 125A as parafeed.
It seems that the output impedance of the SRPP is too high for the primary inductance of 125A.
The bass end begins to fall at some 100 Hz.
Also the gain is a bit too low, some -1.5 dB.
Only half of the primary is used.

An externally hosted image should be here but it was not working when we last tested it.
 
#14
Ah yes the low output impedance of a cathode follower nice! I like the direct coupling of the two stages too 🙂 Thanks artosalo for simulating the circuits.

Although I don't have any 6SL7's I do have a bunch of 12AX7's which is electrically very similar to a 6SL7 I believe. I just have so many 6SN7's (at least 50 Raytheon black plates alone) I don't know what to do with, I can't even give them away believe it or not. I hate to see them just sitting in boxes.

I guess I was thinking SRPP at first because it is basically a small OTL amplifier. But if it won't work with my headphones and I have to buy output transformers than your gain stage/cathode follower is the way to go it looks like.

Thanks
 
#15
150Hz - 15kHz +/-1db is the manufacturer rating on the 125A, some roll off below 100Hz is to be expected. Why use only half the primary when there are enough secondary taps to still get the right impedance? Using the full ~15H of the entire primary will help on the bottom end and make it possible to use a smaller, higher quality coupling capacitor. You could get away with a good 2.2u polyester here instead of an electrolytic. I notice in your sim circuit your using an impedance transform ratio of 100 giving you a load of 3200 ohms on the primary. The ratio should be more in the area of 680 to 700 to get around 20k on the primary.
 
#17
jerluwoo said:
150Hz - 15kHz +/-1db is the manufacturer rating on the 125A, some roll off below 100Hz is to be expected. Why use only half the primary when there are enough secondary taps to still get the right impedance? Using the full ~15H of the entire primary will help on the bottom end and make it possible to use a smaller, higher quality coupling capacitor. You could get away with a good 2.2u polyester here instead of an electrolytic. I notice in your sim circuit your using an impedance transform ratio of 100 giving you a load of 3200 ohms on the primary. The ratio should be more in the area of 680 to 700 to get around 20k on the primary.
Click to expand...

Oh I see it now, I was off by a decimal point and thought he had a primary impedance on the 125a of 32k. I was going to use the 10k primary tap and the 16ohm secondary tap with 32ohm headphones giving me 20k. And yes the spec for 125se series is 100 Hz - 15k Hz
 
#18
jerluwoo said:
150Hz - 15kHz +/-1db is the manufacturer rating on the 125A, some roll off below 100Hz is to be expected. Why use only half the primary when there are enough secondary taps to still get the right impedance? Using the full ~15H of the entire primary will help on the bottom end and make it possible to use a smaller, higher quality coupling capacitor. You could get away with a good 2.2u polyester here instead of an electrolytic. I notice in your sim circuit your using an impedance transform ratio of 100 giving you a load of 3200 ohms on the primary. The ratio should be more in the area of 680 to 700 to get around 20k on the primary.
Click to expand...

The actual frequency response depends on the output impedance of the stage supplying the output transformer. The Hammond specs. 150Hz... is based on the assumption that this stage is a typical PP, but now with cathode follower (with GNFB) the frequency response is much wider and the roll off takes place at some 20 Hz.

The use of half primary is because the turns ration fits better for this application. But it would be possible to use full primary too.
Then the voltage swing at the voltage amplifier ( and at cathode follower) will be doubled and the distortion higher.

Well, I don't see it necessary to have 20k load at the cathode follower.
It works fine and with good linearity with essentially lower impedance too.
 
#19
I see where the confusion is, I was looking at the 125ASE which is for single ended use so the primary doesn't have a center tap. Primary is 2,500-10,000 and secondary is 4 to 32 ohms.

As for the SRPP simulation could it be ran again but this time with a 20k load and the Hammond 125ASE?
 
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#20
I simulated the SRPP again, but with 125A, because I don't know the primary inductance of 125ASE.
With 20k load the gain drops to -6 dB, which means that a pre-amp is required.
The frequency response is still poor, at 20 Hz the drop is 3 dB.

I also run the simulation of the 6SL7_6SN7 with 20k and full primary (15H).
THD is lower, but clipping begins now at 33 mW.
I suggest to use the full primary and select the proper secondary tap by listening test.
This circuit is superior compared to SRPP.
 
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