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#1 Long tail pair with a short tail

Posted: Wed Jun 13, 2018 5:57 pm
by izzy wizzy
I have a question about a long tailed pair with a short tail. Imagine your typical long tail phase splitter. To get decent balance on the anodes with resistor loads, the tail should be a big resistor which implies a negative supply or a constant current source. Now instead of resistive anode loads, there's a PP transformer like so
6n6lineaud1.JPG
And now make the tail short so it's just a regular biasing resistor to ground. So we end up with something like this with one input grounded and the other signal
6n6lineaud2.JPG
6n6lineaud2.JPG (22.81 KiB) Viewed 6209 times
It will be unbalanced by 1dB or so accourding to the model and you can see a rise in 2nd harmonic distortion which is to be expected.

So my question is, with very small signals, phono stage levels, what would be the expected impact on the sound?

cheers,

Stephen

#2 Re: Long tail pair with a short tail

Posted: Wed Jun 13, 2018 6:40 pm
by Nick
I would expect as you say a little 2nd, but no other real problem. FYI, in that case I would be looking at a CCS using a DN2535N3-G instead of a 317, they are only 56p from Farnell.

I just found Farnell had them when I had to repair a ARC phono which used them all over the place in LTP tails.

#3 Re: Long tail pair with a short tail

Posted: Wed Jun 13, 2018 7:05 pm
by izzy wizzy
Thanks Nick. I had used the 317 in the line stage but soon got rid and used an input tx and small tail. I'll look up the Farnel part as Ihave an order for there soon. Will be interesting to see if it's audible.

I did some more in the sim and it looks like unbalanced there's about 0.3% 2nd harmonic and balanced about 0.03%. Is 0.3% at full level audible? I have no idea. I'm only doing this as I don't have an SE transformer but can use a PP transformer.

cheers,

Stephen

#4 Re: Long tail pair with a short tail

Posted: Wed Jun 13, 2018 10:46 pm
by RhythMick
I have some spare SE transformers if you'd like to try them. I can take to Owston if you want.

#5 Re: Long tail pair with a short tail

Posted: Thu Jun 14, 2018 8:29 am
by IslandPink
izzy wizzy wrote: Wed Jun 13, 2018 7:05 pm I did some more in the sim and it looks like unbalanced there's about 0.3% 2nd harmonic and balanced about 0.03%. Is 0.3% at full level audible? I have no idea. I'm only doing this as I don't have an SE transformer but can use a PP transformer.
cheers,

Stephen
What's 0.3% in dB relative to the main signal ?
Is that -25dB or -50dB ?
Either way, that will be audible, when I was doing the jFet phono input , there was a significant difference in tone from what we calculated to be the difference between no distortion, and distortion giving 2nd-harmonic at -70dB . It was better with the small amount of 2nd. These were sims on Spice , not measured, but I expect the delta was more or less as calculated.

#6 Re: Long tail pair with a short tail

Posted: Thu Jun 14, 2018 8:36 am
by Nick
the difference between no distortion, and distortion giving 2nd-harmonic at -70dB . It was better with the small amount of 2nd.
Thing is, the no distortion case wont be no distortion, there will still be higher harmonics down there, but the addition of the 2md will mask them. I found when messing with the monoblocks that distortion down below 0.0005% mattered without the masking of 2nd and 3rd.

#7 Re: Long tail pair with a short tail

Posted: Thu Jun 14, 2018 9:51 am
by IslandPink
Amazing, isn't it !

#8 Re: Long tail pair with a short tail

Posted: Thu Jun 14, 2018 7:43 pm
by izzy wizzy
Interesting stuff.

I don't know if I'm doing this right but anyway I ran 3 circuits. One diff with a short tail, the other with a CCS inthe tail and a single ended version.

This is the FFT traces of all three
5687fft.JPG
The first and 3rd look almost the same with the CCS version having suppressed 2nd harmonic. Is that right?

cheers,

Stephen

#9 Re: Long tail pair with a short tail

Posted: Thu Jun 14, 2018 7:44 pm
by izzy wizzy
RhythMick wrote: Wed Jun 13, 2018 10:46 pm I have some spare SE transformers if you'd like to try them. I can take to Owston if you want.
Thanks for the offer but I won;t be at Owsters ... this time ;)

cheers,

Stephen