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Author Topic: Current sources for input stage and VAS  (Read 2437 times)

shasam

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Current sources for input stage and VAS
« on: November 28, 2020, 02:16:09 PM »
Hi!

I would like to use current source for the input stage and for the VAS of my amplifier, and I see different implementations of these, but I don't realy understand what it does change...

When using current sources for the input stage and for the VAS, with a transistor used for voltage reference, sometimes the base of this transistor is connected on the emiter of the LTP curent source transistor, sometimes it is on the VAS one. Could someone explain me what difference does it make please?

Also, resistors are sometime used on the base of one or more of those three transistors. Randy Slone write 1k is a good value, but I haven't find any further information about this. What is their purpose please?

Thanks a lot for your help!
Please apologise my poor English.

Loudthud

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Re: Current sources for input stage and VAS
« Reply #1 on: November 28, 2020, 08:35:59 PM »
Use the circuit on the left. Problems occur when the VAS clips. The VAS transistor turns off and the VAS current source has no where for the collector current to go, so it sucks a big base current until it's Voltage reference gives up. R7, limits the base current and prevents the input stage current source from going crazy.

Another subtle problem. C1 is intended to filter the reference current for both current sources. But, the way you have it grounded, it doesn't do it's job. It needs to be connected between the plus rail and the junction of R3 and R13 so R3 doesn't see the ripple on the plus rail. Be sure to connect the + side to the plus rail.

This creates another problem, it limits how fast the LTP current source turns on when the amp powers up. That creates a big pulse on the output. If you can make the current source turn on fast, the output will be almost silent when the amp  turns on and off.




shasam

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Re: Current sources for input stage and VAS
« Reply #2 on: November 29, 2020, 09:50:50 AM »
Use the circuit on the left. Problems occur when the VAS clips. The VAS transistor turns off and the VAS current source has no where for the collector current to go, so it sucks a big base current until it's Voltage reference gives up. R7, limits the base current and prevents the input stage current source from going crazy.

Thanks Loudthud! So I should use a circuit like the one in attachement, with no other base resistor?
In a lot of schematics in Douglas Self's book, he use the other circuit (base of the reference transistor connected on the VAS current source emiter). Do you know why, please?


Another subtle problem. C1 is intended to filter the reference current for both current sources. But, the way you have it grounded, it doesn't do it's job. It needs to be connected between the plus rail and the junction of R3 and R13 so R3 doesn't see the ripple on the plus rail. Be sure to connect the + side to the plus rail.

Sorry, my drawing was not clear, but this is how the capacitor is connected. :)


This creates another problem, it limits how fast the LTP current source turns on when the amp powers up. That creates a big pulse on the output. If you can make the current source turn on fast, the output will be almost silent when the amp  turns on and off.

Thanks! I will look for that. Do you have a suggestion for a faster turn on please?

Loudthud

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Re: Current sources for input stage and VAS
« Reply #3 on: November 29, 2020, 11:27:42 PM »
So I should use a circuit like the one in attachment, with no other base resistor?
In a lot of schematics in Douglas Self's book, he use the other circuit (base of the reference transistor connected on the VAS current source emitter). Do you know why, please?

I can't speak for Mr. Self but perhaps he doesn't listen to his amps when they are clipping.  A base resistor on Q90 or Q91 might tweek the temperature coefficient slightly or suppress any tendency of the circuit to oscillate.

Do you have a suggestion for a faster turn on please?

Try one of these:
1) Replace C32 with a zener diode.
2) Replace R156, R157 and C32 with a high impedance current source.
3) Connect the bottom of R157 to the minus rail and change R156 to 22K. Connect the anode of a 1N4007 to the junction of R156, R157 and C32 and the cathode of that diode to ground. This will force C32 to charge fast to the rail Voltage, then slowly charge to about 1.5 times the rail Voltage.


shasam

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Re: Current sources for input stage and VAS
« Reply #4 on: December 01, 2020, 06:54:36 AM »
Thanks a lot Loudthud!
I will look deeper into this next week end, when I get more free time.

shasam

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Re: Current sources for input stage and VAS
« Reply #5 on: December 05, 2020, 04:13:42 AM »
Hi!
I have done some measurements this morning.
With the circuit on the right on my first picture (base connected to VAS current source emitter), I have a THD between 0.04% and 0.09%. (The thermal tracking is not great, and the bias shift, I think this is why the THD varies like this.)

With the one on the left, that you suggested, the THD goes up to 0.6%, 0.7%!   :(

For the moment I have not use any base resistor.

Have a good week end!

shasam

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Re: Current sources for input stage and VAS
« Reply #6 on: December 06, 2020, 03:15:38 PM »
Hi!
I have find some informations about this today  :)

First quote from ESP, that suggest to use the input stage source as reference :
"A special note for the unwary - If one is to use a single current control transistor for both the LTP and Class-A driver, do not use the Class-A (aka VAS - voltage amplifier stage) current as the reference, but rather the LTP.  If not, the varying current in the Class-A circuit will cause modulation of the LTP emitter current, with results that are sure to be as unwelcome as they are unpredictable [ 4 ].  Where the current source reference is based on the VAS (Class-A driver), it's advisable to decouple the voltage reference for the LTP source to minimise interactions."
(https://sound-au.com/amp_design.htm)

And a second one, from Douglas Self, that suggest to use the VAS current source as reference, and give an explanation about the base resistor :
"Bias isolation resistors are not unique to the amplifier of Figure 15.1; they are very commonly used. For an example taken at random, see Meyer.7 My own purpose in adding R23 was not to isolate the two current sources from each other at AC (something it utterly fails to do) but to aid fault-finding. Without this resistor, if the current in either source drops to zero (e.g., if TR1 fails open-circuit), then the reference voltage collapses, turning off both sources, and it can be time-consuming to determine which has died and which has merely come out in sympathy. [...]
The original amplifier used an active tail-source, with feedback control by TR14; this was a mere whim, and a pair of diodes gave identical THD results. It seems likely that reconfiguring the two current-sources so that the VAS source is the active one would make it more resistant to feedthrough, as the current-control loop is now around TR5 rather than TR1, with feedback applied directly to the quantity showing unwanted variations"
The amplifier of figure 15.1 is in attachement.

shasam

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Re: Current sources for input stage and VAS
« Reply #7 on: December 22, 2020, 05:17:14 AM »
Hi! Some news about the amplifier :
I have greatly improve the THD... by disconnecting the multimeter who was tracking the bias on the emitters resistors... I should have think about it before.  :-[
I have made some measurments of how the resistor on the base of the transistor CCS voltage reference influ THD. I have use 1k. With the reference connected to the VAS CCS, the THD without it is 0.08%, with the resistor it is 0.008%, 10 time improvement!  :tu:

With the reference connected to LTP CCS, the THD without it is 0.1%, and with it, again 0.008%. I think the major distortion now come from elsewhere in the circuit, so the difference in the transistor position is not measurable.

I have not improve the thermal tracking for now, so the results are approximate.