LTP vs single first stage

[dgta]

Shoog, I don't follow your logic. Maybe I'm a bit slow today.

If I want that flexibility, I can transformer-couple the input on a simple (common cathode) stage just as well. And shift voltages as desired to get the anode to any DC potential. I don't see how an LTP is inherently superior in that regard. Am I missing something?

[Wavebourn]

I prefer a single tube on input: less noise, less microphonics, less distortions (yes, because only even order distortions cancel each other, whiloe odd order ones increase).
In opamp we want zero volt output / zero volt common mode input bias. I don't use vacuum tube opamps, so it is not an issue.

However, YMMV.

[Shoog]

Originally Posted by dgta

Shoog, I don't follow your logic. Maybe I'm a bit slow today.

If I want that flexibility, I can transformer-couple the input on a simple (common cathode) stage just as well. And shift voltages as desired to get the anode to any DC potential. I don't see how an LTP is inherently superior in that regard. Am I missing something?

I must apologize for misunderstanding you - you are not talking about using a single triode up front to do your phase splitting - just initial voltage gain. I pictured the input triode as concertina style phase splitting - my mistake.
However if you are going to drive only one input of the LTP, it is my opinion that you are not getting the best out of it performance wise, and so the sooner you have your phase split and are in differential mode the better.

Of course that is just my opinion based on what works for me.

Shoog

[Bigun]

My experience with LTPs is from the solid state world only. But I don't like them. I don't plan to use them anymore and I much prefer a single device at the input.

Why ?

The LTP works great on paper when you have two perfectly identical devices but when they are different you can have a fair bit of asymmetry. Invariably you need a current mirror to ensure good balance between the currents through each device, and you often need the additional gain that the current mirror provides (see next point) as well as the improved slew rate to drive the compensation capacitor in a feedback amp.

For transistors at least, the transfer curve for an LTP is rather narrow in terms of differential input voltage. It may look enough on paper, but you suffer under transients - the LTP likes small signals. And I've heard the difference - flattening/expanding the transfer curve by using a complimentary feedback pair for the LTP provides remarkable improvements in bass and overall clarity through having a more accurate error amplifier up front. An LTP feeding another LTP is a also a good solution for feedback amplifiers - the additional gain it provides allows more feedback. This reduces the differential voltage at the input LTP so the transfer curve limitations are less severe. But LTPs inside a global feedback look need frequency compensation.

I wouldn't use an LTP as a phase splitter either as I believe a good quality transformer would be inherently more accurate.

Of course single devices have their own problems - dc offset and thermal drift (bipolars).

[Shoog]

Originally Posted by Bigun

My experience with LTPs is from the solid state world only. But I don't like them. I don't plan to use them anymore and I much prefer a single device at the input.

Why ?

The LTP works great on paper when you have two perfectly identical devices but when they are different you can have a fair bit of asymmetry. Invariably you need a current mirror to ensure good balance between the currents through each device, and you often need the additional gain that the current mirror provides (see next point) as well as the improved slew rate to drive the compensation capacitor in a feedback amp.

For transistors at least, the transfer curve for an LTP is rather narrow in terms of differential input voltage. It may look enough on paper, but you suffer under transients - the LTP likes small signals. And I've heard the difference - flattening/expanding the transfer curve by using a complimentary feedback pair for the LTP provides remarkable improvements in bass and overall clarity through having a more accurate error amplifier up front. An LTP feeding another LTP is a also a good solution for feedback amplifiers - the additional gain it provides allows more feedback. This reduces the differential voltage at the input LTP so the transfer curve limitations are less severe. But LTPs inside a global feedback look need frequency compensation.

I wouldn't use an LTP as a phase splitter either as I believe a good quality transformer would be inherently more accurate.

Of course single devices have their own problems - dc offset and thermal drift (bipolars).

Thats why I always put a transformer in front of the LTP.

Shoog

[the_manta]

Shoog,
Can you name some cheap input transformers ? That sounds interesting. I always thought quality and well designed zobel would be neccesary at least.

[Shoog]

People will turn in their grave at what I am about to say here but I have had success with three types of input transformer;
-OEP microphone input transformer with a 9:1+1 step down ratio for a headphone amplifier
-OEP line input transformer for about €30.00, 2:1+1 step down, a little rolled off beyond about 30khz but adequate.
-mains power toroidals 110V:22V+22V, this is good out to about 60khz but needs loading to stop it ringing out at this frequency - good down to 10hz. This works better than the cheap OEP transformer.

I am currently getting together the parts for a poor mans (cheap as chips) Amnity clone using Talema 115V+115V:15V+15V 7VA power toroidals as both input CT choke and output transformer. I have every reason to expect (from past experience) that this will work perfectly. If using this approach, things to watch out for are, the polarity of the windings as they will ring more easily in one direction, always go for a step down ratio to overcome the inherent interwinding capacitance, and keep all DC out of the windings (Cathode current mirrors will be the used method), and finally go for the smallest VA rating possible to get the most inductance (100H depending on brand). If you expect your transformers to perform flat out to 500khz - then you will have to look elsewhere.

As I said, this is simply to horrific for most people to contemplate, but I have found that I can make a very good sounding product when these details are attended to.

Shoog

[leadbelly]

Morgan Jones covers this in 3rd ed. At the the risk of paraphasing excessively and ruining the message, the balance of a single LTP stage, even if CCS loaded, is significantly lower than a concertina. Multiple CCS loaded LTP stages exhibit comparable balance to a concertina. IIRC this is how the book progresses from the concertina-based Bevois Valley to the 2-LTP-stages-based Crystal Palace.

[Bigun]

Originally Posted by Shoog

Thats why I always put a transformer in front of the LTP.

Shoog

I must be missing something as I don't see how an input transformer is relevant, you are still relying on the accuracy of an LTP made out of two imperfectly matched devices. A phase splitting transformer of good quality should be more accurate.