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Old 4th June 2012, 10:19 AM   #751
sonnya is offline sonnya  Denmark
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Hi All.

I have been very quiet about the TSSA and i have not been responding on the last order of KSA1381ESTU and KSC3503ESTU.

I am waiting on a plain in Helsinki, so i have some time to write you all.

The reason for me not responding is that there is serious issue with the TSSA circuit. We have tried to solve it, and Andrej has come with an solution that i need to try.

But basically what happens is that when we overdrive it with a load, DC levels on the front-end starts to shift and the transistor turns off and generates crossover distortion. The front-end will slowly regenerate from this fault over several seconds.

For this reason i am not going to offer the boards in its present state - They are not up to my standards!

I have attached the measurements.

I will offer an pure 2 stage amp. If the circuit Andrej has drawn will solve the problem it will be an real TSSA 2 stage, if not it will be a 2 stage amp with SSA or diamond buffer input.

No doubt about the sound quality of the 2 stage - It is good, but at the same time don't expect more than 40 - 50Watt @ 8R from it. In most cases this will be enough.

The TSSA modified to an SSA does not have this problem. But it then has the drawbacks of the SSA with pour DC stability.

The diamond buffer is very stable in terms DC stability.

- Sorry. But i thought it was more in place to be honest with your fellows.
Attached Files
File Type: pdf tssav1.5 crossover - 22vp.pdf (22.6 KB, 120 views)
File Type: pdf tssav1.5 crossover - 10vp from 22vp.pdf (24.8 KB, 67 views)
File Type: pdf tssav1.5 crossover - 10vp after 20seconds.pdf (25.2 KB, 85 views)
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Old 4th June 2012, 12:00 PM   #752
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I looking forward to Andrejs solution.
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Old 4th June 2012, 12:19 PM   #753
AKSA is offline AKSA  Australia
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I'm very sorry to hear this, however, amp design is not easy and this is a lesson to all that even the smartest guys are challenged.

OTOH, I'm confident Andrej will find a solution..... some sort of latching diode comes to mind.

Hugh
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Old 4th June 2012, 01:17 PM   #754
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Sonnya,

post your final circuit and pcb layout, lets have a go at it. It could be something is thieving current away from something else.
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Old 4th June 2012, 01:45 PM   #755
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Increasing the value of the feedback resisters and the gain control resistors for same closed loop gain as original should do it.

Last edited by homemodder; 4th June 2012 at 01:53 PM.
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Old 4th June 2012, 02:08 PM   #756
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Hi to all

Here's Sonny's TSSA Basic V1.5.

Also I added Esperado's Crescendo SSA modification with very similar values of DC decoupled feedback bridge and he never reported similar crossover distortions issue.

The difference to SSA is that SSA has DC coupled fb with low DC load impedance (200 Ohm - two 100 Ohm fb gnd resistors), at TSSA the DC load impedance are only two 1 k feedback resistors in series, so 2 k all together. CCS in SSA inject 10 mA into fb bridge, at TSSA only 1,8 mA.

The problem Sonny reported persist only with output load connected, so there's some current leak out/in from bridge's DC potential and as this serves the bias to the input pair and to whole two stages of the TSSA, so it normally should be stable.

I also attached my proposal to hold DC potential, it could also be some small resistor value ie. 100 Ohm resister as well. It should be tested though.
Attached Images
File Type: gif Esperado-Crescendo-SSA mod.gif (34.8 KB, 485 views)
File Type: jpg TSSA FB BRIDGE.jpg (72.0 KB, 481 views)
Attached Files
File Type: pdf TSSA-01010512 by Sonnya.pdf (57.9 KB, 110 views)

Last edited by Lazy Cat; 4th June 2012 at 02:19 PM.
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Old 4th June 2012, 03:09 PM   #757
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Lazy Cat View Post
Hi to all

Here's Sonny's TSSA Basic V1.5.

Also I added Esperado's Crescendo SSA modification with very similar values of DC decoupled feedback bridge and he never reported similar crossover distortions issue.

The difference to SSA is that SSA has DC coupled fb with low DC load impedance (200 Ohm - two 100 Ohm fb gnd resistors), at TSSA the DC load impedance are only two 1 k feedback resistors in series, so 2 k all together. CCS in SSA inject 10 mA into fb bridge, at TSSA only 1,8 mA.

The problem Sonny reported persist only with output load connected, so there's some current leak out/in from bridge's DC potential and as this serves the bias to the input pair and to whole two stages of the TSSA, so it normally should be stable.

I also attached my proposal to hold DC potential, it could also be some small resistor value ie. 100 Ohm resister as well. It should be tested though.
I will get back to this on friday when i am back.
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Old 5th June 2012, 05:17 PM   #758
sonnya is offline sonnya  Denmark
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Ok, after some thinking a simulation where i could recreate the crossover.

I have an reason for the issue and a solution.

Reason:

When driving the input to 2Vpp the current through the feedback network is 20mApp in each feedback network. As long as the output stage can give the input voltage swing multiplied with gain very little current from the feedback network will flow into the feedback network.

But in the case where the output stage reaches its limit the current will start to flow into the input pair and reverse bias the be diode.

To solve it:

We need to raise the input pairs bias current to above the level of current that flows in the feedback network. => 10mA.

Problem that rises from deep:

If we raise the bias current in the input stage we need to lower the collector resistor by the same factor that the current is raised with. And this leads to an openloop gain that is the same factor lower.

Solution:

The soulution (Did i misspell something here???) is to insert a bias current of 9mA across the collector resistor. Now we can raise the collector resistor to 10K which is ~5x more open loop gain...

Now we only need to make a precise balanced non drifting current mirror!

I think this must be the contest of today!

I have attached a simplified schematic and simplified spice model of it.

It took me 3 pints of Finnish beer which is in average not stronger than a danish light beer to get to this solution. Click the image to open in full size.

My favorite beer is Midgård from Dansk Mjød A/S

The label says: Midgårds beer is a Gods beer from the Danish mead (Do you now this beer Michael???) Click the image to open in full size.

The only beer for Gods music through the DIY gear made for gods!!!!!

The FFT that is attached shows the potential of this simplified TSSA V1.6

Comments to the simplified:

It would be a good idea to add the CFP to the input pair as Gerhard has suggested. The FET cascode will be modulated a lot. Even to the point where the vgs voltage gets down to zero shorting the input pair.


Have a look and in the mean time i will change the schematic for the TSSA V1.5
Attached Files
File Type: pdf tssav16-simplified.pdf (12.1 KB, 157 views)
File Type: pdf tssav16-simplified 1w 8R 1KHz.pdf (45.8 KB, 117 views)
File Type: asc tssav16.asc (5.1 KB, 53 views)
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Old 5th June 2012, 06:26 PM   #759
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Sonnya, you are a stubborn fighter. Good that you did not give up and even found a way for improvement. Thanks also that you appreciate my original idea.
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Old 5th June 2012, 06:33 PM   #760
MiiB is offline MiiB  Denmark
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Hey that WAS really cool...nice way to raise open-loop gain and not the output current...This way I believe spec.'s could be really good.. Hats off....

I don't know that beer, but let's have a get together at my place in Aarhus and drink a few.....
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