TGM Amplifier ?

[Bigun]

So next up I tried the LTP boost resistor, providing some direct feedback from the output to the emitters of the LTP.

The first attachment shows the result when the current through this resistor is around 8% of the LTP dc current. And the H2 content is up, not much though.

[Bigun]

Now we crank the LTP boost current up to 14% of the dc level. Now H2 rises as if under my command

[Bigun]

And power corrupts, so we crank it up further...

all those lovely harmonics monotonically decreasing just as the text book asks for.

Of course, it's all theory until I finish the construction...

[destroyer X]

Enjoy this nice topology sonics.

I made i visit, have not readed all thread..i hope you are having good results with this big current into the drivers.

Floating driver emitter resistance, in this kind of emitter follower, have worked better, at my home, when using 150, 180 or 220 ohms.

This avoids the driver saturates before the output.... reduces distortion and works better this way.

regards,

Carlos

[Bigun]

Carlos,

Thanks for the compliment, which has value coming from one of the 'Masters' on this forum.

Unfortunately you will be confused reading my thread by all the changes that I keep making to the design as I learn and then unlearn and then learn again... some time ago I have increased the resistor between driver emitters to 180R in the design. I wish this was because I understand fully the need but it is mostly because of good advice received so far.

[AKSA]

Gareth,

Looking good.

According to an academic friend in the psychoacoustic field, if H2/H3 are slightly above 60dB down, and all higher harmonics are more than 60dB down, there will be masking, and the impression will be one of 'warmth'. Further, the subliminal effects of unhead, objectionable harmonics will be completely masked, and the listening experience will be judged subjectively pleasant.

This is not to say we should try to get the harmonics lower than this, preferably 90dB down or more, but as you have found this is not always easy, though a feedback amp, at the expense of a profusion of harmonics, is pretty good.

Hugh

[Bigun]

Hugh, thanks for the guidance on optimizing this, I'll have to do some work once I have a prototype by trying some different component values. I think your approach to using low order harmonics to 'mask' other artifacts is an under-appreciated approach.

Power Supply
========
I've started looking at the question of cross-talk. I have to build several channels. For stereo it makes good sense to invest in a dual supply so I suspect not very much effort has been devoted to multiple channels. For 5 channels it gets more cumbersome. People have complimented me on my approach of using mono-blocks, but it is a brute-force solution and I thought it worthwhile to dig further.

I set up a simple simulation. It may be too simple and simulations are not the same thing as real life where there are many more parasitics to include than I have bothered with. But it gave me a tool to explore the relative effects of topology on the cross talk from one channel to another. I looked at a two channel arrangement where I set up one channel with zero amplitude input and the other with a 7kHz test signal (I removed the ground-lift resistor as this messes up the reference point for the output) and looked for 7kHz appearing on the power supply rails and subsequently contaminating the other channel.

I've looked at three basic approaches.
1/ Single Supply to all channels
2/ Dual supply, one for all of the input stages, one for all the output stages
3/ Mono block, each one a separate supply

Attached is an example of the Dual - Supply. The main supply is that for the output stages, the intenal resistance is set to 0.5 Ohm and filter capacitors with a series resistance of 0.05 Ohms. The TGM1 amplifier is represented by the triangle.

[nigelwright7557]

Originally posted by Bigun

2/ Dual supply, one for all of the input stages, one for all the output stages

[/B]

Beware of latch up when doing this.
If one rail comes up or drops before another it can cause problems.

They usually have diodes between the rails to stop the lower one discharging before the upper one.

[Bigun]

Here's what I found:

3/ Mono-blocks.
zero ripple induced on neighbouring channel. No surprises.
About 55mV of ripple on the power rails of the channel being driven.

2/ Single Supply
With no filtering on the rails to the front end I get cross talk from one channel to the other. In my particular simulation it works out as -75dB of cross talk on the output.

By introducing RC (100 Ohm, 100uF) filter on the +ve supply to the LTP I see very little improvement. This is my basic TGM1 design so this means I have to improve my design if I want to use a single supply.

By introducing RC filters on both +ve and -ve rails to the amplifier front end (LTP and VAS) the cross talk is greatly reduced. I see around -100dB. This maybe good enough.

3/ Dual Supply.
I feed the front end of both amplifiers from a common supply. When I do this on the +ve rail only (so the front end gets it's -ve feed from the same psu as the output stage) there is no great improvement. When I completely separate the front end with independent +ve and -ve supply I see the cross talk eliminated. Nada. It simulates as good as the mono-blocks.


What I conclude is that a Dual Supply is good enough. For stereo it doesn't make sense, since you may as well have dual mono-blocks. But for multiple channels, dual supply is the best solution (in theory).

What I see from the single supply results is that the cross talk can be brought down to very low values. In a design where I may be using increased H2/3 to 'hide' higher order contamination a cross talk result of -100dB is good enough. For HT application this level of cross talk should be fine. Of course the simulation is most likely underestimating the cross talk and I'm running this at low power levels. Nevertheless, a single supply doesn't look to be the dog that it first appears.

Bottom line - Dual supply (front end and output stages) is the front runner for TGM multi-channel.

[Bigun]

Originally posted by nigelwright7557


Beware of latch up when doing this.
If one rail comes up or drops before another it can cause problems.

They usually have diodes between the rails to stop the lower one discharging before the upper one.

Nice catch - thanks Nigel.