John Curl's Blowtorch preamplifier part II

[Bob Cordell]

We've discussed some about the possibility of distortion caused by program-dependent output stage temperature variations and how to measure the consequences. This is important because it could bring out some causes of audible differences in amplifiers that are not uncovered in conventional lab tests.

A related issue is the different tendencies of amplifiers to be vulnerable to this phenomena. High-bias amplifiers with a large number of output transistors, like the JC-1, will naturally tend to experience less junction temperature variations with program material.

It is also likely that amplifiers built with ThermalTrak output transistors will suffer less from this problem as the thermal feedback to the biasing network is faster and more faithful.

Interestingly it seems likely that amplifiers built with the LT1166 bias controller will have fairly high immunity to this problem, since bias is set with these devices by a real-time monitoring of the voltage across the emitter ballast resistors.

Cheers,
Bob

[john curl]

The 1166 has its own problems. High frequency distortion, for example.

[syn08]

Originally posted by john curl
The 1166 has its own problems. High frequency distortion, for example.

True, but LT166 high frequency distortions can't harm something like the JC-1.

[PMA]

JC-1 does not need it (the 1166).

[Bob Cordell]

Originally posted by john curl
The 1166 has its own problems. High frequency distortion, for example.

Hi John,

Yes, you're right about that. The usual implementations with the LT1166, such as those shown in the datasheet and app notes, can contribute distortion. Some of those arrangements actually pass the signal through the LT1166.

There are better arrangements where the LT1166 can be used only as a dynamic bias spreader where distortion introduced by the 1166 is quite low.

If we can get the distortion down with a better application of the 1166, then I think it may do a pretty good job of mitigating thermal distortion in the output stage.

I continue to believe that such output stage thermal distortion is not likely an issue in a design like the JC-1.

Cheers,
Bob

[BV]

I tried test setup like attached...At TP are with "good" amp only "conventional" products of DUT nelinearity (and so a set of "static" IMD products) and products of non-zero output impedance, fluctuations caused by cycled thermal power are negligible. In my test are "static" IMD products about -110 dB, changes not more than 3-5dB. Square signal can be replaced by any other signal.

[john curl]

We rejected the 1166 more than 10 years ago, after extensive testing by both me and Taiwan. It kept us from meeting THX specs. No remedy offered by LT.

[Bob Cordell]

Originally posted by john curl
We rejected the 1166 more than 10 years ago, after extensive testing by both me and Taiwan. It kept us from meeting THX specs. No remedy offered by LT.

Hi John,

Can you show us how you were using it?

Thanks,
Bob

[john curl]

Look at app note.

[Bob Cordell]

Originally posted by john curl
Look at app note.

Hi John,

I agree; if that's all you did, it will not yield good results.

There are better ways to use the LT1166.

Cheers,
Bob