Will differential share the same THD characteristics as SE, if same triode is used?

[Kenneth Zhu]

I've embarked on a pre-amp project recently. Originally I wanted to use one SE stage plus one SRPP stage, but later I changed my mind to adopt topology as one stage of differential and one stage of SRPP, for differential offers better noice control performance, according to theory.

But I have one puzzle: SRPP, at certain load range, will exhibit higher odd distortions than even. To compromise this, I decide to use a triode that may produce higher even distortions as the first stage. If it is the original SE plus SRPP topology, there is no problem to achieve the compromising effect. But if I use differential, will the distortion characteristisc be the same as using SE? For instance, a 12AU7.

I've checked the school book and found differential seems to have more odd distortions than even. Am I right?

[7N7]

Differential pairs (assuming perfect matching of both the valves and the anode load resistors together with an infinite tail resistance) will halve even order distortion and double odd-order.

The key, as Morgan Jones writes in Valve Amplifiers, is to select valves that produce low odd-order distortion.

Therefore do not use a 12AU7 in your differential pair!

7N7

[Majestic]

I realised they only reduced even-order distortions; but they double odd-orders?! whoa!

[7N7]

Originally posted by Majestic
I realised they only reduced even-order distortions; but they double odd-orders?! whoa!

Yes, but it can still be very good.

For example, I have records of tests I did with 955, which I was planning to use for a pre-amplifier.

Configured as a diff pair, with 50k anode loads and just 148V on the anodes (6.5mA per valve) at +30dB output driving a 30k pad, I got THD of 0.07%. 2nd harmonic was -63dB, 3rd, -55dB. I could live with that!

As a matter of interest, when I prototyped the complete thing: the 955s driving 6AW8 DC coupled using the triode as a cathode follower and the pentode as its sink, I got -3dB at 180KHz.

7N7

[Kenneth Zhu]

Thanks in advance.

[7N7]

Originally posted by Kenneth Zhu
Thanks in advance.

Yes: the most reliable choice is 6SN7/7N712SN7/14N7. Jones demonstrated in his book that these have consistently low THD and in particular 3rd harmonic. If you need a bit more µ then I can recommend 13D3 (µ = 32) which gave good results when I tested it.

I have no information for higher-mu valves. It is vital to remember that operating point is very important; for example, it is well known that the 6SN7 family is best at around 8mA with Va at about 140V.

7N7

[7N7]

Incidentally reading your first post, why are you using SRPP? It makes a good driver, but if distortion is your concern, then you might want to consider another topology.

7N7

[ray_moth]

it is well known that the 6SN7 family is best at around 8mA with Va at about 140V

I didn't know that. I thought 170v was at the lower end of the good performance range.

If you want a high-mu tube with low distortion, the 6SL7 does pretty well.

[7N7]

Originally posted by ray_moth

I didn't know that. I thought 170v was at the lower end of the good performance range.

If you want a high-mu tube with low distortion, the 6SL7 does pretty well.

Yes, in fact anywhere from about 130V to say 220V is good; but of course since one should use a decent value of load resistance, say 47k, running at 170V and 8mA would require HT of 537V which is generally above typical values. Of course even my selected point with a 47k resistor would necessitate 507V!

Active loads is the way to go.

7N7

[ray_moth]

Active loads is the way to go.

It sure is!

Joyeux Noël et Bonne Année