Low TIM, low distortion hybrid front-end

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2-nd channel is up and running

Just soldered and tested the second channel of the front-end.
Works flawlessly from the first power-on - with selected tube and 1% resistors even no need to touch the trimmer - 1.5 mV DC offset right away.
No re-biasing of the power section was required - all parameters are pretty much the same as for the other channel (I tested with the same OPS).
:cheers:
 
Why do you need the second LTP (folded cascode) would it not be possible to go directly to the folded VAS, or is it because you need the extra open-loop gain in order to have good distortion figures..??

Hi Miib,

In fact, most of the gain is provided by VAS.
Open loop gain distribution looks like this:
1) Tube LTP = 10.8db
2) 2-nd LTP = 18.7db
3) Cascoded VAS = 43.2db

Overall OLG = 72.7bd

Assuming CLG = 28.5 db --> LG = 44.2db

The role of the 2-nd LTP is to be a relatively low gain buffer between IPS and VAS, allowing, on one hand, low gain for IPS, and ensuring, on the other hand, low influence of VAS on IPS at high swing. In overall, this topology ensures good transients handling, providing low intermodilation distortion, which is confirmed by live measurements...

Cheers,
Valery
 
That does also translate into app 20dB lower distortion, which may or may not be good. :)
I see what you mean by a current buffer, but do you really get a better drive of the VAS..? As current through the second stage is just a low as in the tube IPS

20db lower distortion is a "side effect" :D
Tail currents are different - Tube LTP = 2.8 mA, 2-nd LTP = 4 mA. 2-nd LTP provides a good isolation of IPS from VAS, providing some gain at the same time. Distributing the gain between 3 stages, keeping the gain of the first two relatively low, allows very good transients handling.

Cheers,
Valery
 
Question - is there any reason to be concerned about having more stages meaning more phase shifts, more junctions with non-linear capacitance ?

From my perspective, based on experience, sometimes it is worth to add some intermediate stage despite the fact it adds phase shifts, non-linear capacitance, etc. It's like adding a EF between the LTP and VAS in blameless - it provides proper "isolation", impedance harmonization (wow! cool expression :p), if you like. The one I use is not a EF, but does the job and some additional gain is also a plus in this particular case.

There may be different opinions ;)

Cheers,
Valery
 
I have never tried a design without global negative feedback so I honestly hadn't looked too closely at it. A hybrid does intrigue me and I do have an earlier iteration of the design in this thread. My boards came from Terry and will need modification to use the latest schematic. I do have two sets of matched and balanced tubes ready to go.
 
The heavier "tube sound" is what has me curious. It has a servo for DC offset control so matching devices isn't as critical.

Yes, the one you mentioned has "pure tube sound" with some 2-nd harmonic at up to -50db. I still have its prototype in my system together with HEX FET OPS with 3 OP pairs - it sounds very, very nice. So now I run two parallel hybrid projects - ultra low distortion and pure tube gain with no global NFB. Either of them would be a valid choice. Some listeners here prefer more natural and transparent first one, some others like the sweeter and warmer second one. I personally like the second one with jazz trios/quartets, blues/rock/progressive bands with clean tones - they sound delicious with pure tubes.
However, big bands or symphonic orchestras playing classic music seem to present more detail and sources separation with the ultra linear one.
Both of them present excellent elastic, pleasant bass, slightly tighter one for ultra-linear design.
So, the best way to hear the difference - build both and listen :p
As soon as you get an appropriate transformer and arrange the proper soft-start - all the rest is pretty easy ;)
 
Yes, the one you mentioned has "pure tube sound" with some 2-nd harmonic at up to -50db. I still have its prototype in my system together with HEX FET OPS with 3 OP pairs - it sounds very, very nice. So now I run two parallel hybrid projects - ultra low distortion and pure tube gain with no global NFB. Either of them would be a valid choice. Some listeners here prefer more natural and transparent first one, some others like the sweeter and warmer second one. I personally like the second one with jazz trios/quartets, blues/rock/progressive bands with clean tones - they sound delicious with pure tubes.
However, big bands or symphonic orchestras playing classic music seem to present more detail and sources separation with the ultra linear one.
Both of them present excellent elastic, pleasant bass, slightly tighter one for ultra-linear design.
So, the best way to hear the difference - build both and listen :p
As soon as you get an appropriate transformer and arrange the proper soft-start - all the rest is pretty easy ;)

I plan to build both but that evil "work" thing is getting in the way at the moment. Not enough hours in a day.
 
I haven't been listening to mine because it is a lot to set up and care must be taken when starting up due to the huge offset until the tube has fully warmed up. Once I get the protection circuit built I may build a case for it so I can just play it whenever I feel like it. I'm not sure I know what the"tube" sound is when it comes to HiFi. I had an old RCA console stereo back in the 60's but since then I have only had SS. This hybrid sounds just like my other IPS to me, which is lovely of course.

Blessings, Terry
 
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