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Tube line stage with unbalanced input, balanced outputs?

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rongon

rongon

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Joined 2009
#1
Hey all,

I built a knock-together line stage with 6FQ7 common cathode stage into a 6DJ8 cathodyne phase splitter. I used it into a couple of low-gain push-pull tube amps with good results. However, I'm playing with a pair of JBL LSR305 powered speakers which have balanced inputs with low 10k input impedance. The 6DJ8 cathodyne doesn't do very well into that low of an input impedance.

I read in MerlinB's chapter on the cathodyne that it's not really suitable as a cable driver. It's prone to picking up hum on the plate (anode) side. I think that's happening in my setup.

But I'd still like to make a tube line stage with gain of about 10, unbalanced inputs and balanced outputs, that can drive 10 foot balanced cables into 10k ohm loads. Is there a good, proven circuit for accomplishing this?

I just bought an Impasse pcb, and I'm still planning on trying that, but would an LTP with MOSFET source follower outputs be better for this particular application? I'm thinking a 6N6P LTP with 10k plate loads and a CCS in the tail, DC coupled to something simple like IRFBC20 MOSFETs, would work well.

Or will the Impasse be perfectly adequate for this?

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#2
Consider something like a 5687 driving a phase splitting transformer. With this setup you get galvanic isolation. The 5687 has better drive capabilities than the 6DJ8. However something like the ECC99 might even be better and then you could consider a bit of a step down to gain some extra drive capabilities.

Shoog
 
#3
You can setup the 6FQ7 as a differential amp with the cathodes tied together an a CCS under the cathodes. In similar projects, I have simply rectified the 6.3V heater winding and grounded the + side to make a -8V DC rail to feed the CCS to help with compliance.

The LTP can drive your 6DJ8s, which I would just setup as a pair of cathode followers.

If you have sufficient voltage, there isn't any reason you couldn't directly couple this. Such a setup will also give you opportunity to use balanced inputs down the road if you so desire.
 
#4
I drive a Rane AC23B, with a similar input resistance to your amp. My linestage is a LTP of triode-strapped EL84's. CCS in the cathodes, and 15k plate loads. B+ just shy of 500V and idle current per cathode is 35 mA. It does a fine job...🙂
cheers,
Douglas
 
#5
Guys, any example of a differential (with a CCS tail) driver stage on 6SN7 and the PP IT 1:2-3, e.g. LL7903 ?
Running like 10 mA each half with 280-300V B+, loaded onto fixed biased EL34, KT66, KT88 etc.
 
#8
Just gathering the opinions/info. I did not say 10K load anywhere ))

Yeah, I thought about 1:4+ too, let's say the fixed bias Rg is 330K, as there are 2 => 165K, with 1+1:4+4 IT it gives perfectly 10K load of each triode, do I get it correct?
 
#9
I'd still like to make a tube line stage with gain of about 10, unbalanced inputs and balanced outputs, that can drive 10 foot balanced cables into 10k ohm loads. Is there a good, proven circuit for accomplishing this?
Click to expand...

Ron, the circuit provided has roughly 10X gain and the cojones of a blue whale. Add a Sowter 3603 to each channel and you have balanced O/Ps and galvanic isolation.
 

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#12
I vote for ImPasse. IMHO, a lot of the ideas being thrown out are fine, but probably wouldn't be comparable in performance to the best line driver IC's. It takes some work to design to optimize both distortion and CMRR. SY did that work.
 
#18
audiowize said:
26dB is a LOT of gain for a typical linestage. The drive requirements of the F4 are rather unusual.
Click to expand...

Fair comment. My point is that some of the ideas being thrown out didn't really embody some thought as to overall performance for the required role, including some suggestions with necessarily bad CMRR.

To give you more of an idea of what I am getting at, a lot of the ideas thrown out for tube stages with SS loads to lower distortion. I had a look at Bill Whitlock's recent paper on bootstrapped line receivers and the tube schematics at Jensen. Very little emphasis on the details of active stages, much more emphasis on circuit topology. Remember, they are recommending circuits for pro audio, so good, quiet performance is necessary, even if they are biased as to transformer application.

As a side note, I noticed that what they call ultra high performance balanced drivers used 2 transformers, 1 before the active device, and 1 after. I'm gonna have to use that sometime 🙂
 
#20
With a good cascode CCS load the gain will be mu, and that is because the AC load is essentially infinite in comparison the rp of the tube, and there is no significant AC loading in the following stage..

Don't forget that Vpp is 6dB greater at the output due to the balanced output. So gain is in fact closer to 32dB input to differential output.
 
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